Sanjoy M. Som

Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints

According to the ‘Faint Young Sun’ paradox, during the late Archaean eon a Sun approximately 20% dimmer warmed the early Earth such that it had liquid water and a clement climate. Explanations for this phenomenon have invoked a denser atmosphere that provided warmth by nitrogen pressure broadening or enhanced greenhouse gas concentrations. Such solutions are allowed by geochemical studies and numerical investigations that place approximate concentration limits on Archaean atmospheric gases, including methane, carbon dioxide and oxygen. But no field data constraining ground-level air density and barometric pressure have been reported, leaving the plausibility of these various hypotheses in doubt. Here we show that raindrop imprints in tuffs of the Ventersdorp Supergroup, South Africa, constrain surface air density 2.7 billion years ago to less than twice modern levels. We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum. Fragmentation following raindrop flattening limits raindrop size to a maximum value independent of air density, whereas raindrop terminal velocity varies as the inverse of the square root of air density. If the Archaean raindrops reached the modern maximum measured size, air density must have been less than 2.3 kg m−3, compared to today’s 1.2 kg m−3, but because such drops rarely occur, air density was more probably below 1.3 kg m−3. The upper estimate for air density renders the pressure broadening explanation possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO2 levels required for hot Archaean climates.

Continental-scale salt tectonics on Mars and the origin of Valles Marineris and associated outflow channels

A synthesis of deformation patterns within and around the Thaumasia Plateau, Mars, points to a new interpretation for regional deformation and the origin of Valles Marineris and associated outflow channels. The morphology of the Thaumasia Plateau is typical of thin-skinned deformation, akin to a “mega-slide,” in which extensional deformation in Syria Planum and Noctis Labyrinthus connects via lateral zones of transtension and strike-slip—Claritas Fossae and Valles Marineris—to a broad zone of compressional uplift and shortening defined by truncated craters and thrust faults along the Coprates Rise and Thaumasia Highlands. However, the low regional slope (~1°) results in gravitational body forces that are too small to deform the basaltic lava flows conventionally thought to compose the flanks of the Tharsis volcanic province. Instead, we conclude that geothermal heating and topographic loading of extensive buried deposits of salts and/or mixtures of salts, ice, and basaltic debris would allow for weak detachments and large-scale gravity spreading. We propose that the generally linear chasmata of Valles Marineris reflect extension, collapse, and excavation along fractures radial to Tharsis, either forming or reactivated as part of one lateral margin of the Thaumasia gravity-spreading system. The other, dextral, lateral margin is a massive splay of extensional faults forming the Claritas Fossae, which resembles a trailing extensional imbricate fan. The compressional mountain belt defined by the Coprates Rise and Thaumasia Highlands forms the toe of the “mega-slide.” Topographic observations and previous structural analyses reveal evidence for a failed volcanic plume below Syria Planum that could have provided further thermal energy and topographic potential for initiating regional deformation, either intrusively through inflation or extrusively through lava flow and/or ash fall emplacement. Higher heat flow during Noachian time, or geothermal heating due to burial by Tharsis-derived volcanic rocks, would have contributed to flow of salt deposits, as well as formation of groundwater from melting ice and dewatering of hydrous salts. We further propose that connection of overpressured groundwater from aquifers near the base of the detachment through the cryosphere to the martian surface created the outflow channels of Echus, Coprates, and Juventae chasmata at relatively uniform source elevations along the northern margin of the “mega-slide,” where regional groundwater flow would have been directed toward the surface. Our hypothesis provides a unifying framework to explain perplexing relationships between the rise of the Tharsis volcanic province, deformation of the Thaumasia Plateau, and the formation of Valles Marineris and associated outflow channels.

Thermal degradation of organic material by portable laser Raman spectrometry

Raman spectrometry has been established as an instrument of choice for studying the structure and bond type of known molecules, and identifying the composition of unknown substances, whether geological or biological. This versatility has led to its strong consideration for planetary exploration. In the context of the ExoGeoLab and ExoHab pilot projects of ESA-ESTEC & ILEWG (International Lunar Exploration Working Group), we investigated samples of astrobiological interest using a portable Raman spectrometer lasing at 785 nm and discuss implications for planetary exploration. We find that biological samples are typically best observed at wavenumbers >1100 cm −1 , but their Raman signals are often affected by fluorescence effects, which lowers their signal-to-noise ratio. Raman signals of minerals are typically found at wavenumbers −1 , and tend to be less affected by fluorescence. While higher power and/or longer signal integration time improve Raman signals, such power settings are detrimental to biological samples due to sample thermal degradation. Care must be taken in selecting the laser wavelength, power level and integration time for unknown samples, particularly if Raman signatures of biological components are anticipated. We include in the Appendices tables of Raman signatures for astrobiologically relevant organic compounds and minerals.

Session 2. Advances in Astrobiological Instrumentation Development

The flux of chemical compounds introduced into the tenuous Europan atmosphere due to energetic charged particle sputtering of the surface ice directly reflects the composition of the ice. In particular, any organic compounds, formed in the sub-ice ocean and entrained in the icy crust, will enter the atmosphere in this sputtering process. This provides the opportunity to remotely determine the surface ice composition through an analysis of the composition of the Europan sputter atmosphere and provides an opportunity to look for astrobiological signatures from Europan orbit. We have developed a preliminary design for an orbiter-based radar spectrometer that can measure minor species in the Europan atmosphere. The radar is sensitive to the total column of molecules between the spacecraft and the surface. Based on conventional radar concepts at longer wavelengths, the instrument concept includes, onboard an orbiter, a transmitter illuminating a spot on the surface and a heterodyne receiver detecting the back-scattered radiation. All molecules in the atmosphere with an electric or magnetic dipole absorb radiation at the millimeter and submillimeter wavelengths at which the spectrometer will operate. Our calculations show that polar species, such as organic Nand S-containing compounds and inorganic salts, if present at ppm levels in the ice, will be detectable. Detection sensitivity is orders of magnitude larger than traditional limb thermal emission spectrometry. In addition, the radar spectrometer observations provide insight into the physical characteristics of the environment (sputter velocities, magnetic field), the surface, and the range between the surface and satellite. 2-02-O. Novel Subcritical Water Extraction Methods for Studying the Mechanisms of Biomarker Preservation in Minerals and Ices

Stratigraphic architectures spotted in southern Melas Chasma, Valles Marineris, Mars: COMMENT COMMENT

[Dromart et al. (2007)][1] describe a spectacular stratigraphic complex within southern Melas Chasma, Vallis Marineris, Mars. Following a rigorous stratigraphic description of the complex, they proceed to interpret the responsible depositional processes as analogous to subaqueous channel-levee