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11.2 Jovian Satellites “Planet--sized” Moons “Planet • Miniature solar systems • • • • Overview of outer solar system moons Geologic activity on Galilean moons The special case of Titan Open question: Why are small icy moons more geologically active than small rocky planets? Outer Solar System Moons Small Moons • Small moons (< 300 km) – No geological activity • Medium-sized moons (300-1,500 km) – Past Geological activity • Large moons (> 1,500 km) – Ongoing geological activity Medium & Large Moons • Enough self-gravity to be spherical • Substantial amounts of ice mixed with rock. • Mostly formed in orbit around jovian planets. • Circular orbits in same direction as planet rotation. • Very numerous g mass • Not enough to self-round • Many are likely captured asteroids or comets. • Unusual orbits common. The Galilean Moons of Jupiter • “Planet-sized” objects in regular orbits 1 Galilean Moons • Follow density vs. distance trend • Easily visible from Earth in small telescopes • Very precise orbits (used as “clocks” in 17th century) Interior Structures Studied by Pioneer (2X), Voyager (2X) and Galileo missions Io Volcanism • Io is the most volcanically active body in the solar system Europa: The Frozen Ocean Planet 2 Europa Jupiter Mission Europa interior LaPlace or JIMO (2020 launch) Ganymede Callisto • Largest moon in the solar system • Clear evidence of geological activity • Evidence of internal heating from tidal flexing plus heat from radioactive decay? 50+ small “moons” • Many likely captured asteroids of comet nuclei • Also impact and collision debris • Transients in unstable orbits • Cratered iceball • No tidal heating, no orbital resonances. • But it has magnetic field ! Out to Saturn Earth 3 Family Portrait Medium-sized Moons of Saturn Enceladus ~500 km @ 4 Rs Density = 1.6 (rock w/ice crust) Tiger Stripes Almost all show evidence of past or current volcanism and/or tectonics Enceladus to scale • Like Europa, a good candidate g for having liquid water beneath a frozen ocean surface Enceladus Eruptions from Tiger Stripes 4 Titan Enceladus cold volcanism 5,150 km @ 20 Rs Density = 1.8 (rock core w/ice crust) Titan’s Atmosphere • Titan is the only moon with a thick (1.5 bar) atmosphere • Consists mostly of nitrogen with some argon, methane, and ethane Cassini Orbital Gallery Surface Lander Optical cameras (VIMS) Radar Imager Titan Structure Titan’s Surface • Huygens probe soft-landed on Titan’s surface in early 2005 • Liquid methane “rocks” made of flammable ice 5 Iapetus Huygens Lander ~1,500 km @ 59 Rs Density = 1.08 (ice) Moons of Uranus Iapetus surface • Only visited by Voyager 2 • Varying amounts of geological activity • Moon Miranda has large tectonic features and very few craters – Miranda likely shattered and reassembled • Total of 27 smaller moons – mostly in irregular orbits Neptune’s Triton Miranda • Mantle & Core visible at surface – The racetracks • Large-scale g tectonic features • Very few craters • Shattered and reassembled in not too distant past? • • • • Thin N2 atmosphere Similar to Pluto, but larger (2,700 km) Almost certainly a captured KBO Active nitrogen geysers Density ~ 2.0 2 0 (rock + ice crust) + 13 small moons. Maybe chunks of each other Geyser plumes 6 Redux: Comparative Planetology Chapter 12 Remnants of Rock and Ice Asteroids, Comets, and the Kuiper Belt • Rock melts at higher • Ices forms lower temp magmas temperatures • Tidal heating melts ice & compounds driving activity • Only large rocky planets have enough internal heat • Much higher impacts rates but lower numbers of craters for tectonic activity 12.1 Asteroids and Meteorites What are asteroids? • Our goals for learning – What are asteroids? – What is the asteroid belt? – The Asteroid = Meteorite connection Vesta – Ceres - Moon • Small Solar System Bodies • Rocky or metallic p composition • Regular orbits • Small (<500 km) • Most are between Mars and Jupiter, but… • ECAs number in the tens of thousands Asteroid Facts • Asteroids are rockyy leftovers of planet p formation. • Largest is Ceres, diameter ~1,000 km • 150,000 in catalogs, and probably over a million with diameter >1 km. • Small asteroids more common than large asteroids. • All the asteroids in the solar system wouldn’t add up to even a small terrestrial planet. Asteroids are cratered and not round 7 Steins flyby: 9/2/2008 • Rosetta - European Space Agency mission • Spacecraft en route to rendezvous with Comet 67P/ChuryumovGerasimenko • Steins is mainbelt asteroid (2.4 AU from Earth) Density of Asteroids Asteroids with Moons • Some large asteroids have their own moon • Asteroid Ida has a tiny moon named Dactyl Orbits • Measuring orbit of asteroid’s moon tells us asteroid’s mass • Most asteroids orbit in a belt between Mars and Jupiter • Mass and size tell us asteroid’s density • Trojan asteroids follow Jupiter’s orbit • Some asteroids are solid rock; others just piles of rubble • Orbits of near-Earth asteroids cross Earth’s orbit Orbital Resonances Origin of Asteroid Belt • Asteroids in orbital resonance with Jupiter experience periodic nudges • Rocky planetesimals between Mars and Jupiter did not accrete into a planet. • Eventually those nudges move asteroids out of resonant orbits, leaving gaps in belt • Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet. 8 Meteorites Meteor Terminology • Meteorite: A rock from space that falls through Earth’s atmosphere • Meteor: M t Th bbright The i ht ttrail il lleft ft by b a meteorite • Both of pieces of solar system debris – Chunks of asteroids or comets Meteorite Types Meteorite Impacts 1) Primitive: Unchanged in composition since they first formed 4.6 billion years ago. g Chicago, March 26, 2003 2) Processed: Younger, have experienced processes like volcanism or differentiation. Ergo…….. Primitive Meteorites Processed Meteorites 9 Meteorites from Moon and Mars The Space Environment • A few meteorites arrive from the Moon and Mars (SNCs) • Composition differs from the asteroid fragments. • A cheap (but slow) way to acquire samples from the Moon and Mars. 1) LDEF left in orbit for 6 years, > 32K orbits 2) Millions of micrometeorite collisions 12.2 Comets • Our goals for learning Comet Facts • Formed beyond the frost line, comets are icy counterparts to asteroids. • Nucleus of comet a “dirty dirty snowball” snowball • Most comets do not have tails tails.. • Most comets remain perpetually frozen in the outer solar system. • Only comets that enter the inner solar system grow tails. – What are comets? – Where do they come from? Nucleus of Comet • A “dirty snowball” • Source of material for comet comet’ss tail Deep Impact mission • Mission to study nucleus of Comet Tempel 1 • Projectile hit surface on July 4. 2005 • Many telescopes studied aftermath of impact 10 Anatomy of a Comet Growth of Tail • Coma is atmosphere that comes from heated nucleus • Plasma tail is gas escaping from coma, pushed by solar wind • Dust tail is pushed by photons Meteor Showers from comet dust From the hinterlands… Only a tiny number of comets enter the inner solar system - most stay far from the Sun Oort cloud: On random orbits extending to about 50,000 AU Kuiper belt: On orderly orbits from 30-100 AU in disk of solar system • Comets eject small particles that follow the comet’s orbit • Cause meteor showers when Earth crosses the orbit. How did they get there? • Kuiper belt comets formed in the Kuiper belt: in a flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets. l • Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction. 12.3 Pluto: Lone Dog No More • Our goals for learning – A long awakening – What are Kuiper Belt Objects? – Planets X, Y, Z…oops, out of letters! 11 Pluto’s Orbit • Pluto will never hit Neptune, even though their orbits cross, because of 3:2 orbital resonance • Neptune orbits three times during the time Pluto orbits twice Other Icy Bodies Is Pluto a Planet? • What is a planet? • Not a gas giant like other outer planets. • Has an icyy composition p like a comet. • Has a very elliptical, inclined orbit. • Pluto has more in common with comets than with the eight major planets Kuiper Belt Objects • These large, icy objects have orbits similar to the smaller objects in the Kuiper Belt that become short period comets • There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune. • The largest of these, “Planet X” was discovered in summer 2005, is even larger than Pluto • So are they very large comets or very small planets? 12 Pluto Best Image of Pluto • Its moon, Charon, is nearly as large as Pluto itself (probably made by a major impact) • Pluto is very cold (40 K) • Pluto has a thin nitrogen atmosphere that will refreeze onto the surface as Pluto’s orbit takes it farther from the Sun. HST view of Pluto & Charon + two smaller moons Other Kuiper Belt Objects Postcards from the Edge “Planet X” a planet? • T.N.O.s = Trans-Neptunian Objects • Most have been discovered very recently so little is known about them. • New Horizons mission will study Pluto and a few other Kuiper Belt object bj in i a 2015 flyby. fl b 13 Pluto and “Planet X” • Pluto’s size was overestimated after its discovery in 1930 • It was considered a planet, and nothing of similar size was discovered for several decades • Now other large objects have been discovered in Kuiper Belt, including “Planet X” • Some scientists consider all of those objects planets; others consider none of them planets. 12.4 Cosmic Collisions: Small Bodies Versus the Planets Have we ever witnessed a major impact? • Our goals for learning – Major impacts in modern times – Did an impact kill the dinosaurs? – Is the impact threat a real danger or media hype? – How do the jovian planets affect impact rates and life on Earth? Comet SL9 caused a stringg of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during a previous encounter with Jupiter This crater chain on Callisto probably came from another comet that tidal forces tore to pieces 14 Impact plume from a fragment of comet SL9 rises high above Jupiter’s surface Dusty debris at an impact site Artist’s conception of SL9 impact Several impact sites Did an impact kill the dinosaurs? Impact sites in infrared light 15 Mass Extinctions • Fossil record shows occasional large dips in the diversity of species: i mass extinctions. • Most recent was 65 million years ago, ending the reign of the dinosaurs. Iridium: Evidence of an Impact • Iridium is very rare in Earth surface rocks but often found in meteorites. • Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact. • Dinosaur fossils all lie below this layer Iridium Layer Consequences of an Impact • Meteorite 10 km in size would send large amounts of debris into atmosphere. • Debris would reduce sunlight reaching Earth’s surface. • Resulting climate change may have caused mass exinction. No dinosaur fossils in upper rock layers Thin layer containing the rare element iridium Dinosaur fossils in lower rock layers Likely K/T Impact Site • Geologists have found a large subsurface crater about 65 million years old in Mexico Comet or asteroid about 10 km in diameter approaches Earth 16 Impact Facts • Asteroids and comets have hit the Earth. • A major impact is only a matter of time: not IF but WHEN. • Major impacts are very rare. • Extinction level events – Millions of years. • Major damage – Every 10 10--100 years. 30 Megaton airburst Meteor Crater, Arizona: 35-50,000 years ago (50 meter object) Is the best-preserved impact on Earth Tunguska, Siberia: June 30, 1908 A ~40 meter object disintegrated and exploded in the atmosphere 17 Frequency of Impacts ECA Families • Small impact happen almost daily. Atens, Earth-crossers • Impacts large enough to cause mass extinctions are many millions of years apart Apollos average orbital Apollos, radii > 1 AU but perihelia inside Earth's orbit. Amors, Mars-crossers Near Misses And Hits… • 1989FC (Asclepius ) missed Earth by 700,000 km in 1989 – Passed through point where Earth was 6 hours previously. – Mean orbital speed = 28 km/sec • Unnamed 10 m object exploded over the Mediterranean Sea on June 6, 2002 – 26 kiloton yield Peru, September 2007 Torino Scale • • • • Apophis (2004MN4) Combines collision probability and Impact Energy Green/Yellow = Normal background impacts level Orange = Very Close Encounter Red = Certain Impact, Localized to Global Destruction April 2029 trajectory • Atens class • 350m diameter • April 13 13, 2036 2036, 1 in 45,000 probability of impact • Was briefly a Torino 4, now a Torino 0 • Not a danger, but each pass changes orbit slightly. 18 An impact with our name on it? • • • • We haven’t seen it yet. Deflection requires detection. detection. More probable with years of advance warning. Control is critical: breakingg a bigg asteroid into a bunch of little asteroids is unlikely to help. • We get less advance warning of a killer comet…can come from any direction and VERY fast What are we doing about it? • LINEAR Program – USAF, NASA, MIT robotic search program – Has detected 211,849 new objects • 1,622 1 622 near earth asteroids • 142 were comets. • Stay tuned to http://impact.arc.nasa.gov • B612 Foundation How do the jovian planets affect impact rates and life on Earth? Influence of Jovian Planets Gravity of a jovian planet (especially Jupiter) can redirect a comet Was Jupiter necessary for life on Earth? water Impacts can and do extinguish life. But were they necessary for “life as we know it”? What have we learned? • Orderly view of 9 planets in empty space is just plain wrong. • Small objects outnumber large objects by millions to one. • The outer solar system is crowded with a new class of objects - KBOs • Most impacts happened early in solar system history; but many are still happening today, esp. in outer solar system. • Solar Systems everywhere! vacuum cleaner 19 Next Week • Extrasolar Planetary Systems • Distribute final averages • Open class period for exam prep and discussion 20