Mars once had a climate that kept it warm enough for water to flow on the surface. But scientists haven’t been able to agree on an atmospheric recipe with enough warming potential. A new proposal says that hot water circulating underground produced a missing ingredient.
Water reacting with iron in Mars’s crust may have delivered a steady supply of hydrogen to the atmosphere for millions of years, says Cin-Ty Lee at Rice University in Texas.
While not a greenhouse gas itself, hydrogen can have a planet-warming effect through its interactions with atmospheric carbon dioxide. Collisions between the H2 and CO2 molecules deform both of them, and this makes them absorb wavelengths of infrared radiation that CO2 by itself doesn’t block, so the molecule pairs enhance the greenhouse effect.
Depending on the total amount of CO2 in the Martian atmosphere, even a few per cent of hydrogen in the atmosphere could be enough to bring the average global temperature above freezing. “I think there’s enough CO2 even now to do it,” says Lee.
The underground hydrogen production plant that Lee proposes is the extremely thick crust, thought to extend as far as 80 kilometres below ground, that underlies Mars’s southern highlands. “Like an iceberg floating on water, the higher the mountain, the thicker the root,” he says.
Just as on Earth, the crust will contain radioactive elements, such as uranium, thorium and potassium, which produce heat as they decay. Thicker crust contains a greater total quantity of these elements, while the surface area available for cooling remains the same. In 2024, Lee published calculations showing that, with sufficiently thick crust, ice in its upper few kilometres would melt and create aquifers.
Once those aquifers exist, Lee told the annual conference of the Geological Society of America in Denver, Colorado, earlier this month, the water will start shedding its heat by circulating up and down through the porous upper layers of the crust. These layers mostly consist of basalt, which is rich in iron. The water would make the iron rust, a process that frees up its constituent hydrogen molecules, which would then find their way up into the atmosphere.
The hydrothermal pump eventually died, leaving Mars the cold desert it is today, because it ran out of both water and power. “For the water, this was a one-way ticket,” says Lee. “And also the radioactivity diminished.”
Ramses Ramirez of the University of Central Florida, who has studied volcanoes as potential sources of hydrogen for the warming of Mars, says that until he has seen a full paper about the idea, he is sceptical the idea can explain Mars’s formerly warm surface “without invoking some extremely favourable circumstances”.
Ramirez also notes that the possibility of water releasing hydrogen from rocks was proposed over a decade ago. But, according to Lee, while earlier studies just supposed that water somehow would be involved, his work shows “how it’s almost inevitable that this has to happen” with a thick enough crust.
He says his idea is testable by looking deep into the Martian crust. “If we find that it is oxidised down to several kilometres, this is the only way it can happen.”
As scientific bycatch, this would then also provide an explanation for the redness of Mars. Meteoroid impacts or other processes would expose some of the rusted rock on the surface. “With the erosion by wind, you are going to generate a lot of dust and it coats everything,” says Lee.