This blog post discusses a new article that my colleagues and I recently published in the journal, Astrobiology. Those with access to this journal can find the article here.
Life on Earth is not limited to the planet’s surface, but reaches high into the atmosphere and deep underground. In the darkness, miles beneath our feet, microorganisms like bacteria continue the great struggle for existence. Astrobiologists are well aware of this. The surface of Mars is an irradiated, frozen desert, but the deep subsurface may be a warm, watery safe haven, where nutrients dissolved in groundwater might sustain life — just as they do on Earth. Unfortunately, we can’t send a probe deep into the martian crust to look for microbes sheltering in the rocks. Such a mission would be immensely costly and difficult. What we can do instead is search for traces of life trapped in ancient minerals that formed deep underground. These minerals can find their up way to the surface — or the surface can find its way down to them — through geological processes of uplift, faulting, and erosion.
High pressures in the deep subsurface can split rocks apart, opening fractures that fill with mineral-laden water. The minerals precipitate into the fractures, forming the thin, sheet-like veins — often seen as streaks of white in pebbles on the beach. NASA rovers have frequently encountered mineral veins on Mars. These tend to be composed of calcium sulfate salts like gypsum (the main ingredient in plaster of Paris). Some of them formed at least a kilometre-deep underground, about four billion years ago. These veins are intriguing for astrobiologists. We know that they precipitated from water that could once have hosted life — not too hot, too cold, or too salty. We also know that this water had to be rich in dissolved sulfate, which forms gypsum when it precipitates. On Earth, many bacteria use dissolved sulfate in the same way that humans and other animals use oxygen — by reacting it with carbon-rich organic compounds to extract energy. The technical name for this process (which effectively transfers electrons to the sulfur atom) is “sulfate reduction”. But could evidence of ancient sulfate reducing microbes ever be found in calcium sulfate veins on Mars?
In our paper, we show that the answer might be “yes”. We studied some calcium sulfate veins from England that are similar in texture, composition, and context to those visited by the NASA rovers Opportunity and Curiosity on Mars. We found that these veins contain iron pyrite (a beautiful mineral, “fool’s gold”), one of the waste products of microbial sulfate reduction. To establish the origin of this pyrite, we used an instrument called an ion microprobe mass spectrometer to measure the isotopic composition of the sulfur in the pyrite. The results revealed a clear isotopic “biosignature” — a fingerprint left behind by bacteria deep underground millions of years ago. Organic compounds were also found, closely associated with the pyrite.
These discoveries show that calcium sulfate veins, which are so common on Mars, might be worth bringing back to Earth for analysis. They could give us a great deal of information about subsurface environments on early Mars, and might even contain evidence of life. We’re not saying that these veins should be the number-one priority — there are plenty of other interesting things on Mars, and we can only collect or examine a limited amount of material. It will be important to figure out whether the veins we looked at are unusual; maybe most gypsum veins are boring, and we just got lucky. We hope to address this in future work.