NASA’s Curiosity rover has been traveling along the bottom of Gale Crater for more than ten years and never ceases to delight us not only with fascinating images of Martian landscapes, but also with no less interesting scientific data about the past and present of the Red Planet. For example, in 2013, the rover’s instruments recorded a surge in methane concentration for the first time, which immediately caused a storm of discussion and speculation about where this simple hydrocarbon came from on Mars. The spectroscopic data on the composition of the atmosphere obtained from the Trace Gas Orbiter orbiter added doubts to scientists – no methane was visible “from above” … Although later this discrepancy seemed to be sorted out: the rover measured methane at night, and the orbiter composition of the atmosphere already in the daytime, when the methane accumulated at the surface during the night, most likely, had already had time to dissipate.
Interest in methane is due to the fact that on Earth this gas has a predominantly biological origin: it is either directly produced by methanogen bacteria, or obtained by chemical (not biochemical) ways from a substance of organic origin. Methane both here and there carries a “biomarker” – a special ratio of carbon isotopes. Life, in the forms that we know, “loves” light atoms. This means that biochemical reactions involving lighter isotopes of the same carbon go a little faster than with heavier ones.
Carbon has two stable isotopes: carbon with mass 12 (light) and mass 13 (heavy). In any substance that was once alive, the proportion of heavy carbon will be slightly less than its average content on the planet. Therefore, even in ancient fossils, in which all organic matter was mineralized a billion years ago, and all the carbon contained in proteins, fats and carbohydrates turned into carbonates, the ratio of light and heavy carbon atoms will indicate that the remains of living matter were fossilized. But this is on Earth, but what is there, on Mars?
During its wanderings, Curiosity drilled many holes in a wide variety of Martian rocks. If the drilled stone dust is placed in a closed container and heated up to 500 degrees, then the organic substances contained in the stone will begin to decompose with the release of our beloved methane. Of course, it will not be much, but it is quite enough for a sensitive device like a mass spectrometer to be able not only to detect them, but also to determine the ratio of isotopes of light and heavy carbon in methane.
As they say in fresh article in PNAS researchers from Pennsylvania State University and several other scientific centers, in 22 of the 24 rock samples collected and studied by Curiosity, the content of heavy carbon-13 was found to be less than the average for the planet (although the range of values is quite large). In other words, the carbon “stuck” in those rocks was enriched in the light isotope by some process. But what kind of process could it be?
Researchers on this topic, as usual, have several hypotheses. It could be ancient Martian life that inhabited the crater when there was water in it a long, long time ago. Moreover, judging by the low concentration of the carbon-13 isotope, there could be both methanogenic organisms producing methane and methanotrophic organisms consuming methane on Mars. That is, carbon had to undergo a “double” enrichment with a light isotope. However, it is too early for supporters of the “once inhabited Mars” hypothesis to rejoice.
Carbon with a reduced content of the heavy isotope could have come to Mars from space along with cosmic dust. It is just very depleted in heavy carbon, and with a successful combination of geophysical circumstances, such as the onset of glaciation, it could well confuse carbon “maps” for astrobiologists. But that’s not all. Photochemical processes in the atmosphere, which can lead to the accumulation of light or heavy isotopes in certain substances, are also quite capable of “mimicking” life in terms of isotopic traces. Add here more various chemical reactions with the participation of minerals in the thickness of the Martian surface, and we will get that again nothing is clear with life on Mars. But, as they say, water wears away a stone. Therefore, the rovers will continue to sharpen Martian stones with great zeal, drill deeper into the surface of the Red Planet and examine the extracted samples with more accurate instruments. Maybe they’ll find it.
Based on materials PNAS.
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