Capability Development for Modeling the Collisional and Dynamical Evolution of Asteroids and Meteoroids in the Inner Solar System, 15-R9644

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Principal Investigator
W. F. Bottke

David Nesvorny
D. Vokroulicky

Inclusive Dates:  08/01/06 – 07/30/07

Background - The objective of this project was to develop the first comprehensive end-to-end numerical model of the meteorite/asteroid/dust delivery process that could be run on the existing computational resources at SwRI. Specifically, SwRI proposed to develop a code with highly-efficient algorithms that can track the collisional and dynamical evolution of millions of individual bodies produced by the breakup of parent bodies across the main asteroid belt. This code would then be used to investigate interesting problems in asteroid/meteorite/dust delivery in the aftermath of large collisions in the asteroid belt. 

Approach - The new code, called TRACKMET, computes how a distribution of migrating bodies en route to Earth is realistically affected by both collisions and dynamical processes. Note that collisions can eliminate existing bodies while creating new, smaller fragments that can also be followed in the code (unless they are smaller than a predetermined threshold).

Accomplishments - The TRACKMET code was successfully created and applied to one problem of interest. To date, the project team has been able to follow the collisional and dynamical evolution of a size distribution of particles ranging from D = 1 m to typically 10 to 15 km, a dynamical range of slightly more than four orders of magnitude. Here objects larger than a particular diameter threshold (50 to 300 m, depending on the steepness of the initial – and fragment – size distributions) were directly tracked in their dynamical evolution across the main asteroid belt via nongravitational forces (e.g., Yarkovsky effect), while smaller objects were tracked by assuming that representative objects would serve as a proxy for a large ensemble of similar-sized objects. An appropriate weight was assigned to the objects, and tests were conducted to ensure the representative objects did not produce pronounced numerical artifacts. These bodies evolved until they reached a resonance that could take them out of the main belt or were struck by an object larger enough to produce a catastrophic disruption. If the latter occurred, the target object was replaced by a fragment size distribution of bodies that continued evolution along the same path. When objects entered the resonance, tables generated from different numerical simulations were used to determine how long the objects would survive collisionally and dynamically in the inner solar system.

For testing purposes, the project team attempted to reproduce the cosmic ray exposure (CRE) ages of the HED meteorite class and found that the Vesta family size distributions tested to date tend to produce a smooth Maxwellian distribution of CRE ages. This provides a good match to the background continuum seen among the HED meteorite classes (and many other meteorite classes), but it does produce the observed "spikes" that exist in a few data sets (e.g., the HED and H chondrite meteorites). This may indicate that the wrong fragment size distribution is being used for these breakup events or, more excitingly, that many meteorites come from prominent multi-km breakup events produced among the near-Earth object population. Regardless, these results do a good enough job of reproducing constraints that production runs can be made. The TRACKMET code can now be termed an asset for future NASA and NSF proposals.

The code also has a second component {\tt TRACKMET} that uses secular analytical theory to model the gravitational perturbations of small particles (D less than 1 mm) evolving under the influence of the planets. Thus, when combined with SwRI's model of how such bodies evolve under the influence of nongravitational forces (e.g., Poynting-Robertson drag), it is now possible to track how such small bodies evolve all the way from the main belt to Earth with all of the pertinent physical included. This gives great hope of using the code {\tt TRACKMET} to model dust evolution in not only our solar system but also extrasolar planetary systems that are still in the earliest phases of their history. This work has led to the publication of a paper currently in press for the Astrophysical Journal.

The code, while highly efficient and capable, is still something of a CPU hog. Typical runs of using size-distributions extending from meters to 10 km take, on average, a week to several weeks to go 2 billion years of simulation time. Disruption events that produce fragment size distributions with larger dynamic ranges (for example, from meters to several 100 km) may take several months of time unless tests show the size of the minimum threshold diameter can be increased to sizes approaching 1 km.

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