Lecture 9: Small Bodies of the Solar System SCI238 W08 Lecture 9: Small Bodies of the Solar System the inner SS is filled with asteroids and other small bodies L9 – Feb 5/08 Small SS Bodies

This week’s events: the Moon: New Moon Feb 7 Venus: visible low in east before sunrise; brightest “morning” star Mars: is visible all night, rises at sunset Jupiter: low in east before sunrise Saturn: rises at 8pm Annular Solar Eclipse visible in far South (Antarctica) NOTE: lunar eclipse on 21 Feb. in the evening L9 – Feb 5/08 Small SS Bodies

Today’s Lecture “left-overs” Moons of Giant/Jovian planets rings today’s topic: small bodies of the Solar System asteroids meteorites comets L9 – Feb 5/08 Small SS Bodies

Saturn’s Moons many moons, but probably a large number still to be discovered Titan: largest of Saturn’s moon’s – has a thick atmosphere, extending far out into space, probably has frozen water (ice) innermost moons tidally locked to planet moons interact with rings L9 – Feb 5/08 Small SS Bodies

Titan: Saturn’s largest moon; has a thick atmosphere methane, nitrogen… smoggy L9 – Feb 5/08 Small SS Bodies

Huygens probe shows some surface features on Titan Titan: icy “rocks”, “river” valleys… L9 – Feb 5/08 Small SS Bodies

Saturn’s Moons are varied, enigmatic deathstar Mimas icy Enceladus cratered Tethys old Dione streaky Rhea two-faced Iapetus L9 – Feb 5/08 Small SS Bodies

Uranus’ moons more than 20 small ones, some irregular in shape, some retrograde five midsized: Miranda, Ariel, Umbriel, Titania, Oberon All icy, varied geological history Miranda is strange – very fractured surface, some cataclysmic event? L9 – Feb 5/08 Small SS Bodies

near IR image shows Neptune’s rings and seven of its moons L9 – Feb 5/08 Small SS Bodies

Urabus’ Miranda shows a fractured surface, obvious violence, but from what? messed-up Miranda L9 – Feb 5/08 Small SS Bodies

Neptune’s moons five small inner moons near ring system two larger, icy… five small outer, retrograde Triton very much larger than any other Neptune moon has thin nitrogen atmosphere surface has nitrogen geysers retrograde (backwards) orbit spiraling in towards Neptune L9 – Feb 5/08 Small SS Bodies

Triton: moon of Neptune Nitrogen atmosphere surface has been reshaped by geological activity L9 – Feb 5/08 Small SS Bodies

Saturn’s Rings: the brightest, best known within “Roche Limit” of Saturn where any large rocks (moons) break up due to tidal effects major rings (“A”, “B”, and “C”) with gaps between them “Cassini’s Division” composed of icy particles with sizes between 1 micron and 10 meters Voyager discoveries: several other rings fine rings within each ring, like record grooves: “spiral density waves” short-lived spokes braided rings “shepherd” satellites rings are very thin – at most a few tens of meters L9 – Feb 5/08 Small SS Bodies

Saturn’s rings are wide, thin, and complex Radio image colours -> particle size purple: mainly >5cm green: mainly <1cm white: particle density too high L9 – Feb 5/08 Small SS Bodies

Shepherd moons control the width of rings… L9 – Feb 5/08 Small SS Bodies

…and also make gaps L9 – Feb 5/08 Small SS Bodies

“spokes” in Saturn’s rings: charged particles briefly suspended above the plane? L9 – Feb 5/08 Small SS Bodies

Galileo spacecraft shows rings around Jupiter ring particles are small and dark: debris from Metis.. L9 – Feb 5/08 Small SS Bodies

Near IR image shows rings around Uranus and some of its moons L9 – Feb 5/08 Small SS Bodies

Neptune’s rings are narrow and bright with dusty regions L9 – Feb 5/08 Small SS Bodies

Asteroids small rocky bodies, no atmosphere, orbit the Sun first asteroid, Ceres, found in 1801: dwarf planet… a=2.77 A.U., P=4.6 years, diameter is 940 km second asteroid, Pallas, found in 1802: a=2.77AU 30,000 asteroids now catalogued – most in “Asteroid Belt” at a=2-3.3 A.U. mostly low eccentricities a few have “Earth-crossing” orbits, most have D<1 km Trojan asteroids are in Jupiter orbit: i.e. a=5.2AU orbital resonances with Jupiter alter asteroid orbits three types are roughly: carbonaceous (very dark, carbon, 75% of all asteroids), siliceous (silicates, 15%), and metallic (iron, rarest type) L9 – Feb 1/07 Small SS bodies

asteroids often found as “star trails” asteroids are seen as streaks in images tracking stars as Earth rotates L9 – Feb 1/07 Small SS bodies

Distribution of asteroids: semimajor axes mainly 2-3.5AU Trojan asteroids: orbital resonance at ±60o Some asteroids have more eccentric orbits bringing them inside Earth’s orbit or beyond Jupiter’s L9 – Feb 1/07 Small SS bodies

distribution of asteroid orbits has “peaks” and “gaps” caused by orbital resonance with Jupiter L9 – Feb 1/07 Small SS bodies

asteroid sizes determined in many ways asteroid sizes determined in many ways... spacecraft closeups confirm ground-based results Gaspra: 16km Ida: 53km Eros: 40km Mathilde: 59km L9 – Feb 1/07 Small SS bodies

masses determined from binary asteroids => densities too... Ida and Dactyl Eugneia + moon Densities => range of composition and structure: rubble, ice, rock L9 – Feb 1/07 Small SS bodies

Meteors and Meteorites a meteor is interplanetary debris that falls toward Earth meteorite is meteor that survives to hit surface of Earth “shooting stars”, occasionally “meteor showers”, rarely “meteor storm” three main meteor types: evidence shows meteors mostly same as asteroid types largest meteorite ever found is ~2000 kg organic materials found in some meteorites L9 – Feb 1/07 Small SS bodies

meteor: a bright streak produced when a piece of interplanetary debris heats the atmosphere L9 – Feb 1/07 Small SS bodies

this car was hit by a 12kg meteorite analysis of the meteor’s path through the atmosphere shows it originated in the asteroid belt L9 – Feb 1/07 Small SS bodies

Iron meteorite Stony meteorite dark, smooth crust created by surface melting characteristic crystalline patterns => slow cooling inside larger body L9 – Feb 1/07 Small SS bodies

primitive meteorites: unchanged since formation; ages ≤4.6x109yr L9 – Feb 1/07 Small SS bodies

Processed meteorites: show evidence of change since original formation L9 – Feb 1/07 Small SS bodies

meteors radiate from same part of the sky occur at same time each year Meteor shower: meteors radiate from same part of the sky occur at same time each year L9 – Feb 1/07 Small SS bodies

meteor showers: due to Earth passing through debris left behind by a passing comet L9 – Feb 1/07 Small SS bodies

L9 – Feb 1/07 Small SS bodies

some meteors (and impacts) are from cometary nuclei…and some of these break up Shoemaker Levy before it hit Jupiter Crater chain on Ganymede: multiple impacts from broken comet? L9 – Feb 1/07 Small SS bodies

impact of Shoemaker-Levy on Jupiter L9 – Feb 1/07 Small SS bodies

Manicouagan Lake: large impact basin formed ~2x108 yr ago Earth has impact craters – they’re just often hard to find! L9 – Feb 1/07 Small SS bodies

Meteor crater – Arizona…formed ~40,000 yr ago L9 – Feb 1/07 Small SS bodies

large (2ookm) and mostly under water Yucatan crater: large (2ookm) and mostly under water L9 – Feb 1/07 Small SS bodies

major impact event ~65x106 yr ago… Yucatan crater: major impact event ~65x106 yr ago… L9 – Feb 1/07 Small SS bodies

Crater size correlates with impactor size… L9 – Feb 1/07 Small SS bodies

Gravity field of jovian planets alters paths of outer SS objects more inner SS impacts without Jupiter… L9 – Feb 1/07 Small SS bodies

Comets distinguished by their appearance: coma, tail… bright ones are not very common virtually all have very eccentric orbits, but there appear to be two populations short period from “Kuiper Belt”: P < 200yr long period from “Oort Cloud”: P > 200yr complicated structure that changes with time relatively low mass objects most of mass is in form of ices (H2O, CO2 ) L9 – Feb 1/07 Small SS bodies

Comet Hale-Bopp over Arizona bright coma and two tails… L9 – Feb 1/07 Small SS bodies

Comets: visible coma and tails; hidden nucleus L9 – Feb 1/07 Small SS bodies

but normally the nucleus is hidden from view by the surrounding “coma” the “nucleus” of a comet is an active place, often with jets of material shooting out on the “sunward” side but normally the nucleus is hidden from view by the surrounding “coma” L9 – Feb 1/07 Small SS bodies

A comet’s appearance changes with distance from the Sun an “ordinary” SS object at large distance Cometary “signature” inside ~3-5AU L9 – Feb 1/07 Small SS bodies

comets “come from” Kuiper Belt… Oort Cloud... Oort Cloud: extends to ~50,000AU main Kuiper Belt: ~30-50AU scattered disk objects beyond Kuiper Belt L9 – Feb 1/07 Small SS bodies

Oort Cloud >> scale than Kuiper Belt >> inner SS Oort Cloud orbits random in inclination Kuiper Belt more disk-like; smaller range of orbital inclination L9 – Feb 1/07 Small SS bodies