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Motivation

EMC/ERH-properties of thin layers of martian analogue soils – First results A. Lorek German Aerospace Center (DLR) (andreas.lorek@dlr.de / Fax: +49 30 67055 303 / Phone: +49 30 67055 390). Motivation.

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Motivation

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  1. EMC/ERH-properties of thin layers of martiananalogue soils – First resultsA. LorekGerman Aerospace Center (DLR)(andreas.lorek@dlr.de / Fax: +49 30 67055 303 / Phone: +49 30 67055 390)

  2. Motivation The water content of soils significantly influences their chemical, physical and biological properties. • [Rivkina et al., 2000] has shown that live its possible below 0°C and at low liquid water contends ( > 2 mono layer of water -- 1 mono layer = 0,3nm thickness) • Chemical experiments at DLR have shown that the Photo-Fenton-Reaction is possible at low water contents (corresponding to a few - n > 2 – mono layers). • Liquid water influences physical properties of soils • Rheology • Electric- and heat conductivity • Dielectric and related spectral properties gullies in a crater wall (at 168.2°W, 37.4°S) (MOC M15-01466) Photo NASA

  3. Mars analogue soil Bentonite (Clay with a high content of montmorillonite) • Montmorillonite was detected on the basis of spectrometer data from Mars Express [Poulet et al., 2005] • Is distinguished as a promising habitat for earlier or current live [Chevrier; Mathe, 2006] • specific surface 50,3 m²/g, absolute density 2,38 g/cm³ (Federal Institute for Materials Research and Testing(BAM)) JSC MARS-1 (Volcanic material from Mauna Kea Vulcan (Hawaii)) • Its spectra is like the Olympus-Amazonis Region of Mars [Allen et al., 1998] • Specific surface 146 m²/g, absolute density 2,64 g/cm³ (BAM) The upper millimeter of the martian surface is of particular interest • It interacts directly with the diurnally varying atmospheric humidity, which can reach saturation during the night and early morning hours • Adsorption/desorption of water in the soil and freezing of water can be a consequence • Could contain several mono layers of liquid water

  4. Equilibrium moisture content EMC/ERH water activity [aw] • Water content in the upper soil layers depends on the relative humidity of the atmosphere • Equilibrium relative humidity (ERH) is related to an equilibrium moisture content (EMC) • An increase of relative humidity leads to an increase of the soil moisture content • The properties of the soil (structure, specific surface, free charges, van der Waals-forces) determine the EMC saturation vapor pressure p0 [Pa] Equilibrium relative humidity Desorption isotherms [BAM]

  5. Theory of the liquid water content in soils (Möhlmann`s formula) • Möhlmann’s formula establish a relationship between ERH and adsorbed (liquid) water content • Theory (Möhlmann) based on Hamaker-Theory of van der Waals interactions (planar surfaces) in combination with the saturation water vapor formula • Hamaker- constant (10-18J to 10-21J ), specific surface and r.h. must be known

  6. Atmospheric humidity and soil moisture on Mars • It is expected that the water content in upper soil is in a range from 2 to 10%wt in mid- and low latitudes • Relative humidity ranges between 100 %r.h. in night - and early morning hours and near 0%r.h. about noon • During 100% r.h. the first millimeter can adsorb a lot of mono layers water • Atmospheric humidity at mid- and low latitudes is in the range of - 80° to -100°C Td (0,12Pa to 0,004 Pa) • There is only a weak increase of soil moisture with decreasing temperature • Sorption isotherm at 20°C corresponds nearly to an isotherm at lower temperatures Feldman, Los Alamos National Laboratory, 2003 Desorption isothermes of Montmorillonite at -16°C, 2°C, 20°C u. 30°C based on data from Jähnchen, Bish, Möhlmann, Stach “Investigation of the water sorption properties of Mars-relevant micro- and mesoporous minerals”, Icarus, 2006 Viking 2 at 47.97° N and 225.74° W (Foto NASA 18 Mai 1979)

  7. Dielectric test plate capacitor (10 Hz - 1MHz) • Measurement system for detection of liquid water content in soils • Distance between capacitor plates is 1,27mm (0.82mm soil, 0.45mm air gap) • Gas in- and output • Possible to evacuate the capacitor • Maximum gas flow through the capacitor is 15 NL/h • Soil adsorb/desorb water from gas flow • Water significantly influence the DK of soil • If formation of ice occurs, the DK decreases to 3,25 at frequencies about 100KHz

  8. Soil moisture measurement at JSC Mars 1 10 Hz 90 Hz 1 KHz 5 KHz 10 KHz 100 KHz 1 MHz • Measurements at 25°C and 1013 hPa for frequencies (10Hz, 90Hz, 1KHz, 5KHz, 10KHz, 100 KHz and 1MHz) • Soil probes were baked out at 105°C over 24h • Results • Considerable and repeatable dependence of the DK on soil moisture content • 3,8 %wt correspond to approximately 1 mono layer of water

  9. Liquid water content in JSC Mars 1 and Bentonitein a temperature range from +25°C to -70°C • JSC Mars 1 shows indication of freezing at -35°C (9,4 %wt curve) • 7,6 %wt curve (close of the 9,4 %wt curve) indicates that, down to -35°C, 2 mono layers are liquid • 6 %wt curve shows no indication of freezing approximately 1,5 mono layers are liquid down to -70°C • 9,4%wt curve for Bentonite shows indication of freezing at -15°C • 7,8%wt curve has a nearly linear decrease over the full temperature range: No freezing is observable • 4,8 %wt curve shows the same behavior: Approximately 3 mono layers are in the liquid state down to -70°C

  10. Diurnal temperature variation (-70°C to +5°C), Gas flow 15Nl/h (air), constant humidity of -80°C Td, pressure1013 hPa, Time period 28 days (Equilibrium not yet reached) • Result • 2,8 %wt soil moisture content at the end of the measurement (0.75 mono layer)

  11. Conclusions • Measurements (under normal pressure at JSC Mars 1) show that adsorbed water in the simulated martian soils is present with contents not less than 3%wt (1 mono layer) • First Measurements lead to the assumption (JSC Mars 1) that, down to -35°C, more than 2 mono layers (8 %wt) are in a liquid state and more than 1 mono layer is liquid at -70°C • A liquid water content of 2 mono layer at -35°C would corresponds to a Hamaker-constant of 3,5x10-19J (Möhlmann’s - formula) • Chemical reactions and live processes can be supported by liquid water below 0°C Future prospects • Further measurements under normal pressure and at temperatures below 0°C • Measurements under martian conditions (CO2 atmosphere, 600 Pa) • Use of a dielectric measurement system (TDR) to confirm and to extend the given results

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