Curiosity finds evidence that Mars was once rich in oxygen
 

Mars may have had a much more Earth-like atmosphere rich in oxygen, according to JPL's Curiosity team. Using its laser-firing instrument in the Gale Crater, the intrepid rover found rocks with manganese oxide, which requires oxygen to form. On Earth,...

https://www.engadget.com/2016/06/28/...ich-in-oxygen/