Transcript Moons, Rings, and Pluto: Small Worlds Among Giants

Moons, Rings, and Pluto:
Small Worlds Among Giants
The Large Moons
The Galilean Moons of Jupiter:
A “Miniature Solar System”
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Four large moons travel on nearly
circular, prograde orbits in Jupiter’s
equatorial plane.
Moving outward from the planet, they
are Io, Europa, Ganymede, and Callisto.
Like a miniature solar system, their
densities decrease with increasing
distance from Jupiter.
A “Miniature Solar
System”
• Io and Europa have large, iron-rich cores
surrounded by thick mantles of rocky
composition.
• Europa has a water / ice outer shell between
100 and 200 km thick.
• Ganymede and Callisto are of more lightweight
overall composition (low-density materials like
water ice).
• Ganymede has a small metallic core topped by
a rocky mantle and a thick icy outer shell.
• Callisto is an undifferentiated mixture of rock
and ice.
Io: The Most Active Moon
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Most geologically active object in entire solar system.
Similar to Earth’s moon in mass and size.
Uncratered surface is collage of oranges, yellows, and blackish browns.
Has active volcanoes. Io’s smooth surface is apparently the result of molten matter
constantly filling in any dents or cracks.
Thin atmosphere made up mainly of sulfur dioxide, produced by volcanic activity and
temporarily retained by the moon’s gravity.
Io: The Most Active Moon
• More than 80 active volcanoes have been identified on Io.
• Largest volcano, named Loki, is larger than the state of Maryland and
emits more energy than all of Earth’s volcanoes combined.
• Io is too small to have all of this activity - it should have died long ago
like our moon. The source of Io’s energy is external - Jupiter’s gravity,
combined with that of the other moons.
• If no other moons were present, it would have suffered the same fate
as our moon. Due to the gravity of the other moons, Io is constantly
“pulled” back and forth creating tidal stresses that continually flex
and squeeze the interior.
• The large amount of heat generated within Io causes huge jets of gas
and molten rock to squirt out of the surface.
• Likely much of the interior is soft or molten, with only a relatively thin
solid crust overlying it.
Europa: Liquid Water Locked in Ice
• Relatively few craters on its surface.
• Surface displays vast network of lines
crisscrossing bright, clear fields of water
ice.
• Ice may be several km thick, with oceans
below up to 100 km deep.
• Again Jupiter’s gravity and the pull of the
other moons is the source of energy, but
the effects here are less extreme due to
increased distance from the planet.
Europa: Liquid Water Locked in Ice
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Icy equivalent of lava flows on Earth - water erupted through the surface and
flowed for many km before solidifying (also called ice volcanism).
Has a weak magnetic field that constantly changes strength and direction
(direct evidence for the saltwater ocean, which would conduct electricity and
thus establish a magnetic field).
May contain more liquid water than exists on Earth.
While still a hostile environment, the large presence of water oceans presents
the possibility of life forming on Europa.
Ganymede and Callisto: Fraternal
Twins
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Ganymede is largest moon in solar system, also larger than
both Mercury and Pluto.
Darker regions on Ganymede are original icy surface,
heavily cratered. Lighter regions are smoother - liquid
water upwelled from the interior, flooding the impact
regions before solidifying.
Ganymede has a system of grooves and ridges indicative of
plate tectonics occurring in the moon’s past - ceased 3
billion years ago as the crust became too thick.
Ganymede and Callisto: Fraternal
Twins
• Callisto is more heavily cratered with fewer fault lines than Ganymede.
• Callisto’s most obvious feature - series of concentric rings surrounding two large
basins. Formed from an impact - upthrust ice partially melted and solidified
before ripples subsided.
• Callisto froze before plate tectonics or other activity could start.
• From cratering rate, Callisto’s surface is maybe 4 billion years old.
Ganymede and Callisto: Fraternal
Twins
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Ganymede’s differentiation indicates it was largely molten at some point,
while Callisto is undifferentiated and apparently never molten.
Ganymede has a weak magnetic field, suggesting the presence of a liquid or
“slushy” water below its surface.
Can’t yet explain all the features of these moons.
Titan: A Moon with an Atmosphere
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Largest moon of Saturn.
Orange color comes from presence of an atmosphere.
A thick, uniform haze layer completely covers the moon.
Surface has icy plateaus smeared with hydrocarbon tar.
Titan: A Moon with an Atmosphere
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Ridges and cracks on surface suggest geological activity in the form of “titanquakes” may
be common.
Evidence for some sort of erosion occurring (possibly wind or volcanic activity).
Fewer craters than expected also suggests geological activity (resurfacing is occurring).
Atmosphere is thicker and denser than Earth’s and more substantial than any other
moon.
Atmospheric composition:
– 90% Nitrogen
– Up to 10% Argon
– Few % Methane
– Trace amounts of other gases.
Surface temperature is a frigid 94 K.
Water ice plays the role of rock on Earth, and liquid water the role of lava (as on
Ganymede and Callisto).
At the temperatures typical of the lower atmosphere, methane and ethane behave
rather like water on Earth - possibly methane rain, snow, fog, and even rivers and
oceans.
Titan: A Moon with an
Atmosphere
• Atmosphere acts like a gigantic chemical factory. Scientists are
interested in it as the same reactions may have preceded the
existence of life on Earth. These reactions would have been
necessary for Earth to have been hospitable for life.
• Huygens probe in 2005 entered Titan’s atmosphere - images
show a network of drainage channels leading to a shoreline. It
landed on solid ground and transmitted images for an hour.
• Titan’s low temperature allows it to retain an atmosphere
easier than other moons like Ganymede and Callisto.
• Don’t yet know if Titan’s differentiated or not.
Triton: Captured from the Kuiper
Belt?
• Neptune’s large moon, about half the
mass of Europa.
• Icy surface that reflects much of the
sunlight reaching it - temperature of only
37 K.
• Thin nitrogen atmosphere and solid frozen
surface probably composed of water ice.
• Nitrogen frost forms and evaporates
seasonally over the polar caps.
• Surface activity has erased most of the
impact craters.
• Large fissures on the surface (like
Ganymede).
• Numerous frozen lakes of water ice,
thought to be volcanic in origin.
Triton: Captured from the Kuiper
Belt?
• Voyager 2 detected great jets of nitrogen gas erupting several km into
the sky.
• Scientists speculate such nitrogen geysers on Titan are common and
perhaps responsible for the thin atmosphere.
• Triton is the only large moon in the solar system to orbit its planet in
retrograde.
• Only large jovian moon to not orbit in the parent planet’s equatorial
plane (i.e. orbit is tilted compared to others).
• Due to odd orbital behavior, it is thought that Triton was captured by
Neptune rather than forming along with Neptune.
• Triton is spiraling in towards Neptune due to retrograde orbit. (Our
moon is slowly spiraling away from Earth.)
• Triton is doomed to be torn apart by Neptune’s gravitational field,
probably within 100 million years due to its inward motion.
The Medium Moons
The Medium-Sized Jovian Moons
• Radii between 200 and 800 km.
• Densities suggest that all are composed mostly of rock and water ice.
• All move on nearly circular orbits and are tidally locked by their
parent planet’s gravity into synchronous rotation.
The Medium-Sized Jovian Moons
• Most show no signs of significant geological activity, with a few exceptions.
• Saturn’s Dione has bright ice cliffs created by tectonic fractures - surface
cracked and buckled as moon cooled. Ice volcanism appears to have erased
older craters.
• Saturn’s Iapetus has a two-faced appearance - one hemisphere is very dark and
the other very light. Don’t know if dark material is coming from inside or
outside the moon.
• Saturn’s Tethys and Uranus’s Ariel both have extensive cracks on their surfaces,
which are most likely the result of meteoritic bombardment.
• Saturn’s Enceladus has ongoing geological activity - possibly coated with ice
crystals, the result of water volcanoes. Much of surface is devoid of craters,
which would have been erased by “lava flows” of water liquefied during
internal upheavals and now again frozen. Don’t know why such a small moon
has so much activity.
The Medium-Sized Jovian Moons
• Most show heavy cratering.
• Saturn’s Mimas and Uranus’s Miranda show clear evidence of violent
meteoritic impacts.
• The impact that caused the large crater on Mimas must have come
very close to shattering the moon.
• Some speculate Miranda’s varied terrains are the result of multiple
major impacts, with the pieces falling back together in a chaotic,
jumbled way.