1) Saturn's large moon Titan
(A) is larger than the Earth, (B) has a rocky crust with many volcanoes, (C) crashed into Jupiter in 1994, (D) is a cratered, desolate world with no atmosphere, (E) may have oceans of ethane, rains of methane, and complex organic molecules on its surface.
Correct: (E). Titan is notable among planetary moons for its thick atmosphere, consisting of nitrogen gas (N2) with a few percent of methane (CH4) and traces of more complex hydrocarbons (molecules composed primarily of carbon and hydrogen, a relatively simple one being ethane: C2H6). The temperature and pressure are right for ethane to be a liquid, possibly resulting in a deep ocean of liquid flooding the low elevations of the surface (p. 527-529). They are also at the border between gas and liquid for methane, so warm drafts from the surface may carry gaseous methane up into the colder upper atmosphere where it cools and condenses into droplets, which fall back toward the surface as "rain", much like water does on Earth. The more complex hydrocarbon molecules (for example, C3H8, or C5H12) detected in Titan's atmosphere are relatively dense and so should settle to the surface to form a tar-like goo on the surface. This is all rather speculative (though it is based on chemistry that has been learned and tested in the laboratory) because we still have only a basic idea of what the temperature, pressure and composition of Titan's atmosphere is like. The Cassini probe should fill in the details in summer 2004 -- watch the paper for amazing details!
2) In terms of its density and internal composition, Titan is most like
(A) the Earth, (B) Jupiter's icy moons Callisto and Ganymede, (C) Jupiter's rocky moon Io, (D) Earth's moon, (E) Jupiter's tiny "captured satellite" moons.
Correct: (B). Titan's average denisty is 1.9 gm/cm3 (p. 527), very similar to that of Jupiter's outer Galilean moons Ganymede and Callisto (densities of 1.9 and 1.8 gm/cm3, respectively, see pp. 514-515). Thus the interior of Titan probably resembles the interiors of those moons -- a mantle of water ice over a rocky core.
3) Uranus and Neptune appear blue because
(A) nitrogen in their atmospheres absorbs blue light very efficiently, (B) methane in their atmospheres absorbs red light very efficiently, (C) their surfaces are covered with liquid water and their clouds are very thin, (D) both have small masses for a Jovian planet, (E) their high wind speeds prevent belt-zone circulation from occurring.
Correct: (B). As described on p. 538, methane absorbs red light. The sunlight falling on Uranus and Neptune, though feeble in strength, is still a mixture of all colors (white light -- remember spectra?). The red is absorbed by the methane and the violet/blue/green light is reflected. The in-between colors orange/yellow are partially reflected and partially absorbed. The mix of photons that is reflected back to Earth appears blue to our eyes.
4) Uranus is peculiar in that its rotation axis is tipped so that it is nearly 90° to the plane of Uranus's orbit around the sun.
(A) This makes the winter and summer seasons much more extreme on Uranus, (B) This disrupts the belt-zone circulation weather pattern, (C) This causes Uranus's magnetic field to have an identical tilt, (D) This caused Uranus to lose all its moons, (E) Answers A and D.
Correct: (A). Being tipped so far on its side means Uranus's southern hemisphere faces the sun almost directly all "summer", with the sun never setting below the horizon, while at the same time the northern hemisphere never sees the sun. The south gains the maximum warmth of the sun, while the north gets no solar heat. As the planet moves in its orbit, the situation graudally reverses. See pp.538-9 and Figure 24-3.
Notice that (D) and hence (E) are incorrect -- Uranus has moons a plenty, at least 21 accoding to p. 543.
5) Why does the belt-zone circulation on the Jovian planets become more difficult to detect on planets more distant from the sun?
(A) The outer planets are colder, so the clouds are buried deeper in the atmosphere, (B) The outer planets are hotter, so the belt-zone circulation is subdued, (C) The outer planets are colder, so the layer of methane haze lying above the clouds is thicker, (D) The outer planets lack a solid, rocky core, and so can not rotate on their axes, (E) Answers A and C.
Correct: (E). Pages 507-511 describes, in the context of Jupiter, what belt-zone circulation is (weather on Jovian planets consisting of dark and light colored bands -- clouds seen at different depths in the atmosphere -- separated by fast-moving winds) and how it is caused (probably by large circulation patterns deep inside Jupiter, see Fig 23-7, rather than sunlight warming the gas and causing winds to blow as on Earth). The concept is extended to Saturn on pp.522-523, where it is noted that because Saturn is farther from the sun than Jupiter and therefore colder, the cloud layers occur deeper in the atmosphere than on Jupiter (see Fig 23-19b). The overlying layer of methane haze are like looking through fog on Earth -- the cloud patterns appear less distinct than the do on Jupiter. On p. 540, the extension is made to Uranus -- colder atmosphere with deeper cloud layers, below so much haze that the light/dark bands are quite difficult to see (best seen when the contrast in the images is enhanced, as in a false color image like Fig 24-4).
6) Though Titan is small, it can retain an atmosphere because
(A) it is very cold, (B) it is very dense, (C) it rotates very fast, (D) it attracts mass from the solar wind, (E) it has a very strong magnetic field.
Correct: (A). A small world (in terms of radius and mass) will have a small escape velocity, meaning atmospheric gases do not have to move very fast in order to escape into space. However, on a cold world like Titan, the gas particles are moving very slowly (remember the correspondence between temperature and particle speed). Heavier molecules like N2 are below the escape velocity of Titan, and so are retained to form an atmosphere (see Fig 22-15).
7) What do the planets Uranus, Neptune and Pluto have in common?
(A) their atmospheres are very cold, (B) they are all "gas giant" planets, (C) they were all unknown to ancient civilizations, (D) they were all discovered using telescopes, (E) Answers A, C, and D.
Correct: (E). They are all very cold, because they are far from the sun and don't receive much light/heat. They are all too faint to see with the unaided eye (well, you might see Uranus on a very dark, clear night if you know exactly where to look) and were not discovered until telescopes were invented (however, astronomers -- being creatures of habit -- named them after Roman gods to be consistent with the names of the brighter planets that were known to ancient peoples).
Incorrect: As for (B), the Jovian planets (Jupiter, Saturn, Uranus and Neptune) are often referred to as "gas giant" planets. A new term, "ice giants", is becoming more commonly used for Uranus and Neptune because they contain more ice than gas. Moreover, Pluto is not a Jovian planet -- it is a tiny world made of rock and ice with very little gas at all.
8) Which of the following is a valid argument for stripping Pluto of its title as a planet?
(A) It is composed of material found nowhere else in the solar system, (B) It is the only terrestrial planet to have a moon, (C) It has no atmosphere, (D) It was discovered by means of a focused search rather than by accident, (E) Its mass and diameter are much smaller than the other planets, and even many moons.
Correct: (E). In terms of mass and diameter, Pluto is much smaller than Mars and Mercury, as well as moons like Titan (Saturn), Triton (Neptune), Luna (Earth), and all four of Jupiter's Galilean moons (see diagram). This makes one a bit uncomfortable about calling this speck of crud a planet. In class, we came up some other definitions of what a planet is, and Pluto fit some of them, but not all. Some other reasons for it not being called a true planet include: (1) its orbit is rather elliptical and tilted to the "plane of the ecliptic" defined by the other planets, (2) it doesn't fit the Jovian or terrestrial planet model, (3) it looks more like a large "Kuiper Belt Object" (icy comet core orbiting outside Neptune) than it does a planet.
Incorrect: As for (A), Pluto is made of the same stuff many of the Jovian moons are made of -- a mix of ice and rock. As for (B), Pluto is not a terrestrial planet (primarily rock), and Earth is an example of a terrestrial planet with a large moon. (C) is wrong -- Pluto has an atmosphere, albeit thin and one that freezes onto the planet's surface when Pluto swings far from the sun in its elliptical orbit. As for (D), Neptune was also discovered as a result of an intentional search, and besides, the method used to discover an object should not determine how the object is classified (its properties should).
9) Which of the following does not correctly describe the rings of Uranus and Neptune?
(A) Very narrow compared to Saturn's broad rings, (B) Composed of coal, (C) Are shepherded by small moons, (D) Appear dark because they do not reflect as much sunlight as Saturn's bright rings, (E) Were discovered when stars passed behind them and "winked out" as its light was absorbed by the material in the rings.
Correct: (B). If you think the rings are composed of coal, you should get some in your stocking! :^) The rings are described on pp.544-545 as being narrow and dark (don't reflect much sunlight). Like Saturn's rings, they are shepherded by the gravitational tug-of-war of several small moons. They were discovered as described in (E). They are "darker than coal", but are probably made out of frozen methane ice from shattered moons and/or rock dust "sandblasted" off rocky moons by micrometeorite impacts.
10) Pluto has a moon, Charon. By watching the orbit of Charon and measuring its period (P) and semi-major axis (a), astronomers can calculate Pluto's mass. The mass and radius (volume) allow astronomers to calculate that Pluto's density is 1.8 gm/cm3. From this, astronomers infer that Pluto
(A) has a large, iron-rich core, (B) is about 50% water ice and 50% rocky material, (C) should have a magnetic field that is much stronger than Earth's, (D) should have a rocky crust like Jupiter's moon Io, (E) must have a system of rings that has not yet been detected.
Correct: (B). I botched this one -- the book says the ice-to-rock ratio is more like 35% to 65%. The other answers are significantly more wrong, so (B) is still the best answer ("about 50% water ice and 50% rocky material"), but I'll give everyone credit for this question to atone for my carelessness. Happy holidays!
Incorrect: (A) an iron rich core would produce a density more like the terrestrial planets: 3.5 to 5 gm/cm3. (C) without a large, liquid iron core (like Earth), or a liquid metallic hydrogen layer (Jupiter & Saturn), Pluto can't create a strong magnetic field. It might produce a very weak one, like several of Jupiter's moon, if some of the water under its icy crust is liquid. (D) Io's rocky crust and smallish iron core result in an average density of around 3.5 gm/cm3 (significantly denser than Pluto is observed to be). (E) No one has ever found rings around a small world (terrestrial planet or moon), so it would be very surprising to find one around Pluto.