The question about water on Mars has shifted from "was there?" to "how much?" According to the latest research from NASA, the answer is "a lot."
After conducting a ground-based analysis of the water in the Red Planet's atmosphere, a team led by Geronimo Villanueva of NASA's Goddard Space Flight Center has determined that not only was Mars once home to an ocean bigger than the Arctic Ocean, but also how much of that water has been lost.
"Our study provides a solid estimate of how much water Mars once had, by determining how much water was lost to space," Villanueva said. "With this work, we can better understand the history of water on Mars."
The volume of this ocean, which existed some 4.3 billion years ago, would have been at least 20 million cubic kilometres (5 million cubic miles). The Arctic Ocean has a volume of just over 18 million cubic kilometres.
On Mars, this volume would have been sufficient to cover the planet's entire surface in a liquid layer 137 metres (450 feet) deep -- however, the more likely scenario is that the ocean covered almost half of the planet's northern hemisphere, the low-lying Northern Plains -- around 19 percent of the surface of Mars -- reaching depths of 1.6 kilometres (1 mile) in places.
The team used the European Southern Observatory's powerful Very Large Telescope in Chile, and the W.M. Keck Observatory and NASA Infrared Telescope Facility in Hawaii to examine two different types of water in Mars' atmosphere: H2O, which makes up most of the water on Earth; and HDO, or "heavy water," in which one of the hydrogen atoms is replaced with a heavy hydrogen isotope called deuterium, which can be found in unusually high proportions on the Mars surface.
The team compared water trapped in a Mars meteorite 4.5 billion years old to water on Mars today, looking for differences in the ratio of HDO to H2O. Over six years, the team mapped the H2O and HDO levels on Mars several times, producing global snapshots of each compound and their ratio.
This produced maps of Mars' regional microclimates, and allowed the team to measure the atmospheric changes on Mars between the time the meteorite left the planet and today -- which, in turn, allowed them to calculate that Mars has lost 87 percent of its water volume since the ocean existed.
They paid particular attention to the permanent ice caps at Mars' north and south poles -- where the planet's largest known water reservoirs remain today. The water in these ice caps is thought to contain information about the evolution of the planet's water from the end of what is known as the wet Noachian period 3.7 billion years ago to the present day.
By measuring the H2O and HDO levels in the atmosphere around the ice caps, the team was able to calculate how much of each is actually in the ice caps. This, in turn, allowed them to calculate the amount of water Mars has lost over the years.
"With Mars losing that much water, the planet was very likely wet for a longer period of time than was previously thought, suggesting it might have been habitable for longer," said study second author Michael Mumma.