Organic Material: Asteroids, Meteorites, and Planetary Satellites

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 originated 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 observations showed that many other asteroids in the main belt have low geometric albedos, and in 1975 it was established 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 location 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 suggested, 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. The systematic decrease in the isotope ratio with increasing C a presolar mechanism, i.e., number reflects a synthesis in which higher homologs were made from lower isotopic fractionation in by progressive addition of single C atoms. interstellar molecular clouds. It is not known whether the interstellar H survived entry into the primordial solar addition of single C atoms. system within the organic The effects of mixing discrete kerogen-like fraction molecules or in some other _ ydrogenand in aminoin the and isotopic components are form from which it was carboxylic acids is illustrated in figure 9-3, which subsequently recycled into shows C and H isotopic considerably enriched in D locally produced organic relative to terrestrial H, and compositions of moieties matter. For example, even more so relative to identified within the kerogen- meteoritic amino acids are like material: the different primordial solar-system H. components cannot be readily These enrichments greatly derived one from another by exceed those that could be isotopic fractionation and are generated by known solar therefore believed to come system processes and are from different sources. therefore generally attributed to fractionation during ion- molecule reactions at very low temperatures in interstellar clouds (fig. 9-4).

163 Asteroids

-10 - asteroids have been B he albedos of the determined largely Aliphatic from infrared radio- m -15 c_ metric observations at 10 and o. Polycyclic aromatics 20 Mm from Earth-based .r- telescopes, and at 25 lam from the Infrared Astronomical -2O (Minor) (Major) Satellite (IRAS). On the basis N of a modest sample of about 180 asteroids, ground-based I I I I radiometry established a +500 +1000 +1500 +2000 bimodal distribution of 6DsMow% asteroid geometric albedos, with peaks in the distribution at 0.04 and 0.15. The IRAS data base has yielded reliable Fig_lre 9-3. Isotopic composition of different moieties present within albedos for over 1800 aster- kerogen-like material in the Murchison carbonaceous meteorite. The values oids; in the sample of objects of the I3c/12C and D/H ratios are expressed in the delta notation as of diameter >40 kin, both the variations, in parts per thousand, relative to terrestrial standards: Pee Dee bimodality and the median Belemnite in the case of C, Standard Mean Ocean Water in the case of H. values of the two peaks are clearly confirmed, but in the sample with diameter <40 kin, the distribution of albedos is more nearly Maxwellian with commonly believed to have the solar nebula, and since it been synthesized by the is most unlikely that the the peak at 0.05 (fig. 9-5). Strecker cyanohydrin reaction organic matter was intro- during aqueous activity in duced after that epoch, very asteroid surface regions, and early organic synthesis is the precursor molecules for indicated. This is obviously that synthesis have all been consistent with either inter- observed in interstellar clouds stellar, nebular, or planetesi- and, in the early solar system, mal production of the organic could plausibly have had such matter. an origin.

Radiometric dating places the epoch of aqueous alteration and impact-induced turnover on the parent asteroids within, at most, a few tens of Myr after solid formation in

164 Telescopic observations have tion bands are seen in the ence of the 3-pro hydrated been used to classify the low- C-type spectra in this region, silicate band appears corre- albedo asteroids into three but farther into the infrared lated with the strength of the basic categories according to some of the asteroids in this violet downturn in reflec- their colors in the photovisual category have a distinct tance, such that the "wet" spectral region (0.3-1.05 l_m). absorption band at 3 pm C-types fit into a subclass In this classification scheme, attributed to H20 and OH in called G. the C asteroids are usually hydrated silicates in the neutral over most of the regolith. Some of the C-types The D-type asteroids are spectral region, with a down- have no bound water, and distinguished from the turn in reflectance toward the there appears to be consider- C-types by their significantly violet end of the spectrum. In able variability among them, redder slopes both in the the near-infrared (1-2.5 _m) with several degrees of hydra- photovisual and near-infrared the C-types are neutral to tion represented. The pres- regions; the D-types include slightly reddish (a small the reddest known asteroids. upward slope in reflectance Hydrated silicates have not toward longer wavelengths). yet been found in asteroids of No discrete spectral absorp- this class, but the D-types are difficult to observe because of their low albedos and their tendency to lie in the outer reaches of the asteroid system.

Figure 9-4. Deuterium fractionation, relative to galactic H2, caused by The P-type asteroids are exchange reactions and ion-molecule reactions, compared with D/H values observed in meteoritic components and in the Orion interstellar cloud. similarly of low albedo, and may represent a transition between the C and D types. They have similar absorption 10-I T r T T- in the UV and featureless red- sloping reflectance throughout the near-infrared. There is ------__ HNC (Orion A) I0"2 currently no information on

--\_o_ HCN (Orion A) the presence of hydrated silicates; as in the case of the ___ Ds, they are very faint and

"1" \_O_ _X _ _ HCO (Orlon A) difficult to observe. 10 3

_:_:_:i:_:_:!:_ Range of values in ::: iiiii:i: :ij:i: Jii

10-4

nge

Galactic H2 - - -- L 1 L 1 ._____J_ 0 100 200 300 400 Temperature °K

165 Figure 9-5. (a) Histogram showing the distribution of the number of 500 asteroids of diameter >40 km with geometric albedo. These data were derived from measurements of the thermal radiation of the asteroids 400 with the h_frared Astronomical Satellite ([RAS). The distribution is bimodal, with the greatest number 300 at about 0.05. (b) The same plot as D > 40 km in (a), but for asteroids of diameter t_ E <40 kin. The bimodality of the z distribution is gone, but the peak 2OO still lies at about 0.05. This and figure 9-5a are adapted from E. F. Tedesco et aI., "fRAS Observations of Asteroids," in Comets to 100 Cosmology, Andrew Lawrence, ed., Springer- Verlag, 1988.

.1 .2 .3 Geometric albedo

(a)

150

125

100 _L_ D<40km

.D E 75

Z 5O

__d ...... n 0 .1 .2 .3 .4 .5 Geometric albedo

(b)

166 he distribution of the dramatic increase in the there is a strong trend toward asteroid types with number of low-albedo objects; increased redness of the distance from the Sun the C-types dominate the spectral reflectances with has proved to be of outer half of the main belt, increasing distance from some significance, once bias- and as they decline beyond the Sun. corrected samples could be 3 AU, the redder objects of studied. A number of studies the P and D classes increase Various investigators have of the heliocentric distribu- toward their peak in the Hilda suggested that the dominance tion of various types that group at 4.0 AU. Color varia- of the low-albedo bodies and culminated in the appear- tions among the reddish types the increased redness in the ance of a clear pattern are have been noted. outer portions of the asteroid shown in figure 9-6. The system are results of a tem- middle of the asteroid main Two clear trends have perature gradient in the solar belt is at about 2.7 AU; inside emerged from this work. First, nebula that is preserved in the this distance most of the the low-albedo asteroids present distribution of the asteroids are of the high- dominate the region beyond asteroids that formed at albedo classes, particularly the about 3 AU, and second, different distances from the S-types (median albedo O. 15). Sun. The appearance of dark Beyond 2.7 AU, there is a bodies at about 3 AU could represent the onset of formation in the solar nebula of carbon-rich minerals and

Figllre 9-6. The distribution of a bias-corrected sample of asteroids by macromolecular carbon taxonomic type with distance from the Sun. This figure shows that the C- compounds (organics) that type objects peak in their distribution at about 3 AU, while the peaks for the color the materials of which P- and D-types are at larger distances. Types E, S, and M are higher-albedo the asteroids are composed. objects not addressed in this paper, f is the estimated fraction of asteroids at Alternatively, 3 AU could a given distance in each class. mark the inner limit in the solar nebula at which such organic compounds formed elsewhere could survive.

2/7 2/5

1/4 / 1/3 3/7 1/2 2/3 3/4 1/1 1.0 I I I I I I /J._

.8 E c D // _

.6 /// S // .4 I / .2

A 2 3 4 5

a (AU)

167 The possible presence of One serious drawback to the he 3.4-t_m absorption complex organics in the outer kerogen hypothesis is that, by band of C-H was solar system bodies in the itself, kerogen, as prepared, sought in the form of kerogen-like material, for example, from coal tar has reflectance spectra of which compose the bulk of the wrong colors, although it asteroids that were selected the organic matter in the is extremely low in albedo. on the basis of their bound- carbonaceous chondrites (up For example, the photovisual water band and their low to 70% in the CI meteorites reflectance of the very red albedo (i.e., the G-types). such as Orgueil) was noted in D-type asteroids could be Tentative identification of the 1980. Kerogen itself is a simulated by a synthetic C-H band was made in mixture of complex molecules mixture of clays and kero- asteroid 130 Elektra, based on which, in the case of terres- gens, the clays giving the red the data shown in figure 9-7. trial petroleum deposits, color and the kerogens the In the laboratory spectra of consists of a waxy substance low albedo. The Trojan carbonaceous meteorites, the that adheres to carbonate asteroid 624 Hektor was C-H band is weak because of grains. At temperatures of matched by a mixture of 85% the high opacity of the finely about 750 K, the kerogen is montmorillonite cIay and granulated sample. This is altered to liquid petroleum. about 8-15% kerogen, plus true for pulverized samples of In the meteorites, it is inter- some magnetite or carbon the bulk meteorite as well as mixed with clay mineral black. In part because the for the concentrated insoluble grains and constitutes a very term kerogen carries a conno- organic residue remaining opaque, black component. tation of origin in terrestrial after removal of the other Kerogen, or kerogen-like petroleum deposits, it is components. Similarly, the material, therefore, appears to perhaps advantageous to band is expected to be very be a plausible cause of the substitute the term "macro- weak in the diffuse reflectance very low reflectivities of molecular carbon" in the spectra of an asteroid, making various solar-system objects, context of meteorite and its detection quite difficult. in part because few other related studies. The intrinsic weakness of the natural substances are so band, plus the inherent black, and in part because the In laboratory reflectance difficulties in registering the widespread occurrence of spectra of carbonaceous spectrum of a weak astro- complex organic molecules chondrites the presence of nomical source in the 3-_m is an acceptable working bound water is revealed by region, account for the hypothesis, given its presence a broad absorption band uncertainties in the detection in interstellar space and the at 3 _m. On the long- of C-H in 130 Elektra. most primitive meteorites, wavelength portion of the and the relative ease of its featureless slope of this band, The importance of establish- genesis in plausible planetary at 3.4 gm, there is super- ing the presence of organic environments. imposed a weak band com- molecules in the regoliths of plex assigned to the stretch- asteroids is multifold. It is ing mode in C-H in the certainly connected with the organic compounds in the general problem of the origin meteorite. of specific kinds of meteorites, but it also bears on the chemistry of the solar nebula on the outer edge of the zone

168 I I I I I Low-Albedo Murchison carbonaceous chondrite Planetary Satellites

The odyssey of the Voyager spacecraft through the outer solar system has resulted in Asteroid 130 elektra the discovery of a number of previously unknown planetary satellites, typically small and close to their parent planets. Together with low- albedo satellites known from telescopic discoveries, there are some 24 of these bodies of

I I I I I a few tens of kilometers in 0 3.2 3.3 3.4 3.5 3.6 3.7 dimension. The impending Wavelength (pm) encounter of Voyager 2 with the Neptune system carries the potential for the discovery Figure 9-7. The reflectance spectnlm of the G-type asteroid 130 Elektra in of more of these small bodies. the region of the 3.4-1_m C-H stretch band, with a laboratory spectrum of the Murchison carbonaceous meteorite for comparison. Effects of continuum The similarity in size, low slope due to sample and asteroid temperature as well as the strong band of bound water in each material have been removed. The correlation of the albedo, and density (where absorption features in the asteroid spectrum with the main feature in known) of some of these Murchison is taken as preliminary evidence for the presence of C-H on an satellites to dark asteroids has asteroid. Adapted from D. P. Ct_:ikshank and R. H. Brown, Science, invited comparison and voI. 238, 1987, p. 183. speculation on commonality of origin. The two satellites of Mars are often regarded as asteroidal in nature, though of terrestrial-planet forma- detection of organic molecu- skepticism has been expressed tion. In the broader sense, the lar material on asteroids will on the issue of dynamical question of the origin of be only the first step, to be capture into their present circular orbits. The outer macromolecular carbon followed by the identification molecules in the interstellar of the species and the study of Jovian satellites are often medium and incorporation the space distribution of compared with the Trojan asteroids (which share into the solar nebula, versus organic-bearing asteroids. the genesis of this material near the forming Sun is of great interest. The confirmed

169 Jupiter's orbit at the Lagrang- The physical evidence for a satellite (except Titan), ian points). Dynamical possible macromolecular though attempts have been considerations of the origin of carbon component of plan- made to detect it on Iapetus, planetary satellites show that etary satellites consists of as we discuss below. asteroids or comets might be broad-band photometric captured by planets under measurements of the colors of The striking bimodal distribu- certain circumstances. Indeed, these objects (0.3-3.5 gm), tion of a very low-albedo the only known way to and in some cases actual surface unit and an expanse acquire such retrograde spectra over limited wave- of water ice or frost on satellites as Saturn's Phoebe length intervals. The limiting Saturn's satellite lapetus has and four of the outer satellites factor in obtaining spectral commanded special attention of Jupiter appears to be data is the faintness of small in the search for organic through dynamical capture. planetary satellites, even material on bodies in the Thus, at least some of the when observed with the outer solar system. Data from small, clark planetary satellites largest telescopes and most Voyagers 1 and 2 confirmed may have originated in a way sensitive spectrometers. the pattern of distribution of similar to the asteroids, Spacecraft sent to the planets the dark material that had though the details seem to with small dark satellites have been derived from telescopic be unique for each body. not carried spectrometers studies showing that the dark suitable for studies of their covering on the leading In discussing the possibility of small dark satellites, with the hemisphere of the satellite is organic materials as the result that only color data centered on the apex of the darkening component of the (from space and from the orbital motion. Such a geom- surfaces of asteroids, and Earth) and imperfect spectra etry argues strongly for a particularly planetary satel- (from the Earth) exist. An method of emplacement of lites, it is worth bearing in added inconvenience is the the dark material that mind the fairly low albedo of absence of strong diagnostic depends in some way on the our own Moon. The overall spectral features in accessible influx of material from space, geometric albedo of the Moon spectral regions, owing partly though the details are is 0.t2, but that of the darkest to the nature of organic unclear. lunar maria is about 0.07. molecules themselves and to Samples returned from the the intrinsic weakness of most Moon attest to the fact that spectral features of low-albedo the regolith on the lunar solids seen in diffuse reflec- maria has no organic compo- tance. The most promising nent, and the low albedo is spectral band is that of CH at attributed to the dark miner- 3.4 gm seen in some carbon- als and texture of the regolith. aceous chondrites, and It is very difficult to make possibly in asteroid natural surfaces with an 130 Elektra, as noted above. albedo half that of the lunar This band has not been maria without resorting to observed on any planetary some other component, and macromolecular carbon compounds are a plausible choice.

170 There are two main ideas It is important to remember tar kerogen had the correct concerning the origin and that very small quantities of spectral reflectance, but composition of the dark dark, opaque powder can mixtures of the kerogen with material on Iapetus. In one, have a strong effect on the clay were produced and a dark material is emplaced by albedo of the material in good match in both spectral deposition from a source which it is incorporated. For reflectance and albedo was outside the orbit of Iapetus, or the case of the dark hemi- achieved. An organic-clay it results from a process of sphere of Iapetus, for example, mixture with about 10 weight impact concentration to the it has been demonstrated percent kerogen produced the surface of dark material that the abundance of water best match, as shown in already present and distrib- ice on that hemisphere could figure 9-8. uted in the upper layers of the range from less than a few satellite. In another model, weight percent to as much as The 10 newly discovered black macromolecular carbon 95 weight percent. Finely small satellites of Uranus all is formed in situ from meth- divided dark material, such as orbit very close to the planet. ane clathrate by solar UV charcoal, lampblack, magne- Only one, Puck, was imaged bombardment. The former tite, or macromolecular at sufficient resolution by model appears to be favored carbon, is extremely effective Voyager 2 to show surface in view of the telescopic data in reducing the overall albedo relief and its slightly non- and the spacecraft images of the mixture and in quench- spherical shape. The albedos showing the manner in which ing the strong ice absorption are all very low, in the range the surface deposits relate to bands normally seen in the of 0.04-0.06, and comparison topography on Iapetus. near infrared. is often made with the particles in the Uranian rings, The dark hemisphere of which are also quite black. Iapetus, which is seen from for the effects of the Furthermore, the ices of the hen fully corrected the Earth face-on for an minor contamina- five major satellites all appear interval of about 48 hours tion of the dark to be contaminated to some once during each 80-day orbit hemisphere with relatively degree with neutral material of the satellite around Saturn, pure water frost in the polar of low albedo which may is sufficiently bright to permit regions, the reflectance of have the same origin. low-resolution spectrophoto- Iapetus between 0.3 and Whether or not this material metric observations in the 2.5 IJm is relatively smooth has any relationship to photovisual and near-infrared and strongly upward sloping macromolecular carbon spectral regions. Such data toward longer wavelengths have been compared with (that is, it is red), as seen in laboratory samples of candi- figure 9-8. Comparisons were date materials, with concen- made using this spectrum of tration on materials of very Iapetus and laboratory data low albedo and red color. for the organic component of the Murchison CM carbonaceous chondrite, and an insoluble coal-tar kerogen. Neither the Murchison organic extract nor the coal-

171 3 I I I I I strong evidence for condensed nitrogen on Triton. Water ice has been found on Pluto's satellite Charon, which has led to some early models of the escape of _2 O r- original methane from the "6 satellite. m

o While neither solid methane nor condensed nitrogen (solid re 1 or liquid) are quite neutral in the photovisual region of the spectrum (0.3-1.0 #.m), both Pluto and Triton are notable for their yellow-red colors.

1 I I I I Pluto's surface is clearly non- .5 1.0 1.5 2.0 2.5 uniform in albedo and color Wavelength (pm) distribution, with a concentration of relatively darker material at the equator and Figure 9-8. Spectral reflectance of the low-albedo hemisphere of Saturn's satellite Iapeh_s (data points) in comparison with the reflectance of a high-albedo deposits at the laboratory sample of 90% clay plus 10% organics (line). The general shapes poles. There is some sugges- and absolute values of the spectra are very similar over this broad wave- tion of surface variegation on length range. From J. F. Bell et al., Ican_s, vol. 61, 1985, p. 192. Triton, but the issue is con- fused by an apparent change in both the spectrum and the photometric lightcurve that occurred sometime in the cannot be established at this Pluto, Charon, early 1980s. time, but it has been shown and Triton that irradiation of surface ices by the ion and electron fluxes observed by Voyager at The low temperatures at the Uranus can darken the sur- surfaces of the outermost faces if those ices contain planets and satellites permit some carbon. While the methane and some other presence of water ice on the simple hydrocarbons to exist larger Uranian satellites is well as solids, though the vapor established, the telescopic pressures are sufficiently high search for other substances, that a certain mobility (subli- such as CH 4, NH 3, CO, and mation and recondensation) CO2 has so far only given a is expected in seasonal cycles. faint hint of NH 3. Methane has been found on Pluto and Triton, and there is

i t 72 i Laboratory Ices of planetary significance optical properties of the (H20, CO2, CO, CH 4, NH 3, candidate materials. Eventu- Studies etc.) all show diagnostic ally, this computational spectral features in diffuse technique can be extended to Numerous laboratory studies reflectance, and a number of mixtures of organic solids in the last few years are these solids have been identi- with ices of planetary interest. pertinent to the outstanding fied on cold planetary bodies. questions of the nature and Similarly, hydrous clays and Work on organic compounds origin of the very dark mate- igneous rock-forming miner- with application to planetary rial on surfaces of some solar als of planetary significance problems is being undertaken system bodies. The experi- (pyroxene, plagioclase, in the context of cometary mental work falls into two orthoclase, and olvine) are nuclei and solids in the general categories: (a) spectro- identified on planets, aster- interstellar medium. Colora- scopic and photometric oids and satellites through tion and darkening of meth- studies of natural materials in their spectral absorption ane clathrate and other ices various mixtures, including features. The laboratory work has been produced by extracts from meteorites, and has stressed spectral regions charged-particle irradiation. (b) similar observations of where actual astronomical Yellow and brown organic synthetic materials produced data exist or can be obtained products are generated when or altered through the effects within the constraints of flux otherwise white ices of of irradiation by ions, pro- level and transparency of the various combinations of tons, electrons, ultraviolet, or Earth's atmosphere. Extension water, methane, and ammo- gamma rays. of the laboratory data into nia are irradiated for times spectral regions from which that are relatively short in Radiation-processed ices and spacecraft data will eventually terms of equivalent exposures organic materials have been come has only begun. in space. Similar experiments studied in several laboratories. have been conducted with the Organic residues from carbon- New work is in progress on production of non-volatile containing gases or ices have combinations of ices and colored organic material from been produced by electrical minerals, which must occur irradiation of methane ice. discharges, UV radiation, because the overall albedos of UV irradiation of ices contain- high-energy ions, and fast some icy bodies are too low ing methane and other simple protons. Some preliminary and the colors are incorrect hydrocarbons can produce planet-related experiments for pure ices. Laboratory work more complex molecules, in with gamma-irradiated is being undertaken in sup- particular a variety of poly- mixtures of nitrogen and port of the computation of cyclic aromatic hydrocarbons, methane have also been done synthetic spectra of pure which show spectral features in the laboratory. In addition, minerals and ices and, most giving a close match to direct studies of meteoritic significantly, combinations of infrared bands in the inter- organics in the context of the these materials in a range of stellar medium and in some interstellar and planetary grain sizes and surface tex- interplanetary dust particles. occurrence of this material tures, through knowledge of have moved steadily forward. the theory of scattering from planetary surfaces and detailed knowledge of the

173 In some cases these experi- Summary needed. More information is ments produce very low- needed on the relationship albedo material that is a between carbonaceous chon- Low-albedo bodies in the plausible constituent of the drites and specific asteroids or solar system appear to come dark planetary surfaces and classes of asteroids. Major meteorites discussed in this in a range of colors, with questions remain about the Iapetus being very red and review, and in other cases, origin of the meteoritic the outer satellites of Jupiter colored material of possible organic matter, particularly. being nearly neutral (nearly relevance to the yellow concerning the role played by fiat reflectance curves). This surfaces of Pluto and Triton presolar, interstellar processes. statement also applies to the are formed. The relative ease This issue will be greatly asteroids in the outer parts of with which complex organics clarified by detailed analysis the main belt and in the are produced from the simple of the organic population of Trojan groupings, and it is raw materials found in many comet nuclei, in which now accepted that the sur- places in the outer solar interstellar molecules should faces of many comet nuclei system virtually ensures a be preserved. In that connec- are extremely low in albedo. significant role for them in tion, the Comet Rendezvous Macromolecular carbon, the surface chemistry of the Asteroid Flyby (CRAF) and which is found in meteorites comets, asteroids, planetary Comet Nucleus Sample and interplanetary dust satellites, and some planets. Return Missions will be of particles, is a plausible low- The current production of prime importance. albedo constituent of these stable and complex hydro- dark asteroid, comet, and carbons in the atmospheres Solid organic matter is pres- satellite surfaces; indeed, the of the outer planets has been ently being synthesized in the presence of organic matter in appreciated for quite some atmospheres of the methane- time. cometary dust has been bearing planets and satellites established. of the outer solar system, and probably on the solid surfaces Insofar as the asteroids are the of Pluto and Triton. Photo- source bodies of the carbon- chemical smog is continu- aceous meteorites, the pres- ously produced in the atmo- ence of organic material on at spheres of Titan and the giant least some classes of asteroids planets, and the presence of is a certainty. Telescopic methane frost or ice on the searches for the CH spectral yellow-colored surfaces of signature strongly suggest its Pluto and Triton virtually presence on dark, hydrous ensure the alteration to more asteroids, but confirmation complex organic solids by the and further exploration are action of UV and the galactic cosmic ray flux.

174 More generally, we wish to Cruikshank, D. P.; Cruikshank, D. P.; and Brown, know which chemical pro- Allamandola, L. J.; Hartmann, R. H.: Organic Matter on cesses have been effective at W. K.; Tholen, D. J.; Brown, Asteroid 130 Elektra. Science, synthesizing organic mol- R. H.; Matthews, C. N.; and vol. 238, 1987, p. 183. ecules from simple inorganic Bell, J. F.: Solid C-N Bearing gases, and what factors have Material on Outer Solar Hartmann, W. K.; Tholen, controlled the extent of System Bodies. Icarus, vol. 94, D. J.; and Cruikshank, D. P.: chemical, pre-biotic evolution 1991, p. 345. The Relationship of Active on different solar system Comets, "Extinct" Comets, bodies. For example, chemical Other Reading and Dark Asteroids. Icarus, evolution led to the emer- vol. 69, 1987, p. 33. gence of life on Earth but not, Allamandola, L. J.; Sandford, Hayatsu, R.; and Anders, E.: apparently, on the carbon- S. A.; and Wopenka, B.: Organic Compounds in aceous chondrite parent Interstellar Polycyclic Aro- Meteorites and Their Origins. asteroids. Were some compo- matic Hydrocarbons and Topics in Current Chemistry, nents of the "primordial Carbon in Interplanetary Dust soup," present on Earth, Particles and Meteorites. vol. 99, 1981, p. 1. missing on the asteroids, and Science, vol. 237, 1987, p. 56. if so, which? Kerridge, J. F.; and Matthews, M. S., eds.: Meteorites and the Clark, R. N.; Fanale, F. P.; and Early Solar System. University Gaffey, M. J.: Surface Compo- of Arizona Press, Tucson, Additional sition of Natural Satellites. In 1988. Satellites, J. A. Burns and M. S. Reading Matthews, eds., University of Lewis, R. S.; and Anders, E.: Arizona Press, Tucson, 1986, The text and references for Interstellar Matter in Meteor- p. 437. this article were up to date at ites. Scientific American, vol. 249, p. 66. the time of writing in 1988. Cruikshank, D. P.: Dark Many additional papers on Matter in the Solar System. Thompson, W. R.; Murray, related topics have been Adv. Space Res., vol. 7, 1987, B. G. J. P. T.; Khare, B. N.; and published since that time; p. 109. references to two of them are Sagan, C.: Coloration and cited here. Cruikshank, D. P.; and Brown, Darkening of Methane Clath- R. H.: Satellites of Uranus and rate and Other Ices by Charged Particle Irradiation: Anders, E.: Organic Matter in Neptune, and the Pluto- Applications to the Outer Meteorites and Comets: Charon System. In Satellites, Possible Origins. Space Sci- J. A. Burns and M. S. Solar System. J. Geophys. Res., vol. 92, 1987, p. 933. ence Reviews, vol. 56, 1991, Matthews, eds., University of p. 157. Arizona Press, Tucson, 1986, p. 836.

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