Capability Development for
Simulating a New Paradigm
Inclusive Dates: 06/26/00 - 10/26/00
Background - Numerous lines of evidence, derived in the 1980s from astrophysical observations of the formative conditions of other stars, and from physical characteristics of our own planetary system, demonstrated that the planets were formed through the accretion of material in the solar nebula, the disk-shaped cloud of gas and particulates (dust) left over from the formation of the Sun itself. In the 1990s, images of newly forming stars obtained by the Hubble Space Telescope (HST) showed them to be surrounded by clouds of gas and dust, in which other planetary systems are presumably forming. These images, and the steadily increasing number of extrasolar planets (i.e., planets orbiting other stars), reinforce the concept that the formation of planets and planetary systems is a natural byproduct of the formation of stars themselves, and that planetary systems are therefore common throughout the galaxy.
In the current model of the planet-building process, the terrestrial (i.e., inner) planets form via a bottom-up process involving accumulation. The first and least well-understood phase of this growth is the so-called 'early-stage' in which grains accumulate into pebbles, which, in turn, grow to boulders and ultimately, 'planetesimals,' approximately one to ten kilometers across. Somewhat better understood is the 'mid-stage' of planetary growth, in which the myriad, kilometer-scale planetesimals grow into a number of roughly 1,000-kilometer-scale protoplanets through accretionary collisions. Finally, during the 'late-stage' of planetary growth, the growth of the protoplanetary embryos sweep up most of the remaining debris and (owing to mutual gravitational perturbations) cross paths and merge via inelastic collisions into a small number of final planets.
One outstanding problem in planetary formation is understanding how the initial stages of planetary growth ran to completion in erosive regions of the early solar system. Researchers hypothesize that large planetesimals scattered from Jupiter's region acted as accretionary 'seeds' that, because of their higher self-gravity, were able to retain impact debris and grow in a region that was collisionally erosive for smaller, native planetesimals. These accretionary seeds would have thus jump-started the planetary growth process in regions such as the inner solar system where the Earth and other terrestrial planets reside.
Approach - To test the 'seeded growth' concept, the research team chose to modify and improve SwRI's existing Mid-stage Accretion Code (MAC). MAC is a published, time-dependent (but single radial zone) code for examining the problem of accretion and erosion of small bodies in the middle stage of accretion. MAC was developed, tested, and applied with NASA funding to examine the growth of large objects in the Kuiper Belt, a reservoir of cometary bodies residing in the 30- to 50-AU region beyond the planet Neptune. To examine the seeded growth hypothesis, MAC needed to be improved 1) to allow accretionary seeds to be introduced with the far higher orbital eccentricities that they will have after injection from the Jupiter-zone than the feed-stock population of small debris in which they must grow, and 2) to treat multiple semi-major axis (i.e., radial) zones so that the time-dependent orbital semi-major axis and eccentricity damping of the orbital eccentricities and semi-major axes of the injected, growing seeds can be realistically treated. Together, these two code improvements (involving approximately 300 lines of code) will allow MAC to conduct the first realistic simulations of the seeded growth hypothesis.
Accomplishments - The coding changes to MAC have been fully implemented. To expand the single-zone MAC code to handle multiple semimajor axis zones, another dimension had to be added to all tracking variables within the code. The new multizone code had to be interfaced with SwRI's existing routines that calculate the collision rates and velocities of particles from one zone when interacting with particles from another zone. Also, SwRI had to develop the formalism governing the semimajor axis and eccentricity evolution of the seeds introduced into the model due to their interaction with lower eccentricity planetesimals in the zone they cross. Finally, the team addressed the memory storage issue inherent in adding yet another dimension to an already memory-intensive computer code by incorporating within the modified code an on-the-fly data compression routine. Test runs of the modified code to study the accretion of material onto seeds introduced from the outer solar system are now underway.