the planet mars
If there's life on Mars, it's pretty salty. NASA/JPL-Caltech
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Finding liquid water on Mars in 2015 bolstered hopes that signs of life lurk somewhere on the red planet. But in practical terms, scientists have more or less ignored whether those specific bodies of water had anything to do with life in the present day. The only way water could exist as a liquid on such a cold planet is if it was saturated in salt, which lowers freezing temperatures. Briny water isn’t considered an ideal place for life to spring up and evolve.

But all that water might be worth another look. New findings published in Nature Geoscience on Monday suggest those pools might harbor more breathable oxygen than we ever imagined—enough for life to exist on or near the surface.

“It’s a really great set of observations that kind of open up a new set of possible opportunities for life on Mars, in particular ones that were not possible on the early Earth,” says Lewis Ward, a geobiologist at Harvard University and a co-author of the new study.

The Martian atmosphere is thought to include a paltry 0.145 percent of oxygen, compared to the nearly 21 percent swirling through Earth. That number doesn’t necessarily kill dreams of Martians; the first lifeforms on Earth, after all, didn’t have access to free oxygen for the first couple billion years (the evolution of photosynthesis fixed that for us). But oxygen enables an organism to use much larger amounts of energy, so a lack of free oxygen clamps down on hopes of finding life that is at least a few notches above basic.

However, the right conditions could allow for vast quantities of oxygen to dissolve into those Martian reserves, especially since oxygen dissolves better in water at lower temperatures. Unfortunately, “no one before had really thought about how much oxygen could be present in liquid water on Mars, because we had no evidence that oxygen had really played a significant role,” says Ward. “This is the first attempt to really understand how much oxygen there could be today.”

The investigation hinges on the presence of manganese oxide on the Martian surface. On Earth, signs of manganese oxide minerals showed up around the time that oxygen first started accumulating in the atmosphere, some 2.5 billion years ago. Unlike iron (which is responsible for the crimson look of the red planet), manganese is pretty difficult to oxidize, and the only ways we find that oxidation taking place on Earth is either very slowly, or with the help of biology.

But in 2014 the Curiosity Rover found very concentrated manganese deposits on Mars, spurring Ward and his colleagues to wonder whether they nearby aqueous environments contained the oxygen required to oxidize the stuff. Manganese oxidation is pretty tightly (though certainly not exclusively) connected to biological activity. The study could be considered a matryoshka doll of questions: first, how much oxygen do you need to oxidize manganese? Then, is this concentration of oxygen capable of existing in Martian waters? And finally, at the core of the study: does this amount of oxygen raise the possibility that oxygen-breathing organisms exist on Mars?

“If there’s enough oxygen to do chemically useful work in oxidizing manganese, it suggests there’s enough to do biologically meaningful work as well,” says Ward.

The new study has encouraging answers to those questions. The team developed models for six different salt concentrations that could maintain liquid temperatures from -207 degrees to around 80 degrees Fahrenheit, and account for various pressures found around the planet. The models say all that salty liquid is more than capable of capturing the pitiful amounts of oxygen strewn above the Martian surface. In fact, the models suggest “there’s actually more oxygen on Mars today than there was on Earth when photosynthesis first began,” says Ward. “That suggests there might actually be enough [oxygen] that you can use it to drive metabolism in microorganisms.” For aerobes that use oxygen to eat carbon for energy, “it turns out there’s actually plenty of oxygen in these brines to support bacteria and some sponges doing just that.” The findings also suggest there is enough oxygen on the surface and subsurface of Mars to support energy harvesting from other sources, like methane and iron.

Sponges are a particularly useful model when thinking about what could survive these environments. “Sponges are one of the earliest sorts of animals to evolve,” says Ward. They’re simple filter feeders able to survive on bacteria, and can get by in very simple ecosystems. Ward does admit “for the sake of looking at the potential of life on Mars and other planets, we’re better off thinking exclusively in terms of microbes,” but the attenuated oxygen requirements of sponges is interesting to think about when we ponder what could evolve on Mars.

https://www.youtube.com/watch?v=2i4D-xSohVA

Ward and his team hope the findings are a first step in helping scientists draft a map of regions on Mars likely to support the largest oxygen concentrations, based on temperature and air pressure, and overlaying it with another map that illustrates the planet’s biggest points of hydrogen and methane that could be consumed for energy. “We could actually point to places on Mars that have the biggest opportunity for detecting biological activity” and potential biomass, he says. The study could also help us shed light on the potential for other worlds like Europa to foster life.

Naturally, there are reasons to be a little cautious about the study’s implications, given that it’s based on computer modeling and not direct observations. It’ll be difficult to confirm the findings—not just on Mars, but also here on Earth, where scientists so far have been unable to run controlled lab experiments that lead to good measurements of oxygen solubility in very cold brines. On Mars itself, we need to be able to study brines that have enough contact with the atmosphere to facilitate gas exchange and allow for oxygen in the air to dissolve into the water.

Jonathan Toner, an astrobiologist at the University of Washington who was not involved with the study, thought the paper did a good job showing that oxygen content in the Martian brines may not be a limiting factor for life, and also found it interesting to see a discussion of atmospheric interactions with the brines, which “could also have implications beyond habitability,” he says. Still, he emphasizes that high oxygen here is only possible with extremely high rates of salinity, which is “very challenging” for life. Combined with average temperatures as low as -67 degrees Fahrenheit, “and you have conditions not known to support any life on Earth. Life needs not just one factor, such as a supply of oxygen, but an array of factors all present at once.” If life exists on present-day Mars, “it would have to adapt to extremely low nutrient supplies, low temperatures, and high salinity, to an extent greater than anything living on Earth is known to tolerate.”

Still, few studies have so far suggested we could dare dream of such complex forms of life on Mars. But with every year, Mars proves to be more tantalizing than we predicted. Scientists tasked with looking for life on the red planet have a lot to check off on their to-do lists.

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