Meteorites, Asteroids, and Comets
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. Story-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.
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. The Allen Hills meteorite, in part triggered the huge increase in Mars missions over the last decade.
How do we know it's from Mars – atmospheric gas trapped in meteorite matches that found by Viking lander, particularly oxygen isotopes.
It was found in 1984 in the Allen Hills area, Antarctica.
some of the evidence for life (put forth originally in 1996)
nanobacteria fossils – may be artifacts and/or too small.
calcium carbonate globules and their surroundings – calcium carbonate can be produced from both life and inorganic processes, and the rinds on the globules were thought to have magnetite grains aligned in a way that could only be produced by life. Most of the details have since been suggested to have been capable of being formed by inorganic processes. See http://www.lpi.usra.edu/lpi/meteorites/mars_meteorite.html for more details.
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 30o.
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 LaGrangian 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 (silicate/stony) - Ordinary chondrite
C,D - Carbonaceous chondrite
M (metallic) - Irons or Stony-irons
E. Some of the details revealed from close encounters
Many have densities and appearance consistent with rubble piles (very loosely put together after a big collision. Note that they are not spherical because they don't have enough gravity to self-deform
Having moons not uncommon.
Fractures and cracks from collision is common.
Lack of very small craters and boulders on Eros is unusual. The former may mean that seismic shaking from a collision settles the dust in the regolith.
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.
B. Where they come from:
Long-term comets have a source region of the Oört cloud, which extends to ~50,000 Au.
They are perturbed by passing nearby stars and other comets and occasionally drop into the solar system.
Short-period comets often come from Kuiper belt, mostly within 50 Au, but extending to 1000 Au.
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.