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sublimation of water ice in low pressure environments:isotopic effects and implications for the martian paleoclimate record

conclusions

An easily detectable 1xVSMOW (~20%) average deviation in the observed D/H ratio will be preserved for 38 billion years and the deuterium peak found in the stratigraphy of the polar cap ice will have a mean width of ~10m. This preservation figure should be viewed as a lower bound for near surface deposits since during low obliquity, the temperature of the polar cap is on average lower [9] than during high obliquity. However, deeper deposits, being somewhat warmer should age more rapidly.This preservation age when combined with the depth of the PLD suggests that it may be possible to preserve a very long record of climate change in the polar caps on Mars provided that vapor diffusion may be suppressed by compaction in the ice sheet. Here the D/H ratio may function analogously to the O18/O16 ratio in ice cores on the earth. This signal would be best detected by in-situ coring, however, since the PLDs are exposed by the polar troughs, it may be possible to detect the climate signal by performing a transect with a rover on the surface.

introduction

Experiments carried out [1] have shown that vacuum sublimation of water ice possesses two distinct complexities. One of these is a bulk isotopic effect which consists of a significant fractionation enhanced by high dust entrainment [2] in prepared samples. Since the polar regions of Mars are made up of dusty ices [3] and are thought to contain an isotopic record of past climate change locked in the ice [4], it is important to ask about the significance of this experimental work in the context of the putative Martian paleoclimate record. It can be shown that even climate changes consisting only of redistribution of ices by sublimation can leave a long lasting isotopic imprint if the amount of material moved is sufficiently great (meters of ice).

simulating martian ice

Samples have been prepared containing various mixtures of insoluble TiO2 dust grains (density = 4.23g/cc) with pure water having concentrations covering 0-50wt% of dust, a range consistent with the northern polar caps of Mars [3]. These dust grains were of similar size to martian dust particles (radius of 1.6 microns), as determined by the Imager for Mars Pathfinder [5] and consistent with atmospheric dust observed by Viking and MER [6]. Next, these ice-dust mixtures were been salted with D2O and flash frozen by pipetting into liquid nitrogen. This prevented the dust from being excluded from the ice matrix.

The resulting samples were then allowed to sublimate under a quartz tungsten halogen lamp set at a low current level to produce heating from above while the base of the sample was kept at 160K using a liquid helium cold finger. Analysis of the initial sublimate gas indicated that the temperature at the top of the sample was 202.88 K and temperature sensors imbedded in the chamber did not detect a significant change over the course of the experiment. During sublimation, the gas pressure was monitored by a capacitance manometer and the composition was recorded using a mass spectrometer.

The actively sublimating surface of the ice cap pictured here is composed of a mixture of ice and fine-grained airfall dust. This material was simulated.

experimental results for dusty ices

Two of our mass spectra are presented here. The first of these [below, top] illustrates the isotopic evolution of a crystalline sample containing no dust at all. In this case, the D/H ratio in the sublimate gas begins below the ratio in the bulk solid near the level predicted by the difference in vapor pressure between deuterated (HDO) and non-deuterated (H2O) water molecules. This preferential sublimation of H2O over HDO causes the surface concentration of HDO to increase resulting in a corresponding increase in the number of HDO molecules escaping the surface.

The classical next argument would be that once the sublimated gas achieves the same D/H ratio as the bulk solid we reach a state of equilibrium where HDO molecules are escaping from the actively sublimating surface as quickly as they are becoming available from the bulk by the advance of the sublimation front down through the sample. However, as can be seen from the figure we never reach a state of equilibrium, in fact by the end of the experiment the concentration of HDO in the sublimate gas has increased past the concentration in the bulk solid. The reason for this lies in the mechanics of solid state diffusion which at the temperature of interest takes place at a rate equivalent to or greater than the rate of advance of the sublimation front [1]. This allows the bulk solid to become enriched in HDO, a fact which eventually shows up in the sublimate gas emitted.

A second sample [Below, bottom] was prepared using 25wt% TiO2 dust. Unlike a clean sample, this type of sample will leave behind a refractory lag deposit as the sublimation front travels through the sample. This creates a barrier to the escape of molecules from the surface of the sample. This has two effects. First, the escape rate of all species is significantly decreased: from 2.0x10‑3 torr total pressure down to 1.5x10-4 torr after 35 days and 160m of sublimation was observed. Secondly since the lighter H2O molecules can diffuse faster though this layer then the heavier HDO molecules, the D/H ratio of the gas escaping the sample decreases as the lag layer increases in thickness. An additional effect comes from our method of creating the dusty sample by flash freezing. This produced a porous material analogous to an ice laid down by precipitation. In such a unit, heavier molecules, once activated by sublimation can diffuse rapidly and preferentially to the colder areas of a sample by simple vapor diffusion, thus depleting the amount of HDO available for the sublimate gas even further.

J.E. Moores , R.H. Brown, D.S. Lauretta and P.H. Smith

Lunar and Planetary Laboratory, 1629 E University Blvd, Tucson, Arizona, 85721 United States

implications for the NPLD

The dusty ice case is most applicable to the northern polar regions of Mars, in particular the Northern Polar Layered Deposits. Like our second sample, these deposits are thought to contain large quantities of airfall dust and also, like our sample, the icy component periodically receives sufficient insolation to cause it to sublimate. Thus, our results indicate that these deposits will also form dust lags which will both decrease their sublimate gas output and concentrate the heavier isotopic species of water in the remaining solid.

This relatively high D/H layer would be laid down at the interface between any dust lag and the residual ice. This enhancement over the course of 35 days (the length of our experimental run) totals 1.47x1021 molecules/m2 and represents 160m of sublimation. If we assume that the of order 10m spacing of the dark units in the PLD represents the typical amount of sublimated material over an obliquity cycle [7], we have a total enhancement of 150mol/m2 each obliquity cycle. This begs the question of how long this isotopic signal can be preserved once this enriched material has been buried and its vapor pathways closed. To answer this question we must consider again solid state diffusion of HDO in H2O. As a parallel we make use of an equation from semiconductor engineering governing diffusion of a fixed amount of doping agent into a semi-infinite substrate:

[12]

Where β is the amount of the D pulse (in mol/m2) diffusing. If we assume that the ice – now buried below the annual temperature wave with most vapor pathways filled in – remains initially close to the surface, it will have a typical temperature (at high obliquity) of 170K [10] and we may use a diffusion coefficient of HDO16 into H2O16 of 6.38x10-18m2/s [9]. Using these figures, the changing distribution of D with time is shown above.

A rover transect of layered deposits such as those pictured here could allow the story of past climate on Mars to be read

Volcanic Caldera like this one are the birthplace of the very fine material which makes heavy fractionation possible

background image credit: ESA/DLR/FU Berlin (G. Neukum) [11]

This oblique image illustrates several important aspects of this poster. Shown prominently is the layered terrain which may contain a record of climate change locked in D/H variations. Also visible is a sublimating water-ice surface rich in dust above calderas which may contain fine volcanic ash [11], the source of future dust slags within the polar layered deposits. Navigate this poster by reading boxes next to the relevant feature.

References: [1] Brown, R.H. et al (2006) Submitted, ApJ. [2] Moores, J.E. et al (2005) LPSC XXXVI abstract n°1973. [3] Clifford et al (2000) Icarus v144 pp210-242. [4] Saunders and Hecht (2002) AGUFM abstract n°P12A-0369 [5] Tomasko et al (1999) JGR vol 104-E4 p8987-9008 [6] Lemmon, M.T. (2004) Science 306 n°5702 pp.1753-1756 [7] Milkovich and Head (2005) JGR 110 E01005 [8] Glicksman, M.E. (2000) Diffusion in Solids. [9] Livingston et al (1998) J. Chem. Phys. 108 2197-2207. [10] Larsen and Dahl-Jensen (2000) Icarus 144 456-462. [11] Ice and Dust at Martian North Pole (2006) ESA Image ID n°SEMKO5D3M5E [12] Schaeffer et al (1999) The Science and Design of Engineering Materials 2nd Ed. P136.