The only terrestrial planets in the solar system that have moons are Earth and Mars. When it comes to the Red Planet, it actually has two moons, Phobos and Deimos. The first person to hypothesize their existence was the famous astronomer Johannes Kepler.
The one to discover them was American astronomer Asaph Hall in 1877. These tiny celestial bodies were hidden in the glare from the planet, but eventually, they were found six days apart from one another. All that being said, here are some facts about Mars’s two little moons.
Facts about Phobos and Deimos:
1. Phobos and Deimos are among the smallest moons in the solar system
Mars is named after the Roman god of war, while its moons, Phobos and Deimos, are named after the horses that pulled the chariot of the god himself. Among the moons of the solar system, Mars’s moons are listed as one of the smallest [1].
The biggest is Phobos, having a diameter of about 25 km / 15.5 mi, while Deimos is around 15 km / 9.3 mi across. They are approximately 140 times smaller than Earth’s Moon.
2. Phobos and Deimos are similar to Earth's Moon
Just like our moon, the Martian moons always present only one side of their face towards the Red Planet. Both these satellites are lumpy, heavily-cratered, and covered in dust and loose rocks.
3. Phobos and Deimos could have been discovered much later
Though the American astronomer Asaph Hall discovered and named the two moons, he was about to give up his search for them. Luckily, his wife, Angelina, urged him on.
With a reinforced heart, Hall continued his observations and discovered Deimos the very next night, and Phobos six nights after that.
3. The Martian moons have an unusual orbit
Phobos orbits Mars at a distance of only 6.000 km / 3.600 mi. Currently, no known moon orbits so close to its parent planet. It whips around Mars around three times a day.
Deimos orbits Mars at a more significant distance than Phobos, completing one orbit around the Red Planet in around 30 hours, or about four times slower than Phobos. It lays at a distance of about 20.069 km / 12.470 mi.
Amusingly, in the late 1950s and 60s, the unusual orbital characteristics of Phobos led many astronomers to speculate that it was hollow inside. A Russian astronomer suggested a “thin sheet metal” structure for Phobos, but this led to even more speculations that the moon was actually of artificial origin.
4. Phobos will one day collide with Mars
Since it is the moon with the closest orbit towards its parent planet, Phobos’s fate is quite interesting. The moon is gradually spiraling inward, drawing about 1.8 meters / 6 ft closer to the Red Planet each century.
Scientists believe that Phobos will one day collide with Mars. Current estimates predict that this will happen in around 50 million years or so. However, there is also the possibility that it will break up before impact and form a planetary ring. Deimos, on the other hand, is slowly drifting away from Mars.
5. Phobos and Deimos moons can be used as a base for astronauts
If someone stood on the Mars-facing side of Phobos, Mars would take up the majority of the sky view. Scientists believe that people will experience this one day.
Many among them have already discussed the possibility of using one of the Martian moons as space station from which astronauts could observe the Red Planet and even launch robots to its surface.
The reason for this is because they would be shielded by miles of rock from cosmic rays and solar radiation for nearly two-thirds of every moon orbit.
6. The Martian moons are among the darkest objects in the solar system
Both moons appear to be made of carbon-rich rock mixed with ice. Many believe that they are actually captured asteroids since they resemble Type I and II carbonaceous chondrites, the substance that makes up asteroids.
From Mars, they don’t appear as moons. Deimos, the most distant moon, looks more like a star in the night sky. When it is at its brightest, it resembles the view of Venus as seen from Earth.
Though Phobos is the closest moon to its host than any other satellite in the solar system, it still appears when viewed from Mars, a third as wide as Earth’s moon.
7. The Martian moons origin is currently unknown
Since they have irregular shapes and strange compositions, many scientists believe that the Martian moons are asteroids. It is possible that Jupiter’s gravity may have nudged them into orbit around Mars, allowing the red planet to capture them.
However, their orbits make this idea improbable. Both moons take stable, almost circular paths around Mars. Captured bodies tend to move more erratically.
An atmosphere may have slowed the pair down and settled them into their present-day orbits; however, Mars’s atmosphere is too thin and insufficient for such a task.
It’s possible that the moons formed like the planet, from debris leftover from when Mars was created. Gravity may had drawn the remaining rocks into the two oddly shaped bodies.
8. Phobos and Deimos may have had a violent birth
Much like Earth’s moon, a collision could have blown chunks of Mars into space, and gravity may have pulled them together into the moons. Another theory proposes that an early moon of Mars could have suffered a collision with a very large object, Phobos and Deimos remained as the only remnants.
The most recent theory about the births of Phobos and Deimos proposes that an early collision once scattered debris into a ring surrounding Mars, and later accreted into the young moons.
9. The moons are hard to explore
Several spacecraft have snapped pictures of the Martian moons during their flybys. The first machine to do this was NASA’s Mariner 9 spacecraft, which orbited the planet in 1971.
It managed to take photos of the moons from afar; they appeared as potatoes from a distance. Many other spacecraft have performed long-range observations; however, one direct mission to the moons failed.
In 2011, Russia attempted to send a spacecraft to Phobos. The mission was called Phobos-Grunt. The probe, unfortunately, became stuck in Earth’s orbit and fell back on the ground in 2012.
The rovers on Mars can’t get close to the moons; however, several proposed missions, most notably to Phobos, are planned. The Aerospace Exploration Agency of Japan, for example, plans to launch in 2024 the Mars Moons eXploration mission to visit both Phobos and Deimos.
The spacecraft will land on the surface of the moons to collect samples and return them to Earth by 2029.
10. Phobos has only (1 / 1,000)th as much gravitational pull as Earth
If you had 68 kilograms / 150 pounds, you would weigh on Phobos 68 grams / two ounces. However, NASA’s Mars Global Surveyor has shown evidence of landslides, boulders rolling and dust that fell back down to the surface after being blasted off the moon by meteorites [1].
11. Phobos has an impact crater which is 9 km / 5.4 mi wide
It is the largest crater discovered on Phobos, covering a significant portion of the moon’s surface. This crater was named Stickney, after the maiden name of Hall’s wife, the discoverer of both Martian moons.
Some scientists believe that this impact must have nearly shattered Phobos.
