What made the kerogens that SAM found in Pahrump Hills rocks?

Biology is one possibility, but the kerogens could also have come from interplanetary dust particles or even igneous rocks (rocks that solidified from a melt).

We decided to test some analogous materials in an Earth copy of the SAM instrument to see if they released collections of smaller carbon-containing compounds that are similar to the collection we observed with SAM on Mars. We ran our experiment on a small sample of the Murchison meteorite (which has a composition similar to interplanetary dust particles) and the Tissint Mars meteorite (which has an igneous origin). Both of these do contain kerogens. Both of these experiments produced carbon-containing compounds like the ones we saw in the Mojave and Confidence Hills samples from Mars, although each rock (Curiosity sample, Murchison, and Tissint) produced different amounts of the different kinds of compounds. So these additional experiments didn't tell us anything about where the carbon compounds we detected on Mars came from, but they do support our favored interpretation, that the Mars rocks contained kerogens.

We wish we could tell you where these large carbon molecules came from. We can't. All options -- biology, Mars geology, meteorites -- are on the table. If biology was a factor, then a lot has happened to these organic molecules over time; there's nothing in what we saw with SAM that makes biology a better interpretation than any other.

Why didn't we see the same materials at Yellowknife Bay that we saw at Pahrump Hills?

We're not sure why the Yellowknife Bay rocks didn't release the same kinds of carbon compounds under the same experimental conditions. Maybe they had less kerogen material to begin with. Maybe the water alteration they experienced a long time ago destroyed the kerogens. Maybe they have been exposed to the harsh Martian environment for too long, and any kerogen molecules that they contained have already broken down. We do know from previous experiments that the Yellowknife Bay mudstones have been exposed at the surface for about 80 million years; maybe the Pahrump Hills rocks have not been exposed for so long, and their kerogens have survived to the present day.

It's actually surprising that the Pahrump Hills rocks preserved kerogens in them for so long. Other work suggests acid fluids circulated through the rocks from the top down about 2.1 billion years ago. We'd ordinarily expect acid fluids to attack large carbon-containing molecules (by oxidizing them and breaking them into tiny molecules like carbon dioxide), so we have to come up with an explanation for how the molecules were preserved for long enough for us to find them.

We can go through a list of possibilities, but we believe that the most important preservation agent was the presence of sulfur in the molecules. The Mojave and Confidence Hills SAM experiment yielded much more carbon bound to sulfur in a variety of different compounds than any other experiments we've done on Mars. The sulfur atoms help link up smaller carbon-containing molecules into much bigger ones, and then prevent them from breaking down. We take advantage of this property of sulfur industrially on Earth to help carbon-containing molecules stay big and unbroken. It's vulcanization, the same process that tire manufacturers use to keep tire rubber from degrading. When those big sulfur-containing carbon-rich molecules get attacked by oxygen, the oxygen is more likely to react with and break off the sulfur, leaving the carbon behind, preserving the big molecules.

The kerogen molecules in the Pahrump Hills rocks could've had this sulfur from their beginnings (whatever their origin), but it's also possible that the molecules didn't start out containing so much sulfur. Sulfur could have come in at the time that the lakebed sediments were turning into rock. In fact, we have evidence from SAM that hydrothermal groundwater that contained sulfides once percolated through these particular rocks. There was probably hydrogen sulfide present, which would've reacted with the carbon-containing molecules to leave behind a sulfur atom in the molecular structure.

Is there anything we can learn about the potential for life on ancient Mars from this study?

When the Curiosity team announced the discovery of a habitable environment preserved in the Sheepbed mudstones, the environment that they discussed is one that could be inhabited by chemolithoautotrophs. (It's a terrifying word but you can sound it out if you go slowly. Chemo, litho, auto, trophs.) Chemolithoautotrophs are organisms that get their carbon from carbon dioxide -- they can build themselves out of carbon that doesn't have a biological origin. But the fact that we've observed large carbon-containing molecules preserved at Pahrump Hills tells us that the Mars lakes could also have supported heterotrophs. Heterotrophs are organisms that build themselves out of organic carbon (that is, big carbon molecules, regardless of their origin). The presence of kerogens preserved from ancient Mars tells us that the environment in which the Pahrump Hills sediments were laid down was a more habitable environment than we've talked about in the past.

While it's disappointing that we can't figure out where the carbon-rich large molecules came from, it's really important that we were able to detect them in Mars material drilled from just beneath the surface. Mars' present-day environment is a very harsh one in which large carbon-containing molecules would be expected to degrade rapidly. If we can dig a little deeper, and look in the right places, we now have evidence-based hope that we can find better-preserved molecules in Mars rocks. At a site selected for better preservation of ancient materials, we just may be able to determine whether these molecules came from space, from igneous rocks, from hydrothermal activity, or -- the most exciting possibility -- ancient Mars life.

Thanks to Jen Eigenbrode for reading this post and offering corrections.