Scientists just found the best evidence yet that liquid water is currently present on Mars.
Using data collected by the Curiosity rover, they determined that tiny amounts of water vapor condense as a liquid in the planet's soil at night and evaporate away in the morning.
The research, published today in Nature Geoscience, shows that high amounts of salt in the soil lower the melting point of water, allowing it to temporarily pool as a liquid despite the subzero temperatures.
"There have been hypotheses and laboratory studies supporting this possibility before," says Javier Martín-Torres, a planetary scientist at Spain's Andalusian Earth Sciences Institute and Sweden's Luleå Technical University, and the lead author of the study, "but this is the first time that we've found evidence that conditions are right for the formation of liquid water on Mars."
Previously, satellite data has provided indirect geological evidence of liquid water — and we have good reason to believe lots of water existed on Mars in the past — but this new finding changes our understanding of the planet's hydrologic cycle. It's still extremely unlikely any life could possibly live in such cold conditions — but the finding could perhaps make it easier for humans to live on Mars in the very distant future.
How liquid water can form on Mars — even in freezing temperatures
Normally, Mars would be far too cold for water to form in a liquid state. So even though scientists detected hints of water vapor being absorbed from the Martian atmosphere into the soil as far back as the 1970s Viking program, they didn't seriously consider that it'd condense in a liquid form.
"We were thinking of the exchange as water vapor going into the surface and being absorbed, or freezing as ice," says Richard Zurek, a NASA scientist who worked on the Viking program.
However, those scientists didn't take into account the heavy amounts of perchlorate salts that are have since been found in Martian soil. These salts can dramatically lower the freezing point of water — enough for the water vapor that gets absorbed into the soil nightly to pool in a liquid state. "This transient liquid water forms within the uppermost five centimeters of the soil overnight, and dries out at sunrise," Martín-Torres says.
Martín-Torres and other scientists determined this was occurring by analyzing data collected by Curiosity on humidity, air pressure, and temperature levels above and below the soil surface. At night, during Mars's spring and winter months, they saw water vapor getting squeezed out of the air and condensing in the soil — because cold air can't hold as much moisture — and in the morning, they saw water vapor rising up from the ground.
"They've got a direct measurement of the relative humidity, and they know there are these salts in the ground," says Zurek, who wasn't involved in the research. "Together, these things tell us that there's liquid water."
So how much water is there on Mars?
It was already known that water exists in all sorts of non-liquid forms on Mars: as ice (both in the ice caps and in glacier belts that encircle both hemispheres), as water molecules bonded to minerals in the planet’s soil, and as water vapor in the atmosphere. The traces of water spotted here by Curiosity are very little, by volume, compared with what we already knew about.
But the finding suggests there might be much more water in the soil than we previously imagined. That's because Curiosity collected this data in Gale Crater — a low-altitude, relatively warm location that has less atmospheric water vapor than most other parts of Mars, so should be least likely to have this sort of condensation.
"The fact that it happens here means it is likely more abundant elsewhere," says Alfred McEwan, a planetary scientist who worked on the new paper.
This phenomenon might be responsible for the dark streaks in Mars' soil that McEwan and other scientists have previously spotted with satellites. These have formed seasonally, and it's been hypothesized that they're evidence of water melting and running downhill over the soil.
"These landslides occur seasonally, in warmer periods, on the slopes of Mars," Martín-Torres says. "One possible explanation for this detachment of material could be changes in state of brines present in the soil."
If these streaks are indeed formed by flowing water, it would mean there's way more liquid water on Mars than we previously imagined. Curiosity hasn't detected similar streaks itself, but it's confirmed a mechanism through which liquid water can form at such cold temperatures: perchlorates.
This doesn't mean there's currently life on Mars
Upon hearing about liquid water, it's tempting to imagine Martian microbes swimming around. But in reality, this finding doesn't make the remote possibility of current life on Mars any more likely.
"We found that the conditions for the formation of liquid water are extremely cold — lower than the threshold needed for the cellular metabolism of life as we know it," says Martín-Torres.
However, many scientists believe Mars was much warmer and wetter billions of years ago, when it may have had a thicker atmosphere to help trap heat. The presence of perchlorates mean it wouldn't have had to be quite as warm for large amounts of liquid water to form, perhaps explaining the geologic evidence of ancient lake beds and other water-related features Curiosity has found so far. If Mars was somewhat warmer long ago, it's possible that life could have evolved.
And regardless of whether there's ever been life on Mars, this finding might make it somewhat easier for humans to someday live there. Though Mars colonization presents all sorts of difficulties, one major challenge would be getting water — for drinking, growing crops, and perhaps refining into rocket fuel.
The most obvious water source would be the ice caps on the poles, as well as the glaciers at higher latitudes, but settling there would mean colder temperatures and less solar power. Refining water that's trapped in soil could present a more appealing option.
Curiosity only found traces of salty water in the top few centimeters of soil. But if there's more liquid water farther down, Zurek says, it could make refining enough water for human use much more economical. Dreams of colonizing Mars remain a distant fantasy, but the bottom line is that a wetter Mars is one we'd have an easier time living on.