Ancient Mars Lost an Ocean with More Water Than Earth's Arctic

It turns out that Mars may have held more water than Earth's Arctic Ocean and covered a great portion of the planet's surface than the Atlantic. Scientists have found that the Red Planet was once watery, and that it lost its ocean over time.

About four billion years ago, Mars would have had enough liquid water to cover its entire surface in a liquid layer about 140 meters deep. It's far more likely, though, that the liquid would have pooled to form an ocean occupying almost half of Mars's northern hemisphere.

"Our study provides a solid estimate of how much water Mars once had, by determining how much water was lost to space," said Geronimo Villanueva, one of the researchers, in a news release. "With this work, we can better understand the history of water on Mars."

The researchers used detailed observations of two slightly different forms of water in Mars's atmosphere. One is the familiar form of water, H2O. The other is HDO, which is semi-heavy water; it's a naturally occurring variation in which one hydrogen atom is replaced by a heavier form, called deuterium.

Since the deurated form of water is heavier than normal water, it's less easily lost to space through evaporation. This means that the greater the water loss from the planet, the greater the ratio of HDO to H2O in the water that remains.

The scientists mapped the distribution of H2O and HDO, producing global snapshots of each as well as their ratio over the course of six years. These maps revealed seasonal changes and microclimates, even though modern Mars is essentially a desert. What was interesting, though, was that the scientists found exactly how much water was lost.

The researchers found that Mars must have lost a volume of water 6.5 times larger than its present day polar caps. In fact, Mars's early ocean must have had a volume of at least 20 million cubic kilometers. It's likely that this water would have been located on the Northern Plains, and an ancient ocean there would have covered 19 percent of the planet's surface. By comparison, the Atlantic Ocean covers 17 percent of the Earth's surface.

"With Mars losing that much water, the planet was very likely wet for a longer period of time than previously thought, suggesting the planet might have been habitable for longer," said Michael Mumma, one of the researchers.

The findings are published in the journal Science.

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