Chapter 11:
Planetary Adornments –
Moons and Rings
Except for Mercury and Venus,
all the planets have Moons
The largest moon, Ganymede,
has evolved since forming
The moons of the outer
planets are either
geologically active,
probably active or dead
Geologically Active Moons
Io is the
innermost
of the
Galilean
Moons
and is
thus
rocky
Io is the most volcanically
active body in the solar system
Many of Io’s volcanoes are
actually sulfur geysers
Lava flows on Io are of extremely
hot magma (ultramafic)
Temperatures of 1450° to 1750°C…400° to 500°
hotter than Earth
Enceladus is a mid-size
moon of Saturn
Enceladus has ice geysers
It is warmer at the poles than
at the equator!
The cryovolcanism is from
the “tiger stripes”
Triton is the largest moon of
Neptune
Triton’s orbit is retrograde, at a
high tilt angle but perfectly circular
and in synchronous rotation.
Nereid is a small moon that orbits in a regular way
Triton has a thin atmosphere
The atmosphere is mostly nitrogen
Nitrogen Geysers on Triton
There are areas on Triton that
look like frozen lakes
Transition
region on
Triton
The transition is from
the polar region with
active geysers to a
“cantaloupe” region of
irregular hilly terrain
Moons that are probably
active
Europa is
the 2nd
Galilean
moon of
Jupiter
Europa has a frozen
surface over a liquid ocean
Ice Rafts of Europa
Titan, the other “probably active”
moon, is the largest moon of Saturn
Titan has a thick atmosphere of
nitrogen with a hydrocarbon
haze which obscures the
surface
The haze is similar in chemical composition to smog.
Think of LA on a really bad day and multiply by 100.
Cassini’s IR & UV cameras
penetrate Titan’s atmosphere
Cassini has found a “sea” near
the north pole of Titan
The liquid is a mixture of ethane and methane
The Huygens Probe
descended to
Titan’s surface
Moons that were once active
Ganymede
is the
largest
moon in
the solar
system
Transition Area
Many areas of Ganymede look like features on
Europa that froze long ago. There may still be
liquid in Ganymede but it is deep in the mantle.
Palimpsests form when impacts
occur over a slushy sub layer
Dione: a world of giant cliffs
Tethys has huge ice cliffs, too
Iapetus is a dark walnut
Miranda, a moon of Uranus, has
been broken and re-assembled
Ariel is another Uranian moon
The stillborn worlds:
geologically dead since birth
Callisto is
the
second
largest
moon of
Jupiter
Valhalla is a multi-ring
impact structure on Callisto
Mimas orbits Saturn
The large impact basin
is named Herschel
Reality is stranger than
fiction. George Lucas
dreamed up the Death
Star 3 years before the
first images of Mimas
from the Voyager mission
Rhea: Ancient and cratered
Hyperion is spongy
Umbriel is a Uranian moon
Oberon
Proteus is another moon of
Neptune
Saturn’s rings were first
observed by Galileo Galilei
“I discovered another very strange wonder, which
I should like to make known to their Highnesses . .
. , keeping it secret, however, until the time when
my work is published . . . . the star of Saturn is not
a single star, but is a composite of three, which
almost touch each other, never change or move
relative to each other, and are arranged in a row
along the zodiac, the middle one being three times
larger than the lateral ones, and they are situated
in this form: oOo.”
Galileo Galilei July 30, 1610
Galileo’s Saturn Drawings
Galileo observed Saturn for over two decades
but never understood what the rings were
With a better telescope,
Christiaan Huygens figured it out
and published his theory in 1659
In 1859 James Clark Maxwell
published a theory on the nature
of the rings that said they are not
solid but composed of small
objects in orbit around the planet
A Saturnian Season by HST
When the rings are edge on
they are almost invisible
All the giant planets have rings
The major rings are
all inside the Roche
limit of their host
planet. Most require
special equipment
and conditions to
detect. Only Saturn’s
rings are easily
visible with a modest
telescope.
Saturn’s rings consist of
thousands of ringlets
The rings are composed of
chunks of ice and rock
While mostly ice, there is
also a lot of dust and rock
Shepherd
Moons
Prometheus
and Pandora
Pan and Encke Division
Ring Spokes
Jupiter’s rings were first
observed by the Voyagers
Jupiter’s rings were extensively
studied by the Galileo probe
Metis,
Adrastea.
Amalthea and
Thebe are all
small moons.
The largest,
Almathea, is
about 100 km
across.
The rings of Uranus were
discovered by occultation
At last count, Uranus has 13 rings
Neptune’s rings are clumpy