Livio L. Tornabene

Chloride-Bearing Materials in the Southern Highlands of Mars

Chlorides commonly precipitate during the evaporation of surface water or groundwater and during volcanic outgassing. Spectrally distinct surface deposits consistent with chloride-bearing materials have been identified and mapped using data from the 2001 Mars Odyssey Thermal Emission Imaging System. These deposits are found throughout regions of low albedo in the southern highlands of Mars. Geomorphologic evidence from orbiting imagery reveals these deposits to be light-toned relative to their surroundings and to be polygonally fractured. The deposits are small (< ∼25 km2) but globally widespread, occurring in middle to late Noachian terrains with a few occurrences in early Hesperian terrains. The identification of chlorides in the ancient southern highlands suggests that near-surface water was available and widespread in early Martian history.

Characterization and petrologic interpretation of olivine‐rich basalts at Gusev Crater, Mars

Additional co-authors: PR Christensen, BC Clark, JA Crisp, DJ DesMarais, T Economou, JD Farmer, W Farrand, A Ghosh, M Golombek, S Gorevan, R Greeley, VE Hamilton, JR Johnson, BL Joliff, G Klingelhofer, AT Knudson, S McLennan, D Ming, JE Moersch, R Rieder, SW Ruff, PA de Souza Jr, SW Squyres, H Wnke, A Wang, A Yen, J Zipfel

A Closer Look at Water-Related Geologic Activity on Mars

Water has supposedly marked the surface of Mars and produced characteristic landforms. To understand the history of water on Mars, we take a close look at key locations with the High-Resolution Imaging Science Experiment on board the Mars Reconnaissance Orbiter, reaching fine spatial scales of 25 to 32 centimeters per pixel. Boulders ranging up to ∼2 meters in diameter are ubiquitous in the middle to high latitudes, which include deposits previously interpreted as finegrained ocean sediments or dusty snow. Bright gully deposits identify six locations with very recent activity, but these lie on steep (20° to 35°) slopes where dry mass wasting could occur. Thus, we cannot confirm the reality of ancient oceans or water in active gullies but do see evidence of fluvial modification of geologically recent mid-latitude gullies and equatorial impact craters.

Columbus crater and other possible groundwater‐fed paleolakes of Terra Sirenum, Mars

Columbus crater in the Terra Sirenum region of the Martian southern highlands contains light-toned layered deposits with interbedded sulfate and phyllosilicate minerals, a rare occurrence on Mars. Here we investigate in detail the morphology, thermophysical properties, mineralogy, and stratigraphy of these deposits; explore their regional context; and interpret the craters aqueous history. Hydrated mineral-bearing deposits occupy a discrete ring around the walls of Columbus crater and are also exposed beneath younger materials, possibly lava flows, on its floor. Widespread minerals identified in the crater include gypsum, polyhydrated and monohydrated Mg/Fe-sulfates, and kaolinite; localized deposits consistent with montmorillonite, Fe/Mg-phyllosilicates, jarosite, alunite, and crystalline ferric oxide or hydroxide are also detected. Thermal emission spectra suggest abundances of these minerals in the tens of percent range. Other craters in northwest Terra Sirenum also contain layered deposits and Al/Fe/Mg-phyllosilicates, but sulfates have so far been found only in Columbus and Cross craters. The regions intercrater plains contain scattered exposures of Al-phyllosilicates and one isolated mound with opaline silica, in addition to more common Fe/Mg-phyllosilicates with chlorides. A Late Noachian age is estimated for the aqueous deposits in Columbus, coinciding with a period of inferred groundwater upwelling and evaporation, which (according to model results reported here) could have formed evaporites in Columbus and other craters in Terra Sirenum. Hypotheses for the origin of these deposits include groundwater cementation of crater-filling sediments and/or direct precipitation from subaerial springs or in a deep (∼900 m) paleolake. Especially under the deep lake scenario, which we prefer, chemical gradients in Columbus crater may have created a habitable environment at this location on early Mars.

Diverse aqueous environments on ancient Mars revealed in the southern highlands

High-resolution spectral images reveal many new exposures of hydrated minerals across the Noachian southern highlands of Mars. Several different phyllosilicates are observed, with inferred accompanying phases ranging from zeolites to sulfates to chlorides, each in distinct geologic settings. Hydrated sulfates are observed in several locations at mid-latitudes, and likely formed independently from the equatorial sulfates. These assemblages suggest a greater diversity of aqueous environments on ancient Mars than has previously been recognized, and their distribution implies that alteration was widespread during the Noachian Period, not restricted to a few locations.

HiRISE imaging of impact megabreccia and sub-meter aqueous strata in Holden Crater, Mars

High Resolution Imaging Science Experiment (HiRISE) images of Holden crater, Mars, resolve impact megabreccia unconformably overlain by sediments deposited during two Noachian-age phases of aqueous activity. A lighter-toned lower unit exhibiting phyllosilicates was deposited in a long-lived, quiescent distal alluvial or lacustrine setting. An overlying darker-toned and often blocky upper unit drapes the sequence and was emplaced during later high-magnitude flooding as an impounded Uzboi Vallis lake overtopped the crater rim. The stratigraphy provides the first geologic context for phyllosilicate deposition during persistent wet and perhaps habitable conditions on early Mars.

Noachian and more recent phyllosilicates in impact craters on Mars

Hundreds of impact craters on Mars contain diverse phyllosilicates, interpreted as excavation products of preexisting subsurface deposits following impact and crater formation. This has been used to argue that the conditions conducive to phyllosilicate synthesis, which require the presence of abundant and long-lasting liquid water, were only met early in the history of the planet, during the Noachian period (> 3.6 Gy ago), and that aqueous environments were widespread then. Here we test this hypothesis by examining the excavation process of hydrated minerals by impact events on Mars and analyzing the stability of phyllosilicates against the impact-induced thermal shock. To do so, we first compare the infrared spectra of thermally altered phyllosilicates with those of hydrated minerals known to occur in craters on Mars and then analyze the postshock temperatures reached during impact crater excavation. Our results show that phyllosilicates can resist the postshock temperatures almost everywhere in the crater, except under particular conditions in a central area in and near the point of impact. We conclude that most phyllosilicates detected inside impact craters on Mars are consistent with excavated preexisting sediments, supporting the hypothesis of a primeval and long-lasting global aqueous environment. When our analyses are applied to specific impact craters on Mars, we are able to identify both pre- and postimpact phyllosilicates, therefore extending the time of local phyllosilicate synthesis to post-Noachian times.

Deep impact craters in the Isidis and southwestern Utopia Planitia regions of Mars: High target material strength as a possible cause

] Using THEMIS, MOC and MOLA data, we havefound 51 craters in the diameter range 6–11.8 km withinsouthwestern Utopia Planitia and Isidis Planitia that aresignificantly deeper than typical fresh craters in the northernlowlands of Mars. The restricted geographic distribution ofthese craters, their simple morphology, and data fromimpact and explosion crater studies suggest that unusuallystrong target materials (as much as a factor of 2 greater thanaverage materials in the Martian lowlands) are the cause ofthe excessive crater depth. We propose that the greater targetmaterial strength acts to delay gravity-dominated collapse tolarger crater sizes. Furthermore, we suggest that a regional,olivine-rich mafic to untramafic rock unit identified by TESand THEMIS is a reasonable candidate for these strongmaterials. The unit is exposed on the southern edge of Isidisbasin and in crater ejecta within the basin, and forms layersthat dip toward the Isidis Basin center.

Louth crater: Evolution of a layered water ice mound

We report on observations made of the ∼36 km diameter crater, Louth, in the north polar region of Mars (at 70° N, 103.2° E). High-resolution imagery from the instruments on the Mars Reconnaissance Orbiter (MRO) spacecraft has been used to map a 15 km diameter water ice deposit in the center of the crater. The water ice mound has surface features that include roughened ice textures and layering similar to that found in the North Polar Layered Deposits. Features we interpret as sastrugi and sand dunes show consistent wind patterns within Louth over recent time. CRISM spectra of the ice mound were modeled to derive quantitative estimates of water ice and contaminant abundance, and associated ice grain size information. These morphologic and spectral results are used to propose a stratigraphy for this deposit and adjoining sand dunes. Our results suggest the edge of the water ice mound is currently in retreat.

Dependence of secondary crater characteristics on downrange distance: High‐resolution morphometry and simulations

On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity (vi) increases with downrange distance (L). We have used high-resolution topography (1–2 m/pixel) to characterize the morphometry of secondary craters as a function of L for several well-preserved primary craters on Mars. The secondaries in this study (N = 2,643) span a range of diameters (25 m ≤D≤400 m) and estimated impact velocities (0.4 km/s ≤vi≤2 km/s). The range of diameter-normalized rim-to-floor depth (d/D) broadens and reaches a ceiling of d/D≈0.22 at L≈280 km (vi=1–1.2 km/s) whereas average rim height shows little dependence on vi for the largest craters (h/D≈0.02,D > 60 m). Populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent: planform elongations (L< 110–160 km), taller downrange rims (L < 280 km), and cavities that are deeper uprange (L< 450–500 km). Populations of secondaries with lopsided ejecta were found to extend to at least L ∼ 700 km. Impact hydrocode simulations with iSALE-2D for strong, intact projectile and target materials predict a ceiling for d/D vs. L whose trend is consistent with our measurements. This study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations. This has applications to understanding the chronology of planetary surfaces and the long-term evolution of small crater populations.