Gypsum on Mars...

Gypsum is a mineral, the stuff that's in the core of drywall.  It's very common on Earth, and had been previously detected on Mars.  What's new here is that the Mars rover Opportunity spotted a vein of gypsum (at right, click to enlarge) – and the only way anybody knows of that such a vein could be formed requires the presence of liquid water.  In other words, this is the equivalent of an old sign saying “Liquid water was once present here!”  Lots of other things found on Mars were already hinting that liquid water was once present on Mars – but this finding makes it all but certain...