ASTR 1210 (O'Connell) Study Guide 20
Jupiter with erupting Io in foreground
(New Horizons Mission image)
"And now for something completely
different," as they used to say on Monty Python. The outer
solar system is different indeed.
The outer solar system we can now explore (out to about 50 AU) is
vast -- over 35,000 times the volume of the terrestrial realm
out to Mars -- and sparsely populated.
The four large "Jovian" planets (Jupiter, Saturn, Uranus, and Neptune) are
entirely unlike the terrestrial planets (see more discussion
in Study Guide 11). They may have rocky
cores, like larger versions of the Earth, at their centers, but these
are enveloped in giant gaseous atmospheres. Only the outermost
skins of these atmospheres can be studied directly. This is
meteorology, instead of the geology/topography we discussed for
the terrestrials. However, it can be just as extreme with respect to
Earth-bound meteorology as are the canyons and mountains of Mars
compared to those of Earth.
Another major distinction of the Jovians is the large number of
satellites they possess. The satellites, observed at close range
by spacecraft, display an astonishing diversity of surface
types and features. Unlike the three terrestrial planet satellites,
the larger Jovian satellites are rich in water ice and exhibit
many different phenomena as a consequence. They may even
harbor biospheres. The ring systems, which are present
around all 4 Jovians, are probably the remnants of distintegrated
Many examples of a third kind of planet have recently been
discovered outside the orbit of Neptune. These are perhaps most aptly
called the "ice dwarf planets," of which Pluto is the
A. History of Discovery/Exploration Beyond Mars
Spacecraft images of the four Jovian planets (from left: Jupiter,
Saturn, Uranus, Neptune),
scaled to their correct relative sizes (but not
distances from the Sun).
B. Jovian Planets: Properties
The four Jovian planets share gross properties. Pluto is entirely
different (see below).
Distant from Sun: 5-30 AU. (Pluto is at 39 AU.)
Large: The Jovians are 4-11 times larger in diameter than Earth.
Jupiter's volume is 1300 times the Earth's. Masses are 15(U)-318(J)
times Earth's. Jupiter contains twice as much mass as all other
planets combined. An animated timelapse image of Jupiter's rotation
and surface features is shown at the right. Click on the image for
Jupiter is midway between terrestrial planets and stars on a
power of ten mass scale. Objects only 13 times more massive are
considered to be small stars, not planets.
- Their low mean densities (~ 1 gram/cc) imply that Jupiter and
Saturn are mainly composed of H and He, with only small rocky
cores, perhaps Earth-size. That is a product of their formation
out of the cool regions of the solar
nebula, which were dominated by icy (H-rich) solids. Uranus and
Neptune have larger fractional complements of heavy elements than
Jupiter and Saturn.
- Internal structures
are entirely different from terrestrial planets because of their
predominant hydrogen composition. Click on the Jupiter
cross-sectional drawing at right for an enlarged version. The high
internal pressures in Jupiter and Saturn convert hydrogen gas
to liquid or "metallic" form in their interiors. The
lower-pressure interiors of Uranus and Neptune contain mantles
composed of ammonia, methane, and water. Unlike Terrestrial planets,
the Jovians have no solid surfaces. They are "gas
- The visible surfaces of the Jovians are cloud layers,
about 150 miles deep. The clouds consist of 3 main types of ice
crystals: ammonia, ammonium hydrosulfide, and water. Colors are from
trace compounds. Thin, white clouds on Neptune are methane
crystals, which freeze out at the lower atmospheric temperature there.
- "Spots": The most famous is Jupiter's Red Spot (the
large oval in the image at right: 22,000 mi long ~ 3x Earth's
diameter). It is a long-lived cyclonic storm. The
transient "Great Dark Spot" on
Neptune was a similar feature, and many smaller spots or ovals,
mostly transient and ranging from dark red to white in color, are
visible in the Jovian atmospheres (most conspicuous on Jupiter; 8 are
present in the image at the right).
- Atmospheric banding is caused by powerful lateral
by rising/falling convection currents. Winds reach
300-600 mph on Jupiter and Saturn and up to 1300 mph on Neptune. See
the enhanced pseudocolor image of Saturn's atmospheric banding below
- The Juno
spacecraft, skimming in an elliptical orbit to within 2600 miles of
Jupiter's cloud layers, is now
returning amazingly detailed
images of the atmosphere.
Here is a beautiful recent
Juno image of Jupiter's Red Spot with Io in the foreground, casting a
- Videos of Jupiter atmosphere:
Special Probes of Jupiter
Strong magnetic fields are generated by motions in the liquid metallic
hydrogen interiors of Jupiter & Saturn. Because the magnetic fields
trap energetic protons and electrons, these
produce powerful radiation
belts, up to 100x stronger than those of Earth.
Pseudo-color infrared image of
C. Ring Systems
Saturn has the brightest rings, but rings are present around all 4
- The rings are not solid: the inner rings revolve faster than the outer
ones, as expected for objects in Keplerian gravitational orbits
- The rings are huge in width but by comparison are remarkably
thin, mostly less than 1 km (3300 feet) thick. They have an aspect
ratio of about 300,000:1. This is over 110 times thinner than
standard printer paper. Here is an article
on the ring geometry by Phil Plait.
- The rings are composed of billions of ice-coated particles (typically about 10 cm in
size). Different particle sizes and coatings produce some of the
structure visible in the rings.
- Origin: the rings are primarily debris from
tidally or collisionally fragmented satellites
The rings lie inside the planet's "Roche Limit". Closer
than this distance from the planet's core, gravity tides would
pull apart a large body, such as a satellite.
- The rings have a complex structure (at right), consisting
of numerous gaps and ringlets. The biggest gaps are "resonance"
effects produced by the cyclical gravitational tug of the satellites
outside the ring. The ringlets may be produced by the self-gravity of
the material in the rings.
Video of Saturn's rings
Spacecraft images of the four Galilean satellites of
Jupiter, shown to scale
(Io, Europa, Ganymede, and Callisto).
D. The Jovian Planet Satellites
In many ways, the satellites of the Jovian planets are more
interesting than the planets themselves. The four largest satellites
of Jupiter (the "Galilean satellites") were discovered by
Galileo and were the first additions to the planetary inventory of the
Solar System in recorded history. Galileo saw them only as points of
light, and good information on their surfaces was not obtained until
the spacecraft flyby missions of the 1970's and 80's.
The composite picture above shows spacecraft images of the four
Galilean satellites (scaled to the correct relative sizes). It
illustrates the outstanding feature of these four satellites: they are
amazingly different from one another. Each is a unique world
in its own right. They are typical of all Jovian satellites in that
they are diverse, and they often have had violent
Here are some of the other characteristics of the Jovian
The satellite complement for a given planet is large: 14-79
Important examples of satellites: (click on the names for additional
Click here for a Java animation of orbits of
the satellites of each planet
Larger moons are mixtures of rocky/icy materials
e.g. Hyperion (Saturn), are
irregular in shape
- Two are larger than Mercury.
See this comparison of
satellites and planets.
- The large moons formed at the same time as their parent planet
- Because of their large ice content, their surfaces are more plastic
than those of the terrestrial planets; some show extensive evidence of
melting and resurfacing.
- Most of these are rocky or icy planetesimals/asteroids, which
were gravitationally captured by a planet over time.
- Io (J): Io is the innermost
of the four Galilean satellites of Jupiter. It suffers
continual volcanic eruptions caused by heating from tidal
flexing in Jupiter's gravitational field. Io is much more active
today than even the Earth. There are presently over 400 active
volcanoes on Io. Its huge volcanic plumes at the time of their
discovery by Voyager 1 in 1979 are shown at the right.
- Europa (J): The second
Galilean satellite is completely ice-coated and
extraordinarily smooth (i.e. has a small range of elevation).
Few craters, indicating a young surface. Most scientists believe the
ice shell covers an underlying
ocean, kept warm by tidal flexing (less severe than for Io).
Long, dark lines on the surface may be places where the shell has
cracked, allowing filling by younger ice. In 2013, NASA announced
vapor plumes jetting off of Europa's surface (like the similar
features found earlier on Enceladus). Because of the presence of
water, there is much speculation about a possible biosphere
on Europa (see Study Guide 23).
- Titan (S):
Saturn's largest moon has a thick atmosphere(!), visible as a
blue haze in the infrared image at the right. It is
mostly nitrogen with a small amount of methane. Titan is the
only object in the Solar System other than Earth to have a
predominantly nitrogen atmosphere. The atmosphere can be retained,
despite Titan's small mass, because of its low temperature at Saturn's
distance from the Sun.
Titan was the main target of the
Cassini-Huygens Mission. While the primary spacecraft stayed in
orbit around Saturn, the Huygens probe was detached and successfully
landed on Titan's surface in January 2005, relaying data during its
descent and for a short period on the ground.
Solar UV light interacting with methane has produced a rich mixture of
clouds and obscuring haze. There is probably hydrocarbon rain
here for an atmospheric profile.
Recent radar data from the orbiting Cassini spacecraft shows that there
are large lakes
on Titan, probably of methane or ethane.
In company with Europa and Enceladus (see below), Titan is now regarded
as a possible site of a biosphere --- but with lifeforms based on
utilizing methane rather than carbon dioxide.
Left: Enceladus; Center: water vapor plume from Enceladus;
Right: possible internal structure of Enceladus
Artist's Concept of Huygens Probe Landing On Titan
- Enceladus(S): Although only a
small satellite, Enceladus was unexpectedly discovered by the Cassini
orbiter to possess huge water/ice geysers jetting into space
from its surface. The plumes contain water vapor, complex
hydrocarbons, and sodium salts. The origin of the plumes is thought
to be warm liquid water reservoirs beneath the surface which
are heated by tidal flexing; jets escape through deep vents. See the
picture above. The outflow from Enceladus feeds Saturn's "E
In April 2014, scientists announced that gravity measurements deduced
from tiny accelerations of the Cassini spacecraft in the vicinity of
Enceladus confirm the presence of a liquid ocean with a volume
comparable to Lake Superior lying under its south pole.
- Miranda (U): has a remarkable
patchwork surface, probably from a shattering collision & reassembly
or possibly surface scars from internal convection
- Triton (N): the largest satellite
of Neptune is a captured KBO. (It has a "retrograde" orbit -- it
orbits in the opposite direction to Neptune's spin -- implying it did
not form with Neptune but was gravitationally captured later.) Triton
has a young, nitrogen-rich surface with diverse structural features
caused by ice melting, shifting, and re-freezing. During Voyager 2's
brief passage, it featured several
"cryovolcanos" of nitrogen, methane, ice, and dust jetting out of its
surface. Its surface has much in common with Pluto's (see
E. Pluto and the Kuiper Belt
Pluto is entirely unlike the four large outer planets. With a
diameter of only 1475 miles, it
is smaller by a factor of 2 than
any of the terrestrial or Jovian planets. It is a rocky/icy object
rather than a gas giant. Its orbit is the most highly inclined to the
ecliptic plane of any of the classical "9 planets."
When first discovered, Pluto was thought to be
isolated at the edge of the Solar System. However, in the last 25
years, after wide-field digital detectors were installed on large
telescopes, astronomers have identified many more similar
bodies, some with sizes comparable to Pluto. These, including Pluto,
are all members of
These discoveries, particularly that of Eris, precipitated the messy
discussion at the International Astronomical Union in the summer of
2006. Astronomers held a debate over the meaning of the term
"planet"---specifically whether or not Pluto and the other large KBO's
should be placed in a separate category. In the end, the IAU voted to
create a new category of "dwarf planet" for these objects but
was then forced to add the asteroid Ceres for consistency. All this
was handled very clumsily, and it generated needless controversy. It
turns out many non-astronomers were
fond of Planet Pluto and protested the demotion.
Long before the discovery of Pluto, we had already identified many
"minor planets" or "asteroids," small,
rocky objects with orbits lying mostly between the orbits of Mars and
Jupiter. Now, we know about many similar, but icy, objects beyond
Neptune. Sensible designations for these types, above some threshold
in size, are as "rock dwarf planets" and "ice dwarf planets,"
- The Kuiper Belt is a
huge volume beyond the orbit of Neptune, centered on the ecliptic
plane, but extending many AU's above and below the plane. It is much
larger than the asteroid belt, which lies
between Mars and Jupiter. Over 1000 "Kuiper Belt Objects" (KBO's)
have been discovered in this volume to date. A number of these are
larger than any of the asteroids; Pluto, for instance, is 2.5 times
the diameter of Ceres, the largest asteroid.
Here is a comparison of the
eight largest KBO's and their satellites to one another.
The plot at the right
shows the known KBOs with the Belt both face-on and edge-on (click for
- The most massive known KBO---yes, it's more massive than
Pluto---is Eris, also the second most distant known KBO (97 AU). It
was discovered in 2005. Its size (1400 miles diameter) is slightly
smaller than Pluto's.
for a page describing Eris by its discoverer, Mike Brown.
- Sedna, a
600-mile diameter object discovered in 2003, has an aphelion (greatest
orbital distance from the Sun) of 937 AU, although at present it is at
only 90 AU. Its orbital period is about 11,400 years. It is distant
enough that it may be a member of
the "Oort Cloud"
rather than the Kuiper Belt. The
VP113 is similar to Sedna, with a perihelion (minimum orbital
distance from the Sun) of 80 AU, currently the most distant perihelion
of any Solar System body. Some of the known KBO orbits are shown
The Pluto Flyby
New Horizons, the first
mission to Pluto and the Kuiper Belt, was launched by NASA in 2006
and, having received a gravity assist from Jupiter, finally reached
Pluto on 14 July 2015. (Yes, nine years later. The fact that New
Horizons was launched with the brisk velocity of over 36,000 miles per
hour gives you some appreciation for the scale of the solar system.)
The spacecraft could not carry enough fuel to decelerate into orbit
around Pluto (that would have made it much more massive and
expensive), so it was always planned as a "flyby," with a closest
approach of about 7800 miles. It took over 15 months for all the data
collected during the brief flyby to be telemetered back to Earth.
New Horizons returned a wealth of data on the surface and atmosphere
of Pluto and the nature of its 5 satellites. Although
the strangeness of Pluto's surface was anticipated by the 1989
Voyager images of Neptune's moon Triton, the close-range, high
resolution imaging of New Horizons revealed amazing features.
Much of the surface is covered by rugged terrain (reddish-brown in the
image above), with mountains ranging up to 16000 feet high. The
surface is not made of rock, however --- rather of a mixture of ices
of nitrogen, water vapor, carbon monoxide, and methane, which have
frozen out at the very low temperature of Pluto (-230C). The
cratering density in the mountainous regions indicates ages up to 4
billion years old.
The smooth, light-colored regions in the image above (part of the
heart-shaped "Tombaugh Regio") almost entirely lack cratering and are
therefore much younger, less than 10 million years old. They consist
mainly of frozen nitrogen and carbon monoxide. The surface is broken
into polygonal cells (see image below right), which indicate
relatively recent convective motions.
The force of the rising
ice was apparently enough to float the mountains of water ice lying at
the margins of the region.
Overall, Pluto is revealed as much more geologically active than
expected, with exchanges continuing to occur between the surface and
interior (including through apparent "cryovolcanos") and between the
surface and the atmosphere. The thin
mainly of nitrogen, methane, and carbon monoxide, partially freezes
out on the surface and then sublimes as Pluto's distance from
the Sun changes.
Charon, the largest (750 miles
diameter) of the five satellites of Pluto, also features surprisingly
complex terrain, with rugged canyons --- in this case almost
exclusively formed of water ice and without much change in the last 4
The Arrokoth Flyby
Following the Pluto encounter, New Horizons was retargeted to fly by
MU69M, which was located in a special 2014 search for objects near
to the spacecraft's outgoing trajectory.
New Horizons performed a highly successful flyby of this second KBO,
now officially named "Arrokoth," on 1 January 2019.
Data taken will be streaming back to Earth through mid-2020. But it was
immediately clear that Arrokoth is a fascinating object. It is a binary,
with two lobes joined at a narrow neck; the largest lobe is about 22km
across. Both lobes are significantly flattened. The reddish color
(also conspicuous on Pluto and Charon) is caused by tholins, organic compounds produced by solar UV irradiation of
carbon-based materials like methane and carbon dioxide.
Arrokoth as imaged by New Horizons
Reading (this material covers 2 lectures):
Study Guide 20
Bennett textbook, Chapter 11
Reading for next lecture:
Study Guide 21
Bennett textbook, Chapter 12
May 2020 by rwo
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copyright © 1998-2020 Robert W. O'Connell. All rights reserved.
These notes are intended for the private, noncommercial use of
students enrolled in Astronomy 1210 at the University of