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Deimos is the smaller and outermost moon of Mars.

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Deimos, is the smaller and outermost of Mars’ two moons. Mars moon Deimos is named after Deimos from Greek mythology. Mars moon Deimos is also known as Mars II.

Discovery of Mars moon Deimos

Deimos Mars moon.
Deimos Mars moon transits the Sun, as seen by Mars Rover Opportunity

Phobos and Deimos were both discovered by American astronomer Asaph Hall. The names were suggested by Henry Madan (1838-1901), Science Master of Eton, from Book XV of the Iliad, where Ares (the Roman god Mars) summons Dread (Deimos) and Fear (Phobos).

Deimos was discovered on August 12, 1877 at about 07:48 UTC (given in contemporary sources as "August 11 14:40" Washington mean time using the old astronomical convention of beginning a day at noon, so 12 hours must be added to get the actual local mean time).

Characteristics of Deimos.

Mars Moon Deimos.
An image of Deimos taken by the Viking 1 orbiter.
Discovery of Deimos.
Discovered by Asaph Hall
Discovered on August 12, 1877
Orbital characteristics of Deimos.
Mean orbital radius 23,460 km
eccentricity 0.0002
Revolution period 1.262 d
Avg. Orbital Speed 0.22 km/s
inclination 0.93º (to Mars' equator)
1.793º (to the local Laplace plane)
27.58º (to the ecliptic)
Satellite of Mars
Physical characteristics of Deimos.
Mean diameter 12.6 km (15.01210.4)
mass 2.2441015 kg (3.8 nEarths)
Mean density 2.2 g/cm
Surface gravity 0.0039 m/s2 (3.9 mm/s2)
Surface Gravity
(Earth = 1)
0.00040 (400 g)
escape velocity 0.0069 km/s (6.9 m/s)
Rotation period synchronous
Albedo 0.07
Surface temp. ˜233 K
Atmospheric pressure no atmosphere

Deimos is probably an asteroid that was perturbed by Jupiter into an orbit that allowed it to be captured by Mars, though this hypothesis is still in some dispute. Like most bodies of its size, Deimos is highly nonspherical with dimensions of 151210 km.

Deimos is composed of rock rich in carbonaceous material, much like C-type asteroids and carbonaceous chondrite meteorites. It is cratered, but the surface is noticeably smoother than that of Phobos, caused by the partial filling of craters with regolith. The two largest craters, Swift and Voltaire, measure about 3 kilometres across (they were given these names, for both writers had supposed the existence of two moons around Mars well before the real moons were discovered).

As seen from Deimos, Mars would be 1000 times larger and 400 times brighter than the full Moon as seen from Earth, taking up a full 1/11 of the width of a celestial hemisphere.

As seen from Mars, Deimos has an angular diameter of no more than 2.5' and would therefore appear starlike to the naked eye. At its brightest ("full moon") it would be about as bright as Venus is from Earth; at the first or third quarter phase it would be about as bright as Vega. When Deimos passes in front of the Sun its angular diameter is only about 2.5 times the angular diameter for Venus during a transit of Venus from Earth. With a small telescope, a Martian observer could see Deimos' phases, which take 1.2648 days to run their course (Deimos' synodic period).

Orbit Deimos.
Orbits of Phobos and Deimos (to scale).

Unlike Phobos, which orbits so fast that it actually rises in the west and sets in the east, Deimos rises in the east and sets in the west. However, the orbital period of Deimos of about 30.4 hours exceeds the Martian solar day ("sol") of about 24.7 hours by such a small amount that it takes 2.7 days between rising and setting for an equatorial observer.

Because Deimos' orbit is relatively close to Mars and has only a very small inclination to Mars' equator, it cannot be seen from Martian latitudes greater than 82.7º.

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