Experimental and theoretical simulation of sublimating dusty water ice with implications for D/H ratios of water ice on Comets and Mars

John E Moores,Peter H Smith,Dante S Lauretta,Robert H Brown

doi:10.1186/2191-2521-1-2

John E Moores, Peter H Smith + Show 2 more

Open Access

https://doi.org/10.1186/2191-2521-1-2

Copy DOI

Abstract

Sublimation experiments have been carried out to determine the effect of the mineral dust content of porous ices on the isotopic composition of the sublimate gas over medium (days to weeks) timescales. Whenever mineral dust of any kind was present, the D/H ratio of the sublimated gas was seen to decrease with time from the bulk ratio. Fractionations of up to 2.5 were observed for dust mixing ratios of 9 wt% and higher of JSC MARS-1 regolith simulant 1-10 μm crushed and sieved fraction. These favored the presence of the light isotope, H2O, in the gas phase. The more dust was added to the mixture, the more pronounced was this effect. Theoretical modeling of gas migration within the porous samples and adsorption on the excavated dust grains was undertaken to explain the results. Adsorption onto the dust grains is able to explain the low D/H ratios in the sublimate gas if adsorption favors retention of HDO over H2O. This leads to significant isotopic enrichment of HDO on the dust over time and depletion in the amount of HDO escaping the system as sublimate gas. This effect is significant for planetary bodies on which water moves mainly through the gas phase and a significant surface reservoir of dust may be found, such as on Comets and Mars. For each of these, inferences about the bulk water D/H ratio as inferred from gas phase measurements needs to be reassessed in light of the volatile cycling history of each body. 98.80.Ft [Isotopes, abundances and evolution (astronomy)], 64.70.Hz [Sublimation], 68.43.-h [Adsorption at solid surfaces]

Highlights

Motivations for simulating HDO The deuterium to hydrogen ratio (D/H) of water is often used to infer the evolution of water inventories on planetary bodies [for instance, [1,2]]
The fractionation factor is the ratio between the bulk D/H ratio of the original solid and the observed minimum gas D/H ratio
Sublimation can itself bring about fractionation in surface reservoirs, as our results show, and much of the highly enriched material will remain close to the surface, likely adsorbed onto dust

Summary

Introduction

Motivations for simulating HDO The deuterium to hydrogen ratio (D/H) of water is often used to infer the evolution of water inventories on planetary bodies [for instance, [1,2]]. There is a high potential for large mass-dependant fractionations upon dissociation as D and H have the largest mass difference of any isotope pair, with D being twice as massive as H. This must be considered along with the tendency of terrestrial planetary atmospheres to evolve towards more oxidated states as hydrogen is lost to space through Jeans escape from the atmospheric exobase [2]. If the water inventory of the planet is exchangeable with the atmosphere, the signature of this missing hydrogen will be partly preserved in the remaining water reservoir as a large enrichment in D/H

Methods

Results

Discussion

Conclusion