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Martian moon: Deimos

Martian moon: Deimos

The Martian moon Deimos was discovered, shortly before its larger companion, Phobos, by Asaph Hall at the Naval Observatory in Washington DC in August 1877. Deimos means 'terror' or 'panic' and is named after one of the sons of Ares, the Greek god of war.

Phobos and Deimos raw (left panel) and processed images (right panel). Credit: ESA/DLR/FU Berlin (G. Neukum)

Deimos has an equatorial, almost circular, orbit. It orbits once every 30h 18 minutes at 20 068 km from the surface of the planet, with the same hemisphere always facing Mars. Both Phobos and Deimos are too light for gravity to make them spherical. Deimos is irregularly shaped with a major diameter of about 12 km. The craters on Deimos' surface are more subdued than those of Phobos, suggesting that it is covered in a layer of regolith even thicker than that of its neighbour. It has two prominent craters, Swift and Voltaire.

Despite remote sensing investigation by Mars Express, Mars Reconnaissance Observer and Mars Global Surveyor, the composition of Deimos has not been resolved. The composition is a key indicator in the search for the origins of Deimos and Phobos. Data from the Mars Express OMEGA spectrometer suggest that Deimos has a primitive composition. Like Phobos, Deimos is thought to be quite porous.

SRC images of Deimos obtained from different orbits. Credit: ESA/ DLR/FU Berlin (G. Neukum)

Mars Express has made close approaches of Deimos, taking images with its High Resolution Stereo Camera from distances of 14 000 km and shorter. The closest approach was in March 2011 when the spacecraft was at a range of about 9600 km. Surface images of Deimos show its odd shape and a general lack of features. However, images obtained with the super resolution channel of the HRSC also provided scientists with a body of data that has resulted in the best orbital model of Deimos to date. These should aid any future missions to the Martian moon. However, the thick layer of regolith is considered an obstacle to future missions, since it covers the most interesting material on Deimos' surface.

Understanding the formation of the Martian moons would contribute to the overall understanding of the formation of the Solar System. Scientists from ESA are studying a possible sample return mission to Phobos.

Key parameters for Deimos
Parameter Deimos
Average distance from centre of Mars 23 458 km
Average distance from surface of Mars 20 068 km
Dimensions 7.5 × 6.1 × 5.2 km
Mass 1.5 × 1015 kg
Rotational Period  Synchronous
Orbital Period 1.26 days (30 h 18 min)
Orbital Eccentricity 0.00024
Orbital Inclination 1.8°
Parameters in this table are taken from Rosenbatt 2011 and references therein


Last Update: 1 September 2019
25-Apr-2024 07:49 UT

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