Of mountains, methane and molecules

CuriosityAfter what has been a relatively quiet period in terms of news from Mars, things are once again proving interesting.

The first uptick came following the start-of-month teleconference Mars Science Laboratory personnel held to summarise the results of the last several months of activities the Curiosity rover has been performing in Gale Crater. In particular, these have allowed scientists to better determine how the 5 kilometre high mound at the centre of the crater may have been formed.

Even before Curiosity arrived on Mars, sufficient evidence had been obtained from orbit to show that features in and round Gale Crater were likely influenced by water-related activity. Curiosity itself found evidence for water once having flowed freely across parts of the crater when it encountered the beds of ancient rivers and streams as it explored the regions dubbed “Glenelg” and “Yellowknife Bay”.

With the journey down to “Mount Sharp”, and NASA call the mound, and the recent explorations of its lower slopes, the science team have been able to piece together the processes that led to its formation.

The first clues came while Curiosity was still en route to the point where examination of the “Mount Sharp’s” lower slopes could begin. As it drove southwards and towards the mound, the rover started to encounter layered sandstone deltas, all inclined towards “Mount Sharp”. On Earth, such layered, angled deposits are found where a river flows into a large lake.

A mosaic of images captured by Curiosity’s Mastcam on March 13, 2014 PDT (Sol 569). White-balanced for natural Earth light, the images show layered sandstone deposits, all pointing towards “Mount Sharp”, indicative of delta sediments dropped by a flowing river as it enters a large lake

Once in the foothills of “Mount Sharp”, in the area dubbed “Pahrump Hills”, Curiosity has repeatedly come across layers of tightly-compacted sedimentary mudstone which are entirely consistent with the sedimentary layering found in the muds and rock in lake beds on Earth. Intriguingly, while most of these layers appear to have been formed by sediments settling out of a large, still body of water, some of them appear to have been affected by wind erosion.

This latter point would indicate that rather than the crater floor once being covered by a single body of water which gradually vanished over time, it was subjected to cycles of wet and dry periods, giving rise to a number of lakes forming within the crater over the ages, each one only a few metres deep. As the water receded / vanished during the dry periods, so the uppermost layers of each lake bed were exposed to the wind, eroding them, before the next wet period started, and a new lake formed, gradually depositing more sediments on top of them.

Thus over a period of millions of years, Gale Crater was home to numerous lakes, each of them fed by assorted rivers and streams flowing into them, giving rise to the alluvial plains around the base of the crater walls, and the sedimentary deltas closer to “Mount Sharp” where these rivers and streams met the standing waters of each lake.

This diagram depicts a vertical cross section through geological layers deposited by rivers, deltas and lakes. A delta builds where a river enters a body of still water, such as a lake, and the current decelerates abruptly so sediment delivered by the river settles to the floor.

This view of Gale Crater is further supported by measurements of the deuterium-to-hydrogen ratio in the rocks sampled by Curiosity. These suggest that the sediments the rover is now examining were laid down during a period when Mars had already started losing its surface water, suggesting an extended period of climatic change on the planet, where the amount of free-standing water may well have been in flux.

Once the water had completely vanished from Gale Crater, it seems likely that “Mount Sharp” was sculpted by wind action within the crater. Thus, it is thought, would have eroded the material of the alluvial plains faster than the more densely compacted mudstone formed under the weight of the successive lakes.

As it might have been: the left image shows the repeated depositing of alluvial and wind-blown matter (light brown) around a series of central lakes which formed in Gale Crater, where material was deposited by water and more heavily compressed due the weight of successive lakes (dark brown). Right: once the water had fully receded / vanished from the crater, wind action took hold, eroding the original alluvial / windblown deposits around the “dry” perimeter of the crater more rapidly than the densely compacted mudstone layers of the successive lake beds, thus forming “Mount Sharp”

 Methane and Organics

The second round of news came on Tuesday, December 16th, when members of the Curiosity science team confirmed that Curiosity has detected odd spikes in the amount of methane within Gale Crater, and has also found evidence of organic compounds in samples obtained from inside Martian rocks.

SAM is an amazingly complex suite of instruments compressed into something the size of a large cardboard box
SAM is an amazingly complex suite of instruments compressed into something the size of a large cardboard box (image: NASA Goddard Space Centre)

Both sets of findings come from the SAM (Sample Analysis at Mars) mini laboratory. This is a powerful set of three experiments that work together to investigate the chemistry of the Martian surface and atmosphere within Gale Crater.

That methane exists on Mars is not news. The first really definitive detection of the gas in the Martian atmosphere came in 2009, although evidence of its presence goes back a good way prior to then.

Methane is of interest to particular interest to scientists because on Mars, it has a relatively short lifespan, only lasting around 340 years before it is broken down by sunlight. So for it to be detected indicates there must be active, methane-producing processes on Mars. Whether these are connected with life or not, however, is highly debatable; methane can be produced by inorganic as well as organic means.

What is intriguing about the SAM findings, however, is that they show two sudden spikes in the amount of methane Curiosity has been monitoring within Gale Crater. These spikes each occurred across two different sets of readings, one obtained towards the end of 2013 and one in early 2014. Each of these pairs of readings indicated methane levels at around seven parts per billion, compared to a nominal detection level by SAM around one-tenth that value. Such spikes strongly suggest that there could be a relatively localised methane-producing process at work within the Gale Crater locale – although what this might be has yet to be determined.

SAM has also detected organic chemicals within rock samples Curiosity gathered in May 2013 from a rock NASA had dubbed “Cumberland”. While not necessarily evidence of life, these compounds are nevertheless some of the essential building blocks of life. The challenge now is to attempt to further identify how they may have been formed.

It may seem a long time between obtaining the samples from the rock and announcing the findings, but there some good reasons for this. The first is that SAM has apparently detected organic elements in previous samples, but a more detailed examination of the data suggested the organics were in fact the result of tiny amounts of contamination carried within SAM from Earth. Another reason for caution and verification has its roots stretching back almost 40 years.

The late Carl Sagan discusses the Viking mission and the controversy surrounding its science results in his ground-breaking television series, Cosmos
The late Carl Sagan discusses the Viking mission and the controversy surrounding its science results in his ground-breaking television series, Cosmos

In 1976, the twin Viking landers arrived on Mars, around half a world apart, with the am of sifting through samples of Martian soil to find evidence of life or at least of organic interactions indicative of life. By criteria drawn-up ahead of the mission, two of the experiments carried by both Viking Landers, notably the Labelled Release (LR) experiment, initially appeared to indicate organic processes at work in the soil samples the landers retrieved.

However, the LR findings have been in dispute ever since they were first announced, with those believing the initial results being false pointing to the fact that neither of the two Gas Chromatograph – Mass Spectrometer (GCMS) experiments carried by Viking yielded positive results, and the fact that a repeat run with the LR experiment was negative. Thus, contamination of the equipment, the possible reaction of inorganic materials to the LR experiment have been indicated as the more likely reason for the initial positive result the experiment gained. Equally, those who favour the view that the LR experiment did produce a genuine positive have used both speculative mathematics and complexity analysis to continue to make their case.

Matters might have rested in 2008, when NASA’s Phoenix lander confirmed the presence of perchlorate salts in the Martian soil. These are an oxidising agent, and any positive results gained by the LR experiment in the 1970 was predicated on there being no oxidising agents present in the Martian soil, a these could influence the results of the tests and lead to incorrect analyses.

NASA's 2008 Phoenix Lander mission confirmed the presence of perchlorate minerals in the Martian soil
NASA’s 2008 Phoenix Lander mission confirmed the presence of perchlorate minerals in the Martian soil

But even the presence of perchlorates failed to resolve matters; they simply added to the debate. Experiments conducted in 2010 showed that any perchlorates present in the Martian soil would destroy any organics also present when heated,  thus preventing the LR experiment from detecting them, again leaving matters open as to whether organic elements were or weren’t present in the Martian soil. However, to complicate matters, the same experiment revealed that the destruction of organics in this way would result in the production of chloromethane and dichloromethane – which were both detected by both of the Viking LR experiments.

Thus, on the one hand, the presence of perchlorates actually seemed to leave the matter of whether or not the Martian soil might contain organics entirely open. On the other, however, it suggested that perhaps the presence of perchlorates indicated organics were present given that the LR experiment did detect chloromethane and dichloromethane. And so the debate has continued.

So, against this background – the risk of contamination in the equipment, the contradictory nature of evidence gathered by Viking and  since put together in the wake of that mission, together with other complexities the presence of perchlorate mineral bring to the table – the SAM science team were understandably cautious. They wanted to eliminate as many possible causes of a false positive set of results as possible before announcing their findings.

Even so, the SAM results shouldn’t be taken to mean actual living microbes have been found on Mars – Curiosity isn’t actually equipped to make such a definitive finding. Rather, what they do show is that Gale Crater once offered an environment with a supply of reduced organic molecules for use as building blocks for life and an energy source for life.

This is the second time that Curiosity has found such and environment, the first being when its explorations of the “Glenelg” / “Yellowknife Bay”. At that time, the rover found strong evidence for the area once having been both wet an offering a mix of chemical elements and a suitable chemical energy source which may have offered a benign enough environment in which life might have arisen. As such, one can’t blame members of Curiosity’s extended science team if they do engage in a little speculation.

“We think life began on Earth around 3.8 billion years ago, and our results shows that places on Mars had the same conditions at that time: liquid water, a warm environment, and organic matter,” Caroline Freissinet, one the the scientists responsible for the SAM equipment, said as the findings were made public. “So if life emerged on Earth in these conditions, why not on Mars as well?”

That’s a very intriguing thought.

All images from NASA / JPL, unless stated otherwise. Video: NASA Goddard.


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