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Capability Development for Modeling Planetary Impacts, 15-9089

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Principal Investigators
Robin Canup
Harold F. Levison
Peter Tamblyn (Consultant)
Erik Asphaug (UCSC Consultant)

Inclusive Dates: 09/01/98 - Current

Background - One fundamental question in planetary science is how the planets in our solar system formed, and, in particular, what processes led to the formation of a habitable Earth. These questions are the foundation of one of NASA’s main scientific directives, the Origins Program, which has received burgeoning support with the discovery of dozens of planetary systems around other stars. Recent work indicates that the final stage of planet formation was dominated by hypervelocity collisions among planet-size bodies. Such impacts were so large that they determined a planet’s spin rate, the tilt of its rotational pole, and its possession of a moon. The specifics of the last impacts that a planet experienced determine many of its end characteristics, including its ability to sustain a life-supporting climate. Many of the planets in Earth's system show evidence of such impacts, for example, Mercury, Uranus, Earth/Moon, and Pluto/Charon.

Despite its fundamental importance, modeling of planet-scale impacts has proven to be difficult, and previous computer models of large impact events are inadequate due primarily to slow computational speed. The main difficulties in modeling large impact events include: 1) their physical scale precludes extrapolation from laboratory experiments, and 2) energies involved are so large that both gravitational dynamics and thermodynamics of the involved materials must be explicitly tracked. Current understanding of the outcome of large impacts to date has been so limited that most planet formation models make the simplifying assumption that planets simply merge when they collide. However, such models have failed to produce Earth-like planets on circular orbits like those of Earth and Venus, and so understanding the outcome of large impact events appears central to resolving fundamental outstanding issues in terrestrial planet formation.

Approach - This project involves developing a new numerical impact algorithm using smoothed-particle hydrodynamics, or SPH. In SPH, objects are represented by a collection of spherical particles whose evolution is tracked as a function of time. The SPH method follows the history of each particle as it moves in space, making it well suited to problems involving deformation and debris ejection. An SPH run’s thermodynamic output is the internal energy of each particle, as well a locally computed pressure and density; the dynamical outputs are the mass, position, and velocity of each particle. To gain expertise in the SPH method (which had not been previously use by the research team), the team involved an outside collaborator, Dr. Erik Asphaug, who is an expert in SPH methods. The model currently being developed can achieve much greater speeds (and therefore much higher numerical resolutions) than existing models, as the team has implemented algorithm improvements and have ported the SPH code onto a parallel cluster of workstations. 

Accomplishments - The team has completed the porting of a parallel version of an SPH code onto its cluster of 16 DEC Alpha workstations and is reconciling the separate trees utilized for the strength and self-gravity calculations to provide a unique capability. Testing indicates that simulations are not communication-limited and that test runs are reproducing results obtained by other SPH codes. Comparison of simulations completed at several numerical resolutions indicates that the resolutions obtainable by this model are sufficiently high to provide convergence of numerical results. In conjunction with this code-development effort, the team has identified a new analytic scaling relationship that appears to generally predict the results of the SPH simulations of planet-scale impacts done to date as a function of impact parameters. The research team intends to test the applicability of this scaling relationship to regimes previously unexplored. SwRI investigators are pursuing two new research efforts involving the first simulations of the hypothesized Pluto-Charon forming impact event, and the first generalized study of impact outcomes in the 1,000-kilometer size range. Proposals based on the capabilities developed in this project to date have been submitted to National Science Foundation’s Planetary Astronomy, NASA Origins of Solar Systems, NASA Planetary Geology and Geophysics, and NASA Applied Information Systems programs.

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