Dry, dusty Mars once had an ocean that held as much water as the Arctic Ocean and covered a larger share of the Red Planet's surface than the Atlantic Ocean does on Earth, according to a surprising new study.
The findings, described online in the journal Science, examined the patterns in the Martian atmosphere to try to understand how much water it has lost in the last few billion years – and finds that the planet may have been wetter and for longer than scientists may have thought.
As the scientists examined their surprising findings, "the story started to make sense," said lead author Geronimo Villanueva, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
Researchers have gone back and forth on whether Mars held enough water for long enough to have given microbial life a sporting chance to emerge on the Red Planet. NASA's Curiosity rover has tasted the air and found that the Martian atmosphere may have been stripped so long ago that there was a slim chance for life; but studies of rocks that the rover has drilled in Gale Crater have revealed signs of a series of lakes that lasted for many millions of years.
To get at this question, an international team of researchers used ground-based telescopes to study the composition of the traces of water in the atmosphere over almost six Earth years (which are roughly three Mars years). Thus, they were able to map the atmospheres, and witness seasonal and microclimate changes over the entire planet.
They specifically looked at two isotopes of water left in the atmosphere: regular water, made of an oxygen and two hydrogens, and semi-heavy water, where one of the hydrogens has an extra neutron in its nucleus. Regular water, which is lighter, tends to rise up and escape the atmosphere at a faster rate, while the heavier water stays put. So over time, the share of heavy water grows – and the greater the share of heavy water today, the more water must have been lost over time.
The scientists took a particular interest in the atmosphere near the polar regions, because much of the Red Planet's water is stored in its polar ice caps. Based on their calculations, the scientists found that the share of heavy water in the atmosphere near the polar areas was about seven times as high as in the water on Earth. At one point, the water reserves must have been about 6.5 times larger than the reserves mostly stored in the Martian polar ice caps today. An early Mars would have held about 20 million cubic kilometers (4.8 million cubic miles) of water.
Where did all this water lie? While it probably could have covered the entire planet with a 450-foot-deep layer, it was probably mostly contained on the low-lying northern plains, and in some places could have gone about a mile deep.
"If you drop all that water on the planet, it will accumulate in the northern part of the planet," Villanueva said. "So that's [where] we think it formed an ocean."
NASA's MAVEN spacecraft is studying what remains of Mars' now-thin atmosphere to see if scientists can learn how much of it escaped – data that will be of great use to planetary scientists looking to solve the mystery of the Martian water. The European Space Agency's first ExoMars mission is also scheduled to arrive at Mars in 2016.
"In the next five years we're going to probably change our perception of what Mars was in the past," Villanueva said.