IWF : Martian Water-Ice Reservoirs



Forschung > Sonnensystem > Planeten > Mars > Mars-Forschung > Mars-Atmosphäre > Atmosphärenflucht > Loss of Water from Mars > Martian Water-Ice Reservoirs

Martian Water-Ice Reservoirs

The discovery of high concentrations of water-ice just below the Martian surface polar areas made by Mars Odyssey has strengthened the debate about the search for life on Mars. Generally it is believed that life on Earth emerged in liquid water from the processing of organic molecules. Thus, the possible origin of life on early Mars should have been related to the evolution of the planetary water inventory (, consequently it is important to know the amount of water-ice stored below the planetary surface. The search and mapping of the present subsurface water and ice reservoirs will be carried out experimentally by Mars Express with its Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) ground penetrating radar in the near future. We estimate the present and past water-ice reservoirs, which are - and were - in exchange with the atmosphere by using the observed D/H ratio in the atmospheric water vapour, measured D/H ratios in Martian SNC meteorites and D/H isotope ratios based on an actual study, regarding asteroid and cometary water delivery to early Mars. By using the results of this study with initial D/H ratios of about 1.2 to 1.6 times the terrestrial sea water (TSW) ratio and the assumption that these ratios were not fractionated by XUV driven hydrodynamic escape due to a more active young Sun before 3.5 Gyr, one gets a present water-ice reservoir, which can exchange with the Martian atmosphere, equivalent to a global ocean layer with a thickness of about 3.3 to 15 m.

: Fig.: Estimated minimal (MIN) and maximal (MAX) present Martian surface water-ice reservoir equivalent to a global ocean with a layer thickness s (dotted lines) which is in isotopic exchange with the atmosphere as a function of total water loss from Mars over the past 3.5 Gyr with various initial D/H isotope ratios. The solid line has an initial D/H ratio equal to the TSW value. The dashed-dotted and dashed lines have D/H ratios of 1.2 times and 1.6 times the TSW value and the dashed-dotted-dotted lines correspond to the average D/H value measured in Martian Shergottite meteorites of about 2.3 times the TSW value which is comparable to the D/H ratio in comets.

By assuming that hydrodynamic escape fractionated the D/H ratio to a value which is stored in the Martian SNC meteorites with a measured average enrichment of about 2.3 times the TSW ratio or cometary value we estimate a present water-ice reservoir equivalent to a global layer with a thickness of about 11 to 27 m. From the obtained range of the estimated present water-ice deposit, we estimate a water-ice reservoir exchangeable with the atmosphere on Mars 3.5 Gyr ago equivalent to a global ocean with a thickness of about 17 to 61 m. All the estimated reservoirs depend on the escape of water from Mars since 3.5 Gyr ago equivalent to a global ocean with a thickness of about 14 m (minimum) or 34 m (maximum). The main uncertainties in the estimation between the minimal and maximal water-ice reservoir is related to the present uncertainties of the efficiency of atmospheric escape rates triggered by plasma instabilities and momentum transfer effects between the solar wind and the ionosphere. However, these uncertainties will be reduced in the near future, since both loss processes will be studied in detail by the Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA-3) on board of Mars Express. The obtained results combined with the discovery of the present water-ice subsurface reservoirs by the MARSIS radar and isotope studies as presented in this work, will also give us an idea how enriched the atmosphere was in D compared to H after the heavy bombardment corresponding to a better understanding of the efficiency of the hydrodynamic escape process due to the young Sun.

Letzte Änderung: 16.11.2007