U11C-01
MESSENGER's Second Flyby of Mercury: A Scientific Overview
MESSENGER is the first spacecraft to visit the planet Mercury in more than 30 years. En route to insertion into orbit about Mercury in March 2011, MESSENGER flew by the innermost planet on 14 January 2008 and will do so again on 6 October 2008. Objectives of the flybys include color imaging of the surface, the first high-resolution spectral reflectance measurements (from ultraviolet to near-infrared wavelengths) of surface composition, the first spacecraft altimetric measurements of surface topography, the first measurements of the abundances and compositions of plasma ions in Mercury's magnetosphere, the deepest penetrations yet into Mercury's magnetosphere, and searches for previously undetected species in Mercury's surface-based exosphere and neutral sodium tail. MESSENGER's first flyby confirmed that Mercury's internal magnetic field is primarily dipolar, documented water-group and other ions in the magnetosphere, mapped a north-south asymmetry in the Na tail and determined the Na/Ca ratio near the tail and near the dawn terminator, detected two outbound current-sheet boundaries that may indicate a planetary ion boundary layer, but did not observe energetic magnetospheric electrons as reported by Mariner 10. The laser altimeter demonstrated that the equatorial topographic relief of Mercury is at least 5 km. MESSENGER's images provided evidence for widespread volcanism, and candidate sites for volcanic centers were identified. Also revealed were newly imaged lobate scarps and other tectonic landforms supportive of the hypothesis that Mercury contracted globally in response to interior cooling and growth of a solid inner core. Reflectance spectra show no evidence for FeO in surface silicates, and MESSENGER's neutron spectrometer yielded an upper bound of 6% on the surface Fe abundance. The reflectance and color imaging observations support earlier inferences that Mercury's surface material consists dominantly of iron-poor, calcium-magnesium silicates with an admixture of spectrally neutral opaque minerals. The October encounter will reveal more than 30% of the planet never before seen at close range, improve knowledge of Mercury's low-degree gravity field and its implications for the structure of the planet's core, and feature targeted observations of the surface, exosphere, and tail that have profited from the experiences of the first flyby.
U11C-02
Mercury Core Properties from the Rotation State
Radar observations have revealed that Mercury's spin axis occupies Cassini state 1 with an obliquity of 2.11± 0.1 arcminutes and a forced 88-day libration of the spin about the 3:2 spin-orbit resonance of 35.8± 2 arcseconds, which determines (B-A)/Cm=(2.03± 0.12)× 10-4 (Margot et al., Science 316, 710-714, 2007). (A<B<C= principal moments of intertia; Cm≈ 0.5C is that of the mantle and crust alone). This large forced libration in longitude implies that Mercury's mantle is decoupled from a core that is at least partially molten. Crucial to this interpretation is the value of the gravitational harmonic C22, whose 50% uncertainty from the Mariner 10 determination limits the constraints on the size of the molten core. In spite of this uncertainty, the most probable value of Cm/C is near 0.5 and it is <1 with 95% confidence. The current most probable value of C22 would have to be increased by more than 3σ to be consistent with a solid inner core. The flyby of Mercury by MESSENGER in January 2008 is followed by a second flyby in October. Together these close approaches reduce the uncertainties in J2 and C22, and thereby the uncertainty in Cm/C. J2 and C22 deduced from the first flyby are not inconsistent with the Mariner 10 values. In the meantime, continued radar observations until orbit insertion in March 2011 will reduce the uncertainties in the pole position and the 88-day forced libration amplitude and constrain long-period variations in the rotation rate due to both planetary perturbations of the orbit and due to a possible, but unlikely, nearly 12-year free libration in longitude. The latter would need, because of damping, a recent excitation to be present. There is an 11.86-year forced libration induced by Jupiter whose amplitude could exceed that of the 88-day forced libration if the free libration period is sufficiently close. The amplitude of this 11.86-year forced libration can further constrain (B-A)/Cm. None of the forced librations will hinder the determination of the core properties from the details of the rotation state. The MESSENGER spacecraft will eventually determine J2 and C22 to 1% uncertainty, which, coupled with occupancy of Cassini state 1, will determine C/MR2 (M, R= mass and radius of Mercury) allowing a rather complete characterization of the core size and state from the rotation state.
U11C-03
The Mercury Gravity Field: MESSENGER Observations
On January 14, 2008, the MESSENGER spacecraft passed within 201 km of the surface of the planet Mercury with a closest approach at 4S, 38E. This was the first of three MESSENGER flybys of the planet, and it provided the first observations of Mercury from a spacecraft since the Mariner 10 flybys in 1974 and 1975. The tracking of the spacecraft during MESSENGER's first flyby provided new data on the gravity field of Mercury from which improved values for the low-degree coefficients in the spherical harmonic expansion of the field have been derived. These new values suggest slightly smaller values for the planet's mass and degree-2 gravity coefficients and the need to solve for a shorter-wavelength gravity anomaly to explain the Doppler tracking observations. On October 6, 2008, the MESSENGER spacecraft will make its second flyby of Mercury and is once more expected to pass within 200 km of the surface, again just south of the equator but over a location almost 170 degrees east of the first flyby. Because the spacecraft will pass over a different part of the planet, the tracking data will provide new and distinct observations of the low-degree gravity field that should significantly reduce the uncertainties in the degree-2 terms, which are important for our understanding of the size and state of Mercury's core.
U11C-04 INVITED
Topography of Equatorial Mercury from MESSENGER Flybys 1 and 2
During the first flyby of Mercury by the MESSENGER spacecraft on January 14, 2008, the Mercury Laser Altimeter (MLA) obtained a 3200-kilometer-long profile that spanned approximately 20% of the near- equatorial region of the planet. Topography along that profile is characterized by a 5.2-kilometer dynamic range and approximately 1-kilometer root-mean-square roughness. Sampled craters are shallower than their counterparts on the Moon, at least in part because of Mercury's higher gravity. Crater floors vary in roughness and slope, which suggests complex modification over a range of length scales. However, the various contributions to crater geometry and the general nature of crater modification have been poorly constrained because no spacecraft images are yet available in the profiled hemisphere. On October 6, 2008, MESSENGER will make its second flyby of Mercury, with closest approach also near the equator but about 170° east of the earlier flyby. Closest approach distances are about 200 km in both cases, and a profile of comparable length to profile 1 is expected from flyby 2. The collective altimetry data, combined with high-resolution and color imaging of both profile regions, will contribute toward understanding Mercury's long- wavelength shape and the geological and geophysical processes that have operated at the planet's surface.
U11C-05
AN OVERVIEW OF ULTRAVIOLET THROUGH INFRARED REFLECTANCE OBSERVATIONS OF MERCURY DURING THE SECOND MESSENGER FLYBY.
During the second MESSENGER flyby of Mercury on October 6, 2008, MESSENGER will pass over part of the planet previously imaged by the Mariner 10 spacecraft. The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) will obtain the first high-spatial-resolution (< 10 km) spectra of this hemisphere, measuring reflectance spectra from Mercury's surface over the wavelength range 220-1450 nm along an equatorial swath from approximately 285 to 360 degrees E. The Visible and Infrared Spectrograph (VIRS) sensor of MASCS, sensitive to wavelengths of 350-1450 nm, will observe the sunlit surface for approximately 8 minutes after the spacecraft passes over Mercury's dawn terminator and will obtain approximately 400 reflectance spectra, about 50 of which will be contemporaneous and co-aligned with Mercury Dual Imaging System (MDIS) Wide Angle Camera color images. MASCS VIRS spectra will sample footprints averaging several kilometers across. MASCS also will obtain five ground spectra in the middle ultraviolet (UV, 220-320 nm) and one discovery scan in the far UV (115-170 nm) using the MASCS Ultraviolet and Visible Spectrometer (UVVS) component in a surface-viewing mode. The UVVS component of MASCS is a scanning grating spectrometer capable of observing one wavelength at a time, so each UVVS spectrum will be spread over a swath of surface area. The MDIS Narrow Angle Camera at higher resolution will image the entire MASCS ground track subsequently. Several days after the flyby, MASCS will look back at the planet to obtain disk-integrated spectra for VIRS and UVVS with Mercury as a near-point source. The MASCS ground track from the first flyby of Mercury in January 2008 crossed a variety of cratered and plains units, revealing a red-sloped, space-weathered surface with only modest spectral variation, and very little indication of the 1- micron absorption band indicative of iron in silicate minerals. Absorption features between 200 and 400 nm in several resolved spectra as well as in the disk-integrated Mercury spectrum indicated the possibility of Fe-Ti minerals on the surface. The ground-track of the second flyby will cross more cratered plains units, including at least one multi-ringed basin viewable with illumination at moderate to low incidence angle. We compare equatorial spectra from the two hemispheres, and we relate them to color images and inferred geologic units.
U11C-06
Cratering on Mercury Interpreted from MESSENGER's First Two Flybys
Images obtained by the Mercury Dual Imaging System (MDIS) during MESSENGER's first flyby of Mercury (January 2008) revealed cratered landscapes never before seen by spacecraft at longitudes generally west of the center of the Caloris basin. Other images showed cratered terrains previously viewed by Mariner 10 but this time with much better lighting conditions. We have studied crater size-frequency distributions (SFDs) in some representative regions from MDIS Narrow Angle Camera frames for craters down to about 1-km diameter as well as from MDIS Wide Angle Camera images covering broader terrains for craters larger than 8-km diameter. Initial results include the revelation that secondary craters often predominate over primary craters at diameters less than 8 km, a larger threshold size than for the Moon or Mars. Considerably larger secondaries from basins may be important in some regions. Many of the smooth plains exterior to Caloris appear to be younger than the plains within Caloris, a result that appears to be inconsistent with the hypothesis that the exterior plains are mainly formed by Caloris ejecta (as in the lunar Cayley plains) and suggests that a period of post-Caloris volcanism occurred in these regions. The small, well-preserved basin Raditladi has an order of magnitude fewer superimposed impact craters (on both its floor and ejecta blanket) than are on the smooth plains west of Caloris, implying an unusually youthful age for this basin of 1 to 2 Ga or less. There are appreciable regional differences in crater SFDs in highland regions, which may shed light on the processes that form intercrater plains. Previously unimaged regions farther west of the Caloris- dominated longitudes viewed during the first flyby will be studied from images obtained during the second flyby (October 2008). They may reveal more regional differences in the SFDs for Mercury's craters.
U11C-07
Mercury's Smooth Plains: Distribution, Origin, and Significance
The origin of Mercury's smooth plains has remained one of the large unanswered questions since the Mariner 10 images were returned more than 30 years ago. The MESSENGER spacecraft's first flyby of Mercury (January 2008) revealed important evidence of a volcanic origin for smooth plains, including volcanic vents, flooding and embayment relations, and distinctive spectral properties. Preliminary geologic mapping based on the January 2008 MESSENGER images (200-1000 m/pixel) shows that like the region imaged by Mariner 10, smooth plains cover over 40% of the surface. The extent of smooth plains on Mercury is significantly greater than lunar smooth plains (maria), which cover only 16% of the Moon's surface. The Caloris basin interior is filled with smooth plains that exceed 1.5 million square km in areal extent. Significantly, much of the newly discovered smooth plains, including those in Caloris, exhibit distinct color boundaries that correspond to their morphologic margins, a characteristic shared with lunar mare deposits, but not lunar highland plains. The new MESSENGER data strengthen the argument that much of Mercury's smooth plains are volcanic in origin but do not prove this hypothesis. However, smooth plains with volcanic signatures, such as the Caloris interior smooth plains, indicate that volcanism was originated from broad zones of partial melting in the upper mantle. The ubiquity of smooth plains (if all are volcanic in origin) demonstrates that global compressive stresses in Mercury's lithosphere were not of sufficient magnitude to preclude widespread volcanic activity, at least for an interval in the Calorian. The second MESSENGER Mercury flyby (6 October 2008) will reveal more unseen terrain and re-image portions of the surface viewed by Mariner 10 with more modern cameras, as well as targeted ultraviolet through near-infrared spectral measurements. The combined MESSENGER and Mariner 10 data will allow us to analyze and present detailed morphologic, color, and spectral characterization of smooth plains across more than 80% of Mercury. Our goal is to determine the emplacement mechanisms for smooth plains and estimate their composition. Volcanic deposits represent our best, and perhaps only, look into the planet's mantle and thus bulk composition. Definitive resolution of the nature and composition of the full suite of smooth plains may be possible only after MESSENGER spends a significant amount of time in orbit about Mercury collecting data with its full complement of science instruments.
U11C-08 INVITED
Volcanism on Mercury: Characteristics and Distribution from the First MESSENGER Flyby
The first MESSENGER flyby of Mercury obtained Mercury Dual Imaging System (MDIS) image data for 21% of the surface unseen by Mariner 10. These data have helped to address and resolve a series of questions related to the existence, nature, and distribution of volcanism outstanding since 1975. Numerous volcanic vents, in the form of irregularly-shaped rimless depressions, are seen around the interior margin of the Caloris basin; some have surrounding shield-like or flow-like structures, in one case a shield in excess of 100 km in diameter. In several cases, the vents appear to be sources of explosive volcanism that formed bright haloes around the vents. The interior of the Caloris basin is filled with plains units spectrally distinctive from the basin rim deposits, and impact crater stratigraphy shows evidence that these units are volcanic in origin. Some of the proximal smooth plains surrounding the exterior of the Caloris basin show distinct differences in color and morphological properties, supporting a volcanic origin; many of the circum-Caloris smooth plains, however, show no spectral distinctiveness compared with surrounding crustal material, and thus an impact- ejecta-related origin for some of these plains remains a viable hypothesis. Some smooth and intercrater plains units distal to the Caloris basin show evidence of flooding and embayment relations unrelated to Caloris ejecta emplacement; local and regional geological and color relationships support a volcanic origin for these plains. Large impact craters show a sequence of embayment of interior floor and exterior ejecta deposits that supports a volcanic origin for the embayment and filling processes. Crater embayment and flooding relationships suggest typical thicknesses of volcanic plains of many hundreds of meters, and local thicknesses inside impact craters of up to several kilometers; these thicknesses are less than the amount thought to be required to cause exterior loading of the Caloris basin that would result in uplift of the basin interior. Evidence is seen for intrusive magmatic activity in the form of a floor-fractured crater with two laccolith-like structures. Pantheon Fossae, a large radial graben swarm originating in the Caloris basin center, can be interpreted as a radial dike swarm linked to the late-stage evolution of the Caloris basin interior, although hypotheses for this feature unrelated to magmatism have also been advanced. These new data provide evidence that volcanism was important in shaping the surface of Mercury.