Some Speculations on Titan|S Past\ Present and Future

$Some Speculations on Titan|S Past\ Present and Future$

Some speculations on Titan|s past\ present and future

Jonathan I[ Lunine\0 Ralph D[ Lorenz0 and William K[ Hartmann1 0Lunar and Planetary Laboratory\ University of Arizona Tucson\ Arizona 74610!9981\ U[S[A[ 1Planetary Sciences Institute\ 519 N[ 5th Ave\ Tucson\ Arizona\ 74694\ U[S[A[

Received 06 March 0886 ^ revised 6 November 0886 ^ accepted 00 November 0886

hidden beneath a high altitude photochemical haze[ Con! Abstract[ The solar system|s second largest natural sat! current with such observations has been a renewed e}ort ellite is shrouded by a thick nitrogen atmosphere\ rich to understand the nature of Titan|s surface\ its interactions in methane\ within which sunlight! and cosmic!ray! with Titan|s atmosphere\ and the resulting long!term evol! driven organic chemistry has gone on for some 3[4 ution of this planet!sized moon[ billion years[ The earliest history of Titan|s atmo! The present article is a review and somewhat speculative sphere\ and speci_cally its origin\ remains unclear until synthesis of recent ideas and new data concerning the the CassiniÐHuygens probe measures the ratio of argon origin and earliest evolution of Titan|s atmosphere\ long! to nitrogen and the abundances of other noble gases term stability of Titan|s atmosphere\ and the role this and isotopes[ However\ the abundance of deuterated moon might play in helping exobiologists understand the methane in the atmosphere today is consistent with an chemical and physical origins of life[ We do not attempt atmosphere that originated in the chemically!processed a review of current understanding of Titan|s atmosphere ^ sub!nebula around Saturn\ rather than in cometary a very recent piece in this regard is that of Lunine "0885#[ material[ Titan|s overall atmospheric history is driven A review of the remote sensing data and its implications by the depletion of methane\ and the mechanisms by for the nature of Titan|s surface is provided by Lorenz which methane might be resupplied from surface or and Lunine "0886#[ external sources[ Remote sensing data mitigate against We begin in Section 1 by considering recent data on a large reservoir of methane at Titan|s surface\ leaving deuterated methane and its implications for the origin of open the possibility that methane is periodically depleted from Titan|s atmosphere on timescales of 096Ð Titan|s atmosphere and for the nature of Titan|s interior[ 097 years ^ under such conditions Titan might oscillate We then move in Section 2 to a discussion of recent models between thin and thick atmospheric epochs[ Titan|s of long!term surfaceÐatmosphere interactions\ with par! surface may provide a repository for complex organic ticular emphasis on atmospheric stability[ Finally\ in Sec! molecules that were synthesized during times when tion 3 we examine one particular issue which is a con! liquid water was temporarily available on the surface\ ceptual sticking!point in understanding the origin of life\ such as after impacts or cryo!volcanic eruptions[ Such namely the origin of bimolecular!handedness\ and con! molecules might provide clues to the resolution of some sider the possibility that organic chemistry on Titan might di.cult issues associated with the origin of life[ Ident! provide a unique perspective on how such handedness ifying the presence and nature of such molecules is a arises[ di.cult exploration problem that must be left to mis! sions which follow!on from the CassiniÐHuygens exploration of Titan[ Þ 0887 Elsevier Science Ltd[ All 1[ Origin of Titan|s atmosphere and implications for the rights reserved interior 0[ Introduction Titan|s atmosphere may be a remnant of the circum! Saturnian gas and planetesimals from which the satellite The past several years have seen an increasing number of itself was derived "Lunine et al[\ 0878#\ or it could be a remote sensing observations of Titan|s surface\ which is product of cometary impacts during or after accretion Correspondence to ] Jonathan I[ Lunine\ Lunar and Planetary "Zahnle et al[\ 0881#[ These are endmember cases\ and it Laboratory\ 0518 East University Blvd[\ University of Arizona\ is possible that the present atmosphere contains con! Tucson\ Arizona 74610!9981\ U[S[A[ Tel ] "0# 419 510 1678 ^ tributions from both[ The di.culty of quantifying how Fax ] "0# 419 510 3822 ^ E!mail ] jlunineÝlpl[arizona[edu much atmosphere might be contributed from cometary 0099 J[ I[ Lunine et al[ ] Some speculations on Titan|s past\ present and future impacts lies in the wide range of cometary velocities to the total nitrogen\ would be re~ected in a very small associated with di}erent dynamical states[ Comets coming argon!to!nitrogen ratio*much less than 0)[ "The con! in from the Oort cloud have much higher velocities than version of ammonia to molecular nitrogen to form the Kuiper Belt comets\ and hence would tend to erode atmo! present atmosphere of Titan is a separate problem ^ vari! sphere o} of Titan\ according to Gri.th and Zahnle ous models are reviewed in Lunine et al[\ 0878[# Current "0884#\ who tried a large number of di}erent simulations Voyager!based upper limits on argon in Titan|s atmo! of cometary population distributions[ They found that sphere are too crude to apply this important test[ typical runs yielded 09) of Titan|s atmosphere being However\ the Huygens Gas Chromatograph Mass Spec! contributed from comets "the remainder assumed to be trometer will be able to measure very small amounts of from the circum!Saturnian nebula#\ but that one out of argon\ and will enable the argon!to!nitrogen ratio to be six simulations yielded the entire atmosphere from comets[ very precisely determined during the descent scheduled One approach to understanding the origin of Titan|s for 1993[ atmosphere is to measure the abundances of species that A complimentary chemical test of the origin of Titan|s are diagnostic of di}erent primordial chemical reservoirs[ atmosphere\ focusing on the methane\ lies in measuring Owen "0871# suggested that the argon!to!nitrogen ratio the deuterium!to!hydrogen ratio\ and can be applied would be diagnostic in this regard[ Laboratory experi! today[ IonÐmolecule reactions in molecular clouds "Van ments and theoretical models "Bar!Nun et al[\ 0877 ^ Lun! Dishoeck et al[\ 0882# lead to an enhancement of the D! ine et al[\ 0880# indicate that argon is similar to that of to!H ratio in a variety of organic and other molecules\ molecular nitrogen in its tendency to incorporate into relative to the value predicted on the basis of ther! solid ice phases at temperatures typical of the UranusÐ modynamics and the rates of chemical reactions between Neptune region during planet formation[ Since comets neutral species[ Insofar as comets contain ices that pre! appear to have much of their elemental nitrogen in the serve much of this enhancement\ a comet!derived atmo! form of molecular nitrogen\ a cometary source for Titan|s sphere on Titan would be correspondingly enhanced in its nitrogen!dominated atmosphere would bring in large D!to!H ratio[ Alternatively\ a Titan atmosphere derived quantities of argon\ as well[ Hence\ as Owen _rst pointed primarily from circum!Saturnian material would exhibit out\ such an atmosphere would have a ratio of argon to little or no deuterium enhancement[ Models for satellite! nitrogen roughly solar\ i[e[\ 9[0[ Small di}erences in the forming disks around Jupiter and Saturn predict that propensity for trapping of these species in ice might lead much or all of the material has undergone processing in to a variation of plus or minus an order of magnitude\ fairly warm environments\ i[e[ above 199 K\ such that i[e[\ argon!to!nitrogen ranging from 9[90Ð0[ strong D!to!H enhancements in molecular species would Alternatively\ if Titan|s atmosphere were derived from be erased by reequilibration with the molecular hydrogen circum!Saturnian gaseous and solid phases associated gas[ In contrast to enhancements of a factor of ten or with the formation of the planet itself\ molecular nitrogen more relative to the solar D!to!H value achieved by ionÐ might not be the principal form of nitrogen[ Physical molecule reactions in the molecular cloud\ enhancements models of the circum!Saturnian nebula by Lunine and of only a factor of two over solar are possible in cir! Stevenson "0871# support the chemical modeling of Prinn cumsatellite disks[ and Fegley "0870#\ who found that ammonia might domi! The predominant hydrogen!bearing species in Titan|s nate\ or at least be comparable to molecular nitrogen\ in atmosphere is methane\ and it is in this species that deu! a circum!Saturnian nebula with densities much higher terium enhancements*or lack thereof*are recorded for than the surrounding solar nebula[ The extent to which Titan[ Pinto et al[ "0875# developed a model in which the ammonia dominates depends upon the kinetics of the progressive photochemical destruction of methane over reactions converting N1 to NH2\ because they determine geologic time leads to an additional deuterium enhance! the lowest temperature at which the reactions proceed in ment on top of any primordial enhancement[ The e}ect the lifetime of the circum!Saturnian nebula[ "Note that derives from the greater magnitude of the binding energy the limiting temperature is well above that in the zone of a deuterium atom to the carbon in methane\ relative to where Titan formed ^ the assumption is made that the the hydrogenÐcarbon binding energy[ Thus\ hydrogen has ammonia is formed closer to Saturn and mixed outward a greater probability of being removed from methane by turbulent processes[# More rapid kinetics\ made poss! during absorption of ultraviolet radiation from the sun[ If ible perhaps by heterogeneous catalysis\ would strongly the current rates of photochemical destruction are typical favor ammonia "Prinn and Fegley\ 0870#[ Ammonia tends over Titan|s history\ then the net tendency of methane to to hydrogen!bond with water ice\ forming a number of retain deuterium vs hydrogen leads to an enhancement of stoichiometric compounds that remove almost all of the deuterated methane "CH2D# relative to CH3 of a factor of ammonia from the nebular gaseous phase[ Molecular two[ Hence if the primordial ratio of deuterium to hydro! nitrogen\ on the other hand\ must rely on physical adsorp! gen in methane was enhanced over solar by a factor of tion to be incorporated in the ice\ and at plausible circum! two\ the current ratio would be four times solar[ Saturnian nebular temperatures near Titan|s present pos! Lecluse et al[ "0885# have reexamined the observations ition is mostly excluded from the solid phase[ Thus\ even of deuterium abundance in the sun\ comets\ the Earth if ammonia is merely comparable to molecular nitrogen and Titan[ The observed value for Titan has decreased in the gas phase of the nebula\ it will dominate in the ice[ somewhat from determinations made in the late 0879|s[

Argon\ like molecular nitrogen\ is incorporated The most recent measured value of CH2DtoCH3 in Tit! ine.ciently in the ice by adsorption[ Thus\ an origin of an|s atmosphere implies a D:H value of 6[64×09−4\ with Titan|s nitrogen atmosphere from ices condensed in a an uncertainty of 229) "Orton\ 0881#[ This value is reducing nebula\ with a high fraction of ammonia relative below the enhancement in comets and\ as noted by Lecluse J[ I[ Lunine et al[ ] Some speculations on Titan|s past\ present and future 0090 et al[ "0885#\ consistent with the photochemical enrich! ment of deuterated methane proposed by Pinto et al[ "0875#\ superimposed on at most a modest "factor of two or less# primordial enhancement relative to the solar D! to!H value[ The lack of a strong primordial enhancement is consistent with the bulk of Titan|s atmospheric methane having been derived locally\ in a circum!Saturnian disk\ rather than from collisions with cometary debris[

However\ earlier determinations of CH2D are a factor of two higher "e[g[\ De Bergh et al[\ 0875 ^ Coustenis et al[\ 0878#\ and therefore ambiguous with respect to con! straining Titan|s source region[ It may be that we must await direct measurement by the Huygens probe to settle the question of Titan|s atmospheric deuterium abun! dance[ If Titan|s atmosphere had a local source\ a high density disk of material formed as a by!product of the growth of Saturn\ then carbon! and nitrogen!bearing molecules Fig[ 0[ Model of the present!day interior of Titan\ assuming would have been dominated by reduced species "Prinn formation of the satellite in a reducing environment\ adapted and Fegley\ 0870# such as methane and ammonia[ If the from Stevenson "0881#[ The Ice I layer might also contain meth! latter were the dominant nitrogen!bearing species\ then ane clathrate hydrate\ as well as trapped pockets of ~uid meth! the ammonia!to!water ratio in the planetesimals making ane[ The uppermost thin layer is the portion of Titan|s crust that up Titan would have been 04)[ Plausibly some of the might be in thermodynamic contact with the atmosphere\ e[g[\ a porous ice layer su}used with liquid methane and other hydro! nitrogen was in the form of more oxidized species\ but the carbons main conclusion is that Titan is likely more ammonia!rich than an object composed of debris derived from the solar nebula or surrounding molecular cloud[ Is such a conclusion consistent with Titan|s density\ sure phases of water ice\ below which is a rock core[ The which at 0[76 g cm−2\ is larger than that predicted for a assumption that interior temperatures were warm enough\ reducing disk\ as reviewed by Lunine and Tittemore during or after accretion\ to soften the ice and allow the "0882#< Accretion models of satellites the mass of Titan\ rock to move to the center of Titan\ is a fairly robust Ganymede and Callisto "Lunine and Stevenson\ 0871# conclusion of thermal evolution models[ suggest that heating during late stages of formation was Density measurements suggest that near!peritectic "on su.cient to vaporize incoming ices\ leading to an enhance! the water!rich side# composition ammoniaÐwater ~uids ment in the silicate component "Stevenson et al[\ 0875#[ are roughly neutrally buoyant relative to Ice I[ This yields Quantitative models of such water loss have yet to appear a situation in Titan|s crust similar to that of the Earth in the literature\ a decade after this suggestion was made\ with respect to magmas[ In the case of Titan\ the magma but the direction of the e}ect\ and the energetics\ both is ammoniaÐwater ~uid which may migrate\ at some time support the notion that Titan lost water during accretion\ and under some conditions\ upward through the ice I crust as did Ganymede and Callisto[ to the surface "Stevenson\ 0871 ^ Lunine and Stevenson\ If the deuterium abundance re~ects the origin of Titan|s 0876#[ Titan|s surface and near!surface crust may contain atmosphere "and hence Titan itself#\ in a reducing environ! ammoniaÐwater solid compounds[ There are exobio! ment\ then the interior of the satellite should be relatively logical implications to this statement which we discuss in rich in ammonia[ In turn\ because ammonia lowers the a later section[ Here\ we point out that ammoniaÐwater melting point of water dramatically\ Titan should have a volcanism has both chemical and morphological impli! liquid mantle of ammoniaÐwater even up to the present cations[ day[ Figure 0\ modi_ed from Stevenson "0881#\ illustrates Owen "0871# made the important point that the rare such a model[ The basic physics which allows such a isotope argon!39\ derived from the decay of potassium! mantle of ammoniaÐwater liquid\ but not liquid water\ is 39 in silicates\ is potentially a measure of the amount the transport of heat outward from the satellite by sub! of outgassing of a silicate core in Titan|s interior\ and solidus convection in the water ice[ The ice self!regulates computed that as much as 09−3 of the present Titan atmo! to a temperature below the ice melting point at which the sphere could be argon!39\ if the relative outgassing rate viscosity is su.ciently low to allow ~ow and transport of was equivalent to terrestrial[ However\ a problem is that accretional and radiogenic heat outward[ This tem! di}usion of the argon!39 through the interior may be too perature happens to be above the liquidus for a com! slow to allow full outgassing over the age of the solar positionally!plausible solution of ammoniaÐwater based system[ Therefore\ the argon!39 content of Titan|s atmo! on recent high!pressure experiments of Cynn et al[ "0878#[ sphere may be principally determined by the leaching of The most recent models of Titan|s interior\ by Grasset and the parent potassium!39 from core silicates into adjacent Sotin "0885#\ also lead to the conclusion that a substantial ammoniaÐwater liquid mantle\ and the volume of that liquid layer exists today if the satellite contains signi_cant material subsequently erupted to Titan|s surface[ Engel et amounts of ammonia[ In the absence of signi_cant al[ "0883# used thermodynamic modeling and exper! amounts of ammonia\ Titan|s interior structure today imental data to compute the solubility of potassium!39 in would be entirely solid\ with Ice I atop several high pres! ammoniaÐwater liquids under kilobar pressures\ and used 0091 J[ I[ Lunine et al[ ] Some speculations on Titan|s past\ present and future this result to determine the amount of argon!39 eventually peratures would occur over geologic time[ A similar result released to the atmosphere as a function of surface ~ow was obtained by McKay et al[ "0882# in the situation volume[ These numbers are not large ^ a kilometer!thick where pools of methane only were present\ such that the ~ow across Titan|s surface would put into the atmosphere methane relative humidity was bu}ered at the surface\ but an argon!39 inventory equivalent to mole fractions of no reservoir of nitrogen was available to exsolve to the between 09−7 and 09−09 of the present atmosphere[ This atmosphere[ signature could be swamped by primordial argon!39\ Remote sensing data "e[g[\ Muhleman et al[\ 0880 ^ which is 0 part in 29\999 of the total argon abundance[ If Gri.th\ 0882 ^ Lemmon et al[\ 0884 ^ Coustenis et al[\ Titan formed of ices containing nitrogen in molecular 0884 ^ Smith et al[\ 0885# fail to detect oceans that were form\ the high argon abundance would imply a primordial hypothesized by Lunine et al[ "0872# to both supply meth! argon!39 abundance "i[e[ that not due to potassium!39 ane and store the major end product of methane pho! decay after Titan core formation# of order 09−5[ However\ tolysis "ethane#[ An alternative model must be considered\ a Titan formed from ices containing nitrogen principally in which methane does periodically become depleted from in the form of ammonia would lack this primordial com! Titan|s atmosphere\ until stochastic resupply mechanisms ponent\ allowing the outgassed component to show "such as cometary impacts or volcanism# restock the atmo! through[ Rheological measurements indicate rather high sphere with methane[ Lorenz et al[ "0886a# have examined viscosities in rapidly!cooled liquid ammoniaÐwater ~ows the implications of such a depletion on Titan|s atmo! "Kargel et al[\ 0880#\ which could produce distinctive types sphere[ They used the non!grey radiativeÐconvective code of ~ow features detectable in Cassini orbiter radar and developed by McKay et al[ "0878# to model the present near!IR images\ and particularly in the Huygens probe Titan atmosphere\ with updated collision!induced absorp! Descent Imager pictures "Lorenz\ 0885#\ allowing for a tion coe.cients and a Mie scattering stratospheric haze[ determination of ~ow volumes independent of the argon! Moist convective transport of energy through the atmo! 39 measurements[ The combination of such ~ow volume sphere was included for model atmospheres in which estimates and measurement of atmospheric argon!39 from nitrogen saturates[ Figure 1\ adapted from their work\ the Huygens probe thus provides a potentially interesting summarizes the surface temperature change[ The results additional constraint on the origin of the atmospheric are rather sensitive to the behavior of the pho! nitrogen[ tochemically!produced haze in the stratosphere\ on the surface albedo\ and on the luminosity of the sun[ Depletions of atmospheric methane early in Titan|s his! tory have a more severe e}ect on the atmosphere because 2[ Long!term stability of Titan|s atmosphere the sun|s luminosity was lower[ In most of the cases stud! ied the atmosphere becomes cold enough for the molec! All post!Voyager published photochemical models "e[g[ ular nitrogen to condense out as rain or snow\ leading to Yung et al[\ 0873 ^ Toublanc et al[\ 0881 ^ Lara et al[\ 0883# potentially complex scenarios of partial collapse of the of Titan|s atmosphere agree that the current inventory atmosphere[ of methane in Titan|s atmosphere will be depleted by Although no single evolutionary timeline is implied by photolysis in less than 097 years[ Various resupply mech! the calculations\ Fig[ 2 shows a possible schematic history anisms have been proposed\ including a surface hydro! of Titan|s atmosphere which assumes no present!day carbon ocean\ methane hiding in a subsurface regolith\ massive surface reservoir of methane[ The _rst few hun! and cometary impacts "see the reviews cited in the intro! dred million years after accretion of Titan are char! duction for the numerous references to these ideas#[ acterized by a massive primordial atmosphere of methane Because the thermal balance of Titan|s atmosphere is and ammonia\ raised by accretional heating[ Escape of dependant on infrared absorption during pairwise col! primordial methane by early\ rapid UV photolysis cools lisions of nitrogen\ methane and hydrogen molecules "the the atmosphere\ forcing ammonia condensation and leav! last being a photochemical product of methane#\ a change ing behind an atmosphere of nitrogen "formed pho! in the methane abundance has a potentially signi_cant tochemically from the ammonia# and methane[ As con! e}ect on the atmosphere[ In other words\ photochemistry ditions cooled su.ciently for the crust of Titan "water may drive climate change on Titan[ ice and some ammonia# to freeze over\ the atmospheric Various studies of the evolution of Titan|s atmosphere methane could have been supported by an ocean of meth! coupled to di}erent models of the surface have been con! ane which today is absent\ but which could have left ducted[ McKay et al[ "0882#\ for example\ considered how behind morphological signatures detectable by Cassini the atmosphere would respond if the atmospheric meth! and Huygens "Lorenz and Lunine\ 0885#[ There may have ane abundance were controlled by surface reservoirs of been a relatively stable time period during which the sur! methane[ If a massive ocean were present "either on the face methane was slowly depleted\ supplying the atmo! surface or as a subsurface reservoir in thermodynamic sphere with methane consumed by solar!ultraviolet! contact with the atmosphere#\ then as methane is con! driven photolysis[ As the surface reservoir depleted\ and verted to heavier hydrocarbons in the atmosphere "whose atmospheric methane was expended\ the atmosphere fate is sedimentation#\ the surface reservoir would gradu! cooled ^ in the _rst quarter of Titan|s history when the ally become less methane!rich\ reducing the near!surface sun|s luminosity was less than 79) of the present!day methane relative humidity but allowing dissolved nitrogen value\ the exhaustion of atmospheric methane could have to evaporate into the atmosphere[ Under such conditions\ led to complete freezeout of the atmosphere[ The surface and imposing the monotonically!increasing luminosity of temperature after atmospheric freezeout is then deter! the sun\ a gradual warming of Titan|s atmospheric tem! mined principally by the albedo of surface frosts and the J[ I[ Lunine et al[ ] Some speculations on Titan|s past\ present and future 0092

Fig[ 1[ The surface temperature of Titan as a function of albedo\ methane gas phase abundance\ solar luminosity and stratospheric haze properties\ adapted from Lorenz et al[ "0886a#[ Haze number are production rate "via methane photolysis# in units of 09−03 gcm−0 s−0[ Numbers for the sun are luminosity in units of the present!day value at SunÐTitan distance[ Methane relative humidities refer to the value just above the surface[ Dashed line includes the e}ect of moist convective transport of heat caused by nitrogen condensation[ An auxiliary vertical scale on the right is the saturation vapor pressure of nitrogen\ tied to the temperature axis on the left

luminosity of the sun at that time\ since the remnant being tested\ but the cratering record has the potential for atmosphere is optically thin[ Several cycles of atmospheric providing evidence of epochs of thin atmosphere[ As on freezeout and resurrection\ as new methane was supplied Venus\ where primary craters have a lower size cuto}\ by volcanism or comet impact\ might have occurred in today|s thick Titan atmosphere prevents smaller bolides the _rst half of Titan|s history ^ only two are shown in the from reaching the surface intact[ Provided old\ heavily schematic history of Fig[ 2[ cratered terrains exist on Titan and are not completely Eventually\ the luminosity of the sun increased obscured by the hundreds of meters of photochemical su.ciently that\ even when methane was depleted from detritus predicted to have fallen over geologic time\ it Titan|s atmosphere\ atmospheric collapse was not as should be possible to determine lower size cuto}s for severe[ The surface temperature during these modest col! cratered terrains[ Engel et al[ "0884# and more recently lapse episodes is determined by the thermal structure of Ivanov et al[ "0886#\ have quanti_ed the shielding by the essentially!pure nitrogen atmosphere and the value of Titan|s atmosphere[ Epochs of thin atmosphere on Titan\ the solar luminosity[ The latter portion of Titan|s geologic particularly during early times when cratering ~uxes were past might then have been characterized by episodes of enhanced\ would show up in a crater size!frequency dis! modest atmospheric de~ation and rein~ation as methane tribution extending below a cuto} of about 5 km to 7 km was exhausted from\ and then added back to\ the atmo! diameter dictated by the thickness of the present atmo! sphere[ The _gure assumes that addition of methane endo! sphere\ as well as in a preponderance of strewn _elds that genically or exogenically restores the atmosphere to a are less likely to form in the current atmospheric epoch state similar to today|s "normalized in temperature by the "Ivanov et al[\ 0886#[ CassiniÐHuygens imaging of Titan\ changing solar luminosity#[ It is conceivable that adding from orbiter and probe\ extends over such a wide range a very large quantity of methane during atmospheric rein! of surface area and resolution\ that analysis of impact ~ation might create a distended atmosphere with a surface crater _elds could o}er sensitive insight into Titan|s atmo! temperature much higher than today|s\ but recent radi! spheric history[ The combination of surface area and res! ative!convective models by Lorenz et al[ "0886b# suggest olution suggests that CassiniÐHuygens could detect crater this is di.cult to achieve for solar luminosities less than densities on the order of a few percent those of the lunar or comparable to the current value[ mare surfaces[ Thus\ if the impact ~uxes are comparable\ Such speculations would seem to have little hope of we may detect measurable cratering records on surfaces 0093 J[ I[ Lunine et al[ ] Some speculations on Titan|s past\ present and future

Fig[ 2[ One possible history of Titan surface temperature\ drawn as equatorial surface temperature vs time ^ the present!day is labelled {{9||[ Duration of collapsed or de~ated atmospheric states depends on methane resupply mechanism and hence is uncertain[ The sketch assumes a monotonically increas! ing solar luminosity beginning at 69) of the present!day value some four billion years ago

more than a few percent of the age of the lunar maria* increasing luminosity of the sun[ Lorenz et al[ "0886b# e[g[ Titan surfaces older than about 49Ð099 My[ Of course\ have considered the habitability of Titan during the onset an independent measure of the impact ~ux\ or crater pro! of the sun|s red giant phase some six billion years hence[ duction rate\ would be needed to convert such data into At a time when the Earth will lose its liquid water\ driven an absolute age ^ Cassini mapping of the icy Saturnian o} by enhanced solar luminosity\ Titan surface tem! satellites\ added to the Voyager data base\ would help peratures will reach above the ammonia!water peritectic accomplish this[ point of 065 K\ and may exceed 199 K for several hundred In addition\ the Titan impact crater data base should million years[ Mirroring a primordial\ post!accretional allow a study of other aspects of the erosive and atmo! warm epoch discussed above\ this future Titan might spheric history of this world[ For example\ absence of any evolve and support a modest biota until mass loss from craters would suggest a geologically very young surface\ the red giant Sun blows the satellite|s atmosphere away[ with active obliteration processes "either internal or atmo! sphere!driven#[ The shape of the size distribution and features of craters above the atmospheric cuto} diameter may be sensitive to speci_c aspects of erosive and impact 3[ Titan|s surface and pre!biological organic chemistry processes[ For example\ the Martian crater population shows a loss of smaller craters apparently associated with Much speculation has been written on the resemblance of aeolian erosion:transport processes\ and dust in_lling "e[g[ the present atmospheric state of Titan to that of the Earth Hartmann\ 0860\ 0863#\ while surface features around before life began[ The current consensus is that the pre! Venus impact craters appear to give evidence of atmo! biotic Earth|s atmosphere was signi_cantly more oxi! sphere shock wave e}ects "Takata et al[\ 0884#[ While dizing than is the present Titan atmosphere\ but much less Titan may well have aeolian erosion\ an additional source so than the present Earth[ Hence\ terrestrial pre!biotic of crater!_ll is the accumulation of photochemical debris ^ conditions might have been intermediate to the present examination of the morphology of craters over a range of state of the two bodies[ Titan has a prodigious amount sizes\ the determination of the composition of the _ll of organic material which participates in chemical cycles material itself by remote sensing spectroscopy on the Cas! powered by sunlight and "at the 09) level relative to solar sini orbiter will constrain regional and global variations ultraviolet# by cosmic rays[ It is worth asking what level of in the depth of such deposits[ complexity might be achieved in Titan|s organic chemistry The ultimate fate of Titan|s atmosphere is tied to the over 3[4 billion years\ particularly at the atmosphereÐ J[ I[ Lunine et al[ ] Some speculations on Titan|s past\ present and future 0094 surface interface[ Some cosmic rays reach the surface\ the asymmetric sugars\ and the same is generally true of in contrast to solar ultraviolet radiation\ but additional amino acids "though there is very recent evidence from sources of surface energy include cryovolcanism and large Cronin "0886# of a predominance of left!handed types in impacts[ Sagan and Thompson "0881# calculated that a selected amino acids in the Murchison meteorite\ which large impact into an ammonia!water crust would produce Engel and Macko "0886# argue from isotopic evidence is pools of ammonia!water liquid lasting of order 092 years ^ not due to terrestrial contamination#[ even pure water pools might last hundreds of years[ The origin of selected handedness among the precursor Such timescales\ while much less than the duration of molecules of biochemistry may involve trial!and!error liquid water on the pre!biotic Earth\ are much longer whereby\ for example\ RNA assembly is truncated by the than timescales available in terrestrial laboratories for wrong!handed ribose\ followed by fragmentation of the pre!biotic chemical experiments[ Therefore\ the organic strand\ assembly of a longer {{homochiralic|| "single! products of reactions in a transient liquid water pool on handed# strand\ truncation by a wrong!handed ribose Titan might be of keen interest to exobiologists studying again\ etc[ Over long periods of time\ some RNA mol! the transition between organic chemistry and biochem! ecules may by chance have been assembled out of homo! istry[ Because the presence of liquid ammoniaÐwater or chiralic sugars and nucleic acids\ but the timescales may water melts on Titan for long durations "compared to the exceed those accessible to the laboratory[ Likewise\ spatial laboratory# is a surprising result\ a crude {{reality!check|| scales larger than practical in the lab might provide a of the Sagan and Thompson "0881# calculation is pre! greater likelihood for such assembly[ While this is specu! sented here[ They suggest around 0) of the crater volume lation\ it points to the issue that laboratory experiments may appear as impact melt[ Melosh "0878# suggests the in pre!biotic synthesis are limited in spatial and temporal 1 ratio of melt mass to projectile mass is ½9[03vi :Em with scales relative to what is available in natural environ! vi the impact velocity and Em the enthalpy of melting\ ments[ about 1×095 J:kg for ice[ Thus for a 499 m radius impac! The record of such experiments on the pre!biotic Earth tor at 09 km s−0\ which would produce a 09Ð19 km diam! is completely erased\ and the crust of Mars is so oxidizing eter crater\ the melt volume is about 0 km2 "factors of that not much may be preserved there\ either[ Pre!biotic order unity are neglected in this analysis#[ Thus the 0) chemistry under the Europan crust is likely too hard to number is about right[ The conduction!limited cooling access for the foreseeable future "at least by direct time of a body of size D is roughly D1:k where k is the sampling#[ The parent bodies of meteorites may have had thermal di}usivity\ typically 09−5 m1 s−0 for solid pre!biotic aqueous chemistry occur which today is partly materials[ Thus the conductive cooling time "ignoring the recorded in the meteorites themselves\ but the degree of mutually!antagonistic e}ects of convection and latent! preservation of this record of very ancient chemistry is heat release# of a 0 km2 melt body is about 0901 s\ or about unknown\ and the environment within which such chem! 094 years*in good agreement with Thompson and Sagan istry occurred is poorly constrained[ "0881#[ "There are terrestrial examples of chemical pro! Titan\ on the other hand\ is undergoing organic chem! cessing of impact melts ] e[g[ the economically!signi_cant istry today throughout its atmosphere and on its surface[ nickel and copper ore deposits at the Sudbury impact Periodic impacts could allow for aqueous organic chem! structure in Canada were produced by gravitational set! istry on timescales much longer than those in the labora! tling of the sul_de!rich component of the impact melt\ see tory\ and larger spatial scales[ It is possible that the surface e[g[ Grieve and Masaitis ^ 0883[# of Titan records numerous truncated experiments in pre! A speci_c example of a problem in experimental exobi! biotic aqueous chemistry\ including the construction of ology is the formation of ribonucleic acid\ or RNA[ RNA homochiralic molecules of biological interest[ While we is generally assumed to have preceded DNA as the storage do not propose here to examine whether ribose formation molecule for the genetic code\ and also acted as a catalyst itself is even possible in transient aqueous solutions on in the absence of enzymes "for a review see Chyba and Titan\ the point is that the identi_cation of asymmetry in McDonald\ 0884#[ The assembly of RNA without the amount of left! and right!handed forms of any organic recourse to biologically!produced precursors requires a molecules found at Titan|s surface would generate intense number of as yet poorly understood steps[ One of these is interest in the pre!biotic scienti_c community[ Whether the biological synthesis of the sugar ribose\ which on the self!organizing chemical systems developed in certain primitive Earth may have involved the so!called formose environments on Titan is at present pure speculation\ but reaction\ in which a wide variety of sugars are generated Titan|s surface has energy sources\ raw organic materials\ in an aqueous solution of formaldehyde "Shapiro\ 0877#[ and a relative accessibility that together make it a worthy Producing a preponderance of ribose from out of this target for exobiological exploration[ {{sugar forest|| "Chyba and McDonald\ 0884# remains The Huygens probe will be unable to do in depth explo! unsolved[ ration of organic chemistry at multiple sites on Titan|s Beyond the synthesis of ribose itself is the constraint surface ^ its mission is to sample directly the atmosphere\ that for the construction of RNA there must be a sig! take images and spectra of the surface\ and conduct a ni_cant predominance of a particular handedness of the limited range of direct experiments at the impact point[ molecule[ Computer models and laboratory experiments However\ the mission will provide crucial information on show that functional RNA cannot be assembled from a the general nature of Titan|s surface\ both at the probe mixture of left! and right!handed ribose molecules\ yet the descent site and globally through Cassini remote sensing[ di}erence in formation energy of the two is exceedingly The data returned should provide much better insight into small "Joyce et al[\ 0873#[ The non!biological natural the prospects for using Titan as a pre!biotic chemical world does not seem to select for a single handedness of laboratory[ Should Titan prove promising\ follow!on mis! 0095 J[ I[ Lunine et al[ ] Some speculations on Titan|s past\ present and future sions might include multiprobes covering a range of sites\ solar system ] current issues[ Ann[ Rev[ Earth Planet\ Sci[ 13\ or an {{aerobot|| with the capability to ascend and descend 104Ð138[ at several sites on Titan|s surface "Lorenz and Nock\ Coustenis\ A[\ Bezard\ B[ and Gautier\ D[ "0878# Titan|s atmo! 0885#[ Regardless of the particular kind of delivery system sphere from Voyager infrared observations[ II[ The CH2D abundance and D:H ratio from the 899Ð0199 cm−0 spectral chosen\ careful consideration must be given to the kinds region[ Icarus 71\ 56Ð79[ of advanced chemical experiments that could be carried Coustenis\ A[\ Lellouch\ E[\ Maillard\ J[ P[ and McKay\ C[ P[ to Titan[ For example\ the capability to detect optical "0884# Titan|s surface ] composition and variability from the activity "chirality# as the signature of molecules with the near infrared albedo[ Icarus 007\ 76Ð093[ same sense of handedness would open up the possibility Cronin\ J[ "0885# Carbonaceous chondrites ] a window on of discovering how and under what conditions a crucial organic chemistry in the early solar system[ In Astrobiolo`y bottleneck is overcome in the assembly of RNA[ Such Workshop ] Preliminary Report\ ed[ D[ DeVincenzi\ pp[ A07Ð investigations represent a daunting challenge at present\ A08 "abstract#[ NASA Ames Research Center[ Cynn\ H[ C[\ Boone\ S[\ Koumvakalis\ A[\ Nicol\ M[ and Stev! but emphasize the need for continued development of enson\ D[ J[ "0878# Phase diagram for ammoniaÐwater mix! analytic techniques that can be carried to other planetary tures at high pressures ] implications for icy satellites[ In surfaces[ Proceedin`s of the 08th Lunar and Planetary Science Confer! ence\ pp[ 322Ð330[ Lunar and Planetary Institute\ Houston[ De Bergh\ C[\ Lutz\ B[\ Owen\ T[ and Chauville\ J[ "0875# Mon! odeuterated methane in the outer solar system[ III[ Its abun! 4[ Conclusion dance on Titan[ Astrophysical Journal 200\ 490Ð409[ Engel\ M[ H[ and Macko\ S[ A[ "0886# Nature 278\ 152Ð157[ This presentation is a mixture of review and speculations Engel\ S[\ Lunine\ J[ I[ and Norton\ D[ L[ "0883# Silicate inter! concerning the formation\ history and signi_cance of Tit! actions with ammoniaÐwater ~uids on early Titan[ J[ Geophys[ Res[ 88\ 2634Ð2641[ an|s surfaceÐatmosphere system[ Much of the speculation Engel\ S[\ Lunine\ J[ I[ and Harmann\ W[ K[ "0884# Cratering is driven by relatively recent remote sensing data that on Titan and implications for Titan|s atmospheric history[ suggests a diverse surface\ while at the same time failing Planetary and Space Science 32\ 0948Ð0955[ to provide a unique determination of surface physical Grasset\ O[ and Sotin\ C[ "0885# The cooling rate of a liquid state and composition[ We are therefore driven back to a shell in Titan|s interior[ Icarus 012\ 090Ð001[ puzzle that surfaced at the time of the Voyager encoun! Grieve\ R[ A[ F[ and Masaitis\ V[ L[ "0883# The economic poten! ter*how methane is supplied long!term to Titan|s atmo! tial of terrestrial impact craters[ International Geolo`y Review sphere*and forced to consider the possibility that Titan|s 25\ 094Ð040[ Gri.th\ C[ A[ "0882# Evidence for surface heterogeneity on atmosphere and surface state have undergone repeated Titan[ Nature 253\ 400Ð403[ profound changes through their history[ The other driver Gri.th\ C[ A[ and Zahnle\ K[ "0884# In~ux of cometary volatiles of the speculations contained herein is the issue of how to planetary moons ] the atmospheres of 0999 possible Titans[ chemical systems evolve into biochemical systems[ While J[ Geophys[ Res[ 099\ 05\896Ð05\811[ it is impossible to gauge the relevance of Titan|s surface Hartmann\ W[ K[ "0860# Martian cratering III ] theory of crater chemistry to addressing this question\ we can at least point obliteration[ Icarus 04\ 309Ð317[ to Titan as a planet!sized system rich in the biogenic Hartmann\ W[ K[ "0863# Martian cratering IV ] Mariner 8 initial elements on which organic chemistry has proceeded for analysis of cratering chronology[ J[ Geophys[ Res[ 67\ 3985Ð 3005[ the age of the solar system\ and still proceeds today[ We Ivanov\ B[ A[\ Basilevsky\ A[ T[ and Jeukum\ G[ "0886# Atmo! have not previously explored a planet with such charac! spheric entry of large meteroids ] implication to Titan[ Plan! teristics "leaving aside the Earth which surface is domi! etary and Space Science 34\ 882Ð0996[ nated by biology#[ Surprises must await us[ Joyce\ G[ F[\ Visser\ G[ M[\ van Boeckel\ C[ A[ A[\ van Boom\ The CassiniÐHuygens mission\ a follow!on to the Voy! J[ H[\ Orgel\ L[ E[ and van Westrenen\ J[ "0873# Chiral selec! ager ~ybys\ is at the same time a precursor to the kinds tion in poly"C#!directed synthesis of oligo"G#[ Nature 209\ of in!depth chemical explorations that will be needed to 591Ð593[ understand fully the processes ongoing at the surface of Kargel\ J[ S[\ Croft\ S[ K[\ Lunine\ J[ I[ and Lewis\ J[ S[ "0880# Rheological properties of ammoniaÐwater liquids and crys! this giant moon[ CassiniÐHuygens is superbly equipped to talÐliquid slurries ] planetological applications[ Icarus 78\ 82Ð provide us with the information needed to assess whether 001[ Titan is a worthy target for further exploration of plan! Lara\ M[\ Lorenz\ R[ D[ and Rodrigo\ R[ "0883# Liquids and etary evolution\ climate change\ and the processes that solids on the surface of Titan ] results of a new photochemical lead to the formation of life[ model[ Planetary and Space Science 31\ 4Ð03[ Lecluse\ C[\ Robert\ F[\ Gautier\ D[ and Guirand\ M[ "0885# Deuterium enrichment in giant planets[ Planetary and Space Acknowled`ements[ The authors acknowledge the support of the Science 33\ 0468Ð0481[ NASA Planetary Atmospheres and Geoscience Programs and Lemmon\ M[ T[\ Karkoschka\ K[ and Tomasko\ M[ "0884# the Cassini project in the preparation of this work[ Titan|s rotational lightcurve[ Icarus 002\ 16Ð27[ Lorenz\ R[ D[ "0885# Pillow lava on Titan ] expectations and constraints on cryovolcanic processes[ Planetary and Space Science 33\ 0910Ð0917[ References Lorenz\ R[ D[ and Lunine\ J[ I[ "0885# Erosion on Titan ] past and present[ Icarus 011\ 68Ð80[ Bar!Nun\ A[\ Kleinfeld\ I[ and Kochavi\ E[ "0877# Trapping of Lorenz\ R[ D[ and Lunine\ J[ I[ "0886# Titan|s surface reviewed ] gas mixtures by amorphous water ice[ Phys[ Rev[ B 27\ 6638Ð the nature of bright and dark terrain[ Planetary and Space 6643[ Science 34\ 870Ð881[ Chyba\ C[ and McDonald\ G[ D[ "0884# The origin of life in the Lorenz\ R[ D[ and Nock\ K[ T[ "0885# BETA!Balloon experi! J[ I[ Lunine et al[ ] Some speculations on Titan|s past\ present and future 0096

ment at Titan[ In Proceedin`s\1nd IAA International Con! Orton\ G[ S[ "0881# Ground!based observations of Titan|s ther! ference on Low!Cost Planetary Missions[ John Hopkins Uni! mal spectrum[ In Proceedin`s Symposium on Titan[ Toulouse\ versity\ Maryland\ 05Ð08 April 0885\ Paper IAA!L!9595[ France\ 8Ð01 September 0880\ ESA SP!227\ ESA Pub! Lorenz\ R[ D[\ McKay\ C[ P[ and Lunine\ J[ I[ "0886a# Pho! lications Division\ ESTEC\ Noordwijk\ Netherlands\ pp[ 70Ð tochemically!driven collapse of Titan|s atmosphere[ Science 74[ 164\ 531Ð533[ Owen\ T[ "0871# The composition and origin of Titan|s atmo! Lorenz\ R[ D[\ Lunine\ J[ I[ and McKay\ C[ P[ "0886b# Titan sphere[ Planetary Space Science 29\ 722Ð727[ under a red giant Sun ] a new kind of {{habitable moon||[ Pinto\ J[ P[\ Lunine\ J[ I[\ Kim\ S[!J[ and Yung\ Y[ L[ "0875# The Geophys[ Res[ Lett[\ 13\ 1894Ð1397[ D and H ratio and the evolution of Titan|s atmosphere[ Lunine\ J[ I[ "0885# Physics and chemistry of the surfaceÐatmo! Nature\ Lond[ 208\ 277Ð289[ sphere systems of Titan\ Triton and Pluto[ In ERCA Vol 1] Prinn\ R[ G[ and Fegley\ B[ "0870# Kinetic inhibition of CO and

Physics and Chemistry of the Earth and Other Objects of the N1 reduction in circumplanetary nebulae ] implications for Solar System\ ed[ C[ Boutron\ pp[ 346Ð363[ Les Editions de satellite composition[ Astrophysical Journal 138\ 297Ð206[ Physique\ Les Ulis\ France[ Shapiro\ R[ "0877# Prebiotic ribose synthesis ] a critical analysis[ Lunine\ J[ I[ "0886# Planet Earth ] Evolution of a Habitable World[ Ori`[ Life Evol[ Biosph[ 07\ 60Ð74[ Cambridge University Press\ in press[ Smith\ P[ H[\ Lemmon\ M[ T[ Lorenz\ R[ D[\ Sromovsky\ L[ A[\ Lunine\ J[ I[ and Lorenz\ R[ D[ "0886# Light and heat in cracks Caldwell\ J[ J[ and Allison\ M[ D[ "0885# Titan|s surface\ on Europa ] implications for pre!biotic synthesis[ LPSC 17\ revealed by HST imaging[ Icarus 008\ 225Ð238[ 744Ð745[ Stevenson\ D[ J[ "0871# Volcanism and igneous processes in Lunine\ J[ I[ and Stevenson\ D[ J[ "0871# Formation of the small icy satellites[ Nature\ Lond[ 187\ 031Ð033[ Galilean satellites in a gaseous nebula[ Icarus 41\ 03Ð28[ Stevenson\ D[ J[ "0881# Interior of Titan[ In Proceedin`s Sym! Lunine\ J[ I[ and Stevenson\ D[ J[ "0876# Clathrate and ammonia posium on Titan\ Toulouse\ France\ 8Ð01 September 0880\ hydrates at high pressure ] application to the origin of meth! ESA SP!227\ ESA Publications Division\ ESTEC\ Noord! ane on Titan[ Icarus 69\ 50Ð66[ wijk\ Netherlands\ pp[ 18Ð22[ Lunine\ J[ I[ and Tittemore\ W[ B[ "0882# Origins of outer planet Stevenson\ D[ J[\ Harris\ A[ W[ and Lunine\ J[ I[ "0875# Origins satellites[ In Protostars and Planets III\ ed[ E[ H[ Levy and J[ I[ of satellites[ In Satellites\ ed[ J[ Burns and M[ S[ Matthews\ Lunine\ pp[ 0038Ð0065\ University of Arizona Press\ Tucson\ pp[ 28Ð77[ University of Arizona Press\ Tucson\ AZ[ AZ[ Takata\ T[\ Ahrens\ T[ J[ and Phillips\ R[ J[ "0884# Atmospheric Lunine\ J[ I[\ Atreya\ S[ K[ and Pollack\ J[ B[ "0878# Evolution cratering e}ects on Venus[ J[ Geophys[ Res[ 099\ 12\218Ð of the atmospheres of Titan\ Triton and Pluto[ In Ori`in and 12\237[ Evolution of Planetary and Satellite Atmospheres\ ed[ S[ K[ Toublanc\ D[\ Parisot\ J[ P[\ Brillet\ J[ and Gautier\ D[ "0881# Atreya\ J[ B[ Pollack and M[ S[ Matthews\ pp[ 594Ð554[ Modeling of the diurnal variations of the atmosphere of University of Arizona Press\ Tucson\ AZ[ Titan[ In Proceedin`s Symposium on Titan\ Toulouse\ France\ Lunine\ J[ I[\ Engel\ S[\ Rizk\ B[ and Horanyi\ M[ "0880# Sub! 8Ð01 September 0880\ ESA SP!227\ ESA Publications limation and reformation of icy grains in the primitive solar Division\ ESTEC\ Noordwijk\ Netherlands\ pp[ 020Ð025[ nebula[ Icarus 83\ 222Ð232[ Van Dishoeck\ E[ F[\ Blake\ G[ A[\ Draine\ B[ T[ and Lunine\ J[ I[ Lunine\ J[ I[\ Stevenson\ D[ J[ and Yung\ Y[ L[ "0872# Ethane "0882# Chemical evolution of protostellar and protoplanetary ocean on Titan[ Science 111\ 008Ð019[ matter[ In Protostars and Planets III\ ed[ E[ H[ Levy and J[ McKay\ C[ P[\ Pollack\ J[ B[ and Courtin\ R[ "0878# The thermal I[ Lunine\ pp[ 052Ð130[ University of Arizona Press\ Tucson\ structure of Titan|s atmosphere[ Icarus 79\ 12Ð42[ AZ[ McKay\ C[ P[\ Pollack\ J[ B[\ Lunine\ J[ I[ and Courtin\ R[ "0882# Yung\ Y[ L[\ Allen\ M[ and Pinto\ J[ P[ "0873# Photochemistry Coupled atmosphereÐocean models of Titan|s past[ Icarus of the atmosphere of Titan ] comparison between model and 091\ 77Ð87[ observations[ Astrophysical Journal Suppl[ Ser[ 44\ 354Ð495[ Muhleman\ D[ O[\ Grossman\ A[ W[\ Butler\ B[ J[ and Slade\ Zahnle\ K[\ Pollack\ J[ B[\ Grinspoon\ D[ and Dones\ L[ "0881# M[ A[ "0880# Radar re~ectivity of Titan[ Science 137\ 864Ð Impact!generated atmospheres over Titan\ Ganymede\ and 879[ Callisto\ Icarus 84\ 0Ð12[