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.


 
This animation shows the 3D (shape) model of Mars’ moon Phobos, built thanks to the data collected by Mars Express’ High Resolution Stereo Camera (53 images obtained by the Super Resolution Channel, or SRC) and NASA’s Viking (16 images). The model is dressed with a mosaic of the same images. The SRC coverage is about 70% of the moon’s surface. The mean resolution is 12 m/pixel.

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.


 
This animation shows how the orbit of Mars Express has been influenced by the gravitational influence of Phobos during the spacecraft’s fly-bys of the moon in Summer 2008. Since the orbital deviation strictly depends on the mass and shape of the moon, scientists could use this very deviation to determine the mass of Phobos with unprecedented accuracy (1.072 x 10¹⁶ kg, or about one-billionth the mass of the Earth).


 
This striking close-up image of Phobos was obtained by the Super Resolution Channel (or SRC, a part of the High Resolution Stereo Camera experiment) on board ESA’s Mars Express on 26 July 2008 (orbit 5861). The distance from the moon’s centre was 2295 km, and the image resolution is 20 m/pixel.

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.


 
This mosaic image is composed by 53 pictures obtained by the Super Resolution Channel (or SRC, a part of the High Resolution Stereo Camera experiment) on board ESA’s Mars Express. The SRC images covered 70% of the moon's surface. The remaining area is filled with 16 images previously obtained by NASA’s Viking mission. The mean resolution is 12 m/pixel.


 
This image, obtained by ESA’s Mars Express on 30 August 2008 (orbit 5984), is the first HRSC colour image of Phobos. It was taken at a distance of 2366 km from the moon’s centre, and it is the composite result of the nadir, green and blue channels. The resolution is 95 m/pixel.

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


 
This image was obtained by the High Resolution Stereo Camera (HRSC) on board ESA’s Mars Express on 23 July 2008 (orbit 5851), at a distance of 93 km from the moon’s centre. The image was taken using the camera’s stereo-1 channel, at a resolution of 3.7 m/pixel.

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.

Note for editors
 

 
Geometry of the Phobos fly-by. Phobos and Mars Express are not to scale.


 
This stereo (anaglyph) view of Phobos has been 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 stereo-2 channel, at a resolution of 15 m/pixel.


 
This series of images was obtained by the Super Resolution Channel (SRC) (left and centre of the panel) and by the High Resolution Stereo Camera (HRSC, right of the panel) on board ESA’s Mars Express. The images were gathered on 28 July 2008 (orbit 5870), at a distance of 351 km from the moon’s centre. The two SRC images show a resolution of 3.2 m/pixel, while the HRSC one has a resolution of 14 m/pixel.

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.



Release date: 10 February 2009