Organic Molecules in the Icy Bodies of Planetary Systems – Accepted Notions and New Ideas

Open Astron. 2018; 27: 341–355

Research Article

Irakli Simonia* and Dale P. Cruikshank Organic Molecules in the Icy Bodies of Planetary Systems – Accepted Notions and New Ideas

https://doi.org/10.1515/astro-2018-0038 Received Feb 26, 2018; accepted Jun 01, 2018 Abstract: Cometary bodies are acknowledged to contain some of the most pristine matter in the Solar System, including ices and minerals. Certain number of previously unidentified spectral emission features detected in comets canbe explained as emission by hydrocarbon molecules enclosed in a Shpolskii matrix and forming frozen hydrocarbon particles. UV-induced photoluminescence spectra of several self-organized molecules exhibit emission lines coincident with unidentified cometary lines, and open the possibility of the presence of this complex organic as components ofthepris- tine organic inventory of comets. Complex organic was detected also in three satellites of Saturn. We describe in this paper results of our investigation of complex organic of the small bodies and present new approaches and hypotheses.

Keywords: comets, planets, ice, organic matter, prebiotic evolution 1 Introduction be rich in complex organic materials (Ehrenfreund and Charnley 2000; Clairemidi et al. 2004; Crovisier and Bockelee-Morvan 2007; Li 2009; Kobayashi and Kawakita The icy halos of comets, which include minerals, con- 2009). In this paper, in accord with other investigators, we sist of shells of micro and nano grains are responsible consider cometary nuclei as the reservoirs of relict organic for the scattering of solar electromagnetic radiation. A matter, that is, primitive material that has survived largely complex mixture of gas and dust around cometary nu- unaltered from an earlier time. The surfaces of icy satel- clei gives rise to the gaseous emissions and the scattering lites and Trans-Neptunian Objects (TNO’s) scatter solar ra- of sunlight. Taken all together, this constitutes the atmo- diation, resulting in the appearance of solar-similar spec- sphere of a comet. The spectra of comets also contain se- tra but with different peculiaritiese.g. ( , specific slopes, ab- ries of unknown emissions. These unidentified emissions sorption bands) reflective of the chemical-mineralogical have been found in the spectra of many comets (Brown composition of a particular surface. The presence of comet al. 1996; Cochran and Cochran 2002; Cremonese et al. plex organics in cometary or planetary ices is character- 2007; Kobayashi and Kawakita 2009; Dello Russo et al. ized by specific colors of surfaces in integrated light or 2013), for example, 109P/Swift-Tutle. These emissions are by the appearance of the specific emissions and absorp- assigned to multiple ionized molecules (Wyckoff et al. tions in the spectral profiles of these bodies. In this paper, 1999; Cochran and Cochran 2002; Kawakita and Watan- we will discuss similarities and differences of complex or- abe 2002). A new theory by Simonia (2004, 2007, 2011a,b, ganic properties for different classes of icy bodies in the 2013) suggests that the luminescence nature of uniden- Solar System. The complexity of the organic components, tified emissions result of photoluminescence by frozen the color and other properties will be the focus of our in- hydrocarbon particles (FHPs) of cometary atmospheres. terest, as well as possible methods for revealing potential Roughly 14% of the previously unidentified emissions prebiotics in the icy bodies. have been identified as photoluminescence of cometary FHP. However 86% of the mentioned emissions remained as unidentified (Simonia 2011b). Comets are known to

Corresponding Author: Irakli Simonia: School of Natural Sciences and Engineering of Ilia State University, Cholokashvili str., 3/5,Tbilisi 0162, Georgia; Email: [email protected]; 995.32 2373468 Dale P. Cruikshank: Astrophysics Branch, NASA Ames Research Center, Moffett Field, CA 94035, United States of America

Open Access. © 2018 I. Simonia and D. P. Cruikshank, published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License 342 Ë I. Simonia and D. P. Cruikshank, Organic in planetary systems

2 Frozen hydrocarbon matter as a rich with various aromatic and aliphatic molecules. The hydrocarbons sequestered in the inner layers or centers of compressed snow – the cometary nuclei may be relict material inherited directly methodological approach from the solar nebula without ever having been processed by the solar electromagnetic radiation or the solar wind. Cometary relicts are the witness of the Solar System An important key to the interpretation of comet spectra formation, resulting from condensation processes, while is derived from the work of Shpolskii (1959, 1960, 1962), the building blocks of complex cometary organic in the who described a matrix with the form of frozen polycrys- form of molecular rings or chains have a presolar origin. talline mixtures of polycyclic aromatic hydrocarbons in n- This means that cometary relicts convey the physical prop- alkanes. Quasilinear photoluminescence spectra of these erties and chemical composition of pristine matter of the matrix have been investigated in detailed. Physical proper- solar nebula. Cometary nuclei might be rich in frozen hy- ties and the exact chemical composition of polycrystalline drocarbons in the form of compressed snow. Physical con- mixtures were measured and analyzed. The results of Sh- ditions of the formation process of each specific cometary polskii’s work could have astrochemical and astrobiologi- nucleus, the local chemical composition of the solar neb- cal significance. We enumerate here experimental results ula and other parameters have conditioned the density, of the mentioned works in the context of the investiga- colors, and other characteristics of frozen hydrocarbon tion of comets and other icy bodies of the planetary sys- matter in the cometary nucleus. The cometary organic ma- tems. 1) Solid solutions of polycyclic aromatic hydrocar- terial in the form of frozen hydrocarbons as a compressed bons (PAH’s) and n-alkanes (from pentane to decane) at 4 snow is probably a dense but rather fragile substance. It < T < 77 K demonstrate photoluminescence spectra in the may be that, layers of frozen hydrocarbons in the form form of a series of very narrow emission lines. In such mix- of molecularly dispersed substance or substitutional solid tures, aromatic hydrocarbons are guest substances and n- solutions carry information on physical processes of the alkanes host substances – solvents, or the so-called ma- evolution of the solar nebula. trix. In the referenced works, the matrix (host substance) When comets approach the Sun, processes of ther- was in the form of straight chains of n-alkanes. An aro- mal destruction and sublimation of their ices begin. Micro matic compound (ring) substituted in the n-alkane ma- fragments of nucleus ices form the spherical halo, which trix is frozen as a rarefied “oriented gas”. Freezing of the becomes larger at shorter heliocentric distances. Shells solvents as crystallized masses, that is as microcrystals, of frozen hydrocarbon particles (FHPs) are the sources of results in the formation of discrete luminescence spectra cometary hydrocarbons that are released from the nucleus when excited by ultraviolet radiation in the range 2500 - to form the coma (Simonia 2004). Each FHP is charac- 3900 A; 2) frozen mixtures of polycyclic aromatic hydro- terized by an inherent size, shape, and color. FHPs with carbons and n-alkanes (pentane, hexane, heptane, etc.) in substitutional solid solution structures have nano dimen- the form of microcrystals similar to fine-grain snow, have sions, but molecularly dispersed FHPs have micro dimen- specific luminescence and absorption spectra, appearing sions. The shape of each FHP depend on structural pecu- as a series of narrow lines. In the case of larger sizes of liarity of icy grain. FHP with substitutional structures may n-alkane molecules narrow luminescence emission lines be flat, but molecular dispersed grains might be bulky and transform into diffuse bands; 3) luminescence spectra of irregular in shape. complex crystalline organic substances in certain cases Solar UV radiation of 2400 – 3950 Å excites photolu- have an anomalous character, resulting in the formation minescence of frozen hydrocarbon particles in the coma of wide structureless bands. Dimers (which consist of two in the wavelength range 3990 – 9000 Å (Simonia 2004, identical monomers) are characterized by such spectra. 2007). The chemical compositions of cometary FHPs and Simonia and Simonia (2013) proposed that comets the physical peculiarities of Shpolskii microcrystals have nuclei are rich with frozen organics as snow-like micro- specific properties of their photoluminescence spectra in- crystals of polycyclic aromatic hydrocarbons in n-alkanes. cluding the positions and profiles of emission lines. Pho- Clusters of numerous snow-like microcrystals congregate toluminescence spectra of the cometary FHPs at low tem- in specific zones irregularly distributed in different layers perature (T< 80K) have the standard quasiline shapes – of the cometary nuclei. These zones may constitute the a series of multiple narrow emission lines with average principal sources of hydrocarbons. In this view, flat snow- line width ∆λ ≤ 1Å. Laboratory analogs of such spectra like microcrystals consist of aromatic molecules fixed in are called quasilinear photoluminescence spectra of the an aliphatic matrix. Hydrocarbons’ sources of comets are Shpolskii matrix (Figures 1, 2). Photoluminescence spec- I. Simonia and D. P. Cruikshank, Organic in planetary systems Ë 343 tra of the cometary frozen hydrocarbons could also show wide, featureless bands (the case of resonance dimmers). Such featureless bands may be observed as a quasicon- tinuum. A similar phenomenon was reported by Churyu- mov and Kleshchenok (1999) and Churyumov et al. (2013). The width of emissions and general character of luminescence spectra of a Shpolskii matrix depends also on the temperature of the polycrystalline mixtures. Microcrys- tals of aliphatic (linear chains) matrix provides a quasilinear nature of the luminescence spectrum of PAHs frozen Figure 1. The spectra of quasilinear photoluminescence of perylene in this matrix (Shpolskii 1959). For a glass-like (the cer- in n-hexane (Nakhimovsky et al. 1989). tain derivatives – for example acphols) matrix, the same PAHs demonstrate featureless luminescence emissions. For comets, it probably means that quasi-continuum luminescence emissions belong to amorphous mixtures of hydrocarbons. Narrow luminescence emissions indicate PAHs in an n-alkane matrix, that is, crystalline FHPs. The detection of luminescence emissions can be a qualitative tool for determination of the state of hydrocarbons in cometary substance. The materials constituting the deep inner parts of comets nuclei, including hydrocarbons, that have never been processed by solar radiation have primor- dial properties. These unaltered materials can be called Figure 2. The spectrum of quasilinear photoluminescence of naph- cometary relicts. We note, however, that energetic cosmic thacene in n-nonane (Nakhimovsky et al. 1989). rays penetrate comet nuclei to a depth of a few meters, causing changes in the structure and chemistry of the ices and minerals. Properties of cometary relict substance are: cleus. Depending on the chemical composition and the 1) low temperature; 2) complex matter with pristine struc- physical properties of the concrete relicts, such lumines- tural features; 3) absence of ionized and implanted com- cence can be in the form of featureless bands or struc- pounds; 4) possible presence of prebiotics; 5) isotopic ra- tured quasilinear spectra. The duration of the lumines- tios for principal elements with characteristic for the Solar cence of relicts may be short, because the intense lumi- System (Simonia and Simonia 2013, 2014). The in situ in- nescence emissions may be quenched by solar infrared ra- vestigation of comet 67P/CG by Rosetta demonstrated that diation. Complex molecules of such relict matter may be some of the outer layers of the nucleus were removed when dissociated into daughter molecules by exposure to the so- the comet was close to the Sun. Both solid particles and gas lar UV. Taking into account the high quantum yield of hy- ejected from deeper layers contain prebiotic molecules (Al- drocarbon luminescence it seems clear that ground-based twegg et al. 2016) and numerous other volatile molecules observations for the detection of relict organic lumines- and refractory organics (Le Roy et al. 2015; Quirico et al. cence must be made at the times of outbursts or immedi- 2016). ately after afterward. Radiative unprocessed cometary re- Cometary relicts, including frozen hydrocarbons, ly- licts may be characterized with high luminescence quan- ing deep inside the nucleus have never been illuminated tum yield and the specific spectral features of such alu- by solar radiation, and the nucleus is too small to have minescence. Laboratory investigations of luminescence of been heated significantly either by gravitational compres- frozen organics demonstrated, that quantum yield of lu- sion or by the inclusion of large quantities of radiogenic minescence of such substance varies from 90% to 100% elements. Comets with long orbital periods are the princi- (Gudipati et al. 2003). In case of low albedo of cometary pal reservoirs of the relict substance of the Solar System. frozen organic relicts ground-based detection of such a In time of active cometary processes, including outbursts, phenomena may be possible. Organics of comets in the fragments of relict hydrocarbon-rich material may be de- form of shells of icy grains may be quickly processed by so- livered into the coma. Incident solar radiations could ex- lar radiation. It means that, within 6-8 hours after outburst cite photoluminescence and cathodoluminescence of the the luminescence properties of the relict material will be relict organics newly delivered from the depths of the nu- lost or changed. Outburst spectra of comets have shown 344 Ë I. Simonia and D. P. Cruikshank, Organic in planetary systems

of such self-organized substance of cometary relicts. In- teraction: solar electromagnetic, corpuscular radiation – cometary relicts, may have high significance within process of formation of potential prebiotics in cometary substance. Bombardment of cometary relicts by the solar electrons and protons may stimulate certain processes in self- organized substance of such relicts. The important conditions for self-assembled processes in cometary relicts might be: 1) penetration of solar radiation in the inner layer of comet nuclei substance. 2) Possibility of absorbed energy accumulation by the cometary relict ices (Simonia 2016). 3) Charges transmission and sharing electrons pairs Figure 3. Spectra of photoluminescence of Pyrene and 3-4 Ben- appearance in molecular systems of cometary relict ices zpyrene (Shpolskii 1962). in conditions of low temperatures (T< 80 K). 4) Presence of aromatic compounds in cometary relicts. For different featureless emissions in range 3990-5800 Å (Cochran et al. comets (for example long periodic) on the different he- 1980). Polycrystalline mixtures may demonstrate the lumi- liocentric distances interaction: solar radiation-cometary nescence bump at the similar spectral range (figure 3). The substance might be different in characteristics. Therefore, presence of complex organics in cometary ice may cause self-assembled processes in molecules of cometary relicts the specific colors of cometary nuclei (e.g. Hartman et al. may have different peculiarities. 1987). The ices of inner layers of icy satellites and certain Possible cometary prebiotics and self-organized TNO’s may have a similar chemical composition and struc- substance search remains significance task. Following tural peculiarities to that of compressed snow with PAHs on the recent results of the Rosetta investigation of in n-alkanes. The presence of frozen hydrocarbon matter 67P/Churyumov-Gerasimenko (Altwegg et al. 2016) it is in icy satellites might be confirmed in measurements of the expected that the nuclei of most, and perhaps all comets colors of the surfaces of planetary satellites, as well as Cen- contain prebiotic compounds, possibly including certain taurs and Kuiper Belt objects. Indeed, the colors of Pluto types of self-organized matter such as heterocycles, pro- and some planetary satellites are most likely caused by the teins, etc. Photoluminescence of self-organized matter processing of hydrocarbons (notably CH4) by solar ultravi- might be excited by solar ultraviolet. FHPs of comets with olet and charged particles. This processing erases the in- prebiotic compounds may luminesce in the optical range trinsic signatures of the hydrocarbons, leaving a refractory in the form of quasilinear spectra (Simonia and Simonia macromolecular material termed “tholin”, as described in 2013, 2014; Gudipati et al. 2015). We have compared spec- a later section of the present paper. tral positions of the cometary unidentified emissions with luminescence emissions of enzymes, pigments and proteins (Simonia and Simonia 2014). Results are given in 3 Frozen organics of comets - Table 1. On the basis of our comparative analysis, we suggest a results of comparative analysis new hypothesis that chlorophyll-like, porphyrin-like, and photoprotein-like molecules might be present in relict ice Frozen hydrocarbon particles as constituent of icy halos of throughout the Galaxy, including cometary relict ices both comets may photoluminesce in the field of solar ultravio- of Solar System comets and exo-comets as well. Our hy- let radiation. Numerous narrow unknown emission lines pothesis suggests that cometary ices, especially frozen or- of the spectra of comets may have such photolumines- ganic relicts, could contain the structural analogs of prebi- cence nature – 4623.31 Å, 4769.04 Å, 4874.10 Å, 5028.42 otics, self-organized matter. The example of such an anal- Å, 5197.20 Å, 5318. 22 Å, 5615.20 Å, 6134.99 Å, 6638.21 Å ogy might be the possible presence of the phthalocyanine (Simonia 2007). Aromatic and aliphatic hydrocarbons are molecule in the cometary ice. Phthalocyanine is the struc- principal components of FHP substance. At the same time, tural analog of the porphyrin molecule (Kadish et al. 2010). complex molecules in the form of rings and their struc- Oxygen atoms provide excitation of luminescence of pre- tures, may compile the principal elements or bases for biotics, including proteins and pigments. Photolumines- self organized substance of FHP. The field of solar radia- cence of the structural analogs (phthalocyanine) is excited tion may provide energy for transformations or evolution by UV photons and solar UV could excite photolumines- I. Simonia and D. P. Cruikshank, Organic in planetary systems Ë 345

Figure 4. Fluorescence spectrum of chlorophyll “b” in n-undecane at 4.2 K. (Personov et al. 1974).

Figure 6. Fluorescence (upper trace) and phosphorescence (lower trace) of Cd-tetrabenzoporphyrines in n-octane at 4.2K. (Platenkamp and Canters 1981).

Figure 5. Fluorescence spectrum of Zn-phthalocyanine in parafin oil at 4.2 K. (Personov et al. 1974). cence of frozen phthalocyanine in the cometary material.

In many cases luminescence emissions of various prebiotics have a featureless character, appearing as rather wide bands (at room temperature). However, the results of several experiments demonstrate that different heterocycles, pigments, and prebiotic molecules dissolved in an organic matrix (T< 80 K) have standard quasilinear luminescence spectra (Platenkamp and Canters 1981; Avarmaa Figure 7. Fluorescence spectra of chlorophyll-a in toluene at 4.2 K. and Rebane 1988) Figure 4, 5, 6, 7. In other words, a Sh- (Avarmaa and Rebane 1988). polskii matrix in the form of frozen polycrystalline mixture (heterocycles, prebiotics in n-alkanes) luminesces as a se- nescence of FHPs with possible prebiotic compounds (Si- ries of very narrow emission lines. In this case, lumines- monia and Simonia 2014). cence of such mixtures is excited by ultraviolet radiation. This suggests that series of very narrow unidentified emission lines found in cometary spectra might represent lumi- 346 Ë I. Simonia and D. P. Cruikshank, Organic in planetary systems Continued on next page PorphirinPorphirinPorphirin Osad’ko 1979 Osad’ko 1979 Osad’ko 1979 2 2 2 Bioluminescence Substance Reference λ unid λ 5710.11 5710 luciferase Ugarova and Brovko 2002 5719.24 5720 luciferase Viviani et al. 2007 5819.92 5820 luciferase Ugarova and Brovko 2002 5879.77 5880 luciferase Viviani et al. 2007 6146.14 6146 H 5759.36 5760 luciferase Ugarova and Brovko 2002 5920.13 5920 luciferase Viviani et al. 2007 5459.97 5460 luciferase Ugarova and Brovko 2002 6122.39 6123 H 5950.72 5950 luciferase Ugarova and Brovko 2002 5730.57 5730 luciferase Viviani et al. 2007 5520.97 5520 luciferase Viviani et al. 2007 6130.63 6130 luciferase Ugarova and Brovko 2002 6109.27 6110 luciferase Ugarova and Brovko 2002 6079.27 6080 luciferase Viviani et al. 2007 6135.00 6135 H 5820.33 5820 luciferase Ugarova and Brovko 2002 5940.52 5940 luciferase Ugarova and Brovko 2002 6070.22 6070 luciferase Viviani et al. 2007 5509.39 5510 luciferase Viviani et al. 2007 5780.54 5780 luciferase Ugarova and Brovko 2002 5790.49 5790 luciferase Ugarova and Brovko 2002 6099.35 6100 luciferase Ugarova and Brovko 2002 5620.68 5620 luciferase Ugarova and Brovko 2002 5930.60 5930 luciferase Ugarova and Brovko 2002 5860.56 5860 luciferase Ugarova and Brovko 2002 5800.10 5800 luciferase Ugarova and Brovko 2002 6060.32 6060 luciferase Ugarova and Brovko 2002 6040.77 6040 luciferase Viviani et al. 2007 5840.66 5840 luciferase Ugarova and Brovko 2002 5649.117 5650 luciferase Viviani et al. 2007 Comets De Vico 5380.88 5380 luciferase Ugarova and Brovko 2002 Unidentified cometary emissions bioluminescence and maxima of the terrestrial prebiotics, heterocycles, and pigments. Possible inventory of organics for exo-comets. Table 1. The firstcolumn of tablethe fourth – comets – specificnames; materials’ second type; column fifth – unidentifiedcolumn emissions – references of comets for luminescence (angstrom); data. thirdcolumn – maxima of luminescenceemissions of various substance; the I. Simonia and D. P. Cruikshank, Organic in planetary systems Ë 347 Continued on next page Shimomura 2006 Shimomura 2006 2+ 2+ (photoprotein) Shimomura 2006 (photoprotein) Shimomura 2006 (photoprotein) Shimomura 2006 (photoprotein) Shimomura 2006 2+ 2+ 2+ 2+ Porphirin Osad’ko 1979 2 Bioluminescence Substance Reference λ unid λ 6219.91 6220 luciferase Ugarova and Brovko 2002 6159.73 6160 luciferase Viviani et al. 2007 7350.53 7350 Chlorophyll Jovanić and Dramićanin 2003 6180.91 6180 luciferase Ugarova and Brovko 2002 6859.33 6860 Chlorophyll a Erokhina et al. 2002 6818.02 6818 Chlorophyll Jovanić and Dramićanin 2003 6210.04 6210 luciferase Viviani et al. 2007 6148.20 6149 H 5359.122 5360 luciferin Shimomura 2006 5339.741 5340 luciferase Shimomura 2006 5350.771 5350 luciferin Ugarova and Brovko 2002 5339.477 5340 luciferase Shimomura 2006 4979.702 4980 Green Fluorescent Protein Shimomura 2006 5350.875 5350 luciferin Shimomura 2006 5350.441 5350 luciferin Ugarova and Brovko 2002 5359.250 5360 luciferin Shimomura 2006 4649.917 4650 Aequorin+Ca 5280.241 5280 Isothiocyanate Shimomura 2006 4899.811 4900 luciferin Shimomura 2006 4750.938 4750 Obelin+Ca 5360.295 5360 luciferin Shimomura 2006 5340.130 5340 luciferin Ugarova and Brovko 2002 5339.280 5340 luciferase Shimomura 2006 4949.918 4950 luciferase-oxyluciferin Shimomura 2006 4969.310 4970 Green Fluorescent Protein Shimomura 2006 4850.474 4850 Photoprotein+Ca 4649.478 4650 Aequorin+Ca 5230.606 5230 Obelin+Ca 4630.388 4630 luciferin Shimomura 2006 5030.540 5030 luciferine Shimomura 2006 4900.788 4900 luciferin Shimomura 2006 4850.680 4850 Photoprotein+Ca 4800.920 4800 Luciferin Shimomura 2006 Comets Ikeya-Zhang 4630.153 4630 luciferin Shimomura 2006 ...continued Table 1. 348 Ë I. Simonia and D. P. Cruikshank, Organic in planetary systems Concluded Bioluminescence Substance Reference λ unid 5830. 5830 Green Fluorescent Protein Shimomura 2006 λ 6818.85 6818 Chlorophyll Jovanić and Dramićanin 2003 5649.117 5650 luciferase Ugarova and Brovko 2002 5379.770 5380 Green Fluorescent Protein Shimomura 2006 5819.947 5820 luciferase Ugarova and Brovko 2002 5369.835 5370 luciferin Ugarova and Brovko 2002 5370.734 5370 luciferin Ugarova and Brovko 2002 5839.545 5840 luciferase Ugarova and Brovko 2002 5620.727 5620 Luciferase-oxylciferin Shimomura 2006 5379.364 5380 Green Fluorescent Protein Shimomura 2006 5940.515 5940 luciferase Shimomura 2006 5401.419 5400 luciferin Shimomura 2006 5520.942 5520 luciferase Ugarova and Brovko 2002 6150.958 6150 luciferase Shimomura 2006 5879.810 5880 luciferase Ugarova and Brovko 2002 7336.250 7336 Chlorophyll Jovanić and Dramićanin 2003 5860.572 5860 luciferase Ugarova and Brovko 2002 6818.013 6818 Chlorophyll Jovanić and Dramićanin 2003 5379.480 5380 Green Fluorescent Protein Shimomura 2006 6130.630 6130 luciferin Shimomura 2006 5950.720 5950 luciferase Shimomura 2006 5459.960 5460 luciferase Ugarova and Brovko 2002 6040.752 6040 luciferase Shimomura 2006 5380.892 5380 Green Fluorescent Protein Shimomura 2006 6070.963 6070 luciferase Shimomura 2006 6239.300 6240 luciferase Shimomura 2006 5790.830 5790 luciferase Shimomura 2006 5450.064 5450 luciferase Shimomura 2006 6060.306 6060 luciferase Shimomura 2006 5800.090 5800 luciferase Ugarova and Brovko 2002 Comets Hale-Bopp 5800.90 5800 luciferase Ugarova and Brovko 2002 ...continued Table 1. I. Simonia and D. P. Cruikshank, Organic in planetary systems Ë 349

We have investigated the spectra of several comets sitions that date their formation to the presolar space en- in search of possible prebiotic emissions of compounds vironment (Wopenka et al. 2013). The ancient organics in of similar structure. We studied optical cometary spectra comets can also be expected to bear isotopic signatures of in the range of 3900 – 8000 ÅÅ. We could not find any ancient origin. prebiotic emissions in the spectra of comets Ikea-Zhang, We consider here the organic solid material that oc- and de Vico (Cochran and Cochran 2002; Cremonese et curs on the surfaces of some planetary satellites, and spec- al. 2007). At the same time, we found possible prebiotic ulate on its origin from the same feedstock as that from emissions in the spectrum of comet C/2004 Q2 Machholz which the comets condensed in the solar nebula. Small (Figure 8). In particular, two earlier unidentified emissions particles of complex organic matter produced by photo- in the spectrum of this comet are correlated in spectral chemical processing of gases occur in the upper atmo- positions and profiles with fluorescence emissions ofCd- spheres of several planetary bodies, such as Titan, Triton, tetrabenzoporphyrine (Figure 6). It will be useful to obtain Saturn, and Pluto, primarily as a consequence of ultra- integral images of cometary coma in these bands. violet processing of methane gas. In certain cases, nitro- We do not consider the results we have obtained so gen is also a key component, and oxygen also contributes far to be the final identification of unknown cometary to the resulting chemical composition. The occurrence of emissions. Instead, our results show the possibilities of refractory macromolecular organic material on the sur- the presence of prebiotic materials and their structural faces of many planetary satellites in the outer Solar Sys- analogs in the cometary ices. At the same time the organic tem (distance from the Sun >5 astronomical units) is in- material noted here, including prebiotics, may exist in icy ferred from the colors of these bodies (i.e. Phoebe, some planetary satellites and TNO’s in the outer Solar System. Centaurs, etc). The spectroscopic signatures of this mate- The next part of this paper explores some aspects of the rial are primarily found in a region of the infrared spectrum presence of primitive organic matter on three of Saturn’s where groundbased astronomical observations of small icy satellites. outer Solar System bodies are not possible. Macromolecu- lar organic solids produced in the laboratory under conditions that reasonably simulate the space environment act- 4 Refractory organics on planets ing on native materials on planetary surfaces and in their atmospheres, are distinctively colored, usually in a range and planetary satellites – data from yellow to red-orange and brown. The measurement of analysis closely similar colors on planetary bodies leads in several cases to the logical inference that those surfaces contain a macromolecular organic component (e.g. Cruikshank et al. The detection of organic complexes, both the soluble and 2005; Kargel 2007; Waite et al. 2009; Scipioni et al. 2014), the insoluble fractions, in the carbonaceous meteorites even if diagnostic spectral bands cannot be detected. (e.g. Alexander et al. 2008), and CHON (carbon, hydrogen, Macromolecular organic solids produced in the lab- oxygen, nitrogen) particles detected in comets (e.g. Jen- oratory by the energetic processing (by ultraviolet, elec- niskens et al. 1991; Lawler and Brownlee 1992; Fomenkova trons, protons, cold-cathode discharges, etc.) of gases and 1999; Briani et al. 2012) reinforces the view that organic ices that include CH and other simple molecules are solids occur throughout the Solar System, and that they 4 called tholins (Khare et al. 1984). Most of the production were incorporated into solid bodies at the time of their for- and analysis of tholin has been undertaken in the gas mation. Flynn et al. (2003) have presented evidence from phase in the context of the photochemical haze in the the analysis of interplanetary dust particles that the bulk atmosphere of Titan, known to have an atmosphere pri- of the prebiotic organic matter in the Solar System did marily of N and CH , but with other molecules in small not form by aqueous processing, but had already achieved 2 4 concentrations (e.g. Coll et al. 1999; Imanaka et al. 2004). its structure at the time of its origin in the interstellar There are many recent papers on Titan tholins, but a com- medium, in the solar nebula, or both. The organic mat- plete review is beyond the scope of the present paper. ter in the carbonaceous meteorites does, however, show Colored refractory residues called ice tholins are pro- evidence of chemical alteration by the presence of liquid duced by the UV photolysis or charged-particle radiolysis water (e.g. Pizzarello and Shock 2010), a condition that is of pure or mixed ices (e.g. McDonald et al. 1996). The analy- unlikely to occur in comet nuclei and on the surfaces of sis of these residues, for example from processing of a mix- most planetary satellites. At the same time, most of the car- ture of N , CH , and CO ices, reveals the presence of carbon compounds found in meteorites carry isotopic compo- 2 4 boxylic acids, urea, HCN and other nitriles, alcohols, ke- 350 Ë I. Simonia and D. P. Cruikshank, Organic in planetary systems

Figure 8. Fragment of the spectrum of comet Machholz (C/2004Q2). We found that 6289.88 Å and 6290.64 Å unidentified emissions (red arrows) are correlates in positions and profiles with fluorescence emissions of Cd-tetrabenzoporphyrine (Figure 6, red arrows) . The spectrum of comet Machholz was obtained by Sung-Won et al. (2009). tones, aldehydes, and amines, with other unidentified ma- direction of its orbital motion around Saturn has a deep terials of high molecular weight (several hundred Da) (e.g. red color and very low reflectivity. This unusual property Materese et al. 2014, 2015). These materials are expected is attributed to the sweeping up of dust particles as the to be present and to provide coloration to the surfaces of body moves through space, and its color properties suggest Neptune’s satellite Triton, and Pluto, where N2, CH4, and that these particles are organic-rich. Prior to the Cassini CO ices are known to be present (Cruikshank et al. 1993; mission, the spectral characteristics of the dark red ma- Owen et al. 1993). terial did not reveal diagnostic bands that would identify Colors in macromolecular solids originate from elec- its composition, nor could the source of the dust particles tronic transitions when σ or π bonding and n non- accumulating on Iapetus’ leading hemisphere be reliably bonding orbitals are promoted to anti-bonding orbitals σ* established (Tosi et al. 2010; Cruikshank et al. 2014). or π* by the absorption of UV photons. Chromophores The source of the dust encountered by Iapetus in its thus produced are efficient in absorbing visible light, es- orbital path is Saturn’s more distant satellite, Phoebe, pecially when conjugated molecules (unsaturated com- which, because of its irregular orbit, is widely presumed pounds with alternating single and double bonds) are to have originated elsewhere in the Solar System (beyond present. In general, the longer the conjugated chain, the 30 AU) and was subsequently captured into its current or- farther into the visible spectral region the absorption ex- bit around the planet (e.g. Johnson and Lunine 2005). Dust tends, thus producing a range of colors from yellow to and other debris ejected from Phoebe by the impact of an- red, and a significant decrease in the overall reflectance other (unknown) asteroidal or cometary body is dispersed (albedo, in astronomical terms). The absorption in solids in an enormous ring centered on Saturn and filling the or- arising in this manner does not occur at narrow or discrete bit of Phoebe (Verbiscer et al. 2009). Small particles (~<10 wavelength intervals, and is therefore not directly diagnos- µm,), spiral inward toward Saturn and encounter Iapetus tic of specific molecules. on route, where they accumulate to a depth of a few tens Clearly, colored organic material currently or recently of centimeters (measured by the Cassini radar experiment) formed in planetary atmospheres or by radiation-induced primarily on the leading hemisphere (Tamayo et al. 2011). chemical reactions on planetary surfaces is not relict ma- The dark red debris from Phoebe is apparently accumulat- terial, but a special case found in the Saturn system may ing in the present epoch because the ring, is clearly evident be relevant to the discussion of ancient organics in comets. from its infrared thermal signature, and the dust layer on A new perspective on the organic content of mate- Iapetus is very young, judged from the paucity of craters rial either created in situ or deposited on planetary sur- left by penetrating meteoroid impacts. faces comes from a study of the satellites of Saturn ac- The imaging spectrometer on the Cassini spacecraft complished with the Cassini spacecraft. Iapetus is the out- obtained spectral images of Iapetus in the range 0.3-5.1 µm ermost large satellite of Saturn (diameter 1492 km), and of surface units as small as a few kilometers in size. The it has long been known that the hemisphere facing the spectrum of the dark red material shows a broad absorp- I. Simonia and D. P. Cruikshank, Organic in planetary systems Ë 351

tion band centered near 3.0 µm attributed to H2O ice and but it is very weakly seen or absent in spectra in the 3.3- OH in the surface materials. Additionally, there are two µm region. Consequently, the appearance of the aromatic absorption band complexes centered at 3.28 and 3.4 µm spectral band on Iapetus indicates a high relative abun- attributed, respectively, to aromatic and aliphatic hydro- dance of this material, of order eight to ten times that of carbons in the solid material covering the surface (Cruik- the aliphatic components. On the other hand, the rela- shank et al. 2008, 2014; Dalle Ore et al. 2012). The spec- tive abundance of -CH2- and -CH3 groups, an index of the tral evidence is insufficient to permit the identification of length of aliphatic chains, is comparable to that of aliphat- specific molecular species, but the relative abundances of ics in the interstellar medium. the aromatic and aliphatic components can be estimated The significance of the organic solids on Iapetus is from the measured band widths and optical thickness, and that their source appears to be the interior of the dis- the relative abundances of –CH3 and -CH2- aliphatic moi- tant Saturn satellite Phoebe, a captured Kuiper Belt ob- eties can be determined. Both of these parameters can ject. Phoebe, in turn, inherited the organic inventory from be compared quantitatively to laboratory compositional the solar nebula in which it accreted, probably at a he- studies of interplanetary dust particles (from asteroids and liocentric distance somewhat greater than 20 AU. The de- comets), dust particles from comet 81P/ Wild returned to gree to which the condensable organics in the solar neb- Earth from the Stardust spacecraft, and meteorites, and ula reflect the original composition of the Solar System’s also to astronomical results for the interstellar medium in nascent molecular cloud and what degree of processing our galaxy and other galaxies. occurred in the nebula are unknown (Pendleton and Cruik- The suite of aromatic molecules that can contribute to shank 2014). However, some primitive meteorites and in- the unresolved absorption envelope in the Iapetus spec- terplanetary dust particles carry a D/H isotopic signature trum includes a suite of neutral polycyclic aromatic hy- very similar to that of interstellar organic molecules (Piz- drocarbons for which theoretical calculations have been zarello et al. 2006), and some meteorites contain unal- made by Bauschlicher et al. 2008, 2009, and Ricca et tered interstellar grains (Davis 2011). As noted above, other al. 2012. Aromatic molecular structures revealed by their isotopes in meteorites date their component materials to peripheral CH or CHn bands can be arbitrarily large in presolar times. These characteristics show that at least size, because only those peripheral bonds with hydrogen some original molecular cloud components were cycled produce detectable bands in the ~3.28-µm region. Con- through the solar nebula and survived during the conden- sequently, the total molecular mass of aromatics can be sation of planetary bodies without being completely chem- large, as it seen with interplanetary dust particles, comets ically altered. samples, meteorites, and micrometeorites. In the spectra The material from which Phoebe accreted should be of astronomical objects that can be observed only in lim- some combination of interstellar ices and mineral grains, ited wavelength regions, a large amount of carbon remains plus solid organic matter and some fraction of the same unseen. The absorption envelope centered at ~3.45 µm material processed in the solar nebula. It is in this sense is characteristic of -CH2- and -CH3 symmetric and asym- that the comparison to relict carbonaceous molecules in metric overtones in aliphatic molecules; these four bands comets may be relevant, since Phoebe can, by virtue of are found in absorption in galactic interstellar dust (e.g. its bulk composition and its capture from a distant re- Pendleton et al. 1994) and in some external galaxies (Dar- gion in the Solar System, be considered a member of the tois et al. 2004). Other molecular materials that may be comet clan, albeit a very large one. The dominant form of contributing to the absorption on Iapetus include CH in cy- carbon in interstellar dust includes hydrogenated amor- cloalkane, olefinic CH and CH2, CH3OH ice, and CH3+lone phous carbon (Pendleton and Allamandola 2002), as well pair electrons in N and O (Cruikshank et al. 2014). as polycyclic aromatic hydrocarbons (PAHs), which are rel- While some extraterrestrial materials studied in the atively refractive. In solar nebula models by Kress et al. laboratory are found to contain aromatic molecular mate- (2010), PAHs are destroyed inward of ~2 AU, while less rial, the remote spectroscopic study with telescopes and stable aliphatic hydrocarbons are destroyed at somewhat spacecraft is the only available technique for the ma- greater solar distance. Thus, the presence of both aromatic jority of Solar System bodies. The detection of organic and aliphatic hydrocarbons in the dust emanating from molecules is thus limited to those with spectroscopi- Phoebe is consistent with their formation and preservation cally active modes that lie within the observable spec- at substantially greater radial distances. Interstellar ices tral ranges. In this respect, many extraterrestrial materi- carry inclusions of small dust particles consisting of hy- als (comet particles, micrometeorites, interplanetary dust drocarbons and more refractory kerogen-like macromolec- particles, etc.) are known to contain aromatic material, ular organic solids. The kerogens originate in interstellar 352 Ë I. Simonia and D. P. Cruikshank, Organic in planetary systems space by UV irradiation of ices on grains of carbon and sil- 5 Discussion icates, and the resulting coated grains were incorporated into the solar nebula. Beyond the zone of terrestrial planet Frozen hydrocarbons are components of ices of comets. It formation, some of the kerogens, together with silicates, can be expected that various layers of nuclei of comets became the refractory component of Kuiper Belt objects, are rich with complex aromatics and aliphatics. Frozen carbonaceous meteorites, and other icy bodies, some of cometary hydrocarbons consist of unified structures in- which are now satellites of the outer planets. Astronomical cluding solid solutions, polycrystalline mixtures, and sub- kerogen is similar to some synthetic tholins; the distinc- stitutional matrices. It has been noted (e.g. Oró et al. 2006), tion between the two classes of organic solids is not great. there may be a connection of the complex organic chem- Prolonged exposure of tholins to the space environment istry of comets that includes prebiotic components to the (both UV and charged particle radiation) causes them to origin of life. Solar ultraviolet radiation, flux of electrons darken and eventually become black as they are dehydro- and protons, and the structure of cometary organics would genated and evolve chemically toward spectroscopically appear to stimulate luminescence phenomena in organic- featureless graphite (Pendleton et al. 2011). A similar strat- rich cometary ice that may be detectable. We have sug- egy of consideration can be applied for another satellite – gested that a Shpolskii matrix in the form of polycrys- Hyperion. talline mixtures (PAHs in n-alkanes) might be a significant Early results of the in situ analysis of 67P/CG comet component of the complex organic substance comprising by the Rosetta mission indicate that the visible surface a major fraction of a comet’s mass. consists largely of a mixture of opaque minerals and com- Hydrocarbon sources irregularly distributed in the in- plex macromolecular organic material containing carbon- ner layers of nuclei of comets that have not been processed hydrogen and/or oxygen-hydrogen chemical groups (Ca- by solar radiation are cometary relicts. In time of cometary paccioni et al. 2015). Direct chemical analysis on the outbursts certain portions of such relict matter is in- comet’s surface by the Philae lander has detected a num- jected into cometary coma where it is subsequently photo- ber of volatile organic molecules with various combina- excited. The de-excitation of previously excited molecules tions of carbon, hydrogen, oxygen, and nitrogen, up to mo- (relict molecules) then gives rise to bright luminescence lar mass 62 (Goesmann et al. 2015). These include some emission. Optical spectra of the cometary bursts could be of the same chemical groups found in the laboratory syn- rich in unknown emission lines or featureless bands es- thesis of organic residues by Materese et al. (2015) noted pecially in the blue range. Photoluminescence spectra of above. The low albedo and reddish color of the surface of polycyclic aromatic hydrocarbons at temperatures T > 100 this and other comets, together with the direct analysis of K have a featureless character in the form of extended blue 67P supports the contention that comet surfaces are gen- emission (3990-4910 ÅÅ). erally dark and red or black in color as a consequence of Our results have demonstrated potential presence of the presence of organic solids, and that they are likely to prebiotics and self-organized substance in ices of comets. be related compositionally to some of the distant asteroids The results of our comparative analysis and evidence of that may be extinct comets (e.g. Hartman et al. 1987). similarities of the luminescence properties of porphyrins There appears to be a wide variety of macromolecu- and their structural analogs might become a qualitative lar organic materials in the Solar System depending on tool for identification of prebiotics and heterocycles dur- the initial composition of the component molecules and ing in situ experiments (comets, icy satellites, and TNOs). the degree of energetic processing and subsequent chem- At the same time, similarities in optical properties of com- ical evolution. Perhaps more significant are the similari- plex organics and prebiotics require substantial additional ties among them in terms of basic chemical groups they laboratory investigations. contain. Their colors are varied, but arise from the same Understanding of process of self-organized substance fundamental molecular mechanism, and they are retained formation in the space environment has high significance. on planetary surfaces when the more volatile ices are re- Formation of self-organized substance in the small bod- moved. In the specific case of Phoebe and Iapetus in the ies of the Solar System might be limited by the following Saturn satellite system, we will gain further insight when pre-conditions: 1) presence of small icy nuclei cloud in observations are capable of determining the inventory of the peripheral part of the Solar System; 2) bombardment organic materials that exist in distant Kuiper Belt objects, of small icy nuclei by the solar and galactic cosmic rays; and thus clarify the possible link to the organics inherited 3) absorption and accumulation of cosmic ray energy by from the Solar System’s nascent molecular cloud. atomic and molecular systems of relict substance of icy I. Simonia and D. P. Cruikshank, Organic in planetary systems Ë 353 nuclei; 4) long time preservation of accumulated energy Further, we summarize recent studies of the organic by relict substance of icy nuclei; 5) expenditure of accu- spectral signature of three satellites of Saturn observed mulated energy for formation of various type of chemical with the Cassini spacecraft in orbit around the planet. bonds in molecular systems of relict substance of icy nu- The unique characteristics of this trio of satellites are clei; 6) abundance of organic compounds in icy nuclei. that they show the spectral signatures of both aromatic Icy cometary nuclei and TNO’s substance may absorbs and aliphatic hydrocarbons, and the material displaying and accumulate also the solar electromagnetic radiation these signatures originated in the interior of the satellite on the various heliocentric distances. Expenditure of ab- Phoebe. Phoebe originated far from the Sun in the Kuiper sorbed energy for providing of the concert type of chemi- Belt region and was later captured by Saturn. Its origin in cal bond may be a variable process depending on heliocen- the same region as the comets are thought to have con- tric distances of icy nuclei. We cannot exclude that forma- densed strongly suggests strong compositional similari- tion of self-organized substance of cometary nuclei might ties, including the inventory of included organics. Thus, be efficient on the large heliocentric distances. The role of material ejected from Phoebe’s interior by impact and sub- radioactive isotopes of cometary ices may be significant in sequently landing on the surfaces of Iapetus and Hyperion context of formation of self-organized substance. may reasonably be expected to carry the same compositional characteristics as the material in comets, as well as the implications of great age and genetic connections to 6 Conclusion the solar nebula. Both the concept of the ejection of frozen hydrocarbon particles from comets and the dispersal of primitive We have considered in this paper, probable presence of organics by impact into a body like Phoebe speak to the frozen hydrocarbon matter in ices of the Solar System, in- widespread distribution of a rich inventory of relict molec- cluding comets, planetary satellites and TNOs. We have de- ular material of prebiotic interest. scribed particularly important physical and chemical peculiarities of such frozen complex organic substance. We Acknowledgment: Authors express their gratitude to have discussed also properties of frozen hydrocarbon par- anonymous reviewer for the valuable discussion. ticles (FHP) in coma of comets. The probable UV-induced luminescence of prebiotic molecules (enzymes, pigments and proteins) in FHPs may produce optical emission lines coincident in wavelength with a number of unidentified References emission lines in comet spectra. This leads to the spec- ulative preliminary conclusion that prebiotics, pigments, Alexander, C. M. O’D., Cody, G. D., Fogel, M., and Yabuta, H. 2008, and heterocycles, including luciferin and molecules simi- In: Kwok S., Sandford S.A. (Eds), In Organic Matter in Space, lar to chlorophyll and porphyrins might be included in the IAU Symp., 251, 293-298. 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