Media credit: Image courtesy NASA/JPL/University of Arizona
Like the other major shield volcanoes on Mars, Arisa Mons has a caldera (large volcanic crater) at its summit.
Calderas form when magma (molten rock) is removed from the magma chamber in the volcano, and the roof of the magma chamber collapses into the resulting void. In the case of Arsia Mons, there are relatively young lava flows that overtop the northeast rim of the caldera.
This HiRISE image samples some of these lava flows. The long elliptical depression is the summit crater of a small shield volcano that fed some of these lava flows. At HiRISE resolution, we see that even these younger lavas are covered by a thick layer of dust. The small dark-rayed crater in the southwest edge of the image shows that the rock under the dust is dark, as expected of lava.
Observation Geometry
Image PSP_002157_1715 was taken by the High Resolution Imaging Science Experiment (HiRISE) camera onboard the Mars Reconnaissance Orbiter spacecraft on 11-Jan-2007. The complete image is centered at -8.4 degrees latitude, 240.1 degrees East longitude. The range to the target site was 244.7 km (153.0 miles). At this distance the image scale is 49.0 cm/pixel (with 2 x 2 binning) so objects ~147 cm across are resolved. The image shown here has been map-projected to 50 cm/pixel and north is up. The image was taken at a local Mars time of 03:41 PM and the scene is illuminated from the west with a solar incidence angle of 57 degrees, thus the sun was about 33 degrees above the horizon. At a solar longitude of 165.0 degrees, the season on Mars is Northern Summer.