1404 Lesson 10 Article 4 Uranus and Neptune
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Article 4: Uranus and Neptune
Discovering Uranus and Neptune
As we reach the edge of the solar system, we encounter the planets Uranus and Neptune and the smaller objects like Pluto, Eris, Sedna, the comets, and many other unusual parts of the solar system. The planets discussed so far are all visible with the naked eye and have been known since antiquity. Uranus was discovered by chance in 1781 by William Herschel, an amateur astronomer
who lived in Bath, England. By profession William was a musician, but his father had interested his children in astronomy. William and his siblings began making telescopes and using them in the small garden of their modest home. On the night of March 13, 1781, William spotted an object whose orbit showed it to be a planet at a distance of 20 AU. Previously, Saturn was the furthest known object at 10 AU, so Herschel's discovery essentially doubled the size of the known solar system. After many suggested names were discarded, the planet was finally named Uranus by the German astronomer Johann Bode in 1850.
William Herschel's garden (left) and telescope (right). Photos courtesy of Anahita Sidhwa
As astronomers began to track Uranus, they noticed its orbit did not quite match the predictions by Newton’s laws. In 1846, Urban Leverrier calculated the position of an eighth planet, which was confirmed by observations from the Berlin Observatory on September 23, 1846. This was Neptune, which is often called “the planet discovered by mathematics” rather than by observation.
Physical Characteristics
Much of what we do know today about Uranus and Neptune has come from the voyages of the twentieth century spacecrafts, Pioneer 10 and 11 and Voyager 2. New 1
Horizons also sent back images on its way to Pluto, but it was further away from the planets than Voyager 2.
Uranus and Neptune are similar in many respects. Each has a radius about four times the size of Earth. While Uranus is slightly larger, Neptune has more mass. The density of Uranus is 1.3 g/cm
3
and that of Neptune is 1.6 g/cm
3
. Both are ice giants, but not as huge as Jupiter or Saturn. They do not have the varied hydrogen layers, and it is possible they did not grow as big as Jupiter or Saturn because they were unable to attract and hold as much hydrogen and helium. While Uranus can barely be seen with the naked eye, Neptune can only be viewed with a telescope. Uranus appears as a greenish-blue disk, whereas Neptune is a definite blue.
The most dramatic thing about Uranus is its strange mode of rotation. While Earth's axis
is tilted at 23.5 degrees from the vertical, and Neptune's is 30 degrees from vertical, Uranus' axis lies at 98 degrees! Think for a moment about what this implies. Uranus' axis lies in the plane of its orbit, or put another way, Uranus rotates on its side! When Voyager 2 flew past the planet in 1986, the south polar region pointed directly at the Sun, but with New Horizons two decades later, the equatorial region was favored. Since
Uranus takes 84 years to make one orbit around the Sun, its polar regions experience the same season for 42 years!
Atmosphere
The color of these planets is due to the amount of methane in the atmosphere, in addition to the usual hydrogen and helium. Methane absorbs the red wavelengths, hence sunlight reflected from these planets is deficient in red. Neptune appears bluer because of having more methane that absorbs more red photons. Gaseous ammonia, which is significant on Jupiter and Saturn, is absent here because the temperatures are cooler than the freezing point of ammonia.
Uranus presented a smooth, featureless appearance to Voyager, indicating its atmosphere had little or no convective motion. This could be due to not having an 2
internal source of energy like Jupiter, Saturn, and Neptune. The cause of Neptune’s internal energy may be gravitational collapse, but not enough data exists to confirm this.
It is interesting to note that despite Uranus’ strange rotation, its wind patterns are still parallel to its equator. And although the planet presents a featureless exterior, the overall temperatures are quite uniform, suggesting there is an efficient means of heat transfer operating in the atmosphere.
In the Voyager photographs dating to 1989, Neptune showed a Great Dark Spot, like Jupiter’s Great Red Spot, in the southern hemisphere. However, this feature has now disappeared, showing the dynamic nature of the atmosphere.
Check Your Understanding
1.
Which planet experiences winter for more than forty years?
a.
Jupiter
b.
Saturn
c.
Uranus d.
Neptune
2.
Which of the following proves that the atmosphere of the Jovian planet changes over
time? a.
Uranus’ atmosphere shows colored bands that change.
b.
Uranus’ magnetic field varies from location to location.
c.
Neptune has dark rings.
d.
Neptune’s great dark spot seen in 1989 has now disappeared.
3.
Which planet has more methane? What is the evidence for this? a.
Uranus, because it rotates on its side.
b.
Uranus, because it does not show any clouds.
c.
Neptune, because it is bluer than Uranus, and methane absorbs blue light.
d.
Neptune, because it is bluer than Uranus, and methane absorbs red light.
Internal Structure and Magnetic Field Uranus and Neptune do not have as many interior layers as Jupiter and Saturn. They probably have a core of rock and metals surrounded by a slushy interior of hydrogen compounds like water, methane, and ammonia, and an exterior layer of molecular hydrogen.
Select link to view the Internal Structure of the Jovian Planets.
Uranus and Neptune are not large enough to produce the kind of internal pressure needed to form metallic hydrogen. Although they do have a magnetic field, it is not as 3
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strong as Jupiter’s. It has been suggested that the magnetic field may be generated at an intermediate depth where the pressure is high enough for water to become conducting.
The orientation of Uranus and Neptune's fields is very strange. Earth, Jupiter, and Saturn have their magnetic axis roughly parallel to the rotation axis. The angle between the magnetic axis and rotation axis for Uranus is 60 and for Neptune it is 55 ! The field ⁰
⁰
also varies across the planet. Uranus probably experienced a catastrophic event that knocked it into the strange rotation we see. Could this same event be responsible for the unusual orientation of its magnetic field?
Select link to view Magnetic Fields of Jovian Planets.
Discovering Rings by Occultation
The Latin word "occultare" means "to conceal." During a solar eclipse, the Moon occults
or conceals the Sun. Planets can similarly cover up our view of stars as they travel along their orbit. Careful study of the star's light as the planet approaches it, covers it up, and then uncovers it gives useful information about the planet's atmosphere, and rings if any.
If the star's light begins to dim gradually, it suggests the planet has an atmosphere. In this case, the starlight is passing through the planet's atmosphere, which will absorb definite wavelengths, depending on the atmospheric chemicals. By analyzing the absorption spectrum from the star, astronomers can deduce the nature and concentration of the chemicals present in the planet's atmosphere. Also, by measuring the amount by which the starlight is dimmed, calculations can be made regarding the depth of the atmosphere. If the planet has a negligible atmosphere, the star's light will not be dimmed as the planet approaches, but the star will blink off abruptly when the edge of the planetary disk covers it up.
Occultation can also be used to measure the diameter of planets. By plotting the position of the planet on successive days with respect to the background stars, its revolution speed can be calculated. Rotation speed can be found from the Doppler shifts described previously. By timing how long an occultation lasts, the size of the planetary disk can be determined.
Uranus' rings were discovered accidentally when it occulted a star in 1977. Astronomers
set up their instruments to measure Uranus' size and upper atmosphere. Since the precise time for the occultation was not known, all instruments were kept ready well in advance. Several photometer pairs were to be used, each pair recording the red light from the red star and the blue light from Uranus. Graphs representing the blue and red output of each pair were to be compared with each other. As Uranus moved in front of the star, it was expected that the red output from the star would dip whereas the blue output from Uranus would remain steady. Surprisingly, the red output dipped sharply long before Uranus covered up the star. Then the red graph went back to its original position, dipped sharply again, went back to original and so on, nine times before Uranus covered up the star. Clearly, there was something present around Uranus that 4
was blocking the starlight. When the dips and rises repeated in reverse order after the occultation, astronomers were convinced they had the first evidence for rings around Uranus. The presence of these and two more dark, sooty rings, which are extremely thin, was confirmed by the Voyager flyby. The rings lie in the equatorial plane of Uranus like they do for Saturn.
Image of Uranus from the Keck Telescope shows several storms (the bright blotches) and Uranus’s thin rings.
Neptune is also surrounded by rings, but they are quite dark and diffuse and incomplete. Jupiter has thin rings, first imaged by Voyager. Thus, rings and strange moons seem to be a signature for all the Jovian planets.
Select link to view Rings of the Jovian Planets.
The Moons of Uranus and Neptune
Uranus has five medium-sized moons with icy surfaces that show signs of volcanism, tectonics, and cratering. The composition is likely to be ammonia, methane, and water ice. Their names are Miranda, Ariel, Umbriel, Titania, and Oberon. Sound familiar? They
are names of characters in Shakespeare’s plays!
The five moons of Uranus
The most significant moon of Neptune is Triton, one of the larger satellites but smaller than our Moon. Unlike other moons, it orbits Neptune in the clockwise direction at a high
angle to its equator. It could be a Kuiper belt object that has been captured by Neptune. An interesting consequence of its retrograde orbit is that it is spiraling in toward Neptune
and will be torn apart to create a ring around it in 100 million years or so!
Triton shows an interesting “cantaloupe-like” terrain, which suggests recent geological activity, and a thin atmosphere, probably from the “nitrogen geysers” detected by Voyager 2 that erupt from its surface.
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Our solar system has many varied inhabitants, and we have much to look forward to as we unravel the mysteries present in our neighborhood.
Check Your Understanding
4.
Which planet has the strongest magnetic field?
a.
Jupiter
b.
Saturn
c.
Uranus
d.
Neptune
5.
Uranus’ rings were discovered by occultation. What else can be found by using this method? a.
The planet’s magnetosphere b.
The chemicals in the planet’s atmosphere
c.
The chemicals in the planet’s core
d.
The planet’s axial tilt
6.
To which planet does Miranda belong? a.
Jupiter
b.
Saturn
c.
Uranus
d.
Neptune Answers to Check Your Understanding
1.
c
Feedback for a, b, d: What is the axial tilt of this planet? Feedback for c: Correct, this happens because its axial tilt is more than 90 , so it rotates
⁰
on its side. 2.
d Feedback for a: This statement is false.
6
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Feedback for b, c: While the statement is true, it is not the reason for the changing atmosphere.
Feedback for d: Correct. This shows the atmosphere changes. 3.
d Feedback for a, b: While the statement is true it is not related to presence of methane.
Feedback for c: Review the wording carefully. Feedback for d: Correct. Since more red light is absorbed, the planet reflects more blue
light. 4.
a
Feedback for a: correct. Feedback for b, c, d: Review the extent of each planet’s magnetosphere.
5.
b Feedback for a, c, d: Review what is meant by occultation and how it is used by astronomers.
Feedback for b: Correct 6.
c Feedback for a, b, d: Review the names of the major moons for each planet.
Feedback for c: Correct. Remember Uranus’ moons are named for Shakespearean characters. 7