![]() Running back the clock in simulations, researchers realized that “At some point in the past, Phobos’ and Deimos’ orbits intersected,” says Efroimsky. “Is it really that certain that a body above the synchronous radius is doomed to fly away, and one below is doomed to fall back toward the planet?” says Efroimsky. Lead researcher in the study Amirhossein Bagheri (Institute of Geophysics, ETH Zurich) carried out hundreds of numerical runs of the evolution of the orbits of Phobos and Deimos back into the far past. They developed a mathematical model of the propagation of the moons’ orbits over time. The research team posed the question of what the Martian moon system might have looked like in the past, based on the snapshot that we see today. ![]() Deimos probably won’t escape from Mars in the lifespan of our solar system. Deimos outward motion is a tiny 2 millimeters (0.08 inches) per year, slower than our Moon’s own 3.8 centimeters (1.5 inches) per year. Deimos is above this limit, and the tidal force leads the tiny moon, pushing it gently outward from Mars. Phobos is well below this limit, with the tidal force of Mars lagging behind it and dragging it downward. Credit: Dave Dickinsonįor Mars, the synchronous radius is at an altitude of 17,032 kilometers (10,583 miles) above the planet’s surface. This is the case with Phobos” The orbits of Phobos and Deimos, versus the Roche Limit for Mars and synchronous orbit. “If the moon is below the synchronous radius, the tides of the planet will drag the moon down. ![]() “There exists a critical altitude which is called the synchronous radius,” Michael Efroimsky (U.S. Tidal forces act on the moons over time, but the total energy in the system always remains constant overall, in accordance with the Law of Conservation of Angular Momentum. Subtle aspects of the overall masses of the moons predict how their orbits will evolve. Yet, because of dynamical problems associated with captured objects, this scenario fell into disfavor, as a result of which, in situ formation models were put forward.” “This is why the Martian moons were originally considered to be captured objects from the nearby asteroid belt. “Most moons are big and round objects, not small and irregularly shaped as Phobos and Deimos,” Amir Khan (Zürich Institute of Geophysics) told Universe Today. Option 2: The moons formed along with Mars early on in the solar system’s history, possibly from a single ‘proto-moon’ or a giant impact on Mars, similar to the impactor proposed in the ‘Theia Hypothesis’ that formed Earth’s large moon. This is not likely, however, from an orbital mechanics perspective, as such events are high energy, and would not produce the neat and tidy orbits with a low ellipticity and a low orbital inclination that we see today. Over the years, Two competing scenarios have emerged to explain the existence of the pair of curious Martian moons: 12 kilometer (7.5 mile)-wide Deimos is just above Mars synchronous orbit, and orbits Mars once every 30 hours and 20 minutes. This also means that unlike Deimos (and our own Moon), Phobos orbits Mars faster than the planet rotates, meaning that viewed from the surface, it actually rises in the west and sets in the east. ![]() The very first looks at Phobos by Mariner 7 in 1969 and Deimos by Mariner 9 in 1971 revealed more a prosaic-looking pair of natural space rocks.Ģ2 kilometer (13.7 mile)-wide Phobos is certainly unique as moons go: it orbits Mars once every 7 hours and 39 minutes, just 6,000 kilometers (3,700 miles) above the Red Planet’s surface, far closer than any other moon in the solar system versus its primary host. Russian astronomer Iosif Shklovsky even made the bizarre claim in 1958 that Phobos was an artificial space station (!), echoing an earlier deliberate April Fool’s hoax perpetrated by astronomy popularizer Walter Scott Houston claiming the same. Mathematically inclined theorists claimed Mars should have two moons even before Hall’s discovery, fitting the exponential cadence of Venus (with zero), Earth (with one moon) and Jupiter (with four)… this assertion famously turns up in Jonathan Swift’s 1726 satire Gulliver’s Travels, though of course, the discovery of Jupiter’s 5 th moon Amalthea in 1892 broke this nifty pattern, tossing this coincidence into the footnote section of astronomical history.īut Phobos and Deimos continued to capture controversy, right on through the 20 th century. The very presence of the two moons has even sparked some outlandish theories over the years.
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