ASTR 1210 (O'Connell) Study Guide 20
Jupiter with Io and Europa in foreground
(Voyager Mission image)
"And now for something completely
different," as they used to say on Monty Python.
The 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, exhibit 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. In many ways, they are
more interesting than their parent planets. 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 dwarfs," of which Pluto is the archetype.
A. History of Discovery/Exploration
B. Jovian Planets (J,S,U,N): Properties
These four share gross properties. Pluto is entirely different (see
Distant from Sun: 5-30 AU. (Pluto is at 39 AU.) The outer solar system is
vast (over 10,000 times the volume of the inner solar system out to
Mars) and sparsely populated.
Large: Radii are 4-11 x Earth. Masses are 15(U)-318(J) x Earth.
J. 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 a more current, high
resolution HD video.
Jupiter is midway between planets and stars on a power of ten
mass scale. Objects only 13 times more massive are considered to be small
- Their low mean densities (~ 1 gr/cc) imply the Jovians
are mainly composed of H and He, with only small rocky cores,
perhaps Earth-size. U,N have larger complements of heavy elements
- Internal structures
are entirely different from terrestrial planets because of their
predominant hydrogen composition. That is a product of their
formation out of the cool regions of the solar nebula, dominated by
icy (H-rich) solids. (Click on the cross section drawing at right for
an enlarged version.)
- High internal pressures in J,S convert hydrogen to liquid or "metallic"
form in their interiors
- No solid surfaces: these are "gas giants"
- Visible surfaces = 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.
- "Spots", e.g. Jupiter's Red Spot (large oval in image at right:
22,000 mi long ~ 3x Earth) are long-lived cyclonic storms. There are similar
features on other Jovians (e.g. the transient "Great Dark Spot" on Neptune).
- Atmospheric banding is caused by lateral windstreams and
rising/falling convection currents. Winds reach 300-600 mph on J,S
and a maximum of 1300 mph on Neptune. See the enhanced pseudocolor
image of Saturn's atmospheric banding below right.
- Videos of Jupiter atmosphere:
Special Probes of Jupiter
Strong magnetic fields are generated by motions in the liquid metallic
hydrogen interiors of Jupiter & Saturn. These produce
strong radiation belts, up to
100x 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
- They 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/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
(Io, Europa, Ganymede, and Callisto).
Each is a unique world in its own right.
D. The Jovian Satellites
Click here for a Java animation of orbits of
the satellites of each planet
Diverse(!) characteristics; often violent histories
Larger moons are mixtures of rocky/icy materials
e.g. Hyperion (Saturn), are
irregular in shape
- 3 are larger than Mercury
- 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.
Interesting examples: (click on the names for additional illustrations)
- Most of these are rocky or icy
planetesimals/asteroids, gravitationally captured over
- Io (J): 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. Io's volcanic plumes at the time of their discovery are shown
- Europa (J):
ice-coated and extraordinarily smooth. 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
the detection of water 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).
(S): Saturn's largest moon has a thick
atmosphere!---mostly nitrogen with a small amount of methane. 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 European 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 possibly hosting 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; 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 ring."
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): shattering
collision & reassembly? or surface scars from internal convection?
- Triton (N): water/ice geysers
E. Pluto and the Kuiper Belt
Pluto is entirely unlike the four large outer planets. It
is smaller by a factor of 2 than
any of the other 8 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 20 years, astronomers have
discovered many more such bodies, some with
sizes comparable to Pluto. These are all members of the "Kuiper
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.
Even before the discovery of Pluto, we had already known of many
small, rocky objects in separate orbits around the Sun---the
"asteroids." Now, we know about many similar, but icy, objects.
Sensible designations for these types, above some threshold in size,
are as "rock dwarf planets" and "ice dwarf planets."
New Horizons, the first mission
to Pluto and the Kuiper Belt, was launched in 2006 and, having received
a gravity assist from Jupiter, is now approaching Pluto with a planned
flyby date of 14 July 2015. The spacecraft was originally intended to
fly within 6500 miles of Pluto's surface, but the discovery of a total
of 5 moons in orbit around Pluto has raised concerns about an invisible
ring system or debris field near Pluto, and the trajectory may have
to be adjusted. Following the flyby, New Horizons will be retargeted
to approach other Kuiper Belt Objects, assuming good candidates can
be found within its limited range of maneuver.
- 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. Over 1000
"Kuiper Belt Objects" (KBOs) have been discovered in this volume to
- The plot at the right shows the distribution of the known KBO's
as of 2012. Click for an enlargement, with scales giving distances
- The most massive known KBO---yes, it's more massive than
Pluto---is Eris, also the most distant known KBO (97 AU). It was
discovered in 2005. Its size (2300 km diameter) is comparable to
for a page describing Eris by its discoverer, Mike Brown.
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 record.
Images of Pluto obtained by New Horizons in April and May, 2015.
Quality will improve greatly toward encounter on 14 July 2015.
Reading for this lecture:
Study Guide 20
Bennett textbook, Chapter 11
Reading for next lecture:
Study Guide 21
Bennett textbook, Chapter 12
June 2015 by rwo
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copyright © 1998-2015 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