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Karen Jean Meech -- Distant Comet Research

Image credit: John Foster

Meech Research

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Basic Solar Nebula Models

Comets are fundamentally important to the study of the planetary science because it is believed that they best represent the primitive material out of which the outer solar system formed. Furthermore, they may have been an important source of volatiles for the terrestrial planets. Cometary origins begin much earlier than the collapse of the protosolar nebula, as the refractory dust grains probably form in the envelopes of evolved stars. The solid carbonaceous matter which is present in comets is believed to have arisen from UV photolysis and high energy polymerization of the icy mantles which have condensed on these interstellar silicate grains. The reactive radicals then combine to form an organic refractory residue after billions of years of processing. These interstellar grains are known from the interstellar extinction curves to range in size from 0.01 to 0.1 microns. These grains may develop an additional icy mantle in the molecular cloud cores prior to their incorporation into comets during collapse of the solar nebula. Unaltered grains in comets may have brought organic materials to the early Earth.

For physical and orbital dynamics reasons it was previously believed that most comets formed in the Uranus-Neptune zone where solar nebula temperatures were low, between about 25-60K. In this scenario, the molecular abundance of the ices found on grains should reflect the interstellar cloud abundances, with depletions only of the most volatile elements. Current models are now suggesting more massive disks out to larger r, however. Precursor grains from the ISM may be heated significantly as they fell through the accretion shock caused by supersonic infall of gas onto the more massive protoplanetary nebula (Lunine & Engle). Inside of 30 AU nearly 90% of the waterice grain mass would sublimate due gas dynamic heating, with the sublimation decreasing with distance. At the ambient temperature of the cold grains in the nebula, the water vapor condenses on the grains. Most of the highly volatile gases in solar abundance (e.g. CH4, CO and N2) will have pressures below their saturated values in the nebula for temperatures above 20K, and cannot condense into their pure solid phases, and instead will be trapped in the condensing water vapor. Laboratory experiments are providing insight into which types of gases might be trapped in the condensing water vapor as a function of temperature, hence distance from the protostar, and at what abundances relative to water. From these experiments, it is clear that the amount of highly volatile materials which can be trapped in comets as the ices recondense is an extremely sensitive function of condensation temperature. Even in the presence of a significant amount of trapped volatile materials in the water-ice, the sublimation of the ices upon heating as the comet approaches the sun are largely controlled by water (because of the strong molecular bonds). However, H2O ice which condensed below 137K (which is certainly the case for conditions in the outer nebula), will condense in an amorphous phase. Upon heating, the guest molecules will have a large effect on the comet outgassing well below the waterice sublimation temperatures as the open structures in the ice close and the ice converts from the amorphous to the crystalline phase. This will occur only once as the comet enters the inner solar system for the first time and should have observable consequences (i.e. exhibit activity) at large r.

Formation Location of Comets

With the development of Whipples icy conglomerate model of the comet nucleus and Oorts inference that a vast reservoir of comets at large heliocentric distances was the source of the long- period comets-, a controversy began to emerge over the dynamical mechanism for the source of the short period comets. Kuiper proposed that the volatile composition of the comets could be explained if they had a source in the outer solar system - at least as far out as the giant planets. The preferred region is the Uranus-Neptune zone because objects ejected into the Oort cloud from the close passage to the giant planets in the JupiterSaturn region would tend to leave the inner solar system on hyperbolic trajectories rather than elliptical trajectories. The current consensus from extensive dynamical model simulations is that the shortperiod comet population is composed of members from two dynamical reservoirs: the Halley-Family comets having evolved from the Oort cloud and the Jupiterfamily comets having come from the Kuiper belt. Comet formation and orbital evolution is now described by a complex dynamical system, where only the long-period comets have sources in the Oort cloud, and some are further perturbed into the high-inclination short-period comet orbits. The JupiterFamily comets preserve the low inclination distribution of their source region - a flattened region of low eccentricity orbits with perihelia outside Neptunes orbit - the Kuiper Belt. These comets formed at larger heliocentric distances than did the Oort cloud comets. With the current efforts in understanding the dynamics of the shortperiod comets, it is clear that the different formation locations may imply differences in composition which must be decoupled from the aging processes. The present research aims are to search for compositional differences in dynamical groups, due either to initial formation differences or to evolutionary processes (aging) with the ultimate goal of understanding the early solar system physics, chemistry and temperature profiles in the outer solar nebula. The program has observationally established, for the first time, that there are significant physical differences between the dynamical comet classes.

Current Research Program

Within the past decade we finally have the technology (highly sensitive CCD cameras on large telescopes) which will allow us to follow comets over a large fraction of their orbits to study the development and evolution of the dust coma created as the sublimating ices leave the nucleus surface. In particular, the activity which is now being seen at large distances, beyond the distance at which water ice can sublimate (6 AU) is giving us our best evidence for the internal structure and physics of comets which can ultimately be closely tied to formation location, conditions and hence solar nebula models (because of the extreme temperature sensitivity of the trapping of guest volatile ices). The observing project is not only challenging because of the need to create a uniform data set over a long time period, but also both because of the need to maintain very accurate orbits for the comets, and most importantly because of their extreme faintness as they move away from the sun. The comets faintness dictates that not only are the worlds largest telescopes are required, but that great care must be taken in both the observation and reduction of the data. Techniques identical to those used for deep extragalactic survey work are used which enable the data frames themselves to be used as part of the calibration process. This allows for the detection of low-surface brightness objects which are only a small fraction of the night sky background to be detected.

ObservatoryTelescope# Clear Nights
Kitt Peak 2.1m 22
Kitt Peak 4.0m 5
Kitt Peak Schmidt 4
Cerro Tololo 0.9m 8
Cerro Tololo 1.5m 12
Cerro Tololo 4.0m 8
Cerro Tololo Schmidt 21
Mauna Kea UH 2.2m 107
Mauna Kea CFHT 3.6m 5
Mauna Kea UKIRT 3.8m 2
Mauna Kea Keck 10m 1
Mt. Bigelow 1.3m 3
European Southern Obs.NTT 3.6m 28
Zelenchukskaja 6m 3
Hubble Space Telescope-- 32 orbits
-- Total Nights229

Fundamental Data Base and Results

The data set comprises observations of a selection of approximately 50 comets over a wide range of r, both as they are approaching the sun and moving away from it. From this data a pattern has emerged which shows for the first time that the comets which are coming in from the Oort cloud are extremely active compared to the short period comets (which originated in the Kuiper Belt). This behavior is shown in Figure 1 (pdf) which shows brightness curves as a function of distance for comets Shoemaker 1987o (a representative Oort comet; filled triangles), P/Halley (open squares) and periodic comets (dots). It is clear from the figure that P/Halley faded much more rapidly at large r than did the Oort comets, and in addition it was fainter than all of the Oort comets at large r.

Because the short period comets formed in situ in the Kuiper Belt, and the Oort comets formed closer in to the sun at higher temperatures, in the Uranus-Neptune region where they should have been able to trap less gas, we would expect that the short period comets would be the most active. The greater activity in the Oort comets is a clear indication that the short period comets have aged, or lost most of their primordial volatile materials, and that the study of the distant Oort comets is the key to understanding the boundary conditions on the early solar system formation.

Figure 2 - Sequence of images of Comet Shoemaker 1987o, an Oort Cloud comet at a range of distances between 5-18AU, showing an extensive tail at all distances.

Last modified: February 4, 2001
Karen Meech
Institute for Astronomy
2680 Woodlawn Drive
Honolulu, HI 96822