Previous Mars Express flybys have already provided the most accurate mass yet for Phobos, and the High Resolution Stereo Camera (HRSC) has provided the volume. When calculating the density, this gives a surprising figure because it seems that parts of Phobos may be hollow. The science team aim to verify this preliminary conclusion.
In particular, the MARSIS radar will operate in a special sequence to try to see inside the moon, looking for structures or some clue to the internal composition. "If we know more about how Phobos is built, we might know more about how it formed," says Witasse.
The origin of Phobos is a mystery. Three scenarios are possible. The first is that the moon is a captured asteroid. The second is that it formed in situ as Mars formed below it. The third is that Phobos formed later than Mars, out of debris flung into Martian orbit when a large meteorite struck the red planet.
All the instruments will be used during the campaign, including HRSC. Although no imaging will be possible during the first five flybys, including the closest one, because Mars Express approaches from the nightside, high-resolution pictures will be possible from 7 March onwards. One task for HRSC is to image the proposed landing sites for the Russian mission Phobos-Grunt.
"It is always busy," says Witasse about running the science mission. "The Phobos flybys make it even more exciting."
This animation shows the 3-D (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 percent 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.
(Photo Credit: ESA/ DLR (K.Willner)/ FU Berlin (G. Neukum))
This is how Phobos looked on July 28, 2008, to the HRSC camera on Mars Express. Then, the spacecraft was 351 km from Phobos. HRSC will take new images during this new sequence of flybys.
(Photo Credit: ESA/ DLR/ FU Berlin (G. Neukum))
Source: European Space Agency