Today's Image of Mars shows exposures of layered rocks in the Argyre Quadrangle on Mars. This bullseye looking feature is created as a result of erosion. Over millions or billions of years the wind on Mars chips away the outer and uppermost layers of these deposits, exposing the older material underneath. Scientists can then study the older, exposed layers and use their composition to determine the climate on Mars when the materials were deposited. Layered deposits like these are thought to be evidence that great lakes or seas once existed on Mars.
The Argyre Quadrangle is home to the Argyre impact basin, which contains a lot of ancient eroded terrain and numerous impact craters, including Galle Crater (aka 'Smiley Face' Crater).
Clicking on this image will take you to the original, captioned HiRISE image.
If you like the HiRISE images showcased in the Mars Photo of the Day posts I encourage you to enter for a free 2012 HiRISE Calendar