Mars should have loads more water – so where has it all gone?

MARS has a real water shortage. It seems we have either misunderstood what its early years were like – or vast amounts of water are hiding beneath its surface.

A lot of evidence points towards Mars being warm and wet early in its history; features that look like rivers, lakes and outflows have been spotted both from orbit and by rovers on the surface, and a lot of the planet’s minerals contain water.

So where did all this water go? The Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft was sent to find the answer. Since its arrival at Mars in 2014, it has been measuring how much atmosphere Mars is losing to space. From that, we can figure out how much it had in the past.

The orbiter keeps track of both the activity of the sun and the ions streaming away from the planet’s atmosphere to build up an inventory of everything that enters and leaves over time. It also estimates the total loss by measuring the fraction of heavier isotopes of certain atoms versus their lighter counterparts. As the lighter versions are easier to knock out into space with a stray cosmic ray or extra energy from solar photons, a higher fraction of heavy isotopes remaining in Mars’s present-day atmosphere means much of the original atmosphere has been lost.

MAVEN focuses on hydrogen and oxygen as ways to trace water and carbon dioxide, and neutral argon as a way to measure the sheer volume of atmosphere loss. Based on measurements of these taken over a full Martian year, the team concludes that about 4 billion years ago, the Red Planet’s atmospheric pressure – currently less than 1 per cent of Earth’s – was up to 1.5 times what Earth’s is today. They also found that it could have had the equivalent of a global ocean between 2 and 40 metres deep in its distant past.

“It’s a consistent story,” said team leader Bruce Jakosky at the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, who presented the findings at the American Geophysical Union meeting in San Francisco in December. “Loss of gas to space is likely a major if not the major process for changing the Mars climate through time.”

The trouble is, that’s less water than expected. In 2015, James Head at Brown University and Michael Carr at the US Geological Survey estimated that the equivalent of a global ocean a few hundred metres deep was needed to explain all the geological features that look like they were formed by water.

“We were counting on their loss rate to explain it,” Head says. “And they didn’t come through.”

One possible reason for the discrepancy is that the long-held notion of Mars being like Earth in the past is wrong. One theory has it that the planet was actually cold and dry, and that streams and rivers formed underneath the ice pack instead of via water flowing on the surface. All that would be needed is a slightly denser CO₂ atmosphere – which MAVEN’s measurements suggest you had.

“Loss of gas to space is likely a major process for changing the Mars climate through time”

The other option is that the water is hidden away somewhere, maybe underground. Dark streaks recently spotted on crater rims that look like they could be liquid water may be fed by underground aquifers, for instance.

“Either it’s hidden somewhere, or there wasn’t that much to start with,” says Carr.

This article appeared in print under the headline “Where did all the Martian water go?”