47th Lunar and Planetary Science Conference (2016) 2234.pdf

OCEAN SURVIVAL, II FORMATION AND RECENT TECTONIC ACTIVITY ON PLUTO. N. P. Hammond1, A. C. Barr2, and E. M. Parmentier1, 1Department of Earth, Environmental and Planetary Sciences, Brown University, 324 Brook Street, Providence RI 02912, [email protected]. 2Planetary Science Insti- tute, Tuscon AZ, 85719.

Introduction: Pluto is one of the largest Kuiper mation of CAIs [8]. We vary the silicate conductivity Belt objects, with a radius of � ≈ 1185 km [1, 2] and a and silicate density to understand their influence on the bulk density of � ≈1.85 g/cm3 [1], suggesting a mixed present day internal structure. The density of the sili- ice-rock composition [3,4]. The New Horizons flyby cate core is varied between 2.45g/cm3 (similar to the on July 14 2015 revealed incredible geologic diversity inferred density of the core of Enceladus [9]) and 3.5 on Pluto’s surface [1]. In addition to very recent geo- g/cm3 (appropriate for compressed carbonaceous logic resurfacing in Sputnik Planum, where nitrogen chondrite [4]). The thickness of the ice shell is varied and carbon monoxide may be flowing, New Hori- with silicate density to match the bulk density of Pluto. zons images also hint at recent geologic activity in the In the core, we use heating from long-lived radioiso- ice shell [1]. Large “mountains” on the western topes with isotopic abundances appropriate for CI and southern margin of Tombaugh Regio as well as Chondrites [6,10]. We vary the silicate conductivity, kr, scarps and troughs in Cthulu Regio suggest tectonic between 1 to 4 W/kg-K [6,7,11,12]. activity in the water ice “bedrock” [1]. We use a temperature dependent thermal conduc- What is the driver of this recent tectonic activity? tivity for ice, �!"# = 651/� W/kgK [13], but limit the One possibility is global volume change due to conductivity in the top 5 km to 1 W/kgK to account for changes in the ice shell [5]. If Pluto has an ocean that the effect of increased porosity [14]. The surface tem- is slowly freezing, it would cause global volume ex- perature is constant at �! = 40 K. Everywhere in the pansion and extensional stresses at the surface. Con- satellite, the initial temperature �! = 150 K. versely, if is converting into a denser phase such Melting at the base of the ice shell can occur if the as ice II, it would lead to global volume contraction. heat flow out of the core exceeds the maximum heat Robuchon and Nimmo [6] modeled the thermal evolu- flow that can be transferred conductively through the tion of Pluto and find that if the ice shell is conductive, ice shell. After an ocean forms, the ocean boundary an ocean would form and survive to the present, lead- moves upward at a rate, !" ! !! ing to a present day extensional tectonic regime. How- = !!! !, ever, this result may be sensitive to the density and !" !" where � is the heat flow at element � and � is the latent thermal conductivity of the silicate core. For example, heat of water ice. We use a pressure dependent melting higher thermal conductivities would allow heat to es- temperature [15] and assume an isothermal ocean. We cape the core more efficiently and lead to colder final benchmark our model against exact solutions to the states [e.g. 7]. It is necessary to vary these parameters Stefan equation [16]. over a range of reasonable values to understand their We also treat the phase transition between ice Ih effect the Pluto’s thermal evolution. and ice II, a dense polymorph of ice with a rhombohe- Here, we simulate the thermal evolution of Pluto to dral structure and a density � = 1.18 g/cm3 [17]. The understand its present day internal structure and tecton- !! equilibrium temperature (T ) between these phases is ic environment. We find that the thermal conductivity eq described by and density of the silicate core strongly control Pluto’s � = � − 14 0.827, thermal evolution. Changes in these parameters lead to !" two different thermal evolution pathways: 1) Pluto where � is pressure in MPa and temperature is in Kel- forms an ocean that survives to present day or 2) the vin [18, 19]. As temperature drops, ice II becomes the ocean freezes and ice II forms at depth. These scenari- stable phase in the ice shell. For likely pressures at the os lead to distinct types of tectonic activity at the sur- base of Pluto’s shell, ice II begins to form when tem- face that may be observable by New Horizons. peratures drop below ~170 K. The reaction of ice Ih to Methods: We have constructed a spherical one- ice II liberates ~1 kJ/mol [20]. dimensional, finite difference model of the thermal Results: So far, we have examined ~50 histories of evolution of Pluto. We assume Pluto is differentiated Pluto’s thermal evolution and tracked several im- into a silicate core and a water ice shell [3,4,6]. Initial- portant parameters over time, including: 1) the surface ly, we assume that both the core and ice shell transmit heat flux, 2) ocean thickness, 3) ice II thickness, 4) heat conductively. We begin our simulations just after surface stress and 5) surface strain. In all of our simu- lations so far, the surface heat flux achieves a maxi- the Pluto-Charon system has finished its tidal evolution 2 into the dual synchronous state, 100 Myr after the for- mum 5 – 6 mW/m , and exponentially decays to ~3 47th Lunar and Planetary Science Conference (2016) 2234.pdf

mW/m2 at present. Figure 1 shows the results of a sim- ulation with rock density 2.75 g/cm3 and thermal con- ductivity 3 W/kg K. An ocean forms at 650 Myr, reaches a thickness of ~100 km but then freezes by 3.5 Gyr. In the last ~200 Myr, ice II begins to form deep in the ice shell. This occurs because the ice shell cools sufficiently to permit the plutotherm to cross the phase boundary. A layer of ice II, 80 km thick, forms. This causes a global surface contraction of ~3% and com- pressional stresses of up to 100 MPa, sufficient to drive the formation of global tectonic features. The persistence of the ocean and the formation of ice II depend strongly on the thermal conductivity and densi- ty of the silicate core.

Figure 2: Present state of the ice shell as a function of silicate con- ductivity and H O layer thickness. Green and blue contours show the 50 2 Ice Ih thickness of the ice II layer and black lines show contours for the ocean thickness. 100 Resurfacing. In colder scenarios where ice II forms, Ice II Ice II large surface contractions occur in the last few 100 150 Depth (km) Myr. This scenario could account for recent tectonic activity suggested by New Horizons if the mountains 200 Ocean on the margin of Tombaugh Regio are the result of compressional uplift. Conversely, if the surface mor- 250 1000 2000 3000 4000 phology suggests recent extensional tectonic activity Time(Myr) on a global scale, it could suggest that global volume Figure 1: A cross section of Pluto’s ice shell through time, with expansion is occurring and an ocean has persisted to � = 3 W/kgK and � =2.75 g/cm3. White, green and blue represent ! ! present day. ice Ih, ice II and liquid water, respectively. Acknowledgements: This work is supported by

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