Space · Moons

Phobos and Deimos

Two moons that look like rocks

You need a telescope

Phobos and Deimos — Phobos, a lumpy rock full of craters and marked by long parallel scratches, above Mars.

The numbers

Diameter of Phobos: 14mi(22.5 km)A rock, not a world
One trip around Mars: 7.7hours
Distance from Mars: 5,827mi(9,377 km)Very close
What will become of it: It will crumble into a ringIn thirty to fifty million years
When they were discovered: In 1877, by Asaph Hall

To get an idea

Phobos goes around Mars in a few hours, faster than Mars spins on itself.

From Mars, Phobos would be seen rising in the west and setting in the east, the opposite of the Sun.

In a few tens of millions of years Phobos will crumble and Mars will have a ring.

Fun facts

These are not in the book: there was no room on paper.

From Mars it would be seen rising in the west and setting in the east, the opposite of the Sun.
Its surface is crossed by long parallel grooves whose origin is still uncertain.
It gets closer to Mars by about three quarters of an inch a year, dragged by tidal effects.
In thirty to fifty million years it will crumble and form a ring around Mars.
They are hard to observe from here, because they are tiny and very close to Mars.

Test yourself

The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.

Question 1 of 5★ is the one from your book

What are the moons of Mars like?

Question 2 of 5

How long does Phobos take to go around Mars?

Question 3 of 5

From Mars, where would Phobos rise?

Question 4 of 5

What will become of Phobos?

Question 5 of 5

Where do their names come from?

Riddles

There are two of us and we have scary names, as suits the sons of the god of war. We are not round like proper moons: we look like two chipped rocks. Who are we?

I travel so close to my planet that I go around it in a few hours, and anyone watching me from down there would see me rise on the wrong side of the sky. What am I?

Every year I sink almost an inch, and sooner or later I will be too close. I will not fall in one piece: I will crumble, and my pieces will become a necklace. What am I?

Try it yourself

Activity 1

Try covering Mars with the tip of a pencil in the eyepiece: it is the trick for making its moons stand out from the glare.

Activity 2

Swing a stone on a string that you keep making shorter: the shorter it gets, the faster the stone goes around. It is why Phobos races so fast.

The words

tidal effect: The stretching a body goes through because of the gravity of another one nearby.

Sources: Wikidata (Q7548), Wikipedia (Fobos (astronomia)).