Download Jovian Planets

Reminders!
Website: http://starsarestellar.blogspot.com/
Lectures 1-7 are available for download as study aids.
Reading: You should have Chapters 1-7 read, and
Chapters 8 by the end of Tuesday.
Homework: Homework #2 is due this Wednesday at the
BEGINNING of class.
Discussion: Next week, we will be reviewing for the
midterm.
Midterm!!!
The midterm is Thursday, June 11th in class. Don’t be late!
The test will cover Chapters 1-8.
It will consist of 40 to 50 questions and you will have until 12
o’clock to complete it.
The test is true-false and multiple choice. Make sure you have a
SCAN-TRON 882 form and a #2 pencil. I will NOT have these
available in class.
Bring your student ID.
The test is closed book, closed notes, and no calculators.
There will be a sheet of equations available.
A practice midterm is available. Take the practice test for 1.5
hours, and see how you do.
EXTRA CREDIT!!! Get 1 extra percentage point for each
mistake you find on the practice test!
Homework Issues
Practice your proportionalities!
L ∝ T4 then T ∝ L1/4
If L goes up by a factor of 2, then T goes up by a factor
of 21/4 ~ 1.2
Unit changes!
Include units!
100 cm or 1 m or 10-3 km is all right. If your answer is
simply 100, then you won’t get full credit!
Jovian Planets
Today’s Lecture:
• Jovian Planets (chapter 7, pages 152-183)
Jupiter and its moons
Saturn and Titan
Uranus
Neptune
The Solar System
Top view of the
solar system
• All the planets orbit
the Sun in the same
direction.
• Most of the planets
rotate in the same
direction too.
Sun
• And so does the Sun.
• Orbits are all nearly in
the same plane.
Side view is
a thin disk
Origin of the Solar System
“Nebular Hypothesis” (Kant, Laplace, 18th century)
• Before the Sun was born, there was a gravitationally
contracting, slowly spinning cloud of dust.
• As it contracts due to gravity, is spins up to conserve
angular momentum (figure skater effect)
• Sun forms in the center, and the outer parts make a
disk.
• Planets form out of the disk.
• Extra gas is blown away by the Sun’s winds.
Terrestrial Planets
• The inner planets: Mercury,
Venus, Earth, and Mars
• All have a smaller radius than
the Earth
• Not very massive (less than
Earth), but dense (>1 g/cm3,
which is the density of water)
• Close to the Sun and few
moons
• Rocky outer parts and iron
cores.
What do you weigh on the moon?
MMoon = MEarth/80, RMoon = REarth/3.7, F = ma
The ratio of surface accelerations for the two bodies is
• Weight = F = ma, so you would weight 1/6 on the Moon of
what you weigh on Earth.
• Note that mass and weight are different. Mass is intrinsic,
weight is a force that depends where that object is
Jovian Planets
• The outer planets: Jupiter, Saturn, Uranus, Neptune
• All have a much larger radii than the Earth
• All are much more massive than the Earth, but they’re
much less dense (0.7-1.6 g/cm3, where 1 g/cm3 is the
density of water)
• Far from the Sun and MANY moons
• Mostly liquid, but have rock/iron cores
• All are rotating very quickly (Jupiter has a radius 11 times
the Earth, but a rotation time of 10 hours!)
Jupiter: King of the Planets
• Radius = 11 RE = 0.1 Rsun
• Mass = 320 ME = 10-3 Msun
• Composed mostly of H and He (like the Sun)
• Colorful bands parallel to the equator. Lots of swirls and
spots, indicating storms.
• Oblate (squashed) shape because of rapid rotation (10
hours rotation period!)
• Famous “Great red spot” is about 2-3 times the Earth’s
diameter! >300 year old storm.
• Visited many times, Voyager 1,2 (1979) Galileo
(1996,1997)
Jupiter’s Moons
• Four Galilean moons: easily seen even through amateur
telescopes as little points of light.
• Io: Erupting volcanoes! Geologically youngest surface
(the “pizza” moon). Interior molten due to tidal forces
during its elliptical orbit around Jupiter.
• Europa: Smooth, narrow, dark stripes and few craters. It
looks like fractured soft ice--is there liquid water (or even
life) below?
• Ganymede, Callisto: old, hard ice with many craters
• Over 50 more moons, and new smaller moons
discovered each year. Many are captured asteroids.
• Very thin ring at a distance of 1.8 time the radius
Saturn: Magnificent Rings
• Density of 0.7 g/cm3. It could float on an ocean of water!
• Mostly H, He, with an Earth-like core. The structure is
similar to Jupiter, with atmospheric bands.
• Surrounded by magnificent rings! Ice chunks and icy
rocks.
• Rings are made of material that failed to form a moon (or
a torn-up moon or comet) due to the tidal forces of Saturn
(it is inside the “Roche limit”)
• Rings in equatorial plane, but inclined 27° with respect to
the orbital plane. They appear “edge-on” twice per 29-year
orbital period (end of 2008)
Saturn (continued)
• Only 4-5 rings are generally visible from Earth. Dark
“Cassini’s division” is between the 2 main rings.
• Voyager (1980) found more than 100,000 ringlets about
20 m thick
• The largest moon Titan: which is a very interesting object.
• Titan has a thick N2 atmosphere like the Earth! Hazy
atmosphere full of methane gas.
• We have not seen methane or ethane oceans, but the
surface appears covered by ice blocks.
• Cassini spacecraft is currently orbiting Saturn 2004-now
• Cassini spacecraft sent the Huygens probe to Titan to
investigate Titan’s surface
Uranus: The Sideways Planet
• Porbit = 84 years
• Not known to ancients;
discovered by William
Herchel (1781)
• H, He mostly; ammonia
and methane clouds
• Very cold: 60 K
• Axis of rotation tipped
98°! (Almost in the
orbital plane.) Extreme
seasons, in terms of the
Sun’s visibility
Sun
Uranus: Occultation of a star by the
planet and its unexpected rings
Star’s brightness vs time
(expected)
Uranus
b
time
(observed)
Uranus
b
time
Rings around Uranus
• 10 very narrow rings discovered in 1977, when Uranus
passed in front of a bright star.
• Some of the rings are very thin <10 km wide. “Shepherd
moons”?
ring
• Voyager 2 (1986) later found the “shepherd moons”
around some rings, as predicted. They keep the rings
narrow (gravitational interactions).
Neptune: Last of the gas giants
• About the same size as
Uranus
• H, He; methane
atmosphere
• Neptune discovered in
1846 (J. Galle) as a result
of analysis of Uranus’
orbit (Leverriere, Adams).
• Neptune perturbs
Uranus
• 1613: Galileo saw it, but
didn’t recognize it as a
planet!
Neptune: end of an odyssey
• Voyager 2: August 1989, got within 5000 km
• Saw complete (but clumpy) rings and an
atmosphere much more dynamic than Uranus.
Winds up to 1100 km/hr!
• “Great Dark Spot” - size of Earth; wispy clouds of
methane skirt around it. (The spot later
disappeared.)
• Triton: large moon with thin methane atmosphere.
Fascinating, varied terrain, and active ice volcanoes!
• Great way to end Voyager’s 12 year odyssey!