Mars could be a great place to live—for sponges

Written by | Science

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.

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.

Last modified: February 8, 2019