The moons of Mars are unequivocally visible from its surface, but the experience differs profoundly from viewing Earth’s Moon. An observer on the Martian plains would see two small, irregularly shaped satellites instead of a single massive, round orb. Phobos and Deimos offer a dynamic spectacle heavily influenced by their proximity and rapid movements, contrasting sharply with the slow progression of our own Moon.
The Moons of Mars: Phobos and Deimos
Mars’s two natural satellites are Phobos (Fear) and Deimos (Dread), named after the sons of the Greek war god Ares. Phobos is the larger, averaging about 14 miles (22.2 km) across, while Deimos measures about 7.8 miles (12.6 km) in diameter. Both moons are irregularly shaped, often described as potato-like, due to their low mass and weak gravity. Scientists suggest they are either captured asteroids or, more likely, formed from debris ejected by a massive impact on Mars.
Appearance in the Martian Sky
From the Martian surface, the visual appearance of the two moons is strikingly different. Phobos, the larger and much closer moon, appears as a noticeable disk, though it remains quite small. When directly overhead, Phobos has an apparent size about one-third the diameter of a full moon viewed from Earth. Due to its non-spherical shape and proximity, an observer might notice that Phobos appears visibly non-round, with its largest craters discernible.
Deimos is a much more diminutive sight, appearing like an exceptionally bright star or distant planet, similar to Venus viewed from Earth. It shines brightly in the night sky, reaching a magnitude of about -5, brighter than any star. Because of its small apparent size, Deimos looks perfectly round to the unaided eye, appearing only about one-nineteenth the apparent width of Earth’s Moon. Phobos is bright enough that it can occasionally be seen in the Martian sky during the daytime.
Unique Orbital Mechanics and Speed
The orbital characteristics of the two moons create a dynamic experience for an observer on Mars. Phobos orbits at an altitude of only about 3,700 miles (6,000 km), completing an orbit in just 7.66 hours. This means Phobos orbits Mars faster than the planet rotates, placing it below the synchronous orbit radius. Consequently, Phobos appears to rise in the west and set in the east, zipping across the sky in just over four hours and sometimes making multiple appearances in a single Martian day.
Deimos orbits much farther away, with an orbital period of 30.35 hours, slightly longer than the Martian day. This proximity to the synchronous orbit means Deimos moves very slowly across the sky, taking about 2.7 days to travel from rising to setting. It rises in the east and sets in the west, moving in the conventional direction. The rapid movement of Phobos is so pronounced that it can noticeably change phases, from crescent to gibbous, during a single brief pass.
Martian Eclipses and Transits
The movement of the moons produces visual phenomena known as transits and eclipses. Since both moons are small relative to the Sun’s apparent size in the Martian sky, neither can cause a total solar eclipse. Instead, they cause transits, where the moon passes directly across the Sun, obscuring only a fraction of its light. Phobos is large enough to cause an annular eclipse, covering a significant portion of the solar disk and appearing as a large black circle moving quickly.
The Phobos transit is extremely brief, typically lasting about 30 seconds due to its rapid orbital speed. Deimos, being much smaller, causes a less dramatic transit, appearing as a tiny black speck moving over the Sun. These events occur frequently near the Martian equator. As the moons pass overhead, they cast a fast-moving shadow, or antumbra, onto the planet’s surface. These transits are scientifically valuable, helping researchers track the moons’ orbits and measure their orbital decay.