Take a deep breath and thank Earth’s atmosphere, which contains enough oxygen for aerobic respiration. Our planet’s life-sustaining conditions are unique in the solar system. Or are they?
Researchers now suggest that some pools of salty water on Mars might also be able to host respiring life. Drawing from estimates of the Red Planet’s temperature and atmospheric pressure, scientists have proposed that some brines near Mars’s surface could contain high enough levels of dissolved oxygen to sustain tiny forms of aerobic life like microbes and sponges.
Tough Conditions for Respiration
Mars has long been targeted as a potential toehold for life in the solar system because it’s a likely place to find water: A study published earlier this year in Science hypothesized that the planet has an underground reservoir of water, and rock formations suggest that water also flowed on Mars in the past.
But its modern-day atmosphere is pitifully thin—less than 1% of Earth’s—and nearly devoid of molecular oxygen (O2). Much of Earth’s oxygen has biological origins, but the scant O2 on Mars is derived from carbon dioxide (CO2) molecules, which make up 96% of the Martian atmosphere. All that adds up to tough conditions for aerobic respiration on the Red Planet.
Vlada Stamenković, a NASA planetary scientist at the Jet Propulsion Laboratory in Pasadena, Calif., recently had an idea: Could tiny forms of respiring life on Mars live in pools of briny water by breathing dissolved oxygen?
After all, he figured, data from NASA’s Curiosity rover on Mars have suggested that brines—mixtures of water and salts—might transiently exist on Mars’s surface. And finding life in the water isn’t too far-fetched: “We all came from the ocean,” Stamenković explained.
Past and Future Climate
Using models of Mars’s climate, Stamenković and his team calculated how much O2 could dissolve in different types of brines exposed to different temperatures and pressures on the planet’s surface and shallow subsurface. The researchers considered not only Mars’s present-day environment but also its likely conditions as far as 20 million years in the past and as long as 10 million years in the future.
These past and future inferences were based on estimates of the planet’s tilt. “The climate strongly changes as the obliquity changes,” Stamenković explained.
The oxygen concentrations that Stamenković and his colleagues calculated ranged widely from roughly 3 millionths of a mole (3 micromoles) of O2 per cubic meter to about a million times higher. Even at the low end of the range, the calculated concentrations exceed 1 micromole of O2 per cubic meter, the minimum concentration required by microbes for aerobic respiration, Stamenković said.
“Mars could offer a wide range of near-surface environments with enough dissolved O2 for aerobic respiration,” the researchers concluded in a paper that was published last week in Nature Geoscience. They added that thanks to the planet’s relatively low obliquity over the last 5 million years, its oxygen solubility levels have likely been particularly high over that time period.
What’s more, some parts of Mars might be particularly likely to host oxygen-rich brines, the researchers found. For instance, near the planet’s poles, the lower temperatures boost oxygen solubility, and dissolved oxygen concentrations would be the highest there, the researchers proposed.
If brines are common on Mars, which Stamenković points out is something we still need to explore with dedicated tools, then these aerobic oases might, in theory and under special circumstances, even be able to supply sufficient oxygen to support simple animals like sponges.
Going to Mars
This work presents “a bold new idea,” said Robina Shaheen, a chemist at the University of California, San Diego, who was not involved in the research. It’s also timely because of the upcoming launch of NASA’s Mars 2020 mission, which is focused on looking for microbial signatures of life on the Red Planet, she said. One of the instruments aboard the Mars 2020 rover will be used to look for subsurface brines.
Stamenković and his team are already thinking ahead to sending their own instrument to Mars. The researchers are currently developing a small probe called TH2OR (Transmissive H2O Reconnaissance) to detect subsurface groundwater on Mars.
TH2OR, which the team hopes to send on a future mission to the Red Planet, relies on low-frequency sounding to detect groundwater at depths as much as a few kilometers. Stamenković and his colleagues will soon begin testing TH2OR at a variety of sites, including the desert near the Jet Propulsion Laboratory.