The truth about Main Belt Comets

Two of the known MBCs imaged using the UH 2.2m telescope. 

More images on Hsieh's website - http://star.pst.qub.ac.uk/~hhh/mbcs.shtml 

As far as we can tell so far, not much separates the MBCs from other asteroids, at least the ones in the same region of the main belt. Main-belt asteroids show a fair amount of variation depending on their distance from the Sun, and so the MBCs are certainly different from objects in the inner belt, but among asteroids beyond 3AU from the Sun (where the MBCs are found), all measurements so far show that they are mostly unremarkable, except for the fact that they emit dust like comets from time to time.  One main difficulty in finding new MBCs is that activity is transient, lasting over only ~1/4 of the orbit for an "activated" asteroid (i.e., that has received an impact recently) and probably only lasting 100-1000 years after an activation has occurred. After that, the activity is thought to die away, and then a new impact is required to renew the activity.  As such, one of our main research goals is to find a "special" characteristic of MBCs that set them apart from other asteroids (e.g. a particular spectral feature, or maybe membership in a young, recently-fragmented asteroid family as mentioned in my talk) that would allow us to rapidly pinpoint new MBC candidates in the absence of actual cometary activity.  Since "dormant" or "inactive" MBCs are really just asteroids with ice that doesn't happen to be currently sublimating, identifying these really just means we will be able to identify icy asteroids and chart their distribution in the asteroid belt, which is actually our primary goal in all this.

AN: Is it likely that they formed in situ in the asteroid belt, or migrated in as e.g. trans-Neptunian Objects (TNOs) from the outer Solar System?

HH: The strong similarity of the MBCs to other main-belt asteroids in addition to their general orbital stability (except for P/Garradd) strongly suggests that they formed in situ in the asteroid belt.  It is extremely unlikely that they recently migrated as TNOs from the outer Solar System. Given the current configuration of the major planets, there is simply no clear and consistent way for an object to make such a orbital transition. When only one MBC was known (133P), people could argue that some unique circumstances (very close approaches to terrestrial planets for example, which are chaotic and are therefore essentially impossible to predict the outcome of) had deposited it into the asteroid belt, but once three MBCs were known (with the likelihood of many more), it became far more difficult to argue for this scenario. Something implausible (in principle) can always happen once, but when it must have happened more than once, you're on much shakier ground.  More likely, they are native to the main belt.

The one possible exception is a scenario suggested by Levison et al.'s so-called "Nice model" which suggests that a large number of Kuiper Belt Objects may have been deposited into the main belt during a period of planetary migration (primarily Jupiter and Saturn) linked to the so-called Late Heavy Bombardment (about 3.8 billion years ago).  This is a purely theoretical result so far though, as no clear links have been observationally demonstrated between KBOs and main-belt asteroids. Nonetheless, it emphasises the need to keep an open mind as to the origin of the MBCs, but for now, we believe them to be native main-belt objects.

AN: Does their presence in the Asteroid Belt have any implications for theories of Solar System formation?

HH: Their presence in the asteroid belt does not actually present a significant challenge to current theories of Solar System formation. Asteroids in the outer belt have been long known to show evidence of past water/ice in the form of hydrated minerals (i.e. minerals that formed in the presence of water).  These minerals were thought to have formed long ago, however, meaning that the water only needed to be present long ago. Over 4.5 billion years (the age of the Solar System), any water was thought to be long baked away by the Sun, so it's probably more accurate to say that the MBCs are telling us something interesting about Solar System evolution, rather than formation, specifically about the survivability of ice in close proximity to the Sun on small (kilometre-scale) bodies.

That said, MBCs provide an opportunity for constraining theories of Solar System formation.  We can see if their pristine ice is consistent with the chemical and isotopic composition of hydrated minerals that we've been using thus far to infer the nature of the early Solar System. Once we find enough MBCs to provide a clear picture of the current distribution of ice in the asteroid belt, we can compare this with predictions of various formation models and weed out the inconsistent ones, and so on.

AN: You mentioned that the observed sublimation of ices from the MBCs was likely triggered by impact. Would these have been recent impact events and on what sorts of scale?

HH: We would expect each "pocket" of subsurface ice that is exposed by a single impact on an MBC to eventually all sublimate away, so yes, current activity must be the result of a recent impact.  We tentatively estimate that metre-sized (as opposed to 10m-sized or cm-size) impacting bodies (which excavate areas of a few hundred square metres) are responsible for MBC activity, and we expect activity to die away after roughly 100-1000 years. Such collisions probably occur roughly every 10,000 years, so if an object is sufficiently icy that each impact actually triggers activity (an excavating impact won't create an MBC if all it excavates is more rock), each object could be active over roughly 1%-10% of its life.

P/2010 A2, discovered earlier this year and captured here by the Hubble Space Telescope, turned out to be two asteroids colliding.

AN: How likely is it that many other asteroids are concealing ices beneath their surfaces and therefore might also be MBCs?

HH: We believe that many other asteroids might be concealing ices beneath their surfaces, at least in the outer main belt (outside of about 3 AU from the Sun).  How much ice is actually contained in these bodies (0.1%? 1%? 10%? 50%?) is unknown though, as is the distance of that ice from the surface of each icy body. Over time, ice in the upper surface layers should sublimate away, even if only slowly, and as ice recedes farther and farther down, it will become more difficult for an impact to excavate it to create an active MBC. I didn't mention this in my talk, but there is also the possibility that large asteroids (10km or larger) may never show observable cometary activity due to their size.  We estimated the speed of the dust being emitted from the current MBCs and found it to be generally fairly slow (1-2 metres per second), which as it turns out, is too slow to actually escape the gravity of asteroids much larger than the known MBCs (about 5 km across and smaller), just as rockets must be launched at a certain speed to escape Earth's gravity.  This slow dust ejection speed is probably due to small amounts of ice that are sublimating.  Much less power is behind MBC dust ejection, relative to other comets, so therefore you get slower ejection speeds. If these speeds are typical of all icy main-belt asteroids, it is entirely possible that large icy asteroids could have their ice excavated and sublimate away all the time, but never produce observable comet-like activity because dust launched by the gas released from ice sublimation simply falls back down to the surface of these larger asteroids.  Again, this is an effect we have to keep in mind when using cometary activity as a proxy for tracing water ice in the asteroid belt.

AN: What sort of quantities of ice might be hosted by the MBCs, and are there any astrobiological implications of MBCs in the inner Solar System?

HH: An excellent question, but unfortunately one for which I have no answers for at the moment!  Best guess, I'd say MBCs could be a few percent to as much as 10% ice, but this is a *pure* guess.  Much more work is needed before we can get anywhere close to a real answer for this.  This is actually one of over-arching goals of this research...figuring out how much present-day ice is present in a single MBC, how many MBCs are in the main belt, and therefore how much present-day ice is in the entire asteroid belt. Astrobiology however is mainly concerned with the primordial water content of the main belt (since most of Earth's water was probably delivered soon after its formation, probably during the aforementioned Late Heavy Bombardment, which doesn't require ice to survive for *that* long in the main belt -- hundreds of millions of years, instead of billions of years, which is a fairly significant difference), so we will need to use thermal modelling to extract the original ice distribution of the asteroid belt from the present-day distribution (once we figure *that* out), since ice is likely to have decreased in abundance over the last 4.5 billion years, though given the existence of the MBCs, clearly not as quickly as we previously thought. As mentioned above, the fact that some water is currently stable as ice in main belt asteroids is not terribly relevant to terrestrial formation or astrobiological scenarios, but it *does* represent an intriguing present-day opportunity to probe the cousins of the ancient objects that *did* participate in the delivery of water to Earth all those billions of years ago.