Chapter 12

How are Triton and Pluto similar?

Both Triton and Pluto are very cold worlds in the outer reaches of the solar system. They are similar in size (between 2000 and 3000 km) and similar in density, and thus similar in the proportion of rock and ice that makes them up. Both have a thin atmosphere of nitrogen, which freezes and sublimates depending on the temperature. Both have irregular or unusual orbits, with Triton moving in a retrograde orbit (unique for a larger-size moon) and Pluto in a tilted, highly elliptical orbit. Astronomers think that Triton may be a captured moon from the realm of trans-neptunian dwarf planets like Pluto.

Compare the geology of Callisto, Ganymede, and Titan.

Callisto is an ice-covered moon whose inner materials have never fully differentiated into different-density layers. It has no inner or outer activity and is basically geologically dead. Ganymede has a central rocky core and shows signs of tectonic activity, including regions of young surface terrain and long cracks in the crust. Titan has similar mass, size, and composition to Callisto and Ganymede, but has an active geology of liquid hydrocarbons on the surface, evaporation into the atmosphere, and rain back onto the surface.

Explain the energy source that powers the volcanoes of Io.

Io is close enough to Jupiter (and caught between Jupiter and the large moons on the other side of Io) to experience significant tidal heating, where the moon alternately stretches and relaxes in its elliptical orbit about the planet. This generates enough heat to produce molten silicate lava that erupts as volcanoes.

List at least three major differences between Pluto and the terrestrial planets.

Pluto is much smaller than all the terrestrial planets (and even smaller than many of the big moons of the giant planets). Its composition is ice and rock as opposed to the composition of rock and metal of the terrestrial planets. Pluto's orbit is highly elliptical and inclined to the plane of the ecliptic as opposed to the circular orbits near the plane of the ecliptic for the terrestrial planets. Pluto has a thin and variable atmosphere of nitrogen, while three of the terrestrial planets have much thicker and warmer permanent atmospheres. (Nitrogen dominates Earth's atmosphere, while carbon dioxide dominates the atmospheres of Venus and Mars. Mercury has only a very thin "temporary" atmosphere, borrowed from the solar wind.) Pluto is but one object of many (including some of the same size as Pluto) in the Kuiper Belt, whereas each of the terrestrial planets dominates the mass in their respective orbits.

How was Pluto discovered? Why did it take so long to find it?

Pluto was discovered at Lowell Observatory by a young, relatively untrained Clyde Tombaugh, from comparisons of photographs taken several days apart that showed the relative motion of Pluto against a background of stars. Pluto was difficult to find because of its small size and great distance from Earth. Although Percival Lowell had suggested that Pluto could be found by its effects on the orbits of other outer planets such as Uranus and Neptune, in fact a planet with so little mass could not be pinpointed by its gravitational effects. It was just luck that Pluto was found where Tombaugh had been told to search.

Describe and compare the rings of Saturn and Uranus, including their possible origins.

Saturn's rings form a wide and complex system, consisting mostly of particles and pieces of ice, and are highly visible. They may have formed from one or more moons that broke up due to a collision, or are left over from early debris that never coalesced into a moon. The rings of Uranus are thin and hard to see, consisting mostly of chunks of carbon and hydrocarbons with very little reflectivity. They may also have formed from the breakup of a small moon due to a collision. They may be kept thin by the presence of shepherd moons.

The Hubble Space Telescope images of Pluto in 2002 showed a bright spot and some darker areas around it. Now that we have the close-up New Horizons images, what did the large bright region on Pluto turn out to be?

The bright spot turned out to be what has been nicknamed Sputnik Planum (the Sputnik Plains). It appears to be a bowl or sea of frozen and perhaps liquid nitrogen, much brighter and younger (no craters visible) than the darker highlands of Pluto.

What are the moons of the outer planets made of, and how is their composition different from that of our Moon?

The moons of the outer planets consist of a mixture of ice and rock, whereas our Moon is just rock.

Why were the rings of Uranus not observed directly from telescopes on the ground on Earth? How were they discovered?

The rings of Uranus consist of dark, coal-like particles in narrow bands, making them almost impossible to see from Earth. They were discovered using a flying infrared telescope during the occultation of a star as Uranus passed in front of it. The light from the star dimmed several times before it was blocked by the disk of Uranus and afterward, indicating the presence of several distinct rings.

What is the evidence for a liquid water ocean on Europa, and why is this interesting to scientists searching for extraterrestrial life?

The surface of Europa features jagged blocks of ice that seem to have rotated and collided with one another, which would not likely happen on a solid moon. Long, straight cracks in the crust are also more likely to happen over a liquid subsurface layer than a solid one. Also, a weak magnetic field implies the presence of a liquid layer below the surface. This is interesting for searchers of extraterrestrial life because liquid water is essential for life as we know it, and life seems to exist in most places on Earth where liquid water is found. For example, life is found near vents on the deep ocean floor where chemical energy from hot springs can serve as a source of energy.

Compare the properties of Titan's atmosphere with those of Earth's atmosphere.

Titan's atmosphere is four times denser than Earth's atmosphere at the surface, even though both atmospheres are mostly nitrogen. However, 20% of Earth's atmosphere is oxygen; Titan's atmosphere is 98% nitrogen, with no free oxygen in it. Methane makes up 1.5% of the atmosphere, and it also has small amounts of various other organic gases, including ethane, carbon monoxide, and hydrogen cyanide. Both atmospheres have a weather cycle of evaporation from surface liquid, formation of clouds, and precipitation back onto the surface, but Earth's cycle is water-based and Titan's cycle is hydrocarbon-based (methane and probably ethane).

Saturn's E ring is broad and thin, and far from Saturn. It requires fresh particles to sustain itself. What is the source of new E-ring particles?

We now know from investigations with the Cassini mission that the new particles in the E ring are produced by the geysers on Saturn's moon Enceladus, which blasts considerable amounts of salt water ice vapor and crystals into space from the "tiger stripe" features on the moon's surface.