Planetary scientist Vlada Stamenković of the NASAJet Propulsion Laboratory and colleagues have developed a new chemical model of how oxygen dissolves in Martian conditions, which raises the possibility of oxygen-rich brines; enough, the work suggests, to support simple animals such as sponges. The model was published in Nature on October 22.
The atmosphere of Mars is far too thin for humans to breathe or for lungs like ours to extract any oxygen at all. It has on average only around 0.6% of the pressure of Earth's atmosphere, and this is mainly carbon dioxide; only 0.145% of the thin Martian atmosphere is oxygen. The new model indicated these minute traces of oxygen should be able to enter salty seeps of water on or near the planet's surface at levels high enough to support life forms comparable to Earth's microbes, possibly even simple sponges. Some life forms can survive without oxygen, but oxygen permits more energy-intensive metabolism. Almost all complex multicellular life on Earth depends on oxygen.
"We were absolutely flabbergasted [...] I went back to recalculate everything like five different times to make sure it's a real thing," Stamenković told National Geographic. "Our work is calling for a complete revision for how we think about the potential for life on Mars, and the work oxygen can do," he told Scientific American, "implying that if life ever existed on Mars it might have been breathing oxygen".
Stamenković cite research from 2014 showing some simple sponges can survive with only 0.002 moles of oxygen per cubic meter (0.064 mg per liter). Some microbes that need oxygen can survive with as little as a millionth of a mole per cubic meter (0.000032 mg per liter). In their model, they found there can be enough oxygen for microbes throughout Mars, and enough for simple sponges in oases near the poles.
In 2014, also suggesting multicellular life could exist on Mars, de Vera et al, using the facilities at the German Aerospace Center (DLR), studied some lichens, including Pleopsidium chlorophanum, which can grow high up in Antarctic mountain ranges. They showed those lichens can also survive and even grow in Mars simulation chambers. The lichens can do this because their algal component is able to produce the oxygen needed by the fungal component. Stamenković et al's research provides a way for oxygen to get into the Martian brines without algae or photosynthesis.
Stamenković found oxygen levels throughout Mars would be high enough for the least demanding aerobic (oxygen-using) microbes, for all the brines they considered, and all the methods of calculation. They published a detailed map of the distributions of solubility for calcium perchlorates for their more optimistic calculations, which they reckoned were closer to the true case, with and without supercooling. The lowest concentrations were shown in the tropical southern uplands. Brine in regions poleward of about 67.5° to the north and about 72.5° to the south could have oxygen concentrations high enough for simple sponges. Closer to the poles, concentrations could go higher, approaching levels typical of sea water on Earth, 0.2 moles per cubic meter (6.4 mg per liter), for calcium perchlorates. On Earth, worms and clams that live in the muddy sea beds require 1 mg per liter, bottom feeders such as crabs and oysters 3 mg per liter, and spawning migratory fish 6 mg per liter, all within 0.2 moles per cubic meter, 6.4 mg per liter.
The research paper is theoretical and is based on a simplified general circulation model of the Mars atmosphere — it ignores distinctions of seasons and the day / night cycle. Stamenković's team combined it with a chemical model of how oxygen would dissolve in the brines and used this to predict oxygen levels in such brines at various locations on Mars. When asked about plans for a future model that might include seasonal timescales, Stamenković responded: "Yes, we are now exploring the kinetics part and want to see what happens on shorter timescales."