Jean-Pierre Williams

Thermal evolution of the Martian core: Implications for an early dynamo

Mars is thought to have possessed a dynamo that ceased ;0.5 b.y. after the formation of the planet. A possible, but ad hoc, explanation is an early episode of plate tectonics, which drove core convection by rapid cooling of the mantle. We present an alternative explanation: that the Martian core was initially hotter than the mantle after core formation, providing an initial high heat flux out of the core. A core initially 150 K hotter than the mantle can explain the early dynamo without requiring plate tectonics. Recent experimental results suggest that potassium is likely to partition into the Martian core, potentially providing an extra source of energy to power a dynamo. We find that the radioactive decay of 40 K cannot explain the inferred dynamo history without the presence of a hot core. Our results also suggest that core solidification is unlikely to have occurred, because this process would have generated a long-lived (.1 b.y.) dynamo. If, as we conclude, the core is entirely liquid, it must contain at least ;5 wt% sulfur. An initially hot core is consistent with geochemical evidence for rapid core formation and incomplete thermal equilibration with the mantle. Thus, the early history of planetary dynamos provides constraints on the processes of accretion and differentiation.

Low palaeopressure of the martian atmosphere estimated from the size distribution of ancient craters

Decay of the CO2-dominated atmosphere is an important component of long-term environmental change on Mars, but direct constraints on paleoatmospheric pressure P are few. Of particular interest is the climate that allowed rivers to flow early in Mars history, which was affected by P via direct and indirect greenhouse effects. The size of craters embedded within ancient layered sediments is a proxy for P: the smaller the minimum-sized craters that form, the thinner the past atmosphere. Here we use high-resolution orthophotos and Digital Terrain Models (DTMs) to identify ancient craters among the river deposits of Aeolis close to Gale crater, and compare their sizes to models of atmospheric filtering of impactors by thicker atmospheres. We obtain an upper limit of P <= 760+/-70 mbar, rising to P <= 1640+/-180 mbar if rimmed circular mesas are excluded. Our work assumes target properties appropriate for desert alluvium: if sediment developed bedrock-like rock-mass strength by early diagenesis, the upper limit increases by a factor of up to 2. If Mars did not have a stable multibar atmosphere at the time that the rivers were flowing, the warm-wet CO2 greenhouse of Pollack et al. (1987) is ruled out, and long-term average temperatures were most likely below freezing.The decay of the martian atmosphere—which is dominated by carbon dioxide—is a component of the long-term environmental change on Mars from a climate that once allowed rivers to flow to the cold and dry conditions of today. The minimum size of craters serves as a proxy for palaeopressure of planetary atmospheres, because thinner atmospheres permit smaller objects to reach the surface at high velocities and form craters. The Aeolis Dorsa region near Gale crater on Mars contains a high density of preserved ancient craters interbedded with river deposits and thus can provide constraints on atmospheric density at the time of fluvial activity. Here we use high-resolution images and digital terrain models from the Mars Reconnaissance Orbiter to identify ancient craters in deposits in Aeolis Dorsa that date to about 3.6 Gyr ago and compare their size distribution with models of atmospheric filtering of impactors. We obtain an upper limit of 0.9 ± 0.1 bar for the martian atmospheric palaeopressure, rising to 1.9 ± 0.2 bar if rimmed circular mesas—interpreted to be erosionally-resistant fills or floors of impact craters—are excluded. We assume target properties appropriate for desert alluvium: if sediment had rock-mass strength similar to bedrock at the time of impact, the paleopressure upper limit increases by a factor of up to two. If Mars did not have a stable multibar atmosphere at the time that the rivers were flowing—as suggested by our results—then a warm and wet CO_2/H_2O greenhouse is ruled out, and long-term average temperatures were most likely below freezing.

Surface properties of Mars' polar layered deposits and polar landing sites

On December 3, 1999, the Mars Polar Lander and Mars Microprobes will land on the planets south polar layered deposits near (76°S, 195°W) and conduct the first in situ studies of the planets polar regions. The scientific goals of these missions address several poorly understood and globally significant issues, such as polar meteorology, the composition and volatile content of the layered deposits, the erosional state and mass balance of their surface, their possible relationship to climate cycles, and the nature of bright and dark aeolian material. Derived thermal inertias of the southern layered deposits are very low (50–100 J m^(−2) s^(−1/2) K^(−1)), suggesting that the surface down to a depth of a few centimeters is generally fine grained or porous and free of an appreciable amount of rock or ice. The landing site region is smoother than typical cratered terrain on ∼1 km pixel^(−1) Viking Orbiter images but contains low-relief texture on ∼5 to 100 m pixel^(−1) Mariner 9 and Mars Global Surveyor images. The surface of the southern deposits is older than that of the northern deposits and appears to be modified by aeolian erosion or ablation of ground ice.

Layering in the wall rock of Valles Marineris: intrusive and extrusive magmatism

[1] High-resolution images of the walls exposed in Valles Marineris reveal variations in appearance and degree of layering indicating various lithologies comprise the Tharsis plateau. The layered wall rock has been proposed to result from effusive flood basalt volcanism or interbedded sediments and volcanics. We present observations of unlayered rock that indicate layering extends to a greater depth in the western half of Valles Marineris and is confined to the Tharsis plateau, a region of thickened, uplifted crust resulting from prolonged intrusive activity. Consistent with this view, we propose that the observed layering may be a manifestation of intrusive rocks resulting from crystal fractionation of intruded basaltic magmas. Terrestrial layered plutons provide analogs for comparison such as those of the North Atlantic Igneous Province (NAIP) a large igneous province associated with crustal rifting and exposures of thick sequences of layered flood basalts and intruded layered cumulates. INDEX TERMS: 5480 Planetology: Solid Surface Planets: Volcanism (8450); 6225 Planetology: Solar System Objects: Mars; 8450 Volcanology: Planetary volcanism (5480). Citation: Williams, J.-P., D. A. Paige, and C. E. Manning, Layering in the wall rock of Valles Marineris: intrusive and extrusive magmatism, Geophys. Res. Lett., 30(12), 1623, doi:10.1029/2003GL017662, 2003.

Acoustic environment of the Martian surface

Prompted by the Mars Microphone aboard the 1998 Mars Polar Lander, a theoretical study of the acoustical environment of the Martian surface has been made to ascertain how the propagation of sound is attenuated under such conditions and to predict what sounds may be detectable by a microphone. Viscous and thermal relaxation (termed classical absorption), molecular relaxation, and geometric attenuation are considered. Classical absorption is stronger under Martian conditions resulting in sounds in the audible frequencies (20 Hz to 20 kHz) being more strongly attenuated than in the terrestrial environment. The higher frequencies (>3000 Hz) will be severely attenuated as the absorption is frequency dependent. At very low infrasound frequencies (i.e., <10 Hz), attenuation will be mostly due to geometric spreading of the propagating wave front and will therefore be more similar to the terrestrial surface environment. Probable sound sources in the landed environment include wind-blown dust and sand from large dust storms, dust devils, and possible associated electrostatic discharge. The sounds most likely to be detected will be sounds generated by the lander itself and aeroacoustic noises generated by winds blowing against the lander and its instruments.

Insolation driven variations of Mercury's lithospheric strength

Mercury’s coupled 3:2 spin‐orbit resonance in conjunction with its relatively high eccentricity of ∼0.2 and near‐zero obliquity results in both a latitudinal and longitudinal variation in annual average solar insolation and thus equatorial hot and cold regions. This results in an asymmetric temperature distribution in the lithosphere and a long wavelength lateral variation in lithosphere structure and strength that mirrors the insolation pattern. We employ a thermal evolution model for Mercury generating strength envelopes of the lithosphere to demonstrate and quantify the possible effects the insolation pattern has on Mercury’s lithosphere. We find the heterogeneity in lithosphere strength is substantial and increases with time. We also find that a crust thicker than that of the Moon or Mars and dry rheologies for the crust and mantle are favorable when compared with estimates of brittle‐ductile transition depths derived from lobate scarps. Regions of stronger and weaker compressive strength imply that the accommodation of radial contraction of Mercury as its interior cooled, manifest as lobate scarps, may not be isotropic, imparting a preferential orientation and distribution to the lobate scarps.

The formation of Tharsis on Mars: What the line-of-sight gravity is telling us

Line-of-sight (LOS) spacecraft acceleration profiles from the Radio Science Experiment and topography from the Mars Orbiter Laser Altimeter (MOLA) instrument of the Mars Global Surveyor (MGS) are analyzed to estimate the effective elastic thickness (Te) for various regions of Tharsis. We identify a buried basin flanking the Thaumasia Highlands at the southeastern margin of Tharsis. Assuming that this basin results from lithospheric flexure from surface loading by the Thaumasia Highlands, we fit LOS profiles across the feature with a thin-shell, elastic flexure model and find the mountain belt to reflect a value of Te ∼ 20 km consistent with a Noachian formation age. We also determine admittances from LOS profiles for five regions across Tharsis and fit them with theoretical admittances calculated using the flexural model. Crater density, surface density, and predominant surface age are found to vary systematically across Tharsis while Te does not. The highest surface density and lowest Te values are obtained for the western portion of Tharsis where crater densities are lowest. Our results imply the majority of the topographic rise was emplaced within the Noachian irrespective of the surface ages. Topographic loading and resurfacing (i.e., volcanic activity) persisted into the Amazonian while becoming increasingly confined to the western margin where the youngest surface ages are found and the eruptive style transitioned from effusive volcanism to shield-forming volcanism as Te increased.

Powering Mercury's dynamo

The presence of the global magnetic field of Mercury has implications for the interior structure of the planet and its thermal evolution. We use a thermal evolution model to explore the conditions under which excess entropy is available to drive a convective dynamo. The current state of the core is strongly affected by its sulfur concentration and the viscosity of the overlying mantle. A present-day dynamo is difficult to achieve. The minimum rate of entropy production required to drive a dynamo is attained in only the most optimistic models, and requires present-day mantle convection. An additional entropy source such as the addition of a radiogenic heat source in the core increases the probability of a present-day dynamo. Given the uncertainty, more specific characterization of the planets interior and magnetic field is required to alleviate ambiguities in the original Mariner 10 observations.

Global Regolith Thermophysical Properties of the Moon From the Diviner Lunar Radiometer Experiment

We used infrared data from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment to globally map thermophysical properties of the Moons regolith fines layer. Thermal conductivity varies from 7.4

Direct evidence of surface exposed water ice in the lunar polar regions

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