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Chemical Optical.Pdf 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 Titan tholin compare. We ATMOSPHERE AND ON comment on the probable existence of polycyclic aro- THE SURFACE OF OUTER matic hydrocarbons in the Titan Haze and how bi- ological and nonbiological racemic amino acids pro- SOLAR SYSTEM BODIES duced from the acid hydrolysis of Titan tholins make these complex organic compounds prime candidates in the evolution of terrestrial life 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 5145 Pholus and the dark hemisphere of Field CA 94035-1000, USA Saturn'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 interstellar medium 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, comets, 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 methanol 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- molecules 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 nitrogen N2 and This supplied the raw materials with which to form less than 10% methane 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, Kuiper Belt Objects and the materials were first recorded during the Voyager 1 gaseous planets and their satellites. Titan, Saturn's and 2 flybys of Titan and showed up as an optically largest satellite, formed from an ammonia-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-water 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 nitriles. 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 amino acid 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 hydrogen 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 (ethane 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 cosmic ray 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 Cosmochemistry 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 ~.
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