Moons of Mars

Mars has two small moons.

They are named Phobos and Deimos. These moons are not round. They look like bumpy rocks.

Phobos is the closer moon. It moves very fast. It rises in the west. Then it sets in the east.

Deimos is the farther moon. It moves much slower. It looks like a bright star. It rises in the east.

Scientists think they might be asteroids. Other people think they came from Mars. A big crash might have made them.

These moons are very small. They are much smaller than our Moon. They are still very cool to see.

Mars has two small moons named Phobos and Deimos.

An astronomer named Asaph Hall found them in 1877.

They are not round like our Moon. Instead, they are irregular in shape, which means they look like bumpy rocks.

Phobos is the moon that orbits closest to Mars. It moves very fast. It circles the planet in only 7.66 hours. Because it is so fast, it rises in the west and sets in the east.

Deimos is the farther moon. It takes about 30.35 hours to orbit Mars. It looks like a bright star in the sky.

Scientists are not sure where the moons came from. One idea is that they are captured asteroids. This means they were rocks floating in space that got caught by Mars. Another idea is that they came from Mars itself. A huge crash might have sent bits of Mars into space. These pieces then stuck together to make moons. Both ideas are still being studied today.

Mars has two small moons named Phobos and Deimos.

These moons are not round like Earth's Moon. Instead, they have irregular shapes that look like bumpy rocks.

Phobos is the moon that orbits closest to the planet. It has a diameter of 22.2 km. Deimos is much smaller and orbits much farther away. It is only 12.6 km across. Because they are so small, they do not cause total solar eclipses on Mars.

These moons move through space in very different ways.

Phobos orbits Mars very quickly. It completes one full circle in only 7.66 hours. Because it moves so fast, it rises in the west and sets in the east. Deimos moves much more slowly. It takes about 30.35 hours to orbit the planet. From the surface of Mars, Deimos looks like a bright star or a small planet. Both moons are tidally locked to Mars. This means they always show the same face to the planet.

People have wondered about these moons for a long time.

Even before they were found, writers guessed they might exist. In 1726, Jonathan Swift wrote about two moons in his book Gulliver's Travels. He guessed they orbited at distances of 3 and 5 Martian diameters. These guesses were close to the real numbers. The American astronomer Asaph Hall finally discovered them in August 1877.

He used a large 26-inch telescope in Washington, D.C. He found Deimos on August 12 and Phobos on August 18.

Scientists are still trying to solve a big mystery.

They want to know how the moons first formed. One idea is that they are captured asteroids. This means they were rocks floating in space that Mars caught. These rocks have colors and types similar to C-type asteroids.

Another idea is that they came from Mars itself. A huge crash might have sent pieces of Mars into space. These pieces could have stuck together to make the moons. Phobos might even be made of materials found on the Martian surface.

Looking at the moons helps us understand our own solar system.

Phobos is slowly moving closer to Mars every year. Eventually, it might break apart or crash into the planet. This could create a ring around Mars like Saturn has. Deimos is moving in a different way. It is slowly moving farther away, just like our Moon does.

Studying these small worlds tells us how planets and moons grow.

Mars is orbited by two small, irregular moons named Phobos and Deimos.

Unlike Earth's Moon, which is a large sphere, these satellites look more like bumpy, irregular rocks. They are quite small in comparison to our own Moon. Phobos has a diameter of 22.2 kilometers (13.8 miles) and a mass of 1.08 kg.

Deimos is even smaller, measuring 12.6 kilometers (7.8 miles) across with a mass of 1.5 kg. These tiny sizes mean they cannot cause total solar eclipses on Mars. They are simply too small to completely cover the Sun from the Martian surface.

The two moons follow very different orbital paths around the red planet.

Phobos orbits very close to Mars with a semi-major axis of 9,377 km. It completes a full revolution in only 7.66 hours. Because Phobos orbits faster than Mars rotates, it appears to rise in the west and set in the east. Deimos orbits much farther away at a semi-major axis of 23,460 km. Its orbital period is much longer, lasting 30.35 hours. From the surface of Mars, Deimos looks like a bright star or a small planet rather than a large disk. Both moons are tidally locked, meaning they always show the same face to Mars.

Humans have speculated about Martian moons long before they were actually seen.

In 1726, Jonathan Swift included two moons in his satire, Gulliver's Travels. He estimated they orbited at 3 and 5 Martian diameters with periods of 10 and 21.5 hours. While these were guesses, they were remarkably close to the real values. Other writers, like Voltaire in his 1752 story "Micromégas," also mentioned two moons. Voltaire suggested that because Mars is further from the Sun than Earth, it required two moons to remain stable. These literary connections are honored today through crater names like Swift, Voltaire, Laputa Regio, and Lagado Planitia.

The actual discovery happened in August 1877 by American astronomer Asaph Hall.

Working at the US Naval Observatory in Washington, D.C., Hall used a 26-inch refractor telescope.

He discovered Deimos first on August 12, followed by Phobos on August 18. The names Phobos and Deimos were suggested by Henry Madan. He chose them from Greek mythology, where the twins represent fear and terror. They were the companions of Ares, the Greek god of war, who is known as Mars in Roman mythology.

Scientists currently debate two main hypotheses regarding the origin of these moons.

The first hypothesis suggests they are captured asteroids from the main belt. Both moons share characteristics with C-type or D-type asteroids, including similar spectra and density. However, capturing an asteroid is difficult because their orbits are very circular and lie in Mars's equatorial plane. This would require a mechanism like atmospheric drag or tidal forces to circularize their paths. Some suggest Phobos might have formed from a binary asteroid that split apart under tidal forces.

The second hypothesis is that the moons formed from Mars itself.

This theory suggests a giant impact event may have ejected material from the planet. This debris could have then re-accreted in orbit to form Phobos and Deimos. Evidence for this includes the high concentration of phyllosilicates in Phobos, which are also found on the Martian surface. Additionally, the interior of Phobos is very porous, with voids making up 25 to 35 percent of its volume. This high porosity is actually inconsistent with an asteroidal origin, supporting the impact theory.

The future of these moons involves very different physical paths. Phobos is moving closer to Mars due to tidal forces. Eventually, it will reach the Roche limit, where the planet's gravity will tear it apart. This could result in Phobos crashing into Mars or forming a ring around the planet. In contrast, Deimos is moving farther away from Mars, similar to how our Moon behaves. Studying these small worlds helps astronomers understand the complex life cycles of planetary systems and the history of our solar system.