Physics 101 - Astronomy - Spring 2019

Class notes for day 14, March 5, 2019

Ch. 11 - The giant planets - Jupiter, Saturn, Uranus, and Neptune

To begin, we looked at a table which lists the planets and some of their properties. These can be divided into two groups: terrestrial planets and Jovian planets. The differences are obvious in Mass, Radius, and Average Density. Differentiation in the early solar system caused most of the lighter material to condense out near Jupiter and Saturn, so they have most of their mass in the form of hydrogen and helium. They are also not so dense as the terrestrial planets, in fact, Saturn has an average density less than water.

Most of what we have learned about the Jovian planets has come from spacecraft: the Voyager I and II spacecraft in the 1980s, the Galileo spacecraft and its atmospheric probe in the 1990s, and the Cassini spacecraft which arrived at Saturn in July 2004 with its Huygens probe that landed on the largest moon of Saturn, Titan, on Jan. 14, 2005. The Cassini-Huygens mission web site has hundreds of pictures of Saturn, its rings, and its moons, all acquired from the Cassini mission:
http://saturn.jpl.nasa.gov/

The New Horizons spacecraft passed Jupiter on its way to Pluto, and sent back newer pictures of Jupiter and some of its moons. These images are on the New Horizons website at: http://www.nasa.gov/mission_pages/newhorizons/main/index.html

I also talked a bit about gravity assists (slingshots) which use the gravity of a planet to speed up a spacecraft so that it can go further out in the solar system. An example of this is the Juno spacecraft, which swung by Earth on Oct. 9, 2013 on its way to Jupiter. It is now in orbit around Jupiter.

We know some things about the upper layers of Jupiter’s atmosphere from the probe that was carried by the Galileo spacecraft. It survived until it was about 150 km below what we call the top of the atmosphere of Jupiter. See the powerpoint for a figure showing the layers of clouds in the atmosphere of Jupiter. Jupiter radiates about twice as much heat as it absorbs from the Sun. All this energy drives Jupiter’s weather system as the heat moves up from the interior. The convection that results from this heat flow has been stretched around the planet in the form of bands. We first observed Jupiter’s Great Red Spot over 300 years ago, and it may be much older. This is a storm which has persisted for hundreds of years. We have also seen other shorter lived structures in Jupiter’s atmosphere, like the Brown oval, which was really just a large hole in the outer cloud layers of Jupiter’s atmosphere. There are also storms which appear as white spots.

All Jovian planets have differential rotation of their atmospheres. So the rotation (and the length of the day) depends on the latitude.

All the Jovian planets have extensive magnetic fields. Jupiter’s extends well past the orbit of Saturn!

Aurorae (plural of aurora) have been seen on both Jupiter and Saturn. These are due to the strong magnetic fields of these planets.

Tidal forces from Jupiter caused the comet called Levy-Shoemaker to break up into fragments as it passed near Jupiter. Then the fragments were captured into an orbit by Jupiter. Then on a later orbit the fragments hit Jupiter in 1994. I showed some pictures of the atmospheric disruptions caused by the comet fragments.

Saturn is believed to have an atmosphere very similar to Jupiter's. However, we have not sent a probe into the atmosphere so the exact differences are not known precisely. The contrast of colors is not so strong as Jupiter, so the atmosphere looks almost uniform to the human eye. However, there are bands and zones similar to the weather patterns on Jupiter.

Just one spacecraft (Voyager II) has visited Uranus and Neptune. Both planets get their bluish color from methane in their atmospheres. Uranus’s seasons are extreme due to its tilt. Neptune’s Dark Spot (seen in 1989) was temporary, and has now disappeared. Uranus’ and Neptune’s magnetic fields have strange orientations, and are not at all aligned with the rotation of the planet. Saturn’s magnetic field is perfectly aligned.