DEIMOS AND PHOBOS

Mars has two small satellites, with an irregular and elongated shape: Deimos (which in Greek means "terror") and Phobos ("fear"). 
They were named after the two servants of Mars ("He orders Terror and Fear to prepare their horses. He wears the shiny armour himself" - Iliad, XV).
 

The existence of these two satellites was hypothesized long before they were actually discovered by the American astronomer Hall, in 1877. 
 

Phobos is an elongated body, with irregular edges, and it measures only 13.5 x 10.8 x 9.4 Km. Its mass reaches 10,800 billion tons (1,08 10¹⁹ g) and the density is 2,0.
Phobos orbits around Mars at a distance of 9,380 Km, with a period of 0.319 days, that is 7 hours 39 minutes. This is also the period of the rotation around its axis: as in the case of the Moon and all the other satellites, in fact, rotation and revolution are synchronized so that the satellite shows always the same side to the planet.  
This is due to a gravitational effect. The surface of Phobos is dark (it has an albedo of barely 0.06) and is full of craters.
 

Image of Phobos taken in 1977 by the Viking probe. You can notice some streaks, maybe caused by the same impact that produced the top left crater.  (NASA)

The Stickney crater, the largest of the craters of Phobos. (NASA/JPL)

 
 

Deimos  has the shape of an egg, with the dimensions of 7.5 x 6.1 x 5.5 Km. Its mass is 1,800 billion tons  (1,8 10¹⁸ g) and its density is 1.7. Its rotation period, equal to the orbital period, is 1.262 days, that is 30 hours and 18 minutes. The average distance from Mars is 23,460 Km.
The surface of Deimos has craters, as that of Phobos. 
The two satellites are similar, as far as the chemical composition is concerned, to the type C   asteroids, in fact some scientists think that they were asteroids that were captured by the martian gravitational field.
 

Image of Deimos taken in 1977 by the Viking Orbiter.  (NSSDC/NASA)

Another image taken from the Viking. (Calvin J. Hamilton)