Experimental results on isotopic fractionation of dusty deuterated water ice during sublimation

1. Experimental results on isotopic fractionation of dusty deuterated water ice during sublimation John E. Moores P.H. Smith, R.H. Brown, D.S. Lauretta, W.V. Boynton, and M.J. Drake LPSC, March 14th, 2008

2. Importance of Fractionation • Fractionation is a record of how water has moved between different reservoirs • Generally, the more water has been removed from a particular location, the higher the expected Deuterium Concentration • Many important estimates of the age, source material and history of water reservoirs in the Solar System are linked to D/H ratios • i.e. Comets which may represent early solar system material and a source for the water of the terrestrial planets • i.e. Mars, where the history of water speaks to the evolution of the Terrestrial Planets • Complicating factors • It is important to have some idea of the D/H ratio of the initial water reservoir • It is important to know the mechanism by which the water is moved

3. Sublimation Fractionation • Main Process active on these bodies in the geologic recent past is sublimation • Can sublimation cause an important fractionation in these reservoirs? • Two factors affect the surface concentration of HDO: • Sublimation • removes material from the surface, preferentially H2O • HDO builds up due to KIE • Solid State Diffusion • Moves the HDO that builds up away from the surface • Attempts to Homogenize the Sample • Depending on the conditions of sublimation ice will fractionate differently. • i.e. Sublimation into Vacuum vs. into an Enclosed Space • Whether or not a regolith is present

4. Three Regimes(solid ice particles) • Rayleigh Fractionation(Solid State Diffusion Wins)(Well Mixed at all times) • Static Lattice(Sublimation Wins)(No movement of molecules) • Dynamic Lattice(Both Effects are Competitive)(Brown et al. 2008)

5. Experimental Apparatus • Which applies to dusty ice? • Cometary Sublimation Fractionation Apparatus: medium volume cryostat (150cc) • Porous ice mixed with dust subliming into vacuum • Basal temperatures ranging from 60K to 210K • Designed to simulate a cometary environment • 5-10cm thick stack • D/HBULK = 5%

6. Mixed Dust and Ice: Observations • Begin without dust • With 1.5 micron TiO2 at 25wt%

7. Mixed Dust and Ice: Observations • More realistic Albedo – JSC Mars-1, 1-10 micron particles obtained by crushing and settling in water 1wt% 3wt% 6wt%

8. Mixed Dust and Ice: Observations • INCREASING DUST CONTENT 9wt% 25wt%

9. How to Explain Sample Behavior • None of the expected mechanisms producesthe observed profiles • All mechanisms produce an increasing D/H ratio in the sublimate gas with time. • What phenomena can potentially produce a declining D/H ratio with time? • Migration of the heavier isotope within the sample • Due to the temperature gradient, it is very difficult to get material to migrate in the sample ~1cm Maximum in actual samples • Migrating material tends to be isotopically light • Gas/Dust and Ice/Dust interactions • Gas/Dust interactions would increase as more dry overburden is exposed

10. What does this mean for Sublimating Bodies? • Comets: • If there is no circulation within the comet, the nucleus should be up to 2.5 times more enriched in HDO then the coma • If there is circulation, the coma may not be representative of the bulk nucleus • Mars: • Ice laid down by precipitation or condensation can not be considered well-mixed • Current atmospheric inventory is dependant on the last sublimated gas • An understanding of the circulation history is required to interpret the D/H ratio

11. Experimental Apparatus • Martian Sublimation Fractionation Apparatus (TUAQ apparatus): large volume cryostat (2000cc) • Solid ice overlain by regolith with different grain sizes, sublimating into 6.1mBar of CO2 • Basal temperatures ranging from 175K to 235K • Designed to simulate the northern polar cap of Mars • Relatively thin ice deposit (3cm)

12. TUAQ setup: Gas/Dust Interactions • TYPICAL RUN: confined grain size distribution of JSC-1 dust in overburden (scale in Hours at a basal temp of -40°C with 1cm of dry overburden of 0.55 to 1mm particles) • For most runs, the change in D/H is broadly consistent with KIE • This may not be as clear-cut at lower temperatures and smaller grain sizes of dust

13. Summary • Sublimation does cause fractionation when ice is mixed directly with dust • The effect of dust mixed with ice is significant but not well understood • Deuterium can be more concentrated in the samples then is explainable by KIE for dusty runs • Interactions with the dust are the likely culprit • Direct Sampling of the solid reservoir is required to unambiguously determine the degree of fractionation • Different histories of circulation between different reservoirs can produce different values for the sublimate gas

14. Thank-you This work funded in part by: Lunar and Planetary Laboratory National Aeronautics and Space Administration National Science and Engineering Research Council of Canada Phoenix Mars Mission University of Arizona