It's important to understand the composition and structure of the rocks found on Mars, both to understand the planet's history, and to find out if it is possible to support humans on extended trips there. One type of rock found on Mars consists of hydrated sulfate minerals, which contain water as part of their structure. Knowing which forms of these minerals are present indicates how much water could be extracted from them if needed.
However, studying these minerals is difficult from 155 million kilometres away. Both the Perseverance rover, which landed on Mars in 2021, and the Rosalind Franklin rover, due to land in 2023, will be able to collect surface rocks and analyse them.
Both rovers will have the capability to carry out Raman spectroscopy, which studies the vibrational motions of materials to characterise them. Every mineral has a unique set of vibrations that is very much like a fingerprint, allowing them to be identified. To be able to do this, however, one first has to find out what the Raman fingerprint of a mineral looks like.
In this study, researchers, including ISIS Facility Development student Johannes Meusburger, focussed on the mineral rozenite, FeSO4·4H2O, which is considered to be a very likely component of martian soil. There is a strong interest in finding rozenite on the martian surface, since on Earth this mineral typically occurs in acidic waters that are thought to be similar to the environments that likely existed billions of years ago when Mars was a wet planet with lakes and rivers on its surface. On Earth, such acid lakes often support thriving microbial communities, making sediments from these early martian waters prime targets in the search for extra-terrestrial life.
To enable the identification of rozenite on the martian surface, Johannes and his colleagues measured the structure and Raman fingerprint of the material using both Raman spectroscopy and neutron diffraction on HRPD under conditions similar to those that would be present on Mars.
Because of the unique interaction of neutrons, the team was able to identify the positions of the hydrogen atoms in the structure, something which had not been done accurately before. They found that the mineral took the same structure across the whole temperature range studied, rather than transforming to other structures as was previously suggested. This was confirmed by accurate computer modelling using STFC's SCARF high performance computing cluster.
Being able to get very accurate Raman fingerprints by combining experimental data and theoretical calculations will be very useful to interpret the data sent back from the rovers and identify extinct lakes and rivers from a time before Mars turned into the dry and inhospitable planet as we know it today.
The article can be found at DOI: 10.2138/am-2022-8502. Currently online as a preprint.