The D/H Ratio of the Martian Water That Formed the Yellowknife Bay Mudstone Rocks Measured By the MSL-SAM Instrument
Abstract:Martian atmospheric loss processes change the isotopic composition of H, C, O, N, and Ar. Escape process to space enrich heavier isotopes in the atmosphere over geological time. The SAM instrument on the Curiosity rover has measured these isotopes in H2O, CO2, N2, and Ar multiple times over the course of the mission. In addition to this present record, volatiles extracted from ancient rocks such as the clays sampled in Yellowknife Bay mudstones of Gale crater may be able to reveal the isotopic composition of light elements much earlier in martian history.
Small samples of mudstone rocks acquired by the Curiosity drill were analyzed by SAM for their volatile content. The SAM Tunable Laser Spectrometer (TLS) measured the D/H ratio in water and the Quadrupole Mass Spectrometer (QMS) determined the D/H ratio in hydrogen released from these samples by stepwise heating.
Clay materials on Earth are known to contain water in several forms. Molecular water bound loosely in pore spaces may be removed by drying under ambient conditions. Water also may be adsorbed on the surface of clay mineral surfaces or reside in interlayer positions or within structural channels in the clays. Finally, clay minerals contain hydroxyl units bound structurally in the minerals that at elevated temperatures (>~ 450°C) produce water and hydrogen of dehydroxylation; this high-temperature component is most likely to reflect the isotopic composition of water (and atmosphere) present at the time of clay formation.
We have designed and implemented a stepped extraction protocol to more precisely measure the D/H ratio in both the low and high temperature water in martian clays. The D/H in the high temperature water reflects the D/H of the water that formed these clay minerals. We will discuss the implications for water loss over the past 3+Ga and compare our findings with those of related martian meteorite studies.
This research was supported by the National Aeronautics and Space Administration.