P23A-0222 1340h
Low Cost Precision Lander for Lunar Exploration
For 60 years the US Defense Department has invested heavily in producing small, low mass, precision guided vehicles. The technologies matured under these programs include terrain-aided navigation, closed loop terminal guidance algorithms, robust autopilots, high thrust-to-weight propulsion, autonomous mission management software, sensors, and data fusion. These technologies will aid NASA in addressing New Millennium Science and Technology goals as well as the requirements flowing from the Vision articulated in January 2004. Establishing and resupplying a long term lunar presence will require automated landing precision not yet demonstrated. Precision landing will increase safety and assure mission success. In the DOD world, such technologies are used routinely and reliably. Hence, it is timely to generate a point design for a precise planetary lander useful for lunar exploration. In this design science instruments amount to 10 kg, 16% of the lander vehicle mass. This compares favorably with 7% for Mars Pathfinder and less than 15% for Surveyor. The mission design flies the lander in an inert configuration to the moon, relying on a cruise stage for navigation and TCMs. The lander activates about a minute before impact. A solid booster reduces the vehicle speed to 300-450 m/s. The lander is now about 2 minutes from touchdown and has 600 to 700 m/s delta-v capability, allowing for about 10 km of vehicle divert during terminal descent. This concept of operations is chosen because it closely mimics missile operational timelines used for decades: the vehicle remains inert in a challenging environment, then must execute its mission flawlessly on a moment's notice. The vehicle design consists of a re-plumbed propulsion system, using propellant tanks and thrusters from exoatmospheric programs. A redesigned truss provides hard points for landing gear, electronics, power supply, and science instruments. A radar altimeter and a Digital Scene Matching Area Correlator (DSMAC) provide data for the terminal guidance algorithms. DSMAC acquires high-resolution images for real-time correlation with a reference map. This system provides ownship position with a resolution comparable to the map. Since the DSMAC can sample at 1.5 mrad, any imaging acquired below 70 km altitude will surpass the resolution available from previous missions. DSMAC has a mode where image data are compressed and downlinked. This capability could be used to downlink live images during terminal guidance. Approximately 500 kbitps telemetry would be required to provide the first live descent imaging sequence since Ranger. This would provide unique geologic context imaging for the landing site. The development path to produce such a vehicle is that used to develop missiles. First, a pathfinder vehicle is designed and built as a test bed for hardware integration including science instruments. Second, a hover test vehicle would be built. Equipped with mass mockups for the science payload, the vehicle would otherwise be an exact copy of the flight vehicle. The hover vehicle would be flown on earth to demonstrate the proper function and integration of the propulsion system, autopilots, navigation algorithms, and guidance sensors. There is sufficient delta-v in the proposed design to take off from the ground, fly a ballistic arc to over 100 m altitude, then guide to a precision soft landing. Once the vehicle has flown safely on earth, then the validated design would be used to produce the flight vehicle. Since this leverages the billions of dollars DOD has invested in these technologies, it should be possible to land useful science payloads precisely on the lunar surface at relatively low cost.
P23A-0223 1340h
Optimization of Deep Moonquake Event Stacks in the Apollo Lunar Seismic Data and Applications to Lunar Structure
Our studies of the Apollo lunar seismic data have focused on the deep moonquakes, which comprise 46% of catalogued events and appear mostly on the long-period channels. Thought to be tidally triggered, deep moonquakes occur periodically at specific source locations, each of which produces its own characteristic waveform. Using cross-correlation techniques, deep moonquakes can be sorted into separate groups, stacked, and analyzed for information on lunar interior structure. An accurate catalog of events is crucial to understanding deep event occurrence characteristics and source locations. This requires identification of all deep events associated with known groups, both in existing catalogs and in the continuous time series. Previous studies of lunar deep events have focused on the event catalog, not the complete continuous data. We search for previously unidentified events by cross-correlating preliminary waveform stacks representing a known group with the continuous data. Our analysis so far has concentrated on deep event group A1, which has the largest number of events (323) of all identified source regions. To generate the preliminary stack every event within the A1 group is cross-correlated with every other event in the group. Those event pairs with cross-correlation coefficients greater than some threshold value ({\it i.e.,} events that are clearly members of the A1 group) are retained. By combining the cross-correlation approach with a robust median despiking algorithm, we have produced improved differential times and amplitudes, enabling us to construct cleaner stacks. Each deep event group stack is then cross-correlated with the continuous time series. Using this method, we have successfully identified 34 new A1 events on stations 12 and 16. The extension of our methods to data from stations 14 and 15, as well as to target stacks from other deep event groups, will provide an improved data set for investigating the temporal occurrence of deep moonquakes. After identifying new events, we optimize stacks from A1 and other groups to obtain the cleanest possible waveforms from which to pick P- and S-wave arrival times. The initial stack is cross-correlated with all members of a given deep event group, a new stack formed by weighting event group members, and the process repeated. We are investigating different weighting schemes, including using the cross-correlation coefficient and measures of signal-to-noise. Convergence is achieved after only a few iterations, and picks can be made from the resulting optimized stack. These picks can be used to test and refine lunar seismic velocity models and location estimates for the deep event groups.
P23A-0224 1340h
Deep Moonquakes: Remaining Problems
We have recently reexamined more than 9000 previously unidentified seismic events catalogued during the Apollo landing missions and positively identified for the first time about 30 deep moonquake nests on the far side of the Moon. Although only a few of them are currently locatable, the relative arrival times among stations for the rest and presence or absence of seismic signals at particular stations suggest that either (a) the region within about $40\deg$ of the antipode is aseismic or (b) the deep interior of the Moon severely attenuates or deflects seismic waves. Aside from the obvious question of how to distinguish between such hypothetical models, this effort raised several more general questions concerning the use of deep moonquake signals to infer the structure and dynamics of the deep interior of the Moon. Among more important ones are: (1) How reliable are the seismic arrival picks from which to compute the seismic velocity variations in the Moon? (2) How do the possible lateral variations in seismic velocity affect the computed radial variation in seismic velocity at depth? (3) Can we tell more about the distribution and mechanism of deep moonquakes from the newly expanded database of identified deep moonquakes? Questions (1) and (2) are especially important because the inferred deep internal structure of the Moon depends critically on their answers. answering these questions may demand additional data collected on future lunar missions, but some may be resolved with further examination of the existing data.
P23A-0225 1340h
Source Mechanisms of Moonquakes Derived From Apollo Data
We study source mechanisms of moonquakes by using amplitude of P and S waves from the long-period records in the special event tape (Nakamura, 1982) of the Apollo missions. In order to evaluate the effect of local structure beneath stations on the observed seismic waves, we first examine the variation of the S-wave motions depending on groups of deep moonquakes. The S-wave arrivals were determined in the seismograms by using eigenvalues and eigenvectors computed in the covariance matrix method (Matsumura, 1981) as well as a traditional technique based on ocular inspection and difference of arrival times between stations (Nakamura, 1983). We estimate the orientations of the S-wave motions in the three methods with deconvolution between horizontal components, particle motions after the S-wave arrivals, and covariance matrixes, respectively. The results from the three methods agree with each other, showing that the S-wave motions at each station vary from source group to group. Thus, the observed polarizations of S-wave motions are not strongly controlled by local structure beneath stations, and are likely to depend on the source characteristics of moonquakes. Assuming that deep moonquakes are caused by shear faulting, we then determine the geometry of the shear faulting for deep moonquakes in the groups A1, A17, A18, and A20. The previous studies only concerned A1. We work with three datasets, including observed amplitude ratios of (1) P/SV, P/SH, and SV/SH at three stations when P waves are visible, (2) SV /SH at three stations, and (3) SV/SH at two stations and SH/SH between the stations. Based on the ray theory, theoretical amplitude ratios are computed from the spherically layered velocity models recently obtained, including the effect of radiation pattern and geometrical spreading. We search for strike, dip, and rake angles of shear faulting in a grid-search technique by comparing the absolute values of the residuals between the theoretical and observed amplitude ratios. Our focal mechanism solutions, which are derived from the angles that minimized the absolute values of the residuals, tend to be characterized by a combination of low- and high-dipping planes. We separately test a hypothesis that deep moonquakes are caused by crack-shaped collapses, searching for the geometry. These methods are also applied to shallow moonquakes.
P23A-0226 1340h
Correlations Between Magnetic Anomalies and Surface Geology Antipodal to Lunar Impact Basins
Previous work has shown that the strongest concentrations of lunar crustal magnetic anomalies are located antipodal to four large, similarly aged impact basins (Orientale, Serenitatis, Imbrium and Crisium). Here, we report results of a correlation study between magnetic anomaly clusters and geology in areas antipodal to Imbrium and Orientale. Only these areas have been selected due to a) limited Lunar Prospector magnetometer data coverage of the area antipodal to Serenitatis, and b) the location of the Crisium antipode, which is dominated by ejecta deposits from the more recent Orientale impact. Unusual geologic terranes have been mapped antipodal to both Orientale (furrowed and pitted terrane) and Imbrium (material of grooves and mounds). These units have been interpreted to be of seismic or ejecta origin associated with the basin forming impacts. Both regions have many high albedo swirl markings, which have been shown to correlate closely with regions of high crustal magnetisation at other locations. Results indicate a strong correlation between the swirl markings and regions of high magnetisation for both the Imbrium and Orientale antipodes. In addition, the furrowed and pitted terrane, material of grooves and mounds and Mare Ingenii (antipodal to Imbrium) show a correlation with high magnitude crustal magnetic anomalies. Mare Ingenii is on the south west edge of the material of grooves and mounds and may overlay that unit, with the anomaly sources beneath the mare materials. This is supported by an observed lack of demagnetisation associated with the 61 km Copernican-aged O'Day crater at the edge of Mare Ingenii, which suggests a deep source for the anomalies. Possible source materials and the origin of the magnetisation will be discussed at the conference.
P23A-0227 1340h
Lunar magnetic anomalies in the solar wind: Possible existence of mini-magnetosphere
It has been suggested that lunar magnetic anomaly fields are interacted with the solar wind plasma to form the mini-magnetosphere on the lunar surface. From the Lunar Prospector (LP) observations of magnetic fields, Lin et al.(1998) pointed out that a mini-magnetosphere was formed in the solar wind downstream of the strong magnetic anomaly in Imbrium antipode region. Harnett et al.(2000, 2002) demonstrated the presence of lunar mini-magnetospheres with MHD and particle simulations. If the mini-magnetosphere exists on the lunar surface and deflects solar wind particles, its role of barrier could produce a high-albedo region around the magnetic anomaly. In this study, we mainly investigate magnetic anomaly fields in the solar wind using the LP MAG low-altitude (15-40 km) data of level1. We detected lunar magnetic anomalies after preprocessing of the level1 data, using Hood's (1981) technique. In the present study, magnetic anomalies were mapped from the data sets in the tail lobe, the moon wake and the solar wind, and were compared with each other. We preliminarily analyzed three typical anomaly regions (Crisium antipode region, Descartes region, and Reiner Gamma region), and all of these three regions show clear magnetic anomalies even in the solar wind. We further carried out the detailed analysis of Reiner Gamma region. Its contour pattern of magnetic field intensities in the tail lobe or the wake is almost symmetrical with respect to the north-south line. However, such symmetry is obviously distorted in the solar wind to show some elongation toward the downstream of the solar wind. Also, the form of distortion seems to be changed when the solar wind conditions (dynamic pressure, the angle of incidence, and so on) are different. These results may support existence of the mini-magnetosphere in Reiner Gamma region. We will discuss the possible mini-magnetosphere comparing the LP MAG data with the ACE data of the solar wind.
P23A-0228 1340h
Lunar Prospector Neutron Spectrometer Response to Shadow-Constrained Distributions of Water at the Lunar Poles
Cold-trapped volatiles at the lunar poles are a potential resource for use in long-term human exploration and would provide extremely valuable scientific information stretching back to the early solar system. Lunar Prospector's neutron spectrometer clearly detected polar enhancements of hydrogen, possibly in the form of shallowly buried ice. These data currently provide the most severe constraints on how much water may be present in the Moon's polar regions, and where it may be found. But these data have a surface spatial resolution of 20-30 km. Constraining the plausible locations of enhanced water ice abundance to permanently shaded areas (based on a model of solar illumination of the polar topography), and then convolving the spectrometer response function with the associated surface neutron flux output, we can predict the response that should be seen from orbit. We then iterate to a solution that provides the best agreement between the simulated LP NS "data" and the actual observations. As the illumination models improve, so does the fidelity of the only locations where truly cold-trapped water can be found. We find that water ice concentrations may be greater than 10 wt% in the floors of some smaller craters. These are sites that would prove fruitful in both exploration and science endeavors at the Moon.
P23A-0229 1340h
Viscoelastic Relaxation of Lunar Basins
The large lunar impact basins provide a unique glimpse into early lunar history. Here we investigate the possibility that the relief of the oldest lunar basins (with the exception of South-Pole Aitken) has decayed through viscous relaxation. We identify nine ancient multi-ring basins with very low relief and low-amplitude Bouguer and free-air gravity anomalies. The characteristics of these basins are consistent with either 1) relaxation of topographic relief by ductile flow (e.g. Solomon et al., 1982) or 2) obliteration of basin topography during crater collapse immediately following impact. Both scenarios require that the basins formed early in lunar history, when the Moon was hot. The latter possibility appears to be unlikely due to the great topographic relief of South Pole-Aitken basin (SPA), the largest and oldest impact basin on the Moon (with the possible exception of the putative Procellarum basin; Wilhelms, 1987). On the other hand, the thin crust beneath SPA may not have allowed ductile flow in its lower portions, even for a hot Moon, implying that a thicker crust is required beneath other ancient basins for the hypothesis of viscous relaxation to be tenable. Using a semi-analytic, self-gravitating viscoelastic model, we investigate the conditions necessary to produce viscous relaxation of lunar basins. We model topographic relaxation for a crustal thickness of 30 km, using a dry diabase flow law for the crust and dry olivine for the mantle. We find that the minimum temperature at the base of the crust ($T_b$) permitting nearly complete relaxation of topography by ductile flow on a timescale $ < $ 10$^{8}$ yrs is 1400 K, corresponding to a heat flow of 55mW/m$^{2}$, into the crust. Ductile flow in the lower crust becomes increasingly difficult as the crustal thickness decreases. The crust beneath SPA, thinned by the impact, is only 15-20 km thick and would require $T_b$ $\geq$ 1550 K for relaxation to occur. The fact that SPA has maintained high-amplitude relief suggests that $T_b$ dropped below $\sim$ 1550 K no later than 100 Myr after the formation of SPA. When did these basins form? The formation of a stable lunar crust ($\sim$4.4 Ga) is clearly an upper limit on their age. The precipitation of ilmenite from the magma ocean is predicted to occur at $\sim$1373 K, which suggests that the lunar magma ocean had not fully crystallized when the oldest basins formed. From thermal evolution models (e.g., Solomon and Longhi, 1977) and crystallization ages of Mg-suite rocks (mostly 4.2-4.4 Ga; Nyquist and Shih, 1992; Snyder et al., 1995) 4.2 Ga appears to be a lower bound on the freezing of the magma ocean (although some residual liquid may have remained in the Procellarum KREEP Terrane). Therefore, the basins that relaxed almost completely due to high crustal base temperatures probably formed between 4.2-4.4 Ga.
P23A-0230 1340h
The formation of the lunar and Martian mascons
Intermediate size basins of the Moon with diameters of about 700 km have excess mass concentrations (mascons), whereas South Pole Aitken with a diameter of about 2500 km and depth of about 8 km, show no sign of a mascon. Also smaller basins and craters have no mascons either. Similar characteristics are detected for the Martian basins. Isidis and Argyre with diameters of about 1200-1500 km have mascons, whereas larger basins such as Hellas and Chryse with diameters over 2000 km have no masons. Two major scenarios have been proposed for the formation of the lunar mascons: a) Partial melting of deeper parts of the Moon and ascending of melt to flood the basin and b) Partial melting of the upper mantle beneath the surrounding ejecta blanket and lateral motion of melt to flood the basin. The impacts had no effect on partial melting in the first scenario but only produced basins as topographic lowlands prone to be filled, whereas the mascons in the second scenario are the direct result of the basin formation process. Both scenarios have major problems. They cannot explain the absence of mascons in Aitken basin, as well as the prolonged flooding of the basins. The geochemical data on lunar rock samples cannot discriminate one scenario against the other, some minerals are produced at depths greater than 500 km but some at shallower depths, about 100 km. We propose a new scenario for the formation of the mascons of the Moon and Mars that alleviates these difficulties. A large impact that produces a large basin results in melting and evaporation of the impactor as well as the near surface strata of the target. The excavated volume gets appreciably filled through isostatic upwelling of the mantle in a geologically short time and the remaining crust is fluidized enough to pose no appreciable resistance against isostatic uplift. A large or intermediate size basin becomes almost completely compensated in a short time. However, the impact induces mantle convection develops in time and allows hot material from deeper parts to ascend at later times and partially melt at certain depths. In the mean time the ejecta blanket hampers heat loss from the mantle beneath the blanket. Both processes occur concurrently and prevail for a considerable time, well over 100 m.y., and generate partial melting and subsequent flooding of the basins. The giant basins retain almost complete compensation, but the remaining crust and upper mantle beneath the intermediate size craters cool and become strong enough to support excess mass. The impacts that produce small basins and craters do not induce appreciable convection and do not produce thick ejecta blankets, thus cannot result in partial melting and basin flooding
P23A-0231 1340h
Earth-based observations of radar-dark crater haloes on the Moon: Implications for regolith properties
Earth-based radar has been used for over 30 years to image the nearside of the Moon and to investigate the physical properties of the lunar regolith. The physical characteristics of the regolith are of great importance to fundamental questions in lunar geology, such as the spatial limits and relative timing of flow units or of materials associated with impacts. In contrast with spectral reflectance methods that are sensitive to chemical variations in the upper several microns of surface materials, radar signals penetrate to greater depths and thus return information representing a larger integrated volume of regolith. Improved resolution in both Earth-based radar and orbital multispectral data provide a new opportunity to explore the physical and chemical properties of the lunar regolith at a variety of scales. In this work, we focus on distinctive radar-dark haloes surrounding nearside impact craters. These haloes show characteristically low radar returns in Earth-based observations at 70 cm wavelength and have not been examined systematically to date, though they were noted in previous work. Using previously existing and newly acquired Earth-based radar observations at 70 cm wavelength in conjunction with UV/VIS multispectral data from the Clementine mission, we investigate variations in physical and/or chemical properties that may be responsible for the presence of radar-dark haloes. The radar-dark haloes examined in the present study are different from the so-called ?dark-haloed cratersO that result from impacts into buried mare material or from volcanism, and which appear dark in optical datasets; the features examined in our study specifically show low radar reflectivity at 70 cm wavelength. This characteristically low radar return could result from (1) the presence of a regolith component with higher loss tangent than the surrounding terrain; or (2) the presence of a relatively block-poor mantling deposit produced by the impact process. In this paper, we use 70-cm radar observations in conjunction with multispectral data to distinguish between these two possible mechanisms. We present results for 39 nearside highlands and mare craters indicating that 70-cm radar-dark haloes are closely tied to the physical properties of impact crater ejecta, and do not result from loss tangent variations in the ejecta or target material. In addition, we investigate the implications of radar-dark crater haloes for impact-related processes and for the longevity of the material comprising the haloes in the face of ongoing bombardment by impactors of various sizes, and whether these haloes can provide clues for inferring the relative ages of impact craters.
P23A-0232 1340h
Global Mapping of Mg-Number Derived from Clementine Data.
The global mapping of the lunar surface using the petrological parameter Mg-number (Mg*) was undertaken because Mg*, or the ratio of Mg to the sum of Mg and Fe on an atomic basis, is an important disciminator in defining and understanding lunar rocks. The dominant lunar rock types, ferroan anorthosites (FAN), high-magnisium suite (HMS), and high-alkali suite (HAS) rocks all vary in Mg* depending upon the petrologic scenario that formed them. Of particular interest are FAN mineralogy and chemistry, which varies from high-Mg# (~70) troctolites to low-Mg# norites (~50) and for some time has been considered to represent a single magma frac-tionation trend. However, recent studies have also shown that the crystallization of FAN rocks may have been more complicated than originally thought. James et al. [1] found that instead of one simple fractionation trend for ferroan anorthosites, there may have been four. Studies by [2], [3], and [4] using Apollo and lunar meteorites for analysis have also eluded to the possibility that FAN rocks may have evolved from a more complex source or process. Therefore a global assessment of lithologies and corresponding Mg* is of great value for lunar petrology. In a remote sensing context, Mg* is the most important control on the spectral properties of lunar mafic silicates. For stoichiometric orthopyroxene and olivine, Mg* is mathematically linked to the Fe content that controls the overall reflectance and intensity of absorption. The changes in band centers and shape that accompany the structural changes as Fe substitutes for Mg along the solid solution series have long been recognized; these changes are highly correlated with Mg*. In clinopyroxene, the strong effect of Ca on structure makes this element important, but Mg* has the dominant effect on reflectance and a comparable effect on spectral shape. In this study, the lunar surface is quantitatively mapped using a theoretical treatment of mineralogic spectra and the effect of environmental space exposure on the optical properties of these minerals. This was accomplished using Clementine ultraviolet and visible (UVVIS) data and a Hapke radiative transfer mixing model. The major features evident in these maps are the strong distinction between mare and highland regions, the former showing low Mg* and the latter generally higher; a large northern highlands unit with low Mg*, and an Mg* high north of South Pole-Aitken basin. Mare units are not universally low, mare Frigoris in particular has elevated Mg* relative to other mare. The strongest variations in the highlands occur in plagioclase rich, low FeO units, that exhibit values ranging near 50 to near 100 in coherent units. The craters Tycho and Aristarchus also exhibit high Mg*; these gabbroic anomalies may indicate more extensive Mg-rich material at depth. Deposits within SPA are unremarkable relative to surroundings, and share the intermediate Mg* of most of the highlands. 1. James, O.B. et al. PLPSC. 1989.; 2.Bersch, M.G., et al., GRL, 1991.; 3. Floss, C., et al., GCA, 1998.; 4. Korotev, R.L., et al., GCA, 2003.
P23A-0233 1340h
Thermal Infrared Spectroscopy of the Moon
In May 2004, we used the University of Hawaii's (UH's) Airborne Hyperspectral Imager (AHI), a 256-band spectrometer covering 7.8-11.5 microns, on the UH 2.2 m telescope on Mauna Kea, Hawaii to image the Moon. At 1-2 days past full Moon, we collected hyperspectral images for the entire near side at diffraction-limited resolution (about 5 km at center of disk). These data, by far, constitute the most extensive hyperspectral imagery ever collected of the Moon at these wavelengths. Thermal infrared wavelengths have been previously shown to have potential for identifying lunar mineralogy and petrology based on laboratory experiments (e.g., [1-3]) and telescopic observations of a handful of locales on the Moon (e.g., [4]). We are currently processing our data for atmospheric and telescopic transmission and emission effects, as well as compensating for unforeseen registration errors due to wavelength dependent refraction in the atmosphere, apparently due to ozone. While the signal is dominated by surface temperature and surface topography compounded by challenging calibration, preliminary analysis indicates that the final signal-to-noise will approximate the value of 1000 reported by [3] as necessary for useful interpretation of lunar thermal infrared spectra. [1] Conel, J. E., JGR, 74: 1614-1634, 1969. [2] Salisbury, J. W. and L. S. Walter, JGR, 94: 9192-9202, 1989. [3] Salisbury, J. W. et al., Icarus, 130: 125-139, 1997. [4] Sprague, A. L. et al., Icarus, 100: 73-84, 1992.
P23A-0234 1340h
Radiating fissure swarms in Beta Regio, Venus: Evidence of formation from impact craters
Venus hosts numerous long and narrow extensional lineaments, with some showing a radiating pattern. A total of 163 large radial extensional systems were identified [1] with two end member mechanisms of origin for these features summarized: (1) domical uplift caused by an ascending mantle diapir; or, (2) emplacement of shallow laterally propagating dikes out from a central magma reservoir. We consider craters located in the Beta Regio (24-45 N, 264-312 E) to further differentiate between models for the 6 radiating graben-fissure structures there. The number of craters on Venus' surface approaches 1000, nearly randomly distributed. The modification of a crater or its ejecta by disruption or embayment by lava from outside documents local tectonic and/or volcanic activity, and the parabolic deposits associated with some craters gives their approximate age, thus dating activity. Also, dip of the crater floor, which forms initially flat, indicates later reorientation. Thus, impact craters have the potential to provide clues about the timing, extent and nature of tectonic and volcanic processes. For this study, we used Magellan's full-resolution and compressed mosaicked radar images to interpret surficial geology, and altimetry to find the dip for each crater. A total of 23 craters are located within this region, with very few showing tectonic or volcanic modification. Venus' radiating fissure swarms have caused surprisingly little modification of impact craters. Almost all craters near the extensional lineaments remain pristine. A single embayed crater lies within the field of one radiating system, the very system that has been identified as the youngest. Dips for 5 craters near the geoid peak on Beta Regio exhibit a discordant pattern. In addition, we find substantial evidence for tectonic disturbance in the northern Beta region. If shallow and recent magma sources had generated the radiating systems, then we would expect to find many more craters embayed by lava. The other scenario for the formation of radiating swarms requires an uplift caused by an ascending diapir. However, the craters within radiating systems do not dip away from the center. Instead, they dip more randomly, often toward the center of the radiating system, suggesting it has collapsed since the time of impact. Thus, the radiating systems may have formed preceding the impacts. [1] Grosfils E.B. and Head J.W., 1994, GRL 21, 701-704.
P23A-0235 1340h
Coronae Formation on Venus by Rayleigh-Taylor Instability
In this study we explore the idea that coronae have formed on Venus as a result of gravitational (Rayleigh-Taylor) instability of the mantle lithosphere. The lithosphere is represented by a system of stratified homogeneous viscous layers (low-density crust over high-density mantle, over a lower density layer beneath the lithosphere). A small harmonic perturbation imposed on the base of the lithosphere is observed to result in gravitational instability under the constraint of assumed axisymmetry. Numerical solutions are obtained for viscous strain on the order of 200 percent or more. Topography develops with time under the influence of dynamic stress associated with downwelling or upwelling, and spatially variable crustal thickening or thinning. Axisymmetric Rayleigh-Taylor instability can produce the scale of uplift and most of the topographic forms displayed by coronae. A central upwelling mechanism is inferred for some coronae (e.g. group 3a, rim surrounding elevated central region coronae), whereas central downwelling is required for others (e.g. Group 8, depressions). Observed average coronae radii are consistent with a lithospheric thickness of only 60 km. The sign of topography is not diagnostic of central upwelling or downwelling because crustal thickness variations induced by the deformation may cause a topographic high above a downwelling or a low above an upwelling when the opposite would be observed in the absence of a crustal layer. The model calculations show, however, that the gravity anomaly is always negative above a downwelling, although its amplitude may be diminished by deformation of the crust. Using the ratio of peak gravity to peak topography anomaly removes uncertainty associated with lithospheric thickness and the density contrast between the crust and mantle. This measure depends primarily on the ratios of crust to lithospheric thickness and strength. The variation of observed gravity to topography ratios suggests spatial variability in the relative strength and thickness of the crust. Average values are consistent however with a low-density crustal layer only 10 to 20 km thick.
P23A-0236 1340h
Venus' Atla and Beta Regiones: Formation of Chasmata and Coronae
Two likely areas of current tectonic and volcanic activity on Venus are Atla and Beta Regiones. Both are marked by pronounced topographic and geoid highs and each lies at the intersection of multiple rifts, i.e. the chasmata system. These regiones may be surface expressions of mantle upwellings. We examine the distribution, style, and attitude of coronae with respect to the two geoid highs. Coronae -- circular features unique to Venus -- could be caused by individual rising diapirs. Unlike Earth, Venus shows little evidence of horizontal motion, resulting in juxtaposition of coronae of all ages. Furthermore, there is little erosion to modify features. In our analysis, we use the three-tiered classification (based on the interior morphology) of 394 coronae, hence termed domal, circular, and calderic. These differing styles may reflect different stages in the evolution of a corona: from domal (youngest, possibly still active) features, progressing through increasing degrees of collapse to the calderic coronae. Comparing locations of these features shows the domal coronae average higher elevations, and calderic at lower elevations, with circular in between. Similar comparisons of other characteristics of the coronae, such as size, elongation, or dip, also show the progression from domal through calderic to circular. Both Atla and Beta are ringed by many coronae, but neither has coronae at or near their crests even within 20 m of their geoid highs. Coronae do occur in many rift segments, yet none occurs at or near these intersection points. Perhaps just as remarkable, Atla has a partial ring of four domal coronae, all within a 10-m geoid range of each other, whereas Beta has a partial ring of 6 or so calderic coronae between three and four 10-m contours from its crest. In both instances, the rings parallel geoid contour lines. These are the nearest coronae of their type to the crests. If corona formation is contemporaneous with the uplift process at Atla and Beta, and if the domal are younger than the calderic coronae, then Atla Regio is a recent feature and more active than Beta. This is in agreement with an independent assessment with modified craters. We use stratigraphy, crater modification, and relative tilt of craters and coronae to further test the timing of events implied in our model.
P23A-0237 1340h
Friction Layer at the Sides of the Plasma Channels in the Venus Nightside Ionosphere
Electron density signatures of the Venus ionosphere measured with the Pioneer Venus Orbiter (PVO) show passes in which the density profile exhibits a plateau shape that extends across a significant section of the upper ionosphere. Such cases are not seen throughout the nightside hemisphere but occur mostly near the midnight plane. A distribution of the number of cases with this peculiarity was obtained from 20 PVO passes of a set of 40 PV orbits that scanned the nightside ionosphere in the first and third season of operation. The distribution shows that the location where the density plateau is observed is slightly shifted from the midnight plane towards the dawn side thus suggesting a displacement in the same direction as that of the distribution of ionospheric holes reported from the early PVO measurements. The peak of the distribution occurs near 1 hr LST and thus its displacement from the midnight plane is slightly smaller than that of the ionospheric holes. With this information it is argued that a friction layer is formed along the sides of the plasma channels with a tendency for the profiles with the density plateau to preferably occur in the dusk side of those features. This result is viewed as implying that the ionospheric material located in that side of the plasma channels, which serve to interpret the ionospheric holes, is more strongly eroded by the solar wind that flows within the channels. It is further suggested that the dusk-dawn difference in the location of the plateau-shaped density profiles across the ionospheric holes may result from the effects of a rotational motion of the upper ionosphere on the solar wind that flows within the plasma channels. The kinetic pressure that is exerted by the rotating ionosphere on the solar wind at the dusk side of the channels implies the large ionospheric erosion seen in that side and hence the profiles with the density plateau.
P23A-0238 1340h
Meteoric Ions in Venus' Atmosphere
From a thorough modeling of the altitude profile of meteoric ionization in the cytherean atmosphere we have deduced that layers of magnesium, iron, and sodium ions should exist between altitudes between 115 and 120~km. Based on the estimated meteoroid mass flux density, a peak ion density of several $10^3$ ions cm${}^{-3}$ is predicted. Allowing for the uncertainties in all of the model parameters, this value is probably within an order of magnitude of the correct density. The peak density is most sensitive to the meteoroid mass flux density, which determines the source function for Mg from the ablating meteoroids, the eddy viscosity coefficient, which determines the effectiveness of mixing the meteoric input downwards, and the presence of sulfuric acid droplets, which is an efficient sink of metallic compunds. We examine the effect of the aerosol layers in the upper mesosphere of Venus' atmosphere on the metallic layers.
P23A-0239 1340h
Hydrogen Loss to the Solar Wind at Venus: Deductions from Proton Cyclotron Waves
Proton cyclotron waves generated by ion pickup processes were detected at Mars, indicating the exospheric loss of hydrogen. Yet, these waves have never been reported at Venus, suggesting the possibility of different or less active loss mechanisms. However, a recent re-examination of the Pioneer Venus records, with more sophisticated data analysis techniques than were initially available, shows the presence of proton cyclotron waves up to at least 12 RV from the planet. A detailed wave analysis indicates that wave generation is controlled by the direction of the interplanetary magnetic field. These waves can provide insight into the nature of the pickup ion transport and acceleration processes at Venus.
P23A-0240 1340h
Planetary Foreshock Radio Emissions
The electron foreshock regions upstream of Earth's bow shock and of travelling interplanetary shocks are known to produce strong radio emissions. A quantitative theoretical model has been developed for radio emissions from the terrestrial foreshock and for coronal and interplanetary type II radio bursts. Here, we present a detailed comparison between the predicted and observed levels of terrestrial foreshock radio emission. We also generalize the theoretical model to other planetary foreshocks, and we compare the predicted levels of radio emissions from the foreshocks of other planets to that from Earth's. Contrary to some expectations, our theoretical results predict exceptionally strong radio emissions from Mercury's foreshock, in particular, and relatively weak emissions from the Jovian and Saturnian foreshocks. These predictions will be testable with forthcoming space missions such as Messenger and Beppi Colombo, as well as existing missions like Galileo and Cassini.
P23A-0241 1340h
Impact of Multiply Charged Heavy Solar Wind Ions on the Surface of Mercury
Mercury is a planet with a relatively weak intrinsic magnetic field without an atmosphere. The surface of the planet is therefore anticipated to be subject to impacting solar wind ions, protons, alpha particles and multiply charged heavy ions such as O$^{6+}$ and O$^{7+}$ ions. Impacting ions are a manifestation of direct plasma-surface interaction at Mercury and it has many consequences to the near Mercury space. Especially, impacting ions kick off neutrals and ions from the surface, thus affecting Hermean exosphere and its magnetospheric plasma. The impact on solar wind protons have recently been studied self-consistently by a quasi-neutral model [Kallio and Janhunen, GRL 2003]. However, effects of impacting multiply charged solar wind ions at Mercury have not yet received much attention. For example, it has been emphasized that impacting multiply charged solar wind ions result in soft-X ray emission, that they can effectively ionize and dissociate the molecular structure of a surface, and that these ions may result in a more effective surface sputtering that impacting solar wind protons [D. E. Shemansky, AIP Conf. Proc. 663, 2003]. In the presentation we (1) Use a quasi-neutral hybrid model to calculate the flux of impacting solar wind alpha particles (He$^{++}$) and O$^{7+}$ ions on the surface of Mercury, and (2) Discuss various consequences of the impact, especially on soft X-ray emission and surface sputtering.
P23A-0242 1340h
Sodium Imager in the BepiColombo Mercury mission
The Mercury's Sodium AtmoSphere Interferometer (MSASI) on BepiColombo will address a wealth of fundamental scientific questions pertaining to the Mercury's exosphere. Together, our measurement on the overall scale will provide ample new information on regolith-exosphere-magnetosphere coupling as well as new understanding of the dynamics governing the surface-bounded exosphere. It arises quite clearly from continuous ground-based observations that the regolith of Mercury releases a fraction of its content to Mercury's exosphere. Some processes are identified up to now as leading to this ejection. These processes are associated with different energies of ejection, behavior in different regions of Mercury's surface and eject different types of population from the surface. The responsible processes are (1) Chemical sputtering, (2) Thermal desorption, (3) Photon-stimulated desorption, (4) Ion sputtering, and (5) Micro-meteoroid impact/vaporization. Each candidate seems to be fairly operative, but any cannot completely explain phenomena observed from the Earth. Also, the fate of ejecta from the regolith is still unknown. Some are expected to return to the lithosphere, the other are lost to interplanetary space. Circulation of lithospheric sodium atoms via exosphere-magnetosphere might bring a significant change in the composition of surface layer on Mercury. The MSASI measurements clearly and definitely can identify the release mechanism, how exospheric sodium is born from the regolith, and bring comprehensive picture of global circulation of regolith materials. Also, MSASI/BepiColombo is the first and unique opportunity to study the formation, circulation, maintenance of the 'surface-bounded exosphere', which is a different type of terrestrial atmosphere. MSASI is a high-dispersion visible spectrometer working in the spectral range around sodium D2 emission (589nm) and devoted to the characterisation of the Mercury_fs exosphere. A tandem Fabry-Perot etalon is used to achieve a compact design. A one degree-of-freedom scanning mirror is employed to allow obtaining full-disk image of the planet and selected region of interest, e.g. polar regions, Caloris Basin, and magnetosphere. Also, simultaneous and complementary data from Limb camera, in-situ measurements of ENA, plasma, and dust, surface composition investigation by X- and gamma-ray are expected to support our mission.
P23A-0243 1340h
Potential Relation Between Caloris Basin And Mercury's Sodium Exosphere
A Mercury's thermal model has been developed, in order to describe theorically the temperature of the surface exposed to solar radiation. This model describes the surface temperature and the first meters below during a whole diurnal cycle. Such a model has been tested with respect to known measurements of Mercury's surface temperature (Chase et al. 1976) and previous models (Hapke and Hale 2002). This model has been coupled to 3-D Monte Carlo model of Mercury's sodium exosphere (Leblanc et al. 2003). This last model describes the sodium adsorption/desorption cycle during a whole diurnal period and has been compared successfully to observations. We here use the thermal model to describe in details the temperature variation induced by the slope associated to Caloris Basin. The results on the sodium exosphere with respect to Mercury positions around the Sun will be described and will discuss the potential role of the particular topography of Mercury's Caloris basin, especially in comparison with exospheric sodium emissions observations (Potter et al.) which revealed North/South hemispheric asymmetries. References: Chase et al. Icarus, 28:655, 1976 - Hale and Hapke, Icarus, 156, 318, 2002 - Leblanc et al., J. Geophys. Res., 108, E12, 5136, 2003 - Potter et al., Plan. Space Sci., 47, 1441, 1999.
P23A-0244 1340h
An ENA Instrument for the Mercury-Exploring BepiColombo mission
The Mercury environment is a closely coupled system, in which three components, surface, exosphere, and magnetosphere interact with each other. In this environment low energy neutral atoms (LENA) with energies of few tens eV to few keV, which can be used for imaging, result from (1) charge-exchange of energetic ions in the near-planet environment with exospheric gasses, (2) sputtering from the planetary surface, and (3) backscattering of precipitating ions. Imaging these LENAs will visualize global distributions of magnetospheric ions, solar-wind ions, and ions of the planetary origin. It will also provide direct measurements to understand contribution of the sputtering sources to the Mercury exosphere. These global measurements are the key in understanding highly dynamic and compact magnetosphere. They will contribute to studies of morphology and dynamics of the precipitation zones, substorms at Mercury, magnetospheric structure and dynamics, and dynamics of planetary ions. We have proposed an ENA instrument (Energetic Neutral atom Analyzer) for Mercury Magnetospheric Orbiter (MMO) of the BepiColombo mission, which is a joint mission of Europe and Japan for Mercury exploration. The ENA instrument is based on surface ionization/reflection technique. A neutral particle is first ionized on a specially-developed surface and is selected in energy in an electrostatic analyzer. After energy analysis the particle hits a reflection surface and goes to detectors. Measuring time-of-flight from the reflection to the detection provides the velocity, which gives the mass of the particle by combining the energy analysis. The instrument is carefully designed to be light-weight ($ < $ 2.0 kg), with high photon suppression, and capable of resolving masses to meet the scientific objectives under tight designing limitations.
P23A-0245 1340h
Single-particle model of Mercury's magnetosphere and exosphere.
The peculiar configuration of the Hermean magnetosphere may allow a solar wind entrance and circulation in Mercury's environment. Cusp regions are extremely large if compared to the Earth's ones, so that intense ion fluxes can be expected there. The solar wind particles are likely to rapidly leave the Hermean magnetosphere or precipitate onto the surface of the planet, thus originating neutral particle emission via ion-sputtering. The exosphere is also refilled by photon-stimulated and thermal desorption processes (also occurring at the planetary surface), and eroded by photon-ionisation and Jeans escape. In the present study, we reconstruct the global distributions of H, O and Na (both neutral and ionised), taking into account the above-mentioned processes, by means of a single-particle model. The Neutral Particle Analyser - Ion Spectrometer experiment (NPA-IS/SERENA), proposed to fly on board of the ESA mission Bepi Colombo, will monitor the circulating ion and neutral particles. The modelled distributions here presented may be considered as a reference tool for the future observations.
P23A-0246 1340h
Modeling The Impulsive Meteoritic Impact Vaporization In The Hermean Exosphere
As for many other aspects of Mercury, our knowledge about the Hermean exosphere and its characteristics is very limited. Many models have been studied to account for the poor observational evidences. Among these, the model by Wurz and Lammer (2003) consists of a Monte Carlo simulation of a large number of trajectories of different species ejected through different surface release processes, following ballistic orbits. Inputs are the distinctive energy and ejection angle distribution of the released particles; then the numerical integration provides a large set of trajectories from which exospheric bulk parameters are derived. The resulting exosphere looks comparable with the observed data. Among the source mechanisms, meteoritic impact vaporization has often been considered to have only a secondary role for exospheric refilling; nevertheless recent works evidenced that it could be the main acting process on the night side of the planet, where the PSD and thermal desorption cannot operate anymore. On this subject almost nothing has been investigated about objects with size larger than 1 cm. Such meteoroids have been shown to be ejected from the Main Belt and to intersect the Hermean orbit; then they are expected to become part of the meteoritic contribution. The present work reviews impact vaporization contribution to the model by Wurz and Lammer. Particularly, it is intended to superimpose on the exospheric model the vapour cloud caused by an impulsive event, and then study both the enhancement of the average exospheric density, than its dynamical evolution with time. In fact, during relatively short time periods, many small meteoroids are expected to fall on Mercury; not so frequent to contribute with a constant rate to the exosphere, but surely enough to be detected by devoted instruments on board of space missions like MESSENGER or BepiColombo.
P23A-0247 1340h
Thin shell dynamo models consistent with Mercury's weak observed magnetic field
Mariner 10 observed Mercury's magnetic field during 2 flybys of the planet between 1974 and 1975, revealing the presence of a magnetic field of internal origin with a dipole moment of $\approx 300$nT$-R_M^3$ ($1 R_M = 2440$ km). Such a field may be too large to be explained easily by remanent magnetization; however, energetic and magnetostrophic balance considerations suggest that it may be too small to be consistent with dynamo action, at least for an Earth-like dynamo. More specifically, energetic and magnetostrophic balance arguments provide an estimate of the toroidal magnetic field strength that the dynamo should generate: if the dynamo is Earth-like in the sense that it has similar toroidal-poloidal field strength scaling as the Earth, then the poloidal field should be much stronger than that observed. Before abandoning a dynamo explanation for Mercury's field, we question whether the same toroidal-poloidal scaling should hold for Mercury. Thermal evolution calculations estimate that Mercury's solid inner core may comprise between 0.5 and 0.8 of the total core radius, much larger than the Earth value of 0.35. As a result, the fluid convecting outer core where the dynamo is generated may be much thinner for Mercury than for Earth. Here we use 3-D numerical dynamo modeling to investigate dynamos operating in thin shell geometries. We examine the ratio of the dipole field at the core-mantle boundary to the toroidal field in the core for various shell thicknesses and Rayleigh numbers and find that some thin shell dynamos can produce magnetic fields with Mercury-like field partitioning. In these dynamos, the toroidal field is produced more efficiently through differential rotation than the poloidal field is produced through interactions of convective upwellings with the toroidal field. The poloidal field is also dominated by smaller-scale structure which was not observable by the Mariner 10 mission, compared to the dipole. This suggests that a hydromagnetic dynamo may be consistent with the weak surface field observed at Mercury and alternative explanations may not be required.
P23A-0248 1340h
Depth of Regolith Cover on Mercury's Polar Volatile Deposits as a Function of Time
Delay Doppler images of Mercury's polar regions show strong evidence for ice in the polar craters (Harmon and Slade, 1992; Harmon et al., 2001) More recent high resolution S-band (12.6 cm) and X-band (3.5 cm radar images show that the ice deposits at the north pole are buried under an average of 36 cm of regolith (Slade et al., 2004; Harcke et al., in preparation). We have modified the model of Crider and Vondrak (2003) initially developed to study lunar ice traps, to calculate the thickness of regolith cover as a function of time. This model treats burial and removal of material from impacts of meteoroids with mass greater than 1 mg. In addition we treat ice deposition and loss by sublimation, sputtering and photon-stimulated desorption by interstellar radiation. We assume realistic values for an ice layer that would be deposited by a comet impact, and we assume continual bombardment by micrometeoritic and cometary dust. We discuss the probable timing of comet impacts at both the north and south poles of Mercury.
P23A-0249 1340h
The MESSENGER Payload
The MErcury, Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched on 3 August 2004, will be the first orbiter of the planet Mercury. After insertion into a near-polar Mercury orbit in March 2011 MESSENGER will make detailed measurements of the planet for one Earth year. During the 6.6-year cruise phase, MESSENGER will fly by Mercury three times and map $ > $90% of the planet. The payload consists of seven instruments, including a dual imaging system with wide-angle and narrow-angle cameras; an integrated ultraviolet, visible, and infrared spectrometer that is sensitive enough to detect atmospheric emissions and robust enough to map spectral absorption features on the sun-lit surface; gamma-ray, neutron, and X-ray spectrometers for remote geochemical mapping; a vector magnetometer to examine the internal and external field sources; a laser altimeter to examine the topography of surface features and determine whether Mercury has a fluid core; and an energetic particle and plasma spectrometer to characterize ionized species in the magnetosphere. The payload was fully calibrated before launch, and all instruments have been successfully operated in space. An Earth flyby one year after launch will be used for extensive in-flight calibration, and two Venus flybys will provide further opportunities for instrument observations. The MESSENGER spacecraft and instruments must cope with the $ > $14 kWm$^{-2}$ solar thermal input and the large velocity change required to reach and survive in Mercury orbit. Several unique technologies have made this mission possible.
P23A-0250 1340h
MESSENGER: The Discovery Mission to Mercury
NASA's MErcury, Surface, Space ENvironment, GEochenistry, and Ranging (MESSENGER) spacecraft, launched on 3 August 2004, has begun its voyage to initiate a new era in our understanding of the terrestrial planets. The mission, spacecraft, and payload are designed to answer six fundamental questions regarding the innermost planet: What planetary formational processes led to Mercury's high metal/silicate ratio? What is the geological history of Mercury? What are the nature and origin of Mercury's magnetic field? What are the structure and state of Mercury's core? What are the radar-reflective materials at Mercury's poles? What are the important volatile species and their sources and sinks on and near Mercury? Planet formational hypotheses will be tested by measuring the surface abundances of major elements by X-ray and gamma-ray spectrometry. The geological history will be determined from high-resolution color imaging of the heavily cratered highlands, intercrater plains, and smooth plains. MESSENGER will provide detailed views of both the Caloris basin and its antipodal terrain. Topographic, mineralogical, and elemental abundance data will be used to seek evidence of volcanic features and units. Measurement of Mercury's magnetic field and its interaction with the solar wind will distinguish the intrinsic dipole and quadrupole components while separating these from the current systems driven by solar-wind-induced convection. The structure of the internal field will put constraints on dynamo models. Such models will also be constrained by measuring Mercury's libration to determine the extent of a fluid outer core. Both water ice and sulfur have been postulated as major constituents of the high-radar-backscatter polar deposits. MESSENGER will combine gamma-ray and neutron spectrometry of the surface with ultraviolet spectrometry and in situ particle measurements to detect both neutral and charged species originating from the surface. Such measurements will address the sources and sinks of volatiles and their couplings with the surface on a global basis as well as the nature of the polar deposits. To broaden scientific participation in the mission, the MESSENGER project is working with NASA to establish a Participating Scientist Program. The MESSENGER team is also continuing its informal interaction with members of the BepiColombo project to maximize the overall scientific return from both missions.
http://messenger.jhuapl.edu