REMOTE SENSING THE WHOLE METEORITE

Remote sensing studies of meteorites have concentrated on measuring the near infrared reflectance spectra of meteorite powders, seeking a reliable, repeatable representation of the most diagnostic absorption features. It is useful, however, to remind ourselves that real meteorites are rocks. Handling and curating the 1400 Vatican Observatory meteorites has made this author aware of some important but sometimes overlooked features of whole-meteorite samples.

In visible light, virtually all meteorites are gray. On cm-scales and larger, most meteorites are remarkably homogeneous. At sub-mm to mm scales, differences in meteorite class can often be seen as a difference in texture: both chondrule and metal grain sizes tend to be uniform within a given meteorite, with different sizes characteristic of different chondrite classes. Density is not a strong discriminant between stony meteorite types, except to indicate the presence of hydration products; differences in asteroid densities and reflected radar power probably indicate differences in porosities, not compositions. However, chemically identical meteorites can be strongly altered in appearance by shock; albedo is not always diagnostic, and what we see in hand specimens in our labs may well be very different from how these materials looked in-situ.

Laboratory experiments on whole rock meteorite samples could be a powerful way of testing innovative remote sensing techniques. Can microwaves discriminate between chondrules and matrix, indicate chondrule size, or indicate the typical size of metal grains in a meteorite? Do different types of meteorite classes or metamorphic grades have distinctive polarization or phase angle reflectance properties? These measurements can go a long way toward answering a basic question: are most stony asteroids chondritic, or are chondrites “ordinary” in our collection only because of special circumstances involving their parent bodies’ orbits or their ability to survive atmospheric passage?