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Project Progress

Survival of ice in Main Belt Comets – a parameter study

The discovery of the new class of objects, known as Main Belt Comets (MBCs), raises a number of questions. These objects exhibit dual characteristics: physically, they appear and behave like active comets, while dynamically they resemble asteroids, residing in the main asteroid belt in stable orbits of relatively low eccentricity. The prevailing concept that comets and asteroids are distinct classes of objects, the former ice-rich and the latter ice-poor, is thus severely challenged. The cometary behavior of MBCs is best illustrated by the earliest discovered such object, 133P/Elst-Pizzaro, which showed similar coma and tail evolution at three consecutive perihelion passages. This implies a thermally driven process, such as activity from water ice sublimation. MBCs most likely formed in-situ, in the outer asteroid belt, although some may have more recently been transported from the outer belt to their current orbits. Previously, any location in the belt had been considered too hot for ice survivability for 4.5 Gyr. The low conductivity of porous cometary material suggests, however, that ice may be retained in the interior of main belt bodies, despite continual insolation. The purpose of this study is to investigate the conditions for survival of water ice, by means of detailed numerical modeling of long-term evolution. The questions that we address are: To what extent and under what conditions may water ice be preserved in MBCs for long durations and how deep below the surface is the ice expected to be found?

We find that large bodies (up to a few km in diameter) may retain ice over the age of the solar system even at shallow depths of the order of 50 m. Smaller bodies in the inner belt, on the other hand, might lose their ices. Members of Collisional families, which may be much younger (as is likely for the currently observed objects), are more likely to have ice at depths as shallow as 10-20 m, accessible to activation by collisions. We have submitted a paper for publication on this project.
Attached are examples of figures from the paper (end of report) with shows the relative depth where water ice can survive as a function of closest approach to the sun (perihelion) and at what distance and size the body complete dehydrates.

Internally-Driven Dust Activity in Comet 22P/Kopff – Thermal and dust-dynamical modeling

Comet 22P/Kopff was one of the targets of the CRAF (Comet Rendezvous Asteroid Flyby) mission, which was canceled by NASA in 1992. As such, it has been observed extensively on different occasions. It was discovered in and experienced about eight close approaches to the giant planets, the most dominant of them with Jupiter. These perturbations affected mostly its inclination and perihelion distance. Its current orbit lies between those of Mars and Jupiter, with a small ecliptic inclination of ~4 degrees and an orbital period of ~6.4 yr. It has a significant coma activity, even before perihelion, and a dust trail, distinguishable as an extended feature in both optical and IR measurements.

Optical CCD observations were obtained during several extensive observing campaigns between 1989-2010, using the UH 2.2-m telescope on Mauna Kea. These were used to make deep composite images to search for dust. Dust-dynamical models were developed for the available observations datasets, using the Finson-Probstein method (F-P). This method models a cometary tail in order to determine onset and cessation of emission, dust particle production rates, sizes, and velocities. These parameters are used as boundary conditions for the thermal models. Thermal evolution models of the nucleus of 22P/Kopff were calculated, by means of a quasi-3D thermal evolution code. For these specific models we have focused on the location of the directed emission, as derived from the dust-dynamical modeling. We examine the dependence of the activity on a few key parameters by following the outgassing of volatiles, ejection of dust and heat transport inside the nucleus. We have found that the models suggest that there was directed activity from the southern hemisphere of the comet, many months after this region of the nucleus would have been out of direct sunlight, suggesting that the source of activity comes from volatiles at depth (e.g. CO or CO₂). This is one of the first in-depth test of utilizing coupled thermal and dust modeling and will be developed as a powerful tool in a thesis project. A paper is in preparation on this project.

The accompanying figure shows the composite image of the comet, from the 1992 observation, and contour plots of the image and the corresponding best-fit dust model. A second figure shows the dust production rates, for several variation of the nucleus model. Each variant model is represented by the two orbits that had the most similar dust activity behavior to the one derived from the F-P analysis.

Dynamical and thermal pathways in the evolution of Centaur objects

The structural and thermal evolution of small Solar system bodies may be strongly dependent on their dynamical history and environment. Objects on planet-crossing orbits are prone to gravitational perturbations that de-stabilize their orbits. Such are the Centaurs, which are the transient population, between the relatively stable trans-Neptunian objects (TNOs) and the short-lived Jupiter family Comets (JFCs). This may indicate that these objects experience intermediate levels of internal processing, at different periods of their lives. Examining the evolution of several these Centaur objects, both in orbital and physical parameters, can help categorize the different states and origin and evolution scenarios in the outer Solar system.

Determining the dynamical evolution of each object is achieved through statistical analysis of the results of multiple N-body integrations. This is achieved by using many clones of specific objects, with known orbital elements. Statistics of large clone samples of specific objects yield valuable information about their current states and future fates. Specifically, and with greater importance to thermal evolution, we focus on the dynamical lifetimes, survivability and mean orbital elements. Profiles of temperature, structure and composition are calculated utilizing our robust thermal evolution code several specific objects, which represent slightly varying dynamical groups, and for different orbits of the same object, which represent specific orbital evolution pathways.
In this study we aim to investigate the scenarios for the origin and evolution of a Centaur object, in terms of its orbital and thermo-physical evolution. The following are the components of this project: (i) Follow the long-term dynamical evolution of several representative samples of unstable orbits in the trans-Neptunian space, in order to identify the source population of Centaurs, formulate a time-dependent injection rate of objects into the giant planet region and constrain the size-distribution of Centaurs; (ii) Study the stability regions in the Centaur orbital space (outer Solar System), by means of statistical analysis of many clones of observed Centaur objects; (iii) Study the interior thermal evolution of various objects, as dependent on their dynamical stability, perihelion distance, size and composition, and its relation to observed surface properties.

Project progress – Modeling efforts for the 3 components of the projects are on their way. Thermal modeling has been completed for ~5 Centaur objects. Dynamical modeling of the observed Centaur population has been completed and preliminary statistical analysis has been done. Dynamical modeling of the trans-Neptunian source population, in collaboration with Dr. Lykawka from Japan’s Kinki University, has been initiated recently.

Attached are 3 figures, representing the different components of the project showing the initial populations for the long-term integrations, the composition and cumulative distribution functions for the lifetimes of the Centaurs