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NITROGEN ION CLUSTERS in TRITON&Apos;S ATMOSPHERE

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NITROGEN ION CLUSTERS in TRITON&Apos;S ATMOSPHERE GEOP}!YSiCAI. RESEARCI! I.ETTERS, VOL. 17, NO. 10, PAGES I725-1728, SEPTEMBER 1990 NITR•EN ION CLU• IN TR1TON'S ATMOSPHERE Mona L Delitsky JetPropulsion Laboratory, California Institute of Technflogy,Pasadena, CA Richard P. Tar• Depaxtmentof Atnx)sphericSciences, University of California,Los Angeles, CA Mark Z. Jacobson Departmereof Atmo•h•c Sciences,University of California,Los Angeles, CA Abstract. The formation of nitrogen cluster ions composition,themodynamics and meteorologyof Triton's [•2*(•h, N*(N2)• shouldocmar easily in Triton'sthin cold atmosphere. •-•gen atmosphere.These ions are formed stepwiseas: •on Ch• As on Earth, the interaction of cosmic-rays or At lowaltitudes, themaxxtynamics (temperature and press•) precipitatingmagnetospheric particles with nitrogem mo•u!es t avc•cluster sizes up to andincluding N2+0•2)4o. Such ions results in the formation of ions and neutral atoms: • closem the threshold(critical size) to nucleate,forming a •-'• Mtrogenice aerosolseen as exteadedhazes in Voyager N2+e* ---> N•+e+e (1) '• .mage•of Tri•n's limb.The critical ion radius for nucleation N2 + e* ---> N + + N + e+e is about 8.3 A. As temperatureincreases and pressure ....deranges with altitude, cluster ion formation becomes less The primaryions, N2+ and N+, are transformedinto f .;•bte and small clustersizes predominate. At altitudes largerions by clust•ing with N2 in a seriesof steps(Bobroe et >200•, clus• are thexmodynamical!yunstable and only al., 1969;HeadIcy et al., !982): •* :andN* am present.Because electron •bination with • 'runs(N +) is very slow,a denseionosp •here should form N2+ + N2+ M '-• N2+(N2)+ M •ere N* recombination via N* clustering with N2 is -•mfavorable. Model calculations yield peak electron coaceatra'uonsabove 200 kin, -asdetected by Voyagerradio ,oe••on measure•ts. The sha• ledgeat the bouomof N2+(N2)+ N2 + M • N2+(N2)2+ M (2) '•:-.•"maos•ea• is causedby the onsetof rapid electron-ion .., • .r•m•inaxkm•ted with cluster ion formation. N2+(N•+ N2+ M • N2+(N2)•t+M The first close-upimages taken by Voyager 2 of (speciessuch as N2+(N2)2can alsobe writtenN•+). Each ,•.:me's satelliteTriton revealeda thin, hazy atmosphere. forwardclusr•ing re,on sty: (k•) hasan associated reverse CIoMa,pa•ehes of fogand local amx•heaSc phenomena such dissociationreaction step (1•). Hence,the clusters will igrow vmting plumeswere obse•ed, •dicating that weather until they reach an equilibrium distribution of s•zes, •• oa thissatellite. The atmosphereis madeup mostlyof con-espondingto the local teanpemtum and pressure, in whi,•ch •..imgen,with small amountsof methane,as predictedby cluster formation and thermal diq•odation rates are in .balance. -.••ank et al. (!983, 1984) six years earlier, and The resultingdistribution of cl.m• size•is gaussian-like,with •.• by 'theVoyager UVS experiment(Broadfoot et al. thepe• correspondingto the thermodymcally "pref--" •,•),These hazes and clouds in theatmosp• resultfrom fluctuationsin the thermodynamicstate of the The thermodynamicsof nitrogencluster formation is •phem. which induce nitrogen to nucleate on ions describedby Hariokaand Nakajima (1988). The enthalpyof ,:•• bycosmic ray- or magnetospirit partiQe-induced formation of NJ from N2+ (reaction1) is very l'd•, 25.8 'ti•, or on solidparticles ejected by Tritons geysers. kcal/mol½,and the reactionis stronglyfavored at low paperdescri•s thenitrogen ion chemistrythat controls !m•aperatm'es.The forward rate coefficient is givenby Boh•r•e '•c composition,and may be responsive for the (!969) as1.9 x 10'e• cmS/mole•-sec at280K; a teanpexamm- dependentrate coefficient,1.7! x 10'2½ (300/T)I's Thevolat-fie gases - in • caseN2 and C• -- will tend cmS/mo!½c2-•c,was derived from the data of Boltme. Highex-ordexclusters have a lowerenthalpy of frm!mtion(for -,mupemunu,which is defied bythe radiative balance •pl½, 2.76kca!/mole for the clustering ofNJ ,.d-•,•s of Triton'satmosph•. The temuperature atthe theirentropy v•ues remainalmost cons.rant from oae s•ze to •• ofTriton w:as detcm•2ned to be roughly 37 K (Conruth tl• neat.Nev•½ss, higherclusters can • easilyform at i:'989),giving a nitr•ea sinfacevapor procure of about the cold temperams of Triton'slower atmo,•:•. The 10•!:'0 's bar, .and• ptussureof roughly 7x10 -m bar. As a •ui!ilxiam conrot for • dusmdngstep .'m givm by, of this:• of • press=cs,nitrogen d.omi• the C•y•i•t 1990 by ,•he American Geophysical Union. whexeK2,, 'm .the .•uilibrium cons/mr for ..the .nth clust•g step,[ ] '•tes a concmtmti.on•m_too. numberß 90GL01732 Gi•bbsfree energychange for '•e reacfoa, • ka is the '•,-8276/90 / 9•L-0173250'3. •, Bo!tzmanncous-rot. it f .o!!owsfrom Eq. (3) 1725 1720 Dclit.,,k5ct ztl.: Nitrogen!on Clustersin Triton's Atmosphere clustersize increaseswith decreasingtemperature and interactionreduces the free energy of the cluster (see below) increasingnitrogen partial pressure, and thus varies rapidly andallows the formation of stableembryos. Heterogenm-m with altitude. nucleationof nitrogenabove geysers would produce locali• Clusteringreactions with N+ asthe coreion (K•,,•, hazes.Ion-induced nucleation could occur wherever clus•m n=l ....) yield a seriesof ionswith anodd number of nitrogen ionsare found, provided the atmosphere is sufficiently atoms -- N3*, Ns+,N7+, etc. This series of clusters has a supersaturated(perhaps as a resultof dynamicalforcing). slightlydifferent rate constant for formation(Bohme, 1969), Hence,global haze layers seen in theVoyager Triton irra•s but in generalbehaves similarly to the N2*-basedclusters. rnayhave theirorigin in ion nucleation. Recombinationof the N + seriesof ionswith electrons yields N The freeenergy expression for ion-inducednuclmti• atoms,which eventually recombine to form N2. However,as upona sphericalcluster of radiusr containingn nitrogen thisprocess is fairly slow,some build-up of N atomsin the molecules(noting that n=4/3•rr3p/mo, where p is thecluster Triton atmosphereis expected.In fact, ultravioletemissions densityand mo is the molecular mass of nitrogen)isgiven •, fromN* as well asN atomswere detected at thetop of the atmosphereby theVoyager UVS experiment(Broadfoot et al., - 1989). AG(r)=-nk•TlnS +4,nrr2o'+ /•-•X 1-••• •) Forward rate coefficients in each cluster series were whereri is the radiusof the ionic core of the cluster,wi-• assumedto be similarfor all clusteringsteps, being equal to chargeq, ande is thedielectric constant. The firstterm re•t• the valuesof Bohme(1969) for the initial N2 association the supersaturation(S), (i.e., theratio of thevap• •a reactionswith N2+ and N +. The reverserate coefficientswere pressureto the thermodynamicequilibrium va•r pressam calculatedusing the forward rate coefficient and equilibrium overthe bulkcondensed material) to the freeenergy, aM t.M constantfor eachclustering step (see Eq. 3). The equilibrium second term takes into account the surface tension of constantswere calculatedusing the thermodynamicdata of nucleation embryo. Nitrogen has a much lower surf• Hariokaand N-_akajima (1989). Largeclusters have enthalpies tension(10.5 dynedcm) than does water (80 dyne/cm);•, and emropiesof reaction,AH andAS, respectively,that nucleationand particle fo,wnation is 'easier' in nitrogencl,-•s approachthe latentheat of condensationand the entropy than in water clouds under the same relative conditionsof change,respectively, for condensationof N2 (Harioka and supersaturation(i.e., the positive surface tension term in F•. 4 Nakajima,1988); the clustersare then actingas liquid is smaller for nitrogen). Since the radius of the clusteris surfacesfor N2condensation. Accordingly, all clust-erslarger always larger than the ionic core radius, the electrostatic thanN24' were assignedbulk valuesfor the enthalpyand conu-ibutionto the free energyis negative,and ion-irdm• entropyof formation. nucleati• is favoredover homogeneous nucleation. The clusteringof N atomswith N2+ was alsoincluded Tbeion-induced nucleation rate can be expressed m: usinga ramcoefficient of 2.5x10-29 cm6/molec2-sec at 300 K (Good, 1975). (The reaction was assumedto have a similar temperature-dependenceasfor clusteringreactions of N2 with j=/½[N2•(r,)Z(r a )Nia 'ex•.AG(ra)-AG(F*)] ksT (5) ions.)Charge exchange reaction rate coefficients (e.g., for N In thisequation, kc is the collisioncoefficient for N2 with + N2+ --> N + + N2) weretaken from Delitsky et al. (1989). clusters,r* is the critical size that a cluster must reach be• The recombination coefficient for N2+ ions with cannucleate and grow into a droplet,ra is theprefened clua• electronsis 3.5x10 -7(300fF) ø• cm3/nmlec-sec(Krasnopolsky, size (i.e., the maximumin the distributionof sizes),Z(r*) 1986). All cluster ion-electron recombination reactions were the Zeldovich factor (which is a correction for the s.• assumedto have a rate coefficientof 3.0x10-6(300,rI')ø-• departurefrom an equilibriumdistribution of clustersizes dm to nucleationof the largestions), Z(ra) accountsf•- crn3_hnolec--•. Atomic ion-electron recombination, occurring equilibriumdistribution of clustersizes, Nion is the at low pressuresvia photonemission, is at least 105 times ion concentration,and the exponential term t•es intoac•omt slowerthan molecular ion-electron dissociative recombination; thedifferen• in .freeenergy between the preferred clus• :• the N + recombinationcoefficient is ~10 '12 cm3/molec-sec. andthe critical size for nucleationand phase change; if Hence,N + losscan be dominated by clusteringto N2 followed differencein energiesis large,then the ion clustersare by electron/cluster-ionrecombination (- 10 -6
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