Adv. Space Res. Vol. 27, No. 2, pp. 299-307, 2001 ~ Pergamon Published by ElsevierScience Ltd on behalfof COSPAR Printed in GreatBritain 0273-1177/01 $20.00 + 0.00 www.elsevier.nl/locate/asr PII: S0273-1177(01)00061-8

SOLID ORGANIC over a wide range of wavelengths. In this paper, we describe the historical background of laboratory re- MATTER IN THE search on this kind of organic matter and how our lab- oratory investigations of tholin compare. We ATMOSPHERE AND ON comment on the probable existence of polycyclic aro- THE SURFACE OF OUTER matic in the Titan Haze and how bi- ological and nonbiological racemic amino acids pro- BODIES duced from the acid hydrolysis of Titan tholins make these complex organic compounds prime candidates in the evolution of terrestrial and extraterrestrial B.N. Khare 1, E.L.O. Bakes 2, D. Cruikshank 3, and life in our own Solar System and beyond. Finally, we C.P. McKay2 also compare the spectrum and scattering properties of our resulting tholin mixtures with those observed 1MS P39-14, NASA Ames Research Center, Moffett on Centaur and the dark hemisphere of Field CA 94035-1000, USA 's satellite Iapetus in order to demonstrate the 2MS 245-3, NASA Ames Research" Center, Moffett widespread distribution of similar organics through- Field CA 94035-1000, USA out the Solar System. Published by Elsevier Science Ltd 3MS 245-6, NASA Ames Research Center, Moffett on behalf of COSPAR. Field CA 94035-1000, USA

INTRODUCTION ABSTRACT Our Solar System was formed from the primor- Many bodies in the outer Solar System display the dial when it collapsed to form presence of low albedo materials. These materiMs, a protoplanetary disk of gas and dust grains. Every- evident on the surface of asteroids, , Kuiper thing that comprises the solid material in our plan- Belt objects and their intermediate evolutionary step, etary system originates from primordial interstellar Centaurs, are related to macromolecular carbon bear- dust, which is an organically rich, low albedo ma- ing materials such as polycyclic aromatic hydrocar- terial whose surface can range from a dark, radia- bons and organic materials such as and re- tively processed, refractory layer to a complex cool lated light hydrocarbons, embedded in a dark, refrac- ice coating. The bodies of the outer Solar System tory, photoprocessed matrix. Many planetary rings in particular formed from the cool outer part of the and satellites around the outer gaseous planets dis- protoplanetary disk and were enriched by complex play such component materials. One example, Sat- sublimed from the surfaces of these grains urn's largest satellite, Titan, whose atmosphere is during protoplanetary disk formation and processing. comprised of around 90% molecular N2 and This supplied the raw materials with which to form less than 10% CH4, displays this kind of low a wide variety of outer Solar system objects such reflectivity material in its atmospheric haze. These as comets, Centaurs, Objects and the materials were first recorded during the gaseous planets and their satellites. Titan, Saturn's and 2 flybys of Titan and showed up as an optically largest satellite, formed from an -methane thick pinkish orange haze layer. These materials are rich region of the circumplanetary nebula that went broadly classified into a chemical group whose labo- on to condense into Saturn and its other satellites ratory analogs are termed "tholins", after the Greek (Prinn and Fegley 1981). Titan is seen by many word for "muddy". Their analogs are produced in as a prebiotic chemical laboratory (see Lunine and the laboratory via the irradiation of gas mixtures and McKay 1995) and its structure is expected to be lay- ice mixtures by radiation simulating Solar ultravio- ered, containing mixed ices and silicates in the core, let (UV) photons or keV charged particles simulating with an overlying silicate carapace and a deep ocean particles trapped in Saturn's magenetosphere. Fair of ammonia- liquid (Sagan and Thompson 1984; analogs of Titan tholin are produced by bombarding Lunine and Stevenson 1987). Titan's surface has a a 9:1 mixture of N2:CH4 with charged particles and pressure of 1.5 bar (Lindal et al. 1983) and a tem- its match to observations of both the spectrum and perature of around 94 K. It is most probably com- scattering properties of the Titan haze is very good posed of water ice coated in accumulated hydrocar- 299 300 B.N. Khare et al. bons and . The action of ionizing radiation formation of Titan Haze tholins. We then com- (Sagan and Thompson 1984), of lightning (Borucki ment on the implications of these experiments for"the et al. 1984), and of meteorite impacts (Jones and formation and evolution of life and briefly describe Lewis 1987; Thompson et al. 1992) is very likely tholin like compounds found in outer Solar System to have driven an exotic prebiological chemistry, per- bodies Iapetus and the Centaur 5145 Pholus. haps as far as the or nucleotide base stage (Fortes 1997). The absence of water provides a se- A BRIEF HISTORY OF EXPERIMENTAL rious stumbling block to the formation of biological SIMULATIONS OF PLANETARY ATMO- molecules. However, impact ejecta are expected to SPHERES contain large quantities of water. Thompson et al. Accurate simulations of planetary atmospheres (1992) calculated that around 70% of Titan's or- have been difficult due to problems in maintaining ganic inventory has been exposed to impact melted an exact gas composition, temperature, pressure, en- water for mean periods of around a century. This ergy cm -3 and above all, keeping the reactions free may have yielded a range of organic compounds by from wall effects. The groundbreaking experiments hydrolysis of tholins, although the products would performed by Sagan and Miller (1960) were under- inevitably freeze again, so that potential biological standably less refined than those performed within activity on Titan's surface is likely to be trapped in the last 5 or 10 years. For instance, the first simu- a permanent stasis. In early Titan, hydrothermal re- lation of a Jupiter atmosphere used a 10:1:3 mixture actions between the ocean and the underlying rock of H2:CH4:NH3 and was sparked with a Tesla coil at layer likely reprocessed a portion of the ocean, plus T,~300 K and Pal bar. The dilution was infalling cometary and chondritic material, into more too low by a factor of 100 and the product molecules complex organic compounds (Shock and McKinnon were repeatedly subjected to sparking, leading to sec- 1993). Significantly, Titan is unique for a satellite in ondary processing of the primary products. In ad- that it is the only moon in our Solar system which dition, wall effects may have been influential. The formed an atmosphere. This atmosphere ranges in product molecules were unsaturated (ethyne C2H2 temperature from around 70 K to 180 K and contains and ethene C2H4) and saturated ( C2H6) hy- mainly methane and nitrogen components which have drocarbons, as well as nitriles (HCN, CH3CN). All been energetically processed via UV radiation and of these molecules, except for CH3CN, have been particles into a complex organic haze in spectroscopically detected in the Jovian atmosphere its stratosphere. Figure 1 shows the power dissi- (B@zard et al. 1995). In all such experiments, be- pated per unit volume of atmosphere by the various sides the gas phase products, complex brown pow- sources of available radiation deposited on Titan as dery or sticky organic solids are always generated on a function of altitude (Sagan and Thompson 1984). the walls of the reaction vessel and in the low temper- These energetically processed haze particles, named ature traps downstream of the energy source. These tholins, can be pyrolysed or hydrolysed to produce solid materials are not pure and they are technically amino acid mixtures. This makes them a prime can- not polymers (a polymer is a repetition of the same didate for facilitation of the evolution of both prebi- monomer unit). Because of their general nature and otic molecules and perhaps, life itself in our own and broad chemical composition, they were simply coined other planetary systems. In addition, because Ti- "tholins", after the Greek for "muddy" (Sagan et al. tan is frozen at a stage of chemical and atmospheric 1992). evolution intermediate between the highly reducing PRESENT DAY LABORATORY SIMULA- (H2/He/CH4/NH3/H20) atmospheres of the Jovian TIONS FOR THE TITAN HAZE planets and the more oxidized (N2/CO2/H20) at- Present day simulations of planetary atmospheres mospheres of the terrestrial planets Mars and Venus are considerably more refined. Previous studies by (Sagan et al. 1992), then Titan's atmospheric chem- the LISA/Paris group and by other groups (Thomp- istry may also provide clues concerning early terres- son et al 1991; Sagan et al 1992; Coll et al 1999) trial atmospheric evolution (Sagan 1974). considered the following conditions necessary for the In this paper, we present a brief historical per- realistic simulation of the Titan Haze. First, the gas spective on the laboratory synthesis of haze analogs mixture must be maintained at the same pressure in planetary atmospheres, followed by a description throughout the simulation and receive a dose of radi- of the comprehensive work performed at the NASA ation and chemical reactants appropriate to the at- Ames Laboratory to simulate the mospheric depth under consideration. This facilitates Solid Organic Matter on Outer Solar SystemBodies 301

log F(erg cm "a s "z) -IX -tO -9 -8 -8.C i , , ,] 3800 -7.O 3700 3600 v~m ¢~1 -6.0 ii P 3500 -5.0 3400 3600 -4.0 32O0 i -3.0 3100 ~ -2.0 3OOO ~ 2950 P -1.1:1 ± f 2900 ~.. 2850 B N~ltwt LN! SlU~ .Quth ~ %%.~°.z%c"4 (77KI K ~llO~ °'°- " S :2650 2.0 2625 3.0I. ..lJ I ooo

40 60 80 100 120 140 160 180 200 Fig. 2. Experimental setup for continuous flow Ternper'atu re, *X plasma discharge irradiation experiment (Thompson et al. 1991)

Fig. 1. Energy deposition regimes in Titan's atmo- sphere F. as a function of altitude r from the center The gas mixture must be ultra high purity and the of Titan, and equivalently, atmospheric pressure P. Al- analysis of the initial gas mixture and the resulting titudes of maximum energy deposition are indicated by products after irradiation should be undertaken using "+" ; altitudes over which the various energy sources are high sensitivity analytical techniques. To meet these within 1 order of magnitude of their peak values are in- requirements, Thompson et al. (1991) have refined dicated by the boxes. Dashed lines indicate altitudes at the continuous flow, inductive plasma discharge tech- which F is within 2 orders of magnitude of peak values. nique, combined with the analysis of the products Titan's surface is shown hatched at the bottom. by gas chromatography mass (GCMS). The apparatus used for this experiment is shown in Figure 2. A premixed 90% N2 to 10% CH4 gas keeping the ratio of uni, bi and trimolecular processes mixture is introduced into the tank at the right and the same as a real atmosphere. Secondly, the flow the flow measured. Contaminant gases are removed rate of the gas mixture must be maintained to provide by the pretrap and an inductive discharge is excited a short residence time through the reaction zone to by a Tesla coil or other radio frequency source as the minimise collisions of molecules with the wall of the gas flows through the glass discharge tube. Gas pres- tube. Thirdly, to keep wall effects to an absolute min- sure is monitored by a thermal conductivity gauge imum, the tube diameter in the reaction zone is large and the products are fractionally trapped in four spi- compared to the mean free path of the molecules. rals of decreasing temperatures, while the unreacted Finally, although the ambient temperature is gener- gas is pumped out continuously at the left. Two ex- ally not important unless key reaction rates display periments performed using this setup are described temperature dependence, the adsorption and conden- below from Thompson et al. (1991) and Sagan et al. sation of volatile organic vapors such as HCN, C2H2, (1992). C2H6 and C3H8 that reach saturation in the lower regions of Titan's stratosphere may coat tholin par- • Higher Pressure, Low Dose Experiment With ticles lowering the contact angle with CH4. This will Cosmic Ray Analog Irradiation facilitate efficient nucleation and this means that any The pressure for this experiment was set at 17 possible temperature dependence of the products and mbar for 49 hours and 34 minutes with 4100 liters of the optical constants of Titan tholins should be in- (N2+CH4) and was irradiated at a delivered power vestigated wherever possible. of around 0.5 W. This simulated the irradiation of The most realistic simulations of a planetary at- the Titan Haze with cosmic rays and produced 62 mosphere combine the above four essential condi- gas phase species including 27 nitriles at a net ra- tions with a quantitative low dose, low pressure, low diation yield (G) of heavy atoms incorporated into CH4 mixing ratio. This is a painstaking experiment products of 4.0(C+N) heV -1, where 1 heV equals and has been conducted by Thompson et al. (1991). 100 eV. The hydrocarbons were significantly satu- 302 B.N. Khare et al.

rated and the highest molecular mass products pro- ~l duced at G<10 -5 molecules heV -1 include 2, 4- ~ ° pentenedienenitrile, several pentenenitrile isomers, 2- methyl-3-butanenitrile and 1,2,4,5-tetraazine, up to ~ ! o o (C+N)=8. The most abundant products in this ] o ° . experiment do not closely resemble the minor con- ~] stituents detected by Voyager in the Titan atmo- sphere.

i ~ i i L i i Lower Pressure~ Low Dose Experiment With C~Hu CZHI C~ll HCN P.zH4 ~ CIN= C..a*N4 C,HI MOrale Saturnian Auroral Electron Analog Irradia- tion The pressure for this experiment was set at 0.24 mbar for 49 hours and 17 minutes with 4900 liters of Fig. 3. Comparison of the mole fraction range com- (N2+CH4) and was irradiated at a delivered power puted from the Thompson et al. 1991 laboratory plus of around 0.04 W. This corresponds to irradiation by eddy mixing model (large rectangles) plus mole frac- Saturnian auroral electrons precipitating in the Titan tions computed by Yung et al. (1984) (diamonds). The mesophere and produced fewer gas phase species were Voyager IRIS derived values for the equator (E) and po- identified, including only 9 hydrocarbons, 6 nitriles lar (P) mole fractions from Coustenis et al. (1989) for and 3 other incompletely identified nitrogen bear- Titan are marked and connected by a line to show their ing compounds, possible including the ring equatorial to polar range. tetraazole. The net yield is 0.79 (C+N) heV -1 and the hydrocarbons are largely unsaturated and include many double and triple bonds. The decline of abun- namely 1) charged particle processes cannot easily dance with heavy atom number at 0.24 mbar tends to be modelled by purely photochemical kinetics, al- be shallow for unsaturated species and steep for sat- though models containing charging processes could, urated species. If this trend continues, then the most of course, be constructed 2) because of N2 bond complex products would be unsaturated species. strength, N2 photochemistry requires )~ <_ 90 nm, which highlights the importance of energetic input Pointers for Future Titan Haze Simulations to the chemistry from charged particles. However, Using the altitude integrated flux of charged parti- Bakes et al. (2001) do find that polycyclic aromatic cle energy deposition at Titan, the laboratory yields chemistry can still occur at these lower at P=0.24 mbar, plus a simple eddy mixing model, UV wavelengths and have a significant effect on the Thompson et al. (1991) and Sagan et al. (1992) es- Chemistry and the conductivity (Borucki et al. 2001) timate the abundance of charged particle radiation of the atmosphere 3) the number of reaction path- products in Titan's upper atmosphere and these are ways increases steeply, nearly factorially, with heavy summarised in Figure 3. While the predicted prod- atom number and errors can propogate if a reaction ucts from these experiments are at molar abundances is overlooked or unmodelled. Bauschlicher and Ricca greater than 10 -s, no gas phase organics were found (2000) and Bauschlicher, Ricca and Bakes (2001) are by Voyager consistent with these values. We conclude carefully and painstakingly studying possible poly- tentative values for future observations of the abun- cyclic aromatic hydrocarbon pathways using quan- dances of CH3CN, CH2CHCHCH2, CH2CCH2 and tum chemical methods to predict the abundance and CH2CHCCH are >10 -9 and CH2CCN, CH3CHCH2 reactivity of larger aromatic molecules in the Titan and CH3CH2CN are >10 -1°. Haze 4) Sagan et al. (1992) found that current com- On the basis of these results, Sagan et al. (1992) putational limits are reached at 4-6 carbon atoms, suggested that properly conducted laboratory exper- while current studies by Bauschlicher, Ricca, Bakes, iments can usefully simulate the organic chemistry McKay and Lebonnois at NASA Ames are surpassing occurring in Titan's atmosphere. Photochemical ki- this limit and they are studying polycyclic aromatic netics is a useful and relevant technique for under- hydrocarbon molecules comprised of 10 or more car- standing possible simple hydrocarbons in a plane- bon atoms via quantum chemical studies of transition tary atmosphere. However, for nitrogen containing states. molecules and more complex molecules, there are In summary, a close collaboration connecting the four main drawbacks found by Sagan et al. (1992), laboratory experiments on Titan tholins performed Solid Organic Matter on Outer Solar SystemBodies 303 by the Cosmochemistry Laboratory at NASA Ames and the precise calculations of the NASA Ames =3 Quantum Chemistry Group, should yield a mutually Titan Thol/n fruitful effort to determine the complex and intrigu- 0.4 ing chemical pathways to macromolecules in the Ti- tan Haze.

Ca = 1.5 LABORATORY MEASURED OPTICAL 0.3 CONSTANTS FOR TITAN TttOLIN AND

THEIR AGREEMENT WITH OBSERVA- ~,...... ¢ i = ] TIONS 0.2 ! Khare et al. (1984) measured the optical constants ii" •~l f "b~ k (imaginary refractive index) and n (real refractive b' Titan index) of Titan tholin produced in the simulations de- scribed above. The optical constants were measured o.I Monod/sperse, from soft X-rays to microwaves through a combina- a = 0.5 p.m tion of transmittance, specular reflectance, interfero- metric, Brewster angle and ellipsometric polarization measurements. The experimental uncertainties in n o~ are 4- 0.05 and for k, 4- 30%. At ), < 550 nm, short 0.40 0.50 0.60 0.70 0.80 0.90 Wavelength {~.m) of the Kuiper CH4 bands, simple radiative transfer theory and a monodisperse size distribution func- tion showed reasonable agreement with both ground based and orbital reflectance spectra of Titan Fig. 4. Early (1983) two stream radiative transfer spec- (Sagan et al. 1985; Sagan et al. 1992; Thompson tral calculations of the reflectivity of monodisperse (r- 1984; Ramirez et al. 2000; Coll et al., this volume; 0.5 /zm) Titan aerosols, compared with observations. see Figures 4 and 5). CH4 absorption ceases at • _< 620 nm. From Sagan et More sophisticated analysis now predicts the opti- al. 1983; Sagan et al. 1985. cal constants expected for a polydisperse sediment- ing haze required to match the ground based and spacecraft observations of Titan (McKay et al. 1989; 0.8 I I I I r I I I O. 1 gm Titan Tholln Samuelson and Mayo 1991; Cabane et al. 1992, 1993; 0.7 L~,ht scattering Rannou et al. 1995, 1997). Results for n are in good Models accord with deductions made by ground based and ~ 0.6 spacecraft observations, where n -~ 1.65 in the near 0.5 UV, visible and near infrared (IR)). Results for k are compared for Titan tholin in Figure 6, and results are i 0.4 // also shown for several other tholins and terrestrial 0.3 (which is spectrally similar to the organic residue from the Murchison 0.2

meteorite). There is a ~ 30% offset which may be 0.I conceivably dose related, although 30% is also the 0.01 probable error in laboratory measurements of k, while 0.0 0.l 0.2 0.3 0.4 0.5 0.6 0.7 0,8 0.9 values extracted from Voyager observations of Titan Wavelength, l~m have an uncertainly of around a factor 2 (Courtin et al. 1995; McKay et al. 1989; Rannou et al). Given the three orders of magnitude variation in Fig. 5. Early (1984) doubling/adding radiative transfer k over the wavelength range of near UV to far IR calculations for monodisperse (r--- 0.i #m) Titan tholin (albeit with no Titan data between 3 and 20 #m), aerosols, compared with observations. The shaded area the agreement of Titan tholin with the Titan haze corresponds to IUE data and the curves are labelled with is striking and it also explains the reddish coloration haze column density in gcm -3 above a dark surface. very well. We note there are windows in the near IR 304 B.N. Khare et al.

pounds float in liquid ethane or methane (Sagan et TttAn Hue al. 1985; Raulin et al. 1987).

10"l THE CHEMICAL COMPLEXITY OF / , I, THOLINS The composition of tholins shows evident C-H, CN 10-2 triple bond and C-C functional groups in its refrac- k tive index k(A) in Figure 6. These are expected from the gas phase products which are presumably tholin 10-3 precursors (Thompson and Sagan 1989). Sagan et al. (1984) reported an elemental composition by weight

tO-4 for Titan tholin of 48% carbon, 6.4% hydrogen, I IIII IIII I I IIIIIII I I IIIIIII I I IIIIIII I I I III 29% nitrogen and 17% , yielding a C:H 0.01 0.1 l.O tO 100 1000 ratio of 1.93. In another study, Coll et al (1995) have reported on tholin produced in a Tesla coil dis- charge and found a tholin composition of CllHllN, while McKay (1996) found an elemental composition Fig. 6. Imaginary part of Titan's complex refractive in- of CllH11N2. Taking into account the broad range dex (stippled strip at center and top right) deduced from of C:N values, this still implies that the tholin is a Voyager observations (McKay et al. 1989; Samuelson large sink of nitrogen atoms and only a small sink for and Mayo 1991) with an assumed dispersion at UV- C atoms in the Titan haze (McKay 1996). Results visible-near IR wavelengths of 4-30% , compared to from the laboratory experiments performed during type II kerogen (Khare et al. 1990). Probable errors the 80's helped to guide the specifications for the in- in our observational knowledge of k for the Titan haze clusion of a GCMS aboard the Cassini Huygens mis- are closer to a factor of 2 (Courtin et al. 1995; McKay sion (for a report on how the mission was assembled, et al. 1989) see ESA SCI (85) 1 and ESA SCI(86)5, Table 6-3- 1: Probe Model Science Payload). In addition, fur- ther in situ analysis of the haze using the Cassini (corresponding to windows also in the gas phase spec- Huygens ACP experiment (SA-CNRS, PI Guy Is- trum) which may permit remote imaging of Titan's rael) will likely shed light not just on the composition surface. Observations have been performed (Smith of Titan tholin, but on likely chemical processes oc- et al. 1996; Combes et al. 1997) using these IR curring on early Earth. For terrestrial experiments, windows. Combes et al. (1997) and Coustenis et al. when the volatile component of Titan tholin was ex- (1999) have calculated the optical behavior of Titan's amined via sequential and nonsequential pyrolytic surface using the complex refractive index of water ice GCMS, over 100 products were detected, including and the complex refractive index of tholins as pub- saturated and unsaturated aliphatic hydrocarbons, lished by Khare et al. (1984). From far IR obser- nitriles, pyrroles, pyrazines, pyridines, vations, the mesospheric tholin abundance above the and the purine adenine (Khare et al. 1984). While main cloud and haze deck at around 2725 km from Ti- some of these products may be conceivably synthe- tan's center is estimated at several #g cm -2 (Thomp- sised during , this is likely not a mQor ef- son and Sagan 1984; McKay et al. 1989; Samuelson fect (Thompson et al. 1991; Khare et al. 1986). The and Mayo 1991). Comparable values emerge from subsequent chemistry of this material, if episodically the International Explorer (IUE) data on flooded with liquid water, plus an interaction with Rayleigh absorbers (Courtin et al. 1991). The sed- possible hydrocarbon oceans, may lead to a rich and imentation time to the surface of these submicron interesting organic chemistry. aerosols is 3x 10 s±l seconds, corresponding to sedi- mentation fluxes of 10 -14±1 gcm -2 s -1 and a sedi- THOLINS AS FACILITATORS OF PREBI- mentary column of tholins accumulated over the life- OLOGY time of Titan of 1-100 meters (McKay et al. 1989; Acid hydrolysis of laboratory synthesised Titan Samuelson and Mayo 1991; Courtin et al. 1991; tholins yields a rich array of racemic biological and Thompson et al. 1989). If there are hydrocarbon abiological amino acids and abundant (Khare et oceans on Titan, the tholins will exist chiefly as a al. 1986). The amino acid yields are ~, 1% by mass submarine deposit because virtually no organic com- of tholin. Their precursors have been suggested to Solid Organic Matter on Outer Solar SystemBodies • 305 form from chain addition reactions of the most abun- an intimate mixture of water ice, amorphous carbon, dant gas phase species (Thompson and Sagan 1989). and nitrogen-rich organic solids (modeled as a Tri- Two step laser mass spectroscopy reveals 10 -4 ton tholin). Owen et al. (2000) suggest that the grams gram -1 of two to four ring polycyclic aro- nitrogen-rich material originate d from Titan (by an matic hydrocarbons in Titan tholin. Indeed, larger unspecified mechanism), which has been producing amounts of PAHs containing more rings than this nitrogen-rich organic compounds for 4.5 Gyr. may also be present (Sagan et al. 1993). In addition to these pure hydrocarbon PAHs, nitrogenated rings ORGANIC SOLIDS ON THE CENTAUR may also form in the Titan haze (Ricca, Banschlicher 5145 PHOLUS and Bakes 2000). The hydrolysis of pure nitro- Centaurs are objects in heliocentric orbits that genated aromatic macromolecules in both water and cross over the orbits of the giant planets, and are an icy matrix can occur via UV irradiation to produce tempor&ry (dynamical lifetimes of order 10 T years) amino acids, as demonstrated by the work of Bern- because of the gravitational action of the giant plan- stein and Dworkin at NASA Ames. Hydrolysis via ets on them. They are thought to be derived from the impact driven, shock heated chemistry of the same Kuiper Disk of icy planetesimals that extends from starting materials could also produce amino acids. 's orbit (about 40 AU from the Sun) to an un- Pierazzo and Chyba (1999) have recently shown that known distance of order 200 AU. The Kuiper Disk the survival of cometary amino acids is plausible for is the source of the short-period comets (orbital pe- large cometary impacts. Blank (private communi- riods <200 y), which have low orbital inclinations cation) has found that when two icy bodies impact to the ecliptic. Thus, Centaurs appear to be in- oneanother, that if one or both bodies contain amino termediate between Kuiper Disk objects and active acids, peptides are formed. If an icy body impacted comets. Some Centaurs exhibit mild cometary ac- Titan, it would already contain aromatics of the sort tivity (a coma of evaporated gas and entrained dust which form via atmospheric gas phase chemistry in particles), and others have no detected activity, sug- the Titan haze. Furthermore, the icy body would gesting that they have not reached a temperature suf- collect aerosols on the way down and possibly aecrete ficient to evaporate significant quantities of their ices. snowed out aerosol material at Titan's surface. When Centaur 5145 Pholus is a particularly important impact occured, then the formation of both amino object because of its extraordinary red color, which acids and peptides by shock heated chemistry may was noticed by several teams of observers who first result, providing a potentially rich source of prebio- studied it. Minerals and ices do not have such logical molecules. Since Pierazzo and Chyba (1999) coloration, but several solid organic materials do. agree that amino acids may plausibly survive large Cruikshank et al. (1998) observed and modeled the cometary impacts, then the complex organic chem- spectrum of sunlight reflected from Pholus (0.3-2.5 istry which could occur via this kind of impact may #m), finding absorption bands in addition to the be highly relevant to present day Titan. distinctive red coloration. The spectrum was mod- eled and the absorption bands very closely matched THOLINS IN THE SATURNIAN SATEL- with a mixture of Titan tholin (which gives the LITE IAPETUS red color), H20 ice, olivine, amorphous carbon, Iapetus is one of the stranger moons of Saturn and methanol ice (or alternatively, hexamethylene (which, as of mid-October, 2000, has 22 satellites). It tetramine). Qualitatively, this model composition for is similar in density to several other Saturnian moons, Pholus is nearly identical to the mean composition of indicating that it has a very small allotment of rocky a nucleus: water ice, silicate dust (olivine and materials and is composed primarily of H20 ice. The pyroxene), organic solid dust grains, and light hydro- hemisphere centered on the apex of Iapetus' orbital carbons (methanol and others). In addition, Khare et motion around Saturn is covered with material of low al. (1994) show that while poly-HCN is a minor com- albedo (~0.07) and red color. In view of its spa- ponent of Titan's aerosols, it may be present on Pho- tial distribution, this material is most likely swept lus. The similarity to cometary composition is fully up from space by Iapetus' orbital motion, but an en- consistent with the view that Centaurs are objects dogenous source cannot be entirely eliminated. Owen derived from a source region for comets, but which et al. (2000) computed rigorous scattering models of have not become fully active. With a diameter of the spectrum (0.3-4.0 #m) of the low-albedo hemi- about 140 km (a typical comet is about 1 kin), Pho- sphere, and found that an excellent fit is given by lus in full cometary activity would be a formidable 306 B.N. Khare et al. object in the inner Solar System! Courtin, R., D. Gautier, and C.P. McKay. Titan's thermal emission spectrum: Re-analysis of the OTHER OBJECTS WITH ORGANIC MAT- Voyager infrared measurements. Icarus 114, 144- TER 162, 1995. 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