Phobos seems to be composed of the same materials as Deimos, but for some reason, the moon has a seriously low density. So low that in the 1960's Russian planetary scientists suggested that the moon was hollow, and might be an artificial satellite...
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The missions were to provide the first close look at Phobos and Deimos, and hopefully answer the question as to why these two moons are unlike anything else in the solar system.
Deimos is the smaller of the two, and orbits mars more distantly than Phobos. We know it has a composition similar to a meteorite sometimes found on earth called a carbonaceous chondrite, a fascinating group of rocks that contain not only the most ancient material known to man, some of which predates the solar system entirely, but also amino acids which are organic molecules that form the basis for life.
Oddly, Deimos is relatively uncratered. In contrast, Phobos is heavily cratered, and orbits Mars in a very bizarre way; it moves so fast that it rises and sets to an observer on Mars twice each day, and due to that speed appears to rise in the west and set in the east, the opposite of earth's own moon. Phobos seems to be composed of the same materials as Deimos, but for some reason, the moon has a seriously low density.
So low that in the 1960's Russian planetary scientists suggested that the moon was hollow, and might be an artificial satellite. Science advisor to President Eisenhower Seigfried Singer supported this theory, as did several NASA scientists at the time. But by the late 1960's, this theory seems to have evaporated, at least openly, and hasn't been seriously discussed since.
Another mystery surrounding Phobos are its chain craters. These have not been seen anywhere else in the solar system, and appear as dark lines with craters in them that crisscross the surface of the moon. Science is still at a loss to explain this phenomenon.
The two Soviet probes, however, are chief among the mysteries of the moons of Mars. Both of the probes mysteriously went silent, Phobos I going silent not long after launch, apparently due to a computer glitch.
Phobos II made it to Mars, and was designed to drop two small probes on the surface of the moon. It never accomplished that, going silent shortly before the maneuver. Again, this was blamed on a computer glitch, however several enigmatic photographs were transmitted several days before the probe failed that have caused wild speculation ever since.
In one of the photos, a large, oblong object is seen close to the moon, and appears to have either a length similar to the moon itself, or is in very close proximity to the spacecraft. The object has been rumored over the years to have been the real cause of the loss of Phobos I, possibly either hitting it, or intentionally destroying the probe to keep it from getting a close look at the moon. Skeptics claim that the object is an electronic artifact, and may not have really been there.
Another bizarre photograph, taken of Mars itself, seems to show an oblong shadow on the surface of the planet. This was apparently caught by two separate cameras on the spacecraft, and seems to suggest an object other than either of the two moons is casting a shadow on Mars from orbit. An enormous object would be needed to do this, so just what was causing this shadow is remains unexplained.
Subsequent missions to Mars have photographed its moons without incident, the most recent being NASA's Mars rovers, one of which has actually photographed Deimos passing in front of the sun from the surface of Mars. Whether or not Phobos II was destroyed by a computer glitch, an unexplained piece of debris, or an alien spacecraft is likely to remain a mystery for the foreseeable future.
Regardless of what happened, the red planet seems to get more mysterious and unusual at every turn, despite the fact that it is one of the most human-explored objects in the solar system.
Kemo D. (a.k.a. no.7) www.beyondgenes.com