Samples taken from two drill holes on Mars support the idea that Mars lost a whole lot of water fairly early in its history.
We know that the Red Planet had wet conditions in the first few hundred million years of its existence. Delta features splay across its surface, and there are mudstone deposits built up by sediments carried in lakes and streams.
But the planet had turned cold and dry by roughly 3 billion years ago. Planetary scientists investigate what happened in part by looking at a particular chemical ratio: the amount of plain old hydrogen (H) compared with the amount of its heavier form, deuterium (D). The D:H ratio differs widely depending on when and where a sample comes from. But it also will lean more in favor of deuterium over time if the parent world loses a lot of regular hydrogen to space. That’s because hydrogen is lighter than deuterium, and so is more easily stripped away from a planet’s atmosphere than deuterium is.
Water molecules can contain both types of hydrogen — and, therefore, can the clay minerals that form in that water. And because hydrogen so often comes from split-up water molecules, the amount of hydrogen lost to space also can reveal the amount of water lost.
The D:H ratio in Mars’s atmosphere today is roughly 6 times the average in Earth’s oceans. But studies of the primitive mantle material in a Martian meteorite called Yamato 980459 reveal a ratio similar to Earth’s, supporting the idea that Mars started out with a lot more water than it has today. (I can’t give you a specific number because the answer depends on several variables, but the initial amount was probably in the ballpark of double the current one.)
Scientists with NASA’s Curiosity rover took a stab at the question of Mars’s freeze-drying history by drilling into clay minerals that solidified in standing water more than 3.6 billion years ago (and likely closer to 4 billion years ago), a period on Mars from which we have no meteorite record. The rover heated the samples, from drill holes called John Klein and Cumberland, in its Sample Analysis at Mars (SAM) experiment and tested the water released. It found that the water had a D:H ratio three times that of Earth’s.
The value is only half that of the present-day deuterium abundance in Mars’s atmosphere, but it’s still higher than expected for early Mars, Paul Mahaffy (NASA Goddard) and colleagues report online December 16th in Science. It implies that Mars has been drying out for a long time.
It’s hard to draw a stringent chronology from the result, though, because it’s unclear whether the samples reflect the D:H ratio across Mars at that time or just in one spot. If they do reveal the planet-wide deuterium levels, then the ratio Curiosity found would imply that Mars has lost to space 100% to 150% as much water as it currently has in its surface and subsurface.
But if the planet has multiple water reservoirs that have been isolated from each other for extended periods of time — such as ice embedded in sediments that hasn’t interacted with the atmosphere, as a recent paper in Earth and Planetary Science Letters suggests — then scientists can’t merely connect the dots. The Curiosity samples would reflect D:H ratios for what’s trapped in subsurface ice and minerals and so would reveal how much water that particular reservoir had lost, but it wouldn’t reveal how much water the planet had lost as a whole. To do that, researchers need the data from NASA’s MAVEN spacecraft, which is studying the current situation in the atmosphere. With those data in hand, scientists can then set to figuring out whether the atmospheric and rock D:H ratios imply the same amount of water loss and, therefore, whether the reservoirs are linked.
What’s interesting to me is that the age of these Curiosity samples falls during the era that Mars started transitioning to cold and dry. Given that the D:H ratio is thrice that on Earth, and given that Mars started freezing out around the time these rocks formed, the new results might support the idea that Mars froze up because it lost a lot of atmosphere to space. Both Mars’s current atmospheric D:H ratio and ratios for other isotopes, such as those of argon and nitrogen, suggest that Mars has lost somewhere between 25% and 90% of its atmosphere (with the data favoring the higher end of that range). So it will be very interesting to see how the D:H story plays out as scientists continue to poke and prod the Red Planet.
P. R. Mahaffy et al. "The imprint of atmospheric evolution in the D/H of Hesperian clay minerals on Mars." Science. December 16, 2014.
T. Usui et al. "Meteoritic evidence for a previously unrecognized hydrogen reservoir on Mars." Earth and Planetary Science Letters. January 15, 2015.