Astronomy 311: Lecture 10 - Asteroids, formation of the Moon and some other solar system specifics.• Vast majority of material in Solar Nebula was swept out into space by thesolar wind: this ended the era of planetary formation.• Winds are much stronger in younger stars.• If gas had not been cleared out, might even have had jovian type planetsin inner solar system. But this could happen in other solar systems• Clearing of excess gas still left many planetesimals between the newlyformed planets.• Asteroids are the rocky leftovers of t he inner solar system.• Asteroid belt between Mars and Jupiter probably contained many moreplanetsimals earlier but most crashed into inner planets or ejected fromSolar System.• Comets are the icy-rich leftover planetesimals from the outer solar system.Total mass of the asteroid belt is only a tiny fraction of the mass of anyterrestrial planet.• Comets beyond Neptune able to g r ow large without disruption - they wouldhave styaed in theior or ig inal orbits on the same plane as the planets. Thesetherefore became the objects of the Kuiper belt.• Comets further out suffered strong gravitational encounters which kickedthem further out and gave them all sort of o r ientatations - the Oort Cloudobjects.• Analysis of meteors supports this.• SOlar nebula theory predicts the existence of both KBO’s and OCO’s.• Exceptions?– Earth’s moon is very large compared to t he host planet. Why? How?– Largest leftover planet collided with the Earth after iron had settledinto core o f planet.– If t his impact had blasted away material from Earth’s outer layersand sent this material into space. Orbiting material could then haveaccreted into a spherical shaped moon.– Overall composition of the moon is similar to the composition of theEarth’s outer layers. Also the moon has a much smaller proportionof easily vaporized ingredients - the heat of the impact would havevaporized such ingredients.1– Other gia nt impacts could explain why Uranus is tilted almost on itsside and why Charon is so big compared to Pluto.• Asteroids– Today about 150,000 asteroids catalogued: largest is Ceres - about1000km in diameter.– Asteroid shapes depend on their gravity.– Shine by reflected light.– Made up of mostly metal and rock (fr om spectroscopy) because theycondensed within the frost line. Those near outer parts of asteroidbelt are made or more carbon-rich material.– Vast majority located between Mars and Jupiter.– Orbit Sun in same direction as the planets but their orbits tend to bemore ellipticaland inclined between 20-30 degrees to the ecliptic plane.– Trojan asteroids share Jupiter’s 12 year robti around the Sun: oneclump always stays 60 degrees in front/behind in its orbit.– A small group of asteroids (Near Earth asteroids) have orbits that passnear the Earth’s orbit.– Orbital resonances are the key to understanding why there is an as-teroid belt.– Orbital resonances mean two objects eg Jupiter and the asteroid willperiodically line up in the same direction and get an ”extra nudge”which can build up over time.– Suhc resonances more often responsible for clear areas: Kirkwood gaps- a lack of asteroids with periods exactly 1/2, 1/3. o r 1/4 of Juputer’s.– Why didnt a planet form in the space occupied by the a steroid belt?– Resonances with Jupiter disrupted the orbits of t he region’s planetes-imals.• Meteors burn up in the atmosphere.• Meteorites are metors that strike the ground.• Types of meteorites– Primitive meteorites - remnants from the birth of the solar system:radiometric dating puts them to be about 4 .6 billion years old. Canbe stony or carbon-rich.2– Processed meteorites - once part of a la r ge object that ”processed” theoriginal solar nebula material into another form. Generally youngerthan primitive meteorites. Can be metal-rich (high density iron, ter-restrial planet cores) or rocky (terrestrial mantles or crusts).• Primitive meteorites: accreted in the solar nebula and orbited the Sunfor billions of years before falling to Earth. Carbon-rich/stony meteoritesformed in the outer/inner solar nebula material respectively.• On Earth, get mostly stony meteorites though carbon-rich are more preva-lent in space - because of orbital resonances.• Processed meteorites must be fragments of larger asteroids that underwnetsome sort of different ia t io n: interiors metled so that metals sank and rocksrose.• Comets– Ice rich because they formed beyond the frots line where hydrogencompounds are abundant.– Nucleus plus tail.– STudy spectra of comets: ice mixed with rocky dust plus some morecomplex chemicals, including some organic molecules, CO2, H2O– Far from the Sun, its frozen, just t he nucleus chunk of ice is present- but still look pretty dark as they have a low reflectivity (less than 5percent).– Halley’s comet has a density much less than water. Comet nucleus isabout 20km across.– As the comet gets closer to the Sun, its surface temp. increases andgases sublimate into gas that escap es the comet’s weak gravity.– This creates a dusty ”atmosphere” called the coma.– Larger than the nucleus it surrounds.– Coma grows as the comet continues into the inner solar system.– Comet tail forms as an extension o f the coma: many hundreds of kmin length, generally point away from sun.– Comet tail can be millions of km long with material shooting out athundreds of meters per second.– Two comet tails, one made of ionized g as - the plasma tail and onemade of dust.– UV light from Sun ionizes gas escaping from the coma. Plasma tailalaway spoints directly away from Sun at all times.3– The dust tail is made up of dust-size particles escaping fro m the coma.– These particles are not a ff ected by the solar wind and instead pushedaway from the Sun by radiation pressure.– Dust tail also generally points away from the Sun but with a slightcurve in the direction the comet came from.– After the comet loops around the Sun and begins to head back out ofthe Sun, sublimation declines, coma dissipates and the tails disappear.– Gas escaping from coma also has pebble-sized material. Too big to beaffected either by the solar wind or radiation pressure - forms a third,invisible tail.– Meteor showers occur when such comet dust enters our atmosphere:when the Earth crosses through a comet’s orbit. Examples are thePerseids (August 12, Swift -Tuttle) Leonids (Nov. 17,