The search for evidence of water on Mars has been going on for so much of my life that it's almost hard to get excited about the evidence. Yes, okay, another sign of water, big whoop. We've mostly moved on to obsessively searching for what that water might have meant: the tiny microbes, or remnants thereof, that would mean Mars did or does support life.
The remains of pits and valleys carved by moving water make its one-time presence on the planet all but certain. But the question of how much liquid water Mars had -- and why liquid water disappeared on Mars while Earth stayed wet -- remains a mystery.
Using data from several large telescopes on Earth (Keck Observatory’s 10-meter Keck II telescope; NASA’s Infrared Telescope Facility, also in Hawaii; and European Southern Observatory’s Very Large Telescope located in Chile) to analyze the ratio of two kinds of water molecules on Mars, the authors of the new study believe they've come up with a solid estimation of Mars's ancient ocean.
By measuring the different amounts of common H2O (which we know as water) and HDO (where one of the hydrogens is replaced by deuterium, a heavier version of the same element), as well as how much of either of them escapes out of the atmosphere over time, researchers can estimate the former water content of an alien body.
It's like looking at a mixture of water and oil, explained the study's first author, Geronimo Villanueva, a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.
"The heavier form of hydrogen sits like water at the bottom of the atmosphere," he said. "The lighter water molecules rise to the top, like oil. So 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."
By measuring the current ratios and knowing the rate at which they escape per year, he said, one can estimate the planet's original water content.
The new research suggests that 4.5 billion years ago, the planet had an ocean covering 20 percent of its surface. Villanueva and his colleagues also found differences in atmosphere at different parts of the planet and during different seasons, revealing that the now universally dry planet once had microclimates and seasonal changes. Because of the planet's geography, they suspect most of the water sat in its low-lying Northern Plains.
The next step is to use measurements taken from an orbiter to confirm and expand the findings, Villanueva said. NASA has already developed an instrument (due to be launched in spring of 2016) to take the same atmospheric measurements from space.