Between 23 July and 15 September 2008 Mars Express performed a series of eight fly-bys of the martian moon Phobos, at distances ranging between 4500 and 93 km from the centre of the moon, conducting some of the most detailed investigations of the Moon to date. In observing Phobos, Mars Express benefits from its highly elliptical orbit which takes it from a closest Mars approach of 270 km above the surface up to a maximum of 10 000 km distant from the planet's centre, crossing the 9 400 km orbit of the moon. Like our Moon, Phobos always shows the same side to the planet, so it is only by flying outside the orbit that it becomes possible to observe the far side. The other spacecraft presently orbiting Mars do so at much lower altitudes, and therefore only see the planet-facing side of the moon.
This image was obtained by the High Resolution Stereo Camera (HRSC) on board ESA’s Mars Express on 28 July 2008 (orbit 5870), at a distance of 351 km from the moon’s centre. The image was taken using the camera’s nadir channel, at a resolution of 14 m/pixel.
The origin of Phobos is debated. While its density, lower than the density of the Martian surface rocks, make it appear to belong to D-class asteroids, the moon appears to share many surface characteristics with the class of carbonaceous C-type asteroids, which suggests it might have been captured from this population. However, it is difficult to explain either the capture mechanism or the following evolution of its orbit into the equatorial plane of Mars. An alternative hypothesis is that it formed in its present position, and is therefore a remnant from the planetary formation period.
