The Moon raises tidal bulges on Earth and as a consequence is being pulled gradually away from Earth. Phobos raises tidal bulges on Mars and as a consequence is being pulled gradually toward Mars. Why this difference?

Question

Quizplus · Accepted Answer

Phobos orbits Mars more quickly than Mars rotates, which means the tidal bulges it creates are always slightly ahead of it in its orbit. This leads to a transfer of energy from Mars to Phobos, slowing down Mars' rotation slightly but accelerating Phobos. However, due to the dynamics of orbital mechanics, this actually causes Phobos to spiral inward towards Mars, unlike the Moon which is spiraling outward away from Earth because it orbits slower than the Earth rotates.

The Moon Raises Tidal Bulges on Earth and as a Consequence