This broad field of barchan sand dunes – dunes with crescent shapes – in the Olympia Undae region near Mars’ northern polar cap was imaged by NASA’s Mars Odyssey orbiter. The image, showing a small segment of the dune field, was taken during summer when the surface was free of frost. The dunes cover an area of some 470,000 square kilometres (181,000 square miles), larger than the U.S. state of California, and surround about a quarter of the northern polar cap. It is the most expansive continuous field of dunes on Mars.
At half the diameter of Pluto, Charon is the largest satellite relative to its planet in the solar system. NASA’s New Horizons spacecraft has returned the best colour and the highest resolution images yet of Charon, showing a landscape covered with mountains, canyons, landslides, surface-colour variations and more — all evidence of a surprisingly complex and violent history.