Download PTYS/ASTR 206 – Section 2 – Spring 2007 Activity #17: 4/10/07

PTYS/ASTR 206 – Section 2 – Spring 2007
Activity #17: 4/10/07
NAME:______KEY______________________________________________________
The purpose of this activity is to go over material covered both in class and in the textbook. This is an
ACTIVITY, so feel free to discuss these with one or two of your neighbors. You must turn in your
own work.
You decide how much the question is worth! You can choose each number (4, 3, 2, 1) only once.
The total must equal 10 pts. Circle the value you wish for each question. If you do not wish to use this
system of scoring, the values underlined will be used (don’t circle anything in this case).
#1. 4 pts 3 pts 2 pts 1 pt The Cassini division in Saturn’s rings is a region between the A and
B rings containing very few ring particles. How was it created?
One of Saturn's satellites exerted a resonant pull on particles within the division, clearing a
gap. That satellite is Mimas, which is in a 2:1 resonance with particles in the Cassini
division. That is, for any particle orbiting Saturn within this “gap”, every two orbits of
Saturn, Mimas will have orbited exactly once. Hence, two orbits these particles will
experience a force that will act to perturb their path. Hence, the region is quite devoid of
particles (but not completely! There are some particles there).
#2. 4 pts 3 pts 2 pts 1 pt Name two things about Saturn’s rings that are unique
compared to those of the other planets.
They are the most massive and the brightest of all of the planetary rings.
#3. 4 pts 3 pts 2 pts 1 pt In terms of the relationship between the period P and semimajor axis, a, of their orbits around the planet, how do the moons of Jupiter behave? (circle one
and provide a brief reason for your answer)
A) The moons do not obey the Keplerian relation P2 = ka3, because they orbit Jupiter and
not the Sun.
B) They obey the relation, P2 = ka3, in which k is the same for planetary motion around
the Sun, because it is a universal constant.
C) They follow the relationship, P2 = ka3, where k is a constant that is different from the
relation governing planetary motion around the Sun.
D) The moons do not obey the relation P2 = ka3, because their motion about Jupiter is
affected by Jupiter's motion about the Sun.
REASON FOR YOUR ANSWER:
The orbits are indeed Keplerian. The constant, k, depends on the mass of the two
bodies in question (for instance, a planet and the Sun; or a moon and its parent
planet, etc.). Since the mass of Jupiter and the mass of the Sun are different, the
constant, k, is different.
Over for 1 more question
#4.
4 pts
3 pts
2 pts
1 pt What is the Roche limit?
The Roche limit gives the distance from a planet at which the tidal force, due to the planet,
between adjacent objects exceeds their mutual attraction. Objects within this limit are unlikely to
accumulate into larger objects. The rings of Saturn occupy the region within Saturn's Roche limit.