Analysis of rocks and sand by NASA's Mars rover Curiosity suggests Mars was once much more like Earth than previously thought.

Curiosity discovered manganese oxides in rocks on the Red Planet, which means the Martian atmosphere may at some point have held much more oxygen that it does now, researchers at Los Alamos National Laboratory say.

Atmospheric oxygen is typically considered a biosignature for life, but the researchers have no reason to suspect microbes or other Earth-like organisms were involved in oxygenating the atmosphere. In Earth's geological record, the appearance of high concentrations of manganese marks the shift to an oxygen-rich atmosphere, produced by microbes.

"These high-manganese materials can't form without lots of liquid water and strongly oxidizing conditions," Dr. Nina Lanza, a planetary scientists at Los Alamost National Laboratory, said in a press release. "Here on Earth, we had lots of water but no widespread deposits of manganese oxides until after the oxygen levels in our atmosphere rose due to photosynthesizing microbes."

Instead, researchers suggest another scenario could have led to an oxygen-rich atmosphere without the help of living organisms.

"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," Lanza said. "It's thought that at this time in Mars' history, water was much more abundant."

Without a magnetic field, ionizing radiation would have split the planet's water into hydrogen and oxygen. While hydrogen escaped Mars' low gravity, oxygen would remain, and settle into rocks.

The rover's findings, published in the journal Geophysical Research Letters, were discovered using the ChemCam instrument on top of the rover, which "zaps" rocks to analyze their chemical makeup.

Curiosity's ChemCam has analyzed about 1,500 rock and soil samples, finding the manganese oxide in the Kimberley region of Gale crater during its fourth year of exploration on Mars.

The Opportunity rover, which has been there since 2004, also has also recently discovered manganese oxide near its landing site, researchers report.

Researchers next hope to learn how to spot whether manganese is biogenic or non-biogenic -- produced by life or non-biological means.

"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 wondering how the heck these could have formed."