If you turned your eyes skyward on an evening in late July, you might have glimpsed the dusky red glow of Mars during its most recent close approach to Earth. Even from millions of miles away, the Red Planet is truly breathtaking—but especially so on its surface, where the asphyxiating atmosphere is 95% carbon dioxide and 100 times thinner than Earth’s.
With such sobering statistics, the habitability of Mars remains under fierce contention. But today, in the journal Nature Geosciences, researchers report that there might just be enough oxygen on Mars to support forms of life that rely on it to breathe—perhaps even multicellular life.
This summer, a team of Italian scientists predicted the presence of a salty underground lake concealed beneath Mars’ frigid southern pole. The announcement made quite a splash: Water, after all, is key to survival on our own planet. But in the equation for complex life, water alone isn’t enough. Almost all multicellular organisms on Earth depend heavily on another crucial ingredient—oxygen—because of its capacity to help all sorts of critters wring more out of their energy sources.
On our home planet, oxygen is released by photosynthesizing plants, bacteria, and algae, and makes up more than 20% of the atmosphere. But this element is in far shorter supply on Mars, where it comprises between just 0.1 and 0.2% of the atmosphere. Because of this, oxygen-dependent, or aerobic, life has been dismissed as an almost guaranteed nonentity on Mars.
But “hardly any oxygen” is a far cry from “no oxygen at all”—and pools of briny Martian liquid could actually be capable of sequestering even tiny traces of oxygen. If they’re close to the Martian surface, they could snatch it from the atmosphere; under other circumstances, oxygen can be split out of water itself. If the oxygen is indeed locked in these liquid lakes, then under the right conditions it might just sustain aerobic life.
To determine if the brines of Mars might meet the mark, a team of researchers led by Vlada Stamenković, a planetary scientist at NASA’s Jet Propulsion Laboratory, decided to model the potential availability of oxygen in theoretical Martian brines.
First, the bad news. Unfortunately, a lot of what Mars has to offer makes it seriously pale in comparison to Earth. For one, water’s capacity to carry oxygen drops off at low pressures, and the atmospheric pressure on Mars is about 6% of what’s here on Earth. Right off the bat, the researchers found this knocks the ability of oxygen to dissolve in Martian water down a few orders of magnitude.
But other aspects of Mars’ harshest conditions actually work in favor of oxygen richness. Extremely cold water is actually very good at clinging to oxygen, below even -100° Celsius. Though untreated water would typically ice over long before reaching these temperatures, the addition of one extra ingredient—salt—can make freezing points plunge. And luckily, Mars water has salt in spades—making its chilly brines surprisingly oxy-capable. With these parameters, the researchers realized, even in worst-case scenarios, the brines held enough oxygen to allow the growth of aerobic bacteria.
But breathing bacteria are still single cells. To push the limits of their model, the researchers added in a few more oxygen-boosting conditions. For instance, the presence of certain minerals like magnesium- and calcium-based salts (both of which are believed to exist on Mars) in briny waters could buoy oxygen availability upwards. To Stamenković’s excitement, when these parameters were met, oxygen levels crept into a range that could, in ideal conditions, even sustain a sponge—a multicellular animal that flourishes in marine environments here on Earth.
“This part [of the model] was astonishing,” says Christopher Reinhard, a biogeochemist who studies planetary atmospheres at the Georgia Institute of Technology and was not involved in the new research. “You have to stack a lot of conditions on top of one another [to have enough oxygen for a sponge], but the fact that you can even be in that ballpark… is amazing.”
Next, the researchers scoured the Martian landscape for optimally oxygenized real estate. As temperature seemed to be a primary driver in their calculations, they zeroed in on the planet’s poles, where temperatures are especially brisk.
Additionally, they noted that Mars’ oxygen availability may still be in flux. Compared to Earth, Mars has a bit more “wiggle,” explains Stamenković, in that it tends to tilt back and forth on its axis as it orbits the sun. That means that at certain times, the poles are hotter or cooler than normal, which could affect their ability to hold oxygen in the theoretical waters below.
But Mars is hardly a spinning top. Even with the wildest possible wobble, it’s unlikely to become Earth 2.0. The reason Earth has far surpassed its ruddier counterpart in oxygen richness is photosynthesis—a phenomenon still conspicuously absent on the Red Planet. Rather, the infrequent flecks of oxygen that dot Mars’ atmosphere come primarily from carbon dioxide being split apart by light, and these additions are meager at best.
At the end of the day, the probability of viable oxygen on the planet is still just that: a probability. And just because a life form like an aerobic bacterium could survive in Mars’ brackish waters doesn’t mean it will (simulations that expose bacteria to presumed Martian conditions don’t always end well). The sponge, sadly, faces even worse odds.
Importantly, the simple combination of oxygen and water doesn’t guarantee life, explains Melissa Floyd, a microbiologist at NASA’s Goddard Space Flight Center who was not involved in the study. What’s more, Floyd points out, there’s a lot about the harsh Martian atmosphere that could forestall burgeoning life, even with ideal oxygenation.
For one thing, if they exist, Martian lagoons wouldn’t exactly make for classic beachside fun: Though in liquid form, water on this planet would be both punishingly cold and highly saline. These extremes—the very characteristics that make Martian water more amenable to oxygenation—could actually nip viability in the bud, says Michael L. Wong, an astrobiologist at the University of Washington who did not participate in the research. To make matters worse, oxygen in water may not just be hanging around, waiting to be sucked up by an eager organism. According to Reinhard, it could be reacting with other chemicals in the brine or with surrounding rocks, making it less available for respiration.
And, of course, Stamenković says, even in the best of times, oxygen availability hinges on an as-yet-unconfirmed find: the presence of liquid Martian water.
Is our best shot the summer’s presumed polar lake? It’s tricky, says Wong. If it exists, this body of water would likely have the mineral content and chill factor necessary to sustain oxygen. But because this polar pool lives about a mile beneath the Martian surface, it’s possible (though not certain) that it’s cut off from an important source of oxygen: the atmosphere. Though, Stamenković notes, it’s still theoretically possible to liberate oxygen from subterranean water.
If that salty subglacial lake exists, however, it’s unlikely to be alone—and its neighbors might be closer to ground level.
In any case, the hunt for habitability may now get a second wind. For some forms of aerobic life, the paucity of oxygen on Mars might not be the deal breaker we used to believe it to be. And if it’s true for Mars, perhaps it’s true for other planets. After all, the rules on existence seem made to be broken.
“The idea that you can have oxygen on a planet without photosynthesis is amazing,” says Reinhard. “I think this will start to diffuse out to other planets… and shift the way we think about the search for life.”