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The Mars Reconnaissance Orbiter has completed its four year of its mission. Here we look at some of the stunning HiRise images it has sent back to Earth...

This image from the HiRise camera shows dark sand dunes and inverted craters in the Arabia Terra region of Mars. The sand was probably derived from basalt, a black volcanic rock that is common on Mars. This crater sticks out because erosion has stripped away the surrounding surface.

Sand dunes, with graceful blue-gray swirls caused by dust devils, which are like mini-tornadoes.

A branch of the Cerberus Fossae - a series of semi-parallel fissures on Mars formed by faults that pulled the crust apart in the region.

The Noctis Labyrinthus region, which is known for its steep valleys. Readings from the orbiter's spectrometer (that uses light to identify materials) has found iron-bearing sulfates and clay minerals. HiRISE observations have revealed exposed layers which are possibly the sources of the signatures seen by CRISM. In the subimage, the layering can be seen in the lower part of the image. To the upper left one can see a dune field which covers other beds.

A thick (2 mile) sequence of ice and dust, stacked like a layer cake, covers the south pole of Mars. Impact craters that form here, like this 0.5mile one, experience slightly different processes of modification and degradation than those that form in rocky areas.

On the floor of this crater where there are no dunes, carbon dioxide ice forms an uninterrupted layer. On the dunes however, dark streaks form as surface material moves around.

These barchan dunes formed because the wind was blowing in one dominant direction. Here several barchans are merging to form an even larger barchan dune.

A huge shield volcano in Syrtis Major. This region exposes Early Noachian bedrock, more than 4 billion years old, and contains a diversity of hydrated minerals. Nasa think this would be an ideal place to explore early Mars.

An area in Arabia Terra, which is a large upland region in the north of Mars and thought by its battered appearance to be one of the oldest terrains.

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