Sciency Words: Chemofossils

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


On Wednesday, I made up a bunch of numbers for the odds that we will find life on (or bring life to) Mars. The odds, in my inexpert judgment, are pretty low for finding anything presently alive on Mars, but I’d say there’s a 50/50 chance we’ll find the fossilized remains of life that existed there in the past. Now if only our rovers were equipped with digging gear!

But after writing Wednesday’s post, I learned that there are, in fact, many different kinds of fossils, including body fossils, trace fossils, ichnofossils, and chemofossils. And according to this article by Claire Cousins, a planetary scientist working on the European Space Agency’s upcoming ExoMars rover, it’s these chemofossils that will probably be our first real evidence of ancient Mars life.

The word chemofossil is, as you may have guessed, a combination of the words chemical and fossil. I was unable to find out who coined the term, but it seems to have happened fairly recently. According to Google Ngrams, it starts appearing in literature in the 1970’s.

Chemofossils are the telltale chemicals left behind by dead and decaying biomaterial. Even if an organism becomes totally decomposed, there may still be a sort of residue that suggests some sort of past biological activity. A good example, which Cousins cites in her article, are amino acids that share the same chirality.

Finding amino acids on Mars would be mildly interesting, but amino acids can come from just about anywhere. However, if those amino acids all have “left-handed” chirality (or “right-handed” chirality), well… the only natural phenomenon we know of that picks and chooses the chirality of amino acids is life.

Now since I’m still in the mood for making up numbers, I’m going to say there’s a 99% chance someone will announce they’ve detected chemofossils on Mars, BUT we will spend the following decade or two arguing about whether or not they really did. As Claire Cousins writes, discovering life on Mars “[…] will be a gradual process, with evidence building up layer by layer until no other explanation exits.”

In other words, I doubt that discovering chemofossils will definitively prove that life once existed on Mars. But I do think chemofossils will be the first “layer of evidence” we find.

8 thoughts on “Sciency Words: Chemofossils

    1. Glad you liked it! Yeah, our understanding of life is very Earth-specific. We really don’t know what life in general might be like, and the science quickly turns into some pretty deep philosophical stuff.


  1. This reminds me of the debate about the earliest life (or purported life) found on Earth. Fossils of microorganisms from 3.5 billion years ago are uncontested. But earlier than that, scientists debate whether they’re looking at the remains of early life or the results of abiotic processes.

    It wouldn’t surprise me if a similar debate doesn’t end up happening for Martian findings. Any finds could leave us debating the boundary between life and non-life.

    Liked by 1 person

    1. I hadn’t thought of that, but that’s a really good parallel. The thing I was thinking of was the Allan Hills meteorite, the one that supposedly had fossilized Martian microbes inside it. The debate over that went on and on for a very long time, and I’m not sure if it’s been definitely settled yet.

      Liked by 1 person

      1. My understanding is that most scientists concluded that there were too many non-biological explanations for the meteorite findings. I wonder how they stack up against the early Earth fossils, which have similar criticisms.

        Liked by 1 person

      2. That’s my understanding as well, but every time I hear something about that meteorite, it sounds like there’s still some discussion going on. I guess if we do find fossils on Mars, someone might want to compare and contrast with the “fossils” from Allan Hills to see if they’re in any way similar.

        Liked by 1 person

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