Chasmata

Chasmata (singular chasma) are large, elongated depressions on the surfaces of planets, moons, and other bodies in the Solar System. The term comes from Greek chasma "opening" and is used in planetary geology to classify canyon-, trough-, or trench-like features that extend for hundreds to thousands of kilometers with steep walls. The International Astronomical Union (IAU) classifies such features as chasmata when they are distinctly long and trough-shaped rather than simple impact craters or pits.

Most chasmata are formed by extensional tectonics that fracture crust and create down-dropped blocks (grabens), producing a trough with steep sides. Other mechanisms include volcanic processes such as caldera collapse, dike intrusion, or crustal collapse around magma reservoirs; erosion by fluid flow, frost, or debris flows can modify the walls and floor. On some bodies with limited atmospheres or no liquid water, chasmata arise primarily from tectonics and volcanic activity rather than fluvial carving.

Chasmata vary greatly in size. Many extend for hundreds to thousands of kilometers and are tens of kilometers wide, with depths of several kilometers. They may be isolated features or part of broader canyon systems that cut across terrains. Some chasmata contain layered deposits, terraces, landslides, or smaller pits and faults along their margins.

On Mars, the Valles Marineris region contains numerous chasmata, such as Ius Chasma, Nili Chasma, Melas Chasma, and Hebes Chasma, illustrating a belt of crustal extension. Similar features occur on Venus and other rocky bodies, reflecting diverse tectonic and volcanic histories.