Astronomy: Chapter 9

Occultation

Blocking of the light of a more distant object, such as the star here, by something between it & us, such as Uranus' rings.

B Ring

Brightest of all of Saturn's rings, which lies closer to the planet.

How was Neptune discovered?

By a careful application of Newton's laws to the somewhat irregular motions of Uranus

C Ring

By the mid-1800s, astronomers using improved technology detected a faint C ring just inside the B ring.

What is the mass of Jupiter compared to other objects in the solar system?

Twice the mass of all other planets combined

Roche Limit

Distance at which a planet creates tides on a moon's solid surface high enough to pull its moon apart.

What is the typical thickness of Saturn's rings?

2 km

How many large, spherical moons are in orbit around Jupiter?

4

How much more massive would Jupiter need to be in order to generate energy by nuclear fusion in its interior?

75 times more massive

What is the Great Red Spot on Jupiter?

A large, long-lived, counterclockwise rotating storm maintained by high-speed wind & circulation in Jupiter's atmosphere

Why are numerous impact craters found on Ganymede and Callisto but not on Io or Europa?

All of them have had many impacts over the past billions of years. But the debris from Io's active volcanoes has covered old impact craters. Liquid water from inside Europa causes the ice to move, erasing many of the impact craters.

Great Dark Spot

At the time Voyager 2 passed it, a giant storm raged in Neptune's atmosphere. Called the Great Dark Spot, the storm was about half as large as Jupiter's Great Red Spot was at that time. The Great Dark Spot was located at about the same latitude of Neptune & occupied a similar proportion of Neptune's surface as the Great Red Spot does on Jupiter. Although these similarities suggested that similar mechanism created the spots, the Hubble Space Telescope in 1994 showed that the Great Dark Spot had disappeared. Then, in April 1995, another spot developed in the opposite hemisphere.

Why are Uranus & Neptune distinctly bluer than Jupiter & Saturn?

Because Uranus & Neptune both have an atmosphere that contains methane where Jupiter & Saturn are 2/3 hydrogen.

Spoke

Data from spacecraft show that Saturn's magnetosphere contains radiation belts similar to Earth's Van Allen belts. Furthermore, dark spokes move around Saturn's rings; these are believed to be created by electric charges on the ring material interacting with the planet's magnetic field. The magnetic field lifts charged particles out of the plane in which the rings orbit. Spreading the particles out decreases the light scattered from them & therefore makes spokelike features in the rings appear darker. The magnetic field, moving with the planet as it rotates, causes the regions of spread particles to change, making the spokes appear to revolve around the planet.

Ringlet

Debris knocked off the moons Adrastea & Metis is responsible for Jupiter's main ring. The outer edge of this ring has 3 well-defined ringlets that are pieces of debris ranging in size from gravel to small boulders.

A Ring

Dimmer of Saturn's rings. There are 4 moonlets in the A ring.

What is the source of Jupiter's intense magnetic field?

Electric currents in Jupiter's liquid hydrogen layer

How were the rings of Uranus discovered?

From the Earth, when each ring momentarily blocked the light from a background star

Encke Division

Gap that exists in the outer portion of the A ring. Unlike the Cassini division, the Encke division is kept clear because a small moon, Pan, orbits within it. Encke's division is named after the German astronomer Johann Franz Encke (1791-1865), who allegedly saw it in 1838. (Many astronomers have argued that Encke's report was enormous, because his telescope was inadequate to resolve such a narrow gap). The 1st undisputed observation of the 270-km-wide division was made by the American astronomer James Keeler (1857-1900) in the late 1880s, with the newly constructed 36-in. refractor at the Lick Observatory in California. A 40-km space near the outer edge of the bright A ring is now named the Keeler gap in honor of his work.

What are the most abundant gases in the atmosphere of Uranus?

Hydrogen & helium, with traces of methane

Cassini Division

In 1675, Giovanni Cassini (1625-1712) discovered an intriguing feature-a dark division in Saturn's rings. This 5000-km-wide gap, called the Cassini division, separates the dimmer A ring from the brighter B ring, which lies closer to the planet. By the mid-1800s, astronomers using improved telescopes detected a faint C ring just inside the B ring. The Cassini division occurs because the gravitational force of Saturn's moon Mimas combines with the gravitational force from the planet to keep the region clear of debris.

Differential Rotation

In 1690, Giovanni Cassini noticed that the speeds of Jupiter's clouds vary with latitude, an effect called differential rotation. Near the poles, the rotation period of Jupiter's atmosphere, 9 hours 55 minutes 30 seconds, is 5 minutes longer than at the equator. Furthermore, clouds at different latitudes circulate in opposite directions-some eastward, some westward. At their boundaries, the clouds rub against each other, creating beautiful swirling patterns. The interactions of clouds at different latitudes also help provide stability for storms like the Great Red Spot. Jupiter's rapid rotation causes its equatorial radius to be 9284 km (5770 mi) greater than its polar radius. All such rotating worlds, wider at the equator than the poles, are said to be oblate.

Shepherd Satellite (Moon)

In 2010, the Cassini spacecraft observed snowballs as large as 20 km (12 mi) in diameter forming in the F ring. These bodies form as small bits in the F ring debris swirl around & collide with each other under the gravitational attractive force of the passing moon Prometheus, which tugs on them. At other times, large pieces collide & break into smaller ones. Because of their confining influence, Prometheus & Pandora are called shepherd satellites or shepherd moons. Among the most curious features of the F ring is that the ringlets are sometimes braided or intertwined & sometimes separate. Although stabilized by the shepherd moons, the F ring does ripple & shows varying brightness. Astronomers theorize that the changes in brightness result from collisions between clumps of matter in the ring.

Compare & contrast the surface features of the 4 Galilean satellites, discussing their geologic activity & their evolution.

Io's surface is full of active volcanoes while the other 3 have icy surfaces. Europa shows signs of movement which could mean that it has a liquid water interior. Ganymede & Callisto have dark, dust-covered icy surfaces with numerous impact craters.

What characteristic of Saturn's satellite Titan makes it different from any other known satellite in the solar system?

It has a permanent & dense atmosphere.

What characteristic of Jupiter's satellite, Io, makes it different from any other known satellite in the solar system?

It is volcanically active, with gas plumes & lava flows.

Which property of Triton, the largest moon of Neptune, makes it significantly different from all other major moons in the solar system?

It orbits in a direction opposite to the rotation & revolution of its mother planet.

Which is the biggest planet in the solar system?

Jupiter

Which is the most massive planet in the solar system?

Jupiter

Which planet is known to have the most moons?

Jupiter

Describe the appearance of Jupiter's atmosphere. Which features are long-lived & which are relatively fleeting?

Jupiter's atmosphere has many bands with a Great Red Spot & a lot of smaller spots. The belts & zones that comprise the bands are long-lived. The Great Red Spot has existed for 3.5 centuries. The smaller spots are younger & relatively fleeting. There are also auroras & often lightning discharges.

Belt

Jupiter's dark , reddish bands. The sources of the colors of the belts are still under investigation. Gas flowing eastward or westward, with very little north-south rotation & provide a framework for turbulent swirling gas patterns, as well as rotating storms similar in structure to hurricanes or cyclones on Earth. These storms are known as white ovals & brown ovals.

Great Red Spot

Jupiter's most striking feature. It is so large that it can be seen through a small telescope. Hurricanelike or typhoonlike storm of swirling gases that extends at least 320 km (200 mi) below the planet's surface. Heat welling upward from inside Jupiter has maintained it for more than 3.5 centuries. It was 1st observed around 1656, either by the English scientist Robert Hooke (1635-1703) or the Italian astronomer Giovanni Cassini (1625-1712). Because earlier telescopes were unlikely to have been able to see it, the Great Red Spot could well have formed long before that time.

The most massive planetary satellite in the solar system is

Jupiter's satellite Ganymede.

What energy source powers Io's volcanoes?

Jupiter's tidal flexing of Io's crust creates heat that melts & boils subsurface sulfur & puts it under pressure.

What characteristic features are seen on the visible surface of Jupiter?

Light & dark bands parallel to the equator

Zone (Atmospheric)

Light-colored bands in Jupiter's clouds that are white due to ammonia vapor at their tops. Gas flowing eastward or westward, with very little north-south rotation & provide a framework for turbulent swirling gas patterns, as well as rotating storms similar in structure to hurricanes or cyclones on Earth. These storms are known as white ovals & brown ovals.

What is liquid metallic hydrogen? Which planets contain this substance? What produces this form of hydrogen?

Liquid metallic hydrogen is a state of matter for hydrogen, where hydrogen becomes a good electrical conductor. The planets that have this substance are Jupiter & Saturn. The immense pressure & high temperatures of the interiors produce this form of hydrogen.

Polymer

Many of the hydrocarbons & carbon-nitrogen compounds in Titan's atmosphere can form complex compounds called polymers. These molecules are long repeating chains of atoms; rubber, cellulose, & plastics are the best known examples on Earth. Scientists hypothesize that droplets of lighter polymers remain suspended in Titan's atmosphere & form a mist, while heavier polymer particles settle down onto Titan's surface.

Which of the following planets does not have rings?

Mars

Which is the least massive planet in the solar system?

Mercury

The Great Dark Spot (which is not the same as the Great Red Spot) was found on which planet or moon?

Neptune

Is Jupiter a "failed star"? Why or why not?

No, Jupiter has 75 times too little mass to shine as a star.

Is Saturn the only planet with rings?

No. All 4 giant planets (Jupiter, Saturn, Uranus, & Neptune) have rings.

Does Jupiter have continents & oceans?

No. Jupiter is surrounded by a thick atmosphere primarily composed of hydrogen & helium that gradually becomes liquid as you move inward. The only solid matter in Jupiter is its core.

Are the rings of Saturn solid ribbons?

No. Saturn's rings are all composed of thin, closely spaced ringlets consisting of particles of ice & ice-coated rocks. If they were solid ribbons, Saturn's gravitational tidal force would tear them apart.

Retrograde Orbit

Remaining outer moons revolve in the opposite direction that Jupiter rotates.

Why do features in Saturn's atmosphere appear much fainter & "washed out" than comparable features in Jupiter's atmosphere?

Saturn has less than 1/3 the mass of Jupiter. Therefore, there is less surface gravity to drive the flows which create the banded structure.

Explain how shepherd satellites affect some planetary rings. Is "shepherd satellite" an appropriate term for these objects? Explain.

Shepherd moons confine the horizontal extent of parts of the rings. If dust in the ring moves away from one of the shepherd's, its gravitational attraction will pull the dust back toward it. The other shepherd moon will do the same, this pull by each of the shepherd moons keeps the rings between their 2 orbits.

Hydrocarbon

Sunlight interacting with methane induces chemical reactions that produce a variety of other carbon-hydrogen compounds, or hydrocarbons. Spacecraft have detected small amounts of nearly 20 hydrocarbons in Titan's atmosphere, including ethane (C2H6), acetylene (C2H2), ethylene (C2H4), & propane (C3H8).

Prograde Orbit

The Galilean moons, along with the smaller moons closer to Jupiter & 6 of the outer moons, orbit in the same direction that Jupiter rotates.

What is Jupiter's Great Red Spot?

The Great Red Spot is a long-lived, oval cloud circulation, similar to a hurricane or typhoon on Earth.

F Ring

The Voyager cameras sent back the 1st high-quality pictures of the F ring, a thin set of ringlets just beyond the outer edge of the A ring. 2 tiny satellites, Prometheus & Pandora, have orbits on either side of the F ring & serve to keep it intact. The outer of the 2 satellites orbits Saturn at a slower speed than do the ice particles in the ring, as governed by Kepler's 3rd law. As the ring particles pass near it, they receive a tiny, backward gravitational tug, which slows them down, causing them to fall into orbits a bit closer to Saturn. Meanwhile, the inner satellite orbits the planet faster than the F ring particles.

What causes the belts and zones in Jupiter's atmosphere?

The belts & zones result from convection in Jupiter's atmosphere combined with its fast rotation.

The rings of Saturn are in which plane with respect to the planetary system?

The equatorial plane of Saturn

Liquid Metallic Hydrogen

The gravitational force created by Jupiter's enormous mass compresses & heats its interior so much that 20,000 km (12,500 mi) below the cloud tops, the pressure is 3 million atmospheres. Below this depth, the pressure is high enough to transform hydrogen into liquid metallic hydrogen. In this state, hydrogen acts like a metal (similar to the copper wiring in a house) in its ability to conduct electricity & heat.

How do the orientations of Uranus' & Neptune's magnetic axes differ from those of the other planets?

The magnetic fields are not aligned with the rotational axis of the planet & they are not located at the center of the planet.

Describe the structure of Saturn's rings. What are they made of?

The rings are highly concentrated in Saturn's equatorial plane but very broad in the radial direction. Saturn's rings are made up of thousands of ringlets of rocky or icy objects that are less than 10 meters in diameter.

Compare the ring systems of Saturn and Uranus. Why were Uranus' rings unnoticed until the 1970's?

The rings of Saturn are largely composed of various types of ice & are very reflective while the rings of Uranus are very thin & comprised of dark material. Because no one would have expected Uranus to have rings but they were discovered on accident when a star was passing behind the planet.

Briefly describe the evidence that supports the idea that Uranus was struck by a large planet-like object several billion years ago.

The tilt of Uranus's rotation axis suggests that it was struck by a large, planet-like object that knocked it on its axis. The appearances of its 5 largest moons also suggest that shattering impacts occurred on them.

Describe Titan's atmosphere. What effect does sunlight have on it?

Titan has a thick, dense atmosphere that has nitrogen, methane, & other hydrocarbons. The sunlight interacting with methane has produced more complicated hydrocarbons.

Describe Titan's surface.

Titan is an icy moon with a surface of rock-hard water ice, but Titan also likely has a liquid water ocean beneath its surface. It has cold temperatures & a thick & hazy atmosphere.

Galilean moon (Satellite)

To the modern astronomer, Jupiter's moons are 4 extraordinary worlds, different both from the rocky terrestrial planets & from hydrogen-rich Jupiter. Now called collectively the Galilean moons or Galilean satellites, they are named after the mythical lovers & companions of the Greek god Zeus (called Jupiter by the Romans). From the innermost moon outward, they are Io, Europa, Ganymede, and Callisto. These 4 worlds were photographed extensively by the Voyager 1, Voyager 2, & Cassini flybys & by the orbiting Galileo & Juno spacecraft. The 2 inner Galilean satellites, Io & Europa, are about the same size as our Moon. The 2 outer satellites, Ganymede & Callisto, are roughly the size of Mercury.

Explain why Triton will never collide with Neptune, even though Triton is spiraling toward that planet.

Triton will never collide with Neptune because Triton will be torn apart when it reaches Neptune's Roche limit.

Which planet has its rotation axis tilted at 98° from the vertical to its orbit?

Uranus

Describe the seasons on Uranus. Why are the Uranian seasons different from those on any other planet?

Uranus spins on its axis so the north pole points at the sun & the northern hemisphere gets all day light for 20 years. Then, it gets equal days & nights for 20 years. Then, the south pole faces the sun & it gets 20 years of continual light. Finally, equal days & nights for another 20 years.

Resonance

Whenever matter drifts into the Cassini division, Mimas (orbiting at a different rate than the matter in the Cassini division) periodically exerts a force on this matter, eventually pulling most of it out of the division. This effect is called a resonance & is similar to what happens when you repeatedly push someone on a swing at the right time, giving them energy that enables them to go higher & higher. Despite the resonance keeping the Cassini division relatively clear of larger (boulder-sized) debris, it is filled with dust-sized particles.

What are the light-colored bands on Jupiter called?

Zones

Europa, one of the Galilean satellites of Jupiter, has a surface consisting of

a relatively young, icy crust covered with a network of streaks and cracks and only a few impact craters.

The physical structure of Saturn's rings is

a sequence of many thousands of separate ringlets consisting of ice blocks & ice-coated rock.

Triton, the largest satellite of Neptune, has

a surface of ice with frozen lakes, plumes of escaping gas, & few craters.

The dark, reddish bands on Jupiter are called

belts.

Detailed observations of Jupiter's rotation suggest that

it is not a rigid object because equatorial regions rotate faster than polar regions.

"Markings" on the surface of Saturn are

similar in appearance to those on Jupiter, but much less distinct.