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Exploring Organic Environments in the Solar System 5 The Giant Planets and Their Satellites LARGE SATELLITES OF THE OUTER SOLAR SYSTEM In addition to the gas-giant planets, the outer solar system hosts at least seven objects with radii greater than 1000 km. These objects include Jupiter’s four large satellites Io, Europa, Ganymede, and Callisto (often referred to as the Galilean satellites), Saturn’s large moon Titan, Neptune’s large satellite Triton, and the planet/Kuiper-belt object Pluto. Each of these objects is remarkably diverse and displays a variety of planetary processes, and six (i.e., all except Io) are sites of interest for the study of organic chemistry in the solar system (Table 5.1). While each object has unique characteristics with regard to organic chemistry in the solar system, they can be arranged in four groups. Jupiter’s icy satellites Ganymede, Callisto, and especially Europa each show evidence of layers of liquid water that offer potentially uniquely interesting environment for organic synthesis. These bodies are also subject to intense radiation bombardment at their locations within Jupiter’s magnetosphere that can potentially affect carbon chemistry in the surface and near-surface ices. Triton and Pluto, while in very different dynamical circumstances, are near-twins in terms of their bulk properties. Both have nitrogen-dominated atmospheres, significant albedo variations on their surfaces, and comparable surface compositions as inferred from infrared spectra. In a group of its own is the organic-rich satellite Titan. With a dense, cloudy atmosphere that processes methane into complex hydrocarbons which condense and precipitate out on the surface, it may be the richest organic environment in the solar system aside from Earth. Lastly, while Io is a fascinatingly complex object, its vigorous volcanism makes it an inhospitable and unlikely site for organics. Many of the molecules listed in Table 5.1 have been identified by remote spectroscopic observations at infrared and ultraviolet wavelengths, both from ground- and space-based platforms. In the cases of Triton and Pluto, the existence of atmospheric CO is inferred from the presence of surface ice and the requirement for vapor pressure equilibrium; upper limits are available from direct measurement. CH4 is thought to be a component of these atmospheres for the same reason and has been directly detected on Pluto.1 The Galileo Near Infrared Mapping Spectrometer (NIMS) experiment has been particularly important for identifying carbon-containing molecules and functional groups on the surfaces of the icy Galilean satellites. Sources of Carbon Europa, Ganymede, and Callisto The Galilean satellites (Io, Europa, Ganymede, and Callisto) have been studied extensively by both spacecraft flybys and remote telescopic observations. The three so-called icy Galileans—Europa, Ganymede, and Callisto—
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Exploring Organic Environments in the Solar System TABLE 5.1 Observed Carbon Inventory in Large Icy Bodies of the Outer Solar System Atmosphere Object Radius (km) Dominant Carbon Surface Energetics Europa 1,560 O2 — CO2 Particle radiation, tidal heating, photodissociation? Ganymede 2,638 O2 — C-H, C≡N, CO2 (organic, –CHO) Callisto 2,410 O2 — Pluto 1,137 N2 CO (<0.06) CH4 (0.0003-0.0045) CH4, CO (red organic) Photodissociation, solid-state greenhouse?, cosmic rays Triton 1,352 N2 CO (<0.59) CH4, CO, CO2 (red organic) Titan 2,575 N2 CH4 (0.015-0.02) C2H2 (10−6) C2H6 (10–5) C3H8 (10–7) C2H4 (10–6-10–8) HCN (10–7) C4H2 (10–9) C3H4 (10–9) HC3N (10–710–9) C2N2 (10–9) CO2 (10–8) CO (10–5) Organic precipitates, source for atmospheric CH4 (liquid hydrocarbons?) Photodissociation, impacts?, cosmic rays have spectra that are characterized by strong infrared bands of water ice.2,3 Models indicate that the water ice fraction varies from 20 percent in some dark regions to nearly 100 percent in others. The remaining material, which sometimes makes up the majority of the surface, is a good match with a variety of hydrated silicates similar to the hydrated silicates found in carbonaceous chondrites.4 In addition, the surfaces have a number of minor constituents that are variable in observability and abundance. At least three different carbon-bearing molecules have been identified in spectra of the surfaces of these objects. Spectra obtained by Galileo’s NIMS show absorption bands indicative of carbon-rich materials. Absorptions are interpreted to indicate C-H and C≡N present in organic compounds. Spectra also indicate CO2, and its spectral characteristics suggest that it is trapped within or bound to dark surface materials. A broad absorption in the ultraviolet is suggestive of trapped O3 and another unidentified band that may be the –CH2O functional group of an organic compound.5 Carbon dioxide shows interesting and suggestive correlations with both albedo and topography in the three icy Galileans. Callisto, the outermost of the four satellites, has an ancient, heavily cratered surface with widespread dark material that is thought to be carbon-rich. Galileo images show that Callisto’s dark material covers the underlying topography, filling in craters and other topographic lows between bright, ice-rich high-standing knobs and crater rims. CO2 is correlated with fresh material such as bright impact craters, suggesting that CO2 is a possible component of Callisto’s icy subsurface. CO2 is strongest on the trailing side of Callisto, suggesting a possible link to irradiation by jovian magnetospheric plasma. Dark heavily cratered terrain constitutes about one-third of Ganymede’s surface. Geological investigations using Galileo’s high-resolution images suggest that the dark material is a relatively thin layer above brighter icy material and has been affected by processes of sublimation, mass wasting, ejecta blanketing, and tectonism. Dark deposits within topographic lows, such as craters,
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Exploring Organic Environments in the Solar System within grooved terrain have a measured albedo as low as 12 percent. As for Callisto, NIMS observations show absorptions in Ganymede’s dark terrain, with bands less intense by about a factor of two than those on Callisto. The CO2 absorption shows a mottled distribution, generally correlating with the darker terrain. CO2 has also been detected by Galileo’s NIMS on Europa, where it is associated with lower-albedo material. Titan In terms of organic chemistry, the premiere destination within the solar system is Saturn’s Mercury-sized satellite Titan. Its 1.5-bar atmosphere contains about 98 percent nitrogen and about 2 percent methane and more complex hydrocarbons. In addition, the atmosphere contains small amounts of nitrogen-containing organics as well as a haze that is believed to be composed of hydrocarbons and polymers formed by the action of solar ultraviolet radiation on this atmosphere. The surface plays an integral, if less well observed, role in this complex system, acting as both a sink for condensed organics and a source needed to replenish atmospheric methane as it is processed. Titan’s carbon chemistry begins with methane present in Titan’s atmosphere. Photons at λ <160 nm are able to remove a hydrogen atom from CH4 to produce a methyl radical, CH3•. A wide variety of hydrocarbons are then produced, and many of these have been detected spectroscopically by both Earth- and space-based instruments (see Table 5.1). Magnetospheric electrons from Saturn convert N2 to N atoms that are then free to react with hydrocarbons to produce nitriles, which are also observed.6-8 Because of Titan’s relatively low mass, H and H2 produced from the breakup of methane escape to space, enabling the chemical evolution of complex organic compounds. Titan’s orange-colored haze is believed to result from the formation of polymers of C2H2, HCN, and C2H4 in the atmosphere initiated through additional photochemical and/or charged-particle reactions.9,10 Many of the observed and predicted organics will condense at the temperatures found in Titan’s stratosphere and accumulate on the surface, where they remain sequestered and/or participate in further chemical processing. Indeed, it has been speculated that organic precipitates could be further processed (and additional organic compounds synthesized) on Titan’s surface through liquid-phase chemistry simply by condensation and accumulation and/or the influence of impact events.11,12 Despite the abundant hydrocarbons observed in the atmosphere and the theoretical predictions of substantial deposits of hydrocarbons on the surface, evidence for liquid or solid organics has been elusive. While Titan’s atmosphere is opaque at most wavelengths due to a combination of particle scattering and gas absorption, there are several infrared windows with sufficiently low optical depths which allow probing of the surface at selected wavelengths.13 These windows have been exploited to detect distinctive, time-stable albedo features on Titan’s surface, ruling out a global ocean. Spectrophotometry can also be carried out within these windows, and the results suggest water ice as an important surface component.14 Earth-based radar observations of Titan are also inconsistent with a global ocean but appear to suggest smaller, relatively smooth features that might be lakes.15,16 Significant advances in understanding of the nature of Titan’s organics will almost certainly result from ongoing studies by the Cassini-Huygens spacecraft. Cassini is currently carrying out radar and near-infrared spectroscopic and mass-spectral measurements in Titan’s atmosphere and observations of the satellite’s surface, and the Huygens probe has conducted imaging and in situ chemical data collection.17,18 Although Cassini’s explorations are far from complete, the successful descent of Huygens through Titan’s atmosphere on January 14, 2005, combined with the bonus of an unexpectedly long period of surface observations, has confirmed some long-standing expectations and revealed some intriguing new characteristics of Saturn’s largest satellite. Images from Huygens’ descent imager, for example, showed features highly reminiscent of drainage channels and shorelines. Similarly, images obtained on the surface show icy pebbles rounded, perhaps, as a result of fluvial activity (see cover illustration). But standing bodies of liquid hydrocarbons were not apparent in any of the images returned by Huygens. Data from Huygens on the variations of temperature and pressure as a function of altitude were virtually indistinguishable from those expected on the basis of models derived from observations made during Voyager 2’s flyby in 1981. However, the atmosphere appears to lack the expected argon (in the form of 36Ar and 38Ar), krypton,
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Exploring Organic Environments in the Solar System and xenon—a possible sign that Titan accreted at a somewhat higher temperature than previously expected. However, the instruments did detect 40Ar, a daughter product of 40K released from Titan’s interior, perhaps as a result of cryovolcanic activity. Chemical analyses performed by Huygens’ gas chromatograph/mass spectrometer (GC/MS) revealed that methane becomes more abundant relative to nitrogen with the approach closer to Titan’s surface. The GC/MS also registered a sharp increase in the methane abundance soon after landing, possibly indicating the presence of liquid methane just below Titan’s surface. The instruments on Cassini itself are also returning important data, the significance of which is still not entirely clear. Images of Titan’s surface show few craters, indicating a geologically active world. However, the surface does not show any significant compositional variations. Data from the initial radar investigations of Titan’s surface, covering just a few percent of the globe, are intriguing and appear to show structures that appear to be lakes of hydrocarbons.19 Continued radar mapping, in conjunction with ground truth provided by the Huygens landing, will improve understanding of Titan as an active world. Another highly unexpected finding is the detection of benzene in Titan’s upper atmosphere, as determined by in situ analysis performed during one of Cassini’s first close flybys. With so many tantalizing initial findings and with Cassini scheduled to make some dozens of additional Titan flybys during the next several years, it is clear that Titan will be a prime objective for additional studies long after Cassini has ceased operation. Triton and Pluto The N2-dominated atmospheres of Triton and Pluto are generally similar to each other. Trace quantities of CH4, CO, and Ar are consistent with their respective vapor-pressure equilibria with ices of CH4, CO, and Ar on the surfaces of these objects. In part because the atmospheres are much less dense than Titan’s, however, the range of expected chemical reactions is more limited. The atmosphere of Triton is so thin that both the thermosphere (where ion-molecule reactions are important) and the surface determine its atmospheric composition. Methane in Triton’s atmosphere is photolyzed and depleted over short time scales and must be replenished from surface or subsurface reservoirs. The CH4 photochemistry is believed to be similar to that on Titan, but there is greater uncertainty due to lack of knowledge of the relevant chemical kinetic parameters for the colder temperatures of Triton’s atmosphere. Moreover, any complex hydrocarbon and nitrile species will easily condense, forming aerosols near Triton’s surface due to temperatures colder than those on Titan.20 Triton is Neptune’s only large satellite, and its retrograde and highly inclined orbit suggests capture by Neptune. Its surface has relatively few craters, indicating a surface age of only ~108 years, which suggests it remains highly active today. The surface shows abundant evidence for tectonism, cryovolcanic activity, and solid-state convection. Active geysers were discovered in Voyager images, with plumes that extend up to ~8 km to the top of the troposphere and are carried by the tenuous winds. The geysers are believed to be powered by N2 gas heated by geothermal or solar energy, and dark surface streaks, some more than 100 km long, mark the fallout of dark dust carried aloft with the geyser plumes. Ground-based infrared spectra show that Triton’s surface is rich in volatile ices, specifically N2, CH4, CO, CO2, and H2O. CH4 and CO are believed to be in solid solution in the more abundant N2 ice. Additional molecules are predicted as surface precipitates resulting from atmospheric photochemistry, specifically HCN, C2H4, C2H6, and C2H2, but these have not yet been confirmed spectroscopically. Triton has a distinctly pink color, leading to the suggestion that ultraviolet and energetic particle irradiation of CH4 ice has created organic chromophores. Organic materials are a suspected, but unconfirmed, component of the dark surface streaks associated with Triton’s geysers. Pluto and its moon Charon have not yet been visited by a spacecraft, although they are expected to be by the middle of the next decade by the New Horizons mission. Recent infrared data indicate the presence of N2, CH4 (both pure and in an icy matrix), CO, and H2O. Ground-based mutual satellite occultation data and modeling, as well as more recent Hubble Space Telescope observations, show broad-scale albedo variations on Pluto’s surface, providing evidence for dark material in its equatorial regions. As on Triton, organic materials are suspected, produced from energetic processing of surface materials or from atmospheric precipitation. This could explain Pluto’s reddish color. Organics are consistent with infrared spectra and could cover approximately 10 to 15 percent
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Exploring Organic Environments in the Solar System of the surface, but these spectra do not require or confirm the presence of organics. Depending on conditions of its formation, Pluto’s interior could contain an organic-rich layer up to 100 km thick. Energetic Processes Radiolysis and Photolysis in Ices Organic molecules can be both synthesized and destroyed in the outer solar system by irradiation. (See the section “Synthesis and Destruction of Organic Materials” in Chapter 4.) Galileo data for the jovian satellites suggest the presence of CO2 and, possibly, CN in the surfaces of the icy satellites, as described above. It is not clear whether that CO2 is from outgassing or is due to decomposition of carbonates at the surface. However, it is clear that CO2 in an ice matrix exposed to radiation may lead to simple organics formed on the icy-satellite surfaces. Radiation processing occuring on such bodies is indicated by the radiation-induced sulfur cycle observed on Europa’s surface and the presence of oxygen and peroxide produced from radiolysis of ice.21,22 The presence of sulfur in H2O exposed to radiation leads to radiation-induced cycling among three primary forms of sulfur— hydrated H2SO4, SO2, and chain sulfur—at equilibrium concentration ratios of about 10:1:1. Such ratios are roughly consistent with reflectance observations of Europa at infrared, ultraviolet, and visible wavelengths, respectively. Depending on the amount of CO2 in ice, similar processing can lead to the formation of frozen carbonic acid and, possibly, more complex compounds. Photolysis in Atmospheres Ultraviolet light from the Sun and, to a lesser extent, energetic cosmic rays provide non-equilibrium energy sources that can drive pathways of chemical synthesis in the atmospheres of large outer solar system objects. The effects of photochemical atmospheric processes are most apparent on Titan,23,24 but these processes are also active in the more tenuous atmospheres of Pluto and Triton.25-27 In all three, CH4 is the principal source for subsequent organic synthesis. The presence of N2 allows for the significant production of nitriles, with HCN as the simplest example. Hydrocarbons and nitriles condense in all three of these atmospheres, resulting in hazes. Precipitation then deposits organics on the surfaces, where they can be further processed as detailed in the previous section. Although the basic outlines of photochemistry in these atmospheres are understood, some species are not adequately modeled. Observations are available to constrain the models for only relatively simple molecules (see Figure 5.1). The uncertainties for larger organic compounds are considerable. Most photochemical models are one-dimensional approximations, and only recently have some models attempted to deal with two-dimensional approximations.28 A follow-up mission to Titan, for example, should be equipped to study the three-dimensional distribution of a variety of organics through both remote sensing and direct sampling of the atmosphere. Laboratory studies may also be conducted under simulated outer solar system conditions, and photolytic products can be studied directly in the laboratory.29 Radiogenic and Tidal Heating The Galileo orbiter made numerous close flybys of each of Jupiter’s large icy satellites, Europa, Ganymede, and Callisto. From tracking of the spacecraft trajectory it is possible to determine the moments of inertia in sufficient detail to constrain the internal structure of these bodies.30 Both Europa and Ganymede are differentiated whereas Callisto is largely undifferentiated, a fact that points to important differences in their formation and evolution within the jovian satellite system. Magnetic field measurements point to the existence of liquid subsurface oceans in all three satellites beneath ice caps that may be tens to hundreds of kilometers thick. Radiogenic heating takes place on all of the bodies in the solar system, with varying degrees of impact on the structure of the object. Tidal heating takes place only for objects that participate in orbital resonances. This restriction is probably only structurally important for the three Galilean satellites, Io, Europa, and Ganymede. Both of these sources of heating are relevant to carbon chemistry primarily by their ability to create and maintain liquid
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Exploring Organic Environments in the Solar System FIGURE 5.1. A schematic representation of the modeled hydrocarbon photosynthetic reaction scheme in Titan’s atmosphere. SOURCE: Y.L. Yung, M. Allen, and J.P. Pinto, “Photochemistry of the Atmosphere of Titan: Comparison Between Model and Observations,” Astrophys. J. Suppl. 55: 465-506, 1984. Copyright 1984. Reprinted with permission of the American Astronomical Society. environments that are conducive to complex carbon chemistry. Secondarily, environmental niches, perhaps analogous to deep ocean vents on Earth, may provide energy gradients that can directly drive carbon chemistry.31 Another important effect of tidal stresses on Europa, in particular, is the creation of cracks in the ice shell that may permit organic enriched material from deeper in the mantle to be transported to the surface where it is accessible for remote observation. Many researchers have noted the asymmetry of the 2.0-µm water band of Europa, an asymmetry that is produced by bound water molecules. While an asymmetry can be a created by hydrated minerals, Dalton32 argues that details of the band structure are more consistent with biogenic material. On the contrary, Clark33 argues that the asymmetric water bands are the product of trapped hydronium ions (H3O+) that create a hostile environment for organics. While the nature of the non-ice surface material on Europa remains uncertain, the notion of investigating material near surface cracks on Europa remains an important goal for possible future missions, including surface landers of the type that were being considered for the now deferred Jupiter Icy Moons Orbiter (JIMO).34 Impacts Impacts of various sizes may play important roles in organic synthesis on outer solar system bodies. Infrequent but very large impacts may produce transient dense atmospheres and/or lead to large-scale resurfacing with fresh material from the interior of the body. Moderate-sized impactors can melt significant volumes of surface ice that may be sufficient to initiate organic synthesis.35 Electrical discharges produced by impacts may also result in organic synthesis.36 Shocks produced by impacts have been shown to result in the formation of polycyclic aromatic hydrocarbons from benzene, a molecule detected during Cassini’s studies of Titan and also observed in Jupiter and Saturn.37 Micrometeorite impacts can be a significant source of material delivered to both atmospheres and surfaces. For example, micrometeorites are thought to be the main source of oxygen delivered to reduced environments like that at Titan. Micrometeorites may also be a direct source of processed organics, as has frequently been speculated for Earth. Titan is, perhaps, the most interesting object with respect to the possible role of large impacts. Model calculations predict the accumulation of 100 to 1000 m of solid and liquid hydrocarbons on Titan’s surface. If such
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Exploring Organic Environments in the Solar System an accumulation were a permanent sink for methane, Titan’s atmospheric methane would be depleted in 10 million years. One proposed solution to the precipitation of organics on the surface and the need for a source of methane to resupply the atmosphere was the theoretical suggestion of a possible global ocean of ethane and methane, which could serve as both a source (methane) and a sink (ethane) for the methane photolytic cycle in the atmosphere. The suggestion of a global ocean appears to be inconsistent, however, with existing observational data.38-41 Currently, these observations (from ground-based radar, near-infrared, and the Hubble Space Telescope), taken together, suggest that liquid-phase hydrocarbons on Titan’s surface are certainly not global but may exist in craters;42 a supposition now potentially confirmed by Cassini’s radar observations.43 If liquid-phase hydrocarbons on Titan’s surface are not sufficient to resupply methane at a steady-state level, it has also been proposed that perhaps Titan is currently in an unusual epoch in its history in which its atmosphere is in an unusually dense state that will eventually become less dense like those found around Triton and Pluto.44 If such a scenario is true, layered deposits of organics on Titan’s surface may preserve a record of these events that would be accessible to advanced missions that might follow Cassini/Huygens.45 Solid-State Greenhouse The Voyager flyby of Neptune and Triton in 1989 revealed the presence of plumes of dark material rising approximately 8 km in the atmosphere, with dark material subsequently blown 100 km downwind leaving surface streaks. The location of the plumes at latitudes receiving maximal seasonal insolation suggested a solid-state greenhouse as the energy source for these geysers.46,47 Geomorphic evidence48 favors the geyser model over an alternative “dust-devil” atmospheric transport model.49 The significance of the relationship between subsurface heating, the dark plume material, and carbon chemistry on Triton is an interesting open question. Although analogs to the Triton plumes have not been found elsewhere in the outer solar system, the New Horizons mission to Pluto will be an important test of the extent of this phenomenon.50 Outer Solar System Satellites: Recommendations Titan Follow-up Mission Titan is a major reservoir of organic materials. Processes occurring in Titan’s atmosphere may provide an ongoing example of the formation of complex abiotic organics from methane, although this example is probably not pertinent to the processes on primitive Earth because, at Titan’s 96 K surface temperature, all water is condensed as ice. Due to the complex abiotic organic synthesis, this moon merits continued and close ground-based observation and modeling and laboratory studies of its atmospheric chemistry. The Huygens probe revealed much new information about the composition of Titan’s atmosphere as it parachuted to the satellite’s icy surface in January 2005 and continued to transmit analyses of the surface organics until radio contact was lost. The 2003 solar system exploration decadal survey singled out a follow-on mission to Titan as a likely priority mission for the decade starting in 2014.51 Recommendation: Planning should start now for a follow-up of the Cassini mission to Titan that would include a lander sent to sample the surface, since the complexity of the organics there is expected to be greater than that of organics in its atmosphere. The lander should have the capability of sampling organics that are solids at 96 K as well as those that are liquids. The Titan Explorer mission considered by the solar system exploration decadal survey is a good starting point for this planning. Galilean Satellites Mission(s) The likely detection of subsurface oceans on Europa, Callisto, and Ganymede has made these bodies prime targets in NASA’s plans to search for biologic processes on solar system bodies. Because the likely environment
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Exploring Organic Environments in the Solar System is covered by kilometers to tens of kilometers of ice, their exploration is problematic. However, the europan surface in particular suggests that there may be an exchange of materials between the surface and subsurface due to geological processes driven by tidal heating. Therefore, plans are underway to search from an orbiting spacecraft for signatures of organic species on the surface or at some depth into the surface that can be probed by an impactor, lasers, or charged particles. The key to success would be the ability, in a relatively intense radiation environment in which destruction of organics is occurring, to study the organic fragments and distinguish delivered organics from intrinsic organics and the occurrence of biotic versus abiotic processes. Such a study would push the envelope on available analytic techniques but would be a critical component of assembling an organic inventory of the solar system. In the late 1990s and early 2000s, NASA’s solar system exploration plans included an Europa Orbiter mission that would undertake flyby observations of Callisto and Ganymede prior to entering orbit about Europa.52 Although excessive cost growth led to the cancellation of this mission, it did not dampen scientific interest in the study of Jupiter’s large, icy satellites. The Europa Geophysical Explorer, a somewhat more elaborate version of the Europa Orbiter, was the highest-priority large mission recommended by the 2003 solar system exploration decadal survey.53 NASA responded to the survey’s recommendation by initiating the development of JIMO, the Jupiter Icy Moons Orbiter mission, the first of a series of advanced-technology spacecraft employing nuclear-electric propulsion systems that would have significantly expanded scientific capabilities compared with previous Europamission concepts. JIMO would have conducted global orbital mapping surveys of all three icy satellites, at resolutions of 10 m or better, and might have included a small Europa lander. Organic materials can be studied by making provisions for high-signal-to-noise-ratio spectroscopy at resolutions adequate to discriminate potential carbon-bearing species in both high- and low-albedo regions. JIMO was indefinitely deferred in 2005, and NASA and the planetary science community are currently assessing plans for a more conventional and very much less expensive alternative.54 Recommendation: The task group reiterates the solar system exploration decadal survey’s findings and conclusions with respect to the exploration of Europa and recommends that NASA and the space science community devise a strategy for the development of a capable Europa orbiter mission and that such a mission be launched as soon as is it is financially and programmatically feasible. Any lander should be equipped with a mass spectrometer capable of identifying simple organics in a background of water and hydrated silicates. Ground-Based Research Additional ground-based infrared observations are needed to provide further information concerning the organics on Triton, Pluto, Charon, Centaurs, and Kuiper belt objects. Laboratory spectral studies of the ices of potential hydrocarbon species must be performed to facilitate the detection of organics. These studies should be done on mixtures of candidate organics alone and together with water ice to see if the spectra are perturbed when admixed with other substances. Laboratory studies of the irradiation of these ices should be performed to determine what other organic compounds are formed. These studies should be carried out with pure ices as well as with mixtures of these substances with other organic compounds and with water ice. It is also extremely important that an optical database be developed for compounds of interest. The only reliable identification of species responsible for the features present in the near-infrared spectra of these bodies derives from spectral modeling. THE GIANT PLANETS The giant planets, Jupiter, Saturn, Uranus, and Neptune, have atmospheres dominated by molecular hydrogen. CH4 is the most abundant carbon-containing molecule in the upper atmospheres of these planets at a concentration ranging from about 0.3 percent in Jupiter to 2 percent in Neptune. The abundance of CH4 relative to H2 increases with distance from the Sun. In terms of mass, the giant planets are, by far, the largest reservoir of carbon in the solar system, except for the Sun. Jupiter alone contains approximately 3 Earth masses of carbon.
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Exploring Organic Environments in the Solar System Organic Compounds in the Atmospheres of the Giant Planets Table 5.2 delineates the current state of knowledge on carbon-containing molecules in the atmospheres of the giant planets. The molecules listed have been identified primarily by remote spectroscopic observations, mainly at infrared and ultraviolet wavelengths, from spacecraft missions (particularly the Voyager 1 and 2, Galileo, and Cassini missions) and space- and ground-based telescopes. In situ measurements were made by the Galileo probe that entered Jupiter’s atmosphere in December 1995. Approximate mixing ratios, defined as the number ratio of each carbon compound relative to molecular hydrogen, are indicated in Table 5.2 in parentheses. The directly observable portions of the atmospheres of the giant planets, their upper troposphere, stratosphere, and mesosphere, are composed mainly of hydrogen and helium with trace quantities of other molecular species. Although the total amount of organic material present in these atmospheres is large, it is diluted at least 1,000-fold by hydrogen and helium. These reducing environments differentiate the organic chemistry that occurs in these atmospheres from the chemistry in others in the present-day solar system. While complex organics are formed in these atmospheres by a variety of processes, they are recycled back to methane on time scales that are short relative to the age of the solar system. Therefore, the organic molecules present in the giant planets provide no history of their solar nebula source, nor do they increase in amounts or complexity with time. This fact can be viewed as both a disadvantage and an advantage. Nevertheless, the relative simplicity of the complex reaction schemes in the atmospheres of the giant planets serves as a test to both understand and model such systems, which may also suggest important conditions for organic synthesis in other reducing environments. Mechanisms of Formation of Organic Compounds in the Atmospheres of the Giant Planets Generally, the atmospheric organic chemistry of the giant planets can be characterized as steady-state systems composed of methane and a series of related simple hydrocarbons resulting from the dissociation of methane by one of several sources of energy and the subsequent recombination of these molecular fragments. The process of creating new carbon-carbon bonds in the atmospheres is initiated by the production of the radical CH3• derived from CH4. Direct photolysis of CH4 by solar photons with λ <160 nm is ubiquitous on the giant planets and drives a complex, non-equilibrium chemistry in their stratospheres.55-57 In terms of both complexity and observability, this process is the most important source of organic compounds in the giant planets. The precipitation of trapped ions near the magnetic poles is a second process that drives organic chemical synthesis. It has been observed best on Jupiter, where an extensive polar haze with increased production of hydrocarbons are observed.58,59 Indeed, estimates of the total organic production indicate that auroral sources and the associated ion-molecule chemistry may dominate the global production of carbon compounds on Jupiter. The magnetic fields of Uranus and Neptune are substantially weaker and have more complex geometries compared with those of Jupiter and Saturn. The contribution of charged-particle precipitation to production of organic molecules on these two planets is not well quantified but is probably less important than the photochemical contribution. Lightning is a more speculative source of energy that can drive non-equilibrium chemistry. Lightning has been observed on Jupiter in localized latitudinal bands by both the Voyager and the Galileo spacecraft,60 although TABLE 5.2 Carbon Compounds Observed in the Atmospheres of the Giant Planets Planet or Satellite Main Carbon Compound (mixing ratio) Trace Carbon Compounds (mixing ratio) Jupiter CH4 (0.003) C2H2 (10–7), C2H6 (10−6), C2H4 (10−9), C4H2 (10–10), C6H6 (10–9), CO (10–9), CO2 Saturn CH4 (0.005) C2H2 (10–7), C2H6 (10–6), C2H4, C4H2, C6H6, CO (10–9), CO2 Uranus CH4 (0.01) C2H2 (10–8), C2H6 (10–9), CO (10–8), CO2 Neptune CH4 (0.02) C2H2 (10–8), C2H6 (10–6), CO (10–6), CO2
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Exploring Organic Environments in the Solar System the vertical and horizontal distribution of the lightning is unknown. Acetylene is a potential tracer of lightning-induced organic synthesis below the region of the atmosphere where CH4 can be photodissociated, but there are only limited observations with the vertical resolution needed to establish the contribution of lightning synthesis to the total organic synthesis. The impact of comet SL9 provided graphic evidence of yet another source of energy that can produce transient impulses of organic synthesis in giant-planet atmospheres. Organic grains totaling some 40 percent of the mass of each impactor were observed at the impact sites.61 However, spectra of the dark material can be matched with the optical constants of Murchison meteorite material, however, suggesting that at least some of this dust was unaltered material from the impactors rather than synthesized in situ.62 Regardless of the energy source—lightning, charged-particle impacts, or photons—the subsequent chemistry of CH4 results in the production of both saturated (e.g., C2H6) and unsaturated (e.g., C2H2, C2H4) hydrocarbons in these atmospheres (Figure 5.2). Model results suggest that about 70 percent of CH4 destruction results in the synthesis of higher hydrocarbons, while the remainder regenerates CH4 through various other photochemical pathways.63 Chemical-transport models have been constructed for all four giant planets. These models make testable predictions, including the vertical distribution of photochemical products and the abundances of more complex hydrocarbons that have not yet been observed. The computer models typically rely on a series of reaction rate coefficients, many of which must be estimated or extrapolated from very different laboratory conditions. The lack of appropriate laboratory data for reactions of interest stands as a significant barrier to further progress in modeling. FIGURE 5.2 The mole fractions (P(×)/P(total)) of observed hydrocarbons (symbols) compared to model predictions (curves) as summarized by Moses et al. SOURCE: J.I. Moses, T. Fouchet, R.V. Yelle, A.J. Friedson, G.S. Orton, B. Bezard, P. Drossart, G.R. Gladstone, T. Kostiuk, and T.A. Livengood, “The Stratosphere of Jupiter,” in Jupiter: The Planet, Satellites, and Magnetosphere (F. Bagenal, T.E. Dowling, and W.B. McKinnon, eds.), Cambridge University Press, 2004. Copyright 2004. Reprinted with the permission of Cambridge University Press.
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Exploring Organic Environments in the Solar System All four giant planets have stratospheric hazes interpreted as being due to condensed hydrocarbons. These aerosols and cloud particles may also provide sites for further processing of hydrocarbons. The existence of polymers or condensed aromatics has been proposed as a possible source of the unidentified yellow-colored compounds evident in the atmosphere’s of Jupiter and Saturn.64 However, no direct observational evidence to either support or refute this suggestion is available to resolve this long-standing problem. It is possible that an eventual resolution will require in situ sampling of aerosol material with a mass spectrometer. Below the tropopause, the increasing temperature and pressure with depth drives chemistry away from kinetic control toward equilibrium control. Under equilibrium conditions, ethane is the most abundant hydrocarbon other than methane, present at a mixing ratio of 10–9 at a temperature of ~1000 K and pressure of ~800 bar in a solar abundance mixture of gases.65 Continuing inward to higher temperatures and pressures, CH4 is replaced by CO as the dominant form of carbon that, in turn, is ultimately replaced by monatomic carbon. It has also been speculated that, at particular depths in giant planet atmospheres, CH4 may pyrolyze to form latticed carbon. However, it must be noted that much of the modeling of carbon chemistry in the deep atmospheres of giant planets involves poorly characterized reactions in dense, hot, non-ideal fluids of hydrocarbons in a hydrogen-rich background. A better theoretical and experimental understanding of these processes is needed to complete understanding of carbon chemistry in such thick atmospheres. Overall, the carbon chemistry of the giant planets is essentially a closed system in steady state. The planets are sufficiently massive that loss of hydrogen is not important. There are no abundant solid surfaces on which photochemical products can be stored. Convection eventually transports hydrocarbons synthesized in the stratosphere to deep layers of the troposphere where they are destroyed by pyrolysis and recycled as methane to the upper atmosphere. Thus, no chemical evolution occurs in the system; the chemistry is controlled by kinetics in the upper layers and by thermodynamics in the lower layers. Advances beyond current understanding of organic synthesis in the atmospheres of giant planets will result from efforts concentrated in three areas: Laboratory studies of reaction rates. Modeling of chemical reaction schemes is currently limited by incomplete knowledge of important reaction pathways and rates at temperatures relevant to giant-planet stratospheres. Remote infrared spectroscopy. Models of the vertical distribution of photochemical products are far more detailed than available remote sensing measurements capable of constraining these models. Improvements in the state of observations could be obtained by increased opportunities to obtain high-resolution, high-signal-to-noise-ratio spectroscopy (R > 3000, S/N > 100) on large telescopes in the near- and mid-infrared (2-15 µm) capable of high spatial resolution. In situ sampling. The unknown identities of the chromophores in the atmospheres of Jupiter and Saturn are a significant gap in knowledge of potential organic chemistry of these planets. Future atmospheric entry probes should consider including experiments designed to identify complex molecules in aerosols and cloud particles that could resolve this long-standing question. NOTES 1. L.A. Young, J.L. Elliot, A. Tokunaga, C. de Bergh, and T. Owen, “Detection of Gaseous Methane on Pluto,” Icarus 127: 258, 1997. 2. C.B. Pilcher, S.T. Ridgeway, and T.B. McCord, “Galilean Satellites: Identification of Water Frost,” Science 178: 1087-1089, 1972. 3. T.B. McCord, G.B. Hansen, and C.A. Hibbitts, “Hydrated Salt Minerals on Ganymede’s Surface: Evidence of an Ocean Below,” Science 292: 1523-1525, 2001. 4. J.M. Moore, C.R. Chapman, E.B. Bierhaus, R. Greeley, F.C. Chuang, J. Klemaszewski, R.N. Clark, J.B. Dalton, C.A. Hibbitts, P.M. Schenk, J.R. Spencer, and R. Wagner, “Callisto,” pp. 397-426 in Jupiter: The Planet, Satellites and Magnetosphere (F. Bagenal, T.E. Dowling, and W.B. McKinnon, eds.), Cambridge University Press, Cambridge, England, 2004. 5. K.S. Noll, H.A. Weaver, and A.M. Gonnella, “The Albedo Spectrum of Europa from 2200 Å to 3300 Å,” Journal of Geophysical Research 100(E9): 19057-19060, 1995. 6. D.W. Clarke and J.P. Ferris, “Titan Haze: Structure and Properties of Cyanoacetylene and Cyanoacetylene-acetylene Photopolymers,” Icarus 127: 158-172, 1997. 7. D.W. Clarke, J.C. Joseph, and J.P. Ferris, “The Design and Use of a Photochemical Flow Reactor: A Laboratory Study of the Atmospheric Chemistry of Cyanoacetylene on Titan,” Icarus 147: 282-291, 2000.
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Exploring Organic Environments in the Solar System 8. B.N. Tran, J.C. Joseph, J.P. Ferris, P. Persans, and J.J. Chera, “Simulation of Titan Haze Formation Using a Photochemical Flow Reactor: The Optical Properties of the Polymer,” Icarus 165: 379-390, 2003. 9. D.W. Clarke and J.P. Ferris, “Titan Haze: Structure and Properties of Cyanoacetylene and Cyanoacetylene-acetylene Photopolymers,” Icarus 127: 158-172, 1997. 10. P. Coll, D. Coscia, N. Smith, M.C. Gazeau, S.I. Ramirez, G. Cernogora, G. Israel, and F. Raulin, “Experimental Laboratory Simulation of Titan’s Atmosphere: Aerosols and Gas Phase,” Planetary and Space Science 47: 1331-1340, 1999. 11. See, for example, D.W. Clarke and J.P. Ferris, “Titan Haze: Structure and Properties of Cyanoacetylene and Cyanoacetylene-acetylene Photopolymers,” Icarus 127: 158-172, 1997. 12. See, for example, J.I. Lunine, R.D. Lornez, and W.K. 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Representative terms from entire chapter: