NASA Astrobiology Institute (NAI)

1. Origin and Activation Mechanism of Main Belt Comets

   Project Investigators: Nader Haghighipour

   Summary

   The newly discovered icy asteroids, known as Main Belt Comets (MBCs), are dynamically similar to the main belt asteroids while present physical characteristics (e.g., dusty tails and comae) that resemble those of comets. This project aims to understand the origin of these objects and the mechanism of their activation. Results indicate that MBCs were most likely formed in situ, and their activation was caused by collision with small meter-sized objects.

   Astrobiology Roadmap Objectives:

   • Objective 1.1: Models of formation and evolution of habitable planets

   Project Progress

   The discovery of main belt comets has raised many questions regarding their origin, regions of existence, and activation mechanism. This project addresses these issues by carrying out an extensive numerical study of the dynamics of MBCs. We have integrated the motions of the three known main belt comets, as well as a synthetic group of their clones. Simulations were carried out for different values of their semimajor axes, eccentricities, and inclinations. Results indicated that MBCs with orbital eccentricities smaller than 0.2 and inclinations lower then 25 deg are stable for at least 1 Gyr. Among the three known MBCs, comet Read falls outside this region and becomes unstable in 22 Myr (Figure 1). Numerical simulations also indicate that MBCs 133P/Elst-Pizzaro and 1999 RE70 may belong to a 50 Myr sub-family of the Themis family of asteroids.

   
   

   Stable (light green) and unstable (purple) MBC clones. The shaded area correspons to the width of 1:2 mean-motion resonance with Jupiter. The three known MBCs are also shown. As seen from this figure, comet read is in the unstable region of the map, while the other two MBCs are in the stable region. This figure also shows that Elst-Pizzaro and RE70 may indeed be members of a Themis sub-family.

   The comet-like activities of MBCs can be attributed to their collisions with meter-sized bodies. Numerical simulations indicate that the rate of collision of km-sized MBCs with meter-sized objects has been high in the past (Figure 2) implying that many MBCs might have existed then and either were eroded by frequent collisions with small bodies, or sublimated themselves out. The current MBCs seem to be the remnants of a break-up event that produced many of these objects. Some might have been ejected from the system, eroded, or sublimated, and some may still be awaiting collisions with meter-sized objects to show their activations. Our results indicate that many such MBCs must exist in the outer region of the asteroid belt.

   
   

   Rates of collisions of m-sized particles with Elst-Pizzaro. Each bin contains 5000 particle. The orange column in each bin shows the averaged rate of collision of particles with Elst-Pizzaro. The blue numbers on top of each orange column represent the contribution of that bin in collision with Elst-Pizzaro. As shown here, most of the collisions come from the vicinity of this MBC, around 3.1-3.2 AU.

Publications

Haghighipour, N.  (2008).  Dynamical Constraints on the Origin and Activation Mechanism of Main Belt Comets.  Asteroids, Comets, and Meteorites.  Baltimore,Maryland.