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The Origin of Main-Belt Comets

by Nader Haghighipour and David Jewitt

The 3 known MBCs Images of the three known MBCs, 133P/Elst-Pizzaro, P/2005 U1 (Read), and 118401 (1999 RE70), taken by Henry Hsieh and David Jewitt at the University of Hawaii 2.2-meter telescope on Mauna Kea.

Recent observations from Mauna Kea have identified a new class of objects among the small bodies of our solar system, dubbed the main-belt comets (MBCs). These objects look like comets (they show comae and tails), but unlike "normal" comets, they orbit the Sun entirely within the asteroid belt between Mars and Jupiter. The MBCs bridge the gap between the classical comets (ice-rich bodies from the Kuiper Belt and beyond) and asteroids (rocks formed at temperatures too high for water ice to survive). This groundbreaking work by former IfA graduate student Henry Hsieh (now at Queen's University in Belfast, Northern Ireland) and David Jewitt was described in Na Kilo Hoku no. 12.

The hybrid nature of the MBCs raises many questions about the origin and activation mechanism of these objects. One possibility is that they formed in place in the outer regions of the asteroid belt, where temperatures in the Sun's accretion disk (the primordial material from which the planets formed) may have been low enough for water to be trapped as ice. Another possibility is that MBCs are comets that formed in the Kuiper Belt (beyond Neptune) and were somehow scattered inward to become trapped among the asteroids. Trapping is effectively impossible in the modern-day solar system, but recent computer simulations suggest that trapping might have been possible during a brief, unstable phase of the solar system that occurred about 3.8 billion years ago. Both possibilities are fascinating: the MBCs either contain primordial samples of ice from the Sun's accretion disk just a few hundred million miles from the orbit of Earth, or they consist of matter from the icy extremities of this disk.

MBC graph

This graph, generated by a computer model, shows the projected eccentricity (the degree to which an orbit is elliptical, with a circle = 0) of the orbits of MBCs plotted over time in millions of years. The orbit of Comet Read will become unstable in approximately 22 million years.

Recent computational simulations of the dynamics of MBCs by Nader Haghighipour point toward the first of these possibilities: If the MBCs were captured from the Kuiper Belt, their orbits would be much more elliptical (as opposed to their current almost-circular orbits), which would render them susceptible to the gravitational influence of nearby planets. These objects would therefore not be able to survive for long periods inside the orbit of Jupiter, as the MBCs do. The simulations suggest that even one of the known MBCs (Comet Read) is barely stable--it will most likely leave its current orbit within 22 million years.

Unlike the classical comets, with their highly elliptical orbits, the nearly circular orbits of the MBCs expose them to essentially continuous heating by the Sun. Ice near the surface of such objects should have been baked away long ago, raising questions as to how their observed activity could be produced. Theoretical work by Norbert Schörghofer, of IfA's Astrobiology Team, has shown a way out of this dilemma. It turns out that even a very thin layer of dirt, perhaps as little as a meter thick, can block the heat of the Sun enough to stifle the loss of ice by sublimation. By this reasoning, all the asteroids in the outer belt could potentially contain ice, provided it remains buried by a modest layer of dirt.

Why, then, do the MBCs look like comets? One answer may be that small boulders in the asteroid belt occasionally strike ice-rich asteroids and punch through their surface rocky layers to expose previously buried ice. The ice then sublimates, producing a weak atmosphere and tail around the body, giving it a cometary appearance. Very modest boulders of only a meter in diameter would be sufficient to puncture the surface debris and expose the ice. While almost nothing is known about the number of such boulders in the asteroid belt, computer models, generated by Nader Haghighipour based on simple assumptions about the distribution of these objects, suggest that suitable collisions occur approximately once every 10,000 years per MBC. Between triggering impacts, the MBCs fall dormant when their surface ices are depleted. If we could make a million-year-long movie of the sky, we might see that MBCs flare up and fade over and over again, like lights blinking on a Christmas tree. Observations from the Pan-STARRS 1 telescope should yield a larger sample of these objects and help to constrain the theories of their formation and evolution.