Tiny tunnels in meteorite whisper about the history of Mars

Peeking inside an ancient Martian meteorite explores the history of the red planet. Analysis of trapped gas bubbles confirm its origin, while tunnel-structures within the clay are similar to the textures we see created by bacteria wriggling around in Earth-rocks.

While the Curiosity and Opportunity rovers continue to explore Mars as robotic geologists, taking photographs, drilling into rocks, and even performing geochemical analysis, the only samples we can directly study ourselves are sky-drop rock-deliveries from space: meteorites. Yamato 000593 is a 30-pound (13.7-kilogram) Martian meteorite discovered in Antarctica.

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Researchers sliced and polished a section of the meteorite into a thin section, allowing them to observe mineral composition and microstructures. The rock formed 1.3 billion years ago from one of the planet's many volcanoes, and was ejected by an impact 12 million years ago. After that, the rock drifted in space until colliding with the Earth, landing in Antarctica about 50,000 years ago.

Lucky us, we actually get pelted with space-rocks fairly frequently, so we have a other meteorites to compare this one to. Nakhla is a Martian meteorite we watched fall in Egypt, so we know it was exposed to a hot, dry, salty terrestrial environment for a very brief time, versus Yamato's 50,000 years in cold, wet visit to Antarctica. Meteorite LEW87051 was also found in Antarctica, but doesn't come from Mars. So, any features that Yamato and Nakhla share are probably things that happened to them on Mars, while any similarities between Yamato and LEW87051 are changes that happened in Antarctica.

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The age ranges for Yamato check out as same-old radiometric dates for most Martian meteorites (165 million to 1.4 billion years old), and the ejection date is within the same "in the last 20 million years" as the other rocks. It's a low-silica mafic to ultramafic rock, which is again, totally normal: all those beautiful shield volcanoes are made of the same style basalts as Hawaii although on a much bigger scale. The gas content trapped in bubbles check out as normal-Martian material. The exact composition of altered minerals suggests the rock was in a lower-temperature brine that evaporated in a relatively short timeframe, but since this is geology "low temperature" translates as less than 300°F (<150°C), and short is hours to weeks. Enough evaporate deposits and alternation minerals have been seen in other meteorites that the "evidence of water," although neat, isn't new.

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Despite getting sued for ignoring a rock getting flipped by a rover-wheel, NASA does constantly hunt for signs of extraterrestrial life. For meteorites, that means hunting for any traces of carbon, then listing all the ways it could form that don't involve life. White and the rest of her team found microscopic spheroids of carbonates (red circle) layered in between iddingsite (blue circle; a water-altered mineral). They aren't groundbreaking discoveries — they're actually consistent with earlier observations of the same type of carbonate spheroids found in the Egyptian Mars-meteorite Nakhla — but do keep growing the pile of evidence of a warm, wet Martian history. The physical overlap of the carbonate spheres and iddingsite even says they formed together during evaporation of mineral-altering brine. The list of purely geologic activities that could produce the spheres without invoking alien life is long enough to be ignored in favour of something much stranger: the texture.

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The texture is downright weird. Throughout olivine crystals in the sample are tiny, wriggly tunnels (black arrows). On Earth, basaltic rocks of the same composition weather into clays when exposed to water, and bacteria carve out microscopic, undulating tunnels. More interestingly, these same tunnels are found in the Martian Nakhla, but not in the non-Martian LEW87051. It's a whispered hint that the texture in these Martian rocks are the result of biotic activity in a wet environment. Or, in the cautious word of an academic who doesn't want the media to scream they've found aliens,

We cannot exclude the possibility that the carbon- rich regions in both sets of features may be the product of abiotic mechanisms; however, textural and compositional similarities to features in terrestrial samples, which have been interpreted as biogenic, imply the intriguing possibility that the martian features were formed by biotic activity.

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which roughly translates into, "maybe something besides bacteria caused these tunnels, but we don't know what that would be." Similarity isn't proof, and "imply the possibility" isn't the strongest declaration of discovery, but finding the same weird features in this new meteorite justifies continuing to hunt for and study meteorites, and keep looking for something that is truly, distinctly alien.

For more, check out the JPL press release, or the original paper (open access). All images from Putative Indigenous Carbon-Bearing Alteration Features in Martian Meteorite Yamato 000593.

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