A Giant Impact Originof Pluto-CharonRobin M. CanupPluto and its moon, Charon, are the most prominent members of the Kuiperbelt, and their existence holds clues to outer solar system formation pro-cesses. Here, hydrodynamic simulations are used to demonstrate that theformation of Pluto-Charon by means of a large collision is quite plausible. Ishow that such an impact probably produced an intact Charon, although it ispossible that a disk of material orbited Pluto from which Charon later accu-mulated. These findings suggest that collisions between 1000-kilometer-classobjects occurred in the early inner Kuiper belt.The Pluto-Charon pair shares key traits withthe Earth-Moon system. Each satellite massis substantial compared to its planet: Charon_smass is 10 to 15% of Pluto_s mass, and theMoon_smassisÈ1% of Earth_s mass. Allother satellite-to-planet mass ratios in our so-lar system are less than È2 10j4. Theorbits of Charon and the Moon are consistentwith a scenario in which each satelliteformed much closer to its planet, and torquesdue to tides raised on the planet by the sat-ellite subsequently caused the satellite_sor-bit to expand to its current separation. Theangular momentum of both pairs is large,within a factor of several of the critical angu-lar momentum for rotational stability of a sin-gle object containing the total system mass.In both cases, an origin by Bgiant impact,[in which a large oblique collision with thegrowing planet produced its satellite andprovided the system with its angular mo-mentum, is favored (1–4). However, to date,the viability of this mode of origin has onlybeen demonstrated for the Earth-Moon case.Models of lunar-forming impacts (4–7)produce disks orbiting the planet containingabout 1 to 3% of the planet_s mass, and assuch the feasibility of forming the propor-tionally more massive Charon has beenunknown. Although giant impacts are believedcommon in the final stage of terrestrial planetformation in the inner solar system (8), theirrole in the outer solar system and the Kuiperbelt Ea disk of objects orbiting exterior toNeptune between about 30 and 50 astronom-ical units (AU)^ is more uncertain (9). Theorigin of Pluto-Charon provides a key con-straint to this issue and to the population ofobjects in the primordial Kuiper belt.Properties of the distant Pluto-Charonpair Esupporting online material (SOM)text^ remain somewhat uncertain. Pluto andCharon have physical radii of RP, 1150 to1200 km and RC, 590 to 620 km, and theirdensities are rP, 1.8 to 2.1 g/cm3and rC,1.6 to 1.8 g/cm3, indicating rock-ice compo-sitions with about 50 to 80% rock by mass(10). Scaling the angular momentum of thePluto-Charon binary, LPC, by the quantityL¶KffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiGM3PCRPCq(where RPCis the radius ofan equivalent spherical object containing thetotal Pluto-Charon system mass, MPC,andG isthe gravitational constant) gives the normal-ized system angular momentum, JPCK LPC/L¶,which is in the range 0.33 G JPCG 0.46 for aCharon-to-Pluto mass ratio, q, in the range0.1 G q G 0.15.Here, I present smooth particle hydrody-namic (SPH) simulations to show that giantimpacts can produce Pluto-Charon–type sys-tems with q 9 0.1 and J È 0.4. In SPH, anobject is described by a multitude of over-lapping particles, each of which represent athree-dimensional (3D) distribution of matterof a specified composition, whose propertiesare tracked in time in a Lagrangian manner.In these simulations, particles are evolved dueto gravity, compressional heating and expan-sional cooling, and shock dissipation (11), andthe Analytic Equation of State, M-ANEOS(12, 13), is used with material constants byPierazzo and Melosh (14). For a full descrip-tion of the SPH technique, see (15), fromwhose work the code used here is directlydescended.The SPH simulations involved betweenN 0 2 104and 1.2 105particles and asimulated time of 1 to 4 days. Given that theappropriate differentiation state and composi-tion of the colliding objects is uncertain, I con-sidered three initial compositions: (i) SER: 100%undifferentiated serpentine EMg3Si2O5(OH)4,a hydrated silicate containing ,14% H2Obyweight^; (ii) IDI: 40% water ice, 42% dunite,and 18% iron by mass and differentiated intoan ice mantle, rock core, and iron inner core;and (iii) SIM: 50% serpentine and 50% waterice in an undifferentiated mixture. These ob-jects range from uniform to highly differen-tiated, with rock mass fractions between 43and 86% and bulk densities between È1.5 to2.5 g/cm3.I modeled a variety of impacts that wereall capable of providing an angular momen-tum within the range for Pluto-Charon. Thecollision of two nonspinning equal densityobjects delivers a normalized angular mo-mentum (16)JcolKLcolL¶0ffiffiffi2pf ðgÞb¶vimpvesc ð1Þwhere b¶Ksin x is the scaled impact parameter,x is the angle between the surface normaland the impact trajectory (a grazing impacthas b¶ 0 1), g is the impactor-to-total massratio, f ðgÞ K gð1 j gÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffig1=3þð1 j gÞ1=3q,and (vimp/vesc) is the ratio of the impact velocityto the mutual escape velocity, with vimp2K vesc2þvV2,wherevVis the relative velocity at largeseparation. Here, vescKffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2GMT=ðRimpþRtarÞp,where MTis the total colliding mass, andRimpand Rtarare the impactor and targetradii, respectively. A preimpact spin Edue toearlier impacts (8, 17)^ that had a compo-nent in the same rotational sense as theimpact (Bprograde[) would increase Jimp.Forthe case of prograde spin vectors normal tothe plane of the impact, the additional con-tribution isJspin0 KimpTminTimpg5=3þ KtarTminTtarð1 j gÞ5=3ð2Þwhere Jimp0 Jcolþ Jspin, Kimpand Ktarare themoment of inertia constants of the collidingobjects, Timpand Ttarare the preimpact spin pe-riods of the impactor and target, and the mini-mum period for rotational stability is TminKffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3p=ðGrÞp0 2:3hoursðr=2g=cm3Þj1=2,where r is the density of the objects. Here, col-lisions with 0.5 G b¶ G 1, 1 G (vimp/vesc) G 2.5 (or0 G vVG 2.5 km/s), 0.13 G g G 0.5, and with (18)and without preimpact spin are simulated.Results. An impact between predifferen-tiated (IDI composition) identical objects, eachof which contained 0.53 MPC(so that g 0 0.5)and had an
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