Our solar system hosts a variety of celestial bodies. Many planets host natural satellites, commonly known as moons, which display a wide range of sizes, shapes, and orbital behaviors. These planetary systems offer insights into our cosmic neighborhood.
Identifying the Unique Celestial Body
Among the planets, Mars stands out for possessing precisely two natural satellites. Known as the “Red Planet” due to the iron minerals on its surface, Mars is the fourth planet from the Sun, orbiting between Earth and Jupiter. It is a terrestrial planet with a rocky surface, roughly half the diameter of Earth. Mars experiences seasons due to its axial tilt.
The Martian Companions
Mars is orbited by two distinct moons, Phobos and Deimos, both discovered in 1877 by American astronomer Asaph Hall. These moons are among the smallest in the solar system, possessing irregular, non-spherical shapes resembling asteroids. They are also among the darkest objects in the solar system, with very low reflectivity.
Phobos, the larger and innermost moon, measures approximately 22 kilometers in diameter. It orbits Mars at about 6,000 kilometers from the surface, completing an orbit in just 7 hours and 39 minutes. Its heavily cratered surface features the prominent Stickney crater, about 9 kilometers wide. Phobos is gradually spiraling inward, nearing Mars by 1.8 meters per century, which could lead to its breakup into a ring or a collision with Mars within 50 million years.
Deimos, the smaller and outermost moon, has an average diameter of about 12.4 kilometers. It orbits Mars at a greater distance of about 23,460 kilometers from the surface, taking about 30 hours and 18 minutes to complete one orbit. Deimos’s surface is smoother than Phobos’s, as many of its craters have been partially filled with regolith. Unlike Phobos, Deimos’s orbit is slowly expanding, gradually receding from Mars. Both moons are tidally locked, always presenting the same face to Mars.
Theories of Their Formation
The origin of Phobos and Deimos is a subject of scientific investigation, with two main hypotheses. One theory suggests both moons are captured asteroids. Their irregular shapes and carbon-rich composition are similar to certain asteroids, supporting this idea. However, their nearly circular orbits, which lie close to Mars’s equatorial plane, challenge the simple capture theory, as captured asteroids typically have more eccentric and inclined orbits.
Another hypothesis proposes the moons formed from debris ejected after a large impact on Mars. This suggests a collision with Mars created a ring of material, from which Phobos and Deimos later coalesced. This impact-origin theory can better explain the moons’ regular, near-equatorial orbits. Recent studies, using data from missions like InSight, have also explored the possibility that both moons originated from the disruption of a common parent body between 1 and 2.7 billion years ago. The ongoing debate, with future missions aiming to collect samples from Phobos, seeks to resolve this mystery.