Download The Moon was born about 4.5 billion years ago

The Moon was born about 4.5 billion years ago. Although there are several theories how a
planet can get a moon, most scientists agree that our Moon was born as a result of a big
collision when a big body the size of Mars bumped into the Earth.
1. Size
The Moon is quite big: it is just about 4 times smaller than the Earth and it is the 5th largest
moon in the Solar System. The radius of the Moon is 1737 km. The gravity on the Moon is about
17% of that on Earth. So if you weight 100 lb on Earth you will weight 17 lb on the Moon.
2. Structure
The Moon, just like the Earth, consists of core, mantle and crust.
3. Surface
The rocky surface of the Moon is heavily cratered. Craters appear on the Moon when small
space bodies (asteroids, meteorites, comets) collide with the lunar surface. We call them impact
craters. There are thousands of impact craters covering the Moon, some of them are billions
years old.
Are there impact craters on the surface of the Earth too? Yes, but not so many. On Earth many
of those space rocks burn as they go through the atmosphere and never reach the surface. Our
planet is very “active” too! There are active volcanoes, strong winds, heavy rains, trees and
plants that constantly change the face of our planet and many of the Earth’s craters change
shape with a time or disappear completely. (There is a special EARTH IMPACT DATABASE
containing a list of 190 confirmed impact craters, their sizes and locations!)
3. Temperature
The temperature on the Moon varies significantly during the day. Remember, that one day is the
time that it takes for the Moon to make a full turn around its axis. It takes about 27 earth days for
the Moon to spin around itself once. Which means that a ‘daytime’ on the Moon lasts for about
13 and a half earth days, followed by 13 and a half earth days of ‘night time’. The side of the
Moon facing the Sun (day side) gets very hot: +123 degrees Celsius. The side of the Moon
facing away from the Sun (night side) cools down to -150 degrees Celsius.
Although the Moon is definitely not the most far away object in the Solar System, this is where
the coldest places in the SS can be found. There are deep craters on the lunar south pole that
stay in the permanent shadow. The temperature on the bottom of those craters is -400 degrees
Celsius. It is colder there than on Uranus, Neptune and even Pluto!
4. Water and air
We used to think that there is no water or air on the Moon. But recently scientists discovered
that the Moon does have an atmosphere made out of the most unusual gases. We do not find
those gases in the atmospheres of rocky planets so it is a bit of a mystery where they came
from. The lunar atmosphere is very thin, its density is close to the density of the air on the very
top of the Earth’s atmosphere where International Space Station orbits the Earth.
Some water has been found on the Moon too. It is water ice, not liquid water (without the thick
atmosphere liquid water cannot stay on the surface for long and escapes into space). Scientist
think that the water ice may have been brought to the Moon by comets (icy bodies of the Solar
System).