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 that was vastly different from its current dry state. But accounting for all the sources of water on the Martian surface and all the ways it could have been taken away found a fundamental discrepancy—we simply don’t know where all that water went.

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

But when Bruce Jakosky at the University of Colorado Boulder and his colleagues tallied up all the ways water could have been removed from the surface since then, finding that it adds up to only a few tens of meters at most. Jakosky introduced this job at the Lunar and Planetary Science Conference (LPSC) in Texas on March 20.

The total water near the surface of Mars, mostly in the form of ice and hydrated minerals, is roughly equivalent to a global ocean only 30 meters deep. “As you get from 150 meters, take a few tens [of metres] and get to 30 meters? You can’t do that. Clearly, there’s something missing in our understanding,” Jakosky said. Even if you take a lower reasonable limit for each process that could have added water to the surface and an upper reasonable limit for each process that removed it, the discrepancy still isn’t completely mitigated, he said.

There are several ideas as to where the water could have gone: it could be that much more of it has evaporated into space since the end of the Noachian than we thought, it could have frozen in as-yet-undiscovered ice deposits, we could be misunderstanding the interactions between ice caps and the atmosphere, or maybe some sources of water really do interact with each other in unexpected ways and we’re miscalculating. It’s most likely some combination of these and possibly other mechanisms, Jakosky said.

While such a large difference may be surprising, it is uncontroversial to say that we do not fully understand the history of water on Mars. In other talks at the LPSC, scientists put forward the idea that instead of one long period of surface water, there could be short periods of rain followed by drought.

“This suggests that the hydrologic cycle on Mars was quite different from Earth, and probably different from terrestrial counterparts,” he said. Eric Hiatt at Washington University in St. Louis during his lecture. His research suggests that groundwater on Mars may not interact with the surface and atmosphere as we previously thought, which could change our view of how much water has actually been added to the surface.

In other words, Bethany Ehlmann at the University of Colorado Boulder suggested that there may now be more water on Mars than we have traditionally thought. All of this highlights that while we know a lot about Mars, we don’t know enough to get a complete picture of its hydrological history.

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

With NASA and SpaceX prioritizing lunar exploration, it could be decades before humans set foot on Mars, so any progress will be incremental for now, 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 in an exciting program that includes an excursion behind the iconic Lovell Telescope.

topics: