Rocks tell of oxygen-rich Martian past

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NASA’s Curiosity Mars rover at a location called “Windjana”, where the rover found rocks containing manganese-oxide minerals, which require abundant water and strongly oxidising conditions to form. The inset images show erosion-resistant fracture fills containing manganese oxides, suggesting the Martian atmosphere might once have contained more free oxygen than it does now.
Credit: NASA/JPL-Caltech/MSSS

NASA’s Curiosity rover has found rocks on Mars that suggest the planet’s atmosphere may once have been more oxygen-rich than now.

High levels of manganese oxides, combined with the mounting evidence of the abundance of ancient lakes, paint a picture of a very Earth-like planet.

The manganese oxides were found in mineral veins in the Gale crater area the rover is investigating.

Curiosity’s Chemistry and Camera (ChemCam) instrument fires laser pulses, which allows scientists to observe the spectrum of resulting flashes of plasma to determine the chemical makeup of the target rock.

“The only ways on Earth that we know how to make these manganese materials involve atmospheric oxygen or microbes,” said Nina Lanza, a planetary scientist at Los Alamos National Laboratory in New Mexico.

“Now we’re seeing manganese oxides on Mars, and we’re wondering how the heck these could have formed?”

Lanza said that higher oxygen concentrations seemed the more feasible explanation. The existence of the minerals also pointed to a time of abundant water.

“These high manganese materials can’t form without lots of liquid water and strongly oxidising conditions.”

On Earth, the appearance of high concentrations of manganese oxide minerals is an important marker of a major shift in our atmosphere’s composition, from relatively low oxygen abundances to today’s oxygen-rich atmosphere.

“Here on Earth, we had lots of water but no widespread deposits of manganese oxides until after the oxygen levels in our atmosphere rose.”

Lanza is the lead author of a new report about the Martian manganese oxides in Geophysical Research Letters.

That still leaves the question of the origin of the oxygen.

“One potential way that oxygen could have gotten into the Martian atmosphere is from the breakdown of water when Mars was losing its magnetic field,” said Lanza.

The theory goes that, without a protective magnetic field to shield the surface, ionising radiation started splitting water molecules into hydrogen and oxygen.

Because of Mars’ relatively low gravity, the planet wasn’t able to hold onto the very light hydrogen atoms, but the heavier oxygen atoms remained.

Much of this oxygen went into rocks, leading to the rusty red dust that covers the surface today.

“It’s hard to confirm whether this scenario for Martian atmospheric oxygen actually occurred,” says Lanza. “But it’s important to note that this idea represents a departure in our understanding for how planetary atmospheres might become oxygenated.”

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