W. Bruce Banerdt

The Borealis basin and the origin of the martian crustal dichotomy

The most prominent feature on the surface of Mars is the near-hemispheric dichotomy between the southern highlands and northern lowlands. The root of this dichotomy is a change in crustal thickness along an apparently irregular boundary, which can be traced around the planet, except where it is presumably buried beneath the Tharsis volcanic rise. The isostatic compensation of these distinct provinces and the ancient population of impact craters buried beneath the young lowlands surface suggest that the dichotomy is one of the most ancient features on the planet. However, the origin of this dichotomy has remained uncertain, with little evidence to distinguish between the suggested causes: a giant impact or mantle convection/overturn. Here we use the gravity and topography of Mars to constrain the location of the dichotomy boundary beneath Tharsis, taking advantage of the different modes of compensation for Tharsis and the dichotomy to separate their effects. We find that the dichotomy boundary along its entire path around the planet is accurately fitted by an ellipse measuring approximately 10,600 by 8,500u2009km, centred at 67°u2009N, 208°u2009E. We suggest that the elliptical nature of the crustal dichotomy is most simply explained by a giant impact, representing the largest such structure thus far identified in the Solar System.

Mars - The regolith-atmosphere-cap system and climate change

Abstract A new model for predicting the behavior of the Martian regolith-atmosphere-cap CO 2 regime and describing its role in climate change is derived. The model describes the time-temperature histories of 90 regolith “chunks” on a latitude-depth grid, accounting for CO 2 exchange by means of laboratory-derived expressions relating temperature, CO 2 pressure ( P co 2 ), and adsorbed CO 2 mass. CO 2 polar cap formation and sublimation is accounted for, subject to the constraint of constant total CO 2 in the atmosphere-regolith-cap system. The influence of differences in regolith adsorption laws for basalt and clay and the influence of variations in regolith depth with latitude, regolith thermal diffusivity, and total exchangeable CO 2 inventory on predicted variations in atmospheric pressure and cap mass are examined. The following results are indicated: (1) The atmosphere acts as a low-capacity conduit, through which flows between 10 and 100 times the current atmospheric mass of CO 2 , between two reservoirs: a regolith “ocean” of adsorbed CO 2 and a polar cap “cryosphere.” (2) Exchange between these reservoirs is driven by variations of obliquity (θ), with the polar cap the dominant CO 2 sink at low θ and the regolith dominating at high θ. Atmospheric pressure ranges between ∼0.1 (low θ ) and ∼15 mbar (high θ ). Pressures of up to 25 mbar could result from pre-Tharsis θ variations. Pressure up to ∼40 mbarare achieved only in models which also call for a huge quasi-permanent present CO 2 cap, which is not apparent. (3) At high θ, the atmospheric pressure is buffered by the regolith and satisfies adsorption equilibrium for the 90 “chunks,” as well as conservation of CO 2 mass. At low θ a potetially large CO 2 cap appears and the atmospheric pressure is buffered by the cap, not the regolith. (4) The periodic reservoir flushing can account for the observation of simultaneous nonenrichment of 18 O and enrichment of 13 N. (5) Mars climate history may be divided into three parts: (a) post-Tharsis formation history as described above, (b) pre-Tharsis formation with qualitatively similar processes but larger oscillations between reservoirs and larger atmospheric pressure and cap mass variation, and (c) the very early period of channel formation. (6) The nature of the response of the three-part regolith-atmosphere-polar cap Martian climate system to surface insolation variations may be analogous to that of Earths ocean-atmosphere-cryosphere system.

Early thermal profiles and lithospheric strength of Ganymede from extensional tectonic features

Abstract Estimates of the brittle lithosphere thickness derived from the width and spacing of extensional tectonic features, coupled with lithospheric strength envelopes (brittle and ductile yield stress versus depth) appropriate for ice, allow the quantitative determination of early thermal profiles and lithospheric strength and stability on Ganymede. Furrows and grooves indicate brittle lithospheric thicknesses of 5–10 and 2–5 km, respectively, assuming that their spacing is controlled by an extensional instability or that their width is controlled by the intersection depth of their bounding faults. Plots of the brittle and ductile yield stress versus depth for the icy lithosphere of Ganymede show a linear increase in brittle strength with depth to a maximum at the brittle-ductile transition, followed by an exponential decrease in ductile yield stress with depth. Because the depth to the brittle-ductile transition depends primarily on the thermal gradient, the thickness of the brittle lithosphere can be used to calculate early thermal profiles of 1.5–6 and 4.5–20°/km during the formation of the furrows and grooves, respectively. Lithospheric strength, the integral of the yield stress versus depth curve, varied from 30–125 GPa m when the furrows formed to 5–30 GPa m when the grooves formed, which correspond to maximum yield stresses of 6–11 and 2.5–6 MPa, respectively. These results indicate that the thermal gradient and lithospheric strength varied laterally by as much as a factor of 5 and that Ganymede cooled in a highly inhomogeneous manner with significant lateral thermal anomalies. Finally, this analysis provides a straightforward explanation for the stability of large remnants of cratered terrain such as Galileo Regio that had a low thermal gradient and strong lithosphere in contrast to small remnants of cratered terrain that were fractured and broken up by grooved terrain as a result of higher thermal gradients and weaker lithospheres.

Evaluating the Wind-Induced Mechanical Noise on the InSight Seismometers

The SEIS (Seismic Experiment for Interior Structures) instrument onboard the InSight mission to Mars is the critical instrument for determining the interior structure of Mars, the current level of tectonic activity and the meteorite flux. Meeting the performance requirements of the SEIS instrument is vital to successfully achieve these mission objectives. Here we analyse in-situ wind measurements from previous Mars space missions to understand the wind environment that we are likely to encounter on Mars, and then we use an elastic ground deformation model to evaluate the mechanical noise contributions on the SEIS instrument due to the interaction between the Martian winds and the InSight lander. Lander mechanical noise maps that will be used to select the best deployment site for SEIS once the InSight lander arrives on Mars are also presented. We find the lander mechanical noise may be a detectable signal on the InSight seismometers. However, for the baseline SEIS deployment position, the noise is expected to be below the total noise requirement >97%

Utopia and Hellas basins, Mars: Twins separated at birth

>97~%

Seismometer Detection of Dust Devil Vortices by Ground Tilt

of the time and is, therefore, not expected to endanger the InSight mission objectives.

Next generation Autonomous Lunar Geophysical Experiment Package

[1]xa0Using topography and gravity data as constraints, we formulate spherical harmonic thin elastic-shell models to determine the subsurface structure of the Hellas and Utopia basins. For Hellas, we show that our model is consistent with the elastic thickness results of McGovern et al. (2002, 2004). The thin elastic lithosphere at the time of formation implies that Hellas is close to isostatic. Since Utopia formed earlier, we argue that an isostatic assumption is justified for the Utopia basin before it was filled. From this supposition, we derive a system of equations that allows us to solve for the amount of fill, the prefill topography, and the amount of flexure due to the fill within the Utopia basin. An analysis of the parameter space shows that the fill density and the amount of fill is strongly dependent on the elastic thickness at the time of infilling. A thinner elastic lithosphere favors a denser fill, while a thicker lithosphere will allow for less dense material. Likewise, larger crustal thickness values lead to smaller fill density values. The presence of quasi-circular depressions, interpreted as impact craters, within the Utopia basin indicates that the majority of the material within Utopia was deposited prior to 4.04–4.11 Ga. The early timing for the deposition combined with the heat imparted by the basin forming event argues for a thinner lithosphere which could, in turn, suggest fill densities that are more consistent with a volcanic load than with pure sediment or ice-rich material. These results are supported using an alternative method of determining the amount of fill and flexure within Utopia. This model assumes that Hellas and Utopia were initially identical and that the only difference in their subsequent evolution was the addition of material in the Utopia basin. The volume of material needed to fill Utopia is immense (on the order of 50 million km3 or more). The high density obtained for the fill requires that it contain a large igneous component, the source of which is problematic. Relaxing the isostatic assumption to a reasonable degree perturbs the density bound only slightly.

Scientific results of the Mars Exploration Rovers , Spirit and Opportunity

We report seismic signals on a desert playa caused by convective vortices and dust devils. The long-period (10–100xa0s) signatures, with tilts of ∼10−7 radians, are correlated with the presence of vortices, detected with nearby sensors as sharp temporary pressure drops (0.2–1xa0mbar) and solar obscuration by dust. We show that the shape and amplitude of the signals, manifesting primarily as horizontal accelerations, can be modeled approximately with a simple quasi-static point-load model of the negative pressure field associated with the vortices acting on the ground as an elastic half-space. We suggest the load imposed by a dust devil of diameter D and core pressure Δ P o is ∼( π /2)Δ P o D 2, or for a typical terrestrial dust devil of 5xa0m diameter and 2xa0mbar, about the weight of a small car. The tilt depends on the inverse square of distance and on the elastic properties of the ground, and the large signals we observe are in part due to the relatively soft playa sediment and the shallow installation of the instrument. Ground tilt may be a particularly sensitive means of detecting dust devils. The simple point-load model fails for large dust devils at short ranges, but more elaborate models incorporating the work of Sorrells (1971) may explain some of the more complex features in such cases, taking the vortex winds and ground velocity into account. We discuss some implications for the InSight mission to Mars.nnOnline Material: Figure of data and geophysical interpretation of seismic refraction line.

Tectonics of the Tharsis Region of Mars: Insights from MGS Topography and Gravity

Geophysical exploration of the interior structure and processes of the Moon has been a high scientific priority dating back to the Apollo days and still remains a high priority even today. The Apollo Lunar Surface Experiments Package (ALSEP) deployed during the Apollo era played an important role in extending knowledge of the Moons history through seismic, magnetic, and geothermal measurements. In this concept study, a contemporary equivalent of ALSEP was investigated. Autonomous Lunar Geophysical Experiment Package (ALGEP) contains instruments similar to ALSEP but has taken advantage of thirty years of technology advances, allowing for development of instruments with increased sensitivity and reduced size and power requirements. Solar powered modular platforms were designed with each instrument equipped with its own miniaturized power, telecom, command and data handling, and thermal subsystems allowing for nighttime data collection. A base station would provide communication with the Earth. The instruments could be deployed long distances from one another and would be essentially wireless, decreasing deployment complexity and risk.

Farside explorer: unique science from a mission to the farside of the moon

Results of the Mars Exploration Rover mission to Mars are summarized.