Outflow channels in the context of Ganges Chasma

Maʼadim Vallis is one of the largest outflow channels on Mars, about 700 km long and significantly larger than Earth's Grand Canyon. It is over 20 km wide and 2 km deep in some places. It runs from a region of southern lowlands thought to have once contained a large group of lakes (see Eridania Lake) north to Gusev crater near the equator. It looks as if water may have collected in Gusev crater, forming a giant lake; the Spirit Rover was sent there to investigate that possibility, but found only volcanic rocks on the floor of Gusev. Any lake deposits were probably covered over by a later deposit of volcanic materials from Apollinaris Mons, a nearby volcano.

Maʼadim Vallis is in the Aeolis quadrangle.

The Hesperian is a geologic system and time period on the planet Mars characterized by widespread volcanic activity and catastrophic flooding that carved immense outflow channels across the surface. The Hesperian is an intermediate and transitional period of Martian history. During the Hesperian, Mars changed from the wetter and perhaps warmer world of the Noachian to the dry, cold, and dusty planet seen today. The absolute age of the Hesperian Period is uncertain. The beginning of the period followed the end of the Late Heavy Bombardment and probably corresponds to the start of the lunar Late Imbrian period, around 3700 million years ago (Mya). The end of the Hesperian Period is much more uncertain and could range anywhere from 3200 to 2000 Mya, with 3000 Mya being frequently cited. The Hesperian Period is roughly coincident with the Earth's early Archean Eon.

With the decline of heavy impacts at the end of the Noachian, volcanism became the primary geologic process on Mars, producing vast plains of flood basalts and broad volcanic constructs (highland paterae). By Hesperian times, all of the large shield volcanoes on Mars, including Olympus Mons, had begun to form. Volcanic outgassing released large amounts of sulfur dioxide (SO₂) and hydrogen sulfide (H₂S) into the atmosphere, causing a transition in the style of weathering from dominantly phyllosilicate (clay) to sulfate mineralogy. Liquid water became more localized in extent and turned more acidic as it interacted with SO₂ and H₂S to form sulfuric acid.