The various probes sent to Mars filmed its two satellites, two tiny and irregularly shaped bodies, which are quite different from the normal roundness of our Moon. More than 300 years ago, the astronomer Kepler had already stated that Mars had to have two satellites, even if he had no way of seeing them. The fact is that Kepler had based his declaration on a simple deduction: since the Earth had only one satellite and Jupter had four (that's all that was known at that time), i.e. four times more than the Earth, the planet in the middle, Mars, had to have twice as many, meaning two. He was right, but astronomers discovered them only in 1877, when they were able to identify two luminous points very close to the planet revolving in a nearly circular orbit. They were given the names of the sons of Ares (the god of war, which the Romans had called Mars), who according to Greek mythology drove their father's war chariot: Phobos (fear) and DΦimos (terror). But they were too small to study them in detail even with the most powerful telescopes. The only thing left to do was to rely on the probes. And it was Mariner 9, while waiting for the huge sandstorm to abate on the surface of Mars, that took advantage of the delay and directed its television cameras on the two moons. Phobos was photographed 27 times and DΦimos 9, and so finally the astronomers could obtain information about their size and shape. The larger of the two is Phobos, an elongated body similar to a potato with a length of 27 kilometers, a width of 21 and a height of 19. It rotates around Mars in 7 hours and 40 minutes. DΦimos, which is farther away, and that looks like a smaller Phobos, has measurements of respectively 15, 12 and 11 kilometers, and instead takes 60 days to complete a full revolution around the planet. From the equator of Mars we would be able to see Phobos, three times smaller than the Moon, move in the night sky, covering the distance in four hours, while DΦimos, which would resemble a very bright star, would take 60. The two satellites would seem to move in opposite directions, Phobos from west to east and DΦimos from east to west, because the first takes less time to revolve around the planet than Mars takes to complete one rotation, while DΦimos takes a few hours longer. Both bodies are pock-marked with small and large craters, with all kinds of shapes: from one that is elongated that looks like a keyhole to one that is perfectly circular. The largest one is found on Phobos and has a diameter of no less than 10 kilometers, about 40% of the maximum diameter of the satellite. The surface of Phobos also has a series of deep parallel trenches whose origin is still being discussed by astronomers. DΦimos resembles the larger moon, but perphaps is not as rugged, and the largest crater has a diameter of just two kilometers. Scientists were particularly impressed by the color of the two moons. In fact, while Mars has a reddish hue caused by oxidation (similar to rusting iron) of the surface layer, Phobos and DΦimos are both very dark grey, meaning that they are probably made of rocks similar to basalt. If this is true, and since basalt is formed through the fusion of minerals on a body that was originally very large, it is likely that the small moons are not asteroids captured by Mars, but the remains of a huge satellite which broke apart in a catastrophic collision in the remote past and that hides one of the many enigmas of the red planet.
The various probes sent to Mars filmed its two satellites, two tiny and irregularly shaped bodies, which are quite different from the normal roundness of our Moon. More than 300 years ago, the astronomer Kepler had already stated that Mars had to have two satellites, even if he had no way of seeing them. The fact is that Kepler had based his declaration on a simple deduction: since the Earth had only one satellite and Jupter had four (that's all that was known at that time), i.e. four times more than the Earth, the planet in the middle, Mars, had to have twice as many, meaning two. He was right, but astronomers discovered them only in 1877, when they were able to identify two luminous points very close to the planet revolving in a nearly circular orbit. They were given the names of the sons of Ares (the god of war, which the Romans had called Mars), who according to Greek mythology drove their father's war chariot: Phobos (fear) and DΦimos (terror). But they were too small to study them in detail even with the most powerful telescopes. The only thing left to do was to rely on the probes. And it was Mariner 9, while waiting for the huge sandstorm to abate on the surface of Mars, that took advantage of the delay and directed its television cameras on the two moons. Phobos was photographed 27 times and DΦimos 9, and so finally the astronomers could obtain information about their size and shape. The larger of the two is Phobos, an elongated body similar to a potato with a length of 27 kilometers, a width of 21 and a height of 19. It rotates around Mars in 7 hours and 40 minutes. DΦimos, which is farther away, and that looks like a smaller Phobos, has measurements of respectively 15, 12 and 11 kilometers, and instead takes 60 days to complete a full revolution around the planet. From the equator of Mars we would be able to see Phobos, three times smaller than the Moon, move in the night sky, covering the distance in four hours, while DΦimos, which would resemble a very bright star, would take 60. The two satellites would seem to move in opposite directions, Phobos from west to east and DΦimos from east to west, because the first takes less time to revolve around the planet than Mars takes to complete one rotation, while DΦimos takes a few hours longer. Both bodies are pock-marked with small and large craters, with all kinds of shapes: from one that is elongated that looks like a keyhole to one that is perfectly circular. The largest one is found on Phobos and has a diameter of no less than 10 kilometers, about 40% of the maximum diameter of the satellite. The surface of Phobos also has a series of deep parallel trenches whose origin is still being discussed by astronomers. DΦimos resembles the larger moon, but perphaps is not as rugged, and the largest crater has a diameter of just two kilometers. Scientists were particularly impressed by the color of the two moons. In fact, while Mars has a reddish hue caused by oxidation (similar to rusting iron) of the surface layer, Phobos and DΦimos are both very dark grey, meaning that they are probably made of rocks similar to basalt. If this is true, and since basalt is formed through the fusion of minerals on a body that was originally very large, it is likely that the small moons are not asteroids captured by Mars, but the remains of a huge satellite which broke apart in a catastrophic collision in the remote past and that hides one of the many enigmas of the red planet.
