Summary: 'Survivability of 1-chloronapthalene during simulated early diagenesis...' (more exciting than the title suggests, honest!)

The shiny full open access version of our latest paper about organic molecules on Mars is now published online at JGR Planets. While we'd like you to read the whole thing to justify the ridiculous cost of publishing open access, if you can't be bothered then here's a summary, which should be relatively understandable.

When missions to Mars are looking for past or even present life, the evidence they are searching for is in the form of organic molecules. While not all organic molecules are formed by biological activity, some compounds, like certain fatty acids, are what we term ‘biomarkers’ and the detection of those would be very exciting as they would be a strong indication of Martian life.

Even detecting non-life-related (abiological) organic molecules on Mars would be exciting, we are sure that they should be present on Mars as the universe is full of them. Meteorites and comets are rich in organic matter, sourced from the primordial material of the early universe, and Mars must have been ‘seeded’ with these ingredients for life in much the same way as Earth.

Most attempts (bar one) to detect organic molecules on Mars, however, have only found simple chlorinated organic compounds – those with one or more chlorine molecules attached. These are not what we would expect from any of the proposed sources of organic matter on the Martian surface, they are too small and simplistic, and the chlorination is a bit weird,  and so something must have altered Martian organic matter to create them.

But when? and how? This is what we have tried to figure out....

Simple chlorinated molecules found on Mars

2 competing hypothesise for the source of these chlorinated molecules have been put forward by previous work:

1) It has been suggested that they may have formed on the Martian surface via reactions between chlorine-bearing salts (especially the perchlorates that I work with) and those organic compounds delivered by meteorites. In this case, the chlorinated organics discovered by the Curiosity Rover would be as 4.5 billion years old, the same age as the rocks they were extracted from. They would have had to survive the increased heat and pressure of being buried to about 3 km depth on a warmer ancient Mars.

The Sheepbed Mudstone, Gale Crater, Mars, where Curiosity has found most of the chlorinated compounds (from Sutter et al., 2017)

2) It has also been suggested that they are formed inside the analysis instrument of the Rover when organic carbon (either from the analysed Martian rocks or just contamination from Earth) is heated along with chlorine-bearing salts that are also known to be in the samples. The salts break down when heated to produce both oxygen and hydrochloric acid which reacts with any organic matter present. In this scenario, anything interesting is mostly burned away to carbon dioxide and carbon monoxide and lost to detection with the few surviving fragments of organic molecules becoming chlorinated. In this case the detected molecules would be formed just before detection and so survivability is not an issue.

To test if the first hypothesis was even possible we examined the ability of simple chlorinated organic molecules to survive the high pressures and temperatures associated with burial over geological timescales. We did this by subjecting these molecules to geologically-relevant pressures in a small reaction vessel, known as a bomblet, which is basically a glorified pressure cooker. Obviously we didn’t have a few billion years to wait around so we had to speed up the process. By increasing the temperature to speed up the reaction rate (remember high school chemistry?) we squeezed most of the history of Mars into a weekend.

How to squeeze 4.5 billion years on Martian geological history into a weekend. Clockwise from top left: Freshly made bomblets ready to be filled with the test chlorinated compounds. Sat inside the larger bomb - the whole thing can be filled with larger volume of sample if necessary. Making sure everything is tight so it doesn't explode when pressurised. Heating up in the reaction vessel to cook for the weekend.

By repeating this experiment at various temperatures and pressures, it was possible for Jonny Tan, one of the PhD students in the research group, to do some fancy maths and computer simulations stability of the molecule under conditions it would be subjected to on Mars. This bit was all over my head, but if you want to play around with his model, its all open source and available on github.

It was found that the bond between the chlorine atom and the rest of the hydrocarbon molecule is quite weak and breaks easily. This means that, on a warmer ancient Mars, when the sediments the chlorinated molecules were detected in were deposited, the increased surface temperatures would have promoted the loss of chlorine relatively rapidly. Intact chlorinated organic molecules would have been unlikely to even survive the first 1 billion years, never mind the 4 billion needed to get to the present day in detectable concentrations. This makes the first hypothesis (that these molecules formed on the Martian surface and are ancient) rather unlikely.

The chlorinated organics detected on Mars in ancient sediments are therefore likely to have been formed very recently, most probably when heating in the Rover's analysis oven promoted reactions between Martian organic matter and chlorine-bearing salts.

Whilst this conclusion isn't going to blow anyone's mind, as most studies had assumed this anyway, it's good to have some actual experimental evidence to back these things up.