Mars is Thirsty: It lost an Ocean of Water

Mars has water.  This statement was in question 15 years ago, but now it’s an accepted idea backed up by proof obtained by the last three Mars rovers.  Not only does Mars have water now, but it had significantly more water in the past, as evidenced by the geological features seen throughout the planet.  Ancient lake and river beds, water erosion, sedimentary rocks, all things that highlight once wet areas.

Artist’s conception of ancient Mars with a water ocean. Credit: ESO/M. KORNMESSER

How much water did Mars have? Where did it all go? A team of astronomers from NASA’s Goddard Space Flight Center in Greenbelt, Maryland has been searching for answers, by looking at two types of water currently present on Mars.  The familiar form of water is made with two hydrogen atoms, but a different type of water present throughout the solar system is called semi-heavy water, where one of the Hydrogen atoms making up the water is actually Deuterium, a Hydrogen atom with an extra neutron, making it slightly heavier.

With the extra weight, the deuterium is less likely to be lost to space to evaporation.  So if we look at the ratio of normal water to semi-heavy Deuterium water on Mars, and compare it to that on Earth, we should see more deuterium water on Mars, and how much more will determine how much water Mars has lost.  It follows the assumption that Earth and Mars started with similar water-deuterium ratios.

The astronomers mapped the distribution of water on Mars for 6 years (3 Mars years).  They were surprised to find seasonal changes, even though Mars is a desert.  Looking at atmospheric water in the polar regions, they found it was enriched in Deuterium by a factor of seven relative to Earth’s ocean water.   This means that Mars must have lost a volume of water 6.5 times larger than the modern polar caps contain, in order to provide this high level of enrichment.

Credit: NASA/GSFC

The real result they were after, the amount of water Mars lost, was staggering.  Mars has lost 20 million cubic Kilometres of water, an amount that could cover the entire planet’s surface in an ocean 140 meters deep.  It is much more likely that the water was contained in a massive ocean that covered much of the northern hemisphere lowlands of Mars, and at points could have been up to 1.6 Km deep.

This is an incredible result.  If Mars contained this much water, the likelihood that life evolved there is much greater.  It shows how much we still have to learn about Mars, and that we are moving forward with amazing science every day.

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