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Phobos and Deimos

Gravity and orbits · level 4, lesson 5 of 6

A question to start

Our Moon is a big round ball that takes a month to go round. Mars has two moons, both tiny and lumpy. One crosses the Martian sky backwards, twice a day. How can that be?

The idea

The two moons of Mars were found in 1877 by the astronomer Asaph Hall, and named Phobos and Deimos, "fear" and "dread", the sons of the war god Mars in Greek myth.

Phobos Deimos Our Moon (for comparison)
Size about 27 × 22 × 18 km about 15 × 12 × 11 km 3,475 km across, round
Distance from its planet's middle 9,376 km 23,463 km 384,400 km
Time to go round once 7 h 39 min 30 h 18 min 27.3 days
Pull at the surface (Earth = 1) about 1/1700 about 1/3000 1/6

Both are too small for their gravity to squash them round, so they are lumpy, like potatoes, and covered in craters. Phobos's biggest crater, Stickney, is about 9 km across, a third of the whole moon.

Phobos crosses the sky backwards. Mars spins once in about 24.6 hours. Phobos goes round in under 8. It overtakes the spinning ground, so from the surface it rises in the west and sets in the east, crossing the sky in about four and a quarter hours and doing it again about 11 hours later. Our Moon is far slower than the Earth's spin, so it rises in the east.

Deimos is slow. It goes round in 30 hours, just a bit slower than Mars spins. So the ground almost keeps up with it. Deimos rises in the east, like our Moon, but drifts so slowly that it stays in the sky for about two and a half days before it sets.

What you would weigh. Phobos's surface pull is about 1/1700 of Earth's. A 40 kg person would read about 23 g on a bathroom scale there, like a few sheets of paper. A careless jump could launch you a long way.

Worked examples

1. Phobos is close. Phobos is only about 6,000 km above the surface of Mars (\(9376 - 3390\)). Our Moon is 60 Earth radii away; Phobos is under 3 Mars radii away. From the surface it looks about a third as wide as our Moon does from Earth.

2. Years on Phobos. In one Mars year of 687 Earth days, Phobos goes round about \(687 \times 24 \div 7.65 \approx 2150\) times.

Try it

  1. Who found the moons of Mars, and when?
  2. Why are Phobos and Deimos lumpy instead of round?
  3. Why does Phobos rise in the west?
  4. Which moon stays in the Martian sky longer, and why?
Answers
  1. Asaph Hall, in 1877.
  2. They are too small: their gravity is too weak to pull their rock into a ball.
  3. It goes round Mars faster than Mars spins, so it overtakes the ground.
  4. Deimos: it goes round only a little slower than Mars spins, so the ground nearly keeps up with it.

Common mistake

"Phobos goes round the wrong way." It goes round the same way Mars spins, eastwards. It only looks backwards from the ground because it is faster than the spin.

Where it leads