Lecture 13: Solar system debris

Lecture 13: Solar system debris

1. Asteroids

   • - asteroids are small rocky bodies in the solar system - most have orbits in the asteroids belt between Mars and Jupiter - some have orbits that cross Mars (Amor) or Earth (Apollo) - Others are captured into Jupiter's orbit (trojans)
   • Asteroid composition
     • - some asteroids may also have icy compounds on the surface - some may be solid, others may be piles of rubble held together - some may be large enough to have differentiated interior  - Vesta shows evidence of volcanism - some are known to have a moon - some are solid, while others are less dense
     • C type
       • - carbonaceous, dark asteroids - appear darker than S type
     • S type
       • - rocky asteroids composed of silicates - appear grey like moon
       • Eros
         • - Mars crosser S-type asteroid - number of large craters, where an impact might have loosened and blown out the debris - boulders and rocks were found on the surface during the descent
     • M type
       • metallic asteroids composed of iron and nickel
   • Asteroid belt
     • - originally thought to be a destroyed planet - gravitational interaction with Jupiter probably prevented a planet from forming instead - asteroids may be a good sample of the early planetesimals - however, it is mostly empty
     • Inner Asteroid belt
       • - orbit closer than 2.50 AU from the sun, 3:1 resonance with Jupiter. largest member is Vesta
     • Middle Asteroid belt
       • - orbit between 2.50 AU to 2.82 AU from the sun - between 3:1 and 5:2 resonance with Jupiter. Largest member is Ceres
     • Outer Asteroid belt
       • - orbits between 2.82 AU and 3.27 AU from the sun. - between 5:2 and 2:1 resonance with Jupiter. Largest member is Hygiea
   • Kirkwood gaps
     • - Resonance with Jupiter's orbit clear out gaps in the asteroid belt - these gaps are known as Kirkwood gaps - asteroids or planetesimals that stray into a gap are perturbed until they are thrown out of the gap or out of the main belt - as a result, most asteroids stay in regions outside the resonances.
   • Asteroid interactions
     • - asteroids sometimes collide in the main belt. these collisions can break up some of the rubble-pile asteroids - asteroids that come from a broken up larger asteroids stay in the same orbit because of larger angular momentum - more solid asteroids stay together but may end up deformed by the impact. - the impact may also throw out some material, which often become more asteroids. - the ejected material stays in a similar orbits as the original asteroid. - this results in asteroid families which share a similar orbit
     • Asteroid families and groups
       • - asteroid families are closely related asteroids in similar orbits - these are probably asteroids that originated from a large collsion or catastrophic breakup of a larger asteroid - they are commonly named after the first member to be discovered  - minor planet groups are looser associations that are somewhat dynamically related - asteroids in the same group have broadly similar orbital characteristics but may not be related. there are groups a;so recognized outside the main asteroid belt
     • Asteroid orbits
       • outside the main belt, there are also asteroids
       • Aten asteroids
         • semi-major axis less than 1AU, crossing the Earth's orbit
       • Apollo asteroids
         • semi-major axis greater than 1 AU, crossing the Earth's orbit
       • Amor asteroids
         • Perihelion just outside the Earth;s orbit
       • Jupiter Trojans
         • - Asteroids that orbit at the L4 and L5 Lagrangian points of Jupiter. these are points where the mutual gravity of Jupiter and the Sun, and the centrifugal forces balance each other out - orbit in a 1:1 resonance with Jupiter
       • Centaurs
         • Minor planets that orbits mostly outside Jupiter's orbit

2. Dwarf Planets

   • a dwarf planet is a celestial body that: - is in orbit around the Sun - has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape - has not cleared the neighborhood around its orbit - is not a satellite
   • Kuiper belt
     • - Kuiper belt is a region of small icy planetesimals beyond the orbit of Neptune - many kuiper belt objects have very eccentric and inclined orbits - they orbit in an ellipse that is often titled out of the planet of the solar system - the kuiper belt objects are mostly ice-rock mixtures of water, methane and ammonia - these are planetesimals that formed outside the frost line - compare this to asteroids which are mostly rocky - some objects may even contain complex organic compounds - other than Ceres, Pluto, and Eris, the other dwarf planets are classified based on their brightness - brightness is somewhat related to size
     • Pluto
       • - orginally classified as a planet, now a dwarf planet - highly eccentric and inclined orbit that crosses Neptune's orbit - 5 moons, 2 are recently found
     • Eris
       • - larger than Pluto, greater mass then Pluto possibly cuz it has a moon - originally thought to be additional planet but more smaller bodies were also found - eventually classified as dwarf planets because they have not cleared out their respective orbits of debris
   • Pluto and Charon
     • - Charon is a large moon compared to Pluto - about half the size and a tenth of Pluto's mass - orbital period is the same as the rotation period of Pluto - Both orbit about their common centre-of-mass and are tidally locked to each other - Pluto-Charon is similar to the Earth-moon system - simulations suggest that a large collision caused surface material to boil off and the two bodies orbit each other - four more moons orbit outside the Pluto-Charon system - Nix, Hydra, Kerberos, and Styx
   • Eccentric orbits
     • - Pluto and Eris have elliptical orbits with and beyond the Kuiper belt - even more eccentric orbits are also known as plunging orbits - these objects seem to fall straight into the centre, swing around and head straight out, - many comets have such orbits

3. Comets

   • - comets are small icy bodies from the outer solar system - they originate mostly in the Kuiper belt - a very eccentric orbit takes a comet into the inner solar system - some comets may also originate from the larger Oort cloud
   • Halley's comet
     • - a well known periodic comet  - orbit very inclined probably due to an encounter with a larger object
   • Comet composition
     • comets have - a very small nucleus of rick, dust, ice and frozen gases - a coma of gas and dust that is the most visible part and can be very large - a hydrogen envelope - a dust tail - an ion tail
     • Comet tails
       • - sunlight heats up material in the icy nucleus - the heated material comes off the nucleus and forms the coma - hydrogen moves out quicker so the hydrogen envelope is larger - the solar wind blows material away, which form tails that points away from the sun - the ion tail is made of ionized gas and is straight - the dust tail curves as the comet gets ahead
     • Cometary nuclei
       • - the nucleus of a comet is a mix of ice and dust  - the nucleus close up look like a rock - every trip close to the sun removes some material - a comet nucleus may sometimes disintegrate violently - close to the sun, jets form on the surface. this means the composition of a comet is not very uniform. material that is more volatile will boil off first: this is because of the jets
   • Breaking up
     • comets are also known to disintegrate due to various reasons: - gravitational interactions and tidal forces may break up a comet - this happens when the comet goes within the Roche limit of a planet - volatile material in its interior may "explode" - this probably happens when the comet is heated by the sun and material inside melts so that pressure builds up  debris from a broken up comet then spreads out over its orbital path
     • Cometary debris
       • - when a comet disintegrates, it leaves a trail of debris in its orbit - if the earth;s orbit intersects the comet;s orbit, the debris will collide with the earth - this happens once a year on the same date until the debris is depleted - most of the colliding debris is small enough to burn up in the atmosphere  - this is the cause of meteor shower
   • Cometary material
     • - the particles were collected in an aerogel sponge and taken back to earth - a wide range of organic compounds were found: hydrocarbons, biologically usable nitrogen compounds and even the amino acid glycine
   • Cometary impacts
     • in 1994, fragments of the comet Shoemaker-Levy 9 collided with Jupiter - the comet was captures into orbit around Jupiter some time ago - it then broke up into fragments as it passed within the Roche limit - the fragments then fell into Jupiter individually - each impact created a fireball and punched through the atmosphere - the largest impact created a dark spot (12000km) and generated energy equivalent to 6,000,000 megatons of TNT
     • Cometary impacts and water
       • - inside the frost line, water remained gaseous. the earth and terrestrial planets formed within the frost line which means that water on earth did not come with it - there was a period of heavy cratering s the outer solar system rearranged - this rearrangement shook up the Kuiper belt, and probably sent comets into the inner solar system  - comets crashing into earth probably brought water and other organic substances

4. Meteoroids

   • - other than cometary debris, small meteoroids collide with the earth regularly - on an average dark night, you can see a few meteors every hour - meteoroids will light up with a flash as they pass through the atmosphere - the flash is caused by heating - most meteors burn up in the atmosphere and do not reach the ground
   • Meteoroids and asteroid collision
     • - meteoroids may also be broken up asteroids - asteroids sometimes collide with each other. large collision ay break up the surface - collisions with a differentiated planetesimals may expose the interior - further collisions can then break up the core
   • Meteororites
     • - meteoroids that actually reach the ground are known as meteorites - meteorites that burn up the earth's atmosphere have densities of 500 - 1000 kg/m3 and are probably comet-like in composition - meteoroids that reach the surface have densities around 5000 kg/m3 and are similar to asteroids
   • Meteor impacts
     • - larger meteoroids are usually start asteroids - these impacts have produced most of the visible craters in the solar system - the Earth has about 100 craters more then 0.1 km in diameter - erosion has made most of them hard to discern
     • Asteroid monitoring
       • constant monitoring is needed to update and refine the asteroid orbits - this helps improve prediction and find smaller (but also dangerous) ones - most efforts have focused on the "continent destroyer" larger asteroids - smaller rocks like the Chelyabinsk meteor are not so easy to see - these are untracked
     • Chelyabinsk meteor
       • - 15 Feb 2013 Russia - classified as an earth-crossing Apollo asteroid
     • Orbit changes
       • - to make things more difficult, the orbit of an asteroid changes when it comes close to a planet - this is because the gravity of the planet pulls on the asteroid and sends it off in a slightly different direction - after an interaction, the new orbit needs to be determined, from more observations