Latest paper summary: Effect of Hydration State of Martian Perchlorate Salts on Their Decomposition Temperatures

Here’s an early Christmas present for everyone (so both of you who read this blog), our latest paper (my first out of this project) is now published online and completely open access for anyone to read.

Effect of Hydration States of Martian Perchlorate Salts on their Decomposition Temperatures

If you can’t be bothered to read it, here is a summary:

Since the Viking landers back in 1976, numerous missions have gone to Mars and attempted to find organic matter on the surface. However, to this date, none have succeeded in finding anything other than simple chlorinated hydrocarbons. We expect there to be a detectable amount of relatively complex organic molecules in the Martian near surface from meteoritic input (meteorites are full of organic matter), hydrothermal processes and maybe even left behind as evidence of extinct (or, unlikely but not impossible, extant) life. This lack of detectable organics was always a bit of a mystery…

Note: the presence of organic matter does not necessarily mean the existence of life, organic molecules are just those which have carbon and hydrogen and they can be formed by numerous non-biological processes – including in deep space.

However, since they were accidentally discovered by the Phoenix mission back in 2008 we’ve known that that there are these salts on Mars called perchlorates. These salts are very rare on Earth, only being known to exist in significant quantities in the Atacama Desert and the Antarctic Dry Valleys; two of the driest places on Earth.  As the name suggests perchlorates have 4 oxygens to each chlorine ion and so are very highly oxidising. While (relatively) stable on the Martian surface, as soon as these are heated within the sample analysis oven of a Martian lander or rover these literally explode, giving off loads of oxygen into the analytical system (which should be a vacuum). This means that any organic matter which may be in the Martian soil is combusted, reacting with the oxygen present and breaking down to carbon dioxide and carbon monoxide,  and the few surviving scraps are heavily broken up and react with the chlorine in the perchlorate, making small chlorinated hydrocarbons. Because of this destruction the molecules we are interested in cannot be detected – This is the perchlorate problem.

My attempt to summarise the Perchlorate Problem on the Sketch Your Science wall at AGU this year

My job as a postdoc researcher at Imperial College is to attempt to find a way around this ‘perchlorate problem’ by researching how these perchlorates (and other similar minerals) react with organic matter. The first step of this, however, is to try to understand a little more about the Martian perchlorates themselves. And this is what this recently published body of work was concerned with.

Since it landed on the Mars in 2012 the Curiosity Rover has drilled 15 holes into the Martian surface and analysed the drilled sample to see what it is made of. One of the techniques used is looking at the gases given off by the sample when it is heated. However, every time the Curiosity Rover has analysed one of these sample holes, the temperature that oxygen gas is released from the sample is very different (Sutter et al., 2016). Most of the oxygen given off is believed to have come from these perchlorates (based on other gases given off at the same time). It has been found by previous laboratory experiments that different types of perchlorate salts (magnesium perchlorate, iron perchlorate, calcium perchlorate, etc) break down and give off oxygen at different temperatures (Glavin et al., 2015), and also break down at different temperatures depending on whether other minerals which may act as catalysts are also present in the soil (Sutter et al., 2014). Therefore it has so far been concluded that different types of perchlorates and/or catalyst minerals have been present at each drill site.

Curiosity on Mars (credit: NASA)

The first 14 drill holes Curiosity drilled on Mars (credit: NASA)

This is a bit problematic. It is believed that most perchlorate forms in the Martian atmosphere and falls out onto the surface (as it does on Earth) and so there is no known mechanism for having different perchlorates in different places – they should all be pretty much the same across the Martian surface.

It is known that perchlorates are highly hydrophilic – they will suck up any water available (they are used as drying agents in industry) and can change their hydration state (how many molecules of water are bound to each molecule of perchlorate) really easily. The temperature and humidity of the Martian surface changes massively throughout the Martian day and year. Photographs taken on Mars by Phoenix show growing blobs on the lander struts which have been interpreted as perchlorate goos as they absorb water throughout the Martian day. So, my boss, Mark Sephton, had an idea that the hydration state of the perchlorates may actually have an important effect on their breakdown temperature and set me to investigate this.

'Blobs' of perchlorate 'goo' moving across the struts of the Phoenix lander (Renno et al., 2009)

I did this by taking a single one of the perchlorate salts (magnesium perchlorate) and drying it out in the glassware drying oven to drive off water in an attempt to reduce its hydration state. After three weeks I removed some sample, flash heated it and analysed the gases given off using pyrolysis-GCMS at 100 °C temperature steps from 200-1000 °C to create oxygen (and other lesser gases) release-temperature profiles. A sample of the perchlorate was also left out on the lab bench to rehydrate and analysed in the same way after 24, 48 and 72 hours of exposure and rehydration. This whole experiment was repeated after another week and then after a fortnight so oxygen release profiles were created for 3, 4 and 5 week drying times and corresponding 24, 48 and 72 hour re-exposure samples.

It was confirmed that these drying and re-exposure experiments were definitely changing the hydration states of the perchlorates by testing them (by X-ray diffraction) next door in the Natural History Museum. This involved transporting them through crowds of tourists under an inert atmosphere (in a sealed lunchbox filled with nitrogen gas) to prevent further hydration state change.

What we found was that different hydration states did indeed affect the temperature of decomposition so that the oxygen release profiles were as different for different hydration states of this one kind of perchlorate as the previous studies had found for all the different kinds they tested. This, therefore, gave a much simpler answer to this puzzle: Curiosity has been analysing different samples with different hydration states of perchlorate in them. This makes sense as the different samples drilled have been analysed at different times of the Martian day and year and samples spend varying amounts of time stashed inside the temperature-controlled innards of the rover before they get round to being analysed – which could allow further changes in their hydration state. Our data show that is it possible for numerous hydration states of perchlorate to exist within a sample and this leads to multiple peaks in the oxygen release profile, some of the Martian samples have multiple peaks in their oxygen release profile and so we suggest that this is due to unstable mixtures of perchlorate hydration states being present on the Martian surface.

This figure from the paper shows how the data from this study (A) compares to select samples analysed on Mars by Curiosity (B) and various types of perchlorate from a study by Glavin et al. (2015) (C). It can clearly be seen that the variation in the samples of magnesium perchlorate that were dried out for various numbers of weeks and analysed in this study is almost as great as the variation in different perchlorates from the Glavin et al. study. So, different hydration states may offer a simpler explanation for the variation seen in the Martian samples. 

We tried to see if the oxygen release profile of the Martian samples (and therefore, based on our findings, the perchlorate hydration state) corresponded to the climate conditions at the time they were sampled at but did not find any relationship. There did seem, however, to be a vague suggestion in the data that they were related to the time of the year that they were sampled at, although more data would be needed to be sure about this.

All in all, we conclude that the hydration state of perchlorate salts is yet another thing to make making sense of Martian data yet more complicated.

Science, Sazerac and Steamers: AGU 17, New Orleans

After a heavy night with a few of the ex-UEA mountaineering club members visiting us in The London it was an unpleasant early morning trudge to the tube station nursing a hangover. With perfect timing it just so happened that I was flying out on the one day that Hell (sorry, London, nope, right the first time) froze over. Heavy snow meant minor panic, as all public transport was buggered and it started to look as though a tube might not turn up at all. Thankfully it did and it turned out that there was no worry of missing my flight as it ended up delayed for 3 hours. There was only one de-icing truck available and it was snowing so heavily that once they’d defrosted one side of the plane the previous side had re-frozen up and needed doing again….This, of course, meant I missed my connecting flight which took off 5 minutes before I landed in Atlanta. Luckily, avoiding a repeat of last time this happened to me, they managed to get me the last seat on the last plane to New Orleans that evening. Although, from the sound of things, plenty were not so lucky, airports around the west coast of the US must have been full of stressed out scientists snuggling up to their poster tubes that night.

AGU was intense, having been to EGU (the European version in Vienna) during my PhD and thinking that was pretty big, I was expecting something of a similar scale. I was wrong, it was unfathomably massive. There were over 25,000 geoscientists in attendance and over 20,000 talks and posters to see….that is a fuck ton of exciting new scientific research being shared. The conference centre was over a kilometre long, it took over 20 minutes to get from one end to the other with all the crowds; thankfully all the Planetary Science talks were clustered together so I didn’t have to run about much – although still managed to average about 15,000 steps a day just getting about!

Due to the sheer number of coevally running sessions it was not possible to see everything I had an interest in (if you’ve read this blog before you may have noticed I get involved in an eclectic mix of sciences) so I stuck with the Planetary Science sessions as that’s what I work with at the moment. Running between talks on Martian surface processes and outer solar system geochemistry – my main 2 things at the moment. There was some exciting results being presented from all the current missions that are on the go at the minute – especially from Mars Curiosity and the recently deceased Cassini probe, so lots of cool space pictures.

Bourbon Street in the French Quarter

I was there to present a poster on our latest paper on the effects of hydration state on Martian perchlorate salts, about which a summary blog post will be coming soon once the open access version of the article is out (you can read the pre-proofed version of the article here, although it is temporarily pay walled). Unfortunately, despite good intentions of being well rested and fresh for the 8am session, the night before got pretty heavy. It is apparently impossible to have a quiet drink in New Orleans and we ended up in a bar with a pretty cool band drinking Sazeracs (the local speciality cocktail, laced with absinthe) with random other scientists until the early hours. This was partly because my mate fancied the band’s singer (oh, shit, I was supposed to never mention that again) but also because this is what happened EVERY SINGLE NIGHT.

Bourbon Street at night, watching people 'get got'

As it turns out, an AGU poster session is not the place to be with an absinthe-derived hangover, they are intense AF. What was cool was that everyone was super positive about the results and I now have a few things to try in the lab for future work that have come out of the chats we had. The argument I was sort of expecting with the research group whose work our work is directly contradicting never came, which I guess is good as it would probably have ended with me spewing up on a senior NASA scientists shoes… I did have a minor fan-girly moment when one of the old professors who’s big on the Curiosity Rover team came up to collect a copy of our paper though.

New Orleans is a great locality for a conference, as somebody who hates being in cities, this is definitely one of the best I’ve been to. Every evening was spent exploring the French Quarter or the Riverside area, hanging out in ridiculously cool bars listening to spectacular live music – lots of Jazz – and drinking great local craft ales and Sazerac. The food was also amazing, blackened fish, shrimp, deep fried catfish and crawdads, so good, although I didn’t get round to trying any ‘gator.

Apparently everything that lives in the river/gulf can and will be deep fried

 While really cool, the area does have quite the seedy side and there were plenty of con-artists out trying to swindle tourists – they must have seen the conference attendees from a mile off, us scientists are not known for our street-smarts. The popular scam is offering a shoe shine then betting the tourist they can tell them exactly where, on what street in which city they got their shoes. The scam answer being ‘On the bottom of yo’ feet, in this street’ and then getting all up in their face and calling over their mates if they don’t hand over the ‘Ten for the shine, and ten for the line’. Must’ve made a killing this week based on the number of guys getting got we spotted while sat up on a balcony overlooking Bourbon Street. Also, someone had altered a pedestrian roadworks diversion to funnel you into a grotty looking strip club, we noticed we were being herded just in time to avoid this one, sneaky bastards.

 On Wednesday afternoon we skipped out the conference to do some touristy stuff. The choice was an airboat swamp tour or a paddle steamer jazz cruise. Sadly at this time of year the alligators would’ve all been hibernating at the bottom of the swamp, so we opted for the steamer. Best. Idea. Ever. Cruising down the Mighty Mississippi, seeing the sights, learning a bit of history from the captain (who had the Louisiana accent ever), listening to jazz and chilling with a few beers was clearly the highlight of the week.

Steaming down the Mississippi

All in, it was an amazing week; exciting science and great times, I wonder if Washington will be able to compare next year? Somehow I doubt it…