Phobos

Definition

Phobos is the larger and closer of the two natural satellites of Mars. It is a small, irregularly-shaped moon that orbits very close to the planet, completing an orbit in just 7 hours and 39 minutes. Phobos is of great interest to astronomers as it provides insights into the formation and evolution of the Martian system.

5 Must Know Facts For Your Next Test

1. Phobos is the larger of Mars' two moons, measuring approximately 22 km in diameter.
2. Phobos orbits just 6,000 km above the surface of Mars, much closer than the Earth's moon is to our planet.
3. Due to its close orbit, Phobos appears to move across the Martian sky faster than the planet rotates, rising and setting multiple times per Martian day.
4. The tidal forces exerted by Mars are causing Phobos to spiral inward and will eventually lead to the moon's destruction, either by crashing into the planet or breaking apart into a ring.
5. Phobos is irregularly shaped, likely due to it being a captured asteroid rather than a moon formed from the same protoplanetary disk as Mars.

Review Questions

• Describe the key features of Phobos and how they compare to the Earth's moon.
  • Phobos is the larger and closer of the two moons of Mars, orbiting just 6,000 km above the Martian surface. This is much closer than the Earth's moon, which orbits about 384,000 km away. As a result, Phobos appears to move rapidly across the Martian sky, rising and setting multiple times per day. Additionally, Phobos is irregularly shaped, likely due to its origin as a captured asteroid, unlike the Earth's spherical moon formed from the same protoplanetary disk.
• Explain the role of tidal forces in the eventual destruction of Phobos.
  • The close orbit of Phobos around Mars results in significant tidal forces exerted by the planet on the moon. These tidal forces are causing Phobos to spiral inward towards Mars over time. Eventually, Phobos will either crash into the planet or be torn apart by the tidal forces, breaking up into a ring of debris around Mars. This process is occurring because Phobos orbits well within the Roche limit, the distance at which a celestial body held together only by gravity will be torn apart by tidal forces.
• Analyze how the characteristics of Phobos provide insights into the formation and evolution of the Martian system.
  • The irregular shape and close orbit of Phobos suggest that it is likely a captured asteroid rather than a moon that formed alongside Mars from the same protoplanetary disk. This provides clues about the early history and evolution of the Martian system, including the possibility of significant impacts or gravitational interactions that led to the capture of Phobos. Additionally, the tidal forces acting on Phobos and its eventual destruction offer a unique window into the long-term dynamics and stability of moons orbiting close to their parent planets, which has implications for understanding the formation and evolution of other planetary systems.

Related terms

Deimos:

Deimos is the smaller and more distant of the two moons of Mars. It is an irregularly-shaped, potato-sized moon that orbits Mars in the opposite direction of the planet's rotation.

Tidal Forces:

Tidal forces are the gravitational forces exerted by one body on another, which can cause distortions or movements in the second body. The close orbit of Phobos around Mars results in significant tidal forces that are gradually causing the moon to spiral inward.

Roche Limit:

The Roche limit is the distance within which a celestial body, held together only by its own gravity, will be torn apart by the tidal forces of another body. Phobos orbits well within the Roche limit of Mars, which is contributing to its eventual destruction.