<<

. 24
( 28)



>>

(b) or atmosphere, from the warm side to the cold
side. The extreme temperature differences from
day to night can introduce stresses that weaken
24.2 Surfaces the surface features.
The total cloud cover on Venus (Fig. 24.2b)
The Mariner 10 spacecraft has mapped 50% of means that we must rely on radar mapping to see
Mercury™s surface. Some images are shown in Fig. the large-scale surface features. Maps have now
24.5. The third and closest encounter shows fea- been made of 93% of the surface, and are shown
tures 50 m across. An obvious feature is extensive in Fig. 24.6. Original maps were made from
cratering. The surface is reminiscent of the Moon, ground-based telescopes, particularly the large
but there are also significant differences. The dish at Aricebo (Fig. 4.27). The more recent data
craters on Mercury are flatter than those on the are from orbiting radar.
Moon. The larger surface gravity on Mercury Venus is relatively flat. Approximately 60% of
means that there is more slumping in the sides of the surface lies within 500 m of the average radius,
the craters. The surface may also be more plastic and only 5% of the surface deviates by more than
than that of the Moon. There is evidence for some 2 km from this radius. However, the total range of
erosion, most likely by micrometeorites. elevation, about 13 km, is comparable to that on
484 PART VI THE SOLAR SYSTEM



Fig 24.6. Radar maps of Venus
from the Magellan spacecraft.
(a) A full hemisphere. (b) Mosaic
of large impact craters.This image
has been processed to resemble a
cloud-free optical image.




(a)




(b)
24 THE INNER PLANETS 485




(c)




(e)
Fig 24.6. (Continued) (c) Three-dimensional perspective of
Maat Mons. (d) Aphrodite Terra.The images are before (left)
and after (right) a landslide. (e) Sag caldera Sachs Patera.
[NASA]




(d)

Earth. About 20% of the planet™s surface is cov- has a rougher terrain. It has a large highland
ered by lowland plains, and about 10% by true region with deep (3 km) depressions, hundreds of
highlands. The remaining 70% is described as kilometers wide and 1000 km long.
rolling uplands. These uplands have a variety of Photographs from the surface show angular
features, some of which may be large impact rocks. This was surprising, since it was expected
craters. There is evidence for volcanoes, some of by some that the thick atmosphere would lead to
which may still be active. considerable erosion, smoothing the rocks.
The maps show two large “continents”. By However, a very low wind speed has been found
using the term “continent” we do not mean to on the surface, so the erosion is not as great as
imply that there is evidence for plate tectonics on originally thought. The soil is basalt, providing
Venus. One continent is called Terra Ishta. It is in additional evidence of volcanic activity. The sur-
the northern hemisphere. It is approximately the face temperature is roughly constant at 750 K.
size of the United States, and several kilometers Even before Viking, many of the early myths
above the mean radius of the planet. It has a high about Mars had been dispelled. Mariner 4 showed
mountain, Maxwell Montes, which is 12 km above a cratered surface. Mariners 6 and 7 showed some
the mean radius (as compared with 9 km for Mt signs of erosion. Mariner 9 arrived during a
Everest on Earth). The western part is a plateau planet wide dust storm. We now know that it is
some 2500 km across. The other continent, the dust storms that we have been interpreting as
Aphrodite Terra, is twice as large as Terra Ishta, and seasonal changes in vegetation. When the dust
486 PART VI THE SOLAR SYSTEM



cleared, the Mariner 9 cameras showed a variety Olympus Mons is much higher than Mauna Kea,
of features, including volcanoes, canyons, craters, an impressive statistic, in view of the fact that
terrace areas, and channels. Many of these are Earth is larger than Mars. We think that this tells
shown in Fig. 24.7. Surface views from landers are us that there are no plate tectonics on Mars. On
shown in Fig. 24.8. Earth, the moving plates keep the sites of
The largest volcano, Olympus Mons, rises 25 km volcanic eruptions moving. New lava goes into
and is some 600 km across at the base. This
means that it has the very shallow slopes typical
of shield volcanoes on Earth. An example of such
volcanoes is the Hawaiian chain. However,




(a)




(c)




(b)
Fig 24.7. Aerial views of Mars. (a) Valles Marinaris, from the
Mars Global Surveyor. On the right is a close-up of the area
outlined on the left. (b) Overhead view of Olympus Mons, in
true color, from the Mars Global Surveyor. (c) Martian crater
on northern Elysium Planitia, from the Mars Global Surveyor.
This is twice the diameter of the Arizona meteor crater.
(d) Three-dimensional view of northern polar cap, from the
Mars Global Surveyor.
(d)
24 THE INNER PLANETS 487



Fig 24.7. (Continued) (e) Hubble
Space Telescope images of
north polar cap. [(a)“(d) NASA;
(e) STScI/NASA]




(e)

Mars shows a difference between its northern
making new mountains, not making old moun-
and southern hemispheres. The southern hemi-
tains larger. Therefore, we see a chain in Hawaii,
sphere has more craters and is 1 to 3 km above
but only one large mountain on Mars.
the mean radius of the planet. It also seems to
The channels are interesting, because they
have an old part, with many craters, and a less old
may have held water in the past. If they did, it
part. The northern hemisphere has volcanic
must have been long ago, since the channels are
plains around large volcanoes.
cratered. This means that water has not eroded
An interesting large feature is the Tharsis ridge,
the channels since the heavy cratering in the Solar
which is a large bulge. It is the largest area of
System, possibly 4 billion years ago.




(a)
Fig 24.8. Surface views of Mars. (a) Enhanced color image
of Viking 1 landing site. (b) Ice at Viking 2 landing site,
(b)
Utopian Planitia.
488 PART VI THE SOLAR SYSTEM




(e)




(f)
Fig 24.8. (Continued) (c) Trench excavated by Viking 1
surface sampler. (d) Twin peaks at Path¬nder landing site.
(e) Flatlands as seen by Path¬nder rover. (f) Sojourner rover
tracks in compressible soil. [NASA]


(c)




(d)
24 THE INNER PLANETS 489



0° 180°










12
60
30




24





°

24




30




°
60
12






180° 0°
km
8
60°


4
30°


0
’30°
’4
’60°

’8
180° 240° 300° 0° 60° 120° 180°
(a)




(b)
Fig 24.9. (a) Maps of Mars global topography as measured
by Mars Global Surveyor orbiter. (b) Hubble Space
Telescope visible and infrared images. (c) Global tempera-
ture map of Mars, as measured by the thermal emission
spectrometer on the Global Surveyor orbiter.
[(a), (c) NASA; (b) STScI/NASA]
(c)

The seasonal changes in the polar caps have
also been of considerable interest. It has long been
volcanic activity, and has many young craters. It
speculated that water is stored there in the winter
also has a number of rills and fractures. A large
and released in the summer. The release is by sub-
equatorial canyon, Valles Marineris, extends away
limation, a phase change directly from the solid to
from Tharsis. There is evidence for wall collapse
gas phase. However, the caps never sublime totally,
and channel formation. The whole ridge may be
with some part remaining through the summer.
uplifted crust.
490 PART VI THE SOLAR SYSTEM



It now appears that the part that never sublimes is Additional information can come from the
composed of water. The growth of the caps in the rotational inertia, I, since it depends on how the
winter is due to the freezing of CO2. (On Earth, we mass is distributed. We can measure the rota-
call frozen CO2 dry ice; it is used to keep things cold tional inertia by seeing the effects that perturb-
because it has a lower freezing point than water.) ing torques have on the planet. The rotational
inertia for the case shown in Fig. 24.10 is

a ba b 3R3 CR2 MRC 4
24.3 Interiors 2 4
R3 2
R3 2
I MRP
C C P C
5 3
24.3.1 Basic considerations
a b3 MRC 4
8 5 5 5
CRC MRP (24.4)
Since we cannot directly observe the interior of a
15
planet, we must come up with indirect methods
RC 5 RC 5
a b R5 e Ca b Mc1 a b df
8
for determining the interior structure. We briefly
15 P RP RP
go over some types of evidence that we can use
for studying planetary interiors. For planets on which we can place seismome-
The average density of a planet can give us ters, we can learn about interior activity. We can
information. For example, consider the simple also learn about how seismic disturbances move
structure shown in Fig. 24.10. The planet has a through the interior. For example, one Apollo
core with density C and radius RC, and a mantle experiment involved allowing a spent Lunar
with density M and radius RM, the radius of the Module to crash into the Moon, going on to meas-
planet. In this case the mass of the planet is ure the seismic disturbances with seismometers
that had been left behind. Seismic waves travel at
a b 3 R3 1R3 R3 2 M4
4
MP different speeds through different materials.
C C P C
3
They will also be reflected from boundaries
The average density of the planet is its mass, between different materials. Analyzing these
divided by its volume, waves can tell us about the composition and size
of various interior sections.
MP
Additional information comes from the mag-
a b R3
4
netic field, which would require a molten iron
P
3 (24.3)
core, and some rotation to stir the core. Heat flow
RC 3 RC 3
Ca b Mc1 a bd
measurements can tell us how close radioactive
material is to the surface. This tells us whether
RP RP
the mantle is well mixed or differentiated.
If we know the material that is likely to make up
In Fig. 24.11 we show the interior structures
the mantle and the core, we can estimate M and
of the four inner planets. In order to make the
C. The average density is easily determined, so
we can find (RC/RP). Earth Venus
Mantle Mantle

Mantle Core Core
RP
Core
Rc
ρM Core
ρC
Mantle
Core
Mantle
Mars Mercury
Fig 24.11. Model interiors for the four inner planets. In
each case, the sizes are relative to the planet™s radius, so the
Fig 24.10. Planet with core and mantle. total radius of each planet is the same in this ¬gure.
24 THE INNER PLANETS 491



comparison more meaningful, we have scaled Earth or Moon. The radius of the core is about
the results to the size of the planet. In each case 1800 km, leaving a 700 km mantle. The core
we can see what fraction of the interior is radius is 72% of that of the planet.
occupied by the core and what fraction by the A surprising result of the Mariner 10 studies
mantle. was the discovery of a weak magnetic field around
We can also carry out theoretical modeling Mercury. This field is about 1% the strength of the
of planetary interiors, just as we do for stellar Earth™s field. It is surprising since Mercury™s low
interiors. Just as in stars, planetary interiors mass would suggest that the core is not hot
must be in hydrostatic equilibrium, meaning enough to be molten. Also, the rotation is so slow
that that the core is not stirred up very much. There is
also radioactivity coming from near the surface,
GM1r2
dP
suggesting a differentiated interior. This differen-
r2
dr
tiation would have also required a period of melt-
ing of the interior.
where M(r) is the mass interior to r. If the density
The surface plains may have resulted from vol-
can be approximated as being constant, then
canic flooding, suggesting an active past. Mercury
a b r3
4
also has scarps, unlike the Moon. These scarps
M1r2 (24.5)
3
may have resulted from a contraction of the sur-
face, something that would have also required a
so the equation of hydrostatic equilibrium takes
molten history for the planet.
the form
The interior structure of Venus is believed to
a b G 2r
dP 4
be very similar to that of the Earth. There are
(24.6)
dr 3
some composition differences. We think that the
core of Venus formed later than that of the Earth.
We can use equation (24.6) to estimate the cen-
The lithosphere is also about twice the thickness
tral pressure of a planet (just as we did for the
of that of the Earth. Venus also has no measura-
Sun), by taking the pressure at the surface to be
ble magnetic field. This may also be an effect of
zero and integrating:
the planet™s slower rotation.
a bG
P1r2 r
4 The density of Mars is 3 g/cm3, much lower
2
r¿ dr¿
dP¿
3 than that of the Earth. This suggests that the core
RP
0

cannot be very large. It is about 1200 km in
to get
radius, meaning that it is only about 40% of the
a bG 1R2 r2 2
2 planetary radius. The core is probably a combina-
2
P1r2 P
3 tion of iron and iron sulfide. If it is all iron, it is
probably even smaller than currently estimated.
In comparing this result with more sophisti-
We have already said that the existence of large
cated models of the Earth, we find that it under-
volcanoes indicates that there are no plate tec-
estimates the central pressure by a factor of about
tonics, also arguing for a cooler interior than the
2. (Among other things, the assumption of con-
Earth.
stant density is an oversimplification.)

24.4 Atmospheres
24.3.2 Results
Even though Mercury™s surface looks like that of
The atmospheric compositions of Venus, Earth
the Moon, Mercury™s density is higher. We esti-
and Mars are shown in Fig. 24.12.
mate that an iron core makes up about 70% of
Not much of an atmosphere was expected on
Mercury™s mass. This is a greater percentage than
Mercury, and only a small amount of gas was
for either the Earth or the Moon. This is because
found. The surface pressure is 10 15 atmospheres.
Mercury formed under higher temperature con-
The gas was detected by ultraviolet spectroscopy.
ditions (being closer to the Sun) than did the
492 PART VI THE SOLAR SYSTEM



atmosphere when Venus passed between the
O2
Others Ar
Earth and across the Sun.
N2 Ar
N2
The surface pressure is 90 atmospheres, much
O2
higher than that of the Earth. The atmosphere is
96% carbon dioxide (CO2) and 3.5% nitrogen (N2).
The total amount of nitrogen on Venus is compa-
rable to that in the Earth™s atmosphere. However,
on Earth, nitrogen is the primary constituent.
Venus also has very small quantities of water, sul-
fur dioxide, argon, carbon monoxide, neon,
hydrogen chloride and hydrogen fluoride.
N2 CO2
CO2
The large amount of CO2 produces a very strong
greenhouse effect on Venus. The 750 K is some
400 K higher than the temperature would be with-
out an atmosphere. It is intriguing that two plan-
ets could start out so close in conditions and end
up so different. The crucial difference between
Venus and Earth seems to be the extra sunlight,
Venus Earth Mars
making Venus initially somewhat warmer than
Fig 24.12. Diagram showing the atmospheric composi-
the Earth. On Earth, water condensed, whereas on
tions of Venus, Earth and Mars.
Venus it remained as a gas, and escaped. On Earth,
the water kept the CO2 bound up in the rocks,
in the form of various carbonates. With the water
This small amount of gas is 98% helium. This
on Venus this couldn™t happen, and the CO2 stayed
light atom should have escaped long ago. This
in the atmosphere. (The amount of CO2 in the
suggests that it is being replaced continuously.
rocks on Earth is comparable to the amount of CO2
Two possible sources are the solar wind and cer-
in the atmosphere of Venus.)
tain types of radioactive decay. Most of the
Once Venus had more CO2 in its atmosphere
remaining 2% of the gas is hydrogen. This may
than the Earth did, the greenhouse effect heated
also come from the solar wind. In addition, there
the lower atmosphere. This heating released
are small traces of oxygen, carbon, argon, nitro-
more CO2 into the atmosphere, increasing the
gen and xenon.
greenhouse effect. This situation is called a run-
The atmosphere of Venus is very interesting,
away greenhouse effect. A small difference in initial
especially in comparison with that of the Earth. It
conditions ends up with a large difference in
was first observed in 1761 by the Russian
final conditions.
astronomer Lomonosov, who saw the backlighted


Fig 24.13. Images of low level
clouds on Venus as seen in the
near infrared by the Galileo
orbiter. [NASA]
24 THE INNER PLANETS 493




100
Altitude (km)




Haze

Clouds Surface
50

Dense Clouds
Cloud
Tops



Subsolar
200 400 600 800
Point
Temperature (K)
(b)
(a)
Fig 24.14. (a) Temperature distribution in the atmosphere of Venus.The region below the dense clouds is clear.
(b) Atmospheric circulation on Venus.There is only one cell from the equator to the pole.There is a break in the east“west
¬‚ow, with the air rising at the point receiving the maximum solar energy.



In Fig. 24.14(a) we show the temperature at dif- atmosphere is 2.7% nitrogen and 1.6% argon.
ferent altitudes. Above the clouds (about 50 km There are traces of oxygen, carbon monoxide,
above the surface), the greenhouse effect is no water vapor, neon, krypton, xenon and ozone.
longer very strong, and the temperature drops to There are blue/white clouds, made up of carbon
about 300 K. The clouds are made up of sulfuric dioxide and water. Most of the water, however, is
acid. The sulfuric acid droplets are about tied up permanently in the northern polar cap.
2 m. There is a lot of sulfur in the atmosphere We think that Mars had a more plentiful atmos-
because there is no water to remove it. The phere in the past, possibly with a pressure equal
haze above these clouds enhances the green- to that on Earth. At that time, there was probably
house effect. There are three cloud layers more water. The earlier atmosphere probably
between 48 and 80 km above the surface. The came from an era of volcanic activity.
presence of some water in the lower altitudes has Strong winds sweep over the planet, lifting
washed the sulfuric acid away, and the lower large amounts of dust into the air. There are a
atmosphere is clear. Many were surprised by the
clarity of the Venera pictures from the surface.
There is also some lightning below the clouds.
There is a general westward circulation of the
winds, as shown in Fig. 24.14(b). The wind speed is
a modest 1 m/s near the ground, growing to a sub-
stantial 100 m/s near the cloud tops. There is a
general pole to equator circulation, but it is not
broken into cells as it is on Earth. This circulation
minimizes the temperature difference between
the equator and the poles to a few kelvin.
The surface pressure on Mars is 0.007 atmos-
phere. This is sufficient for the atmosphere to be
of some importance. The atmosphere is 95% CO2,
leading to a small greenhouse effect. The temper-
Fig 24.15. Clouds on Mars. A mid-summer storm in the
atures are raised by about 5 K above the value
northern hemisphere. [NASA]
they would have without an atmosphere. The
494 PART VI THE SOLAR SYSTEM



number of small dust storms and a few global
dust storms over the Martian year. Some atmos-
pheric scientists have pointed to dust storms on
Mars as providing a test of theories that say that
the aftermath of a nuclear war on Earth would
produce enough dust in the atmosphere to cool
our planet (the so-called “nuclear winter”).
The density in the Martian atmosphere is so
low that the winds are not efficient at energy
transport. Therefore, large temperature variations
can exist across the planet. For example, in the
winter the poles are as cold as 150 K, which is cold
enough to freeze CO2. At its maximum extent, the
CO2 cap extends almost halfway to the equator.
The similarity between the rotation period of
Mars and Earth produces similarities in the cir-
culation patterns. However, since there are no
oceans on Mars, and since the atmosphere is so
thin, the surface responds more quickly to heat-
ing changes. Therefore, the hottest point on Mars
is always the point closest to the Sun, the subsolar
point. This results in a circulation pattern in
which one large cell spans the equator. There is
also a large day/night temperature difference, a
few tens of kelvin.
The Viking landers also carried out a search
Fig 24.16. View of Phobos from the Mars Global Surveyor
for microscopic life on Mars. Three different
orbiter.The largest crater, Stickney, is 10 km in diameter.
experiments were carried out to try to detect
[NASA]
changes in small samples of material as a result of
metabolism of microscopic life. Evidence for some
unusual chemical reactions was found, but there
is no evidence for living organisms. slower, from the surface they appear to move
across the Martian sky in opposite directions.
Since the orbital period of Demos is close to
24.5 Moons Mars™s rotation period, an observer on Mars
would see that it takes 137 hr to make a com-
No moons have been found for Mercury and plete cycle of the sky. It takes almost three days
Venus. Mars has two moons that are much to go from rising to setting, and then another
smaller than the Earth™s. One is shown in Fig. three days to reappear.
24.16. Phobos is 22 km across, and orbits Mars Each moon has an irregular shape. They
in 7 hr 40 min. Demos is 15 km across and appear dark, like certain asteroids. This has led to
orbits in 30 hr. Since one moon orbits faster conjecture that these two satellites were asteroids
than Mars™s rotation period and the other is that were captured by Mars.


Chapter summary
In this chapter we looked at the properties of the We saw what could be learned from ground-
inner planets of the Solar System. based observations. We saw how some of the
24 THE INNER PLANETS 495



observations could lead to misinterpretations, cratering on Mercury, Mars and our Moon.
such as the rotation period of Mercury. We saw However, there are also many differences. For
how radar mapping is a useful tool, both from example, the Earth is the only planet to have an
the Earth and from spacecraft. interior structure that produces plate tectonics.
We discussed the wealth of information that We see very thin atmospheres on Mars and
has been obtained by flybys, orbiters and landers. Mercury, while Venus has a much thicker atmos-
Venus and Mars have been visited extensively. In phere than the Earth. Of course there is a large
the case of Venus, the spacecraft have allowed us to runaway greenhouse effect on Venus, probably
probe the dense cloud cover that hides the surface. initiated by the fact that, with a slightly higher
We have seen certain similarities among the temperature than the Earth, the CO2 was not
inner planets. For example, there is extensive bound into the rocks.


Questions
24.11. If the atmosphere of Mars is so thin, why is
24.1. How do the sizes of the four inner planets
there so much erosion of the surface?
compare? Where does the Moon fit in?
24.12. What causes the seasonal variations in the
24.2. Of the inner planets, which have surfaces
appearance of Mars?
that are most affected by the atmosphere?
24.13. What is the evidence that there was once
24.3. Why is it not surprising that the surface of
water on Mars?
Mercury has similarities with that of the
24.14. What is the evidence that there are no plate
Moon?
tectonics on Mars?
24.4. Once we have a spacecraft orbiting Venus,
24.15. What makes the polar caps on Mars change
why is it better to make a radar map of the
their appearance from summer to winter?
surface than to take a series of photographs?
24.16. How do we learn about the interior of a
*24.5. Why do you think that radar observations of
planet?
a planet become more difficult as the planet
24.17. Compare the significant features of the
is farther from Earth?
interiors of the four inner planets.
24.6. The best Mariner 10 maps of Mercury have a
24.18. Why was the discovery of a magnetic field
resolution of 50 m. If a spacecraft made a
on Mercury surprising?
map of the Earth with the same resolution,
24.19. Describe the runaway greenhouse effect
what types of structures could be discerned?
that took place on Venus.
24.7. If Mercury has no atmosphere, what is the
24.20. The Earth and Venus are similar in many
mechanism for erosion of the surface?
ways, yet their atmospheres evolved so dif-
24.8. What is the difference between continents
ferently. Why is that?
on Venus and on Earth?
24.21. How might an increase of carbon dioxide in
24.9. Venus is often called the Earth™s sister
the Earth™s atmosphere produce a greater
planet. What features do the Earth and
greenhouse effect?
Venus have that are similar? Which features
24.22. Contrast the clouds on Venus with those on
are very different?
Earth.
24.10. Compare the largest mountains on Venus,
Earth and Mars.


Problems
24.1. Compare the tidal effects of the Sun on Venus ing the time delays and Doppler shifts to pro-
with the tidal effects of the Sun and Moon on vide a resolution of 1 km/s on the surface?
the Earth. 24.3. How much would a 1 GHz radar signal be
24.2. For radar mapping of a planet, what time and spread in frequency by the rotation of
frequency resolution are needed in measur- Mercury?
496 PART VI THE SOLAR SYSTEM



24.4. Approximate the central pressures of the four 24.7. Estimate the adiabatic temperature gradients
inner planets and compare them. near the surfaces of Venus and Mars.
24.5. How far under water to we have to go on 24.8. To what altitude do we have to go in the
Earth to obtain a pressure of 90 atmospheres? Venus atmosphere to reach a pressure of
24.6. Using the mass, radius and core radius of the 1 atmosphere?
Earth (given in Chapter 23), find the ratio of 24.9. Show that if a point on the equator of a
planet moves with speed v0 due to rotation, a
the core material to the density of the mantle
point at latitude moves with speed v0 cos .
material.


Computer problems

24.1. Make a graph of pressure vs. altitude in the Venus 24.2. Make a graph of pressure vs. altitude in the Mars
atmosphere. Assume that the surface pressure is atmosphere. Assume that the surface pressure is
90 atmospheres, the main constituent is CO2, and 0.007 atmospheres, the main constituent is CO2,
that the temperature is a constant 600 K. and that the temperature is a constant 200 K.
Chapter 25




The outer planets

In the outer planets, we find a considerable con- The energy output of Jupiter is interesting. It
trast with the four inner planets. We therefore seems to radiate over 50% more energy than it
study them as a group, comparing surfaces, inte- receives from the Sun. In addition it is also a
riors and atmospheres. The relative sizes of the strong source of non-thermal (synchrotron) radio
outer planets (and Earth) are shown in Fig. 25.1. emission.
Space exploration has greatly improved our
view of Jupiter. Pioneers 10 and 11 and Voyagers 1
25.1 Basic features and 2 have provided us with spectacular images
as well as a variety of other observations. These
Jupiter, shown in Fig. 25.2, is by far the most mas- close-up observations have also added to the
sive planet in the Solar System. It is 318 times as already known extensive moon system. They also
massive as the Earth, and is a respectable 0.1% revealed a ring around the planet.
as massive as the Sun. (The rest of the planets Saturn™s mass is 30% that of Jupiter, or 95.2
together only have 129 Earth masses.) Jupiter™s times that of the Earth. It has the lowest density
density is much lower than that of the inner of the planets, 70% the density of liquid water.
planets, 1.3 g/cm3 vs. 5.4, 5.3, 5.5 and 3.9 for With this low density, its acceleration of gravity
Mercury, Venus, Earth and Mars, respectively. Its at the cloud tops is only slightly greater than that
density is only slightly greater than that of liquid on Earth, 1.07g. Its composition is similar to that
water. This suggests that the composition of of Jupiter.
Jupiter is basically different from that of the inner Like Jupiter, it rotates rapidly, in 10.7 hr at the
planets. This is due, in part, to the larger gravity, equator, and slower at the poles. Through a tele-
2.54 g at the cloud tops. The larger gravity means scope, we can see bands in the cloud structure.
that the lighter gases have been retained. However the bands do not show as much contrast
The atmosphere is 85% hydrogen and 15% as those of Jupiter.
helium, with a variety of trace constituents. This Of course, Saturn (Fig. 25.3) is best known for
composition is much closer to that of the Sun its prominent rings. From Earth, we see the rings
than it is to the inner planets. The motions in as three main structures, and can deduce that
the atmosphere are affected by the planet™s they are very thin. Like Jupiter, Saturn has an
rapid rotation. The period is 9.92 hr at the equa- extensive moon system, with 18 moons having
tor. The rotation period is greater at the poles. been identified to date. Pioneers 10 and 11 as well
The rapid rotation produces a large coriolis as Voyagers 1 and 2 have revealed a great com-
force. This is manifested in the appearance of plexity in the structures of the rings in addition
bands and spots, such as the Great Red Spot, which to surprising views of the moons.
is 14 000 by 30 000 km, and has persisted for Uranus (Fig. 25.4) is the closest planet that
centuries. has not been known since ancient times. It was
498 PART VI THE SOLAR SYSTEM



Notice the very faint contrast between the different
Jupiter
bands.
The discovery of Neptune (Fig. 25.5) makes an
interesting story. After following the orbit of
Saturn
Uranus, astronomers found that it did not move
Uranus exactly in its predicted path. The perturbation
Neptune

E




Fig 25.1. Relative sizes of Jupiter, Saturn, Uranus, Neptune
and Earth.


discovered in 1781 by William Herschel. Its mass
is considerably less than that of Jupiter and
Saturn, but at 14.6 Earth masses, it is definitely
not in the class of the inner planets. Its density
(a)
is 1.2 g/cm3, close to that of Jupiter and Saturn.
The difference in sizes reflects a difference in
the composition of Uranus and Jupiter. Its sur-
face gravity is slightly less than that of the
Earth, at 0.87 g.
From ground-based telescopes, we can tell
that Uranus has a high albedo. That is, it reflects
a lot of sunlight. This suggests a coating of
clouds. It is difficult to identify features for the
purpose of measuring the rotation period. Values
from 12 to 24 hr have been proposed, but the
most recently accepted value is 16 hr. The rota-
tion axis is tipped so much that it is almost in the
plane of its orbit. The rotation is retrograde. It is
in the opposite direction from the planet™s
orbital motion. Infrared observations of the
clouds suggest a surface temperature of 58 K, a
very cold place.
Uranus has five main moons and a number of
smaller ones. In addition, a ring system has been (b)
discovered accidentally during an ocultation of a
Fig 25.2. (a) HST image of Jupiter. (b) Voyager 1
star by Uranus. We had our first close-ups from
image of Jupiter taken from a distance of 54 million km.
space when Voyager 2 flew by in January 1986.
[(a) STScI/NASA; (b) NASA]
Some images from Voyager 2 are shown in Fig. 25.4.
25 THE OUTER PLANETS 499




(a) (a)




(b)
Fig 25.4. (a) HST image of Uranus. (b) Uranus taken
from Voyager 2, from a distance of 9.1 million km.The left
image shows Uranus as it would appear to the eye.The
picture on the right is enhanced to show the cloud cover.
[(a) STScI/NASA; (b) NASA]


Adams presented his calculations to the
Astronomer Royal, who was not impressed, and
(b)
did not carry out the easy observations that
Fig 25.3. (a) HST image of Saturn. (b) Saturn image would have been necessary to test the idea.
from Voyager 1, at a distance of 31 million km. [(a) STScI/NASA; Leverier had no better luck in France. Finally,
(b) NASA]
based on Leverier™s calculations, Johannes Galles, in
Germany, carried out the observations, and
found Neptune in 1846. We credit both Adams
of a planet beyond Uranus was suspected. Cal- and Leverier with the successful prediction. As an
culations of the possible location of the planet interesting aside, recent readings of Galileo™s
were carried out independently in 1845 by John C. notes indicate that he may actually have observed
Adams, in England, and Urbain Leverier, in France. Neptune, noting its changing position relative to
500 PART VI THE SOLAR SYSTEM



Fig 25.5. (a) HST images of
Neptune (showing opposite
hemispheres). (b) Voyager 2 image
of Neptune, from a distance of
6.1 million km. [(a) STScI/NASA;
(b) NASA]




(a)


25.2 Atmospheres

Jupiter™s atmosphere contains hydrogen and
helium in the same proportion as in the Sun™s
atmosphere. This suggests that Jupiter has its
original atmosphere. With its large mass, it has
been able to hold even the lightest atoms. A num-
ber of minor constituents have been identified as
well. NH3 (ammonia) and CH4 (methane) are the
most prominent. In addition, C2H6 (ethane), C2H2
(acetylene), H2O, PH3, HCN (hydrogen cyanide)
and CO (carbon monoxide) have been identified.
The temperature distribution is shown in Fig.
25.6. The temperature is 125 K at the cloud tops. As
you go down from there, the temperature
increases by about 2 K for every kilometer that you
drop. Above the cloud tops, the pressure increases
(b)
as you go up. The emitted radiation is approxi-
mately the same at all latitudes. This is true
despite the fact that the solar heating is greatest at
the stars, but did not have enough observations
the equator. This may mean that winds are effec-
to identify it as a new planet.
tive at distributing heat from the equator to the
Neptune™s mass is 25.2 times that of the
poles. Such large winds would require large tem-
Earth, similar to that of Uranus. From occulta-
perature differences to drive them. We do not see
tions, we can tell that its radius is 3.88 Earth
these differences in the upper atmosphere. This
radii. From these numbers, its density turns out
means that the transport must take place in the
to be 1.6 g/cm3, slightly greater than that of
lower atmosphere. Another possibility is that
Uranus. The acceleration of gravity on its sur-
Jupiter has an internal heat source that supplies
face is slightly greater than on Earth at 1.14g. Its
more heat to the poles than to the equator.
rotation period is also hard to determine, with
published values ranging from 17 to 26 hr. The Example 25.1 Energy from Jupiter
currently accepted value is 25.8 hr. We can Compare the energy given off by Jupiter with the
deduce the presence of an atmosphere by the energy it receives from the Sun. Assume that
rate at which starlight dims during occultations. Jupiter radiates like a 125 K blackbody, the temper-
Neptune has two larger and six smaller moons. ature of the cloud tops.
25 THE OUTER PLANETS 501



The situation is even worse than this, because
200
Jupiter doesn™t absorb all of the sunlight that
Altitude (km) Relative to 0.1 Atm Level


strikes it. This indicates that Jupiter gives off more
radiation than it receives from the Sun. Jupiter
100
must have an internal energy source.

The fact that the temperature fall-off with alti-
tude below the clouds is close to the adiabatic
0
rate suggests that convection is an important
Jup
Red form of energy transport. (Remember, we saw
it er
Red
that, on Earth, when there is a rapid drop in tem-
White
-100
White perature with altitude, then convection can be
Brown
very strong.) Because of the temperature distribu-
Blue
Sa




tion, we think that there are three major cloud
tu




Brown
-200 layers, resulting from the fact that different con-
rn




stituents condense at different temperatures and
pressures. We think that the highest cloud layer
Blue
is ammonia ice, the middle layer is ammonium
-300
hydrosulfide (NH4SH), in the form of crystals, and
0 100 200 300 400
the lowest layer is water, in a mixture of liquid
Temperature (K)
and ice.
The varying conditions mean that the chem-
Fig 25.6. Temperature distributions in the atmospheres of
Jupiter and Saturn. Since we don™t know where the surfaces istry is different at different altitudes. We think
are, we use as our reference point the place where the pres- that the different cloud colors reflect different
sure is 0.1 atmosphere.The colors written along each curve
compositions, a result of the varying chemistry.
indicate the dominant color material at that temperature.
The different colors come from different temper-
ature ranges, and therefore from different levels.
For example, the blues correspond to the warmest
SOLUTION
regions, and are therefore closest to the surface.
Jupiter™s luminosity is given by its surface area 4 R2,
J They are probably only seen through holes in the
4
multiplied by the total power per unit area T J , or higher layers. Brown, white and red come from
14 R2 2 1 T 4 2 progressively lower temperatures, meaning they
LJ J J
come from progressively higher levels. We have
The power received from the Sun is the solar lumi- still not identified all of the compounds respon-
nosity 14 R2 2 1 T 4 2 , divided by the area of a sphere sible for the various colorations. Other factors
at the distance of Jupiter from the Sun, 4 d2, besides temperature also affect the chemistry. For
where d is the distance of Jupiter from the Sun, example, some regions have more lightning than
and multiplied by the projected area of Jupiter others, and the energy from the lightning can
R2, or help certain chemical reactions go.
J

14 R2 2 1 T2 2 1 R2 2 The east“west winds are quite substantial.
J

14 d2 2
They flow at about 100 m/s near the equator, and
Prec
at about 25 m/s at the higher latitudes. The winds
flow in alternating east“west and west“east
Taking the ratio of these gives
bands. These alternating wind patterns corre-
LJ TJ 4
d2
4a b a b spond to the alternating color bands. On Jupiter,
Prec T R there are five or six pairs of alternating bands in
125 4 15.2 2 11.5 10 km2 2
each hemisphere. For comparison, on Earth,
8
4a ba b there are only the westward (in the northern
7 105 km
5500
hemisphere) trade winds at low latitudes and the
1.3 eastward jet stream at high latitudes.
502 PART VI THE SOLAR SYSTEM



to have gas falling. The tops of the belts are about
Zone
Zone 20 km lower than the tops of the zones. The belts
(cool)
(cool) are brighter in the infrared, indicating a higher
temperature. The stability of the large-scale cloud
patterns has not been understood. The locations
of the bands are constant, even though their col-
ors occasionally change. Close-up pictures have
shown small circular regions, called eddies, which
are shown in Fig. 25.8. We generally associate
Belt eddies with the dissipation of energy. We would
(warm) therefore expect the patterns to wash out. The
fact that the east“west bands are stable indicates
Fig 25.7. Convection patterns that produce the belts and
that their flow must continue deeper into the
zones.
atmosphere than the eddies.
The most famous cloud feature on Jupiter is
The bright colored bands are called zones. They
the Great Red Spot, shown in Fig. 25.8. It has been
appear to have gas rising, as shown in Fig. 25.7.
observed for more than 300 years. It covers more
The dark colored bands are called belts. They appear




(a)
(b)
Fig 25.8. Images of Jupiter™s atmosphere. (a) Multilevel clouds
from near IR images from Galileo orbiter.These images are at
different IR wavelengths, and the differences are because of the
different opacities at each wavelength.The top left and right
images are at 1.6 and 2.7 m, respectively, and show relatively
clear views deep into the atmosphere.The middle image is at
2.2 m and shows high altitude clouds and haze.The lower left
and center images are at 3 and 5 m, showing deeper clouds.
The false color image at the lower right is a composite showing
the clouds at different layers. (b) True color mosaic of the
belt“zone boundary. (c) The Great Red Spot, from Galileo
orbiter images at four wavelengths: upper left is violet (415 nm);
upper right is IR (757 nm); lower left is IR (732 nm); lower right
is IR (886 nm) at a wavelength of strong methane absorption. (c)
25 THE OUTER PLANETS 503




(a)

(d)




(b)
(e)
Fig 25.9. (a) Jupiter™s northern auroral oval, centered
Fig 25.8. (Continued) (d) White ovals near the Great Red around the north magnetic pole. (b) Saturn aurora (HST
Spot. (e) Watercloud thunderstorms northwest of the Great image). [(a) NASA; (b) STScI/NASA]
Red Spot. [NASA]


water vapor is condensed. This is why hurricanes
than 10 in latitude. It is also surrounded by
intensify when they pass over large bodies of
white ovals, which have flows that would dissi-
water. There are different models for the details
pate energy very quickly. In trying to explain its
of how the red spot works, but it does seem that,
stability, there are two questions that must be
with the conditions in Jupiter™s atmosphere,
answered. (1) How can it be maintained for so
such a storm should be stable for hundreds of
long as a stable fluid flow? (2) What is the energy
years!
source to replace the energy lost in the eddy flow
Jupiter™s has a strong magnetic field, whose
around the spot? The most successful model has
effects are felt far out into space (Fig. 25.9a). At
been to say that the spot is analogous to a hurri-
the top of the cloud layer the magnetic field has
cane on Earth. Large vertical convection currents
ten times the strength of the Earth™s field. As far
allow it to draw energy from the latent heat of
as 7 million km in front of Jupiter, the solar wind
condensing materials below. On Earth, hurri-
is affected by this magnetic field, and is deflected
canes draw energy from that released when
504 PART VI THE SOLAR SYSTEM



(Fig. 25.10) and that of Jupiter. However, there are
certain differences. These differences arise from
the fact that Saturn is farther from the Sun than
is Jupiter. Saturn™s lower gravity and lower rota-
tional speeds are also important.
The temperature is 95 K at the cloud tops.
Saturn gives off approximately twice as much
energy as it receives from the Sun. The temperature
rises as one goes deeper into the atmosphere (as
shown in Fig. 25.6). The rate of temperature change
with altitude is about half of that on Jupiter.
The winds on Saturn are much greater than
on Jupiter. They are about 450 m/s at the equator
and about 100 m/s at higher latitudes. There is
also an alternating pattern (as on Jupiter), with
the speeds alternating between 0 m/s and about
100 m/s. Saturn does not have as many bands as
Jupiter does. The lower temperature means that
(a)
there are chemical differences. This is evidenced
by the fact that the bands don™t show as much
contrast as those of Jupiter. Voyager showed the
equivalent of the Great Red Spot on Saturn. It had
not been seen from the Earth because of the
lower color contrast.
The atmospheres of Uranus (Fig. 25.11) and
Neptune (Fig. 25.12) are hydrogen rich, like those
of Jupiter and Saturn. However, Uranus and
Neptune contain a higher proportion of heavy
materials. This is because those lower mass plan-
ets probably retained less of their original hydro-
gen than did Jupiter and Saturn.
The temperatures of Uranus and Neptune are
(b) almost the same, about 57 K. One would expect
Neptune to be cooler than Uranus, but it appears
Fig 25.10. Images of Saturn™s atmosphere. (a) Voyager 1
to give off more heat than it receives from the
image of Saturn™s red spot. (b) Ribbonlike structure (Voyager
Sun. It is thought that this may be related to the
1 image). [NASA]
way in which the atmosphere traps the heat.
Both atmospheres contain methane. This mol-
ecule has been identified from its infrared spec-
to flow around the planet. The region with no trum. Both planets have a greenish color, and it is
solar wind stretches as much as 700 million km believed that this color comes from the methane.
beyond Jupiter, and even Saturn passes through There is a difference in the cloud content of the
it! (By comparison, the Earth creates a region two atmospheres. Uranus has an atmosphere that
with no solar wind that is only 1% as long.) As is cold and clear to great depths. There appear to
with the Earth™s magnetic field, Jupiter traps be no clouds or haze in the lower atmosphere.
charged particles, creating an active ionosphere. There may be some higher up. Neptune has a vari-
This is the source of strong radio emission. able haze. We think that the haze is composed of
It should not be surprising that we find many aerosol particles or methane ice crystals. A com-
similarities between the atmosphere of Saturn parative study of these two atmospheres, especially
25 THE OUTER PLANETS 505




(a)

(a)




(b)
Fig 25.11. Images of Uranus™s atmosphere. (a) Towards
one rotation pole.The left image is true color, and the right
image is enhanced to show structure in the clouds (Voyager
2 image). (b) A large cloud (Voyager 2 image). [NASA]



(b)
as more data become available, should provide a
good test of our ability to model planetary atmos- Fig 25.12. Images of Neptune™s atmosphere. (a) True color
image of clouds (Voyager 2 image). (b) Changes in great dark
pheres with enough detail to predict or explain
spot (Voyager 2 image). [NASA]
the differences.
Voyager 2 provided a good view of Uranus from
a distance of five planetary radii. There is very lit-
The wind speeds seem to be in the range of tens of
tle structure in the atmosphere. However, there is
kilometers per hour.
a polar haze, probably composed of methane and
Voyager 2 also revealed an interesting temper-
acetylene. There is also an extended hydrogen
ature distribution. The equator is cooler than
(atomic and molecular) corona. Ultraviolet emis-
either of the poles. In addition, the pole facing the
sion from this corona had previously been detected
Sun is cooler than the shadowed pole. This is obvi-
from Earth-orbiting satellites. The rotation period
ously an interesting problem in energy transport.
of the atmosphere appears to be just under 17 hr.
506 PART VI THE SOLAR SYSTEM



continues right to the center. The pressure is
25.3 Interiors 80 million bar; the temperature is about 25 000 K.
The core contains only 4% of Jupiter™s mass.
We cannot directly sample the interiors of these The transition from the gaseous region to the
outer planets. However, we can use physical laws liquid region is probably a gradual one. The tran-
to construct computer models of the interior. We sition from the normal liquid hydrogen to the
can then see which models produce results that metallic hydrogen probably takes place over a
agree with observations. In Fig. 25.13 we show the small change of radius. We think that the interior
internal structures of Jupiter, Saturn, Uranus and has excess energy stored from the time of the col-
Neptune. To allow comparison, they are all scaled lapse of the planet. The energy is so large that it
relative to the size of the planet. In that way, we has not all escaped yet. This is probably the
can compare what fraction of the interior is made source of the excess energy that Jupiter gives off.
up of the various sections. The general structure of Saturn is probably
For Jupiter, the outermost layer is a hydrogen“ very similar to that of Jupiter, as shown in Fig.
helium envelope. With that, extending from a 25.13. There are some differences, however. Saturn
radius 10 000 to 54 000 km, is a liquid region. This has a larger core, containing about 26% of the
liquid is hydrogen. The pressure in this region mass. The central pressure is 50 million bar, and
rises to 40 million times the atmospheric pres- the central temperature is about 20 000 K.
sure on the Earth™s surface. Under these condi- There is a smaller metallic hydrogen zone. The
tions, the hydrogen forms into a metal. This range of radii is from 16 000 to 28 000 km. The tem-
metallic hydrogen contains 73% of the planet™s perature in this zone ranges from 9000 to 12 000 K.
mass. Within the liquid region is the core. The This zone contains about 17% of Saturn™s mass.
core may be made of rock and ice materials, Our understanding of Saturn™s excess heat is
though it has been suggested that the hydrogen not as good as that for Jupiter™s. Some other
explanation is needed. It has been suggested that
some of the energy comes from helium condens-
ing and sinking through the less dense material
170 K
Jupiter Saturn
towards the core.
Uranus and Neptune have higher densities
104 K
Normal
than Jupiter and Saturn. This suggests a different
Normal
H2, He
H2, He
composition. The cores are rock. The rock is
1.4 x 104 K
Metallic
mostly silicon and iron. Over the core is a mantle.
Metallic
H, He
H2, He This mantle probably contains liquid water,
2 x 104 K
8 x 103 K
ammonia and methane. Over the mantle is a
135 K
Rock
Ionic crust of hydrogen and helium. It may be in the
"Ocean"
form of high density gas. The central pressure is
7 x 103 K
about the same for both planets, about 20 million
Molecular
Envelope
bar. The central temperature is about 7000 K.
H2, He,
2.5 x 103 K
CH4

25.4 Rings
80 K

Jupiter, Saturn, Uranus and Neptune have ring
systems. Saturn™s, shown in Fig. 25.14, has been
Uranus Neptune
known since Galileo, while those of Jupiter,
Fig 25.13. Interiors of Jupiter, Saturn, Uranus and Uranus and Neptune are recent discoveries. We
Neptune.These are from model calculations, and are first review the basic properties of the rings, and
expressed relative to each planet™s radius.The numbers at then consider the effects that are important in
each boundary are estimated temperatures.
shaping them.
25 THE OUTER PLANETS 507



25.4.1 Basic properties
Saturn™s rings (Fig. 25.14) shine by reflected sun-
light. Therefore, the brighter areas are those with
more material to reflect the light. Therefore, the
brighter areas are those of higher optical depth,
meaning a greater amount of reflected sunlight.
Three main rings are apparent. The A ring is
the farthest from the planet. It has a width of
20 000 km, ranging from 2.02 to 2.27 times the
(a)
planetary radius. Its thickness is less than 200 m.
Approximately half of the light striking this ring
is reflected back. The B ring is in the middle and
is the brightest. It extends from 1.52 to 1.95 plan-
etary radii. The A and B rings are separated by a
gap, called Cassini™s division. The innermost ring is
the C ring. It is the darkest. It extends from 1.23
to 1.52 planetary radii.
Additional rings have been found from the
ground and from spacecraft. The D ring is a faint
ring inside the C ring (1.11 to 1.23 radii). The E ring
is a very faint ring, outside the A ring (3 to 8 radii).
The F ring is just beyond the A ring (2.37 radii),
(b) and was discovered by Pioneer 11. Finally, the G
ring is a faint ring 2.8 radii from the planet.
The rings are composed of individual particles,
rather than being solid structures. We can tell this
from the spectra of the rings. The Doppler shifts
vary across the rings, as shown in Fig. 25.15. These

Slit



Spectral
Line from
Rings
Red Shift
Wavelength

Blue Shift




Spectral
Line from
Planet
Spectral
Line from
Rings
(c)
Fig 25.14. Saturn™s rings. (a) Voyager 1 image of the unlit Fig 25.15. Simulated spectra of Saturn™s rings, showing the
side, so the B ring appears black. (b) Voyager 1 mosaic. change in Doppler shift as one goes farther out in the ring.
Cassini™s division appears to the right of center as ¬ve bright In the upper diagram the placement of a slit is indicated.
rings with a substantial dark gap. (c) Spokes in the B ring Below, the schematic spectrum shows the Doppler shift of a
(Voyager 1 image). [NASA] particular line as one moves across the slit.
508 PART VI THE SOLAR SYSTEM




Fig 25.16. Jupiter™s ring. Galileo orbiter image of main rings
and halo.The top image is exposed to show the halo. [NASA]



variations are those to be expected for individual
particles orbiting Saturn, with orbital speeds
being given by Kepler™s third law. The particle
sizes range from a few centimeters to 10 meters. Fig 25.17. Uranus™s rings, detailed structure (Voyager
The total mass of the rings is poorly known, but is 2 image). [NASA]
estimated to be in the range 1017 to 1019 kg. This is
only about one-millionth of the Moon™s mass.
the air, in the Kuiper Airborne Observatory.
Infrared observations suggest that the particles
Normally, this observatory is used for infrared
are ices of water and other molecules.
observations. However, in the case of the occulta-
The Voyager flybys produce spectacular pic-
tion, it was used for optical observations, with
tures of the rings. We find that there is an exten-
the airplane providing a means of getting the tel-
sive pattern of detailed structure, as shown in
escope to a favorable viewing point. Shortly
Fig. 25.14. Each ring is divided into smaller rings.

<<

. 24
( 28)



>>