Astronomy 121 Study Guide (6)

What is responsible for the color of Uranus?

The prevalence of methane.

How is the interior of Uranus thought to differ from those of Jupiter and Saturn?

-- Although theoretical models indicate that Uranus has a rocky core similar to those of Jupiter and Saturn, the pressure outside of its core is too low to force hydrogen into the metallic state, so hydrogen stays in its molecular form all the way down into the planet's core. Astronomers theorize that deep below its cloud layers, Uranus has a high-density, "slushy" interior containing thick layers of water clouds.

What is the predicted fate of Triton?

-- Because of its retrograde orbit, the tidal bulge Triton raises on Neptune tends to make the moon spiral toward the planet rather than away from it. Eventually, Triton is doomed to be torn apart by Neptune's gravitational field, probably in no more than 100 million years or so.

Will Pluto and Neptune ever collide?

-- Because of the planets' 3:2 resonance (two orbits of Pluto for every 3 of Neptune) as they orbit the Sun, Pluto and Neptune are in no danger of colliding.

What is special about Miranda? Explain.

-- Miranda displays a wide variety of surface terrains that seems surprising for its small size. These varying surface terrains suggest that Miranda probably experienced one or more catastrophic events.

How do the rings of Neptune differ from those of Uranus and Saturn?

-- Neptune is surrounded by five dark rings. Three are quite narrow, like Uranus', and the other two are broad and diffuse, kind of like Jupiter's ring. Their dark coloration is unlike Saturn's bright rings and are more like Uranus' in that regard. Finally, while all of Uranus' and Neptune's rings lies within their respective Roche limits, Saturn's E-ring and Phoebe ring lie outside of its Roche limit.

In what sense were astronomers fortunate to discover Pluto?

-- Originally, scientists believed that Uranus and Neptune suffered irregularities in their motions and were thus being influenced by another celestial body. Not only are these irregularities nonexistent, but even if they did exist, Pluto is far too small to cause them anyway. Finding Pluto was not a result of celestial mechanics, but rather good luck.

Why was the discover of Uranus in 1781 so surprising? Might there be similar surprises in store for today's astronomers?

-- The discovery of Uranus was the first discovery of a new planet in 2000 years. Due to the relative strength of modern telescopes, it is unlikely that we will discover a new planet in our solar system, though as technology improves it is very likely that we will discover more about extrasolar system, and possibly even Earth-like planets.

Why does Uranus spin 'on its side'?

-- The rotation axis of Uranus is tilted by 98 degrees from the north celestial pole. This tilt may be due to a violent collision in the distant past.

Describe a day on Titania.

-- Titania is a heavily cratered moon with little indication of geological activity. Titania's low reflectivity suggests that the moon's surface is quite dirty. You would be very cold.

What causes Triton's geysers?

-- Triton's geysers are caused by the heating and vaporization of the liquid nitrogen below Triton's surface. Vaporization produces high pressure, which forces the gas through cracks and fissures in the crust, creating a geyser effect.

In what respect is Pluto more like a moon than a Jovian or terrestrial planet?

-- Unlike a J or a T planet, Pluto is made up mostly of water ice. Additionally, it is very similar to Titan's mass and radius.

How does Neptune's moon system differ from those of the other Jovian worlds? What do these differences suggest about the origin of Neptune's moon system?

-- Unlike the other jovian worlds, Neptune has no regular moon system, meaning that it has no moons on roughly circular, equatorial, prograde orbits. This suggests that the moons are actually captured asteroids.

How was Uranus discovered?

-- Uranus was discovered on accident in 1781 by William Herschel.

Why are the rings of Uranus so narrow and sharply defined?

-- Uranus'a rings require shepherd satellites to keep from diffusing. The inner satellites also shape the rings.

How does the magnetic field of Uranus compare with that of Earth?

-- Voyager 2 found that Uranus' magnetic field is about 100 times stronger than Earth's, but that's due to its relative size. For a planet of its size, Uranus' magnetic field is very comparable to Earth's.

How were the mass and radius of Pluto determined?

-- When astronomers discovered Charon, Pluto's satellite, and the fact that it had an orbital period of 6.4 days, they were able to determine Pluto's mass much more accurately than ever before. -- Fortunately, Charon's orbit around Pluto had at the time been oriented in such a way that the astronomers on Earth saw a series of eclipses. The astronomers computed their masses by basing their calculations on the variations in reflected light as Pluto and Charon periodically hid each other.

The rings of Uranus are dark, narrow, and widely spaced. Which of these properties makes them different from the rings of Saturn?

-- While Saturn's rings are bright, wide, and relatively narrowed spaced, Uranus' rings are dark, narrow, and widely spaced.