That's what they say
And here's their spectral analysis to back it up.
Yes, you need water to make a hydroxide like goethite. (here's another site.)
Is that dark patch all goethite? Beats me. I guess it could be. I've seen something like that (ie a rusted hillside) before near the Stillwater Platinum mine in Montana. But it was the result of the weathering of a big pile o' sulfides. I wonder if we'll ever know for sure what that is. Perhaps a comparison of orbital images over time would show if it is a shadow effect.