Jovian planets, their moons & rings
The Moons of the Jovian Planets The 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 Planets The large moons are about the size of the Moon.
The Moons of Jupiter A 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 Jupiter Europa 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. Io Europa 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! Callisto
Surfaces of the Galilean Moons A 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 Jupiter Callisto
A Change in the Tides Jupiter s A body on gravity an elliptical creates orbit tides will in feel the a moons. changing How tidal does force, the tidal so force the body change will flex during response. an orbit? Weak Tides Strong Tides Jupiter (Effect is exaggerated!)
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: Io: Volcanic World 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.
Observations by the Voyagers & Galileo revealed a world with an icy surface riddled with cracks and few craters. Europa: Water World
The Icy Crust of Europa Galileo 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.
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 Europa s Ocean?
Ganymede, Europa s Bigger Sibling Voyager 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 moon Callisto 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 = water ice Io Ganymede Europa Callisto
The Moons of Saturn Saturn s 62 moons appear to be made of water ice and some rock. They exhibit different geologic histories. Saturn has: One large moon, Titan, the second largest moon and 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: Titan Titan 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: Titan Of 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 Atmosphere Surface 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 Titan Titan is shrouded in haze similar to smog particles. Cassini s IR camera can see down to the surface.
Exploring Titan Radar shows the surface of Titan is very Earth-like: mountains, dark sand dunes, river valleys, lakes of liquid, and some recent craters sand dunes mountain ranges river valleys
Exploring Titan Cassini radar images of the north pole show lakes of liquid methane & ethane (liquid natural gas).
Exploring Titan In Jan. 2005, Cassini dropped the Huygens probe into Titan s atmosphere. It safely touched down on the surface, taking pictures and atmospheric measurements along the way. artist s impression
Exploring Titan On 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 Titan In 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 (Moons shown approximately to scale.) Mimas, Enceladus, Tethys, Dione, and Rhea orbit 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!
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 Moon After the discovery, Cassini was flown past and then through the geysers to measure properties: chemistry, temperature, flow rate, speed
Enceladus: The Geyser Moon During these flybys, Cassini found 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 Moon But 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 Neptune Neptune 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 Moon Triton 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 Systems The 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 2 nd process created the main ring system of Saturn and may have created the ring systems of Uranus and Neptune.
Jupiter s Ring Jupiter 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. Galileo looking at night side of Jupiter
Destroying a Moon 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! 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!
Ring Systems and the Roche Limit The ring systems of all four gas giants are inside the Roche limit. (except one of Neptune s rings) planet radius 0 1 2 3 Roche limit (approximate)
Saturn s Ring System Saturn has an extraordinarily large and complex ring system, which was visible even to the first telescopes. This is a view from Cassini: Encke Gap inner edge Cassini Division outer edge
Saturn s Ring System 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 This 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 System Even 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.
Saturn s Ring System The pulling of ring particles by the gravity of the moons sculpt the ring system into a multitude of thin rings. 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 Uranus The 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 Uranus Uranus has 12 small moons that orbit near the ring system and may help shepherd the ring particles, keeping the rings narrow. Hubble image from 1997
The Rings of Neptune Neptune has several faint rings. Differences in width and brightness is a mystery, but may be due to small, unseen shepherd moons. This image shows Neptune s faint rings lit from behind by the Sun. Neptune is too bright, so it has been digitally removed. position of Neptune