• Measuring the plasma environment at Mercury: The fast imaging plasma spectrometer

      Koehn, P. L.; Zurbuchen, T. H.; Gloeckler, G.; Lundgren, R. A.; Fisk, L. A. (The Meteoritical Society, 2002-01-01)
      The plasma environment at Mercury is a rich laboratory for studying the interaction of the solar wind with a planet. Three primary populations of ions exist at Mercury: solar wind, magnetospheric and pickup ions. These pickup ions are generated through the ionization of Mercury's exosphere or are sputtered particles from the Mercury surface. A comprehensive mission to Mercury, such as MESSENGER, should include a sensor that is able to determine the dynamical properties and composition of all these plasma components. An instrument to measure the composition of these ion populations and their three dimensional velocity distribution functions must be lightweight, fast, and have a very large field of view. The Fast Imaging Plasma Spectrometer (FIPS) is an imaging mass spectrometer, part of NASA's MESSENGER mission, the first Mercury orbiter. This versatile instrument has a very small footprint, and has a mass that is about one order of magnitude less than other comparable systems. It maintains a nearly full-hemisphere field of view, suitable for either spinning or three-axis-stabilized platforms. The major piece of innovation to enable this sensor is a new deflection system geometry that enables a large instantaneous (~1.5pi) field of view. This novel electrostatic analyzer system is then combined with a position sensitive time-of-flight system. We discuss the design and prototype tests of the FIPS deflection system and show how this system is expected to address one key problem in Mercury science, that of the nature of the radar-bright regions at the Hermean poles.
    • Source and maintenance of the argon atmospheres of Mercury and the Moon

      Killen, R. M. (The Meteoritical Society, 2002-01-01)
      We propose that argon-40 measured in the lunar atmosphere and that in Mercury's atmosphere is due to current diffusion into connected pore space within the crust. Higher temperatures at Mercury, along with more rapid loss from the atmosphere, will lead to a similar or smaller column abundance of argon at Mercury than at the Moon, given teh same crustal abundance of potassium. Because the noble gas abundance in the mercurian atmosphere represents current effusion, it is a direct measure of the crustal potassium abundance. We assume a fractal distribution of distance to a connected pore space, with the shortest distance increasing with depth. Given this "rock size" distribution, we show that the diffusive flux is not a unique function of temperature. Even though the diffusion coefficient is an exponential function of temperature, the flux to teh surface is fairly insensitive to the temperature.
    • Spectra of extremely reduced assemblages: Implications for Mercury

      Burbine, Thomas H.; McCoy, Timothy J.; Nittler, Larry R.; Benedix, Gretchen K.; Cloutis, Edward A.; Dickinson, Tamara L. (The Meteoritical Society, 2002-01-01)
      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.
    • Identification of mercurian volcanism: Resolution effects and implications for MESSENGER

      Milkovich, S. M.; Head, J. W.; Wilson, L. (The Meteoritical Society, 2002-01-01)
      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.
    • Diurnal variation of sodium and potassium at Mercury

      Hunten, D. M.; Sprague, A. L. (The Meteoritical Society, 2002-01-01)
      A summary is given of our published observations showing a large (3 to 4) morning/afternoon ratio of the abundances of sodium and potassium. The proposed mechanism is deposition of ions and atoms on the cold night side, followed by their outward diffusion and evaporation as the Sun rises. Published criticisms of this mechanism are discussed and answered. The rate at which Na atoms can evaporate from the surfaces of the Moon and Mercury is uncertain, but, after a review of laboratory measurements, we propose that it is substantial at temperatures of 400 K and higher. Possible reasons are discussed why another group does not find the diurnal variation. There are differences in observing geometry, but the matter remains unclear.
    • The sodium tail of Mercury

      Potter, A. E.; Killen, R. M.; Morgan, T. H. (The Meteoritical Society, 2002-01-01)
      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.
    • A procedure for determining the nature of Mercury's core

      Peale, Stanton J.; Phillips, Roger J.; Solomon, Sean C.; Smith, David E.; Zuber, Maria T. (The Meteoritical Society, 2002-01-01)
      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.
    • Crustal properties of Mercury by morphometric analysis of multi-ring basins on the Moon and Mars

      Potts, Laramie V.; von Frese, Ralph R. B.; Shum, C. K. (The Meteoritical Society, 2002-01-01)
      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: Mid-infrared (3-13.5 micrometers) observations show heterogeneous composition, presence of intermediate and basic soil types, and pyroxene

      Sprague, A. L.; Emery, J. P.; Donaldson, K. L.; Russell, R. W.; Lynch, D. K.; Mazuk, A. L. (The Meteoritical Society, 2002-01-01)
      The Aerospace Corporation's Broadband Array Spectrograph System (BASS) mounted on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii was used to obtain spectral measurements of Mercury's thermal emission on March 21, 1998 (45-85 degrees longitude), and on May 12, 1998 (68-108 degrees longitude). The spectra show heterogeneous composition on Mercury's surface between longitudes 45-85 degrees and about 68-108 degrees. These observations include measurements from 3-6 micrometers, a spectral region not previously covered by mid-infrared spectroscopy. Excellent quality data were obtained in the atmospheric windows between 3-4.2 and 4.6-5.5 micrometers. These wavelength regions exhibit high emissivity characteristic of a regolith with strong thermal gradients maintained in a vacuum environment with spectra dominated by grain sizes of about ~30 micrometers. Emission peaks are present at 3.5 and 5 micrometers in the 45-85 degrees longitude data. The 5 micrometer peak has been tentatively attributed to clino-pyroxene. Data were also obtained in the 7.5-13.5 micrometers spectral region. Spectra obtained during both observing periods show well-defined emissivity maxima (EM) in the spectral vicinity (between 7.7-9.2 micrometers) of the Christiansen frequency of silicate soils. The location of the EM for longitudes 45-85 degrees (7.9 micrometers) is consistent with a surface composition of intermediate SiO2 content. The overall spectral shape is similar to that obtained previously at the same location with different instrumentation. In the region 68-108 degrees longitude, three EM are observed at 7.8, 8.2, and 9.2 micrometers, indicating the presence of distinctly different surface composition from the other location. Comparisons of these data to other mid-infrared spectra of Mercury's surface and asteroids, and of the different instrumentation used in observations are included.
    • Lunar pure anorthosite as a spectral analog for Mercury

      Blewett, David T.; Hawke, B. Ray; Lucey, Paul G. (The Meteoritical Society, 2002-01-01)
      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.
    • The basaltic shergottite Northwest Africa 856 (NWA 856): Petrology and chemistry

      Jambon, A.; Barrat, J. A.; Sautter, V.; Gillet, Ph.; Göpel, C.; Javoy, M.; Joron, J. L.; Lesourd, M (The Meteoritical Society, 2002-01-01)
      We report on the discovery of a new shergottite from South Morocco. This single stone weighing 320 g is referenced as Northwest Africa 856 (NWA 856) with Djel Ibone as a synonymous name. It is a fresh, fine-grained basaltic rock consisting mainly of 2 pyroxenes (total ~68 vol%: 45% pigeonite En61-16 Wo9-22 Fs26-68, 23% augite En46-26 Wo34-29 Fs21-43) and plagioclase converted to maskelynite (about 23 %, Ab43-57 Or1-5 An54-36). Accessory minerals include merrillite, Cl-apatite, pyrrhotite, ilmenite, ulvöspinel, silica (stishovite and glass), amorphous K-feldspar and baddeleyite. Amorphous mixtures of maskelynite and silica occur most commonly as median layers inside maskelynite laths. In addition, melt pockets (about 2%) were recognized with relics of maskelynite, pyroxene and both dense silica glass and stishovite occurring as both grains and sub-micrometer needles. The compositions of the melt pockets are consistent with mixtures of maskelynite and pyroxenes with an average of about 50 % maskelynite. The meteorite is highly fractured at all scales. The bulk composition of NWA 856 has been measured for 44 elements. It is an Al-poor ferroan basaltic rock which strongly resembles Shergotty and Zagami in its major and trace element composition. The nearly flat REE pattern (La/Lu)n= 0.9, is similar to that of Shergotty or Zagami and differs significantly from NWA 480, another Moroccan shergottite recently described. According to the U, Ba and Sr abundances, NWA 856 is not significantly weathered. The oxygen isotopes (delta-18O = +5.03 ppm, delta-17O = +3.09 ppm, and Delta-17O = +0.47 ppm) are in agreement with the Martian origin of this meteorite. On the basis of grain size, pyroxene zoning and composition, abundance of silica inclusions associated with maskelynite, trace element abundances, REE pattern and oxygen isotopes, pairing with NWA 480 is excluded. The similarity with Shergotty and Zagami is striking. The only significant differences are a larger grain size, a greater abundance of silica and melt pockets, a slightly more restricted range of pyroxene compositions and the absence of significant mesostasis.