Deuteronilus Cavus is a roughly 120-kilometer-wide depression located in the Ismenius Lacus quadrangle of Mars, within the northern mid-latitudes of Arabia Terra. The name Deuteronilus is derived from Greek and means "second Nile," referencing the nearby Mamers Valles. Mamers Valles is a valley system that stretches over 1,000 kilometers, originating in the southern highlands and extending into the northern lowlands. The valley intersects Deuteronilus Cavus to the east. The Deuteronilus region is commonly referred to as fretted terrain — the transitional zone between the highlands and lowlands — characterized by knobs, mesas, branching channels, and depressions that often exhibit flow-like structures. Deuteronilus Cavus preserves a rich geological history, spanning from ancient Noachian terrain to Amazonian-aged surface deposits.
The nearly circular shape of the depression suggests an impact crater origin, likely formed during the Noachian period. Over time, glacial and fluvial activity significantly reshaped the original crater, enlarging it to nearly twice its initial size. The depression lies within the ancient, cratered highlands of Arabia Terra. Surrounding features include large craters such as Focas Crater (approximately 75 kilometers wide) to the southwest, as well as wrinkle ridges that overlie the ancient terrain. These ridges are surface expressions of lava flows that cooled and contracted, indicating a later period of volcanic activity.
While the surrounding terrain dates back to the Noachian period, the depression itself displays several features indicative of more recent geological activity. The HRSC images reveal that the degraded crater rim is cut in some areas, likely due to mass-wasting resulting from groundwater sapping. Notably, in the southern part of the depression, branching channels, resembling to sapping structures are visible (see annotated image). These channels suggest that drainage ways could have been created by the outflow of subsurface fluids. Along nearly all sections of the crater wall, viscous flow patterns can be observed with a flow direction downward (see annotated image). These features are known as lobate debris aprons and are often interpreted as debris-covered glaciers, consisting of ice-rich material beneath a protective rock layer. These aprons are a typical feature in the mid-latitudes (see April PR - Acheron Fossae) and are believed to form during periods of high obliquity (increased axial tilt) in the Amazonian period, when climatic conditions allowed ice to accumulate.
Inside the depression, the landscape shows a mix of knobs, mesas, and smoother plains. A closer view of one of the knobs near the crater wall shows that it is partially enclosed by lobate debris aprons. The knobs and mesas may be erosional remnants of a formerly continuous rock layer or, particularly toward the center of the depression the remnants of a former central peak structure. However, these “knobby remnants” are a common feature in “fretted” regions.
Toward the center of the depression, darker surface material is visible, likely consisting of aeolian deposits that often accumulate in depressions. The dark material could be of volcanic origin. Spectral analysis also detected the presence of phyllosilicates concentrated within these darker deposits. Phyllosilicates are clay minerals that typically form through aqueous alteration of volcanic ash in marine or lacustrine environments, suggesting that liquid water once played an important role in this region.
