Moons of Mars - Facts for Kids
American astronomer Asaph Hall discovered the two Martian moons.
Hall chose the names Phobos and Deimos (Fear and Panic), because they were the sons of Ares. Ares was the Greek version of the Roman war god Mars. Phobos and Deimos drove Ares's chariot into battle to spread panic. Even today if you have a great fear of something like high places or spiders, it's called a phobia.
Phobos is bigger than Deimos, but they're both tiny.
The moons are too small for gravity to pull them into a rounded shape like our Moon. They're shapeless and lumpy with lots of craters. Phobos is seven times more massive than Deimos, but it's still only 22 km (14 mi) across on average. Tiny Deimos is about 12 km (8 mi) across.
Why did it take so long to find the Martian moons?
Besides being small, they also orbit very near to Mars. Until Asaph Hall searched, no one had looked so close to the planet. For example, the Moon is 384,000 km (240,000 miles) from Earth, and Io is about the same distance from Jupiter. However Deimos is just 23,500 km (14,600 mi) from Mars. Phobos is even closer at 9400 km (5800 mi) – Earth has artificial satellites in higher orbits than that.
Little Phobos has a crater that's over six times as wide as Meteor Crater in Arizona, USA.
In the distant past something big hit Phobos. It was so big that it nearly smashed the moon to bits. The impact left a crater which Asaph Hall called Stickney, named for his wife Angeline Stickney. It's 9 km (5.6 mi) across, while Earth's famous Meteor Crater is only 1.3 km (1 mile).
The moons orbit so close to the surface of Mars that an astronaut on Mars couldn't see them from the polar regions.
The moons are in circular orbits around the Martian equator, so the equator is where you'd get the best view. You might see a full Phobos as big as one-third the size of a full Moon, but Deimos would never be much brighter than Venus is from Earth. If you started traveling away from the equator, you'd see the moons lower and lower in the sky, until they were hidden beneath the horizon.
Moonrises and moonsets on Mars would seem strange to us.
Like Earth and its moons, Mars rotates from west to east, and its moons orbit from west to east. Mars also takes about 24 hours to turn once on its axis. However Deimos takes about 30 hours to orbit. That means it doesn't quite keep up with Mars. If you were an astronaut on Mars, you'd see Deimos slowly moving across the sky. It would rise in the east, be overhead 33 hours later, and then set in the west 33 hours after that. In another 66 hours, it would rise again.
Phobos is a different story. It takes less than eight hours to orbit, and is the only moon we know of that orbits in a shorter time than its planet's day. Phobos zips around so much faster than Mars rotates that it rises in the west and sets in the east. It gets around Mars three times a day.
Since the moons seem to orbit in opposite directions, you might see them going past each other in opposite directions about twice a day.
Astronomers used to think that the shapeless little moons were captured asteroids.
It now seems more likely that they're the result of a collision between Mars and another large body. The impact sent material flying away into an orbiting ring where it clumped together to make the two moons. This may also be how our Moon formed.
In the distant future Mars won't have any moons.
The gravitational forces between planets and their moons make what are called tidal effects. Over many millions of years Earth's spin has slowed down, and the days have become longer. At the same time, the Moon is getting slightly farther away. It's only 4 cm (1.6 in) a year, so nothing to worry about. However Deimos is slowly moving into a higher orbit and astronomers think that it will escape the Martian gravity and become an asteroid millions of years into the future.
Poor Phobos is another story. It's dangerously close to Mars and is being gradually pulled closer to it. It could eventually crash into Mars. However the stress of the tidal forces will probably pull it apart before then. It will once again become a ring of debris orbiting the red planet, making Mars the only inner planet with a ring. That would be a sight worth seeing, but you'd have to wait about 50 million years for it.
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