A cloud forms a white streamer against the orange Mars landscape in this recent image from Mars Express. The cloud is flowing away from Arsia Mons, one of the largest volcanic mountains on the planet. The cloud doesn't emanate from the volcano, though. Instead, it forms as winds flow up the flank of the mountain, allowing the water vapor in the air to condense at high altitude. In this image, the cloud stretches more than 600 miles (900 km). It also casts a dark shadow on the landscape. [ESA/DLR/FU Berlin]