Scientists have long known that Mars once had water, probably filling an ocean in its northern hemisphere. But today, other than its polar ice caps and possible traces in enigmatic gullies, the surface of Mars is dry. So where did that water go?
One theory is that it was lost to space early in the Red Planet’s history, when changes in the deep interior of Mars caused its magnetic field to collapse. This allowed high-energy particles and magnetic fields from the solar wind to hit the planet’s upper atmosphere, knocking molecules, including all-important water vapour, into space.
But the early Mars probably had a lot of water — between 20 million and 200 million cubic kilometres of it, according to recent estimates.
By contrast, Earth’s oceans hold about 1.3 billion cubic kilometres.
And data from NASA’s orbiting Mars Atmosphere and Volatile Evolution (MAVEN) mission — sent in part to measure exactly this atmosphere-eroding process — have not found it to be occurring fast enough to account for the geologically rapid disappearance of that much water from the early Martian surface.
Apparently, something else also happened. And while there are theories that much of the missing water is locked up in permafrost beneath the Martian soil, a new suggestion, published in the journal Nature, is that the water was trapped in hydrated minerals that were then sucked into the Martian interior.
This would have happened, says Jon Wade, an experimental petrologist at the University of Oxford, UK, when iron-rich lava flows encountered surface water and chemically reacted with it. A similar process occurs on Earth, but measurements by Mars rovers have found that Martian basalts contain nearly twice as much iron as their earthly counterparts.
That would have allowed them able to sponge up more water into a variety of iron-rich hydrous minerals, says Wade, who is the lead author of the new study.
“Assuming these water-rock reactions were efficient, it is possible that the process could have consumed in excess of a three-kilometre-thick ocean covering the entire Martian surface,” he says.
As these rocks got buried by successive flows of lava, they would have heated up with depth, just as occurs on Earth. But instead of producing new water-rich magma that returns the water to the surface, as happens on Earth, the different chemistry of the Martian rocks would have caused much of the water to be drawn into the Martian mantle, never to return.
What this means, he says, is that the young, wet Mars was “doomed from the start” due to the high iron content of its lavas. “It was likely inevitable that its surface water would have been sucked back into the mantle,” he says.
It’s an important find not just for understanding the early Mars, but in the search for exoplanets that might be suitable for life. That’s because it may not be enough simply to find a planet that is the right size, right overall composition, and right distance from its sun.
“What we suggest is that the rock chemistry may also play a significant role in setting a planet’s future fate,” Wade says. “Small subtleties such as the amount of iron may have a disproportionate role in deciding a planet’s fate. [They] may play a significant role on whether the planet’s surface can ‘hang on’ to water for lengths of time relevant to the evolution of complex life.”
Other scientists are impressed. Tomohiro Usui, a geologist and geochemist at Tokyo Institute of Technology, calls it a “possible and reasonable” explanation of the fate of the missing Martian water. The reason for the high levels of iron in the Martian basalts, he adds, it that during Mars’s formation, less iron went to its core than did on Earth, leaving more to be included in its upper layers.
Another scientist not involved in Wade’s research is David Brain, a co-investigator on the MAVEN mission from the Laboratory for Atmospheric and Space Physics, in Boulder, Colorado. He calls the new study “an interesting stride forward”.
Whatever the details might have been, there are only two directions in which the Martian water could have gone, he says: “‘up’ to space, or ‘down’ to the subsurface.”
MAVEN, he adds, has been measuring the ‘up’s — showing that the loss of atmosphere to space has been substantial over the course of Martian history.
“The Wade paper,” he says, “suggests that the ‘down’s are important too — substantial water could have been ‘hidden’ in the Martian subsurface. I think the history of Martian climate is best understood by taking the ups and downs together.”