Liquid Water from Ice and Salt on Mars
In 2009, scientists reported that NASA’s Phoenix mission had returned images of what appeared to be droplets of liquid water on a leg of the lander. The series of images were taken shortly after Phoenix touched down on Mars, and appeared to show droplets of water that grew and even coalesced.
The photos stunned scientists because nobody thought liquid water could exist in conditions at the martian surface. No liquid water had ever been found beyond Earth. Water ice had been spotted on Mars, but conventional wisdom predicted that the ice wouldn’t melt if warmed. The low atmospheric pressure of Mars suggested that ice would sublimate (or vaporize) instead, transitioning directly from ice into a gas phase.
After Phoenix’s discovery, scientists supported by the Exobiology and Evolutionary Biology element of the NASA Astrobiology Program began studying how liquid droplets of water could possibly be stable on Mars. They theorized that a missing ingredient had to be present – salt. If the water was briny enough, it might provide the stability needed for it to remain liquid rather than immediately vaporizing.
The problem was that, at the time, no salts had been found on Mars. Further observations of Mars changed that when calcium perchlorate was discovered – a salt also found in places like Chile’s Atacama desert on Earth. Now, thanks to additional data from missions like the Curiosity rover, astrobiologists know that calcium perchlorate could be present on much of Mars’ surface.
Life as we know it requires liquid water to survive, and discovering liquid water on the surface of Mars could have important implications for life’s potential on the red planet – even if the water is very salty.
“On Earth, everywhere there’s liquid water, there is microbial life,” explained Nilton Renno in a 2011 press release from the University of Michigan at Ann Arbor.
Renno is a professor of atmospheric, oceanic and space sciences at the U of Michigan and Principal Investigator on the project, which began three years after the Phoenix images were released. His team used chambers in a lab on Earth to recreate the conditions present on Mars at the Phoenix landing site. Inside the chambers, an atmosphere was maintained with air pressures that were 99 percent lower than that of the Earth at sea level. Temperatures inside the chambers were lowered to between -185 and -5ºF.
The team found that a particular type of salt found on Mars can actually melt ice that it touches. The principle is similar to salt being thrown on sidewalks and roads to melt ice during the winter on Earth. In tests where calcium perchlorate (or a salty, simulated mars soil) was placed on top of water ice, droplets of liquid water formed within minutes at temperature ranges within the conditions of the Phoenix landing site.
The presence of water was confirmed by shining lasers on the surface and examining the reflected light in a process known as Raman scattering spectroscopy.
“For me, the most exciting thing is that I can now understand how the droplets formed on the Phoenix leg,” said Nilton Renno, in a release issued by the American Geophysical Union.
Salty Liquid Life
On Earth, organisms known as halophiles are capable of surviving in water with extremely high concentrations of salt (e.g. hypersaline environments). Halophiles have been found thriving at a range of salinities – from 3% to 30% sodium (NaCl) – and in environments from Antarctica to the Arctic. They’ve been spotted pretty much anywhere we think of as hypersaline, including the San Francisco Bay, the Great Salt Lake, and the Dead Sea.
Halophiles are also a diverse group of organisms that include bacteria and archaea as well as multicellular organisms like diatoms. Astrobiologists study halophiles and the mechanisms they use to survive in order to determine environmental boundaries in which life survives on Earth.
The new study indicates that liquid droplets of water could form on Mars wherever perchlorate comes into contact with water ice, opening up the possibility that environments for life could persist all over the planet. The question now: could droplets of salt water help halophiles similar those on Earth survive within the environmental boundaries on Mars?