Mars is a weird planet. It is cold and extremely dry today, but the geology and topography indicate it was much warmer and wetter in the past. That means its atmosphere must have been thicker a billion or two years ago. To sustain water, there must have been some sort of greenhouse gas to hold in the heat.
On Earth, we have carbon dioxide fulfilling that role (and methane, and water vapor). Scientists have assumed that the Martian atmosphere must have had quite a bit of CO2 back then to keep it mild, and in fact the thin air is mostly CO2 now.
But if that were true, where did it go? The total amount of CO2 must have dropped by a huge amount over the eons, and the most likely place it would be is trapped in carbonate rocks.
One teensy problem: those carbonates aren't there, at least not in the amounts needed. You'd expect there to be limestone all over the planet's surface, but there isn't. Instead, the rovers found evidence of sulfur-rich rocks; jarosite is common on the surface, for example.
This has led scientists at MIT and Harvard to suppose something a bit radical: instead of CO2, maybe there was lots of SO2 -- sulfur dioxide -- in the early Martian air. This idea is a twofer: it explains the high sulfur content on the planet, and it also explains why there are few or no carbonates. Sulfur dioxide inhibits carbonate formation.
Mars clearly had lots of standing water on its surface long ago. If the air had SO2 in it, it would dissolve easily (much like CO2 does). The oceans would have become acidic, a solution of sulfuric acid. When the oceans dried up this sulfur rich water formed the jarosite.
In a sense, this mimics Earth's carbon cycle, where CO2 in the air dissolves in the water and forms carbonates like limestone. But Mars... well, it's different. It's sometimes easy to forget that, which is why scientists have been thinking carbon all this time.
The primitive Mars much have looked very different. Before iron oxide tinted it red, and before the water all disappeared, what did it look like? Orange, or yellow, with its high sulfur content? One thing I can be fairly sure of: it would have smelled bad. Sulfur is the main ingredient in hydrogen sulfide, for one.
Ray Bradbury may have said that Mars is heaven, but I think, given the amount of brimstone that may have been there, he got his theology backwards.