Search

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.05.017 Byline: Denis Lacelle (a), David Fisher (b), Ian D. Clark (c), Alain Berinstain (a) Keywords: Mars; polar geology; Geological processes Abstract: In this study, various approaches that can potentially distinguish between vapor- and liquid-derived ground ice in the martian regolith (petrography, geochemistry, stable OH isotopes, CO.sub.2 O.sub.2 N.sub.2 Ar gas composition) are examined using terrestrial ground ice examples. Although the stable OH isotope composition ratios can distinguish between vapor- and liquid-derived terrestrial ground ice, there might be to much mixing between the various water reservoirs on Mars to effectively use it, and, like on Earth, petrographic and geochemical approaches need to be complemented with additional supporting evidences. Of the different approaches currently being employed to determine the origin of terrestrial massive ground ice and icy sediments, it is the concentration of CO.sub.2 and the O.sub.2/Ar, N.sub.2/Ar and N.sub.2/O.sub.2 ratios of air entrapped in the ice that has proven to be the less ambiguous and most discriminatory. This is because the molar ratios of atmospheric gases change during their dissolution in water due to differences in their relative solubilities, thus providing distinctive ratios for the dissolved gases. The gas composition of air entrapped in the ice not only distinguishes between vapor- and liquid-derived ground ice, but any deviation from the theoretical dissolved values can provide insights into potential physical and biological processes operating in the subsurface, a key component for astrobiology. Author Affiliation: (a) Planetary Exploration and Space Astronomy, Canadian Space Agency, 6767 route de l'aeroport, St-Hubert, QC, J3Y 8Y9, Canada (b) Glaciology Section, Geological Survey of Canada, NRCan, 601 Booth Street, Ottawa, ON, K1A 0E8, Canada (c) Ottawa-Carleton Geoscience Center, Department of Earth Sciences, University of Ottawa, 140 Louis Pasteur, Ottawa, ON, K1N 6N5, Canada Article History: Received 19 March 2008; Revised 22 May 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.10.032 Byline: David Andrew Fisher Keywords: Mars; Ices Abstract: A time varying stable isotope model for the D/H history of Mars water cycle is developed with variable atmosphere, space loss rate, ground and ice cap flux rates. It considers coupled ground reservoirs and traces D/H in the air and reservoirs secularly and over obliquity cycles. The various flux rates are prescribed time variables that simulate surface flux, and solar driven space loss rates. Predicted bulk averages for the ice cap, ground ice reservoirs and atmosphere span the observed ranges reported by Mumma et al. [Mumma, M.J., Novak, R.E., DiSanti, M.A., Bonev, B., Dello Russo, N., Magee-Sauer, K., 2003. The Martian Atmosphere. Conference Reports of 'Sixth International Conference on Mars Atmosphere,' No. 3186]. When the dominant obliquity cycle variations are scaled so that the model delivers present seasonal variations, the present long term bulk D/H average for the ice cap is [approximately equal to]+2.7 (equivalent to +1700a[degrees] in [delta](D) wrt SMOW). The obliquity driven D/H cycle in the ice cap's layers varies between 3 and 6. The smaller more accessible reservoirs have larger bulk averages with the smallest being able to reach D/H values over 9 within [approximately equal to]10.sup.5years. Small hypothetical solar activity driven variations in the escape rate to space and in the fractionation constant [Krasnopolsky, V.A., Feldman, P.D., 2001. Science 294, 1914-1917] for the escape process can produce a 'solar wiggle' whose D/H amplitude can reach 0.1 ([delta](D) amplitude of 100a[degrees]). Because of the temporal variability, a single modern measured atmospheric D/H ratio at a particular Ls cannot tell very much about the total water inventory of Mars. A bulk average for the Northern Ice Cap and better still a dated vertical profile of D/H from the ice cap would, however, go a long way towards illuminating the 'modern' water history of Mars. The age and stability of the Northern Ice Cap and the D/H history locked in the layering is discussed. An ice cap that is very young and exchanges its mass through the atmosphere often will necessarily have a large D/H. Author Affiliation: Glaciology Section, Geological Survey of Canada, NRCan, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Article History: Received 8 May 2006; Revised 22 September 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.07.024 Byline: David Andrew Fisher Keywords: Mars; Regolith; Ice; Temperature Abstract: This paper describes a 'simple standard' model of water transport through regolith that includes diffusive migration and phase changes driven by damped seasonal temperature waves. A hitherto unused first-order process is then added, that can produce ice densities much greater than those allowed by the initial dry porosity. Voids are produced in cooling icy regolith when tensile stresses exceed the cracking threshold ([approximately equal to]2MPa). These stresses build up through an interaction of thermal contraction and elastic-plastic response. When the cracks open up after tensile failure there is purely thermal void enhancement and subsequent reduction as the regolith warms again. When the cracks are open the porosity is increased and they partially fill with ice crystals. Thus the void reduction on warming cannot go back to the original zero point and the bulk density of ice is increased with each temperature cycle. The cracking and thermal adjustment happen at scales of meters to millimeters. The large cracks can occur in pure ice and/or homogeneous icy material and the smaller cracks are produced by rock cobbles, pebbles, and grains having a much smaller coefficient of thermal expansion than ice. Thus a hierarchy of cracks and voids forms each temperature cycle and augments the ice content. The process can take the upper few meters of a pore-saturated icy soil from 28% by mass ice content to 70% in 10 Ma. This mechanism and the seasonal temperature cycle can plausibly produce massive ice deposits in the upper few meters of Mars' high-latitude regolith by diffusion and also keep the massive-ice regolith effectively porous to water vapor transport. The obliquity cycle can produce tensile stresses nearing 2 MPa down to [approximately equal to]100m depth so even deeper cracking could be happening. Author Affiliation: Terrain Sciences Division, Geological Survey of Canada, 601 Booth St., Ottawa, Ontario, K1A 0E8, Canada Article History: Received 4 November 2004; Revised 29 June 2005

Mars Orbiter Camera (MOC) images of the whiter areas of the residual North Polar Cap (P. C. Thomas et al. 2000, Nature 404, 161-164) show a gentle hummocky pitted surface that has been popularly called 'cottage cheese' terrain. The pits are 1 or 2 m deep and tens of meters across. They are typically joined in roughly linear strings or long depressions and these features are referred to here as 'lineations.' The lineations tend to have one or occasionally two preferred directions. We have examined the MOC imagery for the North Cap and using high-resolution images that have good wide-angle context images were able to determine the lineation angles for 31 sites scattered over most of the ice cap. We propose a process that will produce linear features in the white areas, then relate the orientation of the lineations over much of the North Cap to these processes and the inferred ice flow direction. There is first-order agreement between the measured sign of the lineation angles and those predicted assuming ice flow. Higher accumulations and velocities are predicted in the catchment for ice that flows into Chasma Boreale. This comes from the indications that katabatic winds are concentrated in this catchment. Key Words: Mars; North Ice Cap; ice flow; oblquity cycle; lineations; image processing.

The surface of the northern martian ice cap is known to contain alternating white-dark scarp-trough features, and one method of their formation involves spatially coherent patterns of ice accumulation and ablation via sublimation. In this so-called 'accublation' model, the internal ice sheet layers submerge into the ice sheet across the accumulation zone and emerge to the ice surface across the ablation region. Here we report on an Antarctic analogy to the martian accublation model, which demonstrates that surface mass balance conditions actually affect internal ice-sheet structure as predicted by D. A. Fisher (2000, Icarus 144, 289-294). This analogy shows how it is possible for ancient ice to become exposed at the surface of an ice sheet, allowing the paleoclimate record stored within the ice to be measured without the need for deep ice coring.

A new model explains the flow of Mars's northern ice cap similar to terrestrial ice sheets. The northern ice cap of Mars is very thick. The model explains the origin and movement of the ice sheets. The spiral features exhibited by the scarp and trough complexes are investigated. The nature and causes for the structure of the reentrant valleys such as Chasma Boreale and the layered terrain are explained by the model. Scarps are used as standards in the discussion of accumulation and ablation.