Today's Image of Mars was taken by Mars Exploration Rover Opportunity shows a geological region of the rock outcrop at Meridiani Planum. This photo shows an area approximately 1.5cm across. This is one of the famous "blueberries" which are grey hematite concretions and evidence that water once existed on Mars.
This image was taken by Opportunity on Sol 28 of its mission, which was just a few weeks after it landed on Mars. Clicking on this image will take you to images and articles pointing to further evidence that water was once abundant on Mars.