Syrtis Major Planum (formerlySyrtis Major Planitia) is a massiveshield volcano in the eastern hemisphere ofMars. A "dark spot" (analbedo feature), Syrtis Major Planum is located in the boundary between the northern lowlands and southern highlands of Mars just west of theimpact basinIsidis in theSyrtis Major quadrangle. It was formerly believed to be aplain, and was therefore described as aplanitia. Later data from theMars Global Surveyor revealed that it is actually a broad topographic rise.^[2] The dark color of Syrtis Major Planum comes from thebasaltic volcanic rock of the region and the relative lack of dust.

Syrtis Major

Mars digital-image mosaic merged with color of theMC-13 quadrangle, Syrtis Major region of Mars.
Feature type	Shield volcano
Coordinates	8°24′N69°30′E / 8.4°N 69.5°E /8.4; 69.5
Diameter	~1 300 km[1]
Peak	~2.3 km[2]
Discoverer	Christiaan Huygens
Eponym	Gulf of Sidra

The selected landing site for theMars 2020 mission that includes the roverPerseverance and the helicopter droneIngenuity wasJezero crater, at18°51′18″N77°31′08″E / 18.855°N 77.519°E /18.855; 77.519^[3] within the region.^[4] Thenortheastern region of Syrtis Major Planum was also considered a potential landing site.

Syrtis Major is centered near at8°24′N69°30′E / 8.4°N 69.5°E /8.4; 69.5. It extends some 1,500 km (930 mi) north from the planet's equator, and spans 1,000 km (620 mi) from west to east. It is in theSyrtis Major quadrangle. It encompasses a large slope from its western edge atAeria, dropping 4 km (2.5 mi) to its eastern edge atIsidis Planitia. Most of Syrtis Major has slopes of less than 1°, a much lower inclination than the slopes of theTharsis shield volcanoes. It has a 350 km × 150 km north–south elongated central depression containing thecalderas Nili Patera and Meroe Patera, which are about 2 km deep.^[1][2]

The roughly 2,300-meter high peak of Syrtis Major is located northwest of Nili Patera. 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.^[1][2]

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 extinctmagma chamber below, containing dense minerals (probably mainlypyroxene, witholivine also possible) that precipitated out ofmagma before eruptions.^[5] Crater counts date Syrtis Major to theearly Hesperian epoch; it postdates formation of the adjacent Isidisimpact basin.^[2]

The name Syrtis Major is derived from the classicalRoman nameSyrtis maior for theGulf of Sidra on the coast ofLibya (classicalCyrenaica).

Syrtis Major was the first documented surface feature of anotherplanet. It was discovered byChristiaan Huygens, who included it in a drawing of Mars in 1659. He used repeated observations of the feature to estimate the length of day on Mars.^[6] The feature was originally known as theHourglass Sea but has been given different names by differentcartographers. In 1850s,Angelo Secchi called the featureAtlantic Canale. Later he called itScorpion andCook Sea orCook Canal.^[7]

InRichard Proctor's 1867 map it is called theKaiser Sea, afterFrederik Kaiser of theLeiden Observatory. In 1876,Camille Flammarion called it theMer du Sablier (French for "Hourglass Sea") when he revised Proctor's nomenclature. The name "Syrtis Major" was chosen byGiovanni Schiaparelli when he created a map based on observations made during Mars' close approach to Earth in 1877.^[8][9]

Syrtis Major was the object of much observation due to its seasonal and long-term variations. This led to theories that it was a shallow sea and later that its variability was due to seasonal vegetation. In the 1960s and 1970s, theMariner andViking planetary probes led scientists to conclude that the variations were caused by wind blowing dust and sand across the area. It has many windblown deposits that include light-colored halos orplumose streaks that form downwind ofcraters. These streaks are accumulations of dust resulting from disruption of the wind by theelevated rims of the craters ('wind shadows').^[1]

Nili Patera is a 50 km diameter caldera at the center of the Syrtis Major Volcanic Complex.^[10] It and Meroe Patera located to the south are the primary named calderas within a nested caldera complex developed by multiple eruption and collapse events.^[2] In the Northeast quadrant of Nili Patera is a 630 m tall volcanic cone named Nili Tholus,^[10] on and around this cone is a light-tone lava flow of chemically evolved lava^[11] with multiple occurrences of relict silica sinter deposits created by a formerly active hot spring system.^[12]

Nili Patera was the subject of a 2010 study into moving sand dunes and wind ripples. The study showed that dunes are active and that sand ripples are actively migrating on the surface of Mars.^[13] A following study also showed that the sand dunes move at about the same flux (volume per time) as dunes in Antarctica. This was unexpected because of the thin air and the winds which are weaker than Earth winds. It may be due to "saltation" - ballistic movement of sand grains which travel further in the weaker Mars gravity.

The lee fronts of the dunes in this region move on average 0.5 meters per year. The selection may be biased here as they only measured dunes with clear lee edges to measure. The ripples move on average 0.1 meters per year.^[14]

•  
  A MOLA map showing boundaries of Syrtis Major Planum and other regions. Colors indicate elevations.
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  Bright Streaks in Syrtis Major caused by the wind, as seen byTHEMIS.

• List of mountains on Mars by height

• Syrtis Major map atGoogle Mars