What Are Phobos And Deimos?
QUICK ANSWER
Phobos and Deimos are Mars's two small moons. Phobos is larger (about 14 miles across) and orbits closer to Mars. Deimos is smaller (about 8 miles across) and orbits farther out. Both are potato-shaped and likely captured asteroids. They're named for the Greek war horses of Ares (Mars).
Mars's two moons are among the smallest and oddest in the solar system. Both Phobos and Deimos are far smaller than Earth's Moon, oddly shaped like potatoes rather than spheres, and likely originated as asteroids captured by Mars's gravity (though their exact origin is still debated). They orbit very close to Mars and have many interesting features that distinguish them from the typical moons of larger planets.
Where did they come from?
Probably captured asteroids. According to NASA, Mars has two small moons, Phobos and Deimos, that may be captured asteroids. They're potato-shaped because they have too little mass for gravity to make them spherical. The moons were both discovered by American astronomer Asaph Hall in 1877, within days of each other. The names come from the horses that pulled the chariot of the Greek god of war, Ares (whose Roman counterpart is Mars). Phobos means fear and Deimos means terror in Greek.
What is Phobos like?
The larger, closer moon. Phobos is the larger of Mars's two moons, about 14 miles across at its longest. It orbits only about 3,700 miles above the Martian surface, far closer than any other moon orbits its planet. Phobos completes an orbit in just 7 hours and 39 minutes, meaning it goes around Mars three times in a single Martian day. From the Martian surface, Phobos rises in the west and sets in the east (opposite the Sun's direction) because of how fast it orbits. Phobos has a large impact crater named Stickney.
What is Deimos like?
Smaller and farther. Deimos is the smaller moon, only about 8 miles across. It orbits about 14,580 miles above Mars and takes about 30 hours to complete one orbit. From the Martian surface, Deimos appears as a bright point of light rather than a disk, similar to how Venus appears from Earth. Both moons have very dark surfaces (only about 7% reflectivity), making them hard to spot from Earth. NASA's Perseverance rover has photographed Deimos in the Martian sky, capturing time-lapse images of its movement.
What's their future?
Different fates. Phobos is slowly spiraling inward toward Mars due to tidal forces, approaching the planet about 6 feet closer every century. In about 50 million years, Phobos will either crash into Mars or be torn apart by gravitational forces to form a thin ring around the planet, similar to Saturn's rings. Deimos has the opposite fate: it's slowly spiraling outward and will eventually escape Mars's gravity entirely, drifting into independent solar orbit. The two moons offer a glimpse of two opposite endings.
Phobos and Deimos are Mars's two small, potato-shaped moons, likely captured asteroids. Phobos is the larger and closer one, completing an orbit every 7 hours 39 minutes. Deimos is smaller and farther, orbiting in 30 hours. Both were discovered by Asaph Hall in 1877 and named for the Greek war horses of Ares. Phobos is slowly crashing into Mars (or will form a ring) in about 50 million years. Deimos is slowly escaping into solar orbit.
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