What do you get when you cook buried martian mudstone in your oven? The answer appears to be the kind of gases you'd expect if you cooked organic material here on Earth.
A couple of weeks ago the latest results from the Curiosity rover's investigations at Yellowknife Bay in the Gale Crater structure on Mars hit the newsstands in a six paper spread in the journal Science. One of the most intriguing studies involves the 'cooking' of martian material by the Sample Analysis at Mars (SAM) device.
Crudely speaking, the recipe was this: first, place a representative sample of surface dust in an oven, heat it at steadily rising temperatures all the way to just over 800 Celsius and continually measure the composition of gases released (or 'evolved'). Second, drill at a couple of sites 5 centimeters into what appears to have once been the muddy bottom of a lake, scoop that material into a clean oven and repeat the cooking procedure.
The surface dust has been exposed to martian weathering, including the persistent and damaging effects of natural cosmic radiation, nasty surface chemicals like perchlorates, and ultraviolet light. All of which is guaranteed to break down and destroy any long chain organic carbon molecules. The drilled out mudstone on the other hand has been relatively protected by overlaying material.
Both types of sample release a mix of gases at different temperatures. Compounds like water, carbon dioxide, oxygen, sulfur dioxide, hydrogen chloride and chloromethane - to name just a few.
Except the mudstone drill samples (from two locations), release more carbon dioxide at lower temperatures, around 300 Celsius, than the surface sample.
So what's going on? Perhaps the most telling clue - and it is just a clue, none of these results are definitive - is that for one of the mudstone samples, the surge in carbon dioxide coincides with a drop in oxygen being evolved from the breakdown of perchlorates.
The simplest explanation for this behavior would be that something is undergoing combustion, and using up that oxygen. If we did this experiment here on Earth that mystery substance would most likely be composed of organic carbon compounds, the detritus of carbon-fixing life. The concentrations implied by the cooking experiments on Mars indicate a source of carbon at about the 500 parts-per-million level - a healthy quantity.
It's a very exciting find, except there's another potential source of lots of organic carbon on the near surface of Mars - meteorites and interplanetary dust. With a pitifully thin atmosphere to protect it, the surface of Mars is a veritable smorgasbord of carbon-rich material deposited from the surrounding solar system, easily building up a concentration of organics at the few tens to few hundreds of parts-per-million level. The carbon molecules here come from the ancient stewpot of proto-planetary chemistry during the formation of the solar system 4.5 billion years ago.
At this point we can't tell what the source of these potential martian organics is. It could be the remains of an ancient ecosystem in muddy lakewater, or it could just be a layer of primitive meteoritic matter. The rather caustic, oxidizing, nature of martian soil also still presents a real hurdle in chemical analyses, both from reacting in-situ with carbon compounds and in Curiosity's ovens. One of the next major challenges will be to figure out a reliable way to pick through these different factors in order to establish the origins of these carbon compounds.