Where Do Meteorites Come From?

Modified from Rubin A. E. (2002) Disturbing the Solar System: Impacts, Close Encounters, and Coming Attractions. Princeton University Press, Princeton. 361 pp.

Mainly residing in the region between Mars and Jupiter are planetesimals and their fragments that failed to accrete into a decent-size planet. These are the asteroids (typically tens to hundreds of kilometers in diameter) and their smaller cousins, the meteoroids. As they orbit the Sun, they often cross the orbits of other asteroids and occasionally smash into each other. Gravitational interactions with Jupiter send some of the bodies toward the inner solar system.

Some of the errant bodies plunge into the Earth's atmosphere accompanied by sonic booms and whistling sounds. Those objects that survive to reach the surface become meteorites. A few have pelted living beings. About 30 stones fell in New Concord, Ohio on 1 May 1860 and one of them killed a colt. Two stones fell in Sylacauga, Alabama on 30 November 1954; one broke through the roof a house, bounced off a radio stand and hit a sleeping woman in the hip and hand. Hundreds of stones fell in Mbale, Uganda on 14 August 1992; a 3.6-g stone fell through the leaves of a banana tree and struck a boy on the head. The dead horse, bruised woman and uninjured boy were connected to the asteroid belt in a very personal way.

There are 13 well-established chondrite groups with five or more members each. Each group has its own narrow range of mineral compositions, textural characteristics, bulk chemical composition and bulk oxygen-isotopic composition. Mixtures among the chondrite groups are relatively rare, i.e., there are relatively few examples of a fragment of one chondrite group residing within the matrix of another. Therefore, it is likely that each chondrite group was derived from a separate parent body. In addition to the established groups, there are about ten unique chondrites or chondrite grouplets (small groups with fewer than five members each) that were derived from separate parent bodies. There are also diverse groups of achondrites that formed on at least seven different parent bodies that experienced high temperatures and pervasive melting.

Iron meteorites constitute 12 main groups with distinct compositions. The narrow compositional ranges of each of the iron meteorite groups indicate that each was derived from a separate parent body. There are also numerous ungrouped irons and iron grouplets with distinct compositional characteristics, suggesting derivation from 90 to 100 different bodies. (The reason that there are so many iron meteorites is in part sociological; they look so different from terrestrial rocks that they are much more likely to be picked up in the field and brought to an expert for identification.)

Adding up the chondrites, achondrites and irons, major groups, grouplets and unique specimens, we find that the meteorites in our collections were derived from more than 100 or so separate bodies. Any solid body in the solar system is a potential meteorite parent body. The list includes planets, moons, asteroids and comets.

More than 200 meteorites (not taking possible pairings into account) share the petrologic, mineralogical, geochemical and isotopic characteristics of lunar rocks returned from the Moon by the Apollo astronauts and the unmanned Soviet Luna spacecraft. These rocks, known as lunar meteorites, were blasted off the surface of the Moon by energetic meteoroid impacts.

There are about 150 meteorites (not taking possible pairings into account) thought to be derived from Mars. Some are about 1.3 billion years old, approximately the same age as that estimated for the volcanoes on the Tharsis Ridge of Mars. Gas bubbles trapped within glass inclusions of one of these meteorites (EET 79001) have the same chemical and isotopic composition as the martian atmosphere measured at the surface of Mars by the Viking lander in 1976. The presence of highly oxidized (i.e., ferric) iron in some mineral grains in the martian meteorites is consistent with the red dust that covers much of the planet; this dust contains the mineral hematite (Fe2O3), commonly known as rust.

Because the surface gravity of Mars is only 38% as strong as the Earth’s, hydrogen escapes from the top of the martian atmosphere more readily than deuterium (heavy hydrogen atoms containing one proton and one neutron). The deuterium/hydrogen ratios in phosphate grains from these meteorites are about three-to-five times higher than in terrestrial rocks, consistent with the ratio measured in the martian atmosphere.

All the rest of the meteorites, encompassing tens of thousands of individual specimens, are believed to come from asteroids. This inference is based on nine major links between meteorites and asteroids.

1. High-energy nuclear particles known as cosmic rays penetrate silicate rock to depths of about one meter, smashing into atoms in the rock and transforming some of them into radioactive isotopes such as 21Ne (neon atoms with 10 protons and 11 neutrons) and 36Ar (argon atoms with 18 protons and 18 neutrons). The decay of these isotopes can be used as a clock, timing the period the meteoroids existed in space as meter-size objects. The cosmic-ray-exposure ages of stony meteorites range from 30,000 years to 70 million years. This is far too short a time for meteoroids to have traveled to Earth from the vicinity of another star; such interstellar trips would probably take at least several hundred million years. This indicates that meteorites are products of our solar system. Specifically, these cosmic-ray-exposure ages match the typical times expected from theoretical calculations for objects to have traveled to Earth from the asteroid belt.
2. The solar corona (the outer atmosphere of the Sun visible during total eclipses) expands into interplanetary space as the solar wind, carrying with it noble gases such as hydrogen, helium, neon and xenon. Bodies near the Sun are blasted with high concentrations of these noble gases; more distant bodies acquire lower concentrations of solar gas. The abundance of solar-wind-implanted noble gases in some meteorites is consistent with their acquisition at about 3 astronomical units from the Sun, i.e., in the center of the asteroid belt.
3. The very presence of solar-wind-implanted noble gases and solar-flare particle tracks in some meteorites indicates that they were exposed at the surface of an airless body. This is because collisions between solar-wind particles and the molecules in a planet's atmosphere shield the planet surface from exposure to the solar wind. Thus, meteorites containing solar-wind gases cannot have been derived from objects with substantial atmospheres such as Venus and Saturn's moon Titan. Asteroids are too small and have too little gravity to have retained an atmosphere; they are likely candidates for the parent bodies of meteorites containing abundant solar gas.
4. Many meteorites are made up of broken fragments. Some of the fragments are remnants of projectiles that impacted the parent asteroid at fairly low relative velocities. The high proportion of broken fragments in these meteorites is consistent with the dense cratering observed on asteroids that have been photographed by passing spacecraft.
5. Repeated gravitational perturbations by Jupiter cause asteroids in certain orbits to change their orbital characteristics and assume Earth-crossing orbits. There are presently about 700 asteroids in Earth-crossing orbits. Calculations indicate that 7% of them will eventually strike the Earth as meteorites.
6. Analysis of the size and composition of meteorite metal grains and of plutonium fission tracks in phosphate grains indicates that many meteorites cooled from high temperatures at rates between 1 and 100°C per million years. The slowest rates correspond to those expected from material near the centers of rocky bodies 100-300 km in diameter. This size interval matches that of many large asteroids.
7. If small objects and large objects are heated to the same temperature, the small objects cool more rapidly because they have higher surface-area/volume ratios. In other words, relative to their trifling volumes, there is a lot more surface on small bodies from which heat can radiate away. This is why large rocky bodies like the Earth still have enormous reserves of internal heat 4.5 billion years after they formed, while small bodies like asteroids cooled completely shortly after formation. The 4.5-billion-year age of most meteorites thus indicates that they were derived from small, asteroid-size bodies. This conclusion is consistent with the results of a survey of asteroids larger than 10 km in diameter; the survey found that 95% of these objects have diameters less than 200 km.
8. Different minerals absorb and reflect light in characteristic ways at different wavelengths. Comparisons of laboratory studies of the spectral reflectivities of certain meteorites closely match those made telescopically of certain asteroids. These studies reveal that many meteorites have compositions similar to those of some asteroids.
9. Since the late 1950s, a number of chondritic meteorite falls were photographed or videotaped as they plunged through the atmosphere toward the ground. Reconstruction of their orbits reveals close similarities to those of typical Earth-crossing asteroids.

These observations demonstrate that the vast majority of meteorites are from asteroids.