DPS 42nd Meeting Abs..

Session Number: 28Session Title: Comets, including WISE Mission ResultsSession Type: Poster28.01: Survival of ice in Main Belt CometsAuthor Block: Dina Prialnik 1 , G. Sarid 2 , K. J. Meech 21 Tel Aviv Univ., Israel, 2 Instute for Astronomy.Presentaon Time: 10/4/2010 4:00 PM - 4:02 PMLocaon: Exhibit HallAbstract: The discovery of the new class of objects, known as Main Belt Comets (MBCs), raises a number of quesons regarding their structure, composionand origin. Whether MBCs were formed in their present locaon or captured from more distant regions during the early bombardment era is sll debated; eitherway, they have spent most of their lifemes in the main belt, which has been considered too hot for ice to survive for any length of me. The low conducvity ofporous cometary material suggests, however, that ice may be retained in the interior of main belt bodies, despite connual insolaon. Indeed, analycalesmates, as well as numerical computaons, indicate that this is possible. We invesgate the ice survival queson by means of detailed numerical modeling oflong-term evoluon for a range of inial parameters. We consider bodies of radii between 150m and 2.5km, composed of ice and dust, adopng two differentdensity values, and two heliocentric distances.The quesons that we address are: (a) To what extent and under what condions (related to structure and composion) may water ice be preserved in MBCs forthe age of the Solar System? and (b) How deep below the surface is the ice expected to be found? We find that small bodies (< 600m in radius) may completelylose the ice over a me corresponding to the age of the Solar System, especially if the density is low and they are relavely close to the Sun. By contrast, in largerbodies (a few km in size) that may have been captured or collisionally formed only 100 Myr ago, ice may be found at depths of only 10m. Such bodies could beeasily acvated by collisions to exhibit cometary acvity.28.02: Search for Acvity in Comet-Asteroid Transion Object 107P/Wilson-HarringtonAuthor Block: Alain Khayat 1 , K. Meech 1 , J. Pichova 1 , N. Schorghofer 1 , B. Yang 1 , S. Sonne 1 , T. Riesen 1 , J. Kleyna 1 , H. Kaluna 1 , J. Keane 11 Instute for Astronomy, University of Hawaii at Manoa.Presentaon Time: 10/4/2010 4:02 PM - 4:04 PMLocaon: Exhibit HallAbstract: Comet-asteroid transion object 107P/Wilson-Harrington was observed near its October 22, 2009 perihelion passage to search for acvity. No acvitywas detected. Consequently, we place limits on possible dust producon of 0.013 kg/s at 1.23 AU. Furthermore, the data was not sufficient to constrain a rotaonperiod; however, it is clear that the rotaon period is > 4hr. Our data is consistent with the observaons of others (6.1 hr). Phase funcon fing yielded a value ofthe phase coefficient beta= 0.0406 ± .0001 mag/deg, similar to C-type asteroids that have a linear phase curve at large phase angles. Thermal models for107P/Wilson-Harrington show that the average loss rate of exposed crystalline ice at zero latude is in the order of 0.3 meters/year. The derived high loss ratesuggests that 107P/Wilson-Harrington is deprived of surface ice. Our observaons and analysis confirm earlier findings that 107P/Wilson-Harrington is anexample of the very few such objects discovered so far. Such study we made is a crical next step in understanding the life of dormant comets, and a window intothe evoluonary end states of the lives of comets that become exnct. This work has been supported in part by AST-0807521 from the Naonal ScienceFoundaon.28.03: A New Model Of Non-gravitaonal Forces On CometsAuthor Block: Lucie Maquet 1 , F. Colas 1 , J. Crovisier 2 , L. Jorda 31 IM CCE-Observatoire De Paris, France, 2 LESIA-Observatoire de Paris, France, 3 LAM, France.Presentaon Time: 10/4/2010 4:04 PM - 4:06 PMLocaon: Exhibit HallAbstract: The gravitaonal orbit of a comet is affected by the sublimaon of water molecules by the nucleus when the comet approaches perihelion. Thisoutgassing triggers a non-gravitaonal force (NGF) which significantly modifies the orbit of the comet. The amplitude of the perturbaon depends on severalparameters which can be constrained by visible, infrared and radio observaons of the coma and nucleus of the comet. It depends also on the nucleus density,which can in turn be determined by modeling the effect of the NGF on the orbit of a comet. This method is the only method available so far to esmate thedensity of cometary nuclei from the ground. Up to now, the modeling of this effect is mostly based on an empirical model defined in the early 70's which uses asimplified isotropic outgassing model. Aempts have been made to use advanced anisotropic thermal models to calculate the NGF taking into account severalobservaonal constraint and to retrieve the nucleus density, but: (i) this approach is restricted to a handful of cometary nuclei which are sufficiently well-knownto allow this type of modeling, and (ii) the authors usually don't fit directly the astrometric measurements but rather « non-gravitaonal parameters » calculatedwith the above-menoned empirical model. We present a new model for non-gravitaonal forces with the aim of revising the problem of NGF calculaon andnucleus density determinaon. The method is based on the separaon of the surface of the nucleus in several strips located at different latudes. Thecontribuon of each strip to the overall NGF is fied from the astrometric measurements together with the density of the nucleus. This new method will be usedto interpret future astrometric measurements of these prisne objects with GAIA.28.04: The Deep Impact Coma of Comet 9P/Tempel 1 as a Time-of-Flight Experiment Movates DDSCAT Models for Porous Aggregate Grains withSilicate Crystal InclusionsAuthor Block: Diane H. Wooden 1 , S. S. Lindsay 2 , D. E. Harker 3 , M. S. Kelley 4 , C. E. Woodward 5 , D. T. Richard 6 , L. Kolokolova 4 , F. Moreno 71 NASA Ames Research Center, 2 New Mexico State University, 3 UCSD/CASS, 4 University of Maryland, 5 University of Minnesota, 6 San Jose State UniversityResearch Foundaon/NASA Ames, 7 Instuto de Astrofisica de Andalucia, Spain.Presentaon Time: 10/4/2010 4:06 PM - 4:08 PMLocaon: Exhibit HallAbstract.Abstract: Spitzer IRS spectra of short-period Eclipc Comets (ECs) have silicate features, and many have disnct crystalline silicate peaks. TheseSpitzer spectra, when fied with thermal models aer subtracon of the relavely strong contribuon of the nuclear flux to the IR spectrum (e.g., Harker et al.2007), demonstrate ECs have weaker silicate features than long-period Nearly-Isotropic Comets (NICs). There are excepons, however, as some NICs also haveweak features like most ECs. Grains with lower porosies (lower fracon of vacuum) can explain weaker silicate features (Kelley and Wooden 2009; Kolokolova etal. 2007). Alternavely, oming the smallest (submicron) solid grains can reduce the contrast of the silicate feature (Lisse et al. 2006). However, so far, onlymodels for solid submicron crystals fit the crystalline peaks in spectra of comets with weak silicate features. This presents a dilemma: how can the coma bedevoid of small grains except for the crystals? The Spitzer spectra of the Deep Impact event with EC 9P/Tempel 1 provides a data set to model larger porousgrains with crystal inclusions because the post-impact coma was a me-of-flight experiment: an impulsive release of grains were size-sorted in me by theirrespecve gas velocies so that the smaller grains departed the inner coma quicker than larger grains. A velocity law derived from fing small beam Gemini