Main-belt comets (MBCs) are objects that display cometary activity indicative of sublimating ice yet have orbits indistinguishable from those of inert main-belt asteroids. Dynamical analyses suggest that MBCs are likely native to the main asteroid belt, making it now the third currently known source of comets in the Solar System (after the Kuiper Belt and Oort Cloud), and by far, the closest to the Sun. Besides blurring the classical distinctions between asteroids and comets, MBCs provide new constraints to the location of the so-called snow-line (the distance from the Sun beyond which primordial temperatures in the protoplanetary disk were low enough for water to condense as ice and thus be swept up into forming planetesimals) and present an intriguing opportunity to probe a potential ancient source of water on Earth.
Present-day sublimation in the main belt is surprising, as surface ice is unsustainable over the age of the Solar System so close to the Sun. For the MBCs, it is thought that ice remains preserved in subsurface pockets and has only been exposed to direct solar heating recently (within the last 100-1000 years) by impacts from smaller asteroids which excavate these subsurface pockets. Six MBCs are currently known, though the limited observational data used to discover them strongly imply that many more may exist. Exactly how many might exist, how much ice each might contain, and how they are distributed in the main belt are all unknown at the moment though, and are the subject of intense current research at the present time.
What are the known MBCs? There are six currently known MBCs. In order of their discoveries, they are 133P/Elst-Pizarro (133P for short; also known as asteroid 7968), 238P/Read (P/Read for short), 176P/LINEAR (176P for short; also known as asteroid 118401 (1999 RE70)), P/2008 R1 (Garradd) (P/Garradd for short), P/2010 R2 (La Sagra) (P/La Sagra for short), and asteroid 300163 (not yet given a comet designation). In addition, there are two objects --- P/2010 A2 (LINEAR) and (596) Scheila --- in the asteroid belt that have exhibited comet-like activity that was later been determined to most likely have been produced by impacts. Since their activity is likely not sublimation-driven and therefore not "cometary", we do not regard these "disrupted asteroids" as true MBCs.
The orbits of 133P and 176P place them as members of the Themis asteroid family (thought to be produced 1.0-2.5 billion years ago by the break-up of a large parent asteroid from the collision of another asteroid), one of the largest families in the asteroid belt. Both orbits are considered extremely stable, meaning that both objects are likely to be native to their current locations. The orbit of P/Read is similar to those of 133P and 176P, though is considered less stable and has an eccentricity that places it slightly outside the Themis family. The orbit of P/Garradd is quite different from those of the first three MBCs and is not considered stable over more than about 20 million years, much shorter than the ~4.6 billion year age of the solar system, meaning that it is likely not native to its current location. The orbit of P/La Sagra is also different from those of the first three MBCs, though unlike for P/Garradd, the region of the main belt where P/La Sagra is found is not particularly chaotic, which means it may be stable and therefore native to its current location. Asteroid 300163 was just recently discovered to be cometary by the Pan-STARRS 1 survey telescope, and has not yet been thoroughly analyzed.
MBCs with dual comet and asteroid designations (133P and 176P) are so-designated because they were originally known as asteroids before being observed to exhibit cometary activity. Another three currently known MBCs were originally discovered while they were exhibiting cometary activity and so were only designated as comets and did not receive simultaneous asteroid designations. Asteroid 300163 was already known as an asteroid at the time its cometary activity was discovered, and as such, will likely receive a dual comet-asteroid designation.
How were the MBCs discovered? The first-known MBC, 133P, was first discovered in 1979 as an ordinary main-belt asteroid but then serendipitously observed exhibiting a long dust trail on 14 July 1996 by Eric Elst and Guido Pizarro using the European Southern Observatory Schmidt 1.0-m telescope. As the only known asteroid at the time to exhibit cometary activity, its true nature was not well-understood, with some believing it to be an asteroid with sublimating surface ice, and others believing (given the unexpectedness of fresh ice in the asteroid belt) the observed dust emission to be the result of an impact or series of impacts onto the surface of 133P, or that the object was actually an ordinary comet from the outer solar system that had somehow migrated onto a main-belt orbit.
Each scenario carries a certain set of implications. If 133P is an icy asteroid, other asteroids could also be icy and we should see other cometary objects in the main asteroid belt. If 133P's dust emission was the result of an impact, the rarity of such impacts means that such activity would be unlikely to be observed again, particularly for the same object. If 133P is a dynamically-evolved comet, numerical simulations should be able to show a plausible way for this to occur. Observations of renewed 133P activity in 2002 by me and David Jewitt ruled out the second scenario of impact-generated dust emission. This was due to both the implausibility of impacts on the same asteroid in the span of only 6 years (where impacts are not observed occurring with anywhere nearly as often on other asteroids) and numerical dust modeling which showed that an impact generated dust cloud would not persist for the months-long period over which we observed 133P's dust trail. Dynamical simulations showed 133P's orbit to be extremely stable, meaning that it was unlikely to come from elsewhere, particularly not the outer solar system, and furthermore, no plausible dynamical pathway from an outer solar system comet orbit to a main-belt orbit has ever been found. The possibility remained, however, that difficult-to-model chaotic close encounters with the terrestrial planets could have caused such a transition. The extremely low likelihood of such a transition placing 133P exactly onto a main-belt orbit means that 133P could be unique.
Believing that 133P was most likely to be a dynamically ordinary main-belt asteroid that happens to be icy, David Jewitt and I embarked on a deep-imaging survey of selected main-belt objects in search of another 133P-like comet in the asteroid belt, demonstrating that more such objects exist as predicted by the icy asteroid hypothesis. We observed about 600 asteroids between 2004 and 2007, most of which were similar to 133P in their orbital characteristics, physical size, or both. Roughly half-way through the survey, on 26 November 2005, observations of asteroid 118401 (1999 RE70) with the 8-meter Gemini telescope on Mauna Kea, Hawaii showed a faint fan-shaped tail. Coincidentally, just a month earlier on 24 October 2005, another comet with a main-belt orbit designated P/2005 U1 (Read) had been discovered by Michael Read using the Spacewatch 0.9-meter telescope on Kitt Peak in Arizona. Once three known comets had been found in the asteroid belt, it had been clearly demonstrated that 133P was not unique and that we were dealing with a new class of comets, subsequently dubbed main-belt comets.
Since the MBCs were identified as a new class, three more have been discovered. P/2008 R1 (Garradd) was discovered serendipitously by Gordon Garradd on September 2, 2008, using the 0.5-meter Uppsala Schmidt telescope at Siding Spring, Australia. P/2010 R2 (La Sagra) was also discovered serendipitously by Jaime Nomen on September 14, 2010, using the 0.45m telescope at the Observatorio Astronomico de La Sagra in the mountains of Andalusia in Southern Spain. Finally, asteroid 300163 was discovered to be cometary by myself, Larry Denneau, and Richard Wainscoat, during the course of the Pan-STARRS 1 (PS1) sky survey. While this survey does not focus on specific targets, this discovery was made as a result of a concerted comet-finding effort within the survey team, using automated algorithms to automatically flag cometary candidates among the millions of moving objects (including both real and false detections) observed nightly by PS1.
What is the origin of the MBCs? We believe that all the MBCs are native to the main asteroid belt, though P/Read and P/Garradd may not have always been in their exact current locations. Water/ice in the outer main belt (where 133P, P/Read, and 176P are found) is not actually that surprising. Laboratory investigations of meteorites and ground-based spectroscopic studies of asteroids in this region have found evidence of hydrated minerals that could only have formed in the presence of water, indicating that water must have been present, at least in the past when these minerals were formed.
The surprise of the MBCs is that water ice seems to still be present. After ~4.6 billion years (the age of the Solar System), it intuitively seems that any ice should have been long baked away. Recent thermal models have shown that this is not the case, however, finding that ice could in fact survive in shallow subsurface reservoirs on MBC-like asteroids over the age of the solar system, insulated from direct solar heating by no more than a few to a few tens of meters of dusty surface material. This subsurface ice could then be "activated" by impacts from smaller asteroids, each perhaps a meter across (which probably strike every ~10,000 years or so), that either completely expose the subsurface ice to direct solar heating or at least greatly reduce the depth of the insulating layer allowing enough heat to penetrate into the ice pocket to drive sublimation. Each collisional activation is estimated to produce activity for ~100-1000 years before the active site is exhausted of volatile material and/or sufficient non-volatile insulating material has reaccumulated to again quench sublimation.
Whether ice is thermally stable in the subsurface layers of main-belt asteroids may not be the entire story, however. An analysis of the implications of expected collision rates (every ~10,000 years) and likely impactor sizes (~meter-sized, creating active sites a few hundred square meters in area) showed that the surface of a 133P-sized asteroid would become significantly collisionally devolatilized over billion-year timescales (on the order of the age of the Themis family). In other words, over a billion years, the peppering of 133P's surface by meter-sized impactors every 10,000 years should deplete near-subsurface ice as surely as solar heating would do to surface ice. One way to avoid such a situation is if 133P and the other MBCs were in fact formed in recent fragmentations of larger asteroids. Larger asteroids could potentially preserve significant ice reservoirs at large depths where they would be well-protected from depletion by both solar heating and impact excavation, and then be broken up into fragments by a particularly large impact via the same process believed to have originally produced the Themis family itself. If this secondary fragmentation event occurred in the recent past, fragments from deep within the parent asteroid could still contain significant subsurface (or even surface) ice, not having existed long enough to be significantly collisionally devolatilized.
David Nesvorny and his colleagues have recently discovered that 133P is actually in fact a member of a young sub-family within the Themis family that is thought to have formed from just such a recent fragmentation event: the Beagle family. Estimated to be <10 million years old, it could quite easily explain how 133P could still possess enough undepleted subsurface ice to exhibit cometary activity today. The other MBCs have not yet been identified as members of either the Beagle family or any other young families, but as more asteroids, and therefore families, continue to be discovered, it may just be a matter of time before they are.
A final note on the origin of MBCs concerns P/Garradd, which as mentioned above, does not appear to be dynamically stable over more than ~20 million years, and so probably originated elsewhere, perhaps in the outer main belt where the other MBCs are found. Even MBCs in stable orbits can be occasionally knocked into other orbits by collisions with other asteroids, and so this must be kept in mind when using MBCs to trace the distribution of ice in the asteroid belt.
What are the astrobiological implications of the MBCs? The Earth is believed to have formed dry owing to its location inside the snow line, and therefore the water we see today must have been delivered from outside the snow line, perhaps by impacting asteroids and comets. For years, comets were regarded as the most likely water delivery agents due to their high ice content. Measurements of the isotopic composition of comets show that the hydrogen isotope deuterium is roughly twice as abundant in comet ice than in ocean water, however, indicating that comets are unlikely to have been the dominant source of terrestrial water. Deuterium abundances in hydrated minerals in certain meteorites appear to be much better matches to ocean water, suggesting that the outer main asteroid belt may be a more likely source for most of the water we see today. The existence of extant ice on MBCs now means that we no longer need to infer the properties of primordial asteroid ice from hydrated minerals. We can study that ice directly because it is still around today.
To be clear, we are not suggesting that main-belt objects, MBCs or otherwise, are currently striking the Earth, rather that this occurred much earlier in the Solar System's history, perhaps during the period of intense impact activity known as the Late Heavy Bombardment. The value of the MBCs then is that they represent a present-day opportunity to investigate a potential primordial source of Earth's water and other volatile species. Investigation of the chemical and isotopic composition of not only the ice on MBCs, but noble gases that might be trapped in that ice, and drawing comparisons with our own ocean water and atmosphere will yield a great deal of insight into the origin of both, and in turn, the origin of life itself. Such measurements are extremely difficult to do from the ground, and most likely will require spacecraft to visit one or more MBCs to perform in situ measurements.
What current research is being done related to MBCs? There is much we still don't know about MBCs. While a great deal of indirect evidence (particularly the longevity of activity on all MBCs, and the distinct but repeated episodes of activity on 133P) indicate that MBC activity is sublimation-driven, spectroscopic confirmation of water ice on either the surface of an inactive MBC or for an active MBC is needed to absolutely confirm this. We also would like to know what the true distribution of MBCs is in the asteroid belt. The current MBC population of six objects is far too small to make generalizations and so more must be discovered, ideally by deep all-sky surveys like Pan-STARRS (now running) and LSST (expected to be running in the near future). Even more important than the active MBC population may be the dormant MBC population (i.e., asteroids that contain subsurface ice but that have not experienced a recent-enough activating impact to currently exhibit activity) as this may constitute ~90% of the total population of icy main-belt asteroids.
In order to be able to assess the size and distribution of this population, it would be beneficial to develop a means for identifying dormant MBCs (i.e., icy asteroids) in the absence of activity, such as a particular spectroscopic feature or color signature. Such work is particularly important in light of the fact that the distribution of active MBCs is not in fact solely dependent on the distribution of icy asteroids, but is in fact also affected by, for example, asteroid volatility (whether ice is near enough to the surface to be activatable), impact rates in a particular region (some regions of the main belt are denser, and experience higher rates of collisions, than others), dynamical mixing (the process that may have delivered P/Garradd to its current location), and object sizes (there are indications that even if larger asteroids are collisionally activated, they may never appear cometary as their surface gravity is too great for dust particles to be ejected by sublimating ice).
To assess the plausibility of MBC ice as a terrestrial water source, we will need to determine the deuterium abundance in that ice, as well as abundances of other key volatile species such as noble gases, which will probably require in situ measurements by a visiting spacecraft. Once we develop a clearer picture of the size of the icy asteroid population (including both active and dormant MBCs) and the typical amount of ice contained within each member of that population, we will also be better equipped to assess whether there was in fact enough ice in main-belt asteroids to actually supply the Earth with enough water to fill the oceans we see today.