Martian minerals could destroy organic compounds, the molecules necessary for life, as we search for them. A new study shows that heating a mineral called jarosite makes it break up organics before we have a chance to observe them, muddying the results from the Martian rover Curiosity.
Mars fans can be forgiven for a sense of d茅j脿 vu. The same problem may have plagued the 1976 Viking lander, the first spacecraft to search directly for Martian organics by heating powdered rock in its oven. When it failed to find any, researchers were surprised: comets, meteorites and interplanetary dust particles throughout its history. Why weren鈥檛 there any to be found?
In 2008, the Phoenix Mars lander suggested an explanation. The lander discovered perchlorates, a compound combining chlorine and oxygen. When heated, perchlorates break down and release an oxygen atom, which splits organic molecules apart. So instead of seeing organics themselves, Viking may have seen the by-products of their reactions with chlorine and oxygen.
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鈥淲e were wondering if perchlorates were the only minerals on Mars that would have a negative influence on detecting organic compounds,鈥� says James Lewis at Imperial College London. Jarosite, a sulphate salt that forms in the presence of acidic water, also gives up an oxygen atom when heated to about 500 oC.
Climate shift
The presence of the mineral also marks a shift in Martian climate, when a neutral, drinkable water-rich environment gave way to more acidic conditions. Previous rovers and orbiters have seen jarosite in various Martian locales, and Curiosity, which landed in 2012 and is currently climbing a mountain that preserves Mars鈥檚 geological history, just reported earlier this month.
Conveniently for Lewis and his colleagues, jarosite also forms on Brownsea Island in Dorset, UK. 鈥淭here are micro-environments here that are very relevant to Mars,鈥� says Mark Sephton, Lewis鈥檚 PhD advisor. 鈥淲e don鈥檛 have to go all the way to Rio Tinto in Spain or Yosemite in the US to get our samples.鈥�
Lewis removed layers of rock rich in both jarosite and organics from cliff faces on the island and brought them back to the lab to grind them into powder and heat them. He heated the powder from 400 to 1000 oC in 100 oC increments, to find out which compounds appeared at each temperature. This is similar to the technique used in Curiosity鈥檚 SAM (Sample Analysis at Mars) instrument.
鈥淲e never detected the organics we knew were in the sample,鈥� he says. Instead, they saw sulphur dioxide, carbon dioxide, carbon monoxide and water 鈥� what you would expect if the jarosite reacted with the organics and broke them down into other carbon compounds.
鈥淚f Curiosity was doing a similar experiment on Mars, and had a compound with jarosite and organics, it wouldn鈥檛 be able to detect them,鈥� he says.
Curiosity, we have a problem
This isn鈥檛 just a problem for Curiosity, whose primary mission doesn鈥檛 include searching for signs of life today. The European Space Agency鈥檚 rover, due to launch in 2018, will hunt for life 鈥� and it also uses a heating technique to hunt organics.
There is hope, though. Jarosite and related minerals come in many different forms, some of which don鈥檛 fall apart until they reach much higher temperatures. The key, Sephton says, will be choosing the right samples to analyse. It might also be possible to interpret the decomposition products to see if organics were there to begin with.
鈥淚t鈥檚 turning out to be more of a detective story than we first imagined,鈥� Sephton says. 鈥淚t鈥檚 making us work for our science.鈥�
of Curiosity鈥檚 SAM team says she鈥檚 not surprised by the results, but they don鈥檛 mean SAM won鈥檛 see evidence of organics if they are actually there. That鈥檚 because organics come in many forms. Depending on their size and how concentrated they are in the Martian soil, it might be difficult to see them directly 鈥� but easy to see their by-products once sulphates or other minerals destroy them.
鈥淧ersonally, I think if we were to see evidence of high temperature combustion due to the presence of oxygen coming off of a mineral, as they suggest from sulphates, it might be a blessing in disguise,鈥� she says. 鈥淭he key is having a variety of experiments in your back pocket that you can pull out and search for the organics in different ways. That鈥檚 what SAM is doing.鈥�
Journal reference: Astrobiology,
