Bullseye Crater in Elysium Planitia
This HiRISE image allows us to see this unusual Martian geologic feature in three dimensions.
The sequence of events that formed this scene in the equatorial lowlands of Mars are as follows. First, a meteorite impact excavated the 2.2 kilometer (1.4 mile) diameter crater in the center of the picture. Second, the area was flooded by a vast lava flow. However, the lava was not able to overtop the rim of the crater and it remained a large depression.
Third, the area was blanketed by a series of layers of small particles carried by the wind. This deposit makes up what is called the Medusae Fossae Formation and may be composed of volcanic ash. Finally, the Medusae Fossae Formation was largely eroded away by the wind. However, the deposits within the crater were protected from the wind and have remained to this day.
The hills outside the crater are also remnants of the Medusae Fossae Formation.