Today's Mars is more hospitable to life with oxygen breathing than previously thought.
One study suggests that saltwater at or near the surface of the red planet may contain enough dissolved O2 to support oxygenated microbes and more complex organisms such as sponges.
"Nobody thought of Mars as a place where aerobic respiration would work because there is so little oxygen in the atmosphere," said Vlada Stamenkovic, a scientist on Earth and the planet at the Jet Propulsion Laboratory, who was in charge of the work. "We say it's possible that this planet so different from Earth would have given aerobic life a chance."
As part of the report, Stamenkovic and his co-authors also identified which regions of Mars are most likely to contain salt solutions with the highest levels of dissolved oxygen. This could help NASA and other space agencies to plan where to send future missions.
The work was published on October 22 in Nature Geoscience.
On its surface, the planet Mars is not a particularly hospitable place for most earthlings.
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The Martian atmosphere, on the other hand, is only 0.145 percent oxygen, according to the Mars rover.
When no plants produce O2, the smallest amount of oxygen is created on Mars when the sun's radiation interacts with CO2 in the planet's atmosphere.
In addition, the Martian atmosphere is extremely thin – 160 times thinner than Earth's atmosphere. In addition, the surface temperature often drops to minus 100, making the presence of liquid water on the surface of the planet extremely difficult.
Pure, liquid water would either freeze or evaporate on Mars, but salty water or salt solutions could remain in a liquid state at or just below the surface of the planet, the authors said. This is because water mixed with salt has a lower freezing temperature than normal water. (That's why these unfortunate people who live in cold climates use salt to melt the ice on their sidewalks.)
In the first part of the work, the authors show computer models to show that water already exists on Mars Salts may be mixed in a liquid state at or near the surface to be stable.
After the authors were convinced of the existence of this liquid brine, the next step was to determine how much dissolved oxygen they could absorb from the atmosphere.
"If there are saline solutions on Mars, then oxygen would have no choice but to infiltrate them," said Woody Fischer, a geologist at Caltech who helped with the study. "The oxygen would make it anywhere."
In order to calculate how much oxygen the brine could absorb, the researchers had to consider their chemistry as well as the temperature and atmospheric pressure on the Martian surface. Brines absorb more oxygen when the temperature is lower and the air pressure is higher.
Their findings showed that modern Mars can support liquid environments with enough dissolved O2 to support oxygenated microbes around the world. They also found that oxygen levels in brine sheets are particularly high in polar regions where temperatures are cooler.
So far, this work has been done by computer modeling. However, experts still said that the study looks robust.
"The best studies using models for their results are conducting a thorough review of the possible variables that can affect model output," said Kathleen Mandt, planet biologist at the Johns Hopkins University Applied Physics Laboratory. "This study is well suited to investigate a range of possible outcomes."
However, what the study does not do is prove that Mars actually exists on Mars.
"What we do know is that, in theory, brines should be present on Mars, and that they would be able to dissolve enough oxygen to be biologically beneficial," Stamenkovic said.
The next step is twofold.
He hopes that researchers here on Earth will be experimenting to get oxygenated microbes into the brine that could occur on Mars to find out what kind of chemistry they are doing and whether they can thrive. The other step would be to send a lander to Mars looking for salt brine from shallow to deep underground.
"NASA has done amazing work to find evidence of habitable environments of the past," he said. "I'm a big believer in finding current habitable environments, and we can do that by starting to investigate if there's liquid water on Mars."
For this purpose, Stamenkovic is working on the development of a new tool that is no bigger than a shoebox, with which water can be found on Mars and the salt content can be determined, no digging required.
He calls it TH2OR.