Martian sub-surface ionising radiation: biosignature and geology

The surface of Mars, unshielded by thick atmosphere or global magnetic field, is exposed to high levels of cosmic radiation. This ionising radiation field is deleterious to the survival of dormant cells or spores and the persistence of molecular biomarkers in the subsurface, and so its characterisation is of prime astrobiological interest. Here, we present modelling results of the absorbed radiation dose as a function of depth through the Martian subsurface, suitable for calculation of biomarker persistence. A second major implementation of this dose accumulation rate data is in application of the optically stimulated luminescence technique for dating Martian sediments. We present calculations of the dose-depth profile in the Martian subsurface for various scenarios: variations of surface composition (dry regolith, ice, layered permafrost), solar minimum and maximum conditions, locations of different elevation (Olympus Mons, Hellas basin, datum altitude), and increasing atmospheric thickness over geological history. We also model the changing composition of the subsurface radiation field with depth compared between Martian locations with different shielding material, determine the relative dose contributions from primaries of different energies, and discuss particle deflection by the crustal magnetic fields.

Sublimation of water ice mixed with silicates and tholins: Evolution of surface texture and reflectance spectra, with implications for comets

The surfaces of many objects in the Solar System comprise substantial quantities of water ice sometimes mixed with minerals and/or organic molecules. The sublimation of the ice changes the structural and optical properties of these objects. We present laboratory data on the evolution of the structure and the visible and near-infrared spectral reflectance of icy surface analogues of cometary ices, made of water ice, complex organic matter (tholins) and silicates, as they undergo sublimation under low temperature (<-70°C) and pressure (10-⁵mbar) conditions inside the SCITEAS simulation chamber. As the water ice sublimated, we observed in situ the formation of a porous sublimation lag deposit, or sublimation mantle, at the top of the ice. This mantle is a network of filaments made of the non-volatile particles. Organics or phyllosilicates grains, able to interact via stronger inter-particulate forces than olivine grains, can form a foam-like structure having internal cohesiveness, holding olivine grains together. As this mantle builds-up, the band depths of the sub-surface water ice are attenuated until complete extinction under only few millimeters of mantle. Optically thick sublimation mantles are mainly featureless in the near infrared. The absorption bands of the minerals present in the mantle are weak, or even totally absent if minerals are mixed with organics which largely dominate the VIS–NIR reflectance spectrum. During sublimation, ejections of large fragments of mantle, triggered by the gas flow, expose ice particles to the surface. The contrast of brightness between mantled and ice-exposed areas depends on the wavelength range and the dust/ice ratio considered. We describe how the chemical nature of the non-volatiles, the size of their particles, the way they are mixed with the ice and the dust/ice mass ratio influence the texture, activity and spectro-photometric properties of the sublimation mantles. These data provide useful references for interpreting remote-sensing observations of comets and also icy satellites or trans-neptunian objects.

On the seasonal variations of salinity of the tropical Atlantic mixed layer

The physical processes controlling the mixed layer salinity (MLS) seasonal budget in the tropical Atlantic Ocean are investigated using a regional configuration of an ocean general circulation model. The analysis reveals that the MLS cycle is generally weak in comparison of individual physical processes entering in the budget because of strong compensation. In evaporative regions, around the surface salinity maxima, the ocean acts to freshen the mixed layer against the action of evaporation. Poleward of the southern SSS maxima, the freshening is ensured by geostrophic advection, the vertical salinity diffusion and, during winter, a dominant contribution of the convective entrainment. On the equatorward flanks of the SSS maxima, Ekman transport mainly contributes to supply freshwater from ITCZ regions while vertical salinity diffusion adds on the effect of evaporation. All these terms are phase locked through the effect of the wind. Under the seasonal march of the ITCZ and in coastal areas affected by river (7°S:15°N), the upper ocean freshening by precipitations and/or runoff is attenuated by vertical salinity diffusion. In the eastern equatorial regions, seasonal cycle of wind forced surface currents advect freshwaters, which are mixed with subsurface saline water because of the strong vertical turbulent diffusion. In all these regions, the vertical diffusion presents an important contribution to the MLS budget by providing, in general, an upwelling flux of salinity. It is generally due to vertical salinity gradient and mixing due to winds. Furthermore, in the equator where the vertical shear, associated to surface horizontal currents, is developed, the diffusion depends also on the sheared flow stability.

Sharp comparison and maximum principles via horizontal normal mapping in the Heisenberg group

In this paper we solve a problem raised by Gutiérrez and Montanari about comparison principles for H−convex functions on subdomains of Heisenberg groups. Our approach is based on the notion of the sub-Riemannian horizontal normal mapping and uses degree theory for set-valued maps. The statement of the comparison principle combined with a Harnack inequality is applied to prove the Aleksandrov-type maximum principle, describing the correct boundary behavior of continuous H−convex functions vanishing at the boundary of horizontally bounded subdomains of Heisenberg groups. This result answers a question by Garofalo and Tournier. The sharpness of our results are illustrated by examples.

Thermal and mechanical properties of the near-surface layers of comet 67P/Churyumov-Gerasimenko

Thermal and mechanical material properties determine comet evolution and even solar system formation because comets are considered remnant volatile-rich planetesimals. Using data from the Multipurpose Sensors for Surface and Sub-Surface Science (MUPUS) instrument package gathered at the Philae landing site Abydos on comet 67P/Churyumov-Gerasimenko, we found the diurnal temperature to vary between 90 and 130 K. The surface emissivity was 0.97, and the local thermal inertia was 85 +/- 35 J m(-2) K(-1)s(-1/2). The MUPUS thermal probe did not fully penetrate the near-surface layers, suggesting a local resistance of the ground to penetration of >4 megapascals, equivalent to >2 megapascal uniaxial compressive strength. A sintered near-surface microporous dust-ice layer with a porosity of 30 to 65% is consistent with the data.

Geological and hydrological histories of the Argyre province, Mars

The geologic history of the multi-ringed Argyre impact basin and surroundings has been reconstructed on the basis of geologic mapping and relative-age dating of rock materials and structures. The impact formed a primary basin, rim materials, and a complex basement structural fabric including faults and valleys that are radial and concentric about the primary basin, as well as structurally-controlled local basins. Since its formation, the basin has been a regional catchment for volatiles and sedimentary materials as well as a dominant influence on the flow of surface ice, debris flows, and groundwater through and over its basement structures. The basin is interpreted to have been occupied by lakes, including a possible Mediterranean-sized sea that formed in the aftermath of the Argyre impact event The hypothesized lakes froze and diminished through time, though liquid water may have remained beneath the ice cover and sedimentation may have continued for some time. At its deepest, the main Argyre lake may have taken more than a hundred thousand years to freeze to the bottom even absent any heat source besides the Sun, but with impact-induced hydrothermal heat, geothermal heat flow due to long-lived radioactivities in early martian history, and concentration of solutes in sub-ice brine, liquid water may have persisted beneath thick ice for many millions of years. Existence of an ice-covered sea perhaps was long enough for life to originate and evolve with gradually colder and more hypersaline conditions. The Argyre rock materials, diverse in origin and emplacement mechanisms, have been modified by impact, magmatic, eolian, fluvial, lacustrine, glacial, periglacial, alluvial, colluvial, and tectonic processes. Post-impact adjustment of part of the impact-generated basement structural fabric such as concentric faults is apparent. Distinct basin-stratigraphic units are interpreted to be linked to large-scale geologic activity far from the basin, including growth of the Tharsis magmatic-tectonic complex and the growth into southern middle latitudes of south polar ice sheets. Along with the migration of surface and sub-surface volatiles towards the central part of the primaiy basin, the substantial difference in elevation with respect to the surrounding highlands and Tharsis and the Thaumasia highlands result in the trapping of atmospheric volatiles within the basin in the form of fog and regional or local precipitation, even today. In addition, the impact event caused long-term (millions of years) hydrothermal activity, as well as deep-seated basement structures that have tapped the internal heat of Mars, as conduits, for far greater time, possibly even today. This possibility is raised by the observation of putative open-system pingos and nearby gullies that occur in linear depressions with accompanying systems of faults and fractures. Long-term water and heat energy enrichment, complemented by the interaction of the nutrient-enriched primordial crustal and mantle materials favorable to life excavated to the surface and near-surface environs through the Argyre impact event, has not only resulted in distinct geomorphology, but also makes the Argyre basin a potential site of exceptional astrobiological significance. (C) 2015 Elsevier Inc. All rights reserved.

Proton entry into the near-lunar plasma wake for magnetic field aligned flow

We report the first observation of protons in the near-lunar (100-200 km from the surface) and deeper (near anti-subsolar point) plasma wake when the interplanetary magnetic field (IMF) and solar wind velocity (vsw) are parallel (aligned flow; angle between IMF and vsw≤10°). More than 98% of the observations during aligned flow condition showed the presence of protons in the wake. These observations are obtained by the Solar Wind Monitor sensor of the Sub-keV Atom Reflecting Analyser experiment on Chandrayaan-1. The observation cannot be explained by the conventional fluid models for aligned flow. Back tracing of the observed protons suggests that their source is the solar wind. The larger gyroradii of the wake protons compared to that of solar wind suggest that they were part of the tail of the solar wind velocity distribution function. Such protons could enter the wake due to their large gyroradii even when the flow is aligned to IMF. However, the wake boundary electric field may also play a role in the entry of the protons into the wake.