1 Solar System Debris and Formation

Transcription

1 1 Solar System Debris and Formation Chapters 14 and 15 of your textbook Exercises: Do all Review and Discussion and all Conceptual Self-Test 1.1 Solar System Debris Asteroids small rocky bodies Most under 300km across, orbit mostly in asteroid belt 2.1 to 3.3 AU (between orbit of Mars and Jupiter Total mass, less than our moon Ceres, largest 940 km across, 10 4 mass of the Earth Belt probably left over material that never formed a planet Few asteroids can be resolved too small, too dim use indirect methods to determine size Classification: C-type, S-type, M-type C-type, carbonaceous carbon rich S-type, silicate, rocky material M-type, large amounts of iron/nickel S:15%,C:75%;M(and others):10% S-type dominates the inner region of the belt, C-type the outer regions Ceres, Palla, and Vesta largest each over 500km across Only two dozen or so larger than 200km across 99% off asteroids over 100km known and 50% over 10km known Most of the mass in th asteroid belt in the larger objects Vesta appears to have undergone volcanism: odd for a small body Vesta perhaps part of a larger body at one point Galileo probe provided first close up pictures: Gaspra, Ida, both S-type Gaspra: irregular shaped body, 20km across thought to have been part of a larger object at one point Ida: 60km across, has a small moon, Dactyl, total mass about kg, 2.5 g/cm 3 NEAR (Near Earth Asteroid Rendezvous) 1997 visit C-type Mathilde, the S- type Eros Mathilde kg 1.4 gm/cm 3 low density suggests porous, pile of rubble, soft interior NEAR orbited Eros for about a year, then landed on the surface (controlled crash) 1

2 most asteroids have eccentricities between 0.05 to 0.3 (stay between Mars and Jupiter) some asteroids have eccentricities greater than 0.4 can cross Earth s orbit Earth-crossing asteroids can have elliptical orbits that allow the asteroid to leave the main asteroid belt Earth-crossers: Apollo (orbit greater than 1 AU) and Aten Near Earth asteroids deflected by influence of other planets As of 2007 more than 4400 Earth-crossing asteroids known, more than 800 labelled dangerous MN4 350m asteroid will pass within 23,000km (underneath most weather satellites) in April 2029 Most Earth-crossers will collide with the Earth over a long enough timeline On average, over a 1 million year period the Earth is struck by 3 asteroids An asteroid 1km across would be the equivalent to about a hundred times all the nuclear weapons in existence 1km would destroy an area 100km across, and potentially change the climate for many years potential for species extinction 65 million years ago, dinosaurs probably wiped out by 10 to 15 km wide asteroid mass extinction Asteroid impact much more severe that human made climate change life goes on Asteroids have orbital resonances Trojan asteroids stuck at Jupiter-Sun Lagrange points 60 ahead and behind Jupiter Material accumulates at L4 and L5 other Lagrange points unstable Asteroids exhibit holes in the semi-major axes: Kirkwood gaps Kirkwood gaps similar to the orbital resonances in Saturn s rings resonances with Jupiter s orbit, orbits elongate No gap like the Cassini Division: can only see the resonances by examining the semi-major axes of the asteroids Comets Hairy stars Parts: hydrogen envelope, coma, nucleus, dust tail, ion tail Tail appears only when comet is near the sun, ion tail interacts with solar wind, dust tail follows the orbit Tail can easily extend 500,000 km Orbits highly elliptical, short and long period comets comet dirty snowball: loose packed material, water ice, volatile compounds 2

3 Halley s comet perhaps most famous 75 year period (short period comet) Comets originate from the Kuiper belt and the Oort cloud Kuiper belt: region beyond Neptune with many icy bodies, plane close to the ecliptic Oort cloud: thought to be composed of comet, icy bodies extending out to 50,000 AU or more orbits of any orientation, domain of long period comets Mission to comets include Halley s pass in 1986 (USSR, Japan, ESA), Stardust to P/Wild and Deep Impact Mission to Tempel 1 Stardust returned material to earth using areogel technology, Deep Impact fired a projectile into the Tempel 1 Beyond Neptune Kuiper belt region of icy bodies Neptune s gravity influences the Kuiper belt Triton thought to be captured object Pluto probably a Kuiper belt object influenced by Neptune Many Kuiper objects discovered since Pluto some larger than Pluto Kuiper belt objects, leftovers from the solar system s formation To be a planet: orbit sun massive enough to roughly spherical clear the neighbourhood Pluto influenced by Neptune, did not clear neighbourhood, not a planet New Horizons Mission, 2015, learn more about Pluto and the Kuiper Belt Meteoroids Meteor: sudden streak of light Meteoroid: material burning up in the Earth s atmosphere, meteorite if it hits the ground Cometary fragments, can give rise to meteor shower, regular meteor showers associated with a comet Shower appears out of radiant Stray asteroids hit the Earth from time to time (see event rate in textbook) Larger meteoroids not associated with comets Meteors that reach the ground. high density, iron/nickel rich Larger meteoroids probably originate from the asteroid belt, same material composition 3

4 1.2 Formation of Planetary Systems Ten major observations about the solar system: Each planet relatively isolated in space The orbits of the planets are nearly circular The orbits of the planets lie in nearly the same plane The planets orbit in the same sense that the sun rotates Most planets rotate on their axis in the same sense that the sun rotates Most major moons orbit their parent planet in the same sense that the planet rotates Planetary system is highly differentiated The asteroids are old a exhibit properties that are different from the planets Kuiper belt beyond Neptune consists of asteroid sized icy bodies Oort clout of comets, primitive icy bodies residing at great distances from the sun Some irregularities explained by one-off events, (the rotation of Venus, Uranus) Best theory, condensation Nebular theory with interstellar dust dust grains play key role Dust grains for the seed for accumulation Planetesimals protoplanets Differentiation temperature profile, different materials at different distances Condensation temperature dependent High temperatures near the forming sun only metallic, heavy materials condense Grains reformed in the inner solar system only after temperature dropped: inner solar system formed over 100 million years Jovian planets get a head start, form in cooler regions: formed within a few million years Nebular gas ejection, formation stop for the outer worlds Jovians migrate inward from interactions with the nebula Complication from the T Tauri: highly active phase for young stars, too little time for Jovian worlds to form by core-accretion theory Gravitational instability theory proposed to overcome T Tauri phase issue: different predictions for core size Asteroid belt, left overs that could not accumulate, influenced by Jupiter and Mars 4

5 Planetesimals ejected by Jovian influences: band remains behind, Kuiper Belt Lots of traffic in and out Oort cloud and Kuiper Belt populated Gravitational influences move the Jovians over time: Jupiter in slightly inward, Saturn (1 AU), and Uranus (4 AU) outward, and Neptune (up to 10 AU) Neptune maintains resonance with Kuiper Belt objects 3:2 as Neptune move outward Planetary systems discovered around other stars Difficult to see directly (only one so far) must use indirect methods Look for radial fluctuations wobble from gravitational effects Orientation is an issue Sometimes can see planetary transits brightness variations Most planets discovered are comparable to that of Jupiter (some are brown dwarfs) Orbits are generally smaller than those of Jupiter or Saturn less that a few AU Hot Jupiters in close orbits, friction with the nebula brings them in Many orbits are eccentric many way this can happen, may be a common feature (Saturn might have help stabilize Jupiter s orbit!) Certainly selection effects going on Searches go on approximately 200 known or suspected exo-planets 5