What Proof of Life on Mars Might Look Like

Okay, so "conspicuous biomorphic ovoid structure" doesn't sound a likely source of extraterrestrial life, but the job of an astrobiologist mostly involves searching through Martian mineral samples and Earthly proxies (like the bottom of Arctic lakes) to uncover very subtle clues. And tiny clay pockets discovered in a Martian meteorite might prove to be just such a clue: a structure bearing the distinct shape of a biological cell.

The ovoid structure in question was found in the Nakhla Martian meteorite, which was discovered in Egypt in 1911 and began providing Earthlings with many tantalizing clues about our red neighbor well before we'd ever launched the first Martian probe, including structures that hint at the presence of Martian water and, possibly, life.

The meteorite remains a subject of inquiry to this day, obviously, and the ovoid structure itself is a new find, described by a team based at Greece's National Technical University in the current edition of the open-access journal Astrobiology. The mostly clay materials comprising the meteorite, and therefore the Martian geologic environment from whence the meteorite came, make some rather unique sorts of observations possible.

"Clays constitute an important mineral group of which the formation conditions give significant clues not only to the presence of water, since clays are hydrous minerals, but also to the source, type, and volume of fluids, in addition to the timescales involved and the conditions of primary mineral alteration," the paper explains. "Even more important (and relevant to Mars astrobiology) is the notion that clay minerals may also provide clues that pertain to some of the geological processes that are potentially associated with biological activity."

Deep examinations of the microstructures found in Martian clay may give up "textural and chemical biosignatures." The specific means to producing the newly identified Nakhla structures likely involve some "shock event," like an asteroid impact, which caused the region around the future meteorite to heat up, melting the local permafrost, and leading to "aqueous intrusions" into the clay materials that would eventually wind up in Egypt.

The paper describes three plausible abiotic (not life-based) scenarios for the ovoid structure's formation: hydrothermal "corrosion" of an existing mineral structure; the precipitation of new structures during the shock event (the magnification of some preexisting defect); or palagonitization, a process involving the interaction of water and lava. While these are all more likely explanations than the presence of some biology, the possibility remains.

"One reason why we carried out this investigation into the origin of the ovoid structure in Nakhla is because the conspicuous rounded shape of the structure is somewhat reminiscent of a terrestrial cellular microorganism," the authors of the new paper explain. Among the most immediate problems, however, is that the ovoid shapes are the wrong size. Despite "abundant evidence" within the Nakhla that some key preconditions for biological life are present, "the ovoid is very large, and martian microorganisms are expected to be chemotrophic and therefore probably very small."

Finally, the paper concludes, "The consideration of possible biotic scenarios for the origin of the ovoid structure in Nakhla currently lacks any sort of compelling evidence." The available data surrounding the structures—accumulated through intensive examinations involving electron microscopy, x-ray, and spectroscopy—point instead to abiotic processes, not life. At the very least, however, it also doesn't rule out the possibility of biology at work elsewhere within the meteorite's source materials: just not this time.