ESA's Mars Express has photographed a section of Arabia Terra — one of the oldest regions on Mars. The terrain is between 3.7 and 4.1 billion years old, and in that time it has accumulated a striking collection of impact craters.
The most prominent is Trouvelot Crater, stretching 130 km across. Its walls have long been crumbling, its floor is covered in dark volcanic rock rich in iron and magnesium, and the wind has sculpted that material into crescent-shaped barchan dunes. Standing out against this dark landscape is a light-toned mound about 20 km long, marked by ridges and grooves. Similar formations elsewhere on Mars are typically associated with minerals that formed in the presence of water — the mound may once have sat at the bottom of a lake, or built up gradually as groundwater rose and mixed with wind-blown sediments on the crater floor.
Mars Express has been imaging Mars since 2003, and each frame like this one adds another piece to the picture of what the Red Planet looked like billions of years ago.