New evidence for old life on Mars

No one wants to stick their neck out, but it's getting hard to find abiotic explanations

On February 4, a study published in Astrobiology provided some of the strongest evidence that Earthlike microbes were doing their thing on Mars 3.5 billion years ago.

The study follows up a 2025 result that detected long-chain organic molecules in a sedimentary rock formation at Gale Crater. That study found three alkanes (decane, undecane, and dodecane) present in a rock sample at the 30-50 part per billion level.

The rock layers these compounds were found in formed ~3.5 billion years ago¹ and remained buried deep underground until 80 million years ago, when wind erosion exposed them at the surface. The new study makes a quantitative assessment of just how much of these compounds must have been originally present to leave the residual amounts observed by Curiosity.

The answer is “piles and piles of the stuff”, a result that is easy to explain if you have a lake teeming with microbes, but hard to reconcile with the known abiotic pathways for getting organic compounds into Martian mud.

Since the result is so significant, I thought I’d go through it in some detail.

Welcome to Gale Crater

Gale Crater was chosen as the landing site for the Curiosity rover in 2012 in the correct hope that it would turn out to be an ancient lake bed. Years of roving and drilling there have led to exciting discoveries. Thanks to Curiosity, we know that Gale Crater was home to Earth-like freshwater lakes for at least tens of thousands and potentially millions of years, and we know that that sediments there contain complex organic chemicals consistent with the presence of ancient life.

The Cumberland drilling site where alkanes were found in 2025 is a mudstone, which is exactly what it sounds like. Muck gets deposited on a lake bed, and over time the layers compact down and harden into rock. For three billion years or so, this rock stayed peacefully buried, until it was unmarsed by the wind in the recent geological past.

Since Mars has no magnetic field and a thin atmosphere, cosmic rays smash into any rock that is close to the surface. These impacts leave behind trace isotopes that act as a kind of clock. In the case of the Cumberland mudstone, three isotopes (³He, ³⁶Ar and ²¹Ne) give independent values for this exposure time that are in good agreement with one another. The mudstone has been exposed to cosmic rays (that is to say, near the surface) for about 80 million years (78 ± 30 My).

Except for that last interval, the rock has led a sleepy existence that has preserved the evidence of its formation remarkably well, given its enormous age.

What are alkanes?

Like the mudstone they’ve been resting in, the alkanes in the Cumberland sample are sleepy compounds—simple hydrocarbon chains 10-12 atoms long. The shortest of them, decane, is a common constituent of gasoline. Here’s what it looks like in a Kekulé diagram:

(If you’ve never seen one of these diagrams before, each vertex represents a carbon atom, and the lines are single bonds connecting them. The 22 hydrogen atoms needed to fill out the molecule are not shown.)

Alkanes are close relatives of molecules called fatty acids, which have a similar carbon skeleton with a carboxyl group at one end. For example, the fatty acid analogue of decane is decanoic acid, a constituent of coconut oil:

Unfortunately, because of how the samples are heated on Curiosity, the rover can’t tell the difference between alkanes and their fatty acid equivalents. And because of how long these compounds have been entombed, it’s possible that the reactive carboxyl group on the fatty acid had already been reduced to its alkane form before the rover showed up.

Even more unfortunately, the instrument on Curiosity can only detect straight-chain alkanes that are between 10 and 12 carbons long. So what we see in the Cumberland sample is likely constrained by the limitations of our equipment.

This limitation is particularly vexing because terrestrial life shows a clear preference for even-numbered carbon chains. If we could examine a wider range of alkanes in the Cumberland sample, a relative preponderance of even-numbered alkanes would be strong evidence for their biological origin. But you go to Mars with the instruments you have.

Whatever the exact mix of compounds in the mudstone, it’s their abundance that makes it hard to fit an abiotic origin story to their presence. Curiosity detected the three alkanes at the part-per-billion range. But organic compounds exposed to high-energy cosmic rays degrade over time in a process called radiolysis. And since we know the mudstone in question spent 80 million years being zapped by cosmic rays, we can work backward to estimate the original concentration of medium-chain alkanes or fatty acids in the mudstone when it formed.

Doing this with conservative assumptions, the study authors calculate that the original mudstone contained between 120-7200 parts per million of decane, undecane, and dodecane, or their fatty acid equivalents.

Keep in mind that this is almost certainly an undercount. Since the instrument on Curiosity can’t detect alkane chains outside its narrow window, the total organic content of the sample is probably higher (and note that the 7200 ppm end of the published range is already close to 1% by weight of the entire sample).

Fatty acids are molecules intimately associated with life. They are the constituent parts of triglycerides (fats) and of the lipid bilayer that forms the envelope for every known living cell. There are abiotic pathways to synthesizing fatty acids, but the presence of these molecules (or their alkane cousins) in great abundance on an ancient freshwater lake bed is suggestive.

Like a lot of recent discoveries on Mars, we would have no problem calling this discovery a biosignature if we found it on Earth. Looking back from a distance of 3.5 billion years, this is about as unequivocal as evidence for life gets. There’s sunshine, carbon dioxide, a bucolic freshwater lake, piles and piles of sediment, and abundant organic goo in the sediment with no obvious abiotic source. What more do you need?

But the standard of evidence on Mars is high. So the authors of the paper go to some length to consider alternative sources that don’t implicate biology.

Abiotic hypotheses

Without life, there are four ways you can wind up with a bunch of medium-chain alkanes or fatty acids in an old Martian mudstone: