On an impact origin of Pluto-Charon

Walnut Street, Suite 400, Boulder, CO 80302 [email protected], Earth Sciences Department, University of California; Santa Cruz, CA 95064. Introduction: The Pluto-Charon system shares key commonalities with the Earth-Moon system, e.g., its large satellite-to-planet mass ratio, high system angular momentum and a potentially reduced satellite density relative to that of the planet. In both cases an impact origin is favored (e.g., [1-2]). However until recently [3] no quantitative simulations of potential Pluto-Charon forming impacts had been performed. Unlike the relatively well-constrained situation for the formation of Earth's Moon, key properties of Pluto and Charon their rock/ice fractions, mass ratio, and total system angular momentum are somewhat uncertain. However, a primary challenge to an impact origin is obtaining a sufficient yield of material in bound orbit: Charon likely contains ~10% of Pluto's mass, whereas the Moon has only ~ 1% of the Earth's mass. Previous impact simulations involving rocky objects have found a maximum yield of material placed in orbit of only ~ 3 to 4% of the total colliding mass (e.g., [4-5]) Here we report on simulations of potential Pluto-Charon forming impacts using smoothed-particle hydrodynamics and a variety of rock-ice compositions. Results: Charon contains 8 to 15% of the mass of Pluto [6]. The angular momentum of the Pluto-Charon system is not known precisely, but is estimated in the range LP-C = 6 to 8 x 10 37 g-cm/s [7]. This implies a system angular momentum that likely exceeds the critical angular momentum for rotational stability, L*, for a single object containing the total system mass. For an impact between initially non-rotating objects with an impact velocity equal to the mutual escape velocity (vimp = vesc), the resulting system (L/L*) is just a function of K (the moment of inertia factor of the colliding objects), γ (the ratio of the impactor mass to the total colliding mass), and the normalized impact parameter, b (where b = sinξ and ξ is the impact angle),