Michael J. S. Belton

Exposed water ice deposits on the surface of comet 9P/Tempel 1

We report the direct detection of solid water ice deposits exposed on the surface of comet 9P/Tempel 1, as observed by the Deep Impact mission. Three anomalously colored areas are shown to include water ice on the basis of their near-infrared spectra, which include diagnostic water ice absorptions at wavelengths of 1.5 and 2.0 micrometers. These absorptions are well modeled as a mixture of nearby non-ice regions and 3 to 6% water ice particles 10 to 50 micrometers in diameter. These particle sizes are larger than those ejected during the impact experiment, which suggests that the surface deposits are loose aggregates. The total area of exposed water ice is substantially less than that required to support the observed ambient outgassing from the comet, which likely has additional source regions below the surface.

The atmosphere of 2060 Chiron

An explanation for 2060 Chirons behavior, which focuses on the influence of Chirons mass on the development of its dust coma, is presented. It is suggested that dust is entrained by the flow of CO or another gas of similar volatility from an active region. It remains gravitationally bound on orbits confined to a region, roughly 5000 km in extent, that lies between the surface and an exopause imposed by radiation pressure forces. The influence of radiation pressure transforms the initial particle trajectories into satellite orbits with a characteristic period of 20 days and orbital residence time of about 25 revolutions. The particle population in the coma slowly increases, explaining Chirons photometric behavior. 74 refs.

Spitzer Space Telescope Observations of the Nucleus of Comet 103P/Hartley 2

We have used the Spitzer Space Telescope InfraRed Spectrograph (IRS) 22-μm peakup array to observe thermal emission from the nucleus and trail of comet 103P/Hartley 2, the target of NASA’s Deep Impact Extended Investigation (DIXI). The comet was observed on UT 2008 August 12 and 13, while 5.5 AU from the Sun. We obtained two 200 frame sets of photometric imaging over a 2.7 hr period. To within the errors of the measurement, we find no detection of any temporal variation between the two images. The comet showed extended emission beyond a point source in the form of a faint trail directed along the comet’s antivelocity vector. After modeling and removing the trail emission, a NEATM model for the nuclear emission with beaming parameter of 0.95 ± 0.20 indicates a small effective radius for the nucleus of 0.57 ± 0.08 km and low geometric albedo 0.028 ± 0.009 (1σ). With this nucleus size and a water production rate of 3 × 10^(28) molecules s^(-1) at perihelion, we estimate that ~100% of the surface area is actively emitting volatile material at perihelion. Reports of emission activity out to ~5 AU support our finding of a highly active nuclear surface. Compared to Deep Impact’s first target, comet 9P/Tempel 1, Hartley 2’s nucleus is one-fifth as wide (and about one-hundredth the mass) while producing a similar amount of outgassing at perihelion with about 13 times the active surface fraction. Unlike Tempel 1, comet Hartley 2 should be highly susceptible to jet driven spin-up torques, and so could be rotating at a much higher frequency. Since the amplitude of nongravitational forces are surprisingly similar for both comets, close to the ensemble average for ecliptic comets, we conclude that comet Hartley 2 must have a much more isotropic pattern of time-averaged outgassing from its nuclear surface. Barring a catastrophic breakup or major fragmentation event, the comet should be able to survive up to another 100 apparitions (~700 yr) at its current rate of mass loss.

Rotationally Resolved 8-35 Micron Spitzer Space Telescope Observations of the Nucleus of Comet 9P/Tempel 1

We have utilized the Spitzer Space Telescope (SST) Infrared Spectrograph (IRS) to directly observe thermal emission from the nucleus of comet 9P/Tempel 1 on UT 2004 March 25-27. We obtained 8-35 μm low-resolution (R ~ 100) spectra and contemporaneous 16 and 22 μm photometric imaging over a 39 hr period. The comet was 3.7 AU from the Sun at the time, approximately 464 days before perihelion on 2005 July 5, and showed no evidence of extended emission beyond a point source. Visual inspection of the absolute photometry implies a rotation period of 40 ± 2 hr, consistent with earlier results. Snapshot photometry by Spitzer at 8 and 24 μm, taken on UT 2004 March 10 and 15, respectively, are consistent with this light-curve phasing and with the IRS-measured flux. The spectra agree well with the predictions of the standard thermal model for a slowly rotating body with thermal inertia between 0 and 50 J K-1 m-2 s-1/2, and are inconsistent with any rapid rotator model. The mean effective radius at the middle of the light curve is 3.3 ± 0.2 km. The maximum-to-minimum flux ratio of 1.8 in the light curve is consistent with an axial ratio a/b of 3.2 ± 0.4, implying a = 7.2 ± 0.9 km and b = 2.3 ± 0.3 km. Combining our SST infrared light curve with visible observations of the nucleus, we obtain a visible geometric albedo of 0.04 ± 0.01. With this sized nucleus and the published water production rates, we estimate that 9% ± 2% of the surface area is actively emitting volatile material at perihelion.