Day: May 20, 2024

Curiosity, NASA’s Mars Science Laboratory (MSL) rover, arrived on Mars in 2012 – and helped kick-off Space Sunday in this blog. Since then, the mission has been a resounding success; even now the rover continues climbing the flank of “Mount Sharp” (officially designated Aeolis Mons), the 5km high mound of sedimentary and other material towards the centre of Gale Crater where it landed, revealing more and more of the planet’s secrets.

However, there has been one long-running mystery about Curiosity’s findings as it has traversed Gale Crater and climbed “Mount Sharp”. As it has been exploring, the rover has at times been sensing methane in the immediate atmosphere around it. Methane can be produced by both organic (life-related) and inorganic means – so understanding its origins is an important area of study. Unfortunately, Curiosity is ill-equipped to easily detect and investigate potential sources of the gas; that’s more a job for its sibling, Perseverance. As such, the overall cause of the methane Curiosity has detected remains a mystery.

And it is a mystery compounded in several ways. For example: the methane often only seems to “come out” at night; the amount being detected seems to fluctuate with the seasons, suggesting it might be linked to the local environmental changes; but then, and for no apparent reason, Curiosity can sometimes sniff it in concentrations up to 40 times greater than it had a short time before – or after. A further mystery is that whilst Curiosity detects methane in the atmosphere around it, it is the only vehicle on Mars to thus far do so to any significant extent.

Further, the European Space Agency’s (ESA) ExoMars Trace Gas Orbiter (TGO), a vehicle specifically designed to sniff out trace gases like methane throughout the Martian atmosphere has, since 2018 when it started operations, almost totally failed to do so. All of which suggests that whatever Curiosity is encountering is unique to the environment of Gale Crater – and possibly to “Mount Sharp” itself.

Given this, scientists have been trying to determine the source of methane, but so far, they haven’t come up with a specific answer. However, current thinking is that it has something to do with subsurface geological processes involving water – with one avenue of research suggesting that it is curiosity itself that is in part responsible for its release, particularly when it comes to the sudden bursts of methane it detects.

A recent study by planetary scientist at NASA’s Goddard Space Flight Centre has demonstrated that any methane within Gale Crater, whether produced by organic or inorganic means, might actually be following the path outlined in the diagram above – but is getting trapped within the regolith by salt deposits before it can ever be outgassed. However, this was not the original intent of the study, which first started in 2017.

At that time, a team of researchers at NASA’s Goddard Research Centre led by Alexander Pavlov, were investigating whether or not bacteria could survive in an analogue of the kind of regolith Curiosity has encountered across Gale Crater and within environmental conditions the rover has recorded. Their results were inclusive in terms of organic survivability, but they did find that the processes thought to be at work within Gale Crater could lead to the formation of solidified salty lumps within their analogue of Martian regolith.

And there the matter might have rested, but for a report Pavlov read in 2019, as he noted in discussing the results of his team’s more recent work.

We didn’t think much of it at the moment. But then MSL Curiosity detected unexplained bursts of methane on Mars in 2019. That’s when it clicked in my mind. We began testing conditions that could form the hardened salt seals and then break them open to see what might happen.

– Alexander Pavlov, Planetary Scientist, NASA Goddard Research Centre

As a result, Pavlov and his team went back to their work, looking at the nature of the sedimentary layers of “Mount Sharp”, the amount of water ice they might contain, etc., and started testing more regolith analogues to see what might happen with different concentrations of perchlorates within the water ice. Starting with around a 10% suspension (much hight than has ever been found on Mars), the team gradually worked down to under 5% (closer to Curiosity’s findings, but still admittedly high). In all cases, they found that not only did the perchlorates leach out of the escaping water vapour as it passed through the reoglith analogue to form frozen lumps, it tended to do so at a fairly uniform depth the lumps combining over time  – an average of 10 days – to form what is called a “duricrust” layer.

Duricrusts are extensive (in terms of the area they might cover) layers of frozen minerals trapped within the Martian regolith. They were first noted in detail during the NASA InSight lander mission (operational on the surface of Mars between November 2018 and December 2022), significantly impacting the effectiveness of the lander’s HP3 science instrument, which included a tethered “mole” designed to burrow down into the Martian regolith. However, the “mole” kept encountering duricrust layers which, as it broke through, would surround its pencil-like body with a cushion of very loose, fine material which completely absorbed the spring-loaded action of its burrowing mechanism, preventing it from driving itself forward.

In their tests, Pavlov and his team found that the perchlorate duricrust formed in their tests would not only spread across a sample container, it was very effective in trapping neon gas (their methane analogue). Further, when the samples were exposed to the kind of natural expansion and contraction regolith on Mars would experience during a day / night cycle, they found the gas could indeed escape through cracks in the duricrust into the chamber’s atmosphere and be detected – just as with the methane around Curiosity. They also found that if a sample were subject to a pressure analogous to that of the wheel of a 1-tonne rover passing over it, it could be crushed and allow a sudden concentrated venting of any gas under it – again in the manner Curiosity has sometimes encountered.

Whether or not this is what is happening in Gale Crater, however, is open to question – as Pavlov notes. Firm conclusions cannot be drawn from his team’s work simply because scientist have no idea how much methane might actually be trapped within Gale Crater’s regolith, or whether it is being renewed by some source. As already noted, Curiosity is ill-quipped to study methane concentrations in the regolith and rock samples it gathers, because when the one instrument which could do so – the Sample Analysis at Mars (SAM) instrument – was designed, it was believed any methane trapped within Mars would be so deep as to be beyond the rover’s reach, and it thus wasn’t considered as something that would require analysis. While SAM can be configured for the work, it takes considerable time and effort to do so – and that is time and effort taken away from its primary science work, which is more-or-less constant as it handles both rock and atmospheric samples gathered by the rover.

Even so, the Goddard work is compelling for a number of reasons; it points to the fact that howsoever any methane within Gale Crater might be produced (organically or minerally), there is a good chance it is becoming mostly trapped within the regolith, and possibly in concentrated pockets. If this can be shown to be the case, and if these pockets could be localised and reached by a future mission, they might some day give up the secret to their formation – including the potential they are the result of colonies of tiny Martian microbes munching and farting (so so speak!).

Continue reading “Space Sunday(ish!): Mars methane mysteries” →