Ages of Valles Marineris (Mars) landslides and implications for canyon history

The chronology of landslides of Valles Marineris, the equatorial trough system of Mars, has been investigated by a crater population study. Valles Marineris landslides have widespread debris aprons which offer a remarkable opportunity to study the crater population with high resolution images from Mars Orbiter Camera (MOC) and from Mars Odyssey Thermal Emission Imaging System (THEMIS). Sixty-six ages were determined within Valles Marineris including 56 landslide ages and 10 ages of the canyon floor. Results reveal that landslides of Valles Marineris system of canyons occurred during a widespread period of time between 3.5 Gy and 50 My. In some locations, the canyon floor has an apparent age of 3.5 Gy suggesting that at least locally within Valles Marineris no major refreshing processes have occurred for 3.5 Gy. The temporal repetitivity of landslides implies that the triggering mechanisms of the landslides are reproducible in time. Landslides have the same features whatever their age. The dynamic of these landslides is probably the same either with intervention of water up to recently (the last 100 My) or without water since 3.5 Gy.     

Martian landslides in Valles Marineris: Wet or dry?

The objective of this paper is to determine whether martian landslides in Valles Marineris were wet or dry and place constraints on the availability of liquid water in Valles Marineris during the Amazonian, when the landslides occurred. We, thus, statistically compare the power-law relationship between the volume and runout distance of landslides on Earth with those in Valles Marineris, Mars. The exponent of the power-law for martian landslides is similar to that for dry landslides and volcanic flows on Earth, and differs significantly from wet debris flows on Earth. The constant of proportionality in the observed power-law relationship for martian flows is linearly proportional to gravity, as predicted from physical modeling of dry flows in which the dissipation occurs in a layer of uniform thickness. Conversion of gravitational potential energy to heat is insufficient to generate more than a few weight percent of liquid water in the landslide. We thus conclude that water did not significantly influence the dynamics of landslides in Valles Marineris. This implies predominantly dry conditions in Valles Marineris during the Amazonian.     

Rheological constraints on martian landslides

We use a dynamic finite-difference model to simulate martian landslides in the Valles Marineris canyon system and Olympus Mons aureole using three different modal rheologies: frictional, Bingham, and power law. The frictional and Bingham modes are applied individually. Fluidized rheology is treated as a combination of frictional and power-law modes; general fluidization can include pore pressure contributions, whereas acoustic fluidization does not. We find that general fluidization most often produces slides that best match landslide geometry in the Valles Marineris. This implies that some amount of supporting liquid or gas was present in the material during failure. The profile of the Olympus Mons aureole is not well matched by any landslide model, suggesting an alternative genesis. In contrast, acoustic fluidization produces the best match for a lunar slide, a result anticipated for dry crust with no overlying atmosphere. The presence of pressurized fluid during Valles Marineris landsliding may be due to liquid water beneath a thin cryosphere (<1-2 km) or flash sublimation of CO₂.     

Valles Marineris,Mars: Wet debris flows and ground ice

Detailed study of the Valles Marineris equatorial troughs suggests that the landslides in that area contained water and probably were gigantic wet debris flows: one landslide complex generated a channel that has several bends and extends for 250 km. Further support for water or ice in debris masses includes rounded flow lobes and transport of some slide masses in the direction of the local topographic slope. Differences in speed and emplacement efficiency between Martian and terrestrial landslides can be attributed to the entrainment of volatiles on Mars, but they can also be explained by other mechanisms. Support that the wall rock contained water comes from the following observations: (1) the water within the landslide debris must have been derived from wall rock; (2) debris appears to have been transported through tributary canyons; (3) locally, channels emerged from the canyons; (4) the wall rock apprarently disintegrated and flowed easily; and (5) fault zones within the troughs are unusually resistant to erosion. The study further suggests that, in the equatorial region of Mars, material below depths of 400–800 m was not desiccated during the time of landslide activity (within the last billion years of Martian history). Therefore the Martian ground-water or groundice reservoir, if not a relic from ancient times, must have been replenished.     

Landslides in Valles Marineris (Mars): A possible role of basal lubrication by sub-surface ice

There is much interest on the occurrence of water and ice in the past history of Mars. Because landslides on Mars are much better conserved than their terrestrial counterparts, a physical examination and morphological analysis can reveal significant details on the depositional environment at the instant of failure. A study of the landslides in Valles Marineris based on their physical aspect is presented and the velocity of the landslides is calculated with a stretching block model. The results show that the landslides were subject to strong basal lubrication that made them travel at high speed and to long distances. We use physical analysis to explore the four alternative possibilities that the natural lubricant of the landslides in Valles Marineris was either ice, deep water, a shallow carpet of water, or evaporites. Examination of the furrows present on the surface of the landslide deposits shows that either sub-surface ice or evaporites were likely present on the floor of Valles Marineris during the mass failures.     

A simulation of the OMEGA/Mars Express observations: Analysis of the atmospheric contribution

Spectral images of Mars obtained by the Mars Express/OMEGA experiment in the near infrared are the result of a complex combination of atmospheric, aerosol and ground features. Retrieving the atmospheric information from the data is important, not only to decorrelate mineralogical against atmospheric features, but also to retrieve the atmospheric variability. Once the illumination conditions have been taken into account, the main source of variation on the CO₂ absorption is due to the altitude of the surface, which governs atmospheric pressure variation by more than an order of magnitude between the summit of Olympus Mons down to the bottom of Valles Marineris. In this article we present a simplified atmospheric spectral model without scattering, specially developed for the OMEGA observations, which is used to retrieve the local topography through the analysis of the CO₂ band. OMEGA atmospheric observations increase the horizontal resolution compared to MOLA altimetry measurements, and therefore complement the mineralogical studies from the same instrument. Finally, residual variations of the pressure can be related to atmospheric structure variation.     

Evidence for subaqueously resedimented sulphate evaporites on Mars

This article documents the clastic nature of sulphate evaporite beds in the Tithonium Chasma located in the Valles Marineris region of Mars. These beds form a stratified succession characterised by very thick interbedded channel-fill breccia bodies. We infer that the bouldery channel-fills were deposited by voluminous mass-flow processes occurring in a relatively deep subaqueous environment. The redeposition of the coarse-grained evaporite would have responded to phases of high denudation rates in rapidly uplifting hinterlands. Tectonic activity also caused the diapiric uprise and exhumation of evaporite diapirs within the Valles Marineris chasmata, where the apparently young and well development karstic landforms probably formed during the late Amazonian age. These new data strongly suggest the deposition of both primary and resedimented evaporites in a marginal basin area, which effectively restricted ocean access through the proposed “proto-Valles Marineris Strait”. The associated ocean may be the “Ocean Borealis” of Late Noachian-Early Hesperian age.     

Oxygen ion escape at Mars in a hybrid model: High energy and low energy ions

We have studied the escape and energization of several O⁺ populations and an population at Mars by using a hybrid model. The quasi-neutral hybrid model, HYB-Mars model, included five oxygen ion populations making it possible to distinguish photoions from oxygen ions originating from charge exchange processes and from the ionosphere.We have identified two high-energy ion components and one low-energy ion component of oxygen. They have different spatial and energy distributions near Mars. The two high-energy oxygen ion components, consisting of a high-energy “beam” and a high-energy “halo”, have different origins. (1) The high-energy (>∼100 eV) “beam” of O⁺ and ions are originating from the ionosphere. These ions form a highly asymmetric spatial distribution of escaping oxygen ions with respect to the direction of the convective electric field in the solar wind. (2) The high-energy (>∼100 eV) “halo” component contains O⁺ ions which are formed from the oxygen neutral exosphere by extreme ultraviolet radiation (EUV) and by charge exchange processes. These energetic halo ions can be found all around Mars. (3) The low energy O⁺ and ions (<∼100 eV) form a relatively symmetric spatial distribution around the Mars-Sun line. They originate from the ionosphere and from charge exchange processes between protons and exospheric oxygen atoms.The existence of the low- and the high-energy oxygen components is in agreement with recent in situ plasma measurements made by the ASPERA-3 instrument on the Mars Express mission. The analysis of the escaping oxygen ions suggests that the global energization of escaping planetary ions in the martian tail is controlled by the convective electric field.     

Automatic detection of sub-km craters in high resolution planetary images

Impact craters are among the most studied geomorphic planetary features because they yield information about the past geological processes and provide a tool for measuring relative ages of observed geologic formations. Surveying impact craters is an important task which traditionally has been achieved by means of visual inspection of images. The shear number of smaller craters present in high resolution images makes visual counting of such craters impractical. In this paper we present a method that brings together a novel, efficient crater identification algorithm with a data processing pipeline; together they enable a fully automatic detection of sub-km craters in large panchromatic images. The technical details of the method are described and its performance is evaluated using a large, 12.5 m/pixel image centered on the Nanedi Valles on Mars. The detection percentage of the method is ∼70%. The system detects over 35,000 craters in this image; average crater density is , but localized spots of much higher crater density are present. The method is designed to produce “million craters” global catalogs of sub-km craters on Mars and other planets wherever high resolution images are available. Such catalogs could be utilized for deriving high spatial resolution and high temporal precision stratigraphy on regional or even planetary scale.     

Calculation of the enrichment of the giant planet envelopes during the “late heavy bombardment”

The giant planets of our solar system possess envelopes consisting mainly of hydrogen and helium but are also significantly enriched in heavier elements relatively to our Sun. In order to better constrain how these heavy elements have been delivered, we quantify the amount accreted during the so-called “late heavy bombardment”, at a time when planets were fully formed and planetesimals could not sink deep into the planets. On the basis of the “Nice model”, we obtain accreted masses (in terrestrial units) equal to for Jupiter, and for Saturn. For the two other giant planets, the results are found to depend mostly on whether they switched position during the instability phase. For Uranus, the accreted mass is with an inversion and without an inversion. Neptune accretes in models in which it is initially closer to the Sun than Uranus, and otherwise. With well-mixed envelopes, this corresponds to an increase in the enrichment over the solar value of 0.033±0.001 and 0.074±0.007 for Jupiter and Saturn, respectively. For the two other planets, we find the enrichments to be 2.1±1.4 (w/ inversion) or 1.2±0.7 (w/o inversion) for Uranus, and 2.0±1.2 (w/ inversion) or 2.7±1.6 (w/o inversion) for Neptune. This is clearly insufficient to explain the inferred enrichments of ∼4 for Jupiter, ∼7 for Saturn and ∼45 for Uranus and Neptune.     

A solar spectrum for PFS data analysis

A measured calibrated solar radiance in the range 1.2-, with the spectral sampling of does not exist. When studying the measured Planetary Fourier Spectrometer (PFS) spectra of the Earth's or Mars's atmosphere we discover that the most used solar spectrum contains several important errors. Here we present a “calibrated” solar radiance in the wavelength range 1.2-, with the spectral resolution of PFS , which we are going to use for studying Martian spectra. This spectrum has been assembled using measurements from Kitt Peak and from ATMOS Spacelab experiment (uncalibrated high resolution) and theoretical results, together with low resolution calibrated continuum. This is the best we can have in this moment to be used with PFS, while waiting to have good solar calibrated radiances. Examples of solar lines at Mars are given.     

A process to make massive ice in the martian regolith using long-term diffusion and thermal cracking

This paper describes a “simple standard” model of water transport through regolith that includes diffusive migration and phase changes driven by damped seasonal temperature waves. A hitherto unused first-order process is then added, that can produce ice densities much greater than those allowed by the initial dry porosity. Voids are produced in cooling icy regolith when tensile stresses exceed the cracking threshold . These stresses build up through an interaction of thermal contraction and elastic-plastic response. When the cracks open up after tensile failure there is purely thermal void enhancement and subsequent reduction as the regolith warms again. When the cracks are open the porosity is increased and they partially fill with ice crystals. Thus the void reduction on warming cannot go back to the original zero point and the bulk density of ice is increased with each temperature cycle. The cracking and thermal adjustment happen at scales of meters to millimeters. The large cracks can occur in pure ice and/or homogeneous icy material and the smaller cracks are produced by rock cobbles, pebbles, and grains having a much smaller coefficient of thermal expansion than ice. Thus a hierarchy of cracks and voids forms each temperature cycle and augments the ice content. The process can take the upper few meters of a pore-saturated icy soil from 28% by mass ice content to 70% in 10 Ma. This mechanism and the seasonal temperature cycle can plausibly produce massive ice deposits in the upper few meters of Mars' high-latitude regolith by diffusion and also keep the massive-ice regolith effectively porous to water vapor transport. The obliquity cycle can produce tensile stresses nearing 2 MPa down to depth so even deeper cracking could be happening.     

Evidence for ponding and catastrophic floods in central Valles Marineris, Mars

The Valles Marineris canyon system of Mars is closely related to large flood channels, some of which emerge full born from chaotic terrain in canyon floors. Coprates Chasma, one of the largest Valles Marineris canyons, is connected at its west end to Melas Chasma and on its east end to chaotic terrain-filled Capri and Eos Chasmata. The area from central Melas to Eos Chasmata contains a 1500 km long and about 1 km deep depression in its floor. Despite the large volumes of groundwater that likely discharged from chaotic terrain in this depression, no evidence of related fluvial activity has thus far been reported. We present an analysis of the regional topography which, together with photogeologic interpretation of available imagery, suggests that ponding due to late Hesperian discharge of water possibly produced a lake (mean depth 842 m) spanning parts of the Valles Marineris depression (VMD). Overflow of this lake at its eastern end resulted in delivery of water to downstream chaos regions and outflow channels. Our ponding hypothesis is motivated primarily by the identification of scarp and terrace features which, despite a lateral spread of about 1500 km, have similar elevations. Furthermore, these elevations correspond to the maximum ponding elevation of the region (−3560 m). Simulated ponding in the VMD yields an overflow point at its eastern extremity, in Eos Chasma. The neighborhood of this overflow point contains clear indicators of fluvial erosion in a consistent east-west orientation.     

Spectral observations for near-Earth objects including potential target 4660 Nereus : Results from Meudon remote observations at the NASA Infrared Telescope Facility (IRTF)

We present visible and near-infrared spectral measurements for the highly accessible spacecraft target 4660 Nereus and three additional near-Earth objects displaying diverse color characteristics. All near-infrared measurements were carried out during the first remote observing operations between the Observatoire de Paris at Meudon and Mauna Kea, Hawaii. From Meudon, we had fine pointing and guiding control of the NASA Infrared Telescope Facility 3.0-m telescope and the near-infrared spectrograph “SpeX” to measure asteroid spectra in the range 0.8-. The efficiency of the observation was virtually the same as if the observers had been on location. We combine our near-infrared results with complementary 0.4- spectral measurements. Nereus is found to be a rare Xe-type asteroid with a composition that may be analogous to very high albedo enstatite achondrite (aubrite) meteorites, leading to a diameter estimate of less than . 1685 Toro displays a classic S-type spectrum while a steeper visible wavelength slope and a less pronounced absorption feature for 1943 Anteros places it in the L-class. Also unusual is the apparent olivine-rich spectrum for 4142 Dersu-Uzala, which is classified as an A-type.     

Chemical and mineralogical size segregation in the impact disruption of inhomogeneous, anhydrous meteorites

We performed impact disruption experiments on pieces from eight different anhydrous chondritic meteorites—four weathered ordinary chondrite finds from North Africa (NWA791, NWA620, NWA869 and MOR001), three almost unweathered ordinary chondrite falls (Mbale, Gao, and Saratov), and an almost unweathered carbonaceous chondrite fall (Allende). In each case the impactor was a small (1/8 or 1/4 in) aluminum sphere fired at the meteorite target at , comparable to the mean collision speed in the main-belt. Some of the ∼5 to debris from each disruption was collected in aerogel capture cells, and the captured particles were analyzed by in situ synchrotron-based X-ray fluorescence. For each meteorite, many of the smallest particles ( up to in size, depending on the meteorite) exhibit very high Ni/Fe ratios compared to the Ni/Fe ratios measured in the larger particles , a composition consistent with the smallest debris being dominated by matrix material while the larger debris is dominated by fragments from olivine chondrules. These results may explain why the interplanetary dust particles (IDPs) collected from the Earth's stratosphere are C-rich and volatile-rich compared to the presumed solar nebula composition. The IDPs may simply sample the matrix of an inhomogeneous parent body, structurally and mineralogically similar to the chondritic meteorites, which are inhomogeneous assemblages of compact, strong, C- and volatile-poor chondrules that are distributed in a more porous, C- and volatile-rich matrix. In addition, these results may explain why the micrometeorites, which are to millimeters in size, recovered from the polar ices are Ni- and S-poor compared to chondritic meteorites, since these polar micrometeorites may preferentially sample fragments from the Ni- and S-poor olivine chondrules. These results indicate that the average composition of the IDPs may be biased towards the composition of the matrix of the parent body while the average composition of the polar micrometeorites may be more heavily weighted towards the composition of the chondrules and clasts. Thus, neither the IDPs nor the polar micrometeorites may sample the bulk composition of their respective parent bodies.We determined the threshold collisional specific energy for these chondritic meteorites to be 1419 J/kg, about twice the value for terrestrial basalt. Comparison of the mass of the largest fragment produced in the disruption of an sample of the porous ordinary chondrite Saratov with the largest fragment produced in the disruption of an sample of the compact ordinary chondrite MOR001 when each was struck by an impactor having approximately the same kinetic energy confirms that it requires significantly more energy to disrupt a porous target than a non-porous target.These results may also have important implications for the design of spacecraft missions intended to sample the composition and mineralogy of the chondritic asteroids and other inhomogeneous bodies. A Stardust-like spacecraft intended to sample asteroids by collecting only the small debris from a man-made impact onto the asteroid may collect particles that over-sample the matrix of the target and do not provide a representative sample of the bulk composition. The impact collection technique to be employed by the Japanese HAYABUSA (formerly MUSES-C) spacecraft to sample the asteroid Itokawa may result in similar mineral segregation.     

Environmental predictions for the Beagle 2 lander, based on GCM climate simulations

The Mars climate database (MCD) is a database of statistics based on output from physically consistent numerical model simulations which describe the climate and surface environment of Mars. It is used here to predict the meteorological environment of the Beagle 2 lander site. The database was constructed directly on the basis of output from multiannual integrations of two general circulation models, developed jointly at Laboratoire de Météorologie Dynamique du Centre National de la Recherche Scientifique, France, and the University of Oxford, UK. In an atmosphere with dust opacities similar to that observed by Mars Global Surveyor, the predicted surface temperature at the time of landing (L_s=322°, 13:00 local time), is , and varying between ∼186 and over the Martian day. The predicted air temperature at above the surface, the height of the fully extended Beagle 2 robot arm, is at the time of landing. The expected mean wind near the surface on landing is north-westerly in direction, becoming more southerly over the mission. An increase in mean surface pressure is expected during the mission. Heavy global dust storm predictions are discussed; conditions which may only occur in the extreme as the expected time of landing is around the end of the main dust storm period. Past observations show approximately a one in five chance of a large-scale dust storm in a whole Mars year over the landing region, Isidis Planitia. This statistic results from observations of global, encircling, regional and local dust storms but does not include any small-scale dust “events” such as dust devils.     

A ballistics analysis of the Deep Impact ejecta plume: Determining Comet Tempel 1's gravity, mass, and density

In July of 2005, the Deep Impact mission collided a 366 kg impactor with the nucleus of Comet 9P/Tempel 1, at a closing speed of 10.2 km s⁻¹. In this work, we develop a first-order, three-dimensional, forward model of the ejecta plume behavior resulting from this cratering event, and then adjust the model parameters to match the flyby-spacecraft observations of the actual ejecta plume, image by image. This modeling exercise indicates Deep Impact to have been a reasonably “well-behaved” oblique impact, in which the impactor-spacecraft apparently struck a small, westward-facing slope of roughly 1/3-1/2 the size of the final crater produced (determined from initial ejecta plume geometry), and possessing an effective strength of not more than . The resulting ejecta plume followed well-established scaling relationships for cratering in a medium-to-high porosity target, consistent with a transient crater of not more than 85-140 m diameter, formed in not more than 250-550 s, for the case of (gravity-dominated cratering); and not less than 22-26 m diameter, formed in not less than 1-3 s, for the case of (strength-dominated cratering). At , an upper limit to the total ejected mass of 1.8×¹⁰⁷ kg (1.5-2.2×¹⁰⁷ kg) is consistent with measurements made via long-range remote sensing, after taking into account that 90% of this mass would have stayed close to the surface and then landed within 45 min of the impact. However, at , a lower limit to the total ejected mass of 2.3×¹⁰⁵ kg (1.5-2.9×¹⁰⁵ kg) is also consistent with these measurements. The expansion rate of the ejecta plume imaged during the look-back phase of observations leads to an estimate of the comet's mean surface gravity of (0.17-0.90 mm s⁻²), which corresponds to a comet mass of mt=4.5×¹⁰¹³ kg (2.3-12.0×¹⁰¹³ kg) and a bulk density of (200-1000 kg m⁻³), where the large high-end error is due to uncertainties in the magnitude of coma gas pressure effects on the ejecta particles in flight.     

Fragment-collision model for compound chondrule formation: Estimation of collision probability

We propose a new scenario for compound chondrule formation named as “fragment-collision model,” in the framework of the shock-wave heating model. A molten cm-sized dust particle (parent) is disrupted in the high-velocity gas flow. The extracted fragments (ejectors) are scattered behind the parent and the mutual collisions between them will occur. We modeled the disruption event by analytic considerations in order to estimate the probability of the mutual collisions assuming that all ejectors have the same radius. In the typical case, the molten thin () layer of the parent surface will be stripped by the gas flow. The stripped layer is divided into about 200 molten ejectors (assuming that the radius of ejectors is 300 μm) and then they are blown away by the gas flow in a short period of time (). The stripped layer is leaving from the parent with the velocity of depending on the viscosity, and we assumed that the extracted ejectors have a random velocity Δv of the same order of magnitude. Using above values, we can estimate the number density of ejectors behind the parent as . These ejectors occupy ∼9% of the space behind the parent in volume. Considering that the collision rate (number of collisions per unit time experienced by an ejector) is given by Rcoll=σcollneΔv, where σcoll is the cross-section of collision [e.g., Gooding, J.K., Keil, K., 1981. Meteoritics 16, 17-43], we obtain by substituting above values. Since most collisions occur within the short duration () before the ejectors are blown away, we obtain the collision probability of Pcoll∼0.36, which is the probability of collisions experienced by an ejector in one disruption event. The estimated collision probability is about one order of magnitude larger than the observed fraction of compound chondrules. In addition, the model predictions are qualitatively consistent with other observational data (oxygen isotopic composition, textural types, and size ratios of constituents). Based on these results, we concluded that this new model can be one of the strongest candidates for the compound chondrule formation. It should be noted that all collisions do not necessarily lead to the compound chondrule formation. The formation efficiency and the future works which should be investigated in the forthcoming paper are also discussed.