Current estimates suggest that Mars may have had between 100 and 1,500 meters of global equivalent layer (m GEL) of water on its surface. (m GEL refers to a one-meter layer of water that would cover a flat, even surface – Scheller says 1,000 m GEL is roughly half the water in the Atlantic Ocean.) Even the lower end of that estimate is still the water that potential life could have used to build a house.
It is therefore essential to learn how he disappeared. If we know what happened, we might have a better understanding of what locations on Mars might have retained evidence of any life that evolved during that time – and how current and future missions to Mars might seek out that evidence.
In most models of water loss that assume atmospheric loss, the idea has been that UV radiation causes water that is very present in the air to dissociate into hydrogen and oxygen. Both elements – but especially the lighter hydrogen molecules – escape from the atmosphere and head into space. Scientists are measuring this loss of hydrogen (using neutron detectors such as ESA’s FREND instrument and Russia’s trace gas orbiter) as an indicator to determine the rate of water loss on Mars. over time.
However, there are two problems with this theory. For one thing, that doesn’t explain why TGO or other missions are still detecting so much water in the Martian crust. Second, the rate of hydrogen loss measured so far is too small to account for the amount of water we believe Mars originally had. “That could only really explain the lower end of what most geologists think,” says Scheller.
At the same time, we now have a better understanding of the amount of water buried in the Martian crust. Much of this is due in large part to rover missions like Curiosity that have studied Martian rocks directly, as well as laboratory analysis of meteorites from Mars that have landed on Earth. And all of this data slowly led scientists to take the idea that the crust played a bigger role in water loss on Mars more seriously.
Now, Scheller and his colleagues have come up with a new model that uses current data to examine whether the water could have been underground instead.
This water would not have been sucked into huge underground oceans. Instead, water molecules got incorporated into mineral structures like clays as a result of processes like weathering. The same is happening here on Earth.
This process could account for between 30% and 99% of the total water loss during the first 1 to 2 billion years of the planet, depending on the model. Atmospheric loss could make up for the rest.
“It’s an extremely intriguing model,” says Joe Levy, a geologist at Colgate University, who was not involved in the study. “Hydrated minerals and vein-forming minerals are almost everywhere on Mars. Leaking chemical weathering is a truly provocative hypothesis as to what happened to the water on Mars.
A range of 30% to 99% is, of course, huge. This is because we just don’t know enough about the water content of the crust (especially on a global scale), or what the old atmosphere of Mars looked like and to what extent it encouraged or limited the loss of atmospheric water. The model also attempts to take into account how geological activity in the ancient past (such as volcanism) could have affected these mechanisms of water loss.
The model gives us new clues regarding Martian habitability. “The results not only respond to how Mars may have lost its water, but also when he lost his water, ”says Scheller. The authors are certain that the hydrated minerals in the crust are over 3 billion years old, meaning that Mars was potentially the most habitable before that. Any search for evidence of ancient life would be better directed towards rocks that have been preserved from this earlier period.
Scheller suggests that the Curiosity and Perseverance rovers may be able to search for samples within this time range. Perseverance in particular, whose mission is mainly devoted to the search for evidence of Martian life, explore a 3.8 billion year old lake bed. “He’ll be just there to investigate what could have been the mechanisms that caused the sequestration of water in these minerals in the crust,” says Scheller. Even if he can’t do the job on his own, it will capture samples that scientists could study themselves in the lab.
Earth and Mars started out as very similar wet worlds, but ended up taking radically different paths. The loss of water from hydrated minerals in the crust is not unique to Mars; it happens on Earth all the time. But the Earth benefits from the fact that its tectonic plates are actively recycling its crustal rocks in a process that would release this water. In addition, it retained a thick atmosphere which kept the planet at the ideal temperature for life to evolve and prosper. Mars lacks tectonic plates and hemorrhaged from its atmosphere after its magnetic field stopped 4 billion years ago.
“Ultimately, that’s what to keep in mind about habitability on terrestrial planets,” says Scheller. “It’s very fragile.”