Argyre Planitia

Argyre Planitia is a plain located in the Argyre impact basin in the southern highlands of Mars. The basin is approximately 1800 kilometers wide, the second-largest impact basin on Mars after Hellas Planitia, and drops 5.2 kilometers below the surrounding plains.

The basin was formed by a giant impact during the Heavy Bombardment period of the early Sol system, approximately 3.9 billion years ago, and is one of the best preserved ancient impact basins from this time. Argyre is surrounded by rugged massifs which form vague concentric and radial patterns around the basin. Several mountain ranges are present, including Charitum and Nereidum Montes.

Four large Noachian epoch channels lie radial to the basin. Three of these channels (Surius Valles, Dzigai Valles, and Palacopas Valles) flowed into Argyre from the south and east through the rim mountains. The fourth, Uzboi Vallis, appears to have flowed out from the basin's north rim to the Chryse Planitia outflow region and may have drained a lake within the basin. A smaller outflow channel named Nia Valles is relatively fresh-looking, and probably formed during the early Amazonian after the major fluvial and lacustrine episodes had finished.

The original basin floor is buried with friable, partially deflated layered material that may be lake sediment. No inner rings are visible, however isolated massifs within the basin may be remnants of an inner ring.