Donald Brownlee

Department of Astronomy

University of Washington

This paper discusses the collection of samples from primitive solar system bodies. A comet or asteroid sample return mission, though expensive, can be only half the price of a major “flagship,” planetary mission. In addition to mission-targeted bodies, some 20,000 meteorites have landed on Earth at no cost. Over half of known meteorites found on Earth have been recovered from Antarctica. Meteorites are a wonderful resource, though they have some limitations. In addition to the normal meteorites, there are also the invisible meteorites—the cosmic dust particles—which are smaller than normal meteorites but are interesting because they provide an alternate and more representative sampling of bodies in the solar system.

Most of the information we have about primitive solar system bodies comes from spectral reflectivity, which is a useful source of information but is not tied to astrobiology or mineralogy in a straightforward way. Asteroids exhibit different spectral reflectance types, and these have been studied in great detail. One of the most abundant is the so-called C-type, in which the dark material is typically attributed to carbon-rich materials, but in fact any material such as metal or sulfide can make a C-type spectrum. There are also types called P and D, which increase in reflectivity into the red region of the spectrum. In some cases there are matches with spectral reflectance of certain meteorite types. The most successful match is that of Vesta (and the Vestoids that are associated with Vesta) with the so-called howardite-eucrite-diogenite achondrite meteorites.

The meteorites come from the asteroid belt, with a few exceptions that appear to have come from Mars or the Moon. Most meteorites are delivered from the 1:3 resonance (referring to the ratio of the orbital period of the asteroids there to the orbit period of Jupiter) between 2.3 and 2.2 AU. There is a range of meteorite types that come from this small band of the solar system. The asteroid belt delineates the transition between the terrestrial planet and gas giant domains of the solar system, and there indeed appears to be a stratigraphy in the asteroid belt. The main belt is dominated by the C-type, the inner part is dominated by the E- and R-types, and the outer part is dominated by the P- and D-types.

The asteroids in the mid-belt commonly show OH in their reflectance spectrum—denoting the presence of hydrated minerals. Those farther out are thought to be very primitive—the common explanation for P-and D-types is that they have a high organic content, which is responsible for their dark, red higher reflectances. But these do not show OH. It is clear that the closer asteroids were heated above the melting point of water ice.

The black-body temperature today in the middle of the asteroid belt is about 180 K. In the primitive nebula, neither insolation nor the background gas could produce high enough temperatures to melt ice. There must have been an energy source that heated the asteroids and had a radial dependency. One of the most popular explanations is heating from short-lived radioisotopes such as ²⁶Al and ⁶⁰Fe. This imparts a radial dependence because the half-lives of these isotopes are compared to the accretion time of asteroidal-sized bodies, and plausibly those on the inner edge of the belt would form before those on the outer edge. Whatever the explanation, there was a heat source that had a radial dependence. In the 1:3 resonance, which is the source of many of our meteorites, bodies may have formed with a fair amount of ice in them and the ice then melted because of this mysterious heat source. The resulting liquid water then modified these rocks. Many of the primitive meteorites we have were thus relatively warm wet rocks for millions of years, and their parent asteroids may have had a much higher content of water in them than we see today.

In addition to meteorites, there are small particles, called cosmic dust particles, that come from both asteroids and comets. Some of these have quite unusual properties, including very high carbon content —up to 50 percent by