Phobos and Deimos

Abstract

Once the Galilean satellites of Jupiter were discovered and the Copernican model of the solar system became widely accepted, Mars’ apparent lack of a moon was notable. By the end of the seventeenth century, Saturn was known to have at least five satellites, a number that swelled to seven by the end of the eighteenth century. By 1800 a new planet, Uranus, was discovered and found to have at least two satellites. By 1870, two more uranian satellites and another saturnian satellite were detected, along with another new planet (Neptune) with its own satellite. Yet Mars remained moonless. Based on numerology, Kepler predicted that Mars should have two satellites, as two made the most sense when interpolating between Earth’s single moon and Jupiter’s four. Jonathan Swift and Voltaire both “predicted” that Mars would have two moons, but their predictions were based on satire (Hall, 1878). Both William Herschel and Heinrich Louis d’Arrest performed unsuccessful searches for martian moons before Asaph Hall, using the U.S. Naval Observatory 26-inch (66cm) refractor, found first Deimos then Phobos during the excellent Mars apparition of 1877. Because of the primitive state of astrophotography, only visual observations using eyepieces were possible at the time. The satellites were named for characters in the Iliad, Phobos (Fear) and Deimos (Terror), who are the attendants of the god Ares, the Greek equivalent of the Roman god Mars. Despite searches for additional satellites, using both groundand spacecraft-based observations, no additional satellites of Mars have been found within its Hill sphere to a diameter of 180 m assuming an albedo of 0.07 (Sheppard et al., 2004). Given the close proximity of Phobos and Deimos to Mars, positional measurements were typically the only ones undertaken. Pascu et al. (2013) compiled and reviewed these measurements as well as other early observations. A focus of early observations was the secular acceleration of Phobos in its orbit (e.g., Sharpless, 1945), now known to originate from tidal effects that cause the moon to spiral in toward Mars, which eventually will result in Phobos’ impact (Burns, 1978). The first photometric study to estimate the moons’ diameters from their brightnesses, by Edward Charles Pickering of the Harvard College Observatory and reported by Hall (1878), suggested diameters of 9 km for Deimos and 11 km for Phobos. These assumed a Mars-like albedo and hence resulted in diameters smaller than the actual values. Just prior to the first spacecraft encounters with the moons, a combination of photoelectric photometry (Harris, 1961) and albedo estimates from polarimetry (Zellner, 1972) resulted in diameter estimates within 10% of current values (Zellner and Capen, 1974).