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Tucson, AZ, United States

Belton M.J.S.,Belton Space Exploration Initiatives | Belton M.J.S.,Astronomer
Icarus | Year: 2015

I show that the size-distribution of small scattered-disk trans-neptunian objects when derived from the observed size-distribution of Jupiter Family comets (JFCs) and other observational constraints implies that a large percentage (94-97%) of newly arrived active comets within a range of 0.2-15.4km effective radius must physically disrupt, i.e., macroscopically disintegrate, within their median dynamical lifetime. Additional observational constraints include the numbers of dormant and active nuclei in the near-Earth object (NEO) population and the slope of their size distributions. I show that the cumulative power-law slope (-2.86 to -3.15) of the scattered-disk TNO hot population between 0.2 and 15.4km effective radius is only weakly dependent on the size-dependence of the otherwise unknown disruption mechanism. Evidently, as JFC nuclei from the scattered disk evolve into the inner Solar System only a fraction achieve dormancy while the vast majority of small nuclei (e.g., primarily those with effective radius <2km) break-up. The percentage disruption rate appears to be comparable with that of the dynamically distinct Oort cloud and Halley type comets (Levison, H.F., Morbidelli, A., Dones, L., Jedicke, R., Wiegert, P.A., Bottke Jr., W.F. [2002]. Science 296, 2212-2215) suggesting that all types of comet nuclei may have similar structural characteristics even though they may have different source regions and thermal histories. The typical disruption rate for a 1km radius active nucleus is ~5×10-5disruptions/year and the dormancy rate is typically 3 times less. We also estimate that average fragmentation rates range from 0.01 to 0.04events/year/comet, somewhat above the lower limit of 0.01events/year/comet observed by Chen and Jewitt (Chen, J., Jewitt, D.C. [1994]. Icarus 108, 265-271). © 2014 Elsevier Inc.

Lisse C.M.,Johns Hopkins University | Christian D.J.,California State University, Northridge | Wolk S.J.,Harvard - Smithsonian Center for Astrophysics | Dennerl K.,Max Planck Institute for Extraterrestrial Physics | And 8 more authors.
Icarus | Year: 2013

We present results from the Chandra X-ray Observatory's characterization of the X-ray emission from Comet 103P/Hartley 2, in support of NASA's Deep Impact Extended close flyby of the comet on 04 November 2010. The comet was observed 4 times for a total on target time of ∼60ks between the 17th of October and 16th of November 2010, with two of the visits occurring during the EPOXI close approach on 04 November and 05 November 2010. X-ray emission from 103P was qualitatively similar to that observed for collisionally thin Comets 2P/Encke (Lisse, C.M. et al. [2005]. Astrophys. J. 635, 1329-1347) and 9P/Tempel 1 (Lisse, C.M. et al. [2007]. Icarus 190, 391-405). Emission morphology offset sunward but asymmetrical from the nucleus and emission lines produced by charge exchange between highly stripped C, N, and O solar wind minor ions and coma neutral gas species were found. The comet was very under-luminous in the X-ray at all times, representing the 3rd faintest comet ever detected (LX=1.1±0.3×1014ergs-1). The coma was collisionally thin to the solar wind at all times, allowing solar wind ions to flow into the inner coma and interact with the densest neutral coma gas. Localization of the X-ray emission in the regions of the major rotating gas jets was observed, consistent with the major source of cometary neutral gas species being icy coma dust particles. Variable spectral features due to changing solar wind flux densities and charge states were also seen. Modeling of the Chandra observations from the first three visits using observed gas production rates and ACE solar wind ion fluxes with a charge exchange mechanism for the emission is consistent with the temporal and spectral behavior expected for a slow, hot wind typical of low latitude emission from the solar corona interacting with the comet's neutral coma. The X-ray emission during the 4th visit on 16 November 2010 is similar to the unusual behavior seen for Comet 17P/Holmes in 2007 (Christian, D.J. et al. [2010]. Astrophys. J. Suppl. 187, 447-459) as the solar wind became dominated by a less ionized and faster plasma, more typical of outflow from polar coronal hole regions. We postulate that the overall faintness of the comet seen during all visits is due to the unusually well mixed dust and gas content of this hyperactive comet's coma producing Auger electrons rather than X-rays via charge exchange with the solar wind. An alternative possible explanation for the faintness of the comet's X-ray emission, and its unusual high CV and unusually low CVI emission, is that the impinging solar wind was drastically slowed in the inner coma, below 150kms-1, before charge exchanging with cometary neutrals. © 2012 Elsevier Inc.

Thomas P.,Cornell University | A'Hearn M.,University of Maryland University College | Belton M.J.S.,Belton Space Exploration Initiatives | Brownlee D.,University of Washington | And 21 more authors.
Icarus | Year: 2013

The nucleus of comet Tempel 1 has been investigated at close range during two spacecraft missions separated by one comet orbit of the Sun, 51/2. years. The combined imaging covers ∼70% of the surface of this object which has a mean radius of 2.83 ± 0.1. km. The surface can be divided into two terrain types: rough, pitted terrain and smoother regions of varying local topography. The rough surface has round depressions from resolution limits (∼10. m/pixel) up to ∼1. km across, spanning forms from crisp steep-walled pits, to subtle albedo rings, to topographic rings, with all ranges of morphologic gradation. Three gravitationally low regions of the comet have smoother terrain, parts of which appear to be deposits from minimally modified flows, with other parts likely to be heavily eroded portions of multiple layer piles. Changes observed between the two missions are primarily due to backwasting of scarps bounding one of these probable flow deposits. This style of erosion is also suggested by remnant mesa forms in other areas of smoother terrain. The two distinct terrains suggest either an evolutionary change in processes, topographically-controlled processes, or a continuing interaction of erosion and deposition. © 2012 Elsevier Inc.

Groussin O.,French National Center for Scientific Research | A'Hearn M.,University of Maryland University College | Belton M.J.S.,Belton Space Exploration Initiatives | Farnham T.,University of Maryland University College | And 7 more authors.
Icarus | Year: 2010

We present results on the energy balance of the Deep Impact experiment based on analysis of 180 infrared spectra of the ejecta obtained by the Deep Impact spacecraft. We derive an output energy of 16.5 (+9.1/-4.1) GJ. With an input energy of 19.7 GJ, the error bars are large enough so that there may or may not be a balance between the kinetic energy of the impact and that of outflowing materials. Although possible, no other source of energy other than the impactor or the Sun is needed to explain the observations. Most of the energy (85%) goes into the hot plume in the first few seconds, which only represents a very small fraction (<0.01%) of the total ejected mass. The hot plume contains 190 (+263/-71) kg of H2O, 1.6 ± 0.5 kg of CO2, 8.2 (+11.3/3.1) kg of CO (assuming a CO/H2O ratio of 4.3%), 27.9 (+25.0/-8.9) kg of organic material and 255 ± 128 kg of dust, while the ejecta contains ∼107 kg of materials. About 12% of the energy goes into the ejecta (mostly water) and 3% to destroy the impactor. Volatiles species other than H2O (CO2, CO or organic molecules) contribute to <7% of the energy balance. In terms of physical processes, 68% of the energy is used to accelerate grains (kinetic energy), 16% to heat them, 6% to sublimate or melt them and 10% (upper limit) to break and compress dust and/or water ice aggregates into small micron size particles. For the hot plume, we derive a dust/H2O ratio of 1.3 (+1.9/-1.0), a CO2/H2O ratio of 0.008 (+0.009/-0.006), an organics/H2O ratio of 0.15 (+0.29/-0.11) and an organics/dust ratio of 0.11 (+0.30/-0.07). This composition refers to the impact site and is different from that of the bulk nucleus, consistent with the idea of layers of different composition in the nucleus sub-surface. Our results emphasize the importance of laboratory impact experiments to understand the physical processes involved at such a large scale. © 2009 Elsevier Inc.

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