Today Mars is cold and dry, but 3.8–3.6 billion years ago it apparently periodically experienced warm and wet periods. One possible cause of the warming could have been hydrogen emissions entering the atmosphere from meteorite impacts, volcanic eruptions, and chemical reactions in rocks.
The study by scientists from the Tokyo Institute of Science, Tohoku University, and other organizations was published in The Planetary Science Journal. The authors combined a chemical model of the atmosphere with a global climate model of ancient Mars and tested a more detailed version of an earlier hypothesis, which had previously been estimated using simplified models with average values for the entire planet.
They started from an atmosphere twice as dense as Earth’s, dominated by carbon dioxide and 6% hydrogen. The calculations showed that the amount of formaldehyde falling to the surface differed by about 100 times across regions, and the main factor was water vapor: in humid air, ultraviolet radiation more actively broke down water molecules and triggered a chain of reactions.
Rain carried the resulting formaldehyde to the surface and into bodies of water, where it could turn into more complex organic compounds. Snow played almost no role in this process because snowflakes retain formaldehyde 10–100 times less effectively than raindrops.
According to the model, the northern hemisphere received the most of the substance, where there was likely an ocean at the time, and the maxima occurred on the volcanic highlands of Tharsis and Elysium. There, as air rose along the slopes, it cooled and fell as rain, and formaldehyde deposition reached about 50 mg per square meter per year — roughly ten times the planetary average.
The authors also evaluated the landing sites of Mars rovers: in Gale Crater, where Curiosity found organic compounds in ancient sedimentary rocks, and in Jezero Crater, where Perseverance detected signs of such compounds, the values were below average. At the same time, the mountains south of Gale Crater received a lot of formaldehyde, and rivers with groundwater could carry it into the crater; one of the richest regions, the model said, was the landing site of the Chinese rover Zhurong, which does not have instruments for searching for organics.
The researchers emphasize that their calculations show only the delivery of formaldehyde to the surface, not how much of it could have survived to the present day. To look for traces of ancient chemical evolution and the possible origin of life, they suggest choosing lowlands near persistently wet regions, where the substance could accumulate and concentrate as water evaporated.
If organic matter from areas with high and low formaldehyde influx is compared, it will be possible to understand what fraction of it has an atmospheric origin. According to the authors, future missions will help verify the conclusions.
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