How did Mars lose its oceans

Background: Where the lost water of Mars is believed to be

Billions of years ago Mars was a warm home for lakes and oceans. But then, billions of years ago, these huge bodies of water disappeared from its surface. Previously, scientists believed that the water evaporated into space as the planet's atmosphere thinned. As it turns out, the water could have disappeared in exactly the opposite direction, namely into the subsurface. This is the conclusion reached by researchers from the California Institute of Technology (Caltech) in a study that could also have consequences for the search for habitable exoplanets.

"This work rests on the shoulders of decades of work," says Eva L. Scheller, planetary geologist at Caltech and lead author of the new study. "More and more observational results have led us to rethink the water loss on Mars in new ways."

Life on mars

Current estimates assume that Mars had a global equivalent water layer (m GEL) between 100 and 1500 meters on its surface. "M GEL" refers to a layer of one meter of water that would cover a flat surface of the planet. According to Scheller, 1,000 m of GEL is roughly half the amount of water in the Atlantic Ocean. Even the lower value of the Mars estimate range is still enough water with which potential life could have arisen.

Hence, it is important to find out how it disappeared. This will help better understand where on Mars evidence of life that evolved during that time might be preserved - and how current and future Mars missions should look for that evidence.

Most water loss models that assume atmospheric loss assume that UV radiation dissociates water vapor in the air into its components hydrogen and oxygen. Both elements, especially the lighter hydrogen molecules, escape from the atmosphere and end up in space. Scientists measure this hydrogen loss - with neutron detectors such as the ESA FREND instrument and the Russian trace gas orbiter TGO - as a proxy for determining the rate of water loss on Mars over time.

Little water in the crust

There are two problems with this theory, however. For one thing, it doesn't explain why TGO or other missions are still discovering so much water in the Martian crust. Second, the rate of hydrogen loss measured so far is too low to explain how much water Mars originally had. "It could only explain the lower end of what most geologists assumed," says Scheller. In addition, the researchers now have a better understanding of how much water is hidden in the Martian crust. Much of this is thanks to rover missions like Curiosity, which examined Martian rocks directly, as well as laboratory analysis of Martian meteorites that have landed on Earth.

Scheller and her colleagues have now developed a new model that uses current data to investigate whether the water could actually have got underground. It would not have been sucked into vast subterranean oceans. Instead, water molecules have been incorporated into mineral structures such as clays as a result of processes such as weathering. The same thing happens here on earth. According to the model, this process could explain between 30 and 99 percent of all water lost in the planet's first one to two billion years. Atmospheric loss would be responsible for the rest.

"It's an extremely fascinating model," says Joe Levy, a geologist at Colgate University who was not involved in the study. "Hydrated minerals and vein-forming minerals can be seen almost everywhere on Mars. The runaway chemical weathering is a really provocative hypothesis to explain what happened to the waters of Mars."

Billions of years passed

The range from 30 to 99 percent is of course huge. That's because science doesn't know enough about the water content in the crust, least of all on a global scale, or what the ancient Martian atmosphere was like and how it promoted or limited atmospheric water loss. The new model therefore also tries to take into account how geological activities long ago, such as volcanism, could have influenced these water loss mechanisms.

The model also provides new information about the habitability of Mars. "The results also answer when he lost his water," says Scheller. The authors are certain that the hydrated minerals in the crust are more than three billion years old, meaning Mars may have been the most habitable before. Any search for evidence of past life should therefore be directed to rocks that have been preserved from that earlier period.

Scheller suggests that both the Curiosity and Perseverance rovers might be able to search for samples for this period. In particular, the Perseverance rover, whose mission is primarily to search for evidence of life on Mars, will explore a 3.8 billion year old former lake bed.

Rover should help

"It will investigate exactly which mechanisms could have caused the water binding in these minerals in the crust," says Scheller. Even when he can't do the job on his own, he collects samples that scientists could then examine in the laboratory.

Earth and Mars started out as very similar humid worlds, but went drastically different paths. The loss of water to hydrated minerals in the crust is not unique to Mars. This also happens all the time on earth. However, the earth benefits from the fact that its tectonic plates are actively recycling its crustal rocks, releasing this water.

It also maintained a thick atmosphere that kept the planet at the perfect temperature for life to develop and flourish. Mars has no tectonic plates and was bleeding in its atmosphere when its magnetic field went out four billion years ago. "Ultimately, it is this fact that should be considered when considering the habitability of terrestrial planets," says Scheller. "She is very fragile."

(vsz)