The new findings come from a team of researchers who collected data via three large telescopes on Earth: the European Southern Observatory’s Very Large Telescope in Chile, and the Keck II telescope and NASA Infrared Telescope Facility in Hawaii. The scientists measured the amount of water and the amount of HDO — water's heavier form — in Mars' atmosphere. The test was similar to measuring the separation of oil and water, one of the study's authors, Geronimo Villanueva, told The Washington Post. "If you're losing material — which the atmosphere of Mars has been doing since it was formed — you'll lose more of the light material first," he said.

By measuring the ratio of H2O to HDO against their atmospheric rates of escape over nearly six years, the researchers were able to determine the planet's water content billions of years in the past. The research also indicates that Mars once contained several regional atmospheres and a climate divided into seasons.

NASA plans to evaluate these findings from space with the launch of an orbiter in 2016.