Organic Material: Asteroids, Meteorites, and Planetary Satellites
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Organic Material: 93-18555 Asteroids, Meteorites, and Planetary Satellites D. P. Cruikshank and J. F. Kerridge elescopic observations in in situ spacecraft investigations over the last two decades have shown that many planetary satellites, asteroids, and comets have surfaces containing very dark material that is either neutral (black) or red in color. Awareness of the presence of this very dark material probably origi- nated with the 1971 discovery of the very low geometric albedo (0.03) of asteroid 324 Bamberga, determined from its easily detectable thermal radiation. Further infrared observa- tions showed that many other asteroids in the main belt have low geometric albedos, and in 1975 it was estab- lished that dark asteroids are very common, particularly in the outer parts of the main belt. That the Trojan asteroids and the outer satellites of Jupiter are similarly R A V very dark was first shown by infrared observations in 1977. COLOR l"HO 10'a_,.'.,_'.t 159 I Most of the smallest planetary Although comets are not nucleus derived from direct satellites, including the the focus of this paper, the observations. Indeed, the majority of the objects accom- possible relationship of formation of a nonvolatile panying Mars, Jupiter, and comets to asteroids, meteor- crust of low albedo from Uranus, have surfaces of very ites, and interplanetary dust cometary ices appears to be an low albedo, as revealed by (see below) requires that we inescapable consequence of spacecraft encounters and discuss them briefly in the ion irradiation of the outer continued telescopic observa- context of their dark-matter few meters of the nucleus. tions. The outer Jovian component. Evidence has satellites and the two satellites been steadily accumulating Beyond the direct observa- of Mars are comparable in size that at least some, if not all, tions of the nucleus of comet and have low albedos and comet nuclei have surface Halley, the accumulating colors that are similar to the deposits of low-albedo telescopic data on other C-type (dark and neutral) material. Telescopic observa- comets are giving a similar asteroids. The small, inner tions of comet Halley on its picture of the widespread satellites of Uranus discovered approach to perihelion gave occurrence of dark nuclei. by Voyager are also low in indications that its nucleus Many periodic comets have albedo. The small, inner has a dark red surface of low nuclear albedos of 0.10 or satellites of Saturn are an albedo. The direct images of less, with the majority studied exception to the above the nucleus of this comet by so far, less than 0.07. examples because they lie in the Giotto and two Vega the range of geometric albe- spacecraft unambiguously Except for Iapetus, the objects dos from about 0.4 to 1.1. confirmed the pre-encounter noted so far have relatively Phoebe, the outermost known deduction, showing that the uniform surface properties of satellite, has an albedo of 0.06 nucleus has albedo 0.04-0.05. color and albedo. There are and therefore resembles the several other planetary asteroids rather than the ice- Comet Halley contains high- satellites, and probably covered satellites of the molecular-weight organic asteroids, with surfaces that Saturnian system. molecules that have been include some fraction of dark detected both by direct material. For example, the Saturn's large satellite, sampling at the comet by the five large satellites of Uranus Iapetus, is an extraordinary Vega and Giotto spacecraft have reflectance spectra object having a bimodal and from ground-based indicating the dominance of surface distribution of high telescopes. While the identifi- water ice on their surfaces, and low albedo material. The cation of the specific organics but their albedos are too low hemisphere of Iapetus cen- is difficult, the presence of for water ice, ranging from tered on the apex of its orbital aromatics, aliphatics, and Umbriel at about O. 17 to motion has a very low albedo amorphous heavy molecules Ariel at about 0.39. The dark of 0.04 and is red, while the can be reasonably concluded. material mixed with the water trailing hemisphere of the Furthermore, the presence of ice of the Uranian satellites is satellite has an albedo of these heavy organics, which a neutral color, but its albedo about 0.5 and is composed often tend to be black, is remains unknown. of water ice. entirely consistent with the low albedo of the comet 160 Pluto has a nonuniform Cosmic material of low albedo Carbonaceous brightness distribution across that is available to us for its surface, with a wide, study on Earth consists of Chondrites circumplanetary dark band of meteorites of certain classes relatively low albedo at the and interplanetary dust Meteoritic organic matter equator. Models of Pluto's particles collected from the occurs primarily in those surface constrained by the stratosphere, the Greenland meteorites termed carbon- rotational lightcurve and the ice sheets, and the sea floor. It aceous chondrites that have secular change in brightness is natural to consider the the additional property that due to changing observing possible connections of these their chemical compositions aspect predict large surface materials to the comets and are similar to that of the Sun. areas of lower and higher the asteroids, as mechanisms The two properties are related albedo, as well as two polar for their delivery to the Earth at least to the extent that caps. Charon, the satellite of have been established and can preservation of organic Pluto, has an albedo of 0.4 be readily understood. Inter- molecules would have been (compared to Pluto's albedo planetary dust particles are aided by a history that led to of 0.6, both in the blue discussed elsewhere in this minimal chemical fraction- spectral region). We return to volume, but here we discuss ation. Nonetheless, it is these two objects below. the carbonaceous meteorites important to note that even in the context of their organic carbonaceous chondrites have It appears that dark matter content. not been immune to geo- occurs throughout the outer chemical processing during solar system, particularly on their evolution and that the small bodies, but also mixed episodes of localized heating, with other surface compo- impact brecciation, irradia- nents on larger ones. At the tion by solar flares and present time, we know only galactic cosmic rays, and that this low-albedo material aqueous mineralization can be reddish in color recorded in them may well (reflectance increasing at have altered their organic longer wavelengths) or neu- components and may even tral. This simplistic observa- have been implicated in the tion may well belie a tremen- origin of those components. dous diversity in molecular or mineral composition. 161 The molecular and isotopic composition of meteoritic organic matter has been the subject of numerous studies, but considerable work remains to be done, par- CH2OH ticu!arly by application of recently developed analytical techniques to issues that were studied at lower sensitivity OH O_ --_ CH3 and resolution in the 1960s and early 1970s. The principal O N _ O" ""_-_ objective of such studies is to discriminate between the different hypothetical origins that have been proposed for the organic matter. Variables used for that purpose have generally involved the molecular and isotopic distributions within specific Figure 9-1. Schematic representation of the stnlcture of kerogen-Iike classes of compounds, e.g., material in carbonaceous chondrites. Note the three-dimensional array of as a function of C number aromatic moieties with aliphatic bridging units and side chains. within homologous series. Patterns observed so far are difficult to fit within a single mode of synthesis and suggest instead multiple origins for The soluble organic fraction of formation of the organic the organic species now contains many different molecules, whereas others found in meteorites. classes of compounds, some probably reflect admixture of which, e.g., carboxylic and of material from different amino acids, reveal consider- sources. An example of of meteoritic organic able, perhaps complete, the first kind is shown in D he salient properties matter include the structural diversity and a figure 9-2, which shows the following: predominance of branched- distribution of the C isotopes chain molecules, whereas among the homologous series By far the dominant organic others, e.g., aliphatic hydro- of alkanes and monocarboxy- fraction is an insoluble carbons, do not. lic acids: the systematic macromolecular component, decrease in 13C/12C with resembling terrestrial kerogen, increasing C number clearly C, H, and N are not reflects a synthesis in which that consists of polycyclic D he stable isotopes of aromatic moieties distributed distributed uniformly higher homologs were made in a three-dimensional net- throughout the from lower by progressive work with short aliphatic organic matter but show bridging units and side chains variations, some of which are (fig. 9-1). probably related to the mode 162 The mode of synthesis is not the only interesting issue potentially addressable by the .01160 (_ I= I I I meteoritic record. The loca- tion and timing of that .01140 synthesis are also usefully, if not rigorously, constrained by the chondritic data. Thus, it now seems most likely that carbonaceous chondrites, like _ .o_12o all other chondrites, are fragments broken off asteroids .01110 -- _ ........ so that, given the apparently primordial zoning within the ! Terrestrial \ | .01100 ] organic \ "1 asteroid belt, a formation location for those meteorites _ ma.er inside the orbit of Jupiter is .010,0 " ' ' ' ' Y 1 2 3 4 5 6 indicated. An origin for their Number of carbon atoms In molecule organic matter in the inner solar system is thereby sug- gested, but the observed D Figure 9-2. Distribution of carbon isotopes among the homologous series of enrichment mentioned above alkanes and monocarboxylic acids in the organics of the Murchison seems to require operation of meteorite.