Kepler found three laws which governed planetary motion. One
linked their orbital periods and their distance from the sun. The orbital
periods can be found by painstaking observation of the motions of the planets
in the sky. Kepler’s 2nd Law
can then be used to find the radius of orbit. Once this is known Newton’s Law
of gravitation can be used to calculate the mass of a planet. The diameter of a
planet can then be calculated form the angle it subtends when viewed from
Earth.
|
Name
|
Radius km
|
Mass kg
|
|
Sun
|
695000
|
1.99x1030
|
|
Mercury
|
2440
|
3.3x1023
|
|
Venus
|
6052
|
4.87x1024
|
|
Earth
|
6378
|
5.97x1024
|
|
Mars
|
3397
|
6.42x1023
|
|
Jupiter
|
71492
|
1.90x1027
|
|
Saturn
|
60268
|
5.68x1026
|
|
Uranus
|
25559
|
8.68x1025
|
|
Neptune
|
24766
|
1.02x1026
|
|
Pluto
|
1137
|
1.027x1022
|
|
Moon
|
1738
|
7.35x1022
|
|
Ganymede
|
2634
|
1.48x1023
|
1. Using
the table opposite calculate the average densities of the objects listed.
2. The
planets can be divided into two groups called Jovian and Terrestrial. Identify
which belongs to each group, and why they are so named.
3. The
average density of the surface rocks of the earth is 2 800kg m-3. How does this compare with your value. The
earth is thought to have a dense iron core. Why do you think this conclusion
has been drawn?
4. Comment
upon my use of the term average density in question 1.
5. Compare
your values for Mars and the Earth. The
surface rocks on Mars are similar to igneous rocks on Earth. Mars is thought to
have a core rich in sulphur as well as iron, explain why.
6. The
Moon and the Earth are the same age. This has been verified from rocks returned
by American and Soviet missions. Compare the densities of the two bodies.
7. There
are four theories of the formation of the Earth Moon system
a)
That the two bodies formed together at
the same time.
b)
That a proto-planet of the size of Mars
collided with the proto-earth. Both planets were destroyed. The bulk of the
material accreted to form the Earth but a small amount of lighter material spun
out and formed the Moon.
c)
The teardrop theory where the proto
moon was torn away from the Earth leaving a hole now filled by the Pacific
Ocean. (Continental drift?)
d)
The moon was a stray planet that the
earth captured. (compare to other small objects)
8. The
planet mercury superficially resembles the Moon. Do you think its internal
structure is similar? Give reasons for your answer.
9. The
Jovan planets are sometimes called "Gas Giants". With reference to
their densities give reasons why. Neptune and Uranus are both blue gas giants.
Some may call them sister planets. Comment upon this.
10. Should
Pluto be classed as a terrestrial planet?
