We review the assertion that the precise measurement of the second degree gravitational harmonic coefficients, the obliquity, and the amplitude of the physical libration in longitude, C20, C22, theta, and phi0, for Mercury are sufficient to determine whether or not Mercury has a molten core. The conditions for detecting the signature of the molten core are that such a core not follow the 88-day physical libration of the mantle induced by periodic solar torques, but that it does follow the 250,000-year precession of the spin axis that tracks the orbit precession within a Cassini spin state. These conditions are easily satisfied if the coupling between the liquid core and solid mantle is viscous in nature. The alternative coupling mechanisms of pressure forces on irregularities in the core-mantle boundary (CMB), gravitational torques between an axially asymmetric mantle and an assumed axially asymmetric solid inner core, and magnetic coupling between the conducting molten core and a conducting layer in the mantle at the CMB are shown for a reasonable range of assumptions not to frustrate the first condition while making the second condition more secure. Simulations have shown that the combination of spacecraft tracking and laser altimetry during the planned MESSENGER orbiter mission to Mercury will determine C20, C22, and theta to better than 1% and phi0 to better than 8%--sufficient precision to distinguish a molten core and constrain its size. The possible determination of the latter two parameters to 1% or less with Earth-based radar experiments and MESSENGER determination of C20 and C22 to 0.1% would lead to a maximum uncertainty in the ratio of the moment of inertia of the mantle to that of the whole planet, Cm/C, of ~2% with comparable precision in characterizing the extent of the molten core.

Plans are underway for spacecraft missions to the planet Mercury beginning in the latter part of this decade (NASA's MESSENGER and ESA's BepiColombo). Mercury is an airless body whose surface is apparently very low in ferrous iron. Much of the mercurian surface material is expected to be optically mature, a state produced by the "space weathering" process from direct exposure to the space environment. If appropriate analog terrains can be identified on the Moon, then study of their reflectance spectra and composition will improve our understanding of space weathering of low-Fe surfaces and aid in the interpretation of data returned from Mercury by the spacecraft. We have conducted a search for areas of the lunar surface that are optically mature and have very low ferrous iron content using Clementine UVVIS image products. Several regions with these properties have been identified on the farside. These areas, representing mature pure anorthosites (90% plagioclase feldspar), are of interest because only relatively immature pure anorthosites have previously been studied with Earth-based spectrometry. A comparison of Mercury with the lunar analogs reveals similarities in spectral characteristics, and there are hints that the mercurian surface may be even lower in FeO content than the lunar pure anorthosites. We also investigate the potential for use of spectral features other than the commonly studied "1 micrometer" mafic mineral absorption band as tools for compositional assessment when spacecraft spectral measurements of Mercury become available. Most low-Fe minerals plausibly present on Mercury lack absorption bands, but plagioclase possesses an iron-impurity absorption at 1.25 micrometers. Detection of this diagnostic band may be possible in fresh crater deposits.

We investigate the possibility that Mercury's crust is very reduced with FeO concentrations of less than ~0.1 wt%. We believe that such a surface could have a composition of enstatite, plagioclase, diopside, and sulfide, similar to the mineral assemblages found in aubritic meteorites. To test this hypothesis, we investigated the spectra of aubrites and their constituent minerals as analogs for the surface of Mercury. We found that some sulfides have distinctive absorption features in their spectra shortwards of ~0.6 micrometers that may be apparent in the spectrum of such an object. Determination of the surface composition of Mercury using orbital x-ray spectroscopy should easily distinguish between a lunar highlands and enstatite basalt composition since these materials have significant differences in concentrations of Al, Mg, S, and Fe. The strongest argument against Mercury having an enstatite basalt composition is its extreme spectral redness. Significant reddening of the surface of an object (such as Mercury) requires reduction of FeO to nanophase iron, thus requiring a few percent FeO in the material prior to alteration.

The possibility of volcanism on Mercury has been a topic of discussion since Mariner 10 returned images of half the planet's surface showing widespread plains material. These plains could be volcanic or lobate crater ejecta. An assessment of the mechanics of the ascent and eruption of magma shows that it is possible to have widespread volcanism, no volcanism on the surface whatsoever, or some range in between. It is difficult to distinguish between a lava flow and lobate crater ejecta based on morphology and morphometry. No definite volcanic features have been identified on Mercury. However, known lunar volcanic features cannot be identified in images with similar resolutions and viewing geometries as the Mariner 10 dataset. Examination of high resolution, low sun angle Mariner 10 images reveals several features which are interpreted to be flow fronts; it is unclear if these are volcanic flows or ejecta flows. This analysis implies that a clear assessment of volcanism on Mercury must wait for better data. MESSENGER will take images with viewing geometries and resolutions appropriate for the identification of such features.

We use satellite altitude free-air and terrain gravity correlations to differentiate regional variations in crustal viscosity and transient cavity diameters of impact basins on the Moon and Mars that we combine with surface roughness for a rheological assessment of the crust of Mercury. For the Moon and Mars, we separate the free-air anomalies into terrain-correlated and -decorrelated components using the spectral properties of the free-air and computed terrain gravity effects. Adjusting the terrain effects for the terrain-correlated anomalies yields compensated terrain effects that we evaluate for crustal thickness variations of the impact basins. We interpret the terrain-correlated anomalies for uncompensated elements of the crustal thickness variations that we find are strongly correlated with the distribution of basin rings from photogeologic analyses. Hence, we estimate the transient cavity diameter from the innermost diameter of the gravity-inferred rings. Comparing these diameters with the related crustal thickness estimates clearly differentiates regional variations in the crustal rheologies. For the Moon, the analysis points to a farside crust that was significantly more rigid than the nearside crust during bombardment time. For Mars, the growth in transient cavity diameters with apparent crustal age also reflects increased viscosity due to crustal cooling. These results are also consistent with local estimates of surface roughness developed from the root-mean-squared topography over 64 x 64 degree patches centered on the basins. Hence for Mercury where gravity observations are lacking, rheological inferences on its crust may result from comparing photometric estimates of transient cavity diameter and local surface roughness with the lunar and Martian estimates. These results for the Beethoven Basin, a typical multi-ring impact feature of Mercury, suggest that the viscosity of the Mercurian crust was relatively great during bombardment time. This enhanced rigidity, despite crustal temperatures that were probably much hotter than those of the Moon and Mars, may reflect an extremely dry crust for Mercury in its early development.

Mercury is difficult to observe because it is so close to the Sun. However, when the angle of the ecliptic is near maximum in the Northern Hemisphere, and Mercury is near its greatest eastern elongation, it can be seen against the western sky for about a half hour after sunset. During these times, we were able to map sodium D2 emission streaming from the planet, forming a long comet-like tail. On May 26, 2001 (UT) we mapped the tail downstream to a distance of about 40,000 km. Sodium velocities in the tail increased to about 11 km/sec at 40,000 km as the result of radiation pressure acceleration. On June 05, 2000 (UT) we mapped the cross-sectional extent of the tail at a distance of about 17,500 km downstream. At this distance, the half-power full-width of the emission was about 20,000 km. We estimated the transverse velocity of sodium in the tail to range from 2 to 4 km/sec. The velocities we observed imply source velocities from the planet surface of the order of 5 km/sec, or 4 eV. Particle sputtering is a likely candidate for production of sodium atoms at these velocities. The total flux of sodium in the tail was approximately 1 x 1023 atoms/sec, which corresponds to 1 to 10% of the estimated total production rate of sodium on the planet.