Sulfates are salts that form when sulfur, usually in compound form, mixes with other minerals in water.

When the water evaporates, the minerals mix and dry out, leaving the sulfates behind.

These sulfate minerals can tell us a lot about Mars, such as its water history, and how it has weathered over time.

Pure sulfur, on the other hand, only forms under a very narrow set of conditions, which are not known to have occurred in the region of Mars where Curiosity made its discovery.

There are, to be fair, a lot of things we don’t know about the geological history of Mars, but the discovery of scads of pure sulfur just hanging about on the Martian surface suggests that there’s something pretty big that we’re not aware of.