Francois Costard

Standardizing the nomenclature of Martian impact crater ejecta morphologies

The Mars Crater Morphology Consortium recommends the use of a standardized nomenclature system when discussing Martian impact crater ejecta morphologies. The system utilizes nongenetic descriptors to identify the various ejecta morphologies seen on Mars. This system is designed to facilitate communication and collaboration between researchers. Crater morphology databases will be archived through the U.S. Geological Survey in Flagstaff, where a comprehensive catalog of Martian crater morphologic information will be maintained.

Orientation and distribution of recent gullies in the southern hemisphere of Mars: Observations from High Resolution Stereo Camera/Mars Express (HRSC/MEX) and Mars Orbiter Camera/Mars Global Surveyor (MOC/MGS) data

Geologically recent small gullies on Mars display morphologies consistent with erosion by water or by debris flows. Suggested formation models are divided into two main categories: (1) groundwater or (2) melting of near-surface ice/snow sourced from the atmosphere. We have measured location and orientation and recorded the local contexts of gullies to constrain the likely models of gully formation. More than 22,000 Mars Orbiter Camera Narrow Angle (MOC NA) and >120 Mars Express High Resolution Stereo Camera (HRSC) images in the southern hemisphere were searched for gullies. Discrete gullied slope sections with consistent orientation were recorded rather than individual gullies. Slope setting (impact crater, valley wall, etc.), location, and orientation were recorded for each slope section. More than 750 MOC images with gullies (>900 distinct gullied slope sections) and more than 40 HRSC images (>380 distinct gullied slope sections) were identified. From both MOC and HRSC, gullies were found to be most common between 30 and 50 degrees latitude and to have an overall pole facing preference. The preferred gully orientation for HRSC is southeast rather than south in MOC, owing to illumination effects that make gullies difficult to detect on south- to southwest-facing slopes in HRSC. In both MOC and HRSC surveys, higher-latitude gullies show less preference for pole facing than those at mid latitudes. Both data sets produced similar results, demonstrating that our data are reliable. We suggest that the observed latitudinal and orientation distributions of gullies show that insolation and atmospheric conditions play a key role in gully formation.

The spatial distribution of volatiles in the Martian hydrolithosphere

In order to quantify the spatial distribution of volatiles on Mars, 2600 fluidized ejecta craters have been systematically measured, classified and mapped over the planet Mars, using 1 : 2 M scale USGS photomosaics. The latitudinal distribution of ejecta craters reveals that flower ejecta deposits (Type 1), together with low mobility ejecta, are frequently observed in the equatorial region and on ridged plains. Rampart craters (Type 2), with high mobility ejecta, occur at mid latitudes and exhibit a spatial relationship with polygonal patterns and pseudocrater areas. The increase of ejecta mobility with latitude attests for a concentration of volatiles at high latitudes. Statistical analysis shows that cratered uplands and ridged plains contain less volatile material near the surface than the underlying materials. In Chryse Planitia and Utopia Planitia the statistical study and the spatial relationships between polygonally fractured patterns, pseudocraters and the great number of high mobility ejecta deposits suggest the presence of a water-rich alluvial deposit close to the surface near the mouth of Chryse and Elysium channels. This result explains, on a more quantitative basis, the idea that fractured patterns were preferentially developed in a volatile-rich sedimentary deposits. The behaviour of volatiles, at 41 ‡ S, 257 ‡ W near Reull Vallis, exhibits a strong anomaly, with the presence of an abnormally volatile rich layer close to the surface.

Morphology, evolution and tectonics of Valles Marineris wallslopes (Mars)

Abstract Hillslopes up to 11 km in height can be found along the walls of the Valles Marineris troughs. The widest and deepest troughs are grabens, in which tectonics probably exerted the primary control on the wall morphology. Geographical variations in the wall morphology and profiles show that they result from complex, persistent tectonic influences, and that significant changes in erosional processes occurred during this evolution, from late Hesperian to late Amazonian. Preliminary calculations suggest that about 85–95% of the fault-controlled wall relief probably formed in an “ancient” stage prior to this transitional period. A study of the volatile content of the wall rocks, based upon the morphology and distribution of impact craters on the surrounding plateaus, shows that extreme erosional widening of the Central Valles Marineris troughs occurred during the “ancient” stage of high ground ice content. During the subsequent “recent” stage of tectonic and morphological evolution, the wall materials were partly desiccated.

Geomorphologic Evidence for Liquid Water

Besides Earth, Mars is the only planet with a record of resurfacing processes and environmental circumstances that indicate the past operation of a hydrologic cycle. However the present-day conditions on Mars are far apart of supporting liquid water on the surface. Although the large-scale morphology of the Martian channels and valleys show remarkable similarities with fluid-eroded features on Earth, there are major differences in their size, small-scale morphology, inner channel structure and source regions indicating that the erosion on Mars has its own characteristic genesis and evolution. The different landforms related to fluvial, glacial and periglacial activities, their relations with volcanism, and the chronology of water-related processes, are presented.

GPR, a ground‐penetrating radar for the Netlander mission

In the coming decade, several missions are planned that will land on the surface of Mars landers or instrumented geophysical stations. Among the scientific objectives of these projects, one of the most important will be to unravel the many unknowns in the geological and hydrological history of the planet. The Netlander mission offers a unique opportunity to explore the interior of Mars, its subsurface, its atmosphere, and its distant environment from four landing sites that will be selected to offer a variety of different geophysical conditions. We have thus proposed to fly on these four landers a ground-penetrating radar (GPR) to explore the geological characteristics of the subsurface and search for water reservoirs down to a depth which may be sufficient to allow a possible detection of liquid water. We provide in this paper a short description of this radar which is based on a new concept to allow a 3-D imaging of the subsurface by determining the range and direction of the underground reflectors. In order to access to deep layers, it will operate at a low frequency of 2 MHz. Some results obtained by a numerical modeling of the radar operation in an electromagnetic model of the Martian subsurface are presented in order to illustrate the main capabilities of the radar. In the last section, preliminary results from an initial field test are reported. In addition to its primary goal as a ground-penetrating radar, the GPR will also be operated on Mars as an ionospheric sounder and, in a passive mode, as a HF receiver to measure the radio-electric background.

The GPR experiment on NETLANDER

Abstract With the regular access of landers to the surface at each launch window during the next decade, one of the major objectives of the exploration of Mars will be to unravel the many unknown in the geological and hydrological history of the planet. Among the presently planned missions, the NETLANDER project offers a unique opportunity to explore simultaneously the deep interior of the planet and the first layers of the subsurface in 4 landing sites displaying different geophysical morphologies. To take advantage of this opportunity we have proposed to fly on the 4 landers a ground-penetrating radar (GPR); this experiment aims at characterizing the geological structures in the vicinity of the landers and at detecting possible water reservoirs under the form of ground ice or even, at some depth, of liquid water. The GPR will operate at a frequency of ∼2 MHz and will include an original assembly of 3 electric and 3 magnetic antennas as receivers. This design will enable to determine not only the distance at which the signal has been reflected but also its direction and thus will provide a 3D imaging of the uppermost layers of the subsurface down to a depth of ∼2.5 km . A model of the electromagnetic properties of the anticipated layering of the Martian megaregolith including a ground ice layer was built and used to predict through a 3D electromagnetic numerical simulation the performances of the GPR. After a review of the scientific objectives of the experiment, this paper presents a technical description of the instrument and display some of the results of the numerical simulation. The GPR will provide as a by-product a sounding of the ionosphere at the fixed operating frequency and will also be used to measure the radioelectric background expected from possible electrical discharges in the atmosphere during dust storms.

Late Tharsis formation and implications for early Mars

The Tharsis region is the largest volcanic complex on Mars and in the Solar System. Young lava flows cover its surface (from the Amazonian period, less than 3 billion years ago) but its growth started during the Noachian era (more than 3.7 billion years ago). Its position has induced a reorientation of the planet with respect to its spin axis (true polar wander, TPW), which is responsible for the present equatorial position of the volcanic province. It has been suggested that the Tharsis load on the lithosphere influenced the orientation of the Noachian/Early Hesperian (more than 3.5 billion years ago) valley networks and therefore that most of the topography of Tharsis was completed before fluvial incision. Here we calculate the rotational figure of Mars (that is, its equilibrium shape) and its surface topography before Tharsis formed, when the spin axis of the planet was controlled by the difference in elevation between the northern and southern hemispheres (hemispheric dichotomy). We show that the observed directions of valley networks are also consistent with topographic gradients in this configuration and thus do not require the presence of the Tharsis load. Furthermore, the distribution of the valleys along a small circle tilted with respect to the equator is found to correspond to a southern-hemisphere latitudinal band in the pre-TPW geographical frame. Preferential accumulation of ice or water in a south tropical band is predicted by climate model simulations of early Mars applied to the pre-TPW topography. A late growth of Tharsis, contemporaneous with valley incision, has several implications for the early geological history of Mars, including the existence of glacial environments near the locations of the pre-TPW poles of rotation, and a possible link between volcanic outgassing from Tharsis and the stability of liquid water at the surface of Mars.

Formation of recurring slope lineae on Mars by rarefied gas-triggered granular flows

Active dark flows known as recurring slope lineae have been observed on the warmest slopes of equatorial Mars. The morphology, composition and seasonality of the lineae suggest a role of liquid water in their formation. However, internal and atmospheric sources of water appear to be insufficient to sustain the observed slope activity. Experimental evidence suggests that under the low atmospheric pressure at the surface of Mars, gas can flow upwards through porous Martian soil due to thermal creep under surface regions heated by the Sun, and disturb small particles. Here we present numerical simulations to demonstrate that such a dry process involving the pumping of rarefied gas in the Martian soil due to temperature contrasts can explain the formation of the recurring slope lineae. In our simulations, solar irradiation followed by shadow significantly reduces the angle of repose due to the resulting temporary temperature gradients over shaded terrain, and leads to flow at intermediate slope angles. The simulated flow locations are consistent with observed recurring slope lineae that initiate in rough and bouldered terrains with local shadows over the soil. We suggest that this dry avalanche process can explain the formation of the recurring slope lineae on Mars without requiring liquid water or CO2 frost activity. Transient streaks on Martian slopes have been attributed to liquid water. Simulations show that a dry avalanche process involving the flow of gas in the Martian soil due to temperature contrasts can instead explain these recurring features.

Martian fluvial‐thermal erosion: Laboratory simulation

Wide outflow channels occur both in Siberia and on the planet Mars. In Siberia, thermal erosion results from ground thawing produced by the heat transfer from the flow of water to the frozen ground. We suggest that relatively warm floods on Mars could enlarge outflow channels by a combination of thermal and mechanical erosion along frozen river banks. A one-dimensional model is proposed to estimate the thermal erosion efficiency. A first test of this model is a comparison of results with experiments carried out in a cold chamber. A hydraulic channel allows measurements of the thawing line propagation, as well as the thermal erosion rate, in simulated ground ice that is subjected to warm water flow. Various laboratory simulations demonstrate the validity of the mathematical model for the range of laboratory conditions. This model is then applied to a range of possible current and ancient Martian conditions.