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Figure 1.—Differences between UBV indexes of asteroids and the Moon relative to the Sun. Also see figure 1 of the paper by Chapman, Johnson, and McCord."
Goldstein (1968), and Veverka (in this volume”). Included in table I and figure 1 are data for the Moon for comparison.
Because of the faintness of the asteroids, detailed optical data exist only for relatively few bodies. To calculate the geometric albedo from the absolute magnitude, the diameter must be known; but only a few of the largest of the minor planets show disks. (See the paper by Dollfus.*) Detailed reflectivity curves have been published for only one asteroid, Vesta (McCord, Adams, and Johnson, 1970); for the rest, only the UBV indexes are known." The degree to which the optical properties of the larger bodies are representative of all asteroids is not clear. Also, among the fainter asteroids, high-albedo objects, such as Vesta, will be preferentially discovered and observed over low-albedo bodies, such as Pallas.
Because the only optical information available for most of the asteroids are their UBV indexes, the greatest attention will be given to the interpretation of these spectral data. To facilitate comparison with the various laboratory
*See p. 55.
*See pp. 79 and 91.
*See pp. 25, 30, and xv.
“Editorial note: Additional data now are available in the paper by Chapman, Johnson, and McCord, p. 51.
materials, it is necessary to discuss the processes that influence the reflection of light from complex surfaces in the near-UV, visible, and near-IR wavelength region.
Materials of geological interest absorb light by a variety of processes, which will be described briefly. For further information, references such as Garbuny (1965) and Burns (1970) should be consulted. Metallic Conductivity.—Metals contain electrons that are not bound to any particular atom but are free to move about the lattice. The electrons are able to respond rapidly to varying electromagnetic fields and rearrange themselves to prevent the penetration of fields into the interior of the metal. The coefficient for the absorption of electromagnetic waves is thus extremely high in the UV-IR range. Charge Exchange.—A number of nonmetals contain cations that can exist in more than one valence state. If these cations are not separated by too great a distance in the solid-state lattice, then certain electrons, although momentarily bound to a given ion, may nevertheless be able to move about the lattice from one atom to another by a series of oxidation-reduction reactions. The absorption coefficients of these materials are also very high. Important examples are magnetite Fe3O4 and ilmenite FeTiO3. Electronic Transitions.—Several transition elements have d electron shells that are degenerate in the free ion. However, when the ion is in a solid-state lattice, the anisotropy of the electric fields from the surrounding anions removes the degeneracy and may produce states separated by energies corresponding to UV-IR wavelengths. The most important element involved in this type of absorption is iron because of its cosmological abundance. Other elements that also may be significant in determining the colors of certain minerals and glasses are Ti, Cr, and Mn. The ferrous ion Fe2+ has a weak electron-transition band near 1000 nm. This band has been effectively exploited by McCord and his coworkers (McCord and Johnson, 1970; McCord, Adams, and Johnson, 1970; see the paper by Chapman in this volume”) for the remote identification of lunar and asteroidal materials. The ferrous band is especially useful because its exact position depends on the detailed mineralogy of the crystals in which the ion is located and thus often allows identification of the type of rock present on the surface of a body. The ferric ion Fe?” has an extremely strong absorption band near 235 mm; this band has not yet been exploited astrophysically because of the ozone cutoff in Earth's atmosphere. Paradoxically, the presence of iron ions can cause a mineral to be red, green, or blue, depending on the valence states of the iron. Other.—Other processes, such as band-gap conductivity and color centers, are not discussed here either because they are not important for materials of
interest to this paper or because their region of light absorption lies outside the range of UV-IR wavelengths.
Light diffusely reflected from a complex surface consists of rays that have been scattered by two processes: (1) rays that have been reflected from the outer surfaces of grains and (2) rays that have penetrated several wavelengths into the interior of grains and subsequently have been scattered out of the surface by some irregularity. The reflection coefficient for surface-scattered rays is determined, according to the well-known Fresnel laws, by the index of refraction and absorption coefficient of a grain of surface material. The intensity of the volume-scattered ray is determined primarily by the absorption coefficient. For a weakly absorbing material, such as MgO or pure enstatite MgSiO3, the reflection is dominated by volume scattering and the albedo of the substance is high. For a strongly absorbing material, such as a metal, the reflection is almost entirely by surface scattering; the albedo is low because each reflection is a rather inefficient process. A high albedo almost invariably implies a small volume-absorption coefficient.
The UBV properties of a number of terrestrial, lunar, and meteoritic materials are shown in figures 2, 3, and 4, respectively. The enclosed portions of the diagrams are those regions in which asteroids are found. The spectral data were obtained from freshly prepared materials using a Carey 14 spectrophotometer with an attachment to measure nonspecular radiation diffusely scattered at a phase angle of about 60°. The U- B and B - V indexes were calculated from the ratios of reflectivities relative to MgO smoke at wavelengths of 360, 440, and 550 nm and thus they correspond to data taken through narrowband filters. Metallic iron, the Nisfe meteorites, magnetite, and ilmenite are highly absorbing at all wavelengths and thus have UBV difference indexes that are very close to zero and that may even be slightly negative. Magnetite is the major opaque mineral in terrestrial igneous rocks and ilmenite in lunar materials. Several nonopaque, rock-forming minerals, such as pyroxene and olivine, have an absorption edge in the near-UV or visible region; and it is this edge that is partly characterized by the UBV indexes. The nature of this edge is uncertain, but it is known to be strongly affected by the presence of Fe (Shankland, 1968). The edge may involve charge transfer, the tail of the Fe3+ UV band, or both. An extremely important property of this edge is that for most substances it causes the slope of the spectral reflectivity to depend strongly on the size of the particles that make up the reflecting surface. The smaller the size, the
greater the slope and the larger the UBV difference indexes. The reason for this behavior is that a change of particle size has the opposite effect on the reflectivities of opaque and nonopaque materials. As the particle size of an opaque substance is reduced, the surface becomes more complex and a ray requires more reflections, on the average, to escape from the surface. For a weakly absorbing material, as the particle size is decreased, the density of boundary surfaces, which are primarily responsible for scattering the rays out of the medium, increases; thus the average pathlength through the material is decreased, causing reduced absorption. Hence, the effect of decreasing particle size is to reduce the reflectivity on the short-wavelength side of the edge, where surface scattering dominates, and to increase reflectivity longwards of the edge, where most rays are volume scattered, resulting in an increased slope at the edge. This effect is shown by all the materials in figures 2 to 4, where filled symbols represent solid, but unpolished, surfaces and open symbols represent powders. The sole exception is anorthosite, in which the edge is well below 360 nm.
-.] O . .2 .3 .4 .5 .6 B-V Figure 3.—Color difference diagram for lunar materials. Numbers are NASA designations. Filled circles indicate solid surfaces; open circles, powders finer than 37 um in grain
size; half-filled circles, powders coarser than 37 um. Nos. 10017, 10022, and 12018 are crystalline rocks; 10048 and 10065 are breccia; 10084 and 12001 are soil.
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Figure 4.—Color difference diagram for meteorites. Filled symbols indicate solid surfaces; open symbols, powders finer than 37 um in grain size; half-filled symbols, powders coarser than 37 um. Plus signs indicate low-iron chondrite powder finer than 37 um; circles, irons; triangles, high-iron chondrites; squares, achondrites.