Mars may be a famously red, dusty, thinly insulated planet now, but once upon a time, billions of years ago, it almost certainly had huge seas of liquid water and a warmer climate, not unlike our own world. Now, thanks to another glorious find from the Curiosity rover, researchers can confirm that Mars was even more Earth-like than anyone had ever realized.
Using its ChemCam instrument, which probes the geochemistry of any rock samples it comes across, Curiosity has determined that there are extremely high levels of manganese oxide present in surface-level Martian rocks – at least within the Gale crater in which the little mechanical laboratory is based. In order to get such high quantities of this compound, a planet requires just one thing: plenty of free oxygen floating around the atmosphere.
This means that, when these rocks formed, Mars had an atmosphere somewhat like that of Earth’s. Although it’s difficult to determine precisely how much free oxygen there was from this preliminary data, we now know that it must have been somewhat similar conditions to those that existed between 2 and 2.5 billion years ago, when something called the Great Oxygenation Event (GOE) occurred.
Curiosity rover poking around a location known as "Windjana," where rocks containing manganese oxides (inset box) were found. NASA/JPL-Caltech/MSSS
The GOE was most likely caused by the appearance of photosynthesizing life, which slowly converted the carbon dioxide-rich atmosphere on Earth into oxygen. When the surface geology could not chemically react with and subsequently absorb any more free oxygen, the rest was left to build up in the atmosphere, and the world was oxygenated.
Manganese oxide simply cannot form without extremely high quantities of free oxygen in the atmosphere, as observed on our own world at the time of the GOE. This also means that there was abundant liquid water on the surface, which agrees with multiple previous studies.