Syrtis Major Planum is a "dark spot" (an albedo feature) located in the boundary between the northern lowlands and southern highlands of Mars just west of the impact basin Isidis in the Syrtis Major quadrangle. It was discovered, on the basis of data from Mars Global Surveyor, to be a low-relief shield volcano, but was formerly believed to be a plain, and was then known as Syrtis Major Planitia. The dark color comes from the basaltic volcanic rock of the region and the relative lack of dust. A selected landing site for the Mars 2020 rover mission is Jezero crater (at ) within the region. The northeastern region of Syrtis Major Planum was also considered a potential landing site. Geography and geology Syrtis Major is centered near at , extends some north from the planet's equator, and spans from west to east. It is in the Syrtis Major quadrangle. It encompasses a large slope from its western edge at Aeria dropping to its eastern edge at Isidis Planitia. It includes a high-altitude bulge that rises at 310° W. Most of Syrtis Major has slopes of less than 1°, a much lower inclination than the slopes of the Tharsis shield volcanoes. It has a 350 km × 150 km north–south elongated central depression containing the calderas Nili Patera and Meroe Patera, which are about 2 km deep. The floors of the calderas are unique among large Martian volcanoes as they are not elevated relative to the terrain surrounding Syrtis Major. This may account for the high degree of magmatic evolution and hydrothermal activity seen in Nili Patera. The floor of Nili Patera is the less cratered, and therefore the younger, of the two. While most of the rock is basaltic, dacite has also been detected in Nili Patera. Satellite gravity field measurements show a positive gravity anomaly centered on the caldera complex, suggesting the presence of a 600 km × 300 km north–south elongated extinct magma chamber below, containing dense minerals (probably mainly pyroxene, with olivine also possible) that precipitated out of magma before eruptions. Crater counts date Syrtis Major to the early Hesperian epoch; it postdates formation of the adjacent Isidis impact basin. Discovery and name The name Syrtis Major is derived from the classical Roman name Syrtis maior for the Gulf of Sidra on the coast of Libya (classical Cyrenaica). Syrtis Major was the first documented surface feature of