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Jovian planets, their moons & rings
Jovian planets, their moons & rings
The Moons of the Jovian PlanetsThe terrestrial planets have a total of 3 moons.
The jovian planets have a total of 166 moons.
Each collection of moons orbiting the gas giants can be thought of as a “mini solar system”.
All four giant planets have captured many passing objects into permanent or semi-permanent orbits.
More than half of the 166 known moons fall into this category of “irregular moons”. Nearly all of these moons have been discovered in the last 10 years and are completely unexplored.
The Moons of the Jovian PlanetsThe large moons are about the size of the Moon.
The Moons of JupiterA total of 64 moons have now been found orbiting
Jupiter, but only 8 orbit in the same direction and in the plane of Jupiter’s equator.
The four largest regular moons of Jupiter are the Galilean moons: Io, Europa, Ganymede, & Callisto
The Galilean moons are similar to terrestrial planets, but their densities are lower, most contain ice as well as rock and metal.
The Large Moons of JupiterEuropa is smaller than Earth’s Moon; Io is bigger.
Ganymede is larger than Mercury and the largest moon in the Solar System.
Callisto is as large as planet Mercury.
Small, Geologically Dead Worlds?As we’ve discussed with the terrestrial planets, smaller worlds lose their internal heat the fastest.
• Mercury and the Moon cooled long ago and their geologic activity ended billions of years ago.
• The largest moons of the Solar System are the size of Mercury and the Moon. We would predict that they would be cold and geologically dead.
• The major surprise has been the vigorous activity of these small worlds!
Io Europa
Callisto
Surfaces of the Galilean MoonsA closer look at their surfaces show no craters on Io, few craters on Europa, more on Ganymede, and many on Callisto. What’s going on???
Io Europa
CallistoJupiter
Jupiter’s gravity creates tides in the moons. How does the tidal force change during an orbit?
A Change in the Tides
Jupiter
(Effect is exaggerated!)
A body on an elliptical orbit will feel a changing tidal force, so the body will flex in response.
Strong TidesWeak Tides
Io (pronounced eye-oh) is the most volcanically active of any world in the Solar System!
In this view from the New Horizons probe (Feb. 2007), a volcanic plume is visible near the north pole of Io.
The fountain rises over 300 km above the surface.
(This series of 5 frames spans 10 minutes.)
Io: Volcanic World
Io’s volcanic activity:
• Driven by enormous tidal stress. The ground flexes up & down by 100 meters each orbit
• Many active volcanoes, some quite large, rivaling or surpassing volcanoes on Earth and Venus
• Activity can change surface features in a few weeks as eruptions deposit fresh material
• No craters; they fill too fast with lava flows or deposits from eruptions – Io has the youngest surface of any Solar System object
Io: Volcanic World
Each black feature is an active volcano.
Some erupt plumes above the surface.
Others produce huge lava flows of silicate rock.
The yellow colors are due to sulfur chemicals.
Io: Volcanic World
Europa: Water World
Observations by the Voyagers & Galileorevealed a world with an icy surface riddled with cracks and few craters.
The Icy Crust of EuropaGalileo flybys gave us a closer look.
The crust of Europa appears to be a layer of water ice floating on the interior.
Tides or impacts shatter the crust, which freezes again.
These “icebergs” are several km across.
Europa’s Ocean?From the low density of the moon, the amount of water must be large:at least 100 km thick. (Earth ocean 4 km deep.)
Today, data cannot rule out one or the other:
Warm slushy interior
OR
An ocean 100 km deep
Ganymede, Europa’s Bigger SiblingVoyager images showed Ganymede had two terrains described as bright and dark.
A few bright impact craters are visible, but far fewer than on Mercury or the Moon.
Ganymede, Europa’s Bigger Sibling
Galileo images showed two terrain types are fundamentally different:
Dark terrain is saturated with craters and rolling landscape (old!).
Bright terrain is grooved and very lightly cratered (young!).
Life on Europa, Ganymede?Ganymede’s interior may also be warm enough for a subsurface ocean. So Europa and Ganymede may have environments like Earth’s oceans. If conditions are right, perhaps life developed on these watery moons.
Orbiters equipped with radar could look through the ice.
Future plans call for a probe to burrow in and explore what lies below.
Callisto, outer Galilean moonCallisto is slightly smaller than Ganymede and shows no current geologic activity…
Craters cover the surface of Callisto.
Interiors of the Galilean Moons
Composition:
Gray = metal
Brown = rock
Blue = liquid water or slush
White = waterice
Io Europa
Ganymede Callisto
The Moons of Saturn
Saturn’s 62 moons appear to be made of water iceand some rock. They exhibit different geologic histories. Saturn has:
• One large moon, Titan, the second largest moonand the only moon with a thick atmosphere
• Six medium-sized moons – Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus
• And many, many smaller moons
The Moons of Saturn: TitanTitan has been known for many years to have an atmosphere thicker and denser than Earth’s.
Close-up observations by Voyager 1 and 2 show Titan as a blurry orange ball with a thin, blue haze.
The Moons of Saturn: TitanOf the small worlds in the Solar System, only Titan has an atmosphere. What’s special about Titan?
Atmospheres are a balance between high speed atoms and molecules and strength of gravity.
Small worlds have weak gravity, so escape speeds are low. Titan has a mass between Moon/Mercury.
Worlds far from the Sun have colder temperatures. Titan is 9.5 AU from the Sun, average of 95 K.
Weak gravity and slow atoms equals atmosphere!
Titan’s AtmosphereSurface pressure is 1.6 times higher than Earth.
Methane/ethane cycle: evaporation/clouds/rain like water on Earth.
Atmosphere is mostly nitrogen, like Earth.
Surface temperature is 95 Kelvin.
Cold enough that water is as hard as rock.
Exploring TitanTitan is shrouded in haze similar to smog particles. Cassini’s IR camera can see down to the surface.
Exploring TitanRadar shows the surface of Titan is very Earth-like: mountains, dark sand dunes, river valleys, lakes of liquid, and some recent craters
sand dunes
mountainranges
river valleys
Exploring TitanCassini radar images of the north pole show lakes of liquid methane & ethane (liquid natural gas).
Exploring TitanIn Jan. 2005, Cassini dropped the Huygens probeinto Titan’s atmosphere. It safely touched down on the surface, taking pictures and atmospheric measurements along the way.
artist’simpression
Exploring TitanOn the surface, Huygens found many rounded boulders made of water ice.
On Earth, rocks are rounded from banging together in stream beds and river channels.
This site must have seen a lot of flowing liquid at some time in the past: flash floods of methane?
The surface is eerily familiar and very different at the same time…
The Rains of TitanIn March 2011, Cassini scientists announced they had observed rain showers on Titan.
The bright clouds are made of methane and they left behind dark patches, interpreted as wet ground.
Wet doesn’t mean liquid water here… too cold!
It means liquid methane.
Medium-Sized Moons of Saturn
• Mimas, Enceladus, Tethys, Dione, and Rheaorbit close to Saturn. All are tidally locked.
• Iapetus orbits farther out, but is still tidally locked to Saturn.
• Note that all but one are heavily cratered!
(Moons shown approximately to scale.)
Enceladus, Active Moon of Saturn
Recent data from Cassini shows that Enceladus has many of the same features as Europa: banding and cracks.
But there are also regions that show heavy cratering, so geologic activity is not uniform.
Enceladus and the E Ring
In early 2006, these images showed a spray of water ice particles (“geysers”) coming from the south pole of Enceladus. This volcanic activity involving liquid water feeds Saturn’s E ring.
Enceladus: The Geyser MoonAfter the discovery, Cassini was flown past and then through the geysers to measure properties:chemistry, temperature, flow rate, speed…
Enceladus: The Geyser MoonDuring these flybys, Cassinifound the source of the geysers…
Deep cracks in the surface ice allow water to spray into space.
At first, it was thought that warm ice was sublimating (ice to gas directly)…
Enceladus: The Geyser MoonBut based on the sizes of the ice crystals erupted, scientists now think these deep cracks lead down to liquid water below the surface!
Another moon with an ocean?
Or lakes under the icy surface?
The Moons of NeptuneNeptune has 13 known moons, but 12 of them are small and unexplored.
The exception is Triton. This moon has a 6-day, retrograde orbit. It is a very large moon, nearly 80% the size of Earth’s Moon.
Its retrograde orbit suggests Triton was captured rather than forming in orbit around Neptune.
Triton and Neptune raise large tides in each other. This may explain why Neptune is hot and why Triton has a geologically active surface.
Triton, Neptune’s Large MoonTriton is larger than Pluto and may provide clues to Pluto’s geology. Triton’s surface is crater-free, due to volcanic “lava” flows of liquid water…
Ring SystemsThe ring systems of the jovian planets are formed by two different processes:
We’ve seen the first in action with Enceladus and Saturn’s E ring: material lost by moons. Jupiter’s ring is dust blasted from the inner moons.
The second process involves the destruction of a moon by a planet’s gravity.
This 2nd process created the main ring system of Saturn and may have created the ring systems of Uranus and Neptune.
Jupiter’s RingJupiter has a thin ring located inside the orbit of the small, innermost moons. It is made of dust blasted off the small, inner, rocky moons by impacts with small meteorites or interplanetary dust particles.
Galileolookingat nightside ofJupiter
Tides grow stronger closer to a planet.
Within a critical distance, tidal forces are greater than a moon’s own gravity. No moon can form!
Destroying a Moon
This limits the size that moons can grow close to a planet. A “swarm” of small moonlets close to the planet are likely to collide, creating fragments that can cause further collisions → a ring system!
The ring systems of all four gas giants are inside the Roche limit. (except one of Neptune’s rings)
Ring Systems and the Roche Limitpl
anet
radi
us0
12
3
Roche limit (approximate)
Saturn’s Ring SystemSaturn has an extraordinarily large and complex ring system, which was visible even to the first telescopes. This is a view from Cassini:
Cassini Division
Encke Gap inner edge
outer edge
The rings are extraordinarily thin: less than 100 m
Saturn’s Ring System
The rings are transparent, so they cannot be solid!
Instead, they are made of objects ranging in size from snowflakes to large buildings…
Saturn’s Ring System
Saturn’s Ring SystemThis is a close view from Cassini showing the outer edge of the ring system and the faint F ring:
Close inspection shows a moon in the Encke Gap.
The gap is due to the gravitational influence of this moon, called Pan.
It is 20-30 km wide.
Saturn’s Ring SystemEven closer inspection of the outer F ring shows two “shepherd moons”: Prometheus & Pandora
Note the waves in the rings that follow the passage of the moons.
The pulling of ring particles by the gravity of the moons sculpt the ring system into a multitude of thin rings.
Saturn’s Ring System
Each moon couples to the orbital period of particles at certain distances from Saturn.
Here bright indicates lots of particles, dark means there are few particles.
The Rings of UranusThe ring system of Uranus was discovered by accident in 1977. A team was planning to watch a bright star fade as it passed behind the planet.
To their surprise, the star blinked out 5 times before Uranus covered it. The star blinked again after Uranus moved past.
The team members concluded that Uranus must have 5 thin rings orbiting it.
The Rings of Uranus
With the arrival of Voyager 2, more thin rings and several shepherd moons were found.
Altogether, 11 thin rings were identified in images from Voyager 2.
The Small Moons of UranusUranus has 12 small moons that orbit near the ring system and may help shepherd the ring particles, keeping the rings narrow.
Hubbleimage
from 1997
The Rings of NeptuneNeptune has several faint rings. Differences in width and brightness is a mystery, but may be due to small, unseen shepherd moons.
positionof
Neptune
This image shows Neptune’s faint rings lit from behind by the Sun. Neptune is too bright, so it has been digitally removed.
