Was planet Mars once more Earth-like than we previously believed? It may, for instance, have had an atmosphere abundant with oxygen, according to new data from NASA’s Curiosity rover.

The Chemistry and Camera (ChemCam), a laser-spitting instrument on the Curiosity, detected high levels of manganese oxides in Martian rocks, which can be tied to an era when groundwater exists in the Gale Crater area of research. The hint added to other findings — including proof of ancient lakes — that reveal how Earth-resembling the Red Planet once was.

"The only ways on Earth that we know how to make these manganese materials involve atmospheric oxygen or microbes," said lead author and planetary scientist Nina Lanza from New Mexico’s Los Alamos National Laboratory. "Now we're seeing manganese oxides on Mars, and we're wondering how the heck these could have formed?"

According to the researchers, microbes are not the likely reason behind the manganese oxides. The high-manganese objects cannot form without plenty of liquid water as well as highly oxidizing environments, explained Lanza, pointing out that the Earth has sufficient water, but without massive manganese oxide deposits until after its atmospheric oxygen levels climbed.

These minerals mark a major transition in Earth’s atmosphere, from quite low oxygen levels to the oxygen-rich one that it is today. On Mars, the presence of the same materials suggested that its atmosphere once held more oxygen than today — but how did those conditions actually form?

One likely factor: water’s breakdown when the planet was losing its magnetic field, as high oxygen concentrations occurred at a time in its history when water used to be much more plentiful. However, without a magnetic field shielding the surface, ionizing radiation began splitting water molecules into hydrogen and oxygen.

Since Mars has a relatively poor gravity, it did not maintain grip of the very light atoms of hydrogen, but instead kept the heavier oxygen ones. Some of the iron oxides, which offer Mars its popular rusty reddish color, went into the rocks.

While iron oxides do not require a lot of oxygen to exist, manganese oxides can survive when at some point its environment had a greatly oxidizing atmosphere, marking the presence of oxygen.

Lanza clarified that it is difficult to confirm this actual oxygen scenario in the Martian atmosphere, but it could enrich one’s understanding of how planets’ atmospheres can possibly become oxygenated. Rich oxygen levels in the air may be a biosignature or sign of life, but the process itself may not necessarily require life.

The findings were detailed in the journal Geophysical Research Letters.

Data sent back by the Curiosity rover also recently showed evidence of tridymite on Mars — a surprising discovery given that unlike Earth, Mars does not have shifting plates leading to volatile volcanoes and massive earthquakes.

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