The origin of Mars' tiny moon, Phobos (fear in ancient Greek), is a mystery, but three theories are considered plausible. The first is that the moon is a captured asteroid; the second is that it formed in-situ as Mars formed below it, and the third is that Phobos formed later than Mars, from debris flung into martian orbit when a massive meteorite struck the Red Planet. A fourth, far more speculative and controversial (although thoroughly intriguing) theory is one that has been kicking around for decades: that Phobos is a artificial object in Mars orbit -in short, a 1.5-mile-long, extremely ancient and battered spacecraft.
The European Space Agency Mars Express past and future Phobos flybys are designed to provide clues that might solve the mystery of its origins.
A small, odd shaped object, Phobos orbits about 9 km from the center of Mars, closer than any other known planetary moon. Phobos is one of the least-reflective bodies in the solar system, that appears to be 1/3rd hollow and features a large impact crater, Stickney crater. It orbits so close to the planet that it moves around Mars faster than Mars itself rotates. Phobos's orbital radius is decreasing and it will eventually either impact the surface of Mars or break up into a planetary ring.
The next Phobios Mars Express flyby will come in August 2010, with nine flybys within 1200 km from Phobos. The closest approach will be about 403 km over Phobos' night side, on 24 August 2010. Then, between December 2010 and January 2011, 10 flybys are planned, with the closest one coming within 96 km of the moon's dayside, on 9 January 2011.
The powerful cameras, spectrometers and other instruments on Mars Express can glean amazing details; repeated flybys will augment coverage of the surface of Phobos, help confirm or improve previous findings, complement existing data sets, and possibly even make new discoveries.
In describing the internal geometric structure of this "moon" as revealed by MARSIS, ESA sources repeatedly emphasized that "several of these interior Phobos compartments also appear to still be holding some kind of atmosphere ...."
The source repeated this several times ... raising all kinds of fascinating questions regarding "how" the radar could, in fact, determine this -- that some of the vast "rooms" inside Phobos (remember "from a quarter to half-a-mile in diameter ...") were "maintaining an internal pressure."
Image from the recent flyby of Phobos, on 7 March 2010 is shown above. The images show Mars’ rocky moon in exquisite detail, with a resolution of just 4.4 metres per pixel. They show the proposed landing sites for the forthcoming Phobos-Grunt mission.
ESA's Mars Express spacecraft orbits the Red Planet in a highly elliptical, polar orbit that brings it close to Phobos every five months. It is the only spacecraft currently in orbit around Mars whose orbit reaches far enough from the planet to provide a close-up view of Phobos.
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. Mars Express did just this on 7, 10 and 13 March 2010. Mars Express also collected data with other instruments.
Phobos is an irregular body measuring some 27 × 22 × 19 km. Its origin is debated. It 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 subsequent evolution of the orbit into the equatorial plane of Mars. An alternative hypothesis is that it formed around Mars, and is therefore a remnant from the planetary formation period.
In 2011 Russia will send a mission called Phobos–Grunt (meaning Phobos Soil) to land on the martian moon that will enable to make in-situ observations with an important payload which should operates on the surface for one year. Moreover, it will collect samples which will be sent back on Earth for precise analyses.
The mission main objectives are to solve the following :
-the origin of Phobos in relation with the study of the primitive matter,
-its evolution in relation with Mars,
-the role of the asteroids impacts in the formation of the planets and in the evolution of their atmosphere, their crust and in the inventory of the volatile elements.
Some of the more fascinating, and wild speculative theories swirling around the Phobos Gunt Mission at sites like EnterpriseMission.com see it as "the single most important scientific mission in the history of the past half-century of modern solar system exploration .... with Phobos as an ancient solar system relic from an extraordinarly distant, pre-human, apparently solar-system-wide ancient high-tech civilization. One that could ultimately turn out to be directly related to our own.
It is expected that Earth-based ESA stations will take part in controlling Phobos-Grunt, receiving telemetry and making trajectory measurements, including implementation of very long-baseline interferometry (VLBI). This cooperation is realized on the basis of the agreement on collaboration of the Russian Federal Space Agency and ESA in the framework of the Phobos-Grunt and ExoMars projects (see post above).
In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow. Around 1958, Russian astrophysicist Iosif Samuilovich Shklovsky, studying the secular acceleration of Phobos' orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin. Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light.
The density of Phobos has now been directly measured by spacecraft to be 1.887 g/cm³, which is inconsistent with a hollow shell. In addition, images obtained by the Viking probes in the 1970s clearly showed a natural object, not an artificial one, and the "hollow Phobos" speculations have been relegated to the status of a historical curiosity
However, mapping by the Mars Express probe and subsequent volume calculations do suggest the possible presence of vast caverns within the moon and indicate that it is not a solid chunk of rock but a porous body instead. The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third of the moon being hollow, likely in the form of large voids.
In a July 22, 2009 interview with C-Span U.S. astronaut Buzz Aldrin said: "We should go boldly where man has not gone before. Fly by the comets, visit asteroids, visit Phobos, a moon of Mars. There’s a monolith there. A very unusual structure on this potato shaped object that goes around Mars once in seven hours. When people find out about that they’re going to say ‘Who put that there? Who put that there?’ The universe put it there."
Casey Kazan via ESA