David Eugene Smith

An improved solution of the gravity field of Mars (GMM‐2B) from Mars Global Surveyor

A spherical harmonic solution of the Mars gravity field to degree and order 80, Goddard Mars Model 2B (GMM-2B), has been developed using X band tracking data of Mars Global Surveyor (MGS) from October 1997 to February 2000 and altimeter crossovers formed from the Mars Orbiter Laser Altimeter (MOLA) data between March and December 1999. During the mapping mission, MGS was located in a near-polar (92.9° inclination) and near-circular orbit at a mean altitude of 400 km. The tracking data from this orbit provide a detailed, global, and high resolution view of the gravity field of Mars. Mars gravity solutions are stable to 60×60 even without application of a Kaula power law constraint. The Valles Marineris is resolved distinctly with lows reaching −450 mGals. Olympus Mons and its aureole are both separately resolved, and the volcano has a peak anomaly of 2950 mGals. The global correlation of the GMM-2B gravity coefficients with MOLA-derived topography is 0.78 through degree 60, and the correlation remains above 0.6 through degree 62. The global gravity anomaly error predicted from the GMM-2B error covariance through 60×60 is 11 mGal. The global geoid error from GMM-2B through 60×60 is 1.8 m. MGS orbit quality using GMM-2B, as measured by overlapping orbital arcs, is 1 m in the radial direction and 10 m in total position.

The lunar crust: Global structure and signature of major basins

New lunar gravity and topography data from the Clementine Mission provide a global Bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins. Most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar Moho and therefore crustal thickness. The large range of global crustal thickness (-20-120 km) is indicative of major spatial variations in melting of the lunar exterior and/or significant impact-related redistribution. The 61-km average crustal thickness, constrained by a depth-to-Moho measured during the Apollo 12 and 14 missions, is preferentially distributed toward the farside, accounting for much of the offset in center-of-figure from the center-of-mass. While the average farside thickness is 12 km greater than the nearside, the distribution is nonuniform, with dramatic thinning beneath the farside, South Pole-Aitken basin. With the global crustal thickness map as a constraint, regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km. In order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data, the downward continuation of the Bouguer gravity is stabilized by a three- dimensional, minimum-slope and curvature algorithm. Both mare and non-mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims. The inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins. For large (>300 km diameter) basins, moho relief appears uncorrelated with diameter, but is negatively correlated with basin age. In several cases, it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement, suggesting that the lithosphere was strong enough to maintain their state of stress to the present.

Correction to “Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution”

[1] In the paper ‘‘Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution’’ by Patrick J. McGovern, Sean C. Solomon, David E. Smith, Maria T. Zuber, Mark Simons, Mark A. Wieczorek, Roger J. Phillips, Gregory A. Neumann, Oded Aharonson, and James W. Head (Journal of Geophysical Research, 107(E12), 5136, doi:10.1029/ 2002JE001854, 2002), the thickness of the lithosphere and lithospheric heat flow for a number of regions of Mars and as functions of time were inferred on the basis of gravity/topography admittance spectra. Observed admittances, derived from spherical harmonic expansions localized with the scheme of Simons et al. [1997], were compared with those predicted from models for the flexural response to lithospheric loading [e.g., Turcotte et al., 1981]. Gravity was calculated according to the finite-amplitude scheme of Wieczorek and Phillips [1998]. Estimates for the thickness of the elastic lithosphere Te at the time of loading for each region were converted to equivalent thermal gradient dT/dz and heat flux q by means of an elastic-plastic stressenvelope formalism [McNutt, 1984]. Here we describe a correction required in the calculation of the modeled gravity anomalies; we report new estimates of Te, load density rl, dT/dz, and q from corrected model admittances; and we discuss the implications of the new results. [2] The source of the required correction is a difference in reference radius values. As defined by McGovern et al. [2002], the planetary shape was taken to equal the radius from the center of mass of Mars to the Martian surface expressed as a spherical harmonic expansion and referenced to the mean equatorial radius Req = 3396 km:

Crossover analysis of Mars Orbiter Laser Altimeter data

In its first 15 months of continuous operation, the Mars Orbiter Laser Altimeter (MOLA) instrument aboard Mars Global Surveyor ranged to Mars over 330 million times, generating more than 5000 orbital profiles, with a ranging precision of 0.4 m over smooth terrain. The accuracy of the profiles depends on knowledge of the spacecraft position, orientation, and observation time, which are subject to errors. We model these errors via the analysis of over 24 million altimetric crossovers. A quasiperiodic, once per revolution adjustment of the ground tracks as a function of time in three locally orthogonal directions minimizes the altimetric residuals via least-squares. Using a sparse matrix technique, computational effort scales linearly with the number of crossovers and only marginally with the number of parameters. Orbital errors mainly result from poor modeling of spacecraft thrusting events in the absence of tracking. Seasonal effects, likely due to changing thermal environment, as well as residual miscalibrations, are evident in the pointing solutions. Incorporating multiple parameters per revolution significantly improves crossover residuals, and resolves pointing aberrations during orbital transitions from night to day. Altimetry from the adjusted tracks generates a topographic model whose accuracy is typically better than 1 m vertically with respect to the center of mass of Mars. The centroid position of each MOLA shot is typically accurate to ∼100 m horizontally. Terrain models from accurately located lidar data can be gradient-shaded to illuminate geological structures with 1 in 1000 slopes that are invisible to cameras. Temporal changes in elevation (e.g., frost deposition/ablation) at decimeter levels may also be assessed using crossovers, but results must be interpreted with caution due to uncertainties in range walk correction.

Global Distribution of Large Lunar Craters: Implications for Resurfacing and Impactor Populations

Lunar Reconnaissance The Lunar Reconnaissance Orbiter reached lunar orbit on 23 June 2009. Global data acquired since then now tell us about the impact history of the Moon and the igneous processes that shaped it. Using the Lunar Orbiter Laser Altimeter, Head et al. (p. 1504; see the cover) provide a new catalog of large lunar craters. In the lunar highlands, large-impact craters have obliterated preexisting craters of similar size, implying that crater counts in this region cannot be used effectively to determine the age of the underlying terrain. Crater counts based on the global data set indicate that the nature of the Moons impactor population has changed over time. Greenhagen et al. (p. 1507) and Glotch et al. (p. 1510) analyzed data from the Diviner Lunar Radiometer Experiment, which measures emitted thermal radiation and reflected solar radiation at infrared wavelengths. The silicate mineralogy revealed suggests the existence of more complex igneous processes on the Moon than previously assumed. An analysis of high-resolution global topography data advances our understanding of the impact history of the Moon. By using high-resolution altimetric measurements of the Moon, we produced a catalog of all impact craters ≥20 kilometers in diameter on the lunar surface and analyzed their distribution and population characteristics. The most-densely cratered portion of the highlands reached a state of saturation equilibrium. Large impact events, such as Orientale Basin, locally modified the prebasin crater population to ~2 basin radii from the basin center. Basins such as Imbrium, Orientale, and Nectaris, which are important stratigraphic markers in lunar history, are temporally distinguishable on the basis of crater statistics. The characteristics of pre- and postmare crater populations support the hypothesis that there were two populations of impactors in early solar system history and that the transition occurred near the time of the Orientale Basin event.

Mutagenicity of Cycasin Aglycone (Methylazoxymethanol), a Naturally Occurring Carcinogen

Methylazoxymethanol, a carcinogenic and hepatotoxic methylating agent prepared from cycad plants, has been found to be a good mutagen in Salmonella typhimurium.

Constraints on the volatile distribution within Shackleton crater at the lunar south pole

Shackleton crater is nearly coincident with the Moon’s south pole. Its interior receives almost no direct sunlight and is a perennial cold trap, making Shackleton a promising candidate location in which to seek sequestered volatiles. However, previous orbital and Earth-based radar mapping and orbital optical imaging have yielded conflicting interpretations about the existence of volatiles. Here we present observations from the Lunar Orbiter Laser Altimeter on board the Lunar Reconnaissance Orbiter, revealing Shackleton to be an ancient, unusually well-preserved simple crater whose interior walls are fresher than its floor and rim. Shackleton floor deposits are nearly the same age as the rim, suggesting that little floor deposition has occurred since the crater formed more than three billion years ago. At a wavelength of 1,064 nanometres, the floor of Shackleton is brighter than the surrounding terrain and the interiors of nearby craters, but not as bright as the interior walls. The combined observations are explicable primarily by downslope movement of regolith on the walls exposing fresher underlying material. The relatively brighter crater floor is most simply explained by decreased space weathering due to shadowing, but a one-micrometre-thick layer containing about 20 per cent surficial ice is an alternative possibility.

The Mars seasonal CO2 cycle and the time variation of the gravity field : A general circulation model simulation

We investigate the time variation of the long wavelength gravitational field of Mars due to mass redistribution associated with the annual cycle of CO2 exchange between the atmosphere and polar caps. Our analysis utilizes simulated monthly estimates of atmospheric pressure and CO2 polar frost as calculated by the NASA Ames general circulation model (GCM) for a “typical” year. We show that the deposition and sublimation of CO2 polar frost is expected to be the dominant effect on the gravity field at all low degrees. Variations in the amplitude and phase of time variations of gravity are sensitive to multiple factors, including polar elevation and the eccentricity of the Martian orbit. Phase effects associated with additive and competing influences at the summer and winter poles dictate that odd-numbered harmonic coefficients produce an annual signal in the gravity field while even-numbered coefficients show a semiannual signal. The predicted changes in the planetary flattening and pear-shaped terms of the field are at or above the noise level of expected X band Doppler tracking observations from Mars Global Surveyor and so could conceivably be detected from an orbital spacecraft.

The NEAR laser ranging investigation

The objective of the Near-Earth Asteroid Rendezvous (NEAR) laser ranging investigation is to obtain high integrity profiles and grids of topography for use in geophysical, geodetic and geological studies of asteroid 433 Eros. The NEAR laser rangefinder (NLR) will determine the slant range of the NEAR spacecraft to the asteroid surface by measuring precisely the round trip time of flight of individual laser pulses. Ranges will be converted to planetary radii measured with respect to the asteroid center of mass by subtracting the spacecraft orbit determined from X band Doppler tracking. The principal components of the NLR include a 1064 nm Cr:Nd:YAG laser, a gold-coated aluminum Dall-Kirkham Cassegrain telescope, an enhanced silicon avalanche photodiode hybrid detector, a 480-MHz crystal oscillator, and a digital processing unit. The instrument has a continuous in-flight calibration capability using a fiber-optic delay assembly. The single shot vertical resolution of the NLR is <6 m, and the absolute accuracy of the global grid will be ∼ 10 m with respect to the asteroid center of mass. For the current mission orbital scenario, the laser spot size on the surface of Eros will vary from ∼4–11 m, and the along-track resolution for the nominal pulse repetition rate of 1 Hz will be approximately comparable to the spot size, resulting in contiguous along-track profiles. The across-track resolution will depend on the orbital mapping scenario, but will likely be <500 m, which will define the spatial resolution of the global topographic model. Planned science investigations include global-scale analyses related to collisional and impact history and internal density distribution that utilize topographic grids as well as spherical harmonic topographic models that will be analyzed jointly with gravity at commensurate resolution. Attempts will be made to detect possible subtle time variations in internal structure that may be present if Eros is not a single coherent body, by analysis of low degree and order spherical harmonic coefficients. Local- to regional-scale analyses will utilize high-resolution three-dimensional topographic maps of specific surface structures to address surface geologic processses. Results from the NLR investigation will contribute significantly to understanding the origin, structure, and evolution of Eros and other asteroidal bodies.

Extension and uplift at Alba Patera, Mars: Insights from MOLA observations and loading models

High-resolution Mars Orbiter Laser Altimeter (MOLA) profiles of the large shield volcano Alba Patera call for a critical reexamination of models for the growth and evolution of this volcanic edifice. An extensive system of graben cuts across Alba Patera, extending from Ceraunius Fossae to the south to Tantalus Fossae to the northeast. In the vicinity of the edifice, the otherwise generally north-south orientations of these graben are deflected to circumferential directions. MOLA topography reveals that the circumferential graben are located well up the flanks of Alba Patera. Both the type (extensional) and location (midflank) of faulting on Alba Patera are inconsistent with the state of stress predicted for purely surface-loaded flexure models. To constrain the conditions governing the evolution of Alba Patera, we employ finite element models for the volcano and lithospheric stress field under a combination of loading mechanisms. Buoyant sublithospheric loads (representing underplated magma, low-density mantle residuum, or dynamic support from mantle convection) and intralithospheric displacements (representing sills “trapped” by horizontal compressive stresses in the upper lithosphere) can generate the observed midflank slope breaks and circumferential extensional fault zones at radial distances comparable to that of the subsurface loads characteristic radial extent. Both mechanisms moderate stresses in the upper lithosphere in favor of continued magma ascent. However, bottom loading requires a disk-like load geometry and an elastic lithosphere thickness Te sufficiently small to allow the loads shape to be apparent in the surface deformation (Te ≤ 32 km). Such low Te is inconsistent with Mars Global Surveyor (MGS) gravity and topography data. In contrast, intralithospheric sill complexes can produce the observed topography and faulting at Te values consistent with those inferred from gravity and topography (50 ≤ Te ≤ 100 km). For these conditions, principal stress orientations on the lower flanks are also consistent with the orientations of lower-flank graben, provided that faulting was induced by dike intrusion or tensile failure rather than shear failure.