Mars used to have far more oxygen than it does now, scientists have said, potentially making it far more Earth-like (and possibly habitable) than previously believed. Researchers have found high levels of manganese oxides in Martian rocks, which they say can only have come from two sources they know of – microbes or atmospheric oxygen.

Researchers lead by Nina Lanza, from Los Alamos National Laboratory, used the Chemistry and Camera (ChemCam) instrument on board Nasa's Curiosity rover to fire lasers at rocks so they could analyse their chemical make-up. Their findings, published in Geophysical Research Letters, showed high-levels of manganese oxide in mineral-filled cracks of sandstones in the planet's Gale crater.

"The only ways on Earth that we know how to make these manganese materials involve atmospheric oxygen or microbes," Lanza said. "Now we're seeing manganese oxides on Mars, and we're wondering how the heck these could have formed?"

Researchers say the presence of microbes is unlikely at this point, so focused on the more likely scenario - atmospheric oxygen. "These high-manganese materials can't form without lots of liquid water and strongly oxidising conditions," Lanza said. "On Earth, we had lots of water but no widespread deposits of manganese oxides until after oxygen levels in our atmosphere rose."

mars atmospheric oxygen
Features highlighted are 'erosion-resistant fracture fills that are composed of manganese oxides' MSSS/JPL/Nasa

For this reason, the presence of manganese oxides could indicate oxygen levels rose on Mars at some point, before declining to what is seen today. Mars once had oceans and lakes on its surface. The team believes oxygen could have been released into the atmosphere by the breakdown of this water when the planet lost its magnetic field.

With no magnetic field, the water molecules split into hydrogen and oxygen. Low gravity meant that it lost hydrogen, which is very light, but the oxygen remained in the atmosphere, potentially producing these manganese oxides.

Because it is possible to tell the difference between manganese oxides formed by life or through a non-biological process, the scientists are now working to establish their origin. Lanza said: "It's hard to confirm whether this [atmospheric oxygen] scenario for Martian atmospheric oxygen actually occurred. But it's important to note that this idea represents a departure in our understanding for how planetary atmospheres might become oxygenated."