Nakhlite meteorites are igneous rocks from Mars that reacted with water ~630 million years ago while still part of the Martian bedrock, during a time that is often suggested to have been a “dry” period on Mars. However, magmatism and meteorite impacts can trigger so called hydrothermal systems with warm fluids circulating in the bedrock, that on Earth are known to harbor microbial life. Thus, the question is: did the water originate from a large-scale hydrothermal system, or from something else, such as sub-surface ice? The latter, small amounts of ice, that were melted during a short time would not have been a good environment for life to thrive.

“Since water is central to the question of whether life ever existed on Mars, we wanted to investigate how much of the meteorite reacted with water when it was still part of the Martian bedrock. Neutron tomography happened to be the perfect method to do this”, explains Josefin Martell, PhD student in Geology at Lund University.

Having access to nondestructive methods is also of key interest related for the current NASA mission Mars 2020: The rover Perseverance parachuted onto the dusty surface of the planet in February 2021. The vehicle will gather samples over a number of years to try to find out if there has been microbial life on Mars. NASA will send the samples back to Earth around 2030 and hence the current study come timely to establish nondestructive characterization methods.

Figure 1. 3D visualization of the meteorite from Mars studied using high-resolution neutron tomography at the NeXT instrument of the Insitut Laue Langevin combined with laboratory X-ray imaging at Lund University. Olivine (based on x-ray volume) and hydrous constituents (based on neutron volume) have been segmented and combined into one dataset. (A) The sample ‘cut’ showing the surface on the lower half and the inside on the top half. (B) The interior regions with olivine and hydrous phases are highlighted by making other phases are transparent.

An international research team led by Lund University studied a nakhlite meteorite (Miller Range 03346) used nondestructive neutron and x-ray tomography, and in particular the NeXT instrument of the Insitut Laue Langevin (France) and a laboratory x-ray instrument at Lund University (Sweden) respectively. By combining the 3D data from both radiation sources, it was possible to non-destructively determine whether hydrous alteration is interconnected and pervasive. The results reveal confined clusters of hydrous phases within and surrounding a certain mineral, so-called olivine, with limited interconnectivity between the clusters. This implies that the fluids originated from the melting of local subsurface ice following an impact event, rather than from a large-scale hydrothermal system. Consequently, the fluids were likely not present for a long time, meaning that the martian crust sampled by the nakhlites was not a suitable environment to harbor any life during this time period.

The group will continue to explore how neutrons can be used in planetary science with follow-up studies on other samples. Locating hydrogen within sample is one obvious application using neutrons, but combined use of X-rays and neutrons also enable characterization of a whole suite of minerals within the rock samples that would be challenging otherwise.

The researchers are convinced that the results of their study will be helpful when NASA brings back the first samples from Mars, and that neutron and X-ray tomography will be used to study them. ESS can play a key role in investigating samples from Mars brought back to earth by NASA around 2030. At that time, ESS will be a leading neutron source that provides unique research opportunities. The possibility to perform X-ray tomography on the ESS neutron imaging beamline ODIN will be enabled thanks to a recently funded Röntgen-Ångström Cluster project.

This work was carried out by a team from Lund University, led by PhD student Josefin Martell and her supervisor Carl Alwmark, in collaboration with researchers at University of Glasgow, University of Copenhagen, University of Sydney, University of Oxford, Malmö University, the European Spallation Source and with support of beamline scientists at the Institut Laue-Langevin.

Cite:Science Advances 8.19 (2022): eabn3044.

For the original articles, please visit: https://www.science.org/doi/10.1126/sciadv.abn3044