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Two moons

Space and time · level 1, lesson 6 of 6

A question to start

We have one Moon. How many does Mars have, and what would they look like from the ground?

The idea

Mars has two small moons, called Phobos and Deimos. They are not round like our Moon; they are lumpy, like potatoes. Phobos is about 27 km long, smaller than a city.

Phobos is very close to Mars and very fast. It goes round Mars three times a day. It goes round faster than Mars spins, so it does something strange: it rises in the west and sets in the east, the opposite way to the Sun.

Deimos is smaller and farther away. It goes round more slowly, so it drifts slowly across the sky.

Moons Mars Earth
How many 2 1
Biggest Phobos, about 27 km long the Moon, 3,475 km across
Phobos: round 3 times a dayDeimos: slower, farther outMars

The moons are drawn far bigger than they really are, so you can see them.

Worked examples

1. Three times. Phobos rises and sets about twice each sol, as seen from the ground.

2. Size. Our Moon is about 3,475 km across. Phobos is about 27 km long: more than a hundred times smaller.

3. The other way. The Sun rises in the east on Mars, just as on Earth. Phobos rises in the west.

Try it

  1. How many moons does Mars have?
  2. What are their names?
  3. Which of them goes round Mars three times a day?
  4. Where does Phobos rise?
Answers
  1. Two.
  2. Phobos and Deimos.
  3. Phobos.
  4. In the west.

Common mistake

"All moons are round." Our Moon is big enough that its own gravity has pulled it round. Phobos and Deimos are too small, so they stay lumpy.

Where it leads