A NASA rover just exposed something on Mars that eluded orbiters

A NASA rover taking rock samples on Mars has uncovered a plentiful mineral that was invisible to orbiters studying the Red Planet from space.Scientists say the discovery of siderite, a type of iron carbonate, could be crucial evidence to support the theory that Mars once had a thick carbon dioxide-rich atmosphere, allowing a warm enough environment to support oceans, lakes, and streams.  Curiosity, a car-sized lab on six wheels, performed a chemical analysis of four rock samples drille