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.

Credits: ESA/ DLR/ FU Berlin (G. Neukum)

During Mars Express’ fly-bys of Phobos, the MaRS radio science experiment on Mars Express detected the gravitational influence of the moon on the trajectory of the spacecraft, so allowing the determination of the moon’s mass with unprecedented accuracy (1.072 1016 kg, or about one billionth the mass of the Earth). The 3D model of the body built thanks to HRSC data, combined with the mass measurement, is key to calculate the mean density of Phobos. In turn, this can tell scientists a lot about the moon’s composition (how much rock or ice may be present, and how porous the structure may be), helping to close in on Phobos’ origin.

Credits: ESA/ DLR (K.Willner)/ FU Berlin (G. Neukum)

The Super Resolution Channel (SRC), is an additional camera sharing HRSC’s processing electronics. Unlike HRSC, SRC is a framing camera, taking a complete image during a single exposure like a conventional pocket camera. Its 975 mm Maksutov-Cassegrain telescope gives it a pixel-resolution about four times higher than that of HRSC.

Early in the mission, the thermal conditions of the instrument in space caused a distortion in its precision optics. This resulted in a performance lower than expected, with some blur and ghosting visible in the raw images. However, a significant part of the degradation has now been compensated for by analysing the effect of the distortion on test images (with point-like stars as target) and applying a corrective processing.

Credits: ESA/ DLR/ FU Berlin (G. Neukum)

The colour of the moon is very uniform, however it is possible to notice some subtle chromatic variations.

Credits: ESA/ DLR/ FU Berlin (G. Neukum)

The ellipses show the previously (red) and currently (blue) considered landing sites for the Phobos-Grunt Russian mission: based on Mars Express’ image series, the Russian Federal Space Agency is now considering to move the landing location slightly to the north of the previous one, to a safer area located between 7°-21°S and 214°-233°W.

Phobos-Grunt (meaning Phobos Soil) will land on Phobos, collect a soil sample and return it to Earth for analysis. The landing site has been selected to be on the side of the moon facing away from Mars - a place now only imaged by Mars Express after the Viking orbiters in the 1970s.

Credits: ESA/ DLR/ FU Berlin (G. Neukum)

The left SRC image is raw (shown as it has been collected), while the centre SRC one has been corrected for mirror distortion. The right image is a portion of the HRSC image taken using the nadir channel and showing the same portion of the surface imaged by SRC. The latter shows significantly more detail than the HRSC nadir image.

Credits: ESA/ DLR/ FU Berlin (G. Neukum)