The Martian landscape. Image: Mars Pathfinder
In the hunt for life on Mars, NASA’s maxim has always been to follow the water. We haven’t yet found liquid water on Mars, but a new study from a team at the University of Michigan has shown how small amounts of liquid water could exist on the Red Planet despite its below-freezing temperatures. The key ingredient is salt.
Though we’ve never found liquid water flowing on the Martian surface, we have found evidence of such. Weathered rocks and areas suggest ancient rivers, photographs show evidence of gullies flowing down crater rims, and instruments on landers and rovers have found the right chemical signatures of water. Most compelling is the evidence of water in self-portraits from NASA’s Phoenix lander.
But Mars is cold, with an average surface temperature of about -80 degrees Fahrenheit (-60 degrees C). Could liquid water exist on a planet this cold? That’s what the team of researchers at the University of Michigan is answering, by looking at the salt that exists on the planet. Similar to how road salt melts ice on sidewalks in the winter, a type of salt present in Martian soil readily melts ice on contact.
In 2008, Nilton Renno, a professor of atmospheric, oceanic, and space sciences at the University of Michigan, was the first to notice strange globules in photos NASA’s Phoenix lander sent back. As the lander continued sending pictures home, the globules remained, seeming to grow and coalesce.
Evidence of the sublimation of ice captured by the Phoenix mission. Image: NASA/Wikimedia
Though salts had never been found on Mars, Renno suspected these globules were water globules formed in the presence of salts on the surface exposed by the spacecraft’s landing thrusters blasting away the topsoil. That the droplets persisted suggested to Renno that there’s an ice-water cycle happening on Mars that doesn’t need an Earth-based spacecraft to get started.
Renno’s hunch has since found supporting evidence: Phoenix eventually detected calcium perchlorate on Mars, a mixture of calcium, chlorine, and oxygen. It’s a salty mixture that is also found on Earth in arid places like the Atacama Desert in Chile. NASA’s Curiosity rover has also found calcium perchlorate elsewhere on Mars, leading scientists to believe that it and other salts exist across the planet's surface.
The University of Michigan team investigated the possible role of calcium perchlorate in water on Mars by recreating the Phoenix lander’s landing site in a laboratory. Inside two metal cylinders two feet high and five feet long, they duplicated late Martian spring/early summer where the temperature ranges from from -185 to -5 Fahrenheit, the relative humidity is fairly high, and the atmospheric pressure is about 1 percent of what we feel on Earth. With the duplicate Mars environment, they tested two scenarios, one with perchlorate by itself and one with perchlorate on top of water ice.
For the the perchlorate-only experiments, the team put millimeter-thick layers of salt on a temperature-controlled plate or Mars-like soil. Nothing happened. Even hours later there was no evidence of liquid water forming. That means that deliquescence, a process wherein salt sucks vapor out of the air and leaves water behind, wasn't likely a significant process on Mars.
But when the team put calcium perchlorate directly on a 3-millimeter-thick layer of ice, drops of liquid water formed within minutes, even with the chamber stabilized at -100 degrees Fahrenheit. The team confirmed their observed results with Raman scattering spectroscopy that uses lasers to examine the different reflective signatures of substances and states of matter.
The lab results show how small amounts of liquid water could exist on Mars in shallow areas ranging from its polar regions to its mid-latitudes during the daytime hours in the spring and early summer. This cycle would explain the gullies that flow, freeze, and thaw before flowing again, said Renno, as well as lead to water formed just below the surface. And, Renno added, that water wouldn’t need to be in a liquid form to support life. Lattices of frozen saltwater has been found to host microbial organisms on Earth.
"For me, the most exciting thing is that I can now understand how the droplets formed on the Phoenix leg," said Renno of the new work.
So following the water remains a valid way to look for life off of the Earth. We might just have to broaden our expectation of what that water looks like to find some really incredible things.