Mars used to be like Earth, but...

But it did not have a protective magnetic field to shield the surface from ionising radiation.

NASA's Mars Curiosity rover has discovered high levels of manganese oxides in Martian rocks, which may indicate that the red planet was once more Earth-like than previously believed.

Curiosity observed manganese oxides in Mars rocks, which could indicate that higher levels of atmospheric oxygen once existed on our neighbouring planet, NASA researchers said.

This hint of more oxygen in Mars' early atmosphere adds to other Curiosity findings -- such as evidence of ancient lakes -- showing how Earth-like our neighbouring planet once was.

"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.

"Now we're seeing manganese-oxides on Mars and wondering how the heck these could have formed," said Lanza.

Lanza used the Los Alamos-developed ChemCam instrument that sits atop Curiosity to "zap" rocks on Mars and analyse their chemical make-up.

In less than four years since landing on Mars, ChemCam has analysed roughly 1,500 rock and soil samples.

Microbes seem a far-fetched explanation for the manganese oxides at this point, said Lanza, but the idea that the Martian atmosphere contained more oxygen in the past than it does now seems possible.

"These high-manganese materials can't form without lots of liquid water and strongly oxidising conditions," said Lanza.

"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 photosynthesising microbes," she said.

In the Earth's geological record, the appearance of high concentrations of manganese is an important marker of a major shift in our atmosphere's composition, from relatively low oxygen abundances to the oxygen-rich atmosphere we see today.

The presence of the same types of materials on Mars suggests that something similar happened there.

"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.

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

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

Due to Mars' relatively low gravity, it wasn't able to hold onto the very light hydrogen atoms, but the heavier oxygen atoms remained behind, researchers said.

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

While Mars' famous red iron oxides require only a mildly oxidising environment to form, manganese oxides require a strongly oxidising environment.

These results suggest that past conditions were far more oxidising (oxygen-rich) than previously thought.