It is becoming increasingly clear that the planets and the planetesimals (the asteroids and Kuiper belt objects) in the solar system have co-evolved, such that the planets migrate and the planetesimal belts are depleted, while the bombardment rate on the planets evolves. This project will detail one crucial phase in this evolution, associated with the late heavy bombardment (LHB), some 3.9 billion years ago. Building on earlier evidence for the source and the mechanism of this event, this investigation will assess the number, sizes, and changing impact rates of the LHB and post-LHB impactors. Rigorous analysis and numerical modeling combined will identify the dominant and less dominant sources of the LHB and post-LHB impactors, and provide upper and lower limits on the duration of the LHB, its intensity on the Earth, Moon and other terrestrial planets, and the magnitude of depletion of the main asteroid belt.
This research helps to understand the dynamical history of the planets and minor bodies of the solar system, especially the bombardment history of planet Earth, the crater-based geochronology of Mars and Mercury, and the history of the main asteroid belt. It adds impetus to renewed study of the Moon, and is also of interest to extra-solar planetary system studies. Knowledge gained during the study will be integrated into Dr Malhotra's natural sciences class, and meteorite bombardment and the history of early Earth have broad public and community interest.
This research project investigated the history of meteoroidal bombardment on the terrestrial planets, particularly the catastrophic event known as the Late Heavy Bombardment (LHB) of ~3.9 gigayears ago. We had previously demonstrated that the LHB impactors originated in a dynamical instability of the main asteroid belt that launched asteroids into collision paths with the Sun and the planets. We analyzed the distribution of asteroids in the main asteroid belt and discovered that asteroids are missing systematically in regions exterior to the mean motion resonances with Jupiter as well as in a wide zone in the inner half of the asteroid belt. We established that these patterns are not consistent with the action of the gravitational perturbations of the planets in their current orbits, but that they can be reconciled with those perturbations if Jupiter has migrated inward by about 0.3 AU (AU=astronomical unit, the average Earth-Sun distance) and Saturn has migrated outward by about 1 AU from its primordial orbit. Analyzing the orbital eccentricities of the surviving asteroids observed at the present day, we derived that the timescale of planet migration was possibly as short as a few million years. With the help of large-scale numerical orbit integrations, we established for the first time the rate at which large asteroids diffuse out of the asteroid belt and from which specific regions. We calculated that the post-LHB impact flux of Chicxulub-type impactors (i.e., asteroids of diameter ~10 km) on the terrestrial planets is actually an order of magnitude lower than previous estimates that were based on more approximate and uncertain methods. We also discovered an important discrepancy between the observed and the theoretically predicted longitudinal distribution of young impact craters on the Moon. There are a number of possible explanations for this discrepancy, perhaps the most intriguing is that there exists an undiscovered tail of low velocity impactors near the Earth-Moon orbit. Several ideas for observational discovery of these missing near Earth asteroids have been proposed. Our findings have been of much interest in the emerging disciplines of exo-planetary systems and astrobiology in at least three ways: understanding the evolution of the small body populations (asteroids, Kuiper belt objects) in our solar system helps inform the interpretation of recent observations of debris disks around other stars; the evidence for giant planet migration in the solar system helps inform the interpretation of the orbital distribution of exo-planetary systems; and understanding the nature of the LHB helps to constrain the extra-terrrestrial impactor flux and its composition proximate to the emergence of life on Earth in the early Archaean. An important outcome of this research was the PhD dissertation of David A. Minton; Dr. Minton is now employed in an Assistant Professor position at Purdue University. The results of this project were presented at several professional meetings as well as in several public forums; presentations for the general public are posted at the PI’s home page (URL: www.lpl.arizona.edu/~renu/outreach.html). The PI contributed an essay for the 'AstroFascinations' column in Astronomy magazine; this was published in June 2011. An interview with the PI was published in April 2011 in the Women In Planetary Science Blog-'51 Women in Planetary Science' (URL: http://womeninplanetaryscience.wordpress.com/2011/04/25/dr-renu-malhotra/). The PI gave numerous interviews to science journalists, and was quoted in the print, radio, television, and online media on various aspects of solar system science, on at least 16 occasions over the course of this project.
