Introduction
Main belt comets (MBCs) are a newly recognized class of body in the solar system. They are remarkable for having both the orbital characteristics of asteroids and the physical characteristics of comets. What this means is that they look like comets because they show comae and tails but they have orbits interior to Jupiter's (a < a_J) and Tisserand parameters substantially larger than 3, like asteroids. [Note: the MBC definition is an empirical one, based on simple measurable quantities, and does not presume an origin for the comet-like activity by the sublimation of ice. In fact, as described here, there are many possible causes for mass loss from asteroids in addition to sublimation. Another term for these objects is "Active Asteroids".]

The currently known MBCs are 133P/Elst-Pizarro, 176P/LINEAR, 238P/Read, P/2008 R1 (Garradd), P/2010 R2 (La Sagra), P/2010 A2 (LINEAR) , 596 Scheila, 300163 and P/2012 F5 (Gibbs).

In addition, 3200 Phaethon, 2201 Oljato and 107P/Wilson-Harrington are dynamically asteroidal bodies (Tisserand > 3) in which mass loss has been reported, but they are not in the main belt. The distribution of the mass-losing asteroids is shown below, including their relation to the asteroids (orange dots) and classical comets (blue dots).

from Jewitt 2012, Astron. J..

History
The prototype MBC was discovered as asteroid 1979 OW7, rediscovered as asteroid 1996 N2 and then found to be a comet by Eric Elst and Guido Pizarro in 1996. It is now given a cometary designation as 133P/Elst-Pizarro. For some years, the nature of this object was misunderstood. It was first believed to be the signature of the unlikely impact between two asteroids. But 133P has flared into activity on three successive perihelion passages. This substantially discredits the asteroid impact model (because one impact would be unusual enough, three on the same object would be unusual-cubed). We suggested that 133P has an intrinsic source. As other examples of dynamical asteroids with mass-loss were found, the objects were recognized as a new category in 2006 (Hsieh and Jewitt 2006, Science, 312, 561-563, here as a pdf).

Origin of the Activity

We have compelling evidence for the cause of the activity in Scheila (recent impact) and impact might also be the cause of P/2010 A2. Ice sublimation probably drives mass loss from 133P and 238P - the reason to think so is that the activity in these two objects has been found to be repetitive, and other mechanisms do not naturally generate repetitive activity. In other MBCs, the cause of the activity remains unknown, basically because we have not yet accumulated enough data to decide between the following possible causes:

1. Sublimation
Activity in 133P/Elst-Pizarro is recurrent, having been observed at each of the last three perihelia. This is inconsistent with an impact origin but exactly what we would expect from a thermal process such as sublimation. It is what we see in the classical comets, for example, where the driver is the sublimation of water ice. While impacts cannot reasonably explain a repeatedly active body like 133P, I believe that impacts may provide the trigger needed to initiate the activity in some bodies. I made a sketch of this impact trigger mechanism .

2. Impact
Impact is the explanation many people reflexively jump towards when thinking about the MBCs. In fact, though, observable impacts are quite rare, essentially because 'space is big'. The best case is for P/2010 A2, in which the strange morphology (unlike that of other comets) is consistent with impulsive formation in 2009, nearly a year before the object was discovered. I think that such events (disruptions producing A2-like objects) occur annually or perhaps a little more frequently.

3. Electrostatics
On the Moon, dust is levitated by electrostatic charge gradients caused by uneven solar illumination (sunlit regions lose photoelectrons which fly away and stick in the shadowed regions). With the much lower gravity on km sized MBCs, dust could be electrostatically ejected from the body. We cannot rule this process out - however, it is hard to see why it would not apply to all small asteroids, whereas our observations show that the MBC phenomenon is rare. Furthermore, MBC 596 Scheila is too large (110 km diameter) for electrostatics to launch dust.

4. Rotational Bursting
A rapidly rotating asteroid could lose mass centripetally. 133P is indeed quite a rapid rotator (the period is about 4 hrs) but there are many faster rotators that do not lose mass and other MBCs lose mass but do not rotate quickly (e.g. 176P has a period near 20 hr). It is further difficult to see why rotation would eject dust from 133P only when near perihelion: if it's unstable it should be unstable all the time.

Caption: Image of main-belt comet 133P/Elst-Pizarro from the 2.2-meter University of Hawaii telescope on Mauna Kea taken UT 2002 September 7. The long trail of dust particles crosses the image from upper left to lower right. "String-of-sausages" objects in the image are field stars and galaxies smeared-out by tracking the telescope to follow the non-sidereal motion of 133P and chopped up by recording the image in many small pieces. Click on the image for a larger version. From Hsieh et al (2004).

5. Thermal Effects
Thermal effects are capable of producing and launch dust grains on strongly heated asteroid surfaces. Thermal fracture occurs when expansion stresses exceed the tensile strength of the material. Hydrated minerals (clays, serpentine etc) can lose trapped water when heated, causing fracturing and dessication as seen in the cracked mud of a sun-baked lake. Such effects may be responsible for the mass loss found in 3200 Phaethon when near perihelion (q = 0.14 AU, where peak surface temperatures may approach 1000 K). Dehydration can also occur through impact shock compression.

6. Radiation Pressure Sweeping
Radiation pressure competes with gravity to sweep unattached particles from the surfaces of asteroids. Near the sun, radiation pressure can eject dust very readily from km sized bodies and it can work in tandem with the other processes to leak material from the asteroids.

A paper describing these mechanisms is linked here.

Significance
The significance of the MBCs is that at least some appear to be a distinct, third comet reservoir in the Solar system (after the Oort cloud and the Kuiper belt). There is no established dynamical pathway between these other reservoirs and the main-belt in the modern solar system, so any ice in the MBCs probably has led a different history from ice in the other comets. Comparing objects from the three reservoirs allows us to sample the protoplanetary disk of the Sun at three locations (MBCs - near 3 AU and formation temperature near 150 K, Oort Cloud comets - 5 to 30 AU formation temperatures 100 to about 50 K; Kuiper belt - beyond 30 AU, 50 K and colder).

MBCs may illuminate the sources of the terrestrial planet volatiles, including the origin of the Earth's oceans. If Earth formed hot, consistent with its large size and gravitational potential, then water and other volatiles might have been added later, after the surface had cooled down. The outer asteroid belt - home of the MBCs - is a leading candidate source region for this so-called "late veneer" of added volatiles.

Many carbonaceous meteorites (e.g. the CM chondrites) contain hydrated minerals like serpentine, carbonates and even grains of salt have been found. The parents of these meteorites must have contained liquid water. The MBCs may be closely related to these hydrated bodies, except that the ice survived because the temperatures never reached melting. Perhaps the MBCs are pieces of the icy crusts of parent asteroids whose interiors were more thoroughly cooked by embedded short-lived radioactive nuclei.

References

Contact David Jewitt [jewitt@ucla.edu 301-825-2521]

Comet	Jewitt	Kuiper