Phobos

The larger of the two irregularly shaped natural moons of Mars, discovered in 1877 by the American astronomer Asaph Hall. Named after the god of fear (a son of Mars who accompanied his father in battle) in Greco-Roman mythology. The other moon is called Deimos (panic). Phobos is also known as Mars I. With a major axis of 26 km and a minor axis of 18 km, Phobos orbits above the Martian equator at a mean distance of 9,377 km from the center of the planet. Phobos has an orbital period of 0.3189 days (about 7.65 hours) and an orbital eccentricity of 0.015. It is in a synchronous orbit, always keeping the same face toward the Red Planet. This moon has a mass of about 1.08 × 10¹⁶ kg and an estimated mean density 1,900 kg/m3 (or 1.90 g/cm3). The acceleration of gravity on its surface is quite low, at only 0.01 m/s².

Phobos is a dark celestial body, with an albedo of 0.06. Due to the way it reflects the spectrum of sunlight, this moon and its companion moon, Deimos, appear to possess C-type surface materials, similar to that of asteroids found in the outer portions of the asteroid belt. Spacecraft images show Phobos to have a heavily cratered surface. It shows cracks or lineations, which are thought to have originated from shocks produced by past impacts on this body by smaller asteroids, although a tidal cause is also feasible, given Phobos’ proximity to Mars. The surface of Phobos is cratered and covered with regolith - perhaps as deep as 100 meters - formed over millennia as meteorites pulverized the moon’s surface. The largest crater on Phobos is called Stickney and has a diameter of about 10 kilometers. The second-largest crater is called Hall and has a diameter of approximately 6 kilometers.

The most widely held hypothesis concerning the origin of Phobos and Deimos is that the tiny moons are captured asteroids, but this popular hypothesis raises some questions. For example, some scientists think that it is highly improbable for Mars to gravitationally capture two different asteroids. They suggest that perhaps a larger asteroid was initially captured and then suffered a fragmenting collision, leaving Deimos and Phobos as the post collision products. Other scientists bypass the captured asteroid hypothesis altogether and speculate that the tiny moons somehow evolved along with Mars. Expanded exploration of Mars this century should resolve this question. Recent measurements made by Mars orbiting robot spacecraft indicate that the orbit of Phobos is contracting because of tidal forces. Some estimates now suggest that Phobos may decay from orbit within about 100 million years.

Questions to Ponder

• What are the main distinctions between Phobos & Deimos?
• What can you say about Phobos’ orbit?