The Curiosity rover at the Windjana outcrop on Mars, where it found evidence of mangnese oxide on rocks and in rock fissures.  The mineral is formed only in the presence of water and plentiful oxygen. (NASA)

Early in the Curiosity rover’s trek across Gale Crater on Mars, team member and Los Almos National Laboratory  planetary scientist Nina Lanza reported finding surprisingly high concentrations of the mineral manganese oxide.  It was showing up as a blackish-purple fill to cracks in rocks, and possibly as a surface covering to others.

Lanza, who had some experience with the common and much-debated mineral– found in the American Southwest and other arid climes — initially proposed that it just might be related to terrestrial rock varnishes.  This was a bold proposal because manganese-oxide rock varnishes on Earth are almost always associated with microbes, which are known to concentrate the mineral.  So was this a biosignature coming from Mars?

Two years later, Lanza and others on the Curiosity team have published a paper describing in detail the regular detection of Martian manganese oxide, sometimes in concentrations higher than what is found on Earth.  Based on the surrounding geology and geochemistry, the team then concluded that when the mineral was formed, the Mars atmosphere had levels of oxygen much higher than previously imagined.

This conclusion flows from the fact that the mineral is only formed, on Earth at least, when plentiful oxygen and plentiful water are present.  Indeed, manganese oxides (and many other minerals) began forming in earnest here only after the so-called “great oxygenation event” that, through bacterial photosynthesis, delivered vastly more oxygen to Earth’s atmosphere.

On Mars, the manganese oxide was found largely in sedimentary rock cracks, and to geologists that means it was distributed via flowing water after the rocks had solidified.

Finding substantial water and oxygen together on a planet — in our solar system or beyond — has often been described as providing a strong case for a habitable, and perhaps inhabited, planet.

The oxygen in the Earth’s atmosphere increased dramatically around 2.3 billion years ago with the fast spread of cyanobacteria, or blue-green algae, and other photosynthesizing micro organisms in the oceans.  The image is of a plankton bloom (dominated by blue-green algae) off the coast of Washington state, taken by astronauts from the International Space Station. (NASA)

This all sounds suggestive of life on what Lanza calls “middle-aged” Mars.  But here’s where things get tricky.… Read more