The Curiosity rover, in its trip across the surface of Mars, continues to provide data that helps scientists piece together the Red Planet’s past. In its exploration of the 154km-wide Gale Crater, Curiosity recently took samples of some ancient clays. While clay may not sound interesting, these samples contained clues as to the history of the planet’s atmosphere and water—and they paint a sad story.

At the bottom of the Gale Crater, there was once a vast lake. The water’s presence left traces in the clays there, as did the hydrogen that was trapped there. Hydrogen was produced by ultraviolet light from the Sun striking the water, breaking it into its component atoms, hydrogen and oxygen. The hydrogen is then free to diffuse through the Martian atmosphere. But it doesn’t stay there forever. Some of it may escape into space, and some can get absorbed into the surface of Mars, sealed inside hardening clays. Deuterium, an isotope of hydrogen also known as “heavy hydrogen” because it has an extra neutron, also gets infused into the clay.

The hydrogen, being much lighter than its heavy cousin, can escape from the atmosphere much more easily. So if there’s a lot of escaping hydrogen, it means that deuterium is being left behind, which would shift the ratio of deuterium to hydrogen. In other words, the departure of “light” hydrogen might leave deuterium more prominent than before. That’s essentially what Curiosity was investigating—the ratio of deuterium to hydrogen locked in those clays.

The hydrogen isotopes found in the clay reflect their abundances at the time the clay formed, about three billion years ago. That provided a sort of "landmark" for that time period—the relative abundances of hydrogen isotopes at that time suggest the process had been ongoing even then. When combined with an understanding of the planet's initial isotopic abundances in its very early history, it should provide a rate at which the process was happening.

Luckily, data on Mars' initial conditions exists, or can be reasonably inferred in a number of cases. This data comes from meteorites originating from Mars and comparisons with Earth’s oceans (which, by all indications, should be quite similar to the very early Martian ones). Together with that information, Curiosity’s new data allowed the researchers to refine the timeline of the planet’s water and hydrogen loss.

It turns out that escape into space is the dominant mechanism by far, with more hydrogen being lost to space than reabsorbed into the planet. And this process has been going on throughout Martian history, with about 1026 atoms (that’s about 166 grams, or about a third of a pound) leaving the atmosphere every second.

The researchers were also able to conclude that the planet likely took longer to become a dry, arid wasteland than previously predicted. While the process was slower and more dragged out, it started much earlier than predicted.

Researchers hope to learn how much water has been lost from the planet in this way. The water is not all gone—a vast sheet of water ice covers the planet’s North pole, and there’s evidence of subsurface liquid water on the planet. But there's still a key question: how much water was once present?

The history and evolution of the planet’s water is of interest for a variety of reasons. Key among them is determining whether Mars could ever have supported life, and the answer to that question depends on conditions being just right at the right time.

Science, 2014. DOI: 10.1126/science.1260291 (About DOIs)