Mars has a methane mystery.
In June of 2019, NASA’s rover Curiosity detected the carbon-and-hydrogen gas in its largest amount yet: About 21 parts per billion units by volume (ppbv).
So, where is the methane coming from? The answer is either biological or geological – or both.
Methane could be the emissions of microbial life – an exciting sign of life on Mars – or could be created through interactions between rocks and the planet’s environmental factors, like water.
Researchers from Newcastle University, however, have ruled out one cause of Mars’ methane spikes: Wind erosion.
High-resolution imagery over the last decade has shown that winds on Mars drive much higher local rates of sand movement. The whipping sand can potentially wear down rocks and release whatever gases those rocks contain.
In a study published in Scientific Reports this month, the Newcastle team explored (and ultimately debunked) a theory that the levels of detected methane could be produced by the wind erosion of rocks, freeing methane from fluid inclusions and fractures on the planet's surface.
For wind erosion to be a viable mechanism to produce detectable methane in the Martian atmosphere, the study determined that the methane content of any gases trapped within rocks would have to rival those of some of the richest hydrocarbon-containing shales on Earth – a highly unlikely scenario, according to principal investigator Dr. Jon Telling, a geochemist based in the School of Natural and Environmental Sciences at Newcastle University.
“What’s important about this [research] is that it strengthens the argument that the methane must be coming from a different source. Whether or not that’s biological, we still don’t know,” said the lead researcher.
Telling spoke with Tech Briefs about his recent study, and where his team will go next to solve the mystery of methane on Mars.
Tech Briefs: Why is it important to discover the source of methane gas on Mars?
Dr. Jon Telling: On Earth, methane in the atmosphere primarily comes from life – the action of microscopic organisms called methanogenic Archaea. However, methane can also be formed by a variety of other non-biological mechanisms, such as hydrothermal activity.
It is now widely believed that, in the deep past, Mars had the right sort of conditions to support microscopic life on or near the surface, and potentially even today in the deep subsurface. Determining the source, or sources, of methane on Mars today could therefore help determine the likelihood that the methane is a signature of past or present life.
Tech Briefs: How were you able to determine that wind erosion did not cause emissions of methane?
Dr. Telling: In essence, this was very simple. We first conducted a literature survey of estimated present-day rates of wind erosion on the surface of Mars for different rocks with varying susceptibilities to erosion. We then coupled these to the potential concentrations of methane gas trapped within different typical rocks on the surface of Mars.
We based these on gas contents on actual Martian rocks (Martian meteorites that have landed on Earth) and terrestrial rocks that are analogues for rocks on Mars, such as terrestrial basalts and salts. Coupling erosion rates with rock gas contents gave us a range of estimates for the potential methane fluxes that could be released by present-day rates of wind erosion on Mars.
We then used some simple equations to estimate how fast any methane produced at the surface would move up (and be diluted into) into the Martian atmosphere. For these calculations, we used relevant timescales for previous measurements of methane in the Martian atmosphere, ranging from hours to a month.
Tech Briefs: What led you to this study?
Dr. Telling: For most of my research career so far I have been studying glaciers and ice sheets on Earth. As part of this, I have been looking at how gases are released from rocks and minerals by grinding under glaciers, and how mechanochemical reactions, during subglacial rock crushing, can influence water chemistry and help support life under them. The Mars aeolian research is really a crossover from this terrestrial glacier research.
Tech Briefs: What are some other potential theories for the methane that will be investigated next?
Dr. Telling: If the methane is not being generated in the Martian surface, then small, very localized seeps from the deeper subsurface may be a more likely source.
Tech Briefs: What are you working on now, as it relates to this study?
Dr. Telling: One key aspect we are focused on in our current UK Space Agency grant is to look in more detail at potential mechanisms of methane uptake or destruction. Recent analyses of methane in the Martian atmosphere by the ExoMars Trace Gas Orbiter reported earlier this year have been below detection (<0.05 parts per billion), bolstering previous assertions that there may be previously unrecognized mechanisms for the destruction or uptake of methane in the Martian atmosphere. We are currently following up on some previous results from a Danish group to see if mechanochemical reactions may play a role here. Mechanochemical reactions are chemical reactions catalyzed by mechanical grinding, effectively creating highly reactive ‘half bonds’ on freshly fractured mineral surfaces that could have the potential to react with gases in the atmosphere.
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