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The European Space Agency has released images of a grazing occultation of Jupiter by the martian satellite Phobos, that took place on the first of this month.
The only witness of the unusual conjunction was the Mars Express probe, that while keeping its High Resolution Stereo Camera pointed at the gas giant, took a sequence of 104 images (including the 3 above) over a period of 68 seconds. At the time the probe was 11,389 km from Phobos and 529 million km from Jupiter.
The raw images were reduced by the Department of Planetary Sciences and Remote Sensing at the Institute of Geological Sciences, part of the "Freie Universität" in Berlin.
The photographic sequence, apart from its aesthetic appeal (detail can be seen on the surface of Phobos, and the main atmospheric bands on Jupiter are clear), will be useful to better constrain the orbit of Phobos, as theoretical timings can be compared to those actually measured.
Even very small modification to the period could help in resolving doubts that still remain over the origin of the little Phobos (with a mean diameter of about 22 km), and also for the even smaller Deimos (mean diameter less than 13 km).
The asteroid-like looks of both Phobos and Deimos have made it long believed that these satellites originated in the main asteroid belt, and that they were displaced by the gravitational influence of Jupiter, to later be captured by Mars. However, the very long time scales required for the bodies to reach their current, very circular, equatorial orbits, seems inconsistent with the capture hypothesis. The preferred scenario now is that the two satellites were actually formed in martian orbit, from material ejected from the planet after a massive planetesimal impact.
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