Phyllosilicate diversity and past aqueous activity revealed at Mawrth Vallis, Mars

Observations by the Mars Reconnaissance Orbiter/Compact Reconnaissance Imaging Spectrometer for Mars in the Mawrth Vallis region show several phyllosilicate species, indicating a wide range of past aqueous activity. Iron/magnesium (Fe/Mg)-smectite is observed in light-toned outcrops that probably formed via aqueous alteration of basalt of the ancient cratered terrain. This unit is overlain by rocks rich in hydrated silica, montmorillonite, and kaolinite that may have formed via subsequent leaching of Fe and Mg through extended aqueous events or a change in aqueous chemistry. A spectral feature attributed to an Fe2+ phase is present in many locations in the Mawrth Vallis region at the transition from Fe/Mg-smectite to aluminum/silicon (Al/Si)-rich units. Fe2+-bearing materials in terrestrial sediments are typically associated with microorganisms or changes in pH or cations and could be explained here by hydrothermal activity. The stratigraphy of Fe/Mg-smectite overlain by a ferrous phase, hydrated silica, and then Al-phyllosilicates implies a complex aqueous history.     

Environmental effects of large impacts on Mars

The martian valley networks formed near the end of the period of heavy bombardment of the inner solar system, about 3.5 billion years ago. The largest impacts produced global blankets of very hot ejecta, ranging in thickness from meters to hundreds of meters. Our simulations indicated that the ejecta warmed the surface, keeping it above the freezing point of water for periods ranging from decades to millennia, depending on impactor size, and caused shallow subsurface or polar ice to evaporate or melt. Large impacts also injected steam into the atmosphere from the craters or from water innate to the impactors. From all sources, a typical 100-, 200-, or 250-kilometers asteroid injected about 2, 9, or 16 meters, respectively, of precipitable water into the atmosphere, which eventually rained out at a rate of about 2 meters per year. The rains from a large impact formed rivers and contributed to recharging aquifers.     

A wetter, younger Mars emerging

Most researchers have believed that the days were long gone when water splashed on the surface of Mars or even near it. Now continuing analyses of martian meteorites and stunning images from the Mars Global Surveyor (MGS), which has been in orbit since 1997, are breathing new life into the Red Planet. Last month's announcement that the camera aboard MGS had spied signs of geologically recent--possibly even ongoing--water seeps has caught everyone's attention. Other, perhaps more persuasive, signs also suggest that water may even now flow on or beneath the frigid surface.     

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 approximately 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 degrees to 35 degrees) 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.     

Wind-related processes detected by the Spirit Rover at Gusev Crater, Mars

Wind-abraded rocks, ripples, drifts, and other deposits of windblown sediments are seen at the Columbia Memorial Station where the Spirit rover landed. Orientations of these features suggest formative winds from the north-northwest, consistent with predictions from atmospheric models of afternoon winds in Gusev Crater. Cuttings from the rover Rock Abrasion Tool are asymmetrically distributed toward the south-southeast, suggesting active winds from the north-northwest at the time (midday) of the abrasion operations. Characteristics of some rocks, such as a two-toned appearance, suggest that they were possibly buried and exhumed on the order of 5 to 60 centimeters by wind deflation, depending on location.     

Pyroclastic activity at Home Plate in Gusev Crater, Mars

Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarsegrained lower unit lies under a finer-grained upper unit. Textural observations indicate that the lower strata were emplaced in an explosive event, and geochemical considerations favor an explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of pyroclastic materials, whereas the upper unit may represent eolian reworking of the same pyroclastic materials.     

Magnetic Field and Plasma Observations at Mars: Initial Results of the Mars Global Surveyor Mission

The magnetometer and electron reflectometer investigation (MAG/ER) on the Mars Global Surveyor spacecraft has obtained magnetic field and plasma observations throughout the near-Mars environment, from beyond the influence of Mars to just above the surface (at an altitude of approximately 100 kilometers). The solar wind interaction with Mars is in many ways similar to that at Venus and at an active comet, that is, primarily an ionospheric-atmospheric interaction. No significant planetary magnetic field of global scale has been detected to date (<2 x 10(21) Gauss-cubic centimeter), but here the discovery of multiple magnetic anomalies of small spatial scale in the crust of Mars is reported.     

The Spirit Rover's Athena science investigation at Gusev Crater, Mars

The Mars Exploration Rover Spirit and its Athena science payload have been used to investigate a landing site in Gusev crater. Gusev is hypothesized to be the site of a former lake, but no clear evidence for lacustrine sedimentation has been found to date. Instead, the dominant lithology is basalt, and the dominant geologic processes are impact events and eolian transport. Many rocks exhibit coatings and other characteristics that may be evidence for minor aqueous alteration. Any lacustrine sediments that may exist at this location within Gusev apparently have been buried by lavas that have undergone subsequent impact disruption.     

A sporadic third layer in the ionosphere of Mars

The daytime martian ionosphere has been observed as a two-layer structure with electron densities that peak at altitudes between about 110 and 130 kilometers. The Mars Express Orbiter Radio Science Experiment on the European Mars Express spacecraft observed, in 10 out of 120 electron density profiles, a third ionospheric layer at altitude ranges of 65 to 110 kilometers, where electron densities, on average, peaked at 0.8 x 10(10) per cubic meter. Such a layer has been predicted to be permanent and continuous. Its origin has been attributed to ablation of meteors and charge exchange of magnesium and iron. Our observations imply that this layer is present sporadically and locally.     

Stochastic late accretion to Earth, the Moon, and Mars

Core formation should have stripped the terrestrial, lunar, and martian mantles of highly siderophile elements (HSEs). Instead, each world has disparate, yet elevated HSE abundances. Late accretion may offer a solution, provided that ≥0.5% Earth masses of broadly chondritic planetesimals reach Earth's mantle and that ~10 and ~1200 times less mass goes to Mars and the Moon, respectively. We show that leftover planetesimal populations dominated by massive projectiles can explain these additions, with our inferred size distribution matching those derived from the inner asteroid belt, ancient martian impact basins, and planetary accretion models. The largest late terrestrial impactors, at 2500 to 3000 kilometers in diameter, potentially modified Earth's obliquity by ~10°, whereas those for the Moon, at ~250 to 300 kilometers, may have delivered water to its mantle.     

Formation of glaciers on Mars by atmospheric precipitation at high obliquity

Surface conditions on Mars are currently cold and dry, with water ice unstable on the surface except near the poles. However, geologically recent glacierlike landforms have been identified in the tropics and the midlatitudes of Mars. The ice has been proposed to originate from either a subsurface reservoir or the atmosphere. We present high-resolution climate simulations performed with a model designed to simulate the present-day Mars water cycle but assuming a 45 degrees obliquity as experienced by Mars a few million years ago. The model predicts ice accumulation in regions where glacier landforms are observed, on the western flanks of the great volcanoes and in the eastern Hellas region. This agreement points to an atmospheric origin for the ice and reveals how precipitation could have formed glaciers on Mars.     

A prediction of Mars seismicity from surface faulting

The shallow seismicity of Mars has been estimated by measurement of the total slip on faults visible on the surface of the planet throughout geologic time. Seismicity was calibrated with estimates based on surface structures on the moon and measured lunar seismicity that includes the entire seismogenic lithosphere. Results indicate that Mars is seismically active today, with a sufficient number of detectable marsquakes to allow seismic investigations of its interior.     

The complete atomic structure of the large ribosomal subunit at 2.4 A resolution

The large ribosomal subunit catalyzes peptide bond formation and binds initiation, termination, and elongation factors. We have determined the crystal structure of the large ribosomal subunit from Haloarcula marismortui at 2.4 angstrom resolution, and it includes 2833 of the subunit's 3045 nucleotides and 27 of its 31 proteins. The domains of its RNAs all have irregular shapes and fit together in the ribosome like the pieces of a three-dimensional jigsaw puzzle to form a large, monolithic structure. Proteins are abundant everywhere on its surface except in the active site where peptide bond formation occurs and where it contacts the small subunit. Most of the proteins stabilize the structure by interacting with several RNA domains, often using idiosyncratically folded extensions that reach into the subunit's interior.     

Sulfates in the north polar region of Mars detected by OMEGA/Mars Express

The Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) imaging spectrometer observed the northern circumpolar regions of Mars at a resolution of a few kilometers. An extended region at 240 degrees E, 85 degrees N, with an area of 60 kilometers by 200 kilometers, exhibits absorptions at wavelengths of 1.45, 1.75, 1.94, 2.22, 2.26, and 2.48 micrometers. These signatures can be unambiguously attributed to calcium-rich sulfates, most likely gypsum. This region corresponds to the dark longitudinal dunes of Olympia Planitia. These observations reveal that water alteration played a major role in the formation of the constituting minerals of northern circumpolar terrains.     

Composition of the atmosphere at the surface of Mars: detection of argon-36 and preliminary analysis

The composition of the martian atmosphere was determined by the mass spectrometer in the molecular analysis experiment. The presence of argon and nitrogen was confirmed and a value of 1 to 2750 +/- 500 for the ratio of argon-36 to argon-40 was established. A preliminary interpretation of these results suggests that Mars had a slightly more massive atmosphere in the past, but that much less total outgassing has occurred on Mars than on Earth.