Close-up pictures taken by Europe's Mars Express probe of a volcano on the Red Planet reveal water could have flowed on its flanks in the past.
Images of the 5,300m-high mountain, Hecates Tholus, taken 275km above Mars, also show signs of cratering on the slopes caused by volcanic activity.
The caldera, a circular depression from which magma erupts or is withdrawn, can be seen in detail in the pictures.
Scientists say numerous collapses have re-shaped the caldera over time.
Lines seen radiating outwards on the picture are thought to be flow features related to water.
The pictures were taken by the High Resolution Stereo Camera (HRSC) on Mars Express, the European Space Agency probe that is now in orbit around the Red Planet.
The volcano's caldera has a maximum diameter of 10km and a depth of 600m.
This compares with a diameter of 60km and a depth of 3km for the caldera on Olympus Mons which - at around 22km in height - is the tallest volcano in the Solar System.
Hecates Tholus is the northernmost volcano in the Elysium group which lie on the northern plains.