[1] Solar X-ray and UV radiation (0.1-320 nm) received at Earth's surface is an important aspect of the circumstances under which life formed on Earth. The quantity that is received depends on two main variables: the emission of radiation by the young Sun and its extinction through absorption and scattering by the Earth's early atmosphere. The spectrum emitted by the Sun when life formed, between 4 and 3.5 Ga, was modeled here, including the effects of flares and activity cycles, using a solar-like star that has the same age now as the Sun had 4-3.5 Ga. Atmospheric extinction was calculated using the Beer-Lambert law, assuming several density profiles for the atmosphere of the Archean Earth. We found that almost all radiation with a wavelength shorter than 200 nm is attenuated effectively, even by very tenuous atmospheres. Longer-wavelength radiation is progressively less well attenuated, and its extinction is more sensitive to atmospheric composition. Minor atmospheric components, such as methane, ozone, water vapor, etc., have only negligible effects, but changes in CO 2 concentration can cause large differences in surface flux. Differences due to variability in solar emission are small compared to this. In all cases surface radiation levels on the Archean Earth were several orders of magnitude higher in the 200-300 nm wavelength range than current levels in this range. That means that any form of life that might have been present at Earth's surface 4-3.5 Ga must have been exposed to much higher quantities of damaging radiation than at present.

[1] Rocks on the floor of Gusev crater are basaltic in composition, as determined from measurements by the Mars Exploration Rover, Spirit. On the basis of compositional data, models of the basaltic lavas at the time of their emplacement suggest viscosities of 2.3 to 50 Pa Á s (dependent on the number of phenocrysts and vesicles that were present), which would be more fluid than terrestrial tholeiitic lavas and comparable to mare lavas on the Moon or Archean high-Mg basalts on Earth. Morphological data and crater counts derived from the High Resolution Stereo Camera on Mars Express and other orbiters suggest that the lavas flooded Gusev crater at about 3.65 b.y. and postdate older floor materials, such as putative sediments emplaced by Ma'adim Vallis.

[1] We present a large-scale spring hypothesis for the formation of various enigmatic light-toned deposits (LTDs) on Mars. Layered to massive LTDs occur extensively in Valles Marineris, chaotic terrains, and several large craters, in particular, those located in Arabia Terra. Most of these deposits are not easily explained with either a single process or multiple ones, either in combination or occurring sequentially. Spring deposits can have a very wide range of internal facies and exhibit complex architectural variations. We propose the concept of large-scale spring deposits for explaining LTDs on Mars. Stable volcano-tectonic settings, such as the ones typical on Mars, are compatible with a large-scale, long-term, multistage formation of spring deposits. The large-scale spring deposit model can explain the formation of LTDs with a common process, although active in different times and locations, compatible with coeval local or regional processes and deposits, such as volcaniclastic ones. LTDs, if formed as spring deposits derived from subsurface fluids, could potentially offer favorable conditions both to life and to the fossilization of past life forms.

Interior layered deposits (ILDs) within western Candor Chasma were studied by mapping lithologies, measuring layer attitudes and comparing the stratigraphy of two adjacent mounds. Layering tends to dip in the same direction as the local topographic slope, although at different angles. Regionally consistent attitudes do exist, suggesting postdepositional block rotations. The stratigraphy of two adjacent mounds correlates, but the thicknesses of the units differ. Most layered material appears to have been deposited conformably, with one late major unconformity. Several fault populations are identified and correlate well with regional faults associated with the formation of Valles Marineris. The data suggest that here the ILDs predate the faulting and may be early basin fill. According to our model for ILD formation, ILDs are deposited syntectonically during early basin collapse. Subsequent subsidence of surrounding areas, accompanied by little or no sedimentation, left the early deposits as individual mounds, remnants of the former subbasins. Stratigraphic differences between mounds resulted from different subsidence rates of subbasins. A significant change in depositional environment, from depositional to a near cessation of deposition and the onset of a major erosion event, possibly coincided with the opening of the main Valles Marineris troughs. We further suggest that groundwater played an important role in the formation of sulfates. The youngest unit identified, not including surficial deposits, is likely the result of a posttectonic, regionally limited volcanic event. If basin collapse continued following the cessation of deposition, this model can also account for mounds within closed basins, such as Hebes.