Way Out

Uranus and Neptune

Today I will talk about the solar system beyond Saturn. The last two major planets are Uranus at a distance of 19.2 au and Neptune at 30.1 au. Neptune is the more distant and the more interesting. It is also worth noting that these two planets are the only major planets conveniently situated for observing in the evening sky. They are in the constellation of Capricornus, which transits around 8PM these evenings.

First, let’s see where they are. This picture gives a real visual sense of what 19 and 30 astronomical units from the Sun means.

Remember the inverse square law…the flux of sunlight at Neptune is 1/900 as strong as at Earth. As a result, extreme cold dominates at Neptune.

Below is a picture of Neptune in 1989, at time of Voyager flyby. That was the last time we had a good look at this planet.

The Voyager spacecraft flew by Neptune in 1989, returning images of it and its moons. It shows evidence of atmospheric convection, cloud formation, etc, due to the presence of a heat source down deep.

The most interesting aspect of Neptune is its large moon Triton, which is comparable in size to the other big satellites in the solar system. A picture returned from Voyager 2 is shown below.

Obviously this object has had an interesting and varied geological history.

Because of the feeble sunlight, the surface temperature sinks to 35-40K! At this temperature, just about everything is ice; water, carbon monoxide, methane, ammonia, and even nitrogen!

One of the remarkable findings from Voyager is the existence of cryovolcanism (neat term eh?). This is a form of geology that occurs only in the extreme cold of the outer solar system, where a small amount of heat from the distant sun can turn ices into gushers capable of producing volcano-like eruptions. Voyager revealed shadows on the surface of Triton, which are believed to be caused by matter blown up into space by cryovolcanic eruptions.

Triton also shows evidence of geological activity through the ages. Voyager revealed features that seem to be impact basins filled with melted and refrozen liquid of some sort. For example, look at the image of a “skating rink” in an impact basin shown below.

Pluto

When I was in grade school and high school, there was always discussion of the nine planets. Nowadays, some astronomers (yours truly included) only list nine. What happened?

Pluto was discovered in 1932. Most of the time it is the most distant planet. There are oddities with its orbit. Its semimajor axis is 39.5 au (bigger than Neptune), but the eccentricity is 0.248, the biggest of any planet, and way bigger than all except Mercury. Furthermore, the orbit is inclined 17.2 degrees with respect to the ecliptic. All highly unusual. The high eccentricity means that some of the time it is closer to the Sun than Neptune, and the high inclination means something is fishy about it.

About 20 years ago it was found that Pluto is a double planet, consisting of Pluto and its companion Charon.

Think about the following question: why could we learn more about Pluto if it has an object orbiting it?

A Hubble telescope picture of Pluto and Charon is shown below.

The density of Pluto is 2.0 grams/cubic centimeter. What does it mean?

The mass of Pluto is 0.25 percent of the mass of the Earth. Given this number, what do you think about the nature of Pluto?

We now know that that Pluto is merely the largest of a class of very interesting objects called the Kuiper belt objects, which will be discussed further in coming lectures.

Even further out

Uranus, Neptune, and Pluto seem unimaginably far out. It is incredible that we can know much at all about them. The fact that we do know a lot about the outer planets is due to the Voyager 1 and 2 spacecraft, and before them the Pioneer 10 and 11 spacecraft, which flew past the outer planets. Voyager 2 flew past Neptune in 1989.

After flying past the outer planets, these spacecraft kept going, and radio contact has been maintained with all of them. All four contained instruments built at the University of Iowa, and in several cases, still providing data 26 years after launch (Voyagers) and even 30 years (Pioneer).

These spacecraft are now at incredible distances. The most distant is Voyager 1. Last November, it passed 90 astronomical units from the Sun. A picture of the spacecraft is shown below.

The diagram below shows the location of the Voyager spacecraft, and illustrates the fact that they are far beyond even the most distant major planets.

These instruments continue to provide data. The radio wave experiment of the University of Iowa is providing what is probably the most important. Donald Gurnett and William Kurth of this department have studied radio noise at a frequency of a few kilohertz (high audio frequencies) that is measurable only far out in the solar system. It is believed to come from the heliopause, or vast bow wave that separates the solar system from the gas of the interstellar medium, or gas between the stars.

This diagram shows the spectrum of radio emission measured by the University of Iowa experiment on Voyager from 1982 to the present. The bottom scale shows the distance of Voyager from the Sun.