So we might not have any evidence of actual Martian microbes (yet), but if you’re going to search for extraterrestrial life-forms, you should have an idea of what might crawl out.
Early Mars was not the desolate planet it is today. It is thought to have once been habitable, with liquid water on its surface and bearable temperatures, before it lost its atmosphere to extreme space radiation. Scientists have now used a piece of meteorite from ancient Mars to create a prototype of the type of microbial life most likely to survive there (if it exists or ever existed at all). They believe prehistoric Martian life-forms could have possibly been chemolithotrophs, or at least something close. These microbes literally eat rocks and convert them into energy and body mass.
“[Chemolithotrophs] that thrive in geothermal springs metabolize inorganic chemicals, a source of energy that provided the most likely habitable niches for life on early Mars,” said astrobiologist Tetyana Milojevic, who lead a study recently published in Nature. “Hydrothermal settings were widespread during the early geologic history of Mars and continued into the late Amazonian period.”
Somewhere in Northwest Africa, a meteorite that had been ejected from Mars landed around 5 million years ago. Fragments of it are among the rarest substances on Earth. Until the Mars Sample Return Mission flies back with samples collected by Perseverance, which the rover hasn’t even started digging for yet, this meteorite is the closest thing we have. Known as Black Beauty, it is a type of breccia, or sedimentary rock made of angular shards stuck together by mineral cement or smaller particles. It hardly has signs of weathering because it hasn’t really spent that much time on Earth. Several million years is nothing in cosmic terms.
If chemolithotrophs really did exist (or still exist) on Mars, Milojevic believes they could have colonized mineral springs like some types on Earth. Martian rock is the only real way to find out what hypothetical chemolithotrophs would actually feed on and metabolize, so a piece of Black Beauty had to be sacrificed. What she and her team wanted to see was whether the Earth chemolithotroph Metallosphaera sedula could possibly survive on rock that originated on the Red Planet. M. secula hangs out in geothermal hot springs, which are the types of places Martian microbes could have been breeding.
Microbes like this can leave behind traces of their existence on rocks, as was proven in an earlier experiment by Milojevic that involved growing similar microorganisms on simulated Martian regolith.
“In order to properly assess Martian relevant biosignatures, it is crucially important to consider chemolithotrophs in Martian relevant geological and mineralogical settings,” she said.
Would these micro-Earthlings take a bite out of Martian rock? They not only survived, but thrived. M. sedula could metabolize the pyrite-rich Black Beauty fragment NWA 7034, digesting its minerals just like it would break down similar minerals in hot springs. Pyrite (FeS2) is made of iron and sulfur, so that was one source of fuel. They also devoured, magnesium, phosphorus, potassium, and calcium, as well as trace elements embedded in the meteoritic rock. Other minerals provided it with iron ions that gave it the energy to breathe.
Sure enough, M. sedula left behind a “fingerprints” of oxidation and metal breakdown that could mean the existence of life on Mars if found in samples straight from the source. Even if (hypothetical, again) Martian microbes are found in fossil form, or if they left behind detectable biosignatures, it would still be unreal.
Perseverance better persevere through a long, alien journey through the frozen desert that may have once been a hotbed of life. It has a lot of digging ahead.