Why the lack of water on Mars is so mysterious

Planetary scientists agree that Mars once had liquid water on its surface and a water-rich atmosphere, quite different from its current arid state. But an accounting of all the water sources up to the surface of Mars and all the ways it could have been withdrawn revealed a major discrepancy: We simply don’t know where all that water has gone.

The period when Mars is thought to have had liquid water, around 4.5 to 3.7 billion years ago, is known as the Noachian period. Based on our best estimates of how water could have reached the Martian surface, there should have been enough surface water at the end of the Noachian period to cover the entire planet in an ocean between 150 and 250 meters deep.

But when Bruce Jakosky of the University of Colorado at Boulder and his colleagues took stock of all the ways water could have been removed from the surface since then, they found that it only amounted to a few dozen meters at most. Jakosky presented this work at the Lunar and Planetary Science Conference (LPSC) in Texas on March 20.

The total water currently present near the Martian surface, mainly in the form of ice and hydrated minerals, is roughly the equivalent of a global ocean only 30 meters deep. “How do you go from 150 meters, remove a few dozen [of metres] and get to 30 meters? You can’t do that. “It’s clear there’s something missing in our understanding,” Jakosky said. Even if you take the reasonable lower limit of every process that could have added water to the surface and the reasonable upper limit of every process that removed it, the gap is still not completely mitigated, he said.

There are a few ideas about where the water could have gone: it could be that more water has evaporated into space since the end of the Noachian than we thought, it could be frozen in as-yet-unknown ice deposits, we could be misunderstanding the interactions between the ice sheets and the atmosphere, or maybe some of the water sources are actually interacting with each other in unexpected ways and we are exaggerating. It’s most likely a combination of these, and perhaps other mechanisms as well, Jakosky said.

While such a large discrepancy may be surprising, there is no denying that we do not fully understand the history of water on Mars. In other discussions at LPSC, researchers proposed the idea that instead of having a long period of water on the surface, there could have been brief periods of rain followed by drought.

“This suggests that the hydrological cycle on Mars was completely different from that of Earth, and probably distinct from that of its terrestrial analogues,” Eric Hiatt of Washington University in St. Louis said during his presentation. His research suggests that Mars’ groundwater may not interact with the surface and atmosphere in the ways we previously thought, which could change our view of how much water is actually added to the surface.

In another talk, Bethany Ehlmann, of the University of Colorado at Boulder, suggested that there might be even more water on Mars than we traditionally thought. All of this shows that while we know a lot about Mars, we don’t know enough to paint a complete picture of its hydrological history.

Solving the mystery of Mars’ water – and therefore its potential habitability at different times in its history – will be a monumental task. “How can we move forward in this area? We’re not going to do it with more models,” Jakosky said. “If you ask me, I think it really requires troops on the ground. »

With NASA and SpaceX both prioritizing Moon exploration, it could be decades before a human sets foot on Mars. For now, any progress will therefore be gradual, with data from rovers and orbiters.

Mysteries of the Universe: Cheshire, England

Spend a weekend with some of the brightest minds in science and explore the mysteries of the universe on an exciting program that includes an excursion to see the iconic Lovell Telescope.