Physics 5 Astronomy

Physics 5 Astronomy

Homework 5 Solutions

1. Compare Jupiter and Saturn. Please list three ways in which they are similar and three in which they differ.

Ans: Similarities (any 3) between Jupiter and Saturn:

Jovian planets

Contain lots of H and He

lots of moons

encircling ring systems

atmospheric belts, caused by fast rotation

thought to have liquid metallic hydrogen cores

emit significant infrared radiation from internal heat

both have similar diameters: Jupiter 143,000 km, compared to 121,000 for Saturn

Differences (any 3) between Jupiter and Saturn:

Jupiter is 3 times more massive

Saturn’s rings are much more conspicuous

Saturn is flatter (= more oblate), because it is less dense

Jupiter’s "Great Red Spot"; also Jupiter shows more prominent atmospheric features

less helium in Saturn’s atmosphere

Saturn has larger proportions of the ice elements (C, N and O)

Comments: Many answers included the general and obvious similarities and differences (such as Jupiter is larger, Saturn is colorful and Jupiter is more massive, denser). More specific comments are needed

2. Why do we spend little time discussing Neptune and Uranus?

Ans: The main reason is because they are so far away and hence hard to observe from the Earth. Only a single spacecraft has passed relatively nearby (Uranus in 1986 and Neptune in 1989) and the data we have on them is sparse compared to many other planets. They have relatively featureless atmospheres. Many other objects in the Solar System are known to be more interesting.

Comments: Most students try to explain the reason with facts, rather than actually reasoning why.

3. As question 1, but for Pluto and a typical comet.

Ans: Similarities (any 3) between Pluto and a typical comet:

usually far from sun, or extremely far (comets). About 5 comets per year pass within 1 AU of the sun, but these are not typical of all comets.

relatively small bodies

and hence, difficult to study in detail

low density: made of ice and rock

orbiting the Sun in highly non-circular (eccentric) orbits. Most comets have parabolic orbits. The few which stay in the inner solar system have elliptical orbits.

usually not visible with naked eye (except the occasional comet)

Differences (any 3) between Pluto and a typical comet

Pluto is much more massive than any known comet.

Comets spend most time much farther from the sun: 100,000 AU compared to 40 AU.

some comets pass close to the sun, where they grow tails

Pluto has a moon which is large compared to Pluto. We guess that most comets do not have moons, because their masses are too low to keep moons.

Most comets have parabolic orbits, which take them out to near the stars.

Comments: Most students seem to get question; but some contradict themselves by saying "both Pluto and a typical comet are made of ice and rocks," and then they’d say "they are high density objects (which implies iron etc)."

4. Why are asteroids of interest?

Ans: There are 2 major reasons. Firstly, because they are small, many of them comprise relatively unprocessed material, which gives us clues to the formation of the solar system. Secondly, and perhaps more of interest to the non-astronomer, most of those with orbits which pass within 1 AU of the sun will hit the Earth. Once every 10,000 – 1 million years they cause major regional damage. Every 100 Million years they cause a global mass extinction.

Comments: Many did not answer the question: why they are of interest.