Phobos and Deimos are the two small, irregularly shaped moons of the planet Mars. They are named after the Greek mythological twin sons of Ares, the god of war. Phobos represents panic and fear, while Deimos embodies terror and dread. These satellites are among the smallest natural moons in the Solar System, and their true origin has puzzled scientists for decades.
Unique Characteristics of Phobos
Phobos is the larger and innermost of the two Martian moons, with an average diameter of about 22 kilometers. Its proximity to Mars is extraordinary, orbiting at an average distance of only about 6,000 kilometers above the planet’s surface, closer than any other moon to its parent planet. This close orbit causes Phobos to complete a revolution in just 7 hours and 39 minutes, orbiting Mars three times a day and making it the fastest moon in the solar system.
The moon has a highly irregular, potato-like shape and is heavily scarred by impact craters. The most prominent surface feature is Stickney crater, a massive impact site with a diameter of 9 kilometers, which is nearly half the size of the moon itself. The impact that formed Stickney likely came close to shattering Phobos.
Phobos is currently locked in a decaying orbit, spiraling inward toward Mars at a rate of about 1.8 centimeters per year due to tidal forces. Scientists predict this gravitational drag will cause Phobos to either crash into Mars or be torn apart by the planet’s gravity to form a planetary ring within the next 30 to 50 million years. This impending destruction is evidenced by deep grooves that crisscross its surface, thought to be structural “stretch marks” caused by tidal stresses.
Unique Characteristics of Deimos
Deimos is the smaller and outer moon of Mars, measuring only about 12.6 kilometers across at its widest point. It orbits Mars at a much greater distance than Phobos, maintaining an average separation of approximately 23,460 kilometers. This more distant orbit gives Deimos a much slower orbital period of about 30.3 hours, which is only slightly longer than the Martian day.
The moon also has an irregular shape, but its surface appears noticeably smoother than Phobos. This smoother appearance is likely because a thick layer of regolith, or loose surface material, has partially filled in its craters. Deimos’s lower surface gravity means that ejecta from impacts tends to be retained on the surface rather than escaping into space, contributing to this appearance.
Unlike its innermost sibling, Deimos is tidally receding from Mars, following a stable orbit. It is expected to remain a permanent fixture in the Martian sky for billions of years, slowly drifting further away from the planet.
Formation and Discovery
The existence of the Martian moons was first confirmed by American astronomer Asaph Hall in August 1877 using the 26-inch refractor telescope at the U.S. Naval Observatory. He first spotted Deimos on August 12 and then Phobos six nights later, ending a long search for the satellites predicted to exist around Mars.
The precise origin of Phobos and Deimos remains a subject of scientific debate, with two main hypotheses. The first theory suggests they were once captured asteroids, likely C-type, that were captured by the gravitational pull of Mars. This is supported by their irregular shape and low reflectivity, which resemble primitive asteroids found in the outer asteroid belt.
The second prevailing theory proposes that the moons formed in place from a disk of debris ejected into orbit following a massive impact on Mars’s surface. This “giant impact” model is favored by the moons’ nearly circular orbits, which lie close to the Martian equatorial plane. Recent analysis of their composition suggests the material is more similar to Mars itself, lending support to the impact theory.