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Title: The Milky Way Author: Markus Boettcher Last modified by: Instructor Created Date: 2/23/2003 4:28:17 AM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Note%20that%20the%20following%20lectures%20include%20animations%20and%20PowerPoint%20effects%20such%20as%20fly%20ins%20and%20transitions%20that%20require%20you%20to%20be%20in%20PowerPoint's%20Slide%20Show%20mode%20(presentation%20mode).


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Uranus, Neptune, and Pluto
  • Chapter 24

3
Guidepost
In the three previous chapters, we have used our
tools of comparative planetology to study other
worlds, and we continue that theme in this
chapter. A second theme running through this
chapter is the nature of astronomical discovery.
Unlike the other planets in our solar system,
Uranus, Neptune, and Pluto were discovered, and
the story of their discovery helps us understand
how science progresses. As we probe the outer
fringes of our planetary system in this chapter,
we see strong evidence of smaller bodies that
fall through the solar system and impact planets
and satellites. The next chapter will allow us to
study these small bodies in detail and will give
us new evidence that our solar system formed from
a solar nebula.
4
Outline
I. Uranus A. The Discovery of Uranus B. The
Motion of Uranus C. The Atmosphere of Uranus D.
The Interior of Uranus E. The Rings of
Uranus F. The Moons of Uranus G. A History of
Uranus II. Neptune A. The Discovery of
Neptune B. The Atmosphere and Interior of
Neptune C. The Rings of Neptune D. The Moons of
Neptune E. The History of Neptune
5
Outline (continued)
III. Pluto A. The Discovery of Pluto B. Pluto
as a Planet C. The Origin of Pluto and Charon
6
Uranus
Chance discovery by William Herschel in 1781,
while scanning the sky for nearby objects with
measurable parallax discovered Uranus as
slightly extended object, 3.7 arc seconds in
diameter.
7
The Motion of Uranus
Very unusual orientation of rotation axis Almost
in the orbital plane.
97.9o
19.18 AU
Possibly result of impact of a large planetesimal
during the phase of planet formation.
Large portions of the planet exposed to eternal
sunlight for many years, then complete darkness
for many years!
8
The Atmosphere of Uranus
Like other gas giants No surface.
Gradual transition from gas phase to fluid
interior.
Mostly H 15 He, a few Methane, ammonia and
water vapor.
Optical view from Earth Blue color due to
methane, absorbing longer wavelengths
Cloud structures only visible after artificial
computer enhancement of optical images taken from
Voyager spacecraft.
9
The Structure of Uranus Atmosphere
Only one layer of Methane clouds (in contrast to
3 cloud layers on Jupiter and Saturn).
3 cloud layers in Jupiter and Saturn form at
relatively high temperatures that occur only very
deep in Uranus atmosphere.
Uranus cloud layer difficult to see because of
thick atmosphere above it.
Also shows belt-zone structure
? Belt-zone cloud structure must be dominated by
planets rotation, not by incidence angle of sun
light!
10
Grade distribution fall 2011
11
Cloud Structure of Uranus
Hubble Space Telescope image of Uranus shows
cloud structures not present during Voyagers
passage in 1986.
? Possibly due to seasonal changes of the cloud
structures.
12
The Interior of Uranus
Average density 1.29 g/cm3 ? larger portion of
rock and ice than Jupiter and Saturn.
Ices of water, methane, and ammonia, mixed with
hydrogen and silicates
13
The Magnetic Field of Uranus
No metallic core ? no magnetic field was expected.
But actually, magnetic field of 75 of Earths
magnetic field strength was discovered
Offset from center 30 of planets radius!
Inclined by 60o against axis of rotation.
Possibly due to dynamo in liquid-water/ammonia/met
hane solution in Uranus interior.
Magnetosphere with weak radiation belts allows
determination of rotation period 17.24 hr.
14
The Magnetosphere of Uranus
Rapid rotation and large inclination deform
magnetosphere into a corkscrew shape.
UV images
During Voyager 2 flyby Southpole pointed towards
sun direct interaction of solar wind with
magnetosphere ? Bright aurorae!
15
Uranuss Ring Detection
16
The Rings of Uranus
Rings of Uranus and Neptune are similar to
Jupiters rings.
Confined by shepherd moons consist of dark
material.
Rings of Uranus were discovered through
occultations of a background star
Apparent motion of star behind Uranus and rings
17
The Rings of Neptune
Ring material must be regularly re-supplied by
dust from meteorite impacts on the moons.
Interrupted between denser segments (arcs)
Made of dark material, visible in
forward-scattered light.
Focused by small shepherd moons embedded in the
ring structure.
18
The Moons of Uranus
5 largest moons visible from Earth.
10 more discovered by Voyager 2 more are still
being found.
Dark surfaces, probably ice darkened by dust from
meteorite impacts.
5 largest moons all tidally locked to Uranus.
19
Interiors of Uranuss Moons
Large rock cores surrounded by icy mantles.
20
The Surfaces of Uranuss Moons (1)
Oberon
Titania
Largest moon
Old, inactive, cratered surface,
but probably active past.
Heavily cratered surface, but no very large
craters.
Long fault across the surface.
Active phase with internal melting might have
flooded craters.
Dirty water may have flooded floors of some
craters.
21
The Surfaces of Uranuss Moons (2)
Umbriel
Ariel
Dark, cratered surface
Brightest surface of 5 largest moons
Clear signs of geological activity
No faults or other signs of surface activity
Crossed by faults over 10 km deep
Possibly heated by tidal interactions with
Miranda and Umbriel.
22
Uranuss Moon Miranda
Most unusual of the 5 moons detected from Earth
Ovoids Oval groove patterns, probably associated
with convection currents in the mantle, but not
with impacts.
20 km high cliff near the equator
Surface features are old Miranda is no longer
geologically active.
23
Neptune
Discovered in 1846 at position predicted from
gravitational disturbances on Uranuss orbit by
J. C. Adams and U. J. Leverrier.
Blue-green color from methane in the atmosphere
4 times Earths diameter 4 smaller than Uranus
24
The Atmosphere of Neptune
The Great Dark Spot
Cloud-belt structure with high-velocity winds
origin not well understood.
Darker cyclonic disturbances, similar to Great
Red Spot on Jupiter, but not long-lived.
White cloud features of methane ice crystals
25
The Moons of Neptune
Two moons (Triton and Nereid) visible from Earth
6 more discovered by Voyager 2
Unusual orbits
Triton Only satellite in the solar system
orbiting clockwise, i.e. backward.
Nereid Highly eccentric orbit very long orbital
period (359.4 d).
26
The Surface of Triton
Very low temperature (34.5 K)
? Triton can hold a tenuous atmosphere of
nitrogen and some methane 105 times less dense
than Earths atmosphere.
Surface composed of ices nitrogen, methane,
carbon monoxide, carbon dioxide.
Possibly cyclic nitrogen ice deposition and
re-vaporizing on Tritons south pole, similar to
CO2 ice polar cap cycles on Mars.
Dark smudges on the nitrogen ice surface,
probably due to methane rising from below
surface, forming carbon-rich deposits when
exposed to sun light.
27
The Surface of Triton (2)
Ongoing surface activity Surface features
probably not more than 100 million years old.
Large basins might have been flooded multiple
times by liquids from the interior.
Ice equivalent of greenhouse effect may be one of
the heat sources for Tritons geological activity.
28
Pluto
Discovered 1930 by C. Tombaugh.
Existence predicted from orbital disturbances of
Neptune, but Pluto is actually too small to cause
those disturbances.
29
Pluto as a Planet
Virtually no surface features visible from Earth.
65 of size of Earths Moon.
Highly elliptical orbit coming occasionally
closer to the sun than Neptune.
Orbit highly inclined (17o) against other
planets orbits
? Neptune and Pluto will never collide.
Surface covered with nitrogen ice traces of
frozen methane and carbon monoxide.
Daytime temperature (50 K) enough to vaporize
some N and CO to form a very tenuous atmosphere.
30
Plutos Moon Charon
Discovered in 1978 about half the size and 1/12
the mass of Pluto itself.
Tidally locked to Pluto.
Hubble Space Telescope image
31
Pluto and Charon
Orbit highly inclined against orbital plane.
From separation and orbital period Mpluto 0.2
Earth masses.
Density 2 g/cm3 (both Pluto and Charon)
? 35 ice and 65 rock.
Large orbital inclinations ? Large seasonal
changes on Pluto and Charon.
32
The Origin of Pluto and Charon
Probably very different history than neighboring
Jovian planets.
Older theory
Pluto and Charon formed as moons of Neptune,
ejected by interaction with massive planetesimal.
Mostly abandoned today since such interactions
are unlikely.
Modern theory Pluto and Charon members of Kuiper
belt of small, icy objects (see Chapter 25).
Collision between Pluto and Charon may have
caused the peculiar orbital patterns and large
inclination of Plutos rotation axis.
33



A FEW REMARKS ABOUT RINGS Mercury, Venus, Earth
and Mars have no rings. Pluto?.. Who
knows?? Saturn was first with rings and most
prominent one for ring structure. See your
textbook for more detail about the rings of
Saturn observed by Voyager I. Jupiter-Weak ring
structure, not visible from earth. Uranus-Eleven
rings identified. Rings discovered by accident
as light patterns did some funny things when it
was observed. Neptune-Four rings located and
named. See textbook for more details. How do we
discover rings? Two ways Direct observation,
as for Saturn Occultation (dimming) of light from
some light source such as a star as the field of
view is changed.
34
  1. MERCURY-COLD, LIFELESS, CRATEREDNO ATMOSPHERE
  2. VENUS-CLOUD COVERED, EARTH LIKE IN MANY WAYS,
    ANCIENT OBJECT OF WORSHIP AND RESPECT
  3. EARTH-HA HA!!!!!
  4. MARS-RED PLANET, OBJECT OF WORSHIP WAR GOD,
    DEIMOS AND PHOBOS SATELLITES, LITTLE ATMOSPHERE
  5. ASTEROIDS-CLUSTERS OF DEBRIS IN ORBIT ABOUT THE
    SUN, NOT ENOUGH TO MAKE A GOOD SIZED PLANET
  6. JUPITER BIGGEST PLANET, DARK RED SPOT, WHITE
    SPOTS, BANDED, GALILEAN SATELLITES
  7. SATURN-RING STRUCTURE, WELL OBSERVED AND
    OBSERVABLE FROM EARTH
  8. URANUS-FOUND BY ACCIDENT BY GERMAN MUSICIAN
    HERSCHELL
  9. NEPTUNE-FOUND BY LAWS OF CELESTIAL MECHANICS,
    GREAT DARK SPOT
  10. PLUTO-FOUND BY HARD WORK, PREDICTED BY LAWS OF
    MECHANICS BUT A LUCKY GUESS TO FIND IT

35
New Terms
occultation ovoid  
36
Discussion Questions
1. Why might it be unfair to describe William
Herschels discovery of Uranus as accidental? Why
might it be unfair to describe the discovery of
the rings of Uranus as accidental? 2. Suggest a
single phenomenon that could explain the
inclination of the rotation axis of Uranus, the
orbits of Neptunes satellites, and the existence
of Plutos moon.
37
Quiz Questions
1. How do the seasons on Uranus differ from
seasons on Earth? a. Seasons on Uranus are 84
times longer and more extreme than on Earth. b.
Seasons on Uranus are 84 times longer and less
extreme than on Earth. c. Seasons on Uranus are
21 times longer and more extreme than on
Earth. d. Seasons on Uranus are 21 times longer
and less extreme than on Earth. e. Seasons on
Uranus are longer, more extreme, and in reverse
order of the seasons on Earth.
38
Quiz Questions
2. What is our current best hypothesis as to how
the whole Uranian system came to have such a
large inclination? a. A large impact during the
latter stages of planet building tipped Uranus on
its side. b. Tidal interactions between Uranus
and the other Jovian planets pulled Uranus onto
its side. c. Magnetic interactions between the
Sun and Uranus flipped Uranus onto its side. d.
Uranus formed outside of the Solar System and was
captured later. e. The slow rate of rotation of
Uranus gives it such little stability that its
rotation axis precesses wildly.
39
Quiz Questions
3. Both Uranus and Neptune have a blue-green tint
when observed through a telescope. What does
this tell you about their composition? a. Their
atmospheres are composed of mostly hydrogen and
helium. b. Their atmospheres are composed of
mostly carbon dioxide. c. Their atmospheres are
composed of mostly nitrogen. c. Their atmospheres
contain some ammonia. e. Their atmospheres
contain some methane.
40
Quiz Questions
4. How do we get an accurate measurement of the
rotational period of Uranus? a. We measure the
time for one orbit of a dark spoke in the
rings. b. We measure the time for the Great Dark
Spot to travel once around. c. We measure the
time for a particular cloud to rotate once around
the planet. d. We measure the time from one
opposition of Uranus to the next opposition of
Uranus. e. We measure the period of the cyclic
fluctuation in the synchrotron radiation emitted
by Uranus.
41
Quiz Questions
5. In the atmospheres of Jupiter and Saturn we
see ammonia, ammonia hydrosulfide, and water
clouds in three distinct layers. Why don't we
see these same three cloud layers in the
atmospheres of Uranus and Neptune? a. Farther
from the Sun it is too cold for these three
layers of clouds to form. b. These three layers
are likely hidden beneath a higher layer of
methane clouds. c. These chemicals are not
present in the atmospheres of Uranus and
Neptune. d. These condensates form one layer in
the atmospheres of Uranus and Neptune. e. Uranus
and Neptune have no atmosphere.
42
Quiz Questions
6. Which interior zone of Uranus and Neptune do
we suspect contains the electrically conducting
fluid that is responsible for planetary magnetic
fields? a. The zone of liquid water with
dissolved ammonia and methane. b. The liquid
metallic hydrogen zone. c. The liquid
hydrogen-helium zone. d. The liquid outer iron
core. e. The heavy element core.
43
Quiz Questions
7. In what way is Uranus different than the other
Jovian planets? a. Uranus has no rings. b.
Uranus has no moons. c. Uranus has a higher
density. d. Uranus has no metallic hydrogen. e.
Uranus has little remaining heat of formation.
44
Quiz Questions
8. Why is there no liquid metallic hydrogen zone
in the interior of Uranus or Neptune? a. The
temperature is not low enough for hydrogen to
become a superconductor. b. The hydrogen does not
contain sufficient amounts of deuterium. c.
Uranus and Neptune do not contain hydrogen and
helium. d. The pressure is too low for hydrogen
to be metallic. e. No fusion occurs in Uranus and
Neptune.
45
Quiz Questions
9. How did Uranus and Neptune come to have less
hydrogen and helium than Jupiter and Saturn? a.
The mass fraction of light elements like hydrogen
and helium in the solar nebula decreases with
distance from the Sun. b. Much of their original
hydrogen and helium was stripped away by the
gravity of passing stars. c. Much of their
original hydrogen and helium was ionized and
stripped away by the solar wind. d. Much of their
original hydrogen and helium was ionized and
stripped away by interstellar winds. e. Uranus
and Neptune took longer to form than Jupiter and
Saturn.
46
Quiz Questions
10. What difference in the rings of Uranus and
Neptune was first revealed in observations from
Earth-based telescopes? a. The clumpy ring arcs
of Neptune. b. The difference in the albedo of
the ring particles. c. The difference in the size
of the ring particles. d. The elemental
composition of the ring particles. e. The
difference in the dust-to-ice ratio of ring
particles.
47
Quiz Questions
11. What evidence indicates that the rings of
Uranus have little dust and the rings of Neptune
contain a lot of dust? a. The camera lens of
Voyager 2 was dust-free until it passed through
the rings of Neptune. b. The rings of Uranus
appear bright in forward-scattered light, and the
rings of Neptune appear dark in forward-scattered
light. c. The rings of Uranus appear dark in
forward-scattered light, and the rings of Neptune
appear bright in forward-scattered light. d. The
rings of Uranus appear bright in back-scattered
light, and the rings of Neptune appear dark in
back-scattered light. e. The rings of Uranus
appear dark in back-scattered light, and the
rings of Neptune appear bright in back-scattered
light.
48
Quiz Questions
12. How does a thin planetary ring retain its
shape? a. The tidal force of the planet on the
ring particles keeps them together. b. The
magnetic field of the planet traps the ring
particles in a well-defined orbit. c. Small
moons orbiting just inside and outside the rings
shepherd the ring particles. d. The gravitational
attraction of the ring particles on one another
keeps the ring together. e. The electrostatic
attraction of the ring particles on one another
keeps the ring together.
49
Quiz Questions
13. What keeps small shepherd moons from breaking
apart within the Roche Limit of a planet? a.
Gravitational attraction of the moon's
material. b. Gravitational attraction by the ring
particles. c. Electrostatic bonds of the moon's
material. d. Gravitational attraction of larger
moons. e. Tidal forces by the planet.
50
Quiz Questions
14. The discoveries of Uranus, Neptune, and Pluto
all came long after the death of Isaac Newton.
How was Newton involved in the discovery of a new
planet? a. It was the application of Newtonian
gravity to the problem of the orbit of Uranus
that led to the discovery of Neptune. b. It was
with a reflecting telescope (the type invented by
Newton) that the planet Uranus was discovered. c.
It was through perceived perturbations by
Newtonian gravity on the orbit of Neptune that a
search for a ninth planet was begun, which
eventually resulted in the discovery of Pluto. d.
Both b and c above. e. All of the above.
51
Quiz Questions
15. We could divide the Jovian planets into two
subclasses, the Gas Giants and the Ice Giants.
Into which groups should we place the four Jovian
planets? a. The Gas Giants are Uranus Neptune,
and the Ice Giants are Jupiter Saturn. b. The
Gas Giants are Jupiter Saturn, and the Ice
Giants are Uranus Neptune. c. The Gas Giants
are Saturn Uranus, and the Ice Giants are
Jupiter Neptune. d. The Gas Giants are Jupiter
Neptune, and the Ice Giants are Saturn
Uranus. e. The Gas Giants are Saturn Neptune,
and the Ice Giants are Jupiter Uranus.
52
Quiz Questions
16. What is peculiar about the orbits of
Neptune's moons Triton and Nereid? a. Triton's
orbit is around Neptune and Nereid's orbit is
around Triton. b. Triton's orbit is large and
very elliptical, and Nereid's orbit is very small
and circular. c. Triton's orbit is in the
retrograde direction, and Nereid's orbit is large
and very elliptical. d. Triton's orbit places it
inside the Roche Limit of Neptune, and Nereid's
orbit is large and very elliptical. e. They share
similar orbits, and gravitational interactions
cause them to switch orbits each time they meet.
53
Quiz Questions
17. The surface age of Triton is thought to be
about 100 million years. What is the evidence
for such an age determination? a. The
relationship between the rotational and orbital
periods of Triton b. The thickness of nitrogen
snow deposits around the geysers. c. The degree
of tidal heating of Triton due to Neptune. d. Age
dating of meteorites from Triton. e. The density
of impact craters.
54
Quiz Questions
18. How can worlds like Triton and Pluto have
atmospheres when a larger world such as Ganymede
has none? a. Impacts vaporize ices on these cold
bodies. b. Tidal heating releases gases on these
cold bodies. c. In cold environments, gas
molecules have more mass. d. Gas molecules move
more slowly at low temperatures. e. More frozen
gases exist in the colder outer solar system.
55
Quiz Questions
19. What evidence do we have that Pluto and
Charon are made of mixtures of rock and ice? a.
Spectra show that both bodies have some surface
ices. b. Both bodies have a density of 2 grams
per cubic centimeter. c. The Hubble Space
Telescope detected active nitrogen geysers on
Pluto. d. Both a and b above. e. All of the
above.
56
Quiz Questions
20. If you visited Pluto and found Charon a full
moon directly overhead, where would Charon be in
the sky when it was at First Quarter phase? a.
At the west point on the horizon. b. At the east
point on the horizon. c. It depends on the time
of day. d. Directly overhead. e. Either a or b
above.
57
Answers
1. a 2. a 3. e 4. e 5. b 6. a 7. e 8. d 9. e 10. a
11. c 12. c 13. c 14. e 15. b 16. c 17. e 18. d 19
. d 20. d
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