Above, we're seeing a nicely (if artificially) colored field of dunes. A gully, possibly formed by water, is cutting through, meaning that whatever formed that gully happened after the dunes were sculpted by the wind.
Here we've got some layers on the floor of a valley - an offshoot of Vallis Marineris, the largest canyon in the solar system. It's unclear when the layers formed or how - wind, volcanoes, and water all may have had a role.
This one is part of the highest resolution batch - 26 cm pixels. We're looking at a crater - the rocky cliffs of the crater rim is in the upper left, and channels likely formed by flowing water come down off the cliffs toward the bottom right. Most of the gullies are believed to be tens to hundreds of thousands of years old, or older, when liquid water was stable on the planet's surface.
So that's just scratching the surface, so to speak, of an amazing gallery of photos HiRISE has beamed back to Earth. Some have shown the Mars Exploration Rovers and the Viking Landers, providing context for what we're seeing on the ground.
It's becoming apparent that we're getting pretty good at examining the martian landscape with remarkable detail, but a new world awaits beneath the surface - that's where life might still be active and where the history of rock deposition and alteration will tell us about Mars' past. Fortunately, we're about to get there: the Phoenix Lander, slated to arrive in May, will be able to dig about a meter into the surface to look for water ice. ExoMars is equipped with a drill that can penetrate to 2 meters, gathering very intriguing samples for the life-detection instruments on-board.
And finally, here's an interesting article as a follow up to my last post arguing that indeed, the public is getting dumber...
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