AST101- Chapter 12.docx

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Astronomy & Astrophysics
Michael Reid

Asteroids, Comets, and Dwarf Planets [Chapter 12] 12.1 Asteroids and Meteorites  These are the scraps left over from the formation of our solar system  Asteroids “star like” (but only from telescope)  Meteorites are pieces of rock that have fallen to the ground from space (most are pieces of asteroids that orbited the sun before they fell on earth) What are asteroids like?  An asteroids orbit can be calculated from the law of gravity Asteroid sizes, numbers, and shapes  The largest is Ceres (under 1000 km in diameter)  Some are large enough to be considered moons if they orbited a planet  Asteroids don’t add up much in total mass; wed make an object much smaller than our moon  Shapes depend on the gravity; the larger the asteroid the more the gravity can mold its solid rock into a sphere o Only Ceres is large enough for this (why it’s a dwarf planet)  Weak gravity due to smaller sizes is why some are potato shapes  Asteroids have also been battered by impacts because they have craters  They shine with reflected sunlight so the brightness I our sky depends on its size, distance, and reflectivity o Can determine distance form position in its orbit o Can determine reflectivity through comparing brightness in visible light (sunlight) and brightness in infrared light (thermal radiation emitted by asteroid itself) infrared depends on its temperature which depends on how much sunlight it absorbs (tells us the proportions of incoming sunlight that is reflected and absorbed) o Once we know proportions, the brightness and distance tell us the size  Mathilde has a huge central crater which means it’s a “rubber pile” that was able to absorb the shock of the impact without completely falling apart Asteroid compositions  Analysis of light can also tell us the composition  Made mostly of rock and metal since they have condensed within the frost line in the solar nebula  Some are made mostly of metals like iron (fragments of iron cores of shattered worlds), dark carbon rich (near outskirts of belt), some have small amount of water (formed close to frost line) etc Asteroid masses and densities  Direct way to measure mass is through its gravitational effect on another object  For asteroids with moons (smaller asteroids orbiting it), can determine mass through newton’s version of Kepler’s third law to the orbital characteristic of the moon  Once mass and size is known, can calculate density  Eg. Mathilde which survived an impact that left it with a central crater, has a low density, which means it cannot be a solid chunk of rock which confirms that it is a pile of rubber held together by weak gravity Asteroid orbits  All orbit in the same direction and around the sun, but their orbits are more elliptical and inclined to the ecliptic plane than those of planets  They are very far apart though (millions of km)  Not all are located in the asteroid belt though; two sets of asteroids called Trojan asteroids share jupiters 12 year orbit around the sun o Number of Trojan asteroids could be as large as number in asteroid belt but the greater distance to the Trojan asteroids makes it more difficult to study  A small amount of asteroid have orbits that pass through the inner solar system including near- earth asteroids that pass our orbit o These are impacts waiting to happen; many of these kinds of asteroids with orbits have hit our planet in the past Why is there an asteroid belt?  Reason why the asteroid belt didn’t form into a planet is because of the orbital resonances o These occur in the belt between asteroids and jupiter which clear gaps in the asteroid belt o Resonances with young jupiter disrupted the orbits of this regions planetesimals which prevented them from accreting into a full fledged planet  Ongoing orbital disruptions gradually kicked pieces of this “unformed planet” out of the belt altogether o Once happened, they crashed into a planet or flew out of the solar system  Therefore the asteroid belt lost most of its original mass which Is why its less than any terrestrial planet Where do meteorites come from? The difference between meteors and meteorites  Meteor “a thing in the air” is only a flash of light caused by a particle entering our atmosphere at high speed  sometimes called shooting stars or falling stars (people thought they were stars)  They burn up completely before reaching the ground; only in rare cases is the chunk of rock large enough to survive the plunge through our atmosphere and leave a meteorite on the ground  Meteorite is an actual rock that has crashed to earth  Rocks that are large enough to become meteorites are called fireballs Meteorite falls  Are often blasted apart in their fiery descent through our atmosphere, scattering fragments over an area several km across  Most of course fall into our ocean  We sometimes mistaken meteorites for terrestrial rocks, ways to determine if it’s a meteorite is through; dark, pitted crust covering, some have high metal content, often contain elements such as iridium that are rare in earth rocks Types of meteorites  Primitive meteorites: first type of meteorite that formed and radioactive dating shows them to be about 4.6 billion years old; come in 2 subtypes o Stony primitive meteorites: composed of rocky minerals with small fraction of pure metallic flakes mixed in o Carbon rich primitive meteorites: similar to stony ones but they also contain substantial amounts of carbon compounds- some even water  Processed meteorites: were once part of a lager object that “processed” the original material of the solar nebula into another form; radioactive dating shows that are younger; have 2 subtypes o Metal rich processed meteorites: made of high density iron and nickel mixed with small amount of other metals (resemble terrestrial planet cores in composition) o Rocky processed meteorites: have lower densities and made of rock with compositions resembling terrestrial mantles and crusts The origin of primitive meteorites  Pieces of rock that accreted in solar nebula and orbited sun until they fell to earth  Some are different in composition depending where they formed (rock and metal closer in, and after 3 AU from the sun carbon compounds could condense) The origin of processed meteorites  Fragments of larger asteroids that underwent differentiation in which their interiors melted so that metals sank to center and rocks rose to surface  Rocky processed meteorites have been made by lava flows; chipped off the surfaces of asteroids that once had active volcanism (from collisions with other asteroids)  Metal rich processed meteorites are less common and are fragments of larger asteroids that shattered collisions  These shattered worlds lost chance to go into planets Meteorites from the moon and mars  In few cases, meteorites don’t match any known asteroids but instead either the moon or mars o Means they were once part of them  Large impacts can blast surface material form terrestrial worlds into space where the orbit the sun until they come crashing down  “lunar meteorites” or “martian meteorites” 12.2 Comets  Asteroids are one of the 2 major categories of small bodies in the solar system; the other is comets  They are both leftovers and vary in sizes but their compositions are different because of where they formed  Asteroids are rocky due to forming in inner solar system and comets are ice rich because they formed beyond the frost line What are comets like?  They are rarely near earth, vast majority remain perpetually in the far outer reaches of our solar system, orbiting the sun far beyond the orbit of Neptune  The ones we see are rare and because their orbits changed by the gravitational influences of planets, other comets, or stars passing by in the distance o New orbits carry them closer to the sun o This brings them into the inner solar system  Most will not return to the inner solar system for a long time, if ever  A few happen to pass near enough to a planet to have their orbits changed further which end up on elliptical orbits that bring them close to the sun Comets in human history  People figured out through observations that the same comet is seen on numerous prior occasions which means it orbits the sun  Knowing this, you can determine the length of the orbit and when it will return Comet composition  Comets are chunks of ice mixed with rocky dust and some more complex chemicals, and are described as “dirty snowballs”  Distant origin because they have compounds that could have condensed only in colder outer regions of the solar nebula  Show emission features on spectra from hydrogen compounds, carbon dioxide, gases that condensed only in the coldest regions etc.  Not all comets have similar compositions or similar amounts of surface dust  Some dust contains rocky material that formed in the inner solar system and somehow got mixed in with other cometary materials that formed in outer system The flashy lives of comets  Comets grow tails only as they enter the inner solar system where they are heated by the warmth of the sun  Far from the sun it is completely frozen “dirty snowball” and begins to shed gas and dust as it approaches the sun  Comet nuclei are dark and part ice part empty space (not dense)  As it approaches the sun, ice sublimates into gas that escapes from the comets weak gravity  Some escaping gas drags away dust particles from the nucleus, and the gas and dust create a huge, dusty atmosphere called a coma o This coma grows as it continues into the inner solar system and tails form as extensions of the coma that point away from the sun  Has 2 visible tails; one made of ionized gas/plasma, the other made of dust  Plasma tail: consists of gas escaping from the coma (extends almost directly away from the sun at all times); ultraviolet light from the sun ionizes the gas and the solar wind then carries the gas straight outward from the sun  Dust tail: made of dust size particles escaping from the coma (points away from the sun and has a slight curve back in the direction the comet came from); not affected by solar wind and instead are pushed away from the sun by much weaker pressure of sunlight itself  As comet loops around sun and begins to head back outward, sublimation declines, the coma dissipates, and the tail disappears (nothing happens until it comes back sunward, if it even does)  Comets that repeatedly visit eventually don’t last that long and its ices can no longer sublime into gas and escape which means It may disguise as an asteroid or come unglued and break apart  Icy materials of small comet vaporize on entry, and the rocky material in comets takes the form of dust or pebbles too small to survive the passage through earths atmosphere Comet tails and meteor showers  Gas escaping from comets also carry away sand to pebble size pieces of rocky material  These particles are too big to be affected by solar wind or sunlight so they drift away slowly and spread along the orbital path  Form a third invisible tail that follows orbit of comet  We see a meteor lighting up in the sky when one of these small particles burns up in our atmosphere  They make the surrounding air glow with heat since they enter with such a high speed  The particles vaporize and never reach the ground  Particles of comet dust enter the atmosphere worldwide every day, burning up as meteors (the third tails of ejected particles make the dust more concentrated along the orbits of comets)  Meteor showers occur when earth passes through a particular comets orbit at the same time each year (comet dust enter the atmosphere)
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