1. Method To run the simulations of our impacts with Pluto, we used a method known as “Smoothed Particle Hydrodynamics”. SPH is used to model the flow of a continuous fluid by replacing the fluid itself with a set of particles. Once defined, each of these individual particles is each assigned physical attributes (such as pressure and temperature) by an Equation of State (EOS). EOSs are driven either by searching tables of carefully-gathered experimental data, or by artificially-generated curves to relate the physical quantities. Once particles are defined and are given physical meaning, SPH generates a pressure field by calculating interactions between neighboring particles. This sets them in motion during our collisions. Introduction The solar system's Kuiper belt is likely to contain many objects similar in size to Pluto. Pluto's composition, based on its mean density (2030 kg/m^3), is 60% rock and 40% ice. This composition is notably more rich in rock than typical outer solar system satellites, which have rock fractions of 40%. This work investigates the possibility that devolatilization (the removal of ice) of typical Kuiper Belt Objects (KBOs) may occur as a byproduct of large impacts. Our target KBO, named Protopluto, is represented as an object with a 40% rock mass fraction. The impactor is a cometary object composed entirely of ice. We model the collision of the target with a series of impactors, varying the impactor's size and angle. These impacts are simulated using a three-dimensional smoothed-particle hydrodynamics (SPH) code. For each impact, we analyze the fraction of ice thrown off from the target. The impact speed is the escape speed of the target object (~1.22 km/s). Our simulations will constrain the critical impactor size and impact angle ranges required to increase the final rock mass fraction of the target to the 60% value observed for Pluto. 40% 60% 40% 60% The Kuiper Belt is a group of icy bodies of varying sizes of which Pluto is a member. The belt formed from the same disk that the rest of the solar system formed from. The majority of the objects have an icy mantel and rocky core with a mass composition of 60% and 40 % respectively (Schubert et al, 1986). Pluto, however, is significantly different from the other bodies and has is 40% ice and 60% rock by mass (Stern, 1992). This leads us to believe that the dwarf planet suffered a significant impact sometime in the past. Impact Modeling of Kuiper Belt Objects Christopher T. Thompson, Bryce Cummings, Steve Henke, Dr. Paul Thomas UW Eau Claire - Physics and Astronomy Basalt Core Water Ice Mantle IMPACTOR Radius= 804 km Velocity= 1.22 km/s Water Ice Initial Setup We created a Protopluto that is consistent with the composition of typical Kuiper Belt objects observed in the past -having an ice: rock ratio of 60%: 40% by mass. We left the core of Pluto unchanged, but added over 300 km of ice to its mantel to change the overall mass fraction. Also existing in our model was a solid water- ice impactor with a radius equal to that of Pluto’s present day core. The impactor was set at Pluto’s escape velocity of 1.22 km/s, simulating the slowest possible collision between Pluto and another body. We simulated collisions with impact angles varying between 0º and 90º (at 5º increments). The figures above depict the initial setup for our SPH simulations. The impactor (red) can be seen with the stationary Protopluto (blue) in its trajectory. Pictured to the left is a 90º impact and to the right a 30º impact. PROTOPLUTO Radius= 1481 km R core = 804 km Conclusions We found that for the conditions of our simulations, the amount of mass ejected from Pluto was independent from the angle of impact. We consistently calculated a final ice to rock mass ratio of 70% to 30% respectively for each angle tested. As the velocity of impact increases, we fully expect to see the angle of impact to play a much great role in our results. It is apparent that the collision velocity tested is insufficiently low to eject the amount of ice we desire to arrive at Pluto’s present day composition. Future work will consist of refining our model with higher velocities and narrowing the criteria of the likely impact. Results To determine the amount of material remaining post-collision, we summed the masses of all particles with final velocities less than Pluto’s escape velocity of 1.22 km/s. It can be assumed that all particles above this speed will escape Pluto’s gravity. The angle of impact had very little effect on the amount of material that was ejected from Pluto. The mass of ice remaining post-collision increased only very slightly as the impact angle increased. We found consistently for each trial that the resulting composition of Pluto was 70% ice and 30% rock by mass. Shown below is a 90° impact during and after a collision. Particles are colored according to their speeds. It is important to note that each of our simulations was conducted in the absence of gravity. While this may exaggerate the effects of a collision in reality, it does not effect our final results. Acknowledgments Stern, S.A., 1992. The Pluto-Charon System, Ann. Rev. Astron. & Astrophys., 30, 185-233. Schubert, G., T. Spohn, and R.T. Reynolds, 1986. “Thermal histories, compositions, and internal structures of the moons of the solar system.” Satellites (J.A. Burns and M.S. Matthews, eds.) Univ. of Arizona Press, Tucson. Office of Research and Sponsored Programs at UWEC Leigh Brookshaw John Stupak UW Eau Claire Physics and Astronomy Dept. Mary Jo Wagner