Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEpu

Ruiz Pérez, Javier

Torices Hernández, Angélica

Docta Complutense

STRUCTURE OF THE UPPER ICE MANTLE OF TRITON.   J. Ruiz1 and A. Torices2, 1Departamento deGeodinámica, Universidad Complutense de Madrid, 28040 Madrid, Spain, jaruiz@eucmax.sim.ucm.es. 2Facultadde Ciencias Geológicas, Universidad Complutense de Madrid, 28040 Madrid, Spain.Introduction:  Triton’s images, obtained byVoyager 2 spacecraft in 1989, showed that thisNeptune’s satellite have a relatively young surface, ofmaybe ~1 Gyr [1]. Recent analysis of impact cratersabundance and size suggests a lower age; ~100 Myr ifthe impactors come from Kuiper Belt [2]; lesser than10 Myr, if the asymmetry in craters distributionbetween hemispheres, as indicative of cratering whichhas been caused exclusively by a planetocentricpopulation of objects, is taken into account [3].Therefore, it seems probable that Triton isgeologically active at present, and even could have aninternal ocean [2,4]; the materials, resposible of thesurface renovation, would come from it.This cryovolcanic activity is in accordance withthe presence of surface structures that have beeninterpreted as calderas, cryoclastic deposits, flows, etc[1,5]. This activity would include different types andcombinations of explosive and effusive eruptions, andassuming the morphologic similarity betweenterrestrian volcanic structures and Triton’s, it has beensuggested that they have been formed by similarphysical processes, in connection with comparablephysical properties which would control themorphology and behaviour of eruptions [5]. It hasbeen suggested that the likely eutectoid cryomagmacomposition, which is present in Triton couldcorrespond to ammonia-water, ammonia water-methanol, or ammonia-water-non polar gas series [6].Geomorphologic similarity between them can arisepartly from a rheologic similarity between ammonia-water-methanol series and silicate series ranging frombasalt to dacite [6].From estimates of the bulk composition of Tritonand the preservation of topography, the most abundantand likely cryomagma component is H2O [5]. Othersurface components, that are present, are CH4, N2,CO, y CO2 [7]. Even Triton`s crust is covered bynitrogen and other volatiles-rich-frost, is probably thatthe dominant component is H2O because the rest ofcomponents, mentioned above, are non polar orweakly polar molecular liquids which without watercan not formed rigid solids stable against sublimationor melting [6].In this work, we analyse the relation betweenpressure and temperature in the conductive part ofTriton`s ice layer, its consequences in relation tostructure of the upper part of ice mantle, the possibleexistence of an internal ocean and the maintenance ofmagmatic activity until present.Method: It is assumed that Triton is differenciatedin a thick ice layer and a rock and metal core  [1,8]. Ifthere are different substances to water ice in the icelayer, it is considered that they don not affect to den-sity and thermal conductivity. For the calculation ofpressure in depth, it is assumed a constant ice I andice II density: 940 kg m−3 and 1180 kg m−3 respec-tively. Triton’s mass and radius are taken as in [9].In conductive regime, the variation of tempera-tures in function of radius, r, can be described fromFourier’s law, dT = − ( Fr / k ) dr, where Fr is the heatflow in r, and k is the thermal conductivity. If we con-sider a spherical ice layer, heated from below (fromradiogenic heating in the core), in energetic equilib-rium situation, Fr = F R2 / r2, where F is the surfaceheat flow, and R is the body radius. In this work, a Fvalue of 3,3 mW m–2 is adopted [10], which corre-sponds to a chondritic core composition. k is a func-tion of temperature; here the relation k = k0 / T will beused, where k0 is a constant. If we replace both func-tions in the Fourier’s law, the temperature profile isobtained by integration from R until r,T r = TR  exp [ R F ( R / r – 1 ) / k0 ] .                 (1)For ice I k0 = 567 W m–1 [11]. For ice II k0 = 420 Wm–1 is taken, value that adjust properly the thermalconductivity given in [12]. Finally, the boundary be-tween ice I and ice II stability fields is described byPI−II  ( MPa )  =  0.92 T ( K ) − 6.26 ,                 (2)[13], where PI −II is the boundary pressure for T.Possibility of an internal ocean: The radiogenicheating, seems enough to raise, in deep interior, theice temperature over ~176 K  that correspond to thebeginning of melting in the ammonia water system, orover ∼153 K  that is the beginning of melting in am-monia-water-methanol system, and therefore, if am-monia is certain importance mantle component, aninternal ocean of eutectoid composition could exist, orat least a significant melting grade [9]. But on theother hand, due to dependence of the thermal conduc-tivity of Ice I from temperature, Triton’s ice mantlecould be dominated by ice II [14]. The ammonia-watermelt would float easyly in ice II, and should be foundfrozen on the phase transition level between ice I andice II (due to be some denser than water ice I ) [14].Lunar and Planetary Science XXXI 1298.pdfSTRUCTURE OF MANTLE OF TRITON:  J. Ruiz and A. ToricesFigure 1.  P-T curves for Ts values of 38 K (blue) and55 K (red). Convection is not considered in the draw.To have an internal ocean that is stable at presentagainst freezing, it looks necessary that low heat con-ductivity surface ices raise the effective temperatureon surface [9]. In this sense, we find that if we startfrom a surface temperature of 38 K, an ice I layerwould experience transition to ice II at ~60 km undersurface, and at ~54 K of temperature. For surface ef-fective temperatures of ~50 K, the pressure-temperature curve (P-T) in the conductive ice don notcut any more the phase transition between ice I and II.Ice II could be present in the deepest part of mantleonly, and under ice I in convection (with temperaturethat should allow melting in presence of ammonia).P-T curve in a conductive water ice II mantle: Ifeffective surface temperature is lesser than ~50 K,transition from ice I to ice II should be produced torelatively little depth. We have analyzed the relationbetween pressure  and temperature in the conductiveice II under the external ice I layer. Ice II thermalconductivity is approximately ¾ of ice I thermal con-ductivity, therefore, the elevation of temperature withdepth is greater. It leads to a paradoxical situation.The P-T curve cut again the phase transition betweenice II and ice I. It happens to a greater depth and tem-perature how much minor is Ts. For Ts = 38 K, thephase transition would be reached at ~190 km ofdepth and ~196 K of temperature. In medium sizesatellites, like Rhea, it has been suggested that P-Tcurve, once that reach the boundary between stabilityfields of ice I and ice II, would be placed on phaseslimit, due to the change in thermal conductivity [15].But in the case of Triton, its much greater mass donnot allow this possibility.One might ask what real sense this situation has,because it results gravitationally unstable. At first, itcould be avoid if ice II were in convection at lesserdepth and temperature than transition from ice II toice I. But even so, if we start from a Triton`s interior,which is enough hot so that the ice II only can existunder ice I in convection, from (1) and (2) it can beinfered that with the cooling of the satellite, the ice IIphase would begin to grow between conductive ice I.On the other hand, it has been pointed out that certainfeatures, interpreted as having a volcanic origin (aslinear y rugged ridges) could have been formed inrelation with mantle’s plumes [5]. If it is right, thesemantle’s plumes could have its origin in the men-tioned instability, though this possibility must be con-sidered with caution.Conclussions:  The presence or absence of lowthermal conductivity ices on surface or near it, has adecisive importance in the understanding of internalstructure and the maintenance of geological activity inTriton. If these ices have the effect of elevating effec-tive surface temperature in a dozen of degrees ,ormore, the interior would be enough hot to allow theexistence of an internal ocean. If, on the contrary,effective surface temperature is not elevated signifi-cantly, then, the internal structure could evolve to aunstable gravitationally situation.One future work line consists of modeling thecooling of an ice I mantle in Triton, and the possiblegrowth of ice II in the interior of ice I, to investigate ifthe associate instability can make some role in themaintenance of geological activity and the surfacerenovation.References:  [1] Smith, B.A. et al. (1989), Science246, 1422-1449. [2] Stern, S.A. and McKinnon, W.B.(1999), LPSC XXX, #1766. [3] Zahnle, K. et al.(1999), LPSC XXX, #1776. [4] Schenk, P. and Sobi-eszczyk, S. (1999), BAAS 31, No 4. [5] Croft, S.K. etal. (1995), in Neptune and Triton (D.P. Cruikshanked.), 879-947. Univ. Arizona Press, Tucson. [6] Kar-gel, J.S. (1995), Earth Moon Planets 67, 101-113. [7]Brown, R.H. et al. (1995), in Neptune and Triton(D.P. Cruikshank, ed.), 991-1030. Univ. ArizonaPress, Tucson. [8] Shock, E.L. and McKinnon, W.B.(1993), Icarus 106, 464-477. [9] McKinnon, W.B. etal. (1995), in Neptune and Triton (D.P. Cruikshank,ed.), 807-877. Univ. Arizona Press, Tucson. [10]Brown, R.H. et al. (1991), Science 251, 1465-1467.[11] Klinger, J. (1980), Science 209, 271-272. [12]Ross, R.G. and Kargel, J.S. (1998), in Solar systemices, (B. Schmitt, C. De Bergh, and M. Festou, eds.),33-62, Kluwer Academic, Dordrecht. [13] Anderson,J.D. et al. (1987), JGR 92, 14.877-14.883. [14]McKinnon, W.B. and Mueller, S. (1993), BAAS 25.[15] Lupo, M.J. and Lewis, J.S. (1987), Icarus 40,125-135.Ice I 20601001401802200 40 80 120 160 200Pressure (MPa)Temperature (K)Ice IILunar and Planetary Science XXXI 1298.pdf

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Structure of the upper ice mantle of Triton

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Structure of the upper ice mantle of Triton

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