I. Meteorites
Meteorites - something from outer space that hits the ground. When in space, it's a meteoroid; when in the atmosphere, it's a meteor.
A. Possible sources:
1. Asteroids from the asteroid belt. Current earth-crossing asteroids are <10km in diameter, asteroids in the asteroid belt are up to 1000km.
2. Comets, although we have no idea how to tell if a meteorite is of cometary origin.
3. Other planets.
B. Except for 3 above, meteorites are samples of planetesimals left over from planet formation. Possible fates of these planetesimals are:
1. Ejection from the Solar System.
2. Hit a planet (meteorite).
3. Get captured as a moon, or trapped in a LaGrangian pt. (e.g., Trojan asteroids around Jupiter).
4. Hit another planetesimal and fragment.
5. Preservation - the Asteroid Belt between Mars and Jupiter is a nice place to stay.
C. Where are they found? Originally considered sacred religious objects, many meteorites were stored in churches. They were not widely accepted as extraterrestrial until ~1800. Meteorite chunks have been found all over the world, and many sedimentary layers, especially on the sea-floor, have spherules of extraterrestrial origin. The best "hunting-ground" is Antarctica, where meteorite fragments stand out nicely against the ice.
D. Meteorite phenomena:
1. A sonic boom occurs when the meteoroid enters the atmosphere at ~20km/s.
2. A fireball occurs as the meteor ignites the surrounding atmosphere.
3. The outer mm or so of the meteor is heated and forms a "fusion crust".
4. The object either hits the ground or explodes in the atmosphere.
5. No person has ever been killed by a meteorite, but in Nakhla, Egypt, a dog was killed by a meteorite.
E. Classification System:
1. Stony - 95% of meteorites, mostly silicates and generally primitive rocks.
a. chondrites - 85%, have mm-cm sized spherules, called chondrules, that are melt droplets(?). Formed very early in solar system.
i. carbonaceous chondrites - 5% contain many volatiles, and are often taken as bulk solar system composition.
ii. ordinary chondrites - 81%.
b. achondrites - 10% have experienced some heating that has destroyed the chondrules.
2. Stony-irons - 1%, mix of stone and iron.
3. Iron - 4%.
4. Meteorites from the moon and Mars.
F. Some important points:
1. Carbonaceous chondrites appear to have condensed at nebular temperatures intermediate from inner to outer system.
2. The chondrules appear to result from flash heating and then slow cooling.
3. Achondrites, stony-irons, and irons can only arise from heating and differentiation of planetesimals. However, the formation pressure and temperature of observed minerals indicates a parent body <500km. Possible sources of heating:
a. short-lived radioactive isotopes, like Al26.
b. T-tauri phase.
4. The formation age of most of these is ~4.6 Ga, and they appear to have formed over ~20 My, providing constraints on solar system formation.
G. Martian meteorites and Life on Mars - ALH84001 (Allen Hills, 1984, Meteorite 1)
In 1996 a group of scientists claimed to have found multiple lines of evidence for life on Mars in a Martian meteorite. Despite nonbiological explanations having been produced for all these phenomena, their work has at least partially contributed to the current focus of the unmanned space program on the planet Mars. The debate on this has been extensive briefly, the evidence in the meteorite is the following:
1. Shapes that resemble bacteria. Problems: May be too small to be bacteria (< 100 nanometers), no one has much experience looking at rocks at the scale of the alleged bacteria, sample preparation may produce artifacts.
2. Magnetite around carbonate globules was excreted from bacteria. Problems: other processes could produce this.
3. Presence of Polycyclic Aromatic Hydrocarbons near and in carbonate globules. Problems: other processes could produce these.
4. While other processes could produce 1-3 and other lines of evidence, their combined close proximity suggests a life origin. Problem: This is simply a weak argument.
Web sites devoted to the "life on Mars" debate are http://www.lpi.usra.edu/lpi/meteorites/mars_meteorite.html and http://www.psrd.hawaii.edu/Archive/Archive-MarsLife.html
II. Asteroids
A. Why study them?
1. To identify the parent bodies for the meteorites, aiding study of solar system history.
2. To obtain information of the impact flux on the planets.
3. Someday they may be exploitable for mineral resources.
B. A little history of their study:
Bode's rule, ~1772, led to search between Mars and Jupiter for a planet.
1801 - Ceres, the largest asteroid at ~1000 km diameter, was the 1st discovered.
By 1890, 300 discovered.
By 1980, 2095 discovered.
Provisional # is based on year of discovery, e.g., 1978SB.
Officially given # and name, e.g., 1 Ceres.
C. Grouping by location (Note: all have prograde orbits).
1. "Main belt" asteroids: 2.1 - 3.3 Au, eccentricities typically .1-.3 (Planets are <.1) inclinations up to 30°.
Kirkwood gaps-few have been orbits whose semimajor axis is a whole number fraction (e.g., 1/2, 2/3) of Jupiter's orbit.
2. Trojan asteroids: in LaGrangion point ahead and behind Jupiter in its orbit.
3. Amors - asteroids whose orbit crosses Mars.
4. Apollo - Earth crossers 60 known, largest is 8 km diameter. It is estimated that 700±300 > 1 km diameter exist.
5. Atens - Earth approachers, semi-major axis is less than Earth's.
6. Hirayama families - clusters of asteroids in similar orbits, fragments of a parent body.
D. Grouping by Composition -process is to compare the spectra of an asteroid with a set of meteorites.
Asteroid class and associated meteorite:
E,R - Enstatite chondrite
S - Ordinary chondrite
C,D - Carbonaceous chondrite
M - Irons or Stony-irons
III. Comets - from the Greek word Komé, which means hail
A. Parts:
Head - a coma of bright, diffuse material about a solid nucleus, comas are brightest at ~1-2 AU.
Tail - always extends approximately away from the sun
Type I - ionized gas - straight out from the sun.
Type II - broad diffuse dust (bends in direction of orbit)
B. Where they come from:
1. Source region for long-period comets is the Oört cloud, which extends to ~50,000 Au. 2. Short period comets probably from Kuiper belt. The first trans-Neptunian object besides Pluto-Charon was discovered in 1992. Now several hundred discovered.
3. They are perturbed by passing nearby stars (Oort cloud only) and other comets and occasionally drop into the solar system.
4. Meteor showers - the Earth passing through dust from a comet's tail.
C. The basic model of their composition is the "dirty iceberg" - ices of water, methane, and ammonia mixed with some dust and probably covered with soot.