18 December 2018
Mars’ thin atmosphere is mostly carbon dioxide, but something is mysteriously ‘burping’ methane during the Martian spring and summer.
Atmospheric methane as detected during Mars' northern summer. Yellow-red areas show higher concentrations and blue-purple lower.
Scientists using Diamond Light Source have been investigating a form of ice, called a clathrate, to see what it can tell us about the history of water on Mars and how it may have helped to sculpt the Martian surface. The ice could also provide an explanation for the mysterious spikes in methane seen seasonally in the planet’s atmosphere.
The Martian atmosphere, although incredibly thin compared to Earth’s, is made up almost entirely of carbon dioxide, but it does contain traces of other gases. Of particular interest is methane, which on Earth is generally produced as a by-product of geological or biological (life) processes. Mars’ atmosphere contains relatively little methane (averaging 0.5 parts per billion (ppb), compared to Earth’s 1,875 ppb) but the fact it is there at all is a bit of a mystery.
This is because methane doesn’t last long in the atmosphere, so its presence suggests that some process is generating, or releasing, fresh supplies of the gas. Weirder still, there seems to be a seasonal rise in the levels of methane emitted from Mars’ northern hemisphere during the spring and summer, along with random localised spikes – something is ‘burping’ methane into the atmosphere. Given that Mars is thought to be geologically inactive and we have yet to find evidence for life, it begs the question: where is the methane coming from? One answer is that it is being stored away deep underground and being periodically released into the atmosphere, but what might be causing the seasonal timing of the releases remains unknown.
One proposed candidate is a type of ice known as a clathrate. Clathrates are a naturally-occurring ice-like substance that is found on Earth and is thought to be present on Mars and on other planetary bodies in the Solar System. Like ice, clathrates are water-based solids but instead of being made up of water molecules all the way through, clathrates are made up of a cage-like structure of water molecules that form around a trapped gas molecule. If clathrates are locking away large amounts of methane in their icy traps and something is causing their structure to collapse and release the gas, this may help explain where it is coming from.
A colour-enhanced view inside Mars’ Newton Crater, showing evidence for flowing liquid water on the planet’s surface today.
A group of researchers led by Professor Aneurin Evans (Keele University) and Dr Stephen Thompson (Diamond Light Source) have used X-ray diffraction techniques at Diamond Light Source to examine these clathrates. They recreated the pressure and temperature conditions found in the Martian cyrosphere – a region of frozen water ice that extends as deep as 22 km beneath the surface at the planet’s poles (and about 9 km at the equator) – and then used X-ray diffraction to see how clathrate structures form and collapse (known as dissociation) under a range of temperatures. They then examined how adding salts to the clathrates affected these processes.
Mars, it seems, has no shortage of ice, but this water ice is unlikely to be pure and probably contains dissolved salts, such as sodium, magnesium, or calcium salt. Although pure water ice clathrates have been studied before, none of these studies have looked at how dissolved salts affect the stability and dissociation of clathrates. When the team examined clathrates formed in the presence of these dissolved salts, they found that those formed from frozen salt solutions were more unstable (and prone to collapse) than those formed in pure water ice and that those whose ices contained magnesium salts were the least stable of all.
Despite the salty clathrates instability, the team were able to confirm they are likely to exist in the conditions found within the Martian cryosphere. The researchers suggest that, if these salts are not evenly distributed across Mars, it may be that there are regions where clathrates have formed that contain more of one salt or another. Although the team’s experiments focused on clathrates where carbon dioxide was the entrapped gas, on Mars methane clathrates likely exist either as small localised deposits within the more abundant carbon dioxide clathrates, or as regions where the clathrates contain a mixture of the two gases. In regions where magnesium salts prevail, the clathrates might be more unstable and prone to collapse when conditions warm in the Martian spring and summer – explaining the atmosphere’s seasonal methane increases and the variation in locations of the releases.
This process might also help to explain some of Mars’ surface features, which seem to have been sculpted by flows of liquid water. This is because the water molecules in the clathrate’s structure are supported by the gas molecule within it and, when they lose that support, the clathrate collapses – releasing the not just the gas but also, potentially, a flood of liquid water.
Last updated: 25 February 2019