133P/Elst-PizarroThe existence and behavior of 133P/(7968) Elst-Pizarro (EP for short) poses serious problems for our understanding of comets and asteroids and how they relate to one another. Comets are assumed to originate in deep freeze of the outer solar system because of the need to preserve until the present time all the volatile ices that sublimate so rapidly and so spectacularly whenever one of these "dirty snowballs" happens to get knocked into the inner solar system. Asteroids are supposed to be rocky, inert remnants of a terrestrial planet that was perhaps destroyed early on or more likely never was able to form in the first place.
Because of their disparate origins, comets and asteroids are generally seen to occupy distinctly different orbital states. Asteroids typically are on low eccentricity (mostly circular) and low inclination orbits, just like the major planets. Comets on the other hand are known for their highly elliptical orbits that take them from the outer reaches of the solar system into the inner solar system, back out again, and so on. These orbits also tend to be more highly inclined than that of most main belt asteroids.
EP was discovered in 1979 as a perfectly ordinary main belt asteroid (with a prototypically low eccentricity and low inclination) and was largely ignored until 1996 when Eric Elst and Guido Pizarro serendipitously discovered a long thin dust trail emanating from the object. Analysis of this and further observations of the trail revealed that the dust had likely been emanating from EP for many months, making it difficult to explain the activity as a consequence of a simple impact.
Re-observation of EP in 2002 by David Jewitt and me on the University of Hawaii 2.2-meter telescope revealed the return of the dust trail, an observation that due to the expected rarity of even a single impact on a given body, much less two in the span of 6 years, virtually completely ruled out the possibility that the activity could be the result of impacts onto a non-volatile surface (as would be expected for an "ordinary" asteroid).
Even after our extensive analysis of EP in 2002, its precise nature remains an open question. What is certain now is that EP's dust trail is the result of dust ejection via the sublimation of surface volatiles (ices). Considering the intermittency of the dust production, this volatile material is likely distributed on EP's surface in only isolated patches, since if it were distributed uniformly on the surface, we would expect to see dust emission all along EP's essentially circular orbit, which we do not. If the volatile material is confined to a single patch, we could expect to see a seasonal effect by which the patch receives enough solar illumination to produce observable dust emission via sublimation along only a certain segment of its orbit. Currently the two separate observations of an active EP, in 1996 and 2002, roughly one orbit period apart, support this hypothesis, but continued monitoring observations (currently being conducted on the University of Hawaii 2.2-meter and Keck 10-meter telescopes on Mauna Kea) will be necessary to verify its plausibility and also ascertain whether other exposed volatile patches are present elsewhere on EP's surface.
Key questions are how did that volatile material get there and how does one explain its present existence? Is EP an otherwise ordinary comet that has somehow evolved onto a main-belt orbit? If so, the existence of volatiles would be easy to explain: comets are known to contain ice, having spent the majority of their lifetimes in the cold outer solar system in the Kuiper Belt or Oort Cloud, and if EP is only a recent arrival to the asteroid belt, there may not have been enough time for all of its ices to sublimate away. It turns out, however, that the evolution of a comet onto an EP-like orbit is a difficult dynamical trick. No current dynamical models that rely completely on gravitational perturbations (from interactions with the Sun, planets, and other asteroids) are able to reproduce such a transition, and those models that incorporate non-gravitational forces (primarily the Yarkovsky effect and forces imparted by cometary jets or asymmetrical cometary mass loss) have thus far only produced inconclusive results.
Alternatively, EP could be an ordinary asteroid that has managed to preserve ice just below its rocky surface, and has had a small patch of it recently exposed by an impact by another asteroid. Asteroids are known to have once contained water -- hydrated minerals have been found in meteorites found on Earth -- which could have become icy, and as an apparent member of the Themis asteroid family, EP is in fact in a relatively dense region of the asteroid belt and thus has a somewhat elevated risk of collisions. What is currently not clear, however, is if it is in fact possible for ice to be buried deeply enough to survive 4.5 billion years (the age of the solar system, and therefore the asteroid belt) of solar heating only 3 AU from the Sun, but still be shallow enough that an impact from another asteroid could dig it out. Also, if EP contains ice, other asteroids should as well. Why have we not observed such behavior from other asteroids, either from other Themis family members or elsewhere? The answer to this question may lie in the fact that most small EP-sized asteroids, in the Themis family or otherwise, simply haven't been studied very extensively. We are currently conducting a deep optical survey of selected main belt asteroids in an effort to rectify this situation and shed more light on EP's nature.