U21A-0001
Mercury's Internal Magnetic Field: Modeling Core Fields with Smooth Inversions
MESSENGER's second flyby (M2) of Mercury on 6 October 2008 will provide significantly improved geographical sampling of the planet's internal magnetic field over previous measurements. Latitudinal coverage and spacecraft altitudes will be similar to those during MESSENGER's first encounter (M1), but the spacecraft trajectory will be displaced by about 180° in longitude, yielding the first magnetic measurements in the western hemisphere. We investigate spatial structure in Mercury's internal magnetic field by applying methods from inverse theory to construct low-degree-and-order spherical harmonic models. External fields predicted by a parameterized magnetospheric model are subtracted from the vector field observations. The approach takes into account noise contributions from long-wavelength uncertainties in the external field models, unexplained short-wavelength features, and spacecraft attitude errors. We investigate the effect of different regularization (smoothness) constraints on our inversions. Analyses of data from M1 and the two Mariner 10 flybys that penetrated the magnetosphere yield a preferred spherical harmonic solution to degree and order eight with the centered, axial dipole term g10 dominating. The model shows structure at low and mid-latitude regions near the flybys. Terms predicted by an analytical model for long- wavelength crustal fields – namely g10, g30 and g32 – are present, but their relative amplitudes are not consistent with such a field. We conclude that structure in our models is dominated by core, rather than by crustal, fields. We also investigate, through simulations, field morphologies that are recoverable while the spacecraft is in orbit about Mercury, under the assumption that the long-wavelength contributions from external sources can be accurately modeled and removed. Although the elliptical orbit of MESSENGER will impede the recovery of southern hemisphere structure, we obtain excellent recovery of the dipole field and of features at mid-to-high northern latitudes, delivering promising results for the characterization of core fields during the orbit phase.
U21A-0002
Definition of Mercury Dipole Moment and its Offset Using the MESSENGER First Flyby data
Based on the "Paraboloidal" model of Mercury's magnetospheric magnetic field constructed by us previously, parameters of the global magnetospheric current systems are determined for the first MESSENGER flyby. The intrinsic unified planet's dipole moment parameters are derived by fitting the magnetic field measurements obtained by MESSENGER and Mariner 10. During both Mariner 10 first and third flybys on 29 March 1974 and on 16 March 1975, the orientations of the Mercury planet body relative to the Sun were the same. Due to Mercury's 3:2 spin–orbit resonance, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days (two sidereal Mercury orbit periods, or three sidereal Mercury rotation periods). As the time lag between Mariner 10 first and third flybys coincides with two Mercury solar days, the longitudes of the subsolar point on the Mercury surface (Mercury noon longitude) are the same for both these flybys and are equal to about 260 degrees. However, during the MESSENGER first Mercury flyby on 14 January 2008, the noon meridian longitude is different. This fact gives us a possibility to determine not only the dipole offset in a vertical direction (relative to the equatorial plane), but also the dipole offset relative to the origin in the equatorial plane. As it is shown by fitting of all (Mariner 10 and MESSENGER) magnetic field data, this offset is not negligible, as it was proposed previously. The more correct definition of the planet's dipole and magnetospheric source magnetic fields, probably, could be found by introducing of some new additional magnetospheric current systems, specific for Mercury.
U21A-0003
Mercury's internal magnetic field: Constraints on fields of crustal origin
Observations of Mercury's internal magnetic field during MESSENGER's first flyby (M1) and the first and third flybys of Mariner 10 (M10-I, M10-III) suggest that small-scale crustal magnetic fields, if they exist, are at the limit of resolution. Small-scale crustal fields are most easily identified near closest approach (CA) as features with wavelengths comparable to, or larger than, the spacecraft altitude. One small feature (< 4 nT in magnitude) encountered near CA during MESSENGER's first flyby may be either a crustal magnetic field or a plasma pressure effect. By means of Parker's constrained optimization approach, with no assumptions on the direction of magnetization, we can place constraints on the product of magnetization and magnetized layer thickness from such observations. The second flyby (M2) will allow additional constraints to be placed on the presence of small-scale fields, and correlations will be possible among topographic profiles measured by the Mercury Laser Altimeter (MLA), features seen on MESSENGER and Mariner 10 images, and any variations in the internal field. This flyby will acquire the first images of the CA region of M10-III, which has been pivotal in establishing the dipolar character of Mercury's magnetic field. Our ability to isolate small-scale crustal magnetic fields has been hindered by the limited coverage to date, as well as the difficulty in isolating the internal field. Across the terrestrial planets and the Moon, minimum magnetization contrast and iron abundance in the crust show a positive correlation. This correlation suggests that crustal iron content plays a determining role in the strength of crustal magnetization.
U21A-0004
The Effect of Iron "Snow" Layers on Magnetic Field Generation in Mercury
Measurements of Mercury's magnetic field from the Mariner 10 and MESSENGER flybys show a field of internal origin, with a dipole moment on the order of 250 nT-RM3 RM is the radius of Mercury). Although the field is likely caused by a planetary dynamo, a field as weak as Mercury's is difficult to produce with an Earth like dynamo. Owing to its anomalously weak dipole signature, the origin of the Mercurian magnetic field remains a mystery. Recently, Chen et al. (2008) performed experiments which indicate that convection in Mercury's outer core may be driven at multiple locations by an iron precipitate or "snow". Using the Kuang-Bloxham numerical dynamo model, we investigate the effects of these iron snow zones on the dynamo to determine whether these layers can help explain Mercury's weak field. We find that when snow zones are placed both midway through the core, and at the core mantle boundary, the observed dipole field strength is reproduced. Because the geometry of these layers is dependent on the chemical makeup of the core, the results can be used to provide constraints on the sulphur content in Mercury's core.
U21A-0005
A New Orientation Model for Mercury
The IAU Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites (WGCCRE) has published orientation models for Mercury since 1980. The IAU-recommended spin axis orientation (Seidelmann et al, CMDA 98, 2007) is based on assumptions made in 1980 and does not reflect current knowledge. It differs from the measured spin axis orientation (Margot et al, Science 316, 2007) by ~0.04 degrees, an unacceptably large offset for precision work. The resulting positional errors on the surface are of order ~1,500~m, about two orders of magnitude larger than the resolution achievable by MESSENGER cameras. I present an updated orientation model with a formulation for incorporating the forced librations in longitude. An accurate model is crucial for mission planning and data analysis, as well as for characterizing the interior properties of the planet (Peale, Nature 262, 1976). Twenty-one radar measurements obtained from 2002 to 2006 at a wide range of geometries yield the orientation of the spin axis (α=281.0097°, δ=61.4143°) and a robust obliquity value of 2.11 ± 0.1 arcminutes (Margot et al, Science 316, 2007), precisely within the range of theoretical expectations (Peale et al, MAPS 37, 2002). The spin axis uncertainty contours fall on the locus of possible Cassini state positions defined by the orbit pole and the Laplace pole, indicating that Mercury is in Cassini state 1. The occupancy of the Cassini state and the obliquity measurement place important constraints on the moments of inertia and on the interior structure of the planet. This work was supported by the NASA Planetary Astronomy program 07-PAST07-0020.
U21A-0006
Libration and obliquity of Mercury from the BepiColombo radio science and camera experiments
Mercury is the most enigmatic among the terrestrial planets, but the space missions MESSENGER and BepiColombo are expected to advance largely our knowledge of the structure, formation, and evolution of Mercury. In particular, insight into Mercury's deep interior will be obtained from observations of the 88-day forced libration, the obliquity and the degree-two coefficients of the gravity field of Mercury. Of those quantities, the libration is the most difficult to measure and will hence be a limiting factor We report here on aspects of the observational strategy to determine the libration amplitude and obliquity, taking into account the space and ground segment of the experiment. Repeated photographic measurements of selected target positions on the surface of Mercury are central to the strategy to determine the obliquity and libration in the frame of the BepiColombo mission. We simulated these measurements in order to estimate the accuracy of the reconstruction of the orientation and rotational motion of the planet, as a function of the amount of measurements made, the number of different targets considered and their locations on the surface of the planet. From this study, we determine criteria for the distribution and number of target positions to maximize the accuracy on the orientation and rotation determination, from which the obliquity and libration are extracted. We take into account the errors arising from the relative positions of the spacecraft, Mercury and the Earth. We consider various error sources such as the solar thermal influence on the spacecraft bus and the Earth based tracking constraint near solar conjunctions of Mercury. The accuracy on the retrieved parameters is then interpreted in terms of accuracy on the constraints on the interior structure of the planet. Our simulations show that the achievable level of accuracy on the libration amplitude and obliquity will be sufficient to constrain Mercury interior structure models, if the orbiter follows the ESA baseline mission scenario and at least 50 landmarks are imaged at least twice over the mission duration, the libration amplitude can be determined in two Mercury years (176 days) with an accuracy of 3 arcsec or better, which is sufficient to constrain the size and physical state of the planetary core.
U21A-0007
The Global Tectonics of Mercury
The first MESSENGER flyby of Mercury revealed a large number of tectonic landforms in areas not imaged by Mariner 10. The dominant tectonic landforms in the areas imaged by MESSENGER are contractional, consisting of lobate scarps, high-relief ridges, and wrinkle ridges. Among the newly discovered tectonic landforms imaged by MESSENGER is one of the largest contractional landforms seen on Mercury to date. Wrinkle ridges not only occur on the interior smooth plains of the Caloris basin and on exterior plains but also on smooth plains that fill the interior of smaller impact basins and larger craters. In the first complete survey of the Caloris basin, MESSENGER revealed a radial graben complex, Pantheon Fossae, which converges on a zone near the center of the basin. Pantheon Fossae and basin-concentric graben located along the entire outer margin of the Caloris basin floor form a complex pattern of extension unlike anything seen in lunar maria. MESSENGER also revealed the first unmistakable evidence of extensional faulting outside of the Caloris basin, in smooth plains inside the peak ring of the relatively young Raditladi basin. Lobate scarps are the most widely distributed tectonic landform in the areas imaged by MESSENGER and Mariner 10. Areas with relatively few scarps occur in longitudinal bands that correspond to regions with less than optimal lighting geometry for recognizing low-relief morphologic features. MESSENGER images of impact craters crosscut and overthrust by lobate scarps indicate that the horizontal shortening across such features is up to 3 km. The identification of previously unrecognized lobate scarps in areas imaged by Mariner 10 indicates that earlier estimates of average global contractional strain are too low. New estimates for the average strain are about one third greater than the previous values but are still lower limits because of the likelihood that not all lobate scarps in imaged areas have been recognized. The timing of lobate scarp formation, on the basis of crosscutting and embayment relations, indicate that thrust faulting had begun before the end of Calorian- aged smooth plains emplacement and continued after the emplacement of the youngest smooth plains material. The slow, continuous radial contraction that accompanied cooling of Mercury's interior and the likely growth of a solid inner core may be expressed by relatively young lobate scarps.
U21A-0008
Gravity and tectonic patterns of Mercury
We consider the effect of tidal deformation, spin-orbit resonance, non-zero eccentricity, despinning, and reorientation on the global-scale gravity, shape, and tectonic patterns of planetary bodies. Large variations of the gravity and shape coefficients from the synchronous rotation and zero eccentricity values, J2/C22=10/3 and (b-c)/(a-c)=1/4, arise due to non-synchronous rotation and non-zero eccentricity even in the absence of reorientation or despinning. Reorientation or despinning induce additional variations. As an illustration of this theory, we consider the specific example of Mercury. The large gravity coefficients estimated from the Mariner 10 flybys cannot be attributed to the Caloris basin alone since the required mass excess in this case would have caused Caloris to migrate to one of Mercury's hot poles. Similarly, a large remnant bulge due to a smaller semimajor axis and spin-orbit resonance can be dismissed since the required semimajor axis is unphysically small (< 0.1 AU). Reorientation of a large remnant bulge recording an epoch of faster rotation (without significant semimajor axis variations) can explain the large gravity coefficients. This requires initial rotation rates > 20 times the present value and a positive gravity anomaly associated with Caloris capable of driving 10-45° equatorward reorientation. The required gravity anomaly can be explained by infilling of the basin with material of thicknesses > 7 km, or an annulus of volcanic plains emplaced around the basin with annulus width ~ 1200 km and fill thicknesses > 2 km. The predicted tectonic pattern due to these despinning and reorientation scenarios and radial contraction is in good agreement with the observed lobate scarp pattern.
U21A-0009
Constraints on Mercury's radius and limb topography from MESSENGER flyby images
Limb images obtained during MESSENGER's first flyby of Mercury have been used to estimate the planet's radius and to study limb topographic relief. The mean radius (with approach and departure limbs both near longitude 215 deg E) was found to be r = 2439.94 km +/- 0.69 km, in agreement with the results from Mariner 10 occultations and Earth-based radar observations. Separate estimates for polar and equatorial radii are similar to within errors, suggesting that any planetary flattening is minor in comparison with the prevalent topography. The residual topography from our limb profiles shows a dynamic range of 7.8 km and a root- mean-square roughness of 0.75 km, in agreement with the characteristics of the near-equatorial laser altimeter profile and stereo topographic models obtained during the flyby. With new camera calibration measurements and additional limb images from MESSENGER's second Mercury flyby expected this fall, we expect that information on Mercury's size and global shape parameters will steadily improve.
U21A-0010
Prospects for the Representation of Geophysical Fields from MESSENGER Observations of Mercury Using Harmonic Radial Bases
At present, the most common representation of global planetary geophysical data is a spherical harmonic expansion. While this representation is suitable for data that are well distributed globally, it does not adequately represent datasets with highly variable resolution. Many geophysical datasets are acquired in a fashion that produces a very unevenly resolved distribution of observations. For example, the gravity and magnetic field data to be acquired during the orbital phase of the MESSENGER mission to Mercury will have highly variable spatial resolution at the surface because of the spacecraft's strongly elliptical orbit. Due to the nature of potential field continuation, data acquired from higher altitudes will be biased towards longer wavelengths and poorer spatial resolution than data acquired at lower altitudes. Representations of data that can faithfully accommodate variations in resolving power and allow for such data processing operations as interpolation, continuation, differentiation, and integration are necessary make to full use of observations with large variations in resolution. A harmonic radial basis is well suited to these tasks. Further, incorporation of this basis into the spherical spline approach can accurately represent the data, regardless of distribution. Accuracy tests of several reproducing kernels (e.g. Green's functions, Abel-Poisson, logarithmic) demonstrate the advantages and disadvantages of this class of representations relative to spherical harmonics for datasets that are non-uniform over the sphere in either distribution or resolution. We discuss the results of modeling aimed at quantifying how well this approach, relative to spherical harmonics, could recover the gravity field at Mercury based upon the expected orbit of MESSENGER during the orbital phase of the mission.
U21A-0011
Topography of Mercury from Stereophotoclinometric Analysis of MESSENGER Flyby 2 and Mariner 10 Images
Imaging data acquired during the MESSENGER flybys of Mercury are being used to construct preliminary models of the surface topography of the planet. These models will be improved when additional data become available from the orbital phase of the mission. For all three flybys the inbound and outbound views present a crescent and gibbous Mercury, respectively, each bounded by the terminator on one side and the limb on the other. Longitudes in the lit hemisphere between the two limbs will not be visible, and even less will be useful for topographic analysis due to the obliquity of the view. The first MESSENGER flyby on 14 January 2008 provided useful data for about 25 degrees in longitude from inbound imaging and about 80 degrees outbound; the latter became increasingly degraded as the sub-solar longitude was approached. Ideally, high-resolution topography is determined from stereophotoclinometry (SPC), by which images at different illuminations are used to solve for topography and albedo in small "maplets," the centers of which are control points for stereographic analysis and low-resolution topography. During a single flyby, the illumination does not change, and close to the sub-solar point it is difficult to distinguish brightness variations due to topography from those due to albedo variations. The second MESSENGER flyby of Mercury on 6 October 2008 will image longitudes from about 90 degrees W to 90 degrees E. From our experience with data from the first flyby, we will be able to solve for topography from the images in a latitude range from 90 to 10 degrees W (outbound images) and, at lower resolution, from 75 to 90 degrees E (inbound images). Overlap between the MESSENGER and Mariner 10 data sets will allow for a true SPC analysis in much of the former region. Of particular interest will be the topography of the many craters and scarps in this region as well as the hummocky region antipodal to the Caloris basin. During the first MESSENGER flyby, Mercury Laser Altimeter (MLA) observations were made along a path near the equator from 10 to 90 degrees E. Topography from the inbound images will be compared with the MLA data where the two overlap.
U21A-0012
Mercurian Basins Revealed from Mariner 10 Stereo-Derived Topography
Previous studies identified the presence of 23 basins within the Mariner 10 imaged hemisphere of Mercury. Several of the basins were classified as "probable", because these degraded and buried basins were identified based on the remnants of rings that are partially exposed or were recognized by the deflection of tectonic features into arcuate patterns. Without topographic data to identify the main rims and ring locations more precisely, previous studies were unable to confirm if these were actual multi-ring basins. With the completion of a regional topographic dataset derived from Mariner 10 stereo images, the proposed ring locations and main rim diameters of 10 of the 23 previously identified basins were examined. Many of the proposed basin rims and ring locations were revised. In addition, two previously unrecognized basins (530 km and 250 km in diameter) were found. Topographic data from MESSENGER will provide additional verification of these impact basins and further details of basin structure.
U21A-0013
New Crater Depth Data for Mercury Derived From MESSENGER Flyby 1 Imagery
For small (D < 10 km) craters on Mercury, shadow measurements and photoclinometry are the only viable methods for assessing crater rim-floor depth (d) with the Mariner and MESSENGER Flyby data. A limitation of past shadow measurements was the requirement for simple craters that the shadow pass through crater center in order to get a reliable depth estimate. This restriction means that shadow measurements from the Mariner 10 data were taken along the two narrow longitude bands with acceptable sun angles. Recently, Chappelow (LPSC 2008, Abs. #1441) developed a generalized shadow method that allows determination of crater shape and rim-floor depth for any crater with a conic section of revolution. The boundary of the interior shadow defines a portion of an ellipse. The shape of that ellipse and its offset relative to crater center can be used to solve for crater depth and interior shape (cone, ellipse, parabola, hyperbola). The method does not require that the shadow cross the crater center, but it does require that the viewing angle is close enough to nadir that the rim outline is circular. We are using this method with MESSENGER and Mariner data to expand the amount and areal coverage of depth data for small craters, and to determine the interior shapes of those craters. So far we have estimated depths and shapes for 133 craters (1.0 km < D < 8.0 km) in twelve of the frames from MESSENGER Flyby 1 NAC Mosaic #1 (images range from 117 to 150 m/pixel). Mean d/D is 0.17 (sd 0.04) with a range from 0.09 to 0.29. An exponential fit (in km) is d = (0.178±0.08) D0.89±0.04. Pike's previous results with Mariner data (Mercury, UA Press, 165- 273, 1988) show a unity exponent with a d/D ratio of 0.2, and are reasonably consistent with our work. The shapes of the crater interiors are slightly more conical than parabolic. We have observed some spatial clusters of craters with d/D ratios significantly different than the global mean, and we are investigating whether these areas correlate with a particular geomorphology.
U21A-0014
Imaging During MESSENGER's Second Flyby of Mercury
During MESSENGER's second flyby of Mercury on October 6, 2008, the Mercury Dual Imaging System (MDIS) will acquire 1287 images. The images will include coverage of about 30% of Mercury's surface not previously seen by spacecraft. A portion of the newly imaged terrain will be viewed during the inbound portion of the flyby. On the outbound leg, MDIS will image additional previously unseen terrain as well as regions imaged under different illumination geometry by Mariner 10. These new images, when combined with images from Mariner 10 and from MESSENGER's first Mercury flyby, will enable the first regional- resolution global view of Mercury constituting a combined total coverage of about 96% of the planet's surface. MDIS consists of both a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). During MESSENGER's second Mercury flyby, the following imaging activities are planned: about 86 minutes before the spacecraft's closest pass by the planet, the WAC will acquire images through 11 different narrow-band color filters of the approaching crescent planet at a resolution of about 5 km/pixel. At slightly less than 1 hour to closest approach, the NAC will acquire a 4-column x 11-row mosaic with an approximate resolution of 450 m/pixel. At 8 minutes after closest approach, the WAC will obtain the highest-resolution multispectral images to date of Mercury's surface, imaging a portion of the surface through 11 color filters at resolutions of about 250-600 m/pixel. A strip of high-resolution NAC images, with a resolution of approximately 100 m/pixel, will follow these WAC observations. The NAC will next acquire a 15-column x 13- row high-resolution mosaic of the northern hemisphere of the departing planet, beginning approximately 21 minutes after closest approach, with resolutions of 140-300 m/pixel; this mosaic will fill a large gore in the Mariner 10 data. At about 42 minutes following closest approach, the WAC will acquire a 3x3, 11-filter, full- planet mosaic with an average resolution of 2.5 km/pixel. Two NAC mosaics of the entire departing planet will be acquired beginning about 66 minutes after closest approach, with resolutions of 500-700 m/pixel. About 89 minutes following closest approach, the WAC will acquire a multispectral image set with a resolution of about 5 km/pixel. Following this WAC image set, MDIS will continue to acquire occasional images with both the WAC and NAC until 20 hours after closest approach, at which time the flyby data will begin being transmitted to Earth.
U21A-0015
A New Global Mosaic of Mercury
In January 2008 the MESSENGER spacecraft made the first visit to Mercury since Mariner 10 and photographed about 38% of the planet at scales of 1 km per pixel or better. The Mariner 10 spacecraft executed three close-approach flybys of the planet Mercury more than thirty years ago. At each encounter the subsolar point on the planet was nearly identical, so only half the planet was illuminated for the Mariner 10 cameras. Approximately one thousand MESSENGER images were processed and combined into a mosaic that covers about 42% of Mercury's surface at a scale of 1 km per pixel. Combining the image data collected by the two missions yields coverage of about 67% of Mercury's surface. Because a spacecraft flyby happens on a timescale much shorter than Mercury's rotation period, there is no appreciable change in illumination of the surface during the encounter and any given area is seen at a single incidence angle. Combining data from different flybys thus result in large differences in illumination for overlapping areas of the mosaic. While distracting, such different lighting in areas of overlap aids in the interpretation of morphology and albedo variations. MESSENGER will return to Mercury on 6 October 2008 to reveal another large area never before seen by any spacecraft, and the combined coverage after that encounter will be nearly global. Common features in areas of overlap between frames will form the basis for analytic triangulation that will improve camera pointing knowledge and focal length estimates for both the Wide Angle Camera (WAC) and Narrow Angle Camera (NAC). Accuracy of the new mosaic will be checked by comparing coordinates of crater centers in Earth-based radar images of Mercury's poles. The new global mosaic will serve science analysis and provide a critical planning tool for the orbital phase of the MESSENGER mission.
U21A-0016
Lateral Viscosity Variations and the Contractional History of Mercury
The lobate scarps of Mercury bear witness to the history of contraction of a cooling planet. Previous models of the contraction of Mercury were for a one-dimensional mechanical model in which outermost crust of the planet is assumed to be elastic. However, the time-averaged surface temperature of Mercury varies strongly with both latitude and longitude due to the pattern of solar insolation over the planet, influenced by the planet's 3:2 spin-orbit resonant state, near-zero obliquity, and eccentric orbit about the Sun. The variation in surface temperature produces variations in lithospheric thickness and viscosity structure that may affect the patterns of deformation and faulting due to contraction. We explore this issue using a semi-analytic, spherical viscoelastic model that incorporates lateral variations of viscosity. Results suggest that temporal and spatial variations in lithospheric thickness during contraction affect the response of the planet. In particular, for contraction occurring relatively late in the presence of a thick lithosphere, the lithosphere deforms at the longest wavelengths, bowing outward where it is thinnest –at low latitudes for Mercury, particularly the areas around the equatorial "hot poles" that face the Sun at perihelion. We couple this model to thermal evolution calculations to simulate the effects of different core evolution scenarios. Topographic data returned by the Mercury Laser Altimeter (MLA) from MESSENGER's first flyby of Mercury in January demonstrate the existence of a long-wavelength slope along the equator, and many new lobate scarps were documented from MESSENGER images. After MESSENGER's second flyby in October, nearly the entire planet will have been imaged by spacecraft. Information on the long-wavelength shape of the planet, the distribution and orientation of lobate scarps, and their relation to other geological features, together with model results, will provide new constraints on the timing of global contraction and the heterogeneous response of the planet to interior cooling.
U21A-0017
The Effect of Heterogeneous Lithospheric Structure on Surface Stress and Tectonics on Mercury
Observations of Mercury's surface from the Mariner 10 and MESSENGER spacecraft reveal a planet-wide distribution of lobate scarps, which are interpreted to be the surface expression of thrust faults due to global contraction. Statistical analysis of scarps mapped in images from both missions suggests that the orientations of these features are not consistent with a uniform distribution, the expected outcome for a contracting lithosphere of constant thickness. One possible explanation for this observation considers Mercury's thermal state, which is driven by the insolation pattern. Mercury's 3:2 spin-orbit resonance, in concert with its substantial orbital eccentricity of e = 0.20, causes long-wavelength surface temperature variations of more than 130 K. Corresponding variations in lithospheric thickness are hence expected. If the contraction occurred while Mercury was in this dynamical state, the resulting stress distribution recorded on the surface during global cooling and inner core solidification may reflect this heterogeneity. We seek to determine whether contraction of a lithosphere with lateral thickness variations can explain the observed lobate scarp orientations on Mercury's surface. We employ the three-dimensional, viscoelastic, finite element model CitcomSVE along with a thermal evolution model to study Mercury's response to cooling, inner core formation, and lithosphere growth and to track the accumulation of stress in the lithosphere. The resulting stress pattern can then be compared to the orientations and distribution of scarps mapped on the surface of Mercury. Preliminary results indicate systematic variations between the expected orientation of faults in the "cold poles," where the lithosphere is thicker and the "hot poles," where the lithosphere is weaker. As MESSENGER returns more data, comparisons between model results and surface features will be refined.
U21A-0018
On the difficulty to detect Mercury's mantle convection from future gravity data
Thermo-chemical evolution and convection models of Mercury suggest that its thin mantle may still exhibit thermal convection. To confirm or disregard this assumption, future gravity and topography data can be in principle used. In the case of present-day mantle convection the dynamic flow in the mantle not only generates a gravity signal possibly detectable above the surface of the planet, but also modifies the observed planetary topography. To estimate the possible amplitudes of these dynamic signatures self- consistently, we have employed a viscous mantle flow model coupled with an overlaying thick elastic lithosphere. For a wide range of present-day elastic thicknesses obtained using a parameterized thermal evolution model and maximal temperature anomalies in the present mantle of Δ T =± 100 K, we find that the dynamic geoid and topography do not exceed significantly ± 10 and ± 250 meters, respectively. Unless the present-day elastic thickness is significantly smaller than the results of thermal- evolution models suggest, we will not be able to distinguish between a convecting or a non-convecting mantle. This is in contrast to earlier findings of Spohn et al. [2001], Breuer et al. [2007] and Redmond and King [2007]. References: Spohn et al. (2001), Planet. Space Sci., 49, 1561--1570. Redmond and King (2007), Phys. Earth Planet. Int., 164, 221--231. Breuer et al. (2007), Space Sci. Rev., 132(2-4), 229--260.
U21A-0019
Caloris Impact Basin: Exterior Geomorphology, Stratigraphy, Morphometry, Radial Sculpture, and Smooth Plains Deposits
New observations of the Caloris basin from the January 2008 flyby of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have helped address important questions about the extent of the basin, its filling and modification history, the nature of its ejecta deposits, and the origin of smooth plains on both its interior and exterior. These data reveal that the basin is ~15% larger than was originally estimated by Mariner 10 and is moderately elliptical (1525 x 1315 km). Prominent basin-related sculpture and secondary craters are observed around Caloris to distances of at least 1700 km from the basin rim (extending over at least 20% of the surface area of the planet), emphasizing the significance of this event as a stratigraphic marker. MESSENGER data indicate that the entire basin interior is partially filled, embaying the Caloris Montes rim material with plains that are likely of volcanic origin. Some areas of smooth plains on the Caloris exterior have characteristics suggesting that they embay both surrounding highlands and other smooth plains, which is strong evidence that they were also emplaced by volcanism rather than impact. However, much of the circum-Caloris plains display limited color contrast with their surroundings and have morphological characteristics consistent with their emplacement as impact melt or fluidized ejecta, similar to the Cayley Plains on the Moon. The stratigraphy of the Caloris basin region that was inferred on the basis of Mariner 10 data is generally supported by new observations, as the main facies of the Caloris Group (the Caloris Montes, Odin Formation, and Van Eyck Formation) are apparent in MESSENGER images. These findings re-emphasize the importance of understanding the Caloris basin for interpreting the detailed geological history of Mercury.
U21A-0020
Geology of the unusual double-ring Raditladi basin on Mercury
During its first flyby of Mercury in January 2008, MESSENGER viewed a large portion of the surface of the
planet not previously seen by spacecraft. From high-resolution images obtained during the flyby we are
investigating the geology of the Raditladi basin, a ~~250-km diameter impact feature located west of the
Caloris basin and centered at 27°N, 119°E. On the basis of impact crater density, approximately
an order of magnitude less than is found on smooth plains to the west of Caloris, Raditladi is thought to be
one of the youngest impact basins on Mercury, with an age of 1-2 Ga or less. Raditladi contains an interior
peak-ring structure ~~125 km in diameter, and its floor is filled with smooth plains material that clearly
embays the central peak ring. The basin walls have undergone modification, with terraces most pronounced
within the north and west sides of the rim. The plains are smoother on the eastern and southern side of the
basin and appear to have covered more of the underlying material. This difference suggests that the fill may
be slightly deeper there than elsewhere in the basin. The plains within the basin floor contain a number of
discontinuous, arcuate, flat-floored troughs, interpreted to be graben. The graben are arranged in an
approximately circumferential pattern about 70 km in diameter, centered ~~10 km south of the basin
center. The graben represent the only major extension identified on Mercury to date outside of the Caloris
basin. It is likely that the troughs formed as the result of uplift and extension of the basin floor, but defining
the source of uplift at a relatively recent stage in Mercury's history when interior cooling models predict a
thick, strong lithosphere presents an interesting challenge. The fill within Raditladi may have a volcanic
origin, as appears to be the case for plains in the nearby Caloris basin, but an alternative interpretation is
that the fill consists of impact melt. Color analyses are underway to help understand the origin of the plains
material, and further studies of the cratering record, topography, and geology will elucidate the history of this
enigmatic basin.
http://messenger.jhuapl.edu/
U21A-0021
Pit-floor Craters on Mercury: Characteristics and Distribution From the First and Second MESSENGER Flyby
From images obtained by the MESSENGER spacecraft during its first flyby of Mercury on 14 January 2008, we have identified five impact craters that exhibit pit craters on their floors. Pit craters are rimless depressions that are determined to have formed by non-impact process. Among the arguments in favor of this interpretation are that the features are rimless steep-sided craters that exhibit no observable ejecta and that they are irregularly shaped, with the long axis of the pit crater often concentric to the host impact crater. Impact craters hosting pit craters we term pit-floor craters, which vary in size from 52 to 120 km in diameter. The associated pit craters range in size from 20 to 38 km in diameter, but there is no evident correlation between pit-floor crater diameter and size of pit crater. Pit craters are found on other planets, occurring along rift zones and commonly in chains. The majority of pit craters observed on Mercury, however, do not match the characteristics of pit craters in these other planetary settings. There are three important differences: First, pit craters on Mercury have only been found of the floors of impact craters and within basins, which suggests that pit craters must be related to the impact process in some way. Second, pit craters identified on Mercury to date are distinctive for their large size. Pit craters on other planetary objects are smaller, typically <2 km in diameter. By comparison Mercury's pit craters range upward from 6 km wide and can be nearly 40 km in length. Pit craters in this size range on the other planets are actually calderas. And third, they occur as isolated craters rather than within chains. Lava flows are not observed to be associated with pit craters on Mercury, and there is little evidence to support formation of the pit craters observed via explosive venting of volatiles. They are however, proximally located to smooth plains deposits. A spatial association between the pit-floor craters and smooth plains deposits nonetheless suggests a genetic relationship. Our interpretation of this relation is that pit craters provide evidence for a process of endogenic crater modification, whereby shallow subsurface magma bodies form collapse pits or calderas in association with near-surface magma reservoirs. In this scenario for pit formation, the mechanism of formation is the result of piston-like collapse over a broad magma body or evacuated chamber. Stoping of roof material by magma below pit-floor craters creates a large cavity, into which the highly fractured crater floor above collapses after magma is withdrawn. At present there is not enough evidence to indicate whether an individual pit crater formed all at once (i.e., in a single collapse or explosion) or in episodes over a protracted period of time. Moreover, it is not known whether all the pit-floor craters were made over a brief and contemporaneous interval or at different times. During its second Mercury flyby on 6 October 2008, MESSENGER will view about 30% of the surface at close range for the first time and may yield additional examples of these enigmatic features.
U21A-0022
Explosive Volcanic Eruptions on Mercury: Eruption Conditions, Magma Volatile Content, and Implications for Mantle Volatile Abundances
During its first flyby of Mercury, MESSENGER imaged several candidate volcanic centers inside the rim of the Caloris impact basin. The most prominent of these features is a broad shield-like structure over 100 km in diameter, with a near-central, irregularly shaped depression surrounded by a bright, spectrally distinctive deposit interpreted to have a pyroclastic origin. The candidate pyroclastic deposit on Mercury has a mean radius of ~24 km, equivalent in size to the third largest lunar pyroclastic deposit when mapped to lunar gravity conditions. From the extent of the candidate pyroclastic deposit, it is possible to characterize the eruption parameters of the event that emplaced it, including vent speed and estimates of magmatic volatile content for candidate volatile species. The minimum vent speed is found to be ~300 m/s, and the pre- eruptive magmatic volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million (ppm) by weight for volatiles of the type that typically propel volcanic eruptions on other planetary bodies (e.g., CO, CO2, H2O, SO2). On Earth, measurements of the exsolution of volatiles (H2O, CO2, S) from basaltic magmas during eruptive episodes at Kilauea volcano, Hawaii, indicate values of ~1300-6500 ppm for the mantle source. Evidence for the presence of significant amounts of volatiles in partial melts derived from the crust or mantle of Mercury is an unexpected result given that most models for the formation of the innermost, metal-rich planet predict extreme volatile depletion.
U21A-0023
Comparison of the Color Properties of Selected Features on Mercury from Mariner 10 and MESSENGER Multispectral Images
During the second flyby of Mercury by the MESSENGER spacecraft (October 6, 2008), the Mercury Dual Imaging System (MDIS) will collect high-spatial-resolution broadband and lower-spatial-resolution multispectral images. The MDIS wide-angle camera (WAC) has 11 narrow-band color filters with center wavelengths in the range 430 to 1020 nm. The portion of the planet that will be illuminated by the Sun and visible to MDIS during the spacecraft's departure trajectory includes a region that was covered by Mariner 10 two-color vidicon imagery in the ultraviolet (355 nm) and orange (575 nm), which has provided color-ratio and spectral parameter images with which a number of workers have made inferences about the composition of surface units. The region of interest is the Mariner 10 first-encounter "incoming" hemisphere. Prominent features in this area include the crater Kuiper, the Rudaki plains, the Homer basin, and the craters Lermontov and Mistral. Kuiper (62-km diameter), the base of the Kuiperian time-stratigraphic system, is superimposed on the older crater Murasaki. Kuiper has an extensive bright ray network, and spectral parameter images indicate that Kuiper has excavated material that is lower in opaque content than the typical surface in the area. The Rudaki plains are low in opaque minerals compared with the surroundings that they embay and may represent a deposit of the High Reflectance Plains (HRP) described on the basis of MESSENGER flyby 1 imagery. Dark, high-opaque material is disposed along a linear ring segment of the Homer basin and may have been emplaced by pyroclastic activity. Lermontov and Mistral have high- reflectance, blue material on their floors, which may correspond to the Bright Crater Floor Deposits (BCFDs) present in the MESSENGER first-flyby departing hemisphere. The new MESSENGER data, which offers greater wavelength range in addition to improved radiometric calibration, co-registration, and signal-to-noise ratio compared with Mariner 10, will provide the opportunity to re-evaluate and extend the conclusions from the prior work.
U21A-0024
Identification of Color Units From the Ultraviolet to the Near Infrared During the Second Messenger Flyby of Mercury
The first flyby of the planet Mercury by the MESSENGER spacecraft on January 14, 2008, provided a wealth of new, high-spatial-resolution remote sensing data. Analysis of spectra (wavelength range 220 – 1450 nm) from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) produced several important new findings. Fresh crater deposits sampled by MASCS did not exhibit spectral features indicative of ferrous- iron-bearing silicates. The average visible to near-infrared spectral slope of the surface of Mercury was found to be significantly lower than that of the farside Moon. This slope difference is attributed to a greater abundance of opaque minerals in Mercury's regolith, a greater fraction of larger-sized particles of metallic iron, or a combination of these two effects. Finally, an absorption feature in Mercury's middle-ultraviolet spectrum, not seen in a lunar spectrum acquired with the same instrument, is interpreted as evidence for a low abundance of ferrous-iron-bearing silicates as well as a relatively lower modification of spectral properties by space weathering. The second flyby of Mercury will provide additional spatial coverage that will allow us to test and refine these initial conclusions. The brief opportunity to view the surface of Mercury during the first flyby allowed only a narrow strip of terrain to be sampled by MASCS. The two end-member color units (low-reflectance materials, and high-reflectance smooth plains) identified through an analysis of multispectral imaging from the Mercury Dual Imaging System (MDIS) acquired during the first flyby were not unambiguously sampled by MASCS. The hemisphere available during the second flyby was imaged previously by the Mariner 10 spacecraft, and these distinct units will now be observed by MASCS. In particular, MASCS will sample the plains areas west of Rudaki crater and the interior of the crater Homer. The Rudaki plains have been interpreted as a deposit of volcanically emplaced material embaying a local region of greater opaque content but similar iron abundance. The wide wavelength coverage and high spatial resolution of MASCS will provide a new dataset from which we will attempt to test these earlier inferences and elicit additional compositional details.
U21A-0025
Low-Reflectance Material in Mercury's Crust
Mercury's reflectance and spectral slope are broadly similar to that of the lunar nearside. However, a host of Earth-based measurements and spacecraft data indicate that the composition and physical makeup of their surfaces may exhibit significant differences. Apollo samples and orbital remote sensing show that the lunar nearside surface is generally high in FeO (6 to 20 wt %) while the farside surface abundance is somewhat lower (3 to 10 wt %). Earth-based remote sensing of Mercury indicates that its surface contains less than 6 wt % FeO and perhaps even lower than 3 wt %. The reflectance of the Moon, and most likely Mercury, is controlled to first order by variations in iron and titanium abundances. If Mercury's ferrous iron content is much lower than that of the lunar nearside, then why is its reflectance comparable (0.019 vs. 0.021 at phase angle 65°, respectively)? Two-color vidicon observations by Mariner 10 revealed patchy low-reflectance, relatively blue units within Mercury's crust. Hapke and coworkers first speculated that opaque minerals (most likely ilmenite) could explain the color and reflectance of this low-reflectance component. Multispectral image data obtained by MESSENGER during its January 2008 flyby of Mercury covered new terrain and provided higher resolution, better signal-to-noise ratio, and extended wavelength coverage above that obtained by Mariner 10. The new data confirmed the existence of the low-reflectance material (LRM) and its relatively blue color and provide much better geologic context to interpret the origin of this material. MESSENGER multispectral data show the LRM to be widespread across the surface and to occur at depth within the crust. Three key observations show the vertical distribution of LRM. First the rims and floors of 100-km-scale craters within the Caloris basin are composed of LRM, streamers of LRM occur in ejecta traceable back to outcrops in crater floors (e.g. Mozart), and large continuous sections of ejecta blankets of several craters and basins are composed of LRM (Tolstoj, Basho, Neruda). What is the composition and origin of the LRM? A significant abundance of ilmenite (low in reflectance and spectrally neutral) within the LRM is the most likely candidate material on the basis of MESSENGER multispectral data, laboratory spectral measurements of analog materials, lunar analogy, and cosmochemical arguments. Its widespread distribution deep within the crust is consistent with LRM forming as an original component of the crust or as a result of widespread later intrusion by magma. We note that no occurrences of LRM as surface flows or volcanic edifices have yet been identified. The widespread LRM material works to lower Mercury's overall reflectance along with space weathering processes. MESSENGER's second Mercury flyby (6 October 2008) will further elucidate the areal distribution of LRM and provide an opportunity for remote sensing over an extended wavelength range with the onboard spectrometer.
U21A-0026
Turn the heat up – A first look at MESSENGER's near-infrared spectra of Mercury using new high-temperature emissivity measurements
Analyzing the surface composition of Mercury's regolith from remote-sensing measurements is a challenging task. In support of the National Aeronautics and Space Agency's MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission and especially in preparation for the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) instrument on the BepiColombo mission of the European Space Agency and the Japan Aerospace Exploration Agency, we are developing a Planetary Emissivity Laboratory (PEL) at Deutsches Zentrum für Luft- und Rahmfahrt (DLR) in Berlin. The PEL allows measurement of the emissivity of Mercury-analogue materials at grain sizes smaller than 25 μm and at temperatures of more than 400°C, typical for Mercury's low-latitude dayside. The PEL development follows a multi-step approach. We have already obtained emissivity data at mid-infrared wavelengths that show significant changes in spectral behavior with temperature indicative of changes in the crystal structure of the samples. We are currently installing a new calibration target that will allow the acquisition of emissivity data over the full wavelength range from 1 to 50 micrometer with good signal-to-noise ratio. Here we present initial data in the range 1 to 1.4 micrometer, the near-infrared wavelength coverage of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument on MESSENGER. Even these early PEL measurements have important implications for the analysis of the spectral observations obtained during MESSENGER's first Mercury flyby on 14 January 2008 as well as the data to be obtained during the probe's second Mercury flyby on 6 October.
U21A-0027
First Estimate of Mercury's Surface Iron Abundance from Neutron Spectroscopy
Thermal neutrons, which are produced on Mercury's surface by galactic cosmic rays and measured by the MESSENGER Neutron Spectrometer (NS), are highly sensitive to the presence of iron and titanium. This high sensitivity is due to the fact that both iron and titanium have large absorption cross sections for thermal neutrons. By a form of Doppler Filter Spectroscopy, the NS successfully measured thermal neutrons from Mercury during the MESSENGER's first flyby of the planet in January 2008. These first thermal neutron data from Mercury were combined with neutron transport models of analog lunar soils to deduce that the average iron abundance of the Mercury surface material is less than ~6% by weight. This upper limit result is broadly consistent with previous estimates of iron abundances that suggest Mercury's surface has low iron abundances. Uncertainties in this iron abundance estimate are due mainly to systematic uncertainties in our understanding of the NS spatial response on the MESSENGER spacecraft. To reduce these uncertainties, we are conducting a detailed study of the neutron transport properties of the MESSENGER NS and spacecraft using the particle transport code MCNPX. These calculations, along with expected new data from MESSENGER's second Mercury flyby in October 2008 will enable us to: 1) achieve a more constrained estimate for or upper bound on Mercury's surface iron abundance from flyby data; and 2) refine the models and techniques for making maps of iron abundances using NS data from the mapping phase of the MESSENGER mission.
U21A-0028
Gamma-Ray Spectrometer Measurements at Mercury during MESSENGER's First Two Flybys
Determining the surface composition of Mercury is important for an understanding the origin and evolution of the planet. A Gamma-ray Spectrometer (GRS) with a high-purity germanium detector is part of the scientific payload on the MESSENGER spacecraft used to measure the elemental composition of the Mercury's surface. The MESSENGER spacecraft flew by Mercury for the first time on 14 January 2008 and will fly by again on 6 October 2008. During the first flyby the GRS was powered on starting about 35 hours prior to closest approach. Gamma-ray spectrometry is typically is photon limited, as the fluxes from a planet are relatively low. Simulations, nonetheless, indicated that some of the strongest peaks from Mercury might be measurable during the flyby. Spectra of the planet were taken while the spacecraft was within 2500 km range (for period of 22 minutes). Gamma-ray peaks from K, Si, Al, and Fe were identified and analyzed. All of these peaks were also measured during the longer period prior to closest approach. To be significant, any net signal from Mercury must be resolvable above this background level. Estimates based on the decay of 40K, indicated that abundances of K greater than 1% by weight on Mercury would be detectable, even for this short accumulation time. The first flyby measurement gave only an upper limit for K of 0.5% for the equatorial region viewed during closest approach. This result adds a new constraint on the abundance of K- rich minerals, at least for this region on the planet. The only peak that was statistically significant above background was the Si inelastic scatter gamma ray at 1779 keV. The second fly-by will allow further measurements of these elements for a different area on Mercury. These measurements will also be combined with those from the first flyby to improve the statistics for estimating the average elemental abundances in the equatorial regions of Mercury.
U21A-0029
MESSENGER X-Ray Spectrometer Observations during the Second Mercury Flyby
Information on the surface elemental composition of Mercury is limited, but will be greatly expanded by observations to be made by instruments onboard the MESSENGER spacecraft. One of these instruments, the X-Ray Spectrometer (XRS), will determine elemental abundances on the surface of Mercury by measuring fluorescent X-ray emissions induced on the planet's surface by the incident solar flux. The most prominent fluorescent lines are the Kα lines from the elements Mg, Al, Si, S, Ca, Ti, and Fe (1-10 keV). The sampling depth is less than 100 microns. Prior to entering orbit about Mercury in March 2011, MESSENGER will have flown by the planet three times for spacecraft trajectory corrections and scientific observations. At the first Mercury flyby (14 January 2008), no X-ray fluorescence was detected from the planet during the 5 minutes of observing time available to the XRS. Calculations based on typical solar X-ray emissions and likely models for surface composition suggested that even for quiet Sun conditions (no flare activity) and limited viewing time, flux from the Mg and Si lines (1.25 and 1.74 keV, respectively) would be detectable above background. But solar X-ray emissions during the flyby were an order of magnitude less than expected and no X-ray signal was detected from the planet. Observing parameters for the second Mercury flyby (6 October 2008) will be similar to those in January with about 5 minutes of observing time and a sunlit portion of the planet filling the XRS field of view. More typical solar X-ray emissions should provide sufficient signal for a statistically significant measurement of surface composition.
U21A-0030
Mercury: a prediction for bulk chemical composition and internal structure in readiness for new MESSENGER data
The MESSENGER spacecraft has confirmed that Mercury's magnetic field is dominantly dipolar and due to
an active dynamo in a molten outer core (Solomon et al, 2008 Science 321 59). An energy source is needed
to maintain this dynamo. Either liquid iron is freezing at the surface of an inner solid core (as proposed here)
or solid iron is precipitating within an outer sulphur-rich core (Chen et al, 2008 GRL 35 L07201). If the outer
core does not contain sulphur and consists solely of pure metal (Fe, Ni, Cr,..), then an active dynamo is
inconsistent with previous numerical models for the radiogenic thermal evolution of the planet. Those earlier
models found that the present temperature at the core/mantle boundary (CMB) is ~ 500 K below the
melting temperature of metal ~ 2030 K for a CMB pressure of 70 kbar. The earlier calculations were
based on low lunar abundances of U and Th.
Here I present a new model for the bulk chemical composition, thermal evolution and current internal
structure of Mercury. The model is based on the modern Laplacian theory of solar system origin (Prentice,
1978 Moon Planets 19 341; 2001 Earth Moon & Planets 87 11; 2006 Publ. Astron. Soc. Aust. (PASA) 23 1;
2008 - URL below). A key feature of this theory is that the planets formed from a concentric system of gas
rings (n = 0, 1, 2,..) that were shed by the contracting protosolar cloud. The temperatures Tn of the
rings scale with mean orbital radius Rn closely as Tn ~ Rn-0.9. Mercury plays a crucial
role in calibrating this relationship because of a condensation process of metal/silicate fractionation (Lewis,
1972 EPSL 15 286). Choosing Tn ~ 1630 K for mean orbit gas ring pressure of 0.17 bar, the
condensate consists mostly of Fe-Ni-Cr (mass fraction 0.671), gehlenite (0.190) and Mg-silicates (0.081). It
has mean density 5.30 g/cm3. Na, K and S are absent. The mass fractions of U and Th, namely 5.66
× 10-8 & 2.08 × 10-7, are a factor of 4.3 times greater than those of the proto-Earth
condensate.
The interior thermal profile of Mercury has been computed with the above bulk composition. The planet is
assumed to be a 2-zone structure (core/mantle) with initial central temperature 2500 K and constant surface
value 350 K. The temperature profile T(r) at time 0 is fitted smoothly against radius r so that TCMB is a
fixed fraction φ of the local melting temperature of gehlenite, namely Tm(p)/K = 1863 + 6.8p/kbar
for pressure p (Hirschberg, 1970). The rock is assumed to locally convect if T(r) exceeds φTm. For
model 1, φ = 0.90 and the outer 13% of the cooling core is still molten today. The mean density of the
model, 5.25 g/cm3, falls short of the observed value 5.43 g/cm3 (Anderson et al, 1987 Icarus 71
337). Increasing the core mass fraction to 0.701 to fix this problem, the melt mass drops to 10% of the core
mass. Taking φ = 0.95 and core mass fraction 0.707, the melt mass increases to 30%. The core is
everywhere cooling, except at its outer edge where TCMB is steady. The mass of the solid inner core
continues to grow as molten metal freezes out at its surface. The predicted polar moment-of-inertia factor of
Mercury is C/MR2 = 0.332 ± 0.002.
I thank George Null (JPL) and David & Michelle Warren (Hobart) for continued support.
http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1945.pdf
U21A-0031
Mercury Atmospheric and Surface Composition Spectrometer Observations of Sodium and Calcium in Mercury's Exosphere During the Second MESSENGER Flyby
MESSENGER will make its second flyby of Mercury on October 6, 2008, with exospheric sodium and calcium being among several species that will be observed. The observations of sodium begin in the extended tail region ~ 100,000 km in the anti-sunward direction and continue up to ~ 5000 km from the planet. The spacecraft will oscillate back-and-forth about the spacecraft-Sun vector, resulting in scanning the field of view up and down in the Z direction (i.e., north/south) in a Mercury-fixed coordinate system. The extent of the scan in the Z direction will be five Mercury radii above and below the anti-sunward vector at the beginning of the tail sweep and decrease to 3 Mercury radii as the spacecraft nears the planet. Calcium observations begin in the extended tail region ~ 35,000 km behind the planet and continue along with the sodium observations. After the extended tail-region observations, MESSENGER will enter into the shadow of the planet where sodium and calcium nightside observations begin. The spacecraft will be rotated ~ 180° around the spacecraft-Sun vector, resulting in a fan-tail pattern that begins with the field of view pointing in the equatorial plane toward the dawn-side hemisphere of the planet, rotates to the northern polar region, and then continues rotating until pointing in the equatorial plane toward the dusk-side hemisphere of the planet. The field of view will then begin to intersect the planet on the nightside where the observations continue up to the dawn-side terminator region. As the line-of-sight moves out in front of the sub-solar point during the outbound leg of the flyby, dayside observations of sodium begin above a 1000-km tangent height and continue up to several thousand kilometers. A portion of these observations will provide measurements similar to those during the first Mercury flyby and will provide a basis to compare morphological similarities and differences in sodium and calcium between the two sets of observations. Observations performed during this flyby that were not performed previously include measurements of the sodium tail at a much greater distance from the planet, providing additional data on the extent of the sodium tail. Moreover, because calcium was observed on the nightside hemisphere of the planet during the first flyby, this flyby will include a search for calcium in the extended tail region that will provide information concerning the release energy and transport for calcium. Finally, this flyby will search for extended sodium on the dayside exosphere and possibly help to constrain the release processes for sodium on the dayside.
U21A-0032
Variations in Mercury's Sodium Exosphere Taken at the McMath-Pierce Solar Telescope in January 2008 in Support of the MESSENGER Flybys
Ground-based observations of the sodium exospheric emission at Mercury taken at the McMath-Pierce Solar Telescope at Kitt Peak, Arizona, were conducted during the period of January 10-18, 2008. This dataset brackets observations taken with the Ultraviolet and Visible Spectrometer (UVVS) on The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument on board the MESSENGER spacecraft during the first flyby of the planet, January 14, 2008. From the ground, the distribution of sodium D2 was mapped using a 5"x5" image slicer. Strong temporal and spatial variability of the sodium emission was observed on the dayside of the planet. One particularly interesting feature was seen on January 12, when exceptionally good seeing allowed a data coverage at intervals of 2 to 3 hours. Variations of the sodium D2 emission were observed peaking at different regions of the dayside exosphere near the sub solar point and/or near one or both poles. There was little north-south asymmetry in the ground-based dayside sodium exosphere on January 14 but there was an indication of a possible asymmetry observed in the tail by MASCS. Our goal is to model the observed sodium exospheric variations. Photon-stimulated desorption of sodium is expected to induce a maximum sodium density near the sub solar point. Sputtering in the polar cusps should produce high latitude sodium emissions, but solar radiation pressure may also sweep sodium to high latitudes, and impact vaporization by meteorites and meteors would induce local and transient density enhancements in the exosphere. We discuss the observed variations, and present results of the modeling of the sodium distribution using a Monte Carlo model developed by Crider and Killen.
U21A-0033
The Morphology of Mercury's Ca Exosphere as Observed With Keck-1/HIRES
We present results from observations of Mercury's Ca exosphere with the HIRES spectrograph on the Keck-1 telescope in May 2008. The Ca exosphere exhibited a distinct N/S asymmetry at this time, with higher densities observed in the northern regions. The extended Ca exosphere was observed to contain significant tangent densities of 5.6 x 107/cm2 in the tail region at 10000 km radius from planet center, and 2 x 108/cm2 at 5500 km radius on the subsolar side of the planet. The species was observed to be most dynamic at low planetary altitude and in the near tail region, as measured through its persistent high LOS blue-shifted velocity. Relative to the sodium exosphere, which was measured concurrently, the temperature, scale height, and velocities of Ca are all greatly enhanced. We also present observations of Mercury's atmosphere with Keck-1/HIRES from June 2005 and May 2007, including new, low upper limits for other metals. We discuss the implications of these observations in the source and loss processes for Ca and other refractory species.
U21A-0034
A View of Mercury's Neutral Hydrogen Exosphere Through the First Two MESSENGER Flybys
Because of the difficulties of observing H Lyman α at Mercury from Earth orbit, observations of this emission by the Ultraviolet and Visible Spectrometer (UVVS) component of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument onboard MESSENGER represent the first chance in over thirty years to revisit the neutral H exosphere of Mercury that was first explored by the Mariner 10 Ultraviolet Spectrometer. The Mariner 10 observations revealed two interesting features of the H distribution in Mercury's exosphere. The first was a "bump" near 200 km altitude that may have been an instrumental artifact but has otherwise defied explanation. The second was a two-component nature to the distribution, which has been ascribed to a "cold" (~ 110 K) population of H atoms near the surface and a "warm" (~ 420 K) population at higher altitudes. However, issues exist with this explanation, and the true origin of this two-component H distribution remains one of the biggest questions in understanding Mercury's exosphere. During the first MESSENGER flyby of Mercury, the UVVS conducted observations of H Lyman α at both the tail and dayside regions of Mercury. While the tail-region observations revealed no H signal above the interplanetary background, the dayside observations showed an exospheric distribution consistent with the Mariner 10 "warm" component. Spacecraft priorities precluded observations of the H distribution at the low-altitude regions of the Mariner 10 "cold" distribution and "bump." During the second flyby, however, observations of the low-altitude H distribution are a high priority, and a dedicated, high- spatial-resolution H Lyman α observational sequence will occur radially from the surface to roughly 800 km altitude. Dayside observations of H will also be conducted at higher altitudes but will again be subject to spacecraft constraints, and additional H observations will be carried out in the tail region. We present an analysis of the H Lyman α observations from MESSENGER's second flyby and a synthesis of observations from both flybys in an attempt to answer the two outstanding questions: "What is the nature of the two-component H distribution in the exosphere?," and "If it exists, what is the origin of the 'bump' near 200 km altitude?" The UVVS data will be compared and contrasted with the Mariner 10 data to address these questions as well as to reveal changes that have occurred over the intervening 33 years and between the first and second MESSENGER flybys. We note that the UVVS data represent only a glimpse at this aspect of the complicated Mercury exospheric system. A full understanding of the exosphere, and of the H Lyman α emission in particular, will require many observations spanning a variety of observing conditions, as will be provided by the UVVS during the orbital phase of the MESSENGER mission.
U21A-0035
Upstream Solar Wind Conditions at Mercury During the First two MESSENGER Flybys
The knowledge of upstream solar wind conditions at Mercury is essential not only for modeling the Hermian magnetosphere-exosphere-surface system but also for interpreting the pioneering in situ observations made by MESSENGER during the January and October 2008 flybys. For this reason, and due to the fact that the MESSENGER plasma instruments cannot see the solar wind, we intend to provide upstream solar wind conditions at Mercury to the entire MESSENGER community. We do this using a combination of two independent solar wind models. The first is a steady-state 3-D magnetohydrodynamic (MHD) model of the solar corona and inner heliosphere, which simulates the solar wind propagation from the source surface outward to Mercury, using synoptic charts of the photospheric magnetic field as input. The second model is a time-dependent 1-D MHD model of solar wind propagation that employs actual solar wind data at 1 AU as boundary conditions and propagates the solar wind backward in time to Mercury. Although the latter approach is a rather unorthodox way of simulating the solar wind, the method is well tested. We first test the reversibility of the method by propagating the simulation results at Mercury back to Earth (forward in time). The backward and forward propagated solar wind at Earth was found to be practically identical with the original input indicating that the backward propagation does not result in problems with conservation. As a second test, we compare results from the two models. The relatively good agreement between the predictions of the two models verifies that the reverse propagation method can be successfully applied in estimating solar wind conditions at Mercury. The agreement between the two models is best for the solar wind plasma data (speed, density and temperature) and less convincing for the magnetic field data, which can be attributed to the necessary a priori constraints of the magnetic field inherent in both models. Limitations and advantages of both modeling approaches are discussed.
U21A-0036
Energetic Particles in Mercury's Magnetosphere During MESSENGER's Second Flyby
The MESSENGER spacecraft made the first direct observations of Mercury's magnetosphere in over 30 years during its first flyby on 14 January 2008. The interplanetary magnetic field (IMF) was northward immediately prior to and following MESSENGER's equatorial passage through this small magnetosphere. The Energetic Particle Spectrometer (EPS) did not observe the intense bursts of energetic electrons reported by Mariner 10 in 1974. EPS, which responds to electrons from ~15 keV to ~1 MeV and ions from ~15 keV to ~3 MeV, did not see any increases in particle intensity above instrumental background (a generous upper limit is 0.1% of those reported by Mariner 10) during its passage through Mercury's magnetosphere. In contrast, a low- energy, multi-component plasma was measured throughout the magnetospheric traversal. MESSENGER's next flyby will be on 6 October 2008. Although the Sun has been very quiet during the months leading up to the second flyby with almost no sunspots or active regions visible on the solar disk, strong high-speed solar wind stream structures indicative of solar minimum have been observed for many solar rotations. Under an extended period of southward IMF, high-speed solar wind enhances reconnection, which in turn is expected to lead to energetic particle acceleration events. If there is significant solar flare activity, solar energetic particles should have easy access into Mercury's magnetosphere. Even in the absence of solar flare activity during MESSENGER's second flyby, however, high-speed solar wind may produce bursts of energetic particles in Mercury's magnetosphere. Direct observations of these bursts could resolve the controversy over the true composition of these particles that has existed for over 30 years because of questions about the response of the Mariner 10 instrument.
U21A-0037
Narrow-Band Ultra-Low-Frequency Wave Observations During the October 6, 2008, Flyby of Mercury.
During MESSENGER's first flyby of Mercury on 14 January 2008 (M1), numerous observations were made of narrow-band ultra-low frequency (ULF) waves having frequencies between the He+ and H+ cyclotron frequencies. These magnetospheric waves were observed primarily outbound between closest approach and the magnetopause and exhibited systematic variations in frequency and amplitude with distance from the planet, while the polarization properties of these waves were quite variable. The trajectories of the M1 encounter and the second flyby (M2, 6 October 2008) are similar, both located in the equatorial nightside magnetosphere crossing from dusk to dawn. However, the M2 trajectory will enter the magnetosphere ~1.5 RM (Mercury radii) tailward and exit the magnetosphere ~0.5 RM sunward of the penetration points for M1. The flybys M1 and Mariner 10 I (29 March 1974) sampled similar flux tubes during their magnetospheric passages, while the Mariner 10 III (16 April 1975) flyby sampled flux tubes at much higher latitudes. Only one brief burst of these waves was detected during the Mariner 10 I flyby. The difference in prevalence of ULF waves found for M1 relative to that of the Mariner 10 flybys is intriguing, and we use observations from M2 to investigate whether this prevalence is consistent along comparable trajectories. In this paper the ULF waves detected during M2 will be analyzed and compared with the previous observations of ULF waves in Mercury's magnetosphere to assess, to the extent possible, whether their occurrence or properties depend on prevailing IMF and activity conditions within Mercury's magnetosphere.
U21A-0038
Resonant absorption of ULF waves at Mercury's magnetosphere
Ultra low frequency (ULF) waves, which are assumed to be standing waves on the field, are observed by the Mariner 10 spacecraft at Mercury. These waves are oscillating at 38% of the proton gyrofrequency. It is well known that heavy ions, such as Na+, are abundant in Mercury's magnetosphere. Because the presence of different ion species has an influence on plasma dispersion characteristics near the ion gyro- frequencies, magnetospheric eigenoscillations observed at Mercury with frequency in the gyrofrequency range require a multi-fluid treatment for the plasma. Thus ULF waves at Mercury may have a distinct difference from typical ULF oscillations at Earth, which are often described in terms of magnetohydrodynamics (MHD). By adopting a multi-fluid numerical wave model, we examine how magnetic eigenoscillations occur in Mercury's magnetosphere. Because protons and sodium ions are the main constituents at Mercury, we assume an electron-proton-sodium plasma in our model. Our results show: (1) The observed ULF waves are likely compressional waves rather than standing oscillations such as field line resonances (FLRs), (2) FLRs at Mercury are expected to occur when the ion-ion hybrid and/or Alfvén resonance conditions are satisfied, (3) The magnetic field of FLRs at Mercury's magnetosphere oscillates linearly in the east-west (azimuthal) meridian when the frequency is located between two ion gyrofrequencies, and (4) The resonance frequency enables us to estimate the local heavy ion concentration ratio.
U21A-0039
Comparative MHD Models of the First two MESSENGER Flybys of Mercury
The second MESSENGER flyby of Mercury on October 6, 2008, will provide new insight into the structure and dynamics of Mercury's small but complex magnetosphere. In this paper, we present an improved global magnetohydrodynamic (MHD) model of the magnetosphere of Mercury that takes into account the data from this most recent flyby. This model uses an updated estimate of the intrinsic magnetic field of the planet by combining fits from two of the Mariner 10 flybys and the two MESSENGER flybys. The results with this new improved dipole will be compared with the pre-encounter dipole derived using only the first MESSENGER flyby data. Our new model also provides clues on how the structure of magnetosphere of Mercury will have changed between the two MESSENGER encounters to adapt to the two different sets of solar wind conditions for each flyby. These changes can be mainly seen in the bow shock and magnetopause positions. But we also show that the effect of the solar wind condition is visible in the reconnection rates and patterns between the solar wind and the intrinsic magnetic field lines, and in the way particles from the plasma sheet convect to the day side of the inner magnetosphere. Finally, we introduce through this model results from including the effect of heavy ions in the MHD scheme. The physical and technical validity of the results will be discussed and compared with results obtained with test particle techniques and hybrid codes.
U21A-0040
Multi-Fluid Modeling of Mercury's Magnetosphere in Advance of the 2nd MESSENGER Flyby
In preparation for the second MESSENGER flyby of Mercury on October 6, 2008, 3D multi-fluid simulations are used to predict Mercury's magnetospheric response to many possible solar wind and interplanetary magnetic field (IMF) conditions. Magnetic field components, synthetic spectrograms and the temperature and densities of planetary ions including He+ and Na+ will be plotted along the MESSENGER II flyby trajectory through Mercury's magnetotail. Although solar wind activity may be increased compared to the MESSENGER I flyby in January, we will model both quiet and active solar wind conditions in preparation for the 2nd flyby. Specifically, we look to examine the location and time scales of any flux ropes that form and the location of the bow shock and magnetopause. Additionally, we are interested in determining under what external conditions asymmetric sodium outflow occurs, as well as examining the presence of any field-aligned currents. We will also provide model comparisons with ground-based observations and data from the MESSENGER I flyby.
U21A-0041
Kinetic simulation of Mercury's magnetosphere compared with observations during MESSENGER's first Mercury flybys on 14 January and 6 October 2008
MESSENGER's 14 January and 6 October 2008 encounters with Mercury have provided new in situ observations of its small magnetosphere. Global three-dimensional kinetic simulations of Mercury's magnetosphere have revealed its basic structure, including a bow shock, magnetopause, well-pronounced cusp regions, and a closed ion ring that forms around the planet within the magnetosphere. In this paper we compare and interpret MESSENGER's observations with the results obtained using such a global kinetic model; in particular, we focus on waves observed in the foot region of the inbound quasiperpendicular shock, the wavetrain in the downstream region of the shock, magnetosheath turbulence, possible signatures in MESSENGER's data related to the presence of Mercury's belt of quasitrapped particles, and large-amplitude oscillations in the region of Mercury's quasiparallel shock. The ions in Mercury's belt remain quasitrapped for several cyclotron periods before they are either absorbed by Mercury's surface or escape from the magnetosphere. We also examine the formation of vortices driven by the Kelvin-Helmholtz instability close to Mercury's magnetopause as well as the location of reconnection points within Mercury's magnetosphere. It was established during MESSENGER's Mercury encounters that heavy ions including sodium (Na+) and potassium (K+) populate the magnetosphere. Therefore we have also undertaken a study to examine the transport, distribution, and energization of these heavy ions during the solar wind conditions corresponding to those found by MESSENGER. For this purpose we employ a particle-tracing technique using results from the three-dimensional global kinetic simulations of Mercury's magnetosphere for the self-consistent electric and magnetic field configuration at the the time of these flybys. To examine solar wind sputtering as a source for ion ejection from the planet, heavy ions are launched outward from regions near the planet where hybrid simulations show strong particle precipitation. Their trajectories are then followed until they either hit the planet or are picked up by the solar wind and lost downstream. The heavy ions can be transported throughout the magnetosphere of Mercury and become accelerated by non-adiabatic processes in the magnetotail current sheet, as well as near reconnection regions. The nature of this transport depends significantly on the upstream parameters of the solar wind. The distribution of heavy ions and their energy profile in the magnetosphere will be compared with MESSENGER data from the two flybys.
U21A-0042
Particle Tracing of Heavy Ions in Hermean Exosphere With Applications to the MESSENGER.
We carry out Monte Carlo simulations of heavy particle species which are released from the surface of the planet Mercury as neutral particles and later on they are ionized. We consider three major sources of particle injection, namely Photon stimulated desorption, Solar wind sputtering and Micro-meteoroid vaporisation to build neutral exosphere. Then, we investigate the resulting exosphere and identify regions of particle acceleration due to the electromagnetic fields as well as distribution of ionization in the simulation box. The results are put into the context of the MESSENGER mission which measure the in situ data during its first fly by on January, 14 2008.
U21A-0043
Distribution of sodium pickup ions during MESSENGER's first and second Mercury flybys: constraints on exospheric models
A test particle model of sodium pickup ions at Mercury is used to investigate the likely distribution of neutral sodium about the planet during the time of the first MESSENGER flyby. The test particles are weighted by exosphere models corresponding to the release processes of ion sputtering and photon-stimulated desorption (PSD). The ionization of the sodium exosphere followed by pick-up by the magnetosheath convection flow is modeled using magnetospheric fields from a magnetohydrodynamic simulation. The Maxwellian PSD component is gravitationally bound and does not appreciably contribute to the pickup ion distribution above the magnetopause. Furthermore, the neutral model PSD distribution provided by a Weibull function would substantially "mass-load" the upstream solar wind contrary to what is observed by MESSENGER. It is concluded that the sodium magnetosheath ions originate mainly from an escaping neutral cloud due to ion sputtering and/or to a partially escaping PSD distribution modified by regolith trapping. These results will be compared with ion measurements during the second MESSENGER flyby because the overlapping of the exosphere with the magnetosphere is expected to be more favorable as Mercury approaches its perihelion.
U21A-0044
Statistical trajectory tracings of sodium ions with a new MHD model of Mercury's magnetosphere
From the observations by Mariner 10, it has been suggested that the Mercury's magnetosphere might be an analogous to the Earth's magnetosphere. Observation by MESSENGER in Janurary 2008 seems to support this assumption under a quiet solar wind condition. On the other hand, the temporal and spatial scales of the Mercury's magnetosphere are much smaller than those in the Earth's magnetosphere because of its week intrinsic magnetic field and strong dynamic pressure of the solar wind at the Mercury's orbit. The MHD simulation is one of the powerful methods to understand global structure of the magnetosphere. However, in the Mercury's magnetosphere, it should be pointed out that the kinetic effects of plasma might not be negligible because of a large gyro-radius of heavy ions. Statistical trajectory tracing of test particles is an important scheme to investigate the kinetic effects of particles. Previous studies by Delcourt et al. [2003; 2005] used analytical models of electric and magnetic fields that are obtained by rescaling the Earth's magnetosphere and calculated the motion of planetary sodium ions. Although this approach is efficient to see the dynamics of heavy ions, resultant properties largely depend on the field models. Therefore, it is important to examine the particle motion in the self-consistent MHD magnetic field. Delcourt and Seki [2006] suggested that dynamics of sodium ions is sensitive to the solar wind conditions based on trajectory tracings of the sodium ions in the magnetic and electric fields obtained from MHD simulation with southward IMF. In this study, we will present results of statistical trajectory tracing of sodium ions in the electric and magnetic field configuration obtained by a new MHD simulation code that automatically satisfies divB=0 condition [Yagi et al. Comput. Phys. Commun., submitted, 2008] to avoid artificial acceleration/deceleration. Initial states of sodium ions are given from an exospheric model [Leblanc and Johnson, 2003], which is also used in the previous studies of the statistical trajectory tracings. In this presentation, we discuss the effect of the solar wind conditions on the sodium dynamics in the Mercury's magnetosphere.