Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Mars, Phobos and Deimos
Note added Nov 11: Tidal Effects on Orbits
     Tidal interactions between planets and moons can affect the rotation of each object (usually causing one side of the moon to always face the planet), and the orbital motion of the moon. In the case of the Earth and Moon, where the orbital period of the Moon is much longer than the rotation period of the Earth, the same effect that causes one side of the Moon to always face the Earth is causing the Earth's rotation to slow by 1 or 2 thousandths of a second per day, each century. In other words, a century from now the Earth will take 1 or 2 thousandths of a second longer to rotate once than it now does, and a century ago the Earth took 1 or 2 thousandths of a second less to rotate once than now.
     The rotational momentum that the Earth is losing as a result of its slowing is transferred to the orbital motion of the Moon (in a way to be discussed in a future webpage), causing it to gradually move further away from the Earth (currently, by about an inch or so per century). The same thing is happening with Mars and its moon Deimos, which is very slowly moving further and further from Mars as a result of their interaction.
     However, in the case of Phobos, which is exceptionally close to Mars, its orbital period is much shorter (only about 9 hours) than the planet's rotational period (about 24 hours 37 minutes), so the effects are to a certain extent reversed. Like our Moon and Deimos, Phobos always keeps one face to Mars, with its elongated axis parallel to its orbit, so that the shadow it casts on the Martian surface is elongated. However, since its orbital period is shorter than Mars' rotational period, the tidal interaction between Mars and Phobos is making Mars rotate faster instead of slower, and is making Phobos' orbit smaller instead of larger (the best current estimate is that it is moving about 6 feet closer to Mars each century, much faster than our Moon's change in orbital size). As a result, tidal stresses on Phobos (the difference between the gravitational pull of Mars on the nearer side of Phobos and the gravitational pull on Mars on the farther side of the moon) are gradually increasing. If Phobos were a stiff, solid body it might be able to move considerably closer to Mars before the tidal stress could do much more than slightly deform it; but its low density (considerably less than that of typical rocks) indicates that it is more like a loosely aggregated pile of rubble than a solid rock. As a result, it is now believed that some of the linear features on the surface of Phobos may be stress fractures caused by tidal stress, and if so, that Phobos could gradually spiral in toward Mars and be torn to bits in as little as 30 or 40 million years, forming a ring of very dark, dust and rubble-sized objects that would eventually disperse over a period of another few tens of millions of years.

     1977 Viking orbiter image of Phobos. The large crater on the left is Stickney. If the impact that created Stickney had been much stronger, Phobos would probably have been broken into many much smaller pieces. (Viking Project, JPL, NASA, apod980531)

     A 1978 Viking orbiter image, showing a slightly different view of the same part of Phobos (Viking Project, JPL, NASA, Edwin Bell II, NSSDC)

     Phobos imaged by the Mars Global Surveyor. The darkest moon in the solar system, Phobos may be a captured asteroid made of a loose mixture of ice and dark rock.
(Malin Space Science Systems, MGS, JPL, NASA, apod030701)

     A color image of Phobos taken by the Mars Express shows the inner moon of Mars in exceptional detail. (G. Neukum (FU Berlin) et al., Mars Express, DLR, ESA, apod061203)

Above, an image of Phobos near the limb of Mars, taken by the Mars Express. The surface features are distorted by foreshortening, and by the motion of the moon and the spacecraft's camera as it followed the moon. The image clearly shows how much darker Phobos is than Mars. Phobos is the darkest moon in the solar system, and of great interest because its structure and composition may well be unique, and the hope that it will soon be a target for direct exploration by a lander. (Image Credit ESA / DLR / FU Berlin (G. Neukum), apod101201)

Above, an image of Phobos taken by the Mars Express. (Image Credit G. Neukum (FU Berlin) et al., Mars Express, DLR, ESA, apod100317)

Above, a portion of a Mars Express image of the region near Gusev Crater (the Spirit rover landing site) shows the river-like structure which led to hopes that the crater might contain evidence of ancient water flow, an ancient shield volcano to the north of the crater, and at the bottom an elongated shadow cast on the surface of the planet by Phobos. Phobos can never totally eclipse the Sun, being far smaller than our Moon, and even as close to Mars as it is, never covering more than about half the solar disk. Despite that, as shown in this image (and the one below), the penumbral shadow on the moon is clearly visible on the Martian surface. In the image above the shadow is elongated because of the shape of the moon, the angle at which its shadow struck the surface, and the fact that the image was built up line by line, and during the time it took to form the image the moon (and its shadow) moved relative to the surface. In the image below, which was taken in more nearly real-time, the shape of the shadow is not quite as distorted. (Image Credit ESA / DLR / FU Berlin (G. Neukum) / Stuart Atkinson,

Above, a 1999 Mars Global Surveyor photo of the shadow of Phobos on western Xanthe Terra. Refer to the caption for the previous image for notes about the shape of the shadow. (Malin Space Science, MGS, JPL, NASA, apod030329)

Above, an animated view of Phobos transiting (passing in front of) the Sun as seen from the surface of Mars by the Opportunity Rover on its 45th Martian day ("sol") after landing on Mars in 2004. Despite its proximity to the planet Phobos is too small to completely cover the Sun, but as shown in the previous images the partial eclipse casts a noticeable shadow on the surface of the planet. A comparison of the images also shows that Phobos has its long axis aligned along its orbit, as its elongated shape is obvious both in the shape of its shadow and in its appearance as it passes in front of the Sun. Note: The original animation shows the moon passing from left to right, presumably due to the orientation of the rover camera at the time the images were taken; however, on this page the direction is reversed to show the view that a northern hemisphere observer would have as Phobos moves eastward across the face of the Sun (in other words, the image is oriented so that north is on top and east on the left). (Image Credit NASA/JPL/Cornell, Planetary Photojournal)

An old image of the "other" side of Phobos, meaning a portion of the moon not covered by the images shown above. There should be much newer images of most if not all of the moon now available, so this image will probably be replaced in the next iteration of this page.


A mosaic of Deimos as photographed by Viking orbiter in 1970's
(Viking Project, JPL, NASA, solarviews)

A 2008 enhanced color image of Deimos taken by the HIRISE spacecraft. Note the double crater at top left and the indentation on the limb to its right. Those same features are shown at top right on the previous image. A comparison of their position on the two images demonstrates that the HIRISE image shows a view of Deimos almost completely different than the portion of the moon shown on the Viking image. The portion shown here is the side of Deimos which always faces Mars (like most moons it rotates synchronously, meaning it always keep the same face to its planet). Since the HIRISE spacecraft orbits Mars only about 200 miles above its surface and Deimos is about 12 thousand miles away, the view from the spacecraft is the same as the view from the surface -- namely, the "near" side of the moon. (Image Credit NASA/JPL/University of Arizona, LPL)

1977 Viking 2 orbiter view of Deimos from only 18 miles away, showing an area less than a mile square
(Viking Project, JPL, NASA, apod961222)

An old image of the "other" side of Deimos (in comparison to the previous image)