MARSVOLATILES · Investigations into Martian Volatiles

Investigations into Martian Volatiles

The aim of this research was to determine how the volatile element content of the interior, surface and atmosphere of the planet Mars has changed over geological time. This aim was achieved via sub-microscopic mineralogical analyses, and measurements of volatile element isotopic ratios, in Martian meteorites with different geological ages. As a recent Martian meteorite fall, Tissint is ideal for studying the different water reservoirs on Mars, which can be distinguished by their contrasting hydrogen isotope (D/H) ratios. The Martian atmosphere has high D/H ratios, whereas the Martian mantle is reported to have low D/H ratios similar to Earth. I analysed the water content and D/H ratios of various Tissint mineral types using the Cameca 1280 ion microprobe at the University of Hawaii. The mineral types were identified using the transmission electron microscope (TEM) at the University of Glasgow. My research suggests that the nominally anhydrous mineral olivine has an unusually high water content in Tissint, suggesting this meteorite has experienced alteration on the surface of Mars. Tissint is a member of the youngest group of Martian meteorites, the shergottites (<500 million years old), meaning it crystallised during the current dry period in Mars’ history. Therefore, the discovery of alteration via fluid processes is surprising, and may suggest Mars is not currently as dry beneath the surface as we previously believed. The complex effects of the high shock pressures experienced by Tissint (during the impact event that blasted it from the Martian surface to Earth) can give new insights into how these high impact pressures affect the hydrogen isotopic composition and water content of different minerals. I used the transmission electron microscope at the University of Glasgow to determine the magnitude of pressure that each mineral had experienced. My research suggests that Tissint minerals formed via high impact pressures contain high D/H ratios, indicating significant implantation of hydrogen from the Martian atmosphere during the impact event. However, in low pressure minerals, such as olivine, which have not been structurally altered by high shock pressures, the hydrogen isotope ratios remain low. These low ratios are characteristic of the Martian mantle, and so indicate that the water in olivine was present before the impact event (e.g., via alteration on the surface of Mars). The above research results relating to the Tissint Martian meteorite have been submitted for publication in Geochimica et Cosmochimca Acta. Another avenue of study involved Martian meteorite Dhofar 019. Found in the Oman desert in 2000, Dhofar 019 was exposed to a hot desert environment for an unknown period of time, and has experienced significant weathering whilst on Earth. The aim of this study was to determine whether any of the alteration minerals in Dhofar 019 have a pre-terrestrial origin. We found that the alteration minerals had a very different composition to those found on Mars and in other Martian meteorites (e.g., in the nakhlite group of meteorites, or ALH84001). Hence, in contrast to previous publications, we suggest all alteration in Dhofar 019 is terrestrial. This research has been submitted for publication in Meteoritics and Planetary Science. During 2015 I visited the NanoSIMS facility at the Open University to study the effect of degassing on Martian meteorite MIL 090136. This nakhlite-type meteorite is special because it contains the hydrous mineral apatite both within olivine-hosted melt inclusions (which crystal early during the rocks solidification from lava) and within the matrix (which is one of the last areas of the meteorite to crystallise). Differences in D/H ratio between the early and late crystallising apatite can give an indication of the amount of water the lava lost as steam before it completely solidified. This true value can then be used to infer the abundance of water in the parent magma of this lava, and ultimately can indicate the amount of water present in the Martian mantle. Data processing is currently underway in anticipation of publication by the end of the year. Investigations of carbon-rich inclusions in Martian meteorites have resulted in the identification of several target inclusions in Tissint and Dhofar 019 (both shergottites, <500 million years old), Nakhla and Lafayette (nakhlites, 1.3 billion years old), Chassigny (chassignite, 1.3 billion years old) and NWA 7034 (ungrouped, ~2 billion years old). The presence of these inclusions in meteorites of all ages suggests the source of carbon is not being depleted at the surface over time, and that it is ubiquitous all over the Martian globe. The source of carbon could be from space (e.g., from carbon-rich meteorites bombarding the Martian surface over time), or from the Martian mantle. Study of the composition of these inclusions is ongoing, via the fourier transform infra-red spectroscopy and X-ray microscopy facilities at the Diamond lightsource synchrotron, Harwell, UK. In addition to producing my own volatile element data from Martian meteorites, I have produced a database of all previously published hydrogen isotope and water content data for Martian meteorites. This database highlights the relationship between the separate water reservoirs on Mars (atmosphere vs. mantle), and shows how the Martian atmosphere has evolved over geological time. This database is part of an invited review paper entitled ‘The D/H ratio of the inner Solar System’, which has been submitted for publication in Philosophical Transactions A. As well as the above main focuses of research, I have been involved with several other Martian volatile-element related projects, and am listed as a co-author on three related publications. I have also written a first author paper not directly relating to this project, but still on the theme of volatile elements in planetary bodies (Hallis et al., 2015 Evidence for primordial water in Earth’s deep mantle Science). My research into hydrogen and carbon abundances of Mars is highly relevant to current and future missions searching for evidence of life (e.g., NASA Mars Science Laboratory and Mars 2020, ESA ExoMars). Information relating to the hydrogen and carbon cycles is also valuable for possible future human exploration.

Data: CORDIS, © European Union