Transcript Lec22-041007 - Lunar and Planetary Laboratory

Planetary Rings (cont.)
+ Galilean Satellites
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Announcements
•
Reading Assignment
– Finish Chapter 15
•
5th homework due Thursday.
•
Reminder about term paper – due April 17.
– A sample “planet fact sheet” has been posted on the website
– Details of turnitin.com
•
•
•
•
•
Go to www.turnit.com
Click on “new users”
Usertype  student
Class ID: 1868418
Password: Section2
•
Mission Updates?
•
Next study-group session is tomorrow, from 10:30AM-12:00Noon – in room
330.
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
All Rings Have Gaps:
Why?
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Rings, Gaps, and Resonances
• Gaps can occur in rings
because the positions of
particles in that ring are in a
resonance with the planet and
a moon
• Example: The Cassini division
is located at a point that is in a
2:1 resonance with Saturn’s
moon Mimas
– Note that it is not
completely devoid of
particles!!
PTYS/ASTR 206
The Cassini division between
Saturn’s A and B rings
Rings / Galilean Satellites
4/10/07
• The gaps in rings are
similar to Kirkwood gaps
– Gaps in the number of
asteroids in the
asteroid belt
– We will discuss this
more in two weeks
Distribution of asteroid periods
showing gaps, like in planetary rings
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Cassini Found Density Waves in Saturn’s
Rings
Cassini images of spiral density waves in Saturn’s Rings
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
• Small moons can also
act to “shepherd” the
ring particles (by gravity)
into certain locations.
Shepherd moons
• Such moons are known
as shepherd moons
• Saturn may have many
undiscovered shepherd
satellites
Saturn’s faint F ring, which is just outside
the A ring, is kept narrow by the
gravitational pull of shepherd satellites
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Moons can even orbit a
planet within one of the
gaps in the rings, as in
the case of Saturn’s
moon Pan which orbits
within the Encke gap
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Different parts of the Rings
have different colors
• Different ring colors are caused by light
scattering off of different sized particles
– Light scattering depends on the size
and composition of the particles in
the ring
PTYS/ASTR 206
The outer ring of Uranus has a moon
embedded in it. The moon sweeps
up larger pieces of material, leaving
dust.
Dust more effectively scatters
Rings / Galilean
Satellites
4/10/07
blue light
The Color of Saturn’s Rings reveal that the ring
particles do not migrate outside of their orbits
Voyager 1 image
Colors are strongly enhanced
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Cassini image
Natural color
The Roche Limit
• Particles in a ring never
coalesce to form a moon.
– Planetary rings are within
the Roche limit of the
planet.
• Tidal forces are greater than
the gravitational forces among
the ring particles
• Question: why don’t the ring
particles (chunks of ice and
rock) break apart?
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Rings Are Probably Short-Lived
• Gravity of the Sun and
planet’s satellites constantly
disturbs orbits of particles
• Collisions probably happen
frequently
• Particles should drift out of
the ring
• Rings around outer planets
have probably come and
gone repeatedly during the
history of Solar System
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
• Most spectacular
– Most massive (by 105)
– Brightest
• albedo of 0.8
– The planet itself has an
albedo of 0.5
• Probably composed of ices
– Youngest?
– Sizes of the particles range
from micrometers to a about
ten meters
• Besides gaps, get spokes, waves
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Saturn’s Rings
Earth-based observations reveal three broad rings
encircling Saturn
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Spokes in Saturn’s Rings
• Voyager I found changing
dark structures (directed
outward) in the rings.
– given the term "spokes"
• They rotate with the magnetic
field
• It has been proposed that
electrostatic repulsion
between ring particles may
play a role.
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Jupiter and Saturn’s Moons
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Galilean Satellites
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Earth-based views
• Easily seen through a small
telescope
– Even binoculars
• Their movement can be seen
throughout the night
– They make shadows on
Jupiter when they transit
– They can also disappear
behind Jupiter during an
occulation
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Made by amateur -- Antonio Cidado
(the movie was made using individual frames
taken during a single night’s observing)
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Galilean Satellites and Kepler’s Laws
• Jupiter’s moons obey Kepler’s laws, just as the planets
do in orbit about the Sun.
– The orbital period is related to the semi-major axis
(Newton’s form of Kepler’s 3rd law)
– Note that this is slightly DIFFERENT in form from
what we used for the planets (why?)
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Orbital Resonances among Galilean Satelites
• Io, Europa, and Ganymede are in an orbital
resonance such their orbital periods are in the
proportion 1:2:4
– Io revolves around Jupiter twice for every
complete orbit of Europa
– Europa revolves around Jupiter twice for
every complete orbit of Ganymede
• Io completes 4 orbits in this time
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Jupiter’s Satellites
Io
Europa
Ganymede
Callisto
Distance
(J. radii)
5.9
9.4
15.0
26.4
Period
(days)
1.769
3.551
7.154
16.69
Mass
(Lunar)
1.22
0.65
2.02
1.47
Density
(g/cc)
3.6
3.0
1.9
1.9
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
You may notice …
•
Io’s orbital period: 1.769 days
• Europa’s orbital period: 3.551 days
but … 3.551/1.769 = 2.00735 ??? (not 2)
We know that the orbital resonance is exactly 1:2. So
what is the explanation for this apparent discrepancy?
(The answer is subtle and you are not required to know it)
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Jupiter’s Satellites
Io
Europa
Ganymede
Callisto
Distance
(J. radii)
5.9
9.4
15.0
26.4
Period
(days)
1.769
3.551
7.154
16.69
Mass
(Lunar)
1.22
0.65
2.02
1.47
Density
(g/cc)
3.6
3.0
1.9
1.9
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Solar-system in Miniature ?
• The density of the Galilean
satellites decreases with
distance from Jupiter
• This is analogous to
planets in the solar system
– Close to the Sun are
the Terrestrial planets
(high density)
– Farther away are the
Gas Giants (low
density)
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07
Planet densities
The Galilean satellites probably formed in a similar
fashion to our solar system but on a smaller scale
PTYS/ASTR 206
Rings / Galilean Satellites
4/10/07