A recently published study of a 30-pound martian meteorite found in Antarctica suggests the presence of indigenous carbon-rich material, ancient water erosion, and a number of tiny structures that resemble the sort of features that we see rock-eating microbes leaving in basaltic glasses here on Earth.
This rock, Yamato 000593, appears to have formed 1.3 billion years ago from martian lava. About 12 million years ago it got blasted from the surface of Mars as part of the spall from an asteroid impact, before winding up falling to Earth's surface as a meteorite some 50,000 years ago.
Perhaps the most intriguing data from the study is this image of a thin, polished section of the interior of the meteorite:
These tubule and microtubule-like structures emanating from a mineral vein strongly resemble structures in material taken from places like the side of Mauna Loa and oceanic crust on Earth. Previous analyses of these Earthly samples have attributed the features to so-called 'autolithotrophic' organisms - microbes that eat raw inorganic material, seeking out elements like iron or manganese in particular to drive their metabolism.
This example (2nd figure) shows the microscopic tunnels that terrestrial microbes appear to have bored in a piece of volcanic glass. Fissures allow the organisms to be carried into the rock by water, but once there they can cause their own bio-erosion. The form of these tunnels is not the same as those known to be caused by water reacting with the minerals, and DNA traces have been retrieved from these sites.
So what's happening in Yamato 000593? The authors use a decent amount of caution, but do claim to find - in addition to carbon-rich (but not carbonate) enhancements along tunnel walls - a number of nano-meter to micro-meter scale carbon-rich spherules, structures seen also in a famous martian meteorite called Nakhla - as well as the curious microtunnels or tubules.
It's certainly intriguing, although fingers have been burnt before, with bolder claims of micro-fossils in martian meteorites back in 1996 that didn't pan out (a study that included authors from the present one). One thing is certain, whatever processes have been at play in Yamato 000593 are worth understanding. If they turn out to be non-biological we'll still learn more about Mars. If they turn out to be the result of terrestrial bio-contamination (a possibility that's hard to rule out at this stage), we'll - rather ironically - learn how Earth organisms could get a foothold on Mars!
And of course, it is possible that these really are the ancient signatures of life on another world...