Phobos (the large moon) and Deimos captured together by ESA’s Mars Express Orbiter. [Credit: ESA/DLR/FU Berlin]

Before 1610 our Moon was the Moon. So it was sensational – and to many people unbelievable – when Galileo discovered moons orbiting Jupiter. As time passed, the other outer planets were also found to have moons. When Neptune was discovered in 1846, it was a only a matter of weeks before its moon Triton was discovered by English astronomer William Lassell. Yet it wasn't until thirty years later that Asaph Hall spotted the Martian moons, Phobos [FOE.bəs] and Deimos [DYE.mos].

Features of Phobos
Phobos is one of the least reflective bodies in the Solar System, its surface slightly darker than asphalt. Since it absorbs sunlight so well and has no atmosphere as a blanket, there's a substantial difference between the sunlit side's -4 °C (25 °F) and the side in shadow at -112 °C (-170 °F).

This image from ESA's Mars Express shows the irregular shape and extensive cratering of Phobos. Although Phobos has over six times more mass than Deimos and is twice as big across, nonetheless its dimensions are still only about 27x22x19 km (17x14x12 mi). That's a tiny moon.

Not only does the moon have lots of impact craters, it has a whopper of a crater named Stickney. It's named for Asaph Hall's wife Angeline Stickney Hall, and is 9 km (5.6 mi) across. Even on Earth this would be an impressive hole in the ground. The famous Meteor Crater in Arizona is only 1.3 km across. Whatever hit Phobos must have come close to smashing the moon to bits. Here is an image of Stickney taken by NASA's Mars Reconnaissance Orbiter showing the impact crater and within it another crater named Limtoc.

Moonrise. Moonrise.
Only 6000 km (3750 mi) above the Martian surface, Phobos orbits closer to Mars than any other known moon does to its planet. In fact, Earth has artificial satellites orbiting at higher altitudes than Phobos does.

The best view of Phobos would be from the Martian equator, because the moon is in a circular equatorial orbit. But despite the moon's closeness to the surface, its small size means it wouldn't appear any bigger than a third of the size of our full Moon. If you traveled away from the equator, Phobos would seem smaller and lower in the sky until it disappeared below the horizon. Neither of Mars's moons would be visible from the Martian polar regions.

Even though Phobos is small, it's a mover, whizzing around Mars in less than eight hours. Although Mars is smaller than Earth, that's still astounding. As Phobos orbits, Mars rotates. And Phobos is moving three times faster than Mars, so a Martian would see the moon rising in the west and setting in the east – opposite to Deimos, and to Earth's Moon. After setting in the east, it would rise again eleven hours later.

It's breaking up
Early space probes found mysterious grooves on the surface of Phobos. People thought that they had resulted from an impact, possibly related to the one that created Stickney crater. However many scientists now think that they're signs of tidal stress on the rocky moon.

Gravitational interactions between planet and moon, known as tidal effects, are causing Deimos to move gradually away from Mars. However Phobos is so close to Mars, the opposite is happening. Phobos is being pulled towards Mars at the rate of about two meters every hundred years. It's a slow process, but within 50 million years or so Phobos will no longer exist.

Would the moon break up and form a ring around Mars, or would it crash into the planet? Scientific opinion had been divided, but it appears that the most likely outcome is Phobos breaking up and making Mars the only rocky planet with a ring. Observational data has been used in a computer model to work out how strong Phobos is. It seems that the moon is so weakly held together it will break up before it can slam into Mars.

Questions
But what do we know about the other Martian moon Deimos? And how were these strange little moons formed? People had assumed for a long time that they were captured asteroids, but evidence gathered by space probes suggests otherwise. Click on the link below this article to find out more about Deimos.