Last January, NASA announced the discovery that several of the rock samples analyzed by the rover curiosity they are enriched in light isotopes of carbon. On Earth, this geochemical signal would be immediately interpreted as almost indisputable evidence of the presence of microbial life in the remote past. The reason is that carbon exists in two stable isotopic forms: carbon-12, or light, and carbon-13, whose nucleus contains an extra neutron. Due to this extra neutron, carbon-13 is heavier and bulkier, which has prompted the terrestrial biosphere to develop a lazy tendency to use the light form, and thus virtually all of the organic compounds made by life. on Earth they are enriched in carbon-12.
For your research, curiosity analyzed 24 different rock samples, all taken from the floor of the Gale Crater, where he has been working since 2012. The rocks studied were formed by the compaction of sediments from the bottom of an ancient dammed lake in the crater 3.5 billion years ago. curiosity he heated the samples to 850 degrees to release the gases trapped inside, so he could measure their isotopic composition. The results varied widely between samples, but at six specific sites, the amount of carbon-12 detected was significantly higher than what has been measured in the current Martian atmosphere and in meteorites from Mars that we have in our collections on Earth.
The explanation for this light carbon enrichment, in the absence of biological processes, is not simple. In fact, on Earth it would be interpreted as the result of the metabolic activity of microorganisms inhabiting the deep subsoil, and feeding on carbon from magma, with a preference for trapping the light isotope. These microbes would give off methane as a metabolic by-product, and other microorganisms settled in more superficial areas would feed on the gas, also selecting the methane (CH4) formed with light carbon. As a result of this biological cycle of active carbon over a long period of time, the inhabited areas at the bottom of the primitive Gale Lake would have generated a fossil record enriched in light carbon, which Curiosity is now unearthing.
Of course, being another planet, it is necessary to firmly rule out possible non-biological processes as an alternative origin of isotopic fractionation, before talking about extraterrestrial life. Two abiotic explanations have been proposed to understand the observations of curiosity, involving ultraviolet (UV) radiation and stardust. On the one hand, UV radiation can react with carbon dioxide, which today makes up 96% of the Martian atmosphere, to produce organic compounds; but the process by which these organic compounds would become enriched in carbon-12 is not clear, since the current Martian atmosphere is actually enriched in carbon-13, and we know that it already exhibited a similar isotopic imbalance when the sediments that are being deposited were deposited. analyzing curiosity. And, on the other hand, perhaps the young Solar System passed through an interstellar cloud of gas and dust, which are precisely characterized by containing carbon-12; the cloud could have blocked out sunlight and plunged Mars into a deep freeze, causing widespread glaciation and preventing light carbon from the cosmic dust shower from being diluted with other sources of Martian carbon. Are these processes less exotic than the existence of microorganisms at the bottom of a lake on a terrestrial planet?
The reality is that non-biological alternatives are at least as unlikely as a scenario in which the subsurface of a lake on early Mars harbored microorganisms with a methane-based metabolism, and around the same time that we know for sure that these biological processes occurred on Earth. Therefore, and for the first time, the biological interpretation to understand the Martian data is not the most incredible. It certainly requires many additional explanations, including independent evidence of the presence of life and a better understanding of the carbon cycle on Mars; but verifying the alternative hypotheses will not require less work. Isotopic enrichment in Martian rocks finally provides an extraordinary clue that forces us to turn our approach to the problem of life on Mars on its head: it is time to show that the discovery of curiosity It has nothing to do with biology.
Alberto Gonzalez Fairen He is a researcher at the Center for Astrobiology (CSIC-INTA) in Madrid and at the Department of Astronomy at Cornell University in New York.
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