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p355-6 1-20
1. How was Uranus discovered?
When you’re alone in the bathtub as a little kid…. (KIDDING!) Uranus was discovered by British astronomer
William Herschel in 1781, He was charting the faint stars in the sky at the time.
2. Why did astronomers suspect an eighth planet beyond Uranus?
Small but measurable discrepancies in Uranus’s orbit mean there must have been SOME object exerting a
gravitational force on Uranus.
3. How did astronomers determine where to look for Neptune?
In 1845 Englishman John Adams and (in 1846) Frenchman Urbain Leverrier independently calculated where
Neptune should be based on perturbations in Uranus’s orbit. German Johann Galle was the first person to see
Neptune and know what it was. (We can see from Galileo’s notes that he probably observed Neptune and
thought it was just another fuzzy star.)
4. How did Uranus come to be spinning on its “side?”
No one knows. Astronomers speculate about a catastrophic impact (as they do about a similar hypothetical
impact that knocked almost all rotational momentum out of Venus, but one wonders about the difference
between an impact strong enough to knock the planet almost 100˚ on its side, and an impact strong enough
simply to obliterate the planet.)
5. What is responsible for the overall colors of Uranus and Neptune?
Both planets atmospheres are dominated by H2 and He (as Jupiter and Saturn’s atmospheres are) but Uranus
and Neptune have quite a bit more CH4 (methane), 2% and 3% respectively. As CH4 absorbs longer
wavelengths of light (the red end of spectrum) it follows that that the planets would show colors of
shorter wavelengths. Neptune has 50% more methane Uranus has, so it’s noticeably bluer.
6. Why are storms and atmospheric features more easily seen on Neptune than on Uranus?
Uranus is slightly larger in size than Neptune, but Neptune has more mass* and stronger gravity than Uranus.
So the clouds that make up storms and atmospheric features are blanketed by LESS foggy gas layers, as
Uranus’s clouds are. Think of it this way, Neptune’s grater gravity pulls the foggy layers in tighter so they are
thinner, and you can see the cloud-tops more easily.
*If you ever see Neptune listed as the 7th planet and Uranus as the 8th – as I did once in a science museum of all
places – it is most likely because whomever arranged the planets did so by mass and not S-M axis (distance
from sun) as is customary. (NOTE: Many people see the phrase “more massive” and equate it with “being
bigger,” which is OK colloquially/conversationally for the layperson, but not for scientists. Technically
speaking “more massive” means “has more mass” which you can read as “has more molecules” or even
“weighs more” (which, though “weighs more” is also technically incorrect, it’s much closer to the true meaning
of “more massive” than “bigger” is!)
7. How are the interiors of Uranus and Neptune thought to differ from those of Jupiter and Saturn?
Theoretical models indicate that Uranus and Neptune have rocky cores similar to those found in Jupiter and
Saturn—about the size of Earth and perhaps 10 times more massive. However, the pressure outside the cores of
Uranus and Neptune is too low to force hydrogen into the metallic state, so hydrogen stays in its molecular form
all the way in to the planets' cores. Astronomers theorize that deep below the cloud layers, Uranus and Neptune
may have high-density, "slushy" interiors containing thick layers of water clouds.
8. How do the magnetic fields of Uranus and Neptune compare with Earth’s?
Voyager 2 found that both Uranus and Neptune have fairly strong internal magnetic fields—about 100 times
stronger than Earth's field and 1/10 as strong as Saturn's. However, because the radii of Uranus and Neptune
are so much larger than the radius of Earth, the magnetic fields at their cloud tops are actually comparable in
strength to Earth's field. ALSO, their magnetic fields are inclined much more to their respective rotational axes:
Uranus 60˚ ; Neptune 46˚; Earth 11˚
9. Describe a day on Titania.
Unimaginably cold. Even if you were in Titania’s northern hemisphere during Uranian summer when the sun is
up for over four consecutive decades.
10. What is unique about Miranda? Give a possible explanation.
Miranda displays a wide range of surface features: ridges, valleys, large oval faults. This might be because
Miranda was obliterated by an asteroid impact long ago and the pieces came back together in a jumbled mass.
11. Why are the icy moons of the outer planets so dark?
Astronomers currently favor the “radiation darkening” theory: solar radiation reacts with the icy surfaces of
these moons to form a sooty layer of organic (carbon based) molecules.
12. 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?
Neptune has no moon “system” to speak of. None of its moons’ orbits are even generally circular, nor do they
orbit in Neptune’s equatorial plane. It’s biggest moon Triton has a retrograde orbit.
13. What causes Triton’s geysers?
Subsurface liquid nitrogen that is somehow warmed and vaporized, and ejected through fissures in Triton’s
crust.
14. What is the predicted fate of Triton?
Grim. Its retrograde orbit induces counter-rotational tidal bugles in Neptune’s surface which infinitesimally
slows Neptune’s rotation, but markedly slows Triton in its orbit, and as we know, orbiting bodies that slow
down do not remain in orbit for long. Triton probably has a hundred million years until it crosses
Neptune’s Roche limit and is shredded. (It has been proposed that by that time Saturn’s ring system may have
destabilized and shrunk, and the new Neptunian ring system created by Triton’s remains could take over as the
most spectacular ring system in the solar system.)
15. How were the rings of Uranus discovered?
During a stellar occultation in 1977: Neptune passed in front of a star. Scientists noticed the star’s light dim the
littlest bit just before it went behind Uranus, and then again just after it emerged. Scientists figured, correctly,
that this dimming was caused by a ring system slightly blocking/dimming the star.
16. How were the rings of Neptune discovered?
A stellar occultation hinted at the presence of rings around Neptune, but their existence was only confirmed
during the Voyager 2 flyby in 1989.
17. The rings of Uranus are dark, narrow, and widely spaced. Which of these properties makes them
different from the rings of Saturn?
All three. Saturn's rings are bright and wide with relatively narrow gaps in between.
18. Why are the Uranian rings so narrow and sharply defined?
They are “corralled” by shepherd moons.
19. How do the rings of Neptune differ from those of Uranus and Saturn?
Neptune’s rings are similar to Uranus’s but narrower and darker, and there are fewer of them. (One of
Neptune’s rings isn’t even a complete ring but a collection of ring sections.)
20. Why was the discovery of Uranus in 1781 so surprising? Might there be similar surprises in store for
today's astronomers?
Uranus was the first planet discovered in over 2000 years so it was quite a big deal. (No one was looking for it
- no one had any reason to believe there could be more planets, so Uranus’s discovery really shook peoples’
idea of the constancy of reality. It was the first of many humbling blows to human arrogance. When you
consider how little we know of the outer planets – to say nothing of the Kuiper Belt objects and other TransNeptunian Objects, it seems as if what we do know is quite a bit less that what we don’t know, an idea scientists
sort of take for granted now, but not back then when many serious thinkers actually thought they were getting
close to knowing everything there was to know.