A color image of Hadriaca Patera on the northeast rim of Hellas basin of Mars; north toward top. The scene shows a central circular depression surrounded by low radial ridges and, at the bottom of the image, the channel of Dao Vallis. A patera (Latin for shallow dish or saucer) is a volcano of broad areal extent with little vertical relief.

This image is a composite of Viking medium-resolution images in black and white and low-resolution images in color. The image extends from latitude 27 degrees S. to 37 degrees S. and from longitude 263 degrees to 273 degrees; Mercator projection.

Hadriaca Patera is less than 2 km high, has a 60-km-diameter caldera at its center, and is surrounded by a 300-km-wide ring of low ridges. The radial ridges may be lava flows with lava channels at their crests. South of Hadriaca, Dao Vallis begins at a steep-walled depression 40 km across but forms a much shallower channel that extends 800 km southwest into the floor of the Hellas basin. The channel is very likely fluvial in origin, with the release of water being triggered by volcanic activity.