Chapter 11: Moons, Rings and Pluto

Ring and Satellite Systems General properties. –Composition different from objects in the inner solar system  Most contain dark, organic compounds mixed with ice and rock  Dark materials imply small reflectivity –Most satellites in regular orbits  West-to-east direction  In the plane of the planet’s equator  Likely to have formed at about the same time as the planet –Irregular satellites  Retrograde (east-to-west), or else have orbits of high eccentricity, or high inclination  Usually smaller satellites, located relatively far from their planet  Formed subsequently or captured

Jupiter's Moons 52 known satellites (as of about 2 years ago) –Four large moons (Callisto, Ganymede, Europa, Io)  Europa, Io are the size of our moon  Ganymede, Callisto – the largest – are bigger than mercury –Many smaller moons.  Note: moon and satellite mean the same thing in astronomy.

Large "Galilean" Moons Io Europa Ganymede Callisto –Callisto, Ganymede, Europa, Io –first seen by Galileo (the astronomer) –studied by the Galileo space probe –Hubble Space Telescope observations –combined data has found important similarities to the terrestrial planets –differences between moons mostly due to distance from Jupiter

Callisto: Crater Packed  Outermost of the Galilean satellites.  2 million km from Jupiter.  Noontime surface temperature: 130 K (140 o C below freezing!)  Diameter 4820 km ~ same as Mercury  Mass ~ 1/3 Mercury’s mass  Not as dense as Mercury  Composed largely of ice.  Not fully differentiated. –From details of gravitational pull on Galileo spacecraft. –Surprising! –Yet appears to be “frozen”  Covered with craters  Absence of interior forces – geologically dead.

Craters on Callisto

Ganymede: Largest, Most Varied  Largest satellite  Cratered, but less so than Callisto  ¾ of the surface seem to have formed “recently” –1 billion years old – rather than 4.4  Differentiated –Rock and metals sank to form a core about the size of the Moon –Mantle + crust floating above core. –Magnetic Field – partly molten interior

Ganymede  Ganymede is the largest moon in the solar system –Diameter = 5262 km –Slightly larger than Mercury  Ganymede should have little tidal heating due to its distance from Jupiter –But, Ganymede shows evidence for surface alteration  Ganymede must have had more geological activity in the past –Did it once have a more eccentric orbit?

Old Dark Terrain  Old: it is covered with craters  Dark: ice covered with dust from meteoroid impacts

New Bright Terrain  New due to fewer craters  Bright due to fracturing of the icy surface

Craters – varying degradation Geologically active. –Younger terrain result of tectonic and volcanic forces. –Extensive Mountain ranges formed by compression of the crust. –Some indication of large scale crustal movements.

Why is Ganymede different from Callisto?  Small difference in –size. –Internal heating.  Gravity of Jupiter –Ganymede close enough to Jupiter to have tidal force effects – episodically heating the crust.

Europa: Ice-Covered Ocean real colorenhanced color

Europa  Europa’s surface is covered with a layer of ice  Under the ice is water or warm fluid ice  Water flows up to the top continually resurfacing Europa  Tidal heating produces the internal energy –It also has differentiated Europa into an iron core, a rocky mantle and an icy crust  Tidal flex may also crack the surface

Evidence for Warm Oceans on Europa  Europa has ice rafts where the surface has been broken up and reassembled  Galileo has imaged faults where the ice has pulled apart and water as flowed up  Europa also has smooth areas where water has flowed up and re-frozen  On Earth simple life forms evolved under water at warm deep ocean vents –Could something similar have happened on Europa?

Io

Io: Volcano World   Io is the most volcanically active world in the solar system   Io has an elliptical orbit, so the tidal forces on it vary with time   These changing forces squeeze and flex Io producing heat   The hot interior produces massive volcanism   The interior heat has also produced a differentiated interior – –Io has an iron core surrounded by a molten, rocky mantle Io Jupiter

Volcanism on Io   Io has no impact craters – –They have been eradicated by lava   Volcanoes produce plumes of material that extend up to 280 km above the surface   The colors on Io come from sulfur (yellow, black, red) and from sulfur dioxide (SO2, white)   Volcanoes can be very long lived – –Some have been observed for 20 years Loki volcano erupts

Pele Volcano

Io’s Plasma Torus   Io’s volcanoes put lots of ions into its orbit – –Ions are atoms that have lost an electron giving them a net electrical charge   The ions are effected by Jupiter’s magnetic field producing a plasma torus   As Jupiter rotates its changing magnetic field produces an electrical current through the torus and interior

Saturn’s Moons and Rings  30 known satellites (before Cassini) –Titan: largest of Saturn’s satellites  Almost as big as Ganymede  Only satellite with substantial atmosphere  Rings of Saturn

Titan  The second largest moon in the solar system –Hint: on the test, don’t be fooled by the name!  Only moon with a significant atmosphere  Thick atmosphere makes the surface impossible to see  Why does Titan have an atmosphere? –Titan is large enough to have a strong gravitational field –Titan is cold enough so that the gas in the atmosphere is slow moving

Dense atmosphere:  pressure 1.5 times Earth's  mostly nitrogen plus 6% argon and a few percent methane.  trace amounts of organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water  water is formed when methane in Titan's upper atmosphere is exposed to sunlight.  chemical activity despite low surface temperature, 94 K (-290 F).  like the smog found over large cities, but much thicker. Conditions like Earth early in its history when life was first getting started. May have the necessary building blocks for life! Titan: Cloud World

Huygens Probe Lands on Titan

Uranus System  Ring and Satellite tilted at 98 o just like the planet itself.  11 rings –Composed of very dark particles –Discovered 1977 –Narrow ribbons of material with broad gaps  very different from the rings of Saturn  20 known satellites – none really large

Neptune System  8 known satellites –6 regular close to the planet –2 irregular farther out  Triton (remember, Neptune carries a trident; in mythology, Triton is Neptune’s son) –large moon in retrograde orbit, –has an atmosphere, –active volcanism, –Bears some resemblance to Pluto

Triton: Ice World

orbit is retrograde: didn’t form with Neptune! Perhaps a captured Kuiper Belt object Capture may have shattered another Neptunian moon Capture scenario accounts for: Triton's orbit unusual orbit of Nereid provides energy to melt and differentiate Triton's interior Historical connection to Pluto? similar bulk properties Pluto has eccentric Neptune- crossing orbit Is Triton a captured comet? What about Pluto? Triton: Ice World

Ice volcanoes on Triton: plume rising 8 km above the surface and extending 140 km "downwind" Triton: eruptions of volatile gases like nitrogen or methane driven by seasonal heating from the Sun. Earth, Venus, Mars: rocky magma driven by internal heat. Io: sulfur compounds driven by tidal interactions with Jupiter. plume

Triton's Atmosphere  Triton’s Atmosphere  Triton has a very thin nitrogen atmosphere (1.6 X atmospheres of pressure)  Triton is very cold (37 K) and thus nitrogen is mostly frozen on the surface  A little bit of nitrogen evaporates to produce the atmosphere  Motions of the atmosphere (wind) seem to effect the plumes

Tenuous Clouds

Summary: Six Large Moons  The six large moons of the gas giants resemble the terrestrial planets of the inner solar system –They can have volcanoes, atmospheres, and evidence of resurfacing  In general they are cold and have rocky interiors and icy exteriors  Some produce internal energy through tidal heating  Europa and Titan may have the conditions for life to exist

Pluto  Discovered through systematic search. –At P. Lowell observatory in –Named Pluto after the roman god of the underworld (also PL are the initials of Percival Lowell).  Highest inclination to the ecliptic (17 o ).  Largest eccentricity ~  Average distance ~40 AU = 5.9 billion km. –Perihelion closer than Neptune  Orbital period ~ earth years.  Rotation : ~ 6.4 days on its side.  Pluto's diameter 2240 km  Largest satellite: Charon –Charon’s orbit is locked to Pluto: Charon revolves and rotates in the same time as Pluto rotates.  Also two smaller satellites found. HST Picture Pluto Charon

Pluto Basics  Not visited by spacecraft, – very faint, –observation requires best telescopes  Diameter ~ 2190 km (60% of the Moon)  Density ~ 2.1 g/cm3  Mixture of rocky material and water ice  Similar to Triton (Neptune)  Highly reflective surface – –frozen methane, carbon monoxide, nitrogen  Surface temperature 50 K/ 60 K  Tenuous atmosphere.

Quaoar – new planet?  orbit more circular than Pluto's  closer to the ecliptic – 7.9 degree inclination compared to Pluto's 17 degrees.  diameter 1280 km vs. Pluto's 2240 km  possibly Pluto and Quaoar are both Kuiper belt objects

Pluto's Orbit

The Nature of Pluto  Pluto is not like the Terrestrial or Jovian planets.  Pluto, Quaoar, Xena, Charon, and possibly Triton, are examples of Kuiper belt objects.

Rings  All four giant planets have rings  Each ring is a system of billions of small particles (moonlets).  Each ring displays complicated structure related to the interaction between the rings and satellites.  The four ring systems are quite different.

Ring Basics  Saturn –Made up of icy particles spread out into several vast, flat rings, with a great deal of fine structure.  Neptune/Uranus –Made up of dark particles, confined to a few narrow rings, with broad empty gaps.  Jupiter –Rings are transient dust bands, constantly renewed by erosion of dust grains from small satellites

What causes Rings?  Ring = collection of vast numbers of particles –Each particle obeys Kepler’s laws. –Inner particles revolve faster –Ring does not rotate as a solid body. –Better to consider the revolution of individual moonlets.  Particles within the ring are close to one another. –Exert mutual gravitational influence, even collide in low speed collisions. –Gives rise to waves that move across the rings.  Two basic theories –Breakup theory, remains of a shattered satellite. –Make up of particles that did not fuse into a single body

Ring Causes Continued  Gravitation of the planet –Tidal forces for orbits close to the planet, can tear bodies apart, or inhibit loose particles to come together.  Rings of Saturn, Uranus are close to the planet…  Breakup – a satellite, or a passing comet may have come too close and torn apart under tidal forces, or through some collision.  It is believed that some of the rings are young, and must therefore be the result of a breakup.

Rings of Saturn  Many rings and sub-rings: A, B, C  B-Ring : Brightest, most closely packed particles  A/C-rings : translucent.  Total mass of B ~ that of icy satellite 250 km in a diameter.  A & B separated by a wide gap called Cassini division.  Rings are broad and very thin.  Main ring ~ km, thickness ~ 20 m.  Composed of water ice.  Particles range from grains the size of sand up to house-sized boulders  A handful of narrow rings ~ 100 km, in addition to the main rings.

 Narrow and black  Almost invisible from Earth  Nine rings discovered (1977) during observation of a star – occultation  First seen by Voyager (1986)  Outermost and most massive called Epsilon –100 km wide, ~ 100 m thick, km from the planet.  Other rings much smaller : 10 km wide.  Particles are very dark; black carbon and hydrocarbon compounds.  Rings of Neptune are similar but even more tenuous. Rings of Uranus and Neptune

Satellite-Ring Interactions  Rings have intricate structure as discovered by Voyager.  Structures due to mainly gravitational effects of satellites. –Without satellites, the rings would be flat and featureless. –There could even be no rings at all… –Gaps in Saturn A-ring result from gravitational resonances with smaller inner satellites. (Mimas)

Uranus' moon Miranda: Uranus' moon Miranda: innermost and smallest of the five major satellites, just 480 kilometers (about 300 miles) in diameter. Two major strikingly different types of terrain:  old, heavily cratered, rolling terrain with relatively uniform reflectivity.  young, complex terrain characterized by sets of bright and dark bands, scarps and ridges (ovoid regions at right and left and the distinctive chevron feature below and right of center).  likely due to upwelling of partially melted ices

April 11, 2006Astronomy Discussion Question  Astronomers wish to search for life in the ocean believed to lie beneath the ice of Europa. How should we approach this exploration to avoid possible cross contamination of Earth and Europa with organisms (DNA) from each other?