Average score was 75.6
There was a larger range in scores on Exam 2 than on Exam 1. Some (many?) of you found the test to be too long, although some people managed to complete the exam successfully within the time allotted. YOU WILL SEE ONE OF THE SHORT-ANSWER PROBLEMS AGAIN ON THE FINAL EXAM!
Average score was 75.6
In addition to the large number of moons that orbit each of the four giant planets (Jupiter currently has 63, Saturn has 31, Uranus has 27, and Neptune has 13), there is also dust, ice, and rock debris that orbit each of the four gas giants. These materials make up the rings of these planets.
Image from http://www.ifa.hawaii.edu/%7Esheppard/satellites/.
Jupiter's ring is made almost entirely of dust. It is very thin and dark, so it is difficult to see except in certain lighting conditions.
Image from Galileo spacecraft.
Image from Galileo spacecraft.
Saturn's ring system is the most extensive. It is made of millions of fragments of water ice (and rock), all orbiting Saturn. It extremely vast, but also extremely thin. [Scale model of 1 km across, thickness of a piece of paper.] The rings are "held in place" by small satellites that orbit Saturn near the rings. The gravitational influence of these "shepherd moons" keeps the rings in line.
Image from Voyager 1 spacecraft.
Image from Voyager 2 spacecraft.
Image from the Hubble Space Telescope.
Uranus' rings are narrow and dark in color. They are difficult to see from the Earth for this reason.
Image from Voyager 2 spacecraft.
Image from Voyager 2 spacecraft.
Neptune's rings are similar to those of Uranus, but even thinner. They have uneven distributions of particles, which is still not well-understood.
Image from Voyager 2 spacecraft.
Image from Voyager 2 spacecraft.
The origin of the planetary rings is under debate. One possibility is that they are remnants of satellites (moons) that broke apart by a collision with a large asteroid or comet. The tidal forces due to Saturn and the other moons may have prevented the particles from coalescing back into a satellite, and they dispersed into a disk. The other idea is that they are a "failed satellite," meaning that the particles were never able to come together to form a satellite in the first place. The gravity of the planet (and tidal forces on the particles) is an extremely important factor in either hypothesis.
Pluto is the smallest planet in our solar system, being smaller than the 7 largest planetary moons. It is "different" for several reasons:
From The Cosmic Perspective by Bennett et al.
From the Hubble Space Telescope.
Pluto was discovered in 1930 by Clyde Tombaugh, who was working at the Lowell Observatory in Flagstaff, AZ. It is the only planet discovered by an American. People thought that there might be another planet beyond the orbit of Neptune that was perturbing its orbit; the calculations were all wrong, but thanks to a careful search, Tombaugh discovered Pluto anyway.
Due to its odd nature (see above), Pluto is sometimes categorized as a Kuiper Belt Object (we will talk more about these objects next week). However, it has historically been categorized as a planet. This subject is still under debate, although here is an official statement claiming that Pluto will remain a planet.
Pluto's average density is 2 g/cm3, indicating that it is made of rock and ice. Clearly it is quite different from the gas giants, and in fact it more closely resembles the icy satellites of the outer solar system or the cometary nuclei of the Kuiper Belt.
From Hubble Space Telescope.
"True color" image of Pluto by Young et al., using Hubble Space Telescope data.
From Hubble Space Telescope.
Pluto's moon, Charon, was discovered in 1978, after careful examination of some Pluto images showed a "bump" on its side.
USNO Photograph.
The New Horizons mission is planned as the first spacecraft exploration of Pluto and Charon. It is scheduled for launch in early 2006, with an arrival date at Pluto of 2015. It will fly by Pluto and Charon and continue on into the Kuiper Belt to study an object there.