Energy balance of the Deep Impact experiment

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.     

Swift ultraviolet photometry of the Deep Impact encounter with Comet 9P/Tempel 1

We report time-resolved imaging UV photometry of Comet 9P/Tempel 1 during the interval 2005 June 29-2005 July 21, including intensive coverage of the collision with the Deep Impact probe and its immediate aftermath. The nuclear flux of the comet begins to rise within minutes of the collision, and peaks about 3 h after impact. There is no evidence for a prompt flash at the time of impact. The comet exhibits a significant re-brightening about 40 h after the initial outburst, consistent with the rotation period of the comet, with evidence for further periodic re-brightenings on subsequent rotations. Modelling of the brightness profile of the coma as a function of time suggests two distinct velocity systems in the ejecta, at de-projected expansion speeds of 190 and 550 m/s, which we suggest are due to dust and gas, respectively. There is a distinct asymmetry in the slower-moving (dust) component as a function of position angle on the sky. This is confirmed by direct imaging analysis, which reveals an expanding plume of material concentrated in the impact hemisphere. The projected expansion velocity of the leading edge of this plume, measured directly from the imaging data, is 190 m/s, consistent with the velocity of the dust component determined from the photometric analysis. From our data we determine that a total of (1.4±0.2)×¹⁰³² water molecules were ejected in the impact, together with a total scattering area of dust at 300 nm of 190±20 km².     

Imaging polarimetry of the dust coma of Comet Tempel 1 before and after Deep Impact at Haute-Provence Observatory

We have observed the coma of Comet 9P/Tempel 1, the target of the Deep Impact mission, by the polarization imaging technique, before and after the impact event (−32, −7, +43 and +65 h). Our observations were conducted in the red wavelength domain from Haute-Provence Observatory (France), with the 80-cm telescope. The overall polarization of 9P/Tempel 1, as obtained near 41° phase angle, is monitored and compared to data from other (active and less active) comets studied by the same technique. The linear polarization of the dust ejected by the impact is compared to previous observations of dust present in jets, ejected during outbursts or released when comets happen to split. At phase angles of about 41°, the difference in polarization between the comets with a low maximum in polarization and the comets with a high maximum in polarization is about 1%; it may thus be difficult to conclude about the classification. Nevertheless, the overall polarization after the impact rapidly reached a value corresponding to the high polarization class of comets, and later progressively decreased to its initial value. The polarization was measured to be slightly lower (about 1%) before the impact than after it in a 26,000-km aperture. The plume formed from dust ejected by the impact was still present 65 h after it. The variations of the intensity and the polarization in the coma provide some clues to variations of the physical properties of the particles; comparison with other techniques corroborates the presence of large particles and of submicron-sized grains in aggregates.     

Investigations of the pre-Deep Impact morphology of the dust coma of Comet Tempel-1

The pre-Deep Impact images of Comet Tempel-1 obtained at the Indian Astronomical Observatory are used to investigate the morphology of the dust coma of the comet. We show that the trajectory of a cometary grain under the influence of solar radiation pressure is a reliable diagnostic to estimate its initial velocity. Four main active regions at mean latitudes +45° ± 5°(D), 0° ± 5° (E),−30° ± 5°(A) and−60° ± 5°(F) are found to explain the morphology of the dust coma in the ground-based and published images obtained by the High Resolution Instrument(HRI) cameras aboard the Deep Impact flyby spacecraft. From a χ² fit of the intensity distribution in the observed and the simulated images, we derive the fraction of the productivity of the active vents to the total dust emission of the comet to be 27%. Of this the southern source alone accounts for 19.8%. The grains are found to be ejected with a velocity distribution with an upper limit of 70 ± 7 m s⁻¹. However, the broad region ‘A’ appears to eject slower grains with an upper limit of 24 ± 2.5 m s⁻¹. This source, that is active throughout the cycle is likely to be driven by CO₂ sublimation. We compute the dependence of the percentage contribution of the southern source on the heliocentric distance and show that this ratio varies over the apparition and reaches a maximum at around 260 days before perihelion. The published images of the nucleus of Comet Tempel-1 show significant departure from sphericity. Therefore, the torque exerted by the enhanced activity of the southern region may be significant enough to produce changes in the rotational state of the nucleus before each perihelion passage.     

Palomar and Table Mountain observations of 9P/Tempel 1 during the Deep Impact encounter: First results

We present the first results of the Palomar Adaptive Optics observations taken during the Deep Impact encounter with 9P/Tempel 1 in July 2005. We have combined the Palomar near-IR imaging data with our visual wavelength images obtained simultaneously at JPL's Table Mountain Observatory to cover the total wavelength range from 0.4 to 2.3 μm in the B, V, R, I, J, H, and K filter bands, spanning the dates from 2005 July 03-07. We also include in our overall analysis images taken on the pre-encounter dates of June 1 and June 15, 2005. The broad wavelength range of our observations, along with high temporal resolution, near-IR sensitivity, and spatial resolution of our imaging, have enabled us to place constraints on the temperature of the impact flash and incandescent plume of >700 K, and to provide mean dust velocities of order approximately 1.25 h after impact derived from our 1.64 μm observations. Our ejected dust mass estimates, as derived from our near-IR observations, are an order of magnitude less than those previously reported for visual wavelength observations.     

Spectropolarimetry of the Deep Impact target Comet 9P/Tempel 1 with HiVIS

High resolution spectropolarimetry of the Deep Impact target, Comet 9P/Tempel 1, was performed during the impact event on July 4th, 2005 with the HiVIS spectropolarimeter and the AEOS 3.67-m telescope on Haleakala, Maui. We observed atypical polarization spectra that changed significantly in the few hours after the impact. The polarization of scattered light as a function of wavelength is very sensitive to the size and composition (complex refractive index) of the scattering particles as well as the scattering geometry. As opposed to most observations of cometary dust, which show an increase in the linear polarization with the wavelength (at least in the visible domain and for phase angles greater than about 30, a red polarization spectrum) observations of 9P/Tempel 1 at a phase angle of 41° beginning 8 min after impact and centered at 6:30 UT showed a polarization of 4% at 650 nm falling to 3% at 950 nm. The next observation, centered an hour later showed a polarization of 7% at 650 nm falling to 2% at 950 nm. This corresponds to a spectropolarimetric gradient, or slope, of −0.9% per 1000 Å 40 min after impact, decreasing to a slope of −2.3% per 1000 Å an hour and a half after impact. This is an atypical blue polarization slope, which became more blue 1 h after impact. The polarization values of 4 and 7% at 650 nm are typical for comets at this scattering angle, whereas the low polarization of 2 and 3% at 950 nm is not. We compare observations of Comet 9P/Tempel 1 to that of a typical comet, C/2004 Machholz, at a phase angle of 30° which showed a typical red slope, rising from 2% at 650 nm to 3% at 950 nm in two different observations (+1.0 and +0.9% per 1000 Å).     

The gas production of Comet 9P/Tempel 1 around the Deep Impact date

The target of the Deep Impact space mission (NASA), Comet 9P/Tempel 1, was observed from two nights before impact to eight nights after impact using the FORS spectrographs at the ESO VLT UT1 and UT2 telescopes. Low resolution optical long-slit spectra were obtained to study the evolution of the gas coma around the Deep Impact event. Following first results of this observing campaign on the CN and dust activity [Rauer, H., Weiler, M., Sterken, C., Jehin, E., Knollenberg, J., Hainaut, O., 2006. Astron. Astrophys. 459, 257-263], this work presents a study of the complete dataset on CN, C₂, C₃, and NH₂ activity of Comet 9P/Tempel 1. An extended impact gas cloud was observed moving radially outwards. No compositional differences between this impact cloud and the undisturbed coma were found as far as the observed radicals are concerned. The gas production rates before and well after impact indicate no change in the cometary activity on an intermediate time scale. Over the observing period, the activity of Comet 9P/Tempel 1 was found to be related to the rotation of the cometary nucleus. The rotational lightcurve for different gaseous species provides indications for compositional differences among different parts of the nucleus surface.     

Imaging polarimetry of Comet 9P/Tempel before and after the Deep Impact

The NASA's Deep Impact mission was the first impact experiment to a cometary nucleus. The target of the mission was Comet 9P/Tempel, one of the Jupiter family comets. The impact was performed on July 4th, 2005. Imaging polarimetric observations were carried out by Polarimetric Imager for COmets (PICO) mounted on the Lulin One-meter Telescope (LOT) at Lulin Observatory, Taiwan. Intensity and linear polarization degree maps were obtained on July 3-5, 2005. Impact ejecta plume was clearly recognized in the enhanced intensity map. Furthermore, arc-shaped region of high polarization was recognized in the polarization map. Dust grains in this region had larger expansion velocity than the grains which provided the brightest area in the intensity map. comparing our results with the MIR spectroscopy obtained by Subaru Telescope we conclude that very small carbonaceous grains might be responsible for the region of high polarization.     

The shape, topography, and geology of Tempel 1 from Deep Impact observations

Deep Impact images of the nucleus of Comet Tempel 1 reveal pervasive layering, possible impact craters, flows with smooth upper surfaces, and erosional stripping of material. There are at least 3 layers 50-200 m thick that appear to extend deep into the nucleus, and several layers 1-20 m thick that parallel the surface and are being eroded laterally. Circular depressions show geographical variation in their forms and suggest differences in erosion rates or style over scales >1 km. The stratigraphic arrangement of these features suggests that the comet experienced substantial periods of little erosion. Smooth surfaces trending downslope suggest some form of eruption of materials from this highly porous object. The Deep Impact images show that the nucleus of Tempel 1 cannot be modeled simply as either an onion-layer or rubble pile structure.     

Hubble Space Telescope observations of Comet 9P/Tempel 1 during the Deep Impact encounter

We report on the Hubble Space Telescope program to observe periodic Comet 9P/Tempel 1 in conjunction with NASA's Deep Impact Mission. Our objectives were to study the generation and evolution of the coma resulting from the impact and to obtain wide-band images of the visual outburst generated by the impact. Two observing campaigns utilizing a total of 17 HST orbits were carried out: the first occurred on 2005 June 13-14 and fortuitously recorded the appearance of a new, short-lived fan in the sunward direction on June 14. The principal campaign began two days before impact and was followed by contiguous orbits through impact plus several hours and then snapshots one, seven, and twelve days later. All of the observations were made using the Advanced Camera for Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a spatial resolution of 36 km (16 km pixel⁻¹) at the comet at the time of impact. Baseline images of the comet, made prior to impact, photometrically resolved the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement with the 6.0±0.2 km diameter derived from the spacecraft imagers. Following the impact, the HRC images illustrate the temporal and spatial evolution of the ejecta cloud and allow for a determination of its expansion velocity distribution. One day after impact the ejecta cloud had passed out of the field-of-view of the HRC.     

A crater and its ejecta: An interpretation of Deep Impact

We apply recently updated scaling laws for impact cratering and ejecta to interpret observations of the Deep Impact event. An important question is whether the cratering event was gravity or strength-dominated; the answer gives important clues about the properties of the surface material of Tempel 1. Gravity scaling was assumed in pre-event calculations and has been asserted in initial studies of the mission results. Because the gravity field of Tempel 1 is extremely weak, a gravity-dominated event necessarily implies a surface with essentially zero strength. The conclusion of gravity scaling was based mainly on the interpretation that the impact ejecta plume remained attached to the comet during its evolution. We address that feature here, and conclude that even strength-dominated craters would result in a plume that appeared to remain attached to the surface. We then calculate the plume characteristics from scaling laws for a variety of material types, and for gravity and strength-dominated cases. We find that no model of cratering alone can match the reported observation of plume mass and brightness history. Instead, comet-like acceleration mechanisms such as expanding vapor clouds are required to move the ejected mass to the far field in a few-hour time frame. With such mechanisms, and to within the large uncertainties, either gravity or strength craters can provide the levels of estimated observed mass. Thus, the observations are unlikely to answer the questions about the mechanical nature of the Tempel 1 surface.     

Observations of Comet 9P/Tempel 1 around the Deep Impact event by the OSIRIS cameras onboard Rosetta

The OSIRIS cameras on the Rosetta spacecraft observed Comet 9P/Tempel 1 from 5 days before to 10 days after it was hit by the Deep Impact projectile. The Narrow Angle Camera (NAC) monitored the cometary dust in 5 different filters. The Wide Angle Camera (WAC) observed through filters sensitive to emissions from OH, CN, Na, and OI together with the associated continuum. Before and after the impact the comet showed regular variations in intensity. The period of the brightness changes is consistent with the rotation period of Tempel 1. The overall brightness of Tempel 1 decreased by about 10% during the OSIRIS observations. The analysis of the impact ejecta shows that no new permanent coma structures were created by the impact. Most of the material moved with . Much of it left the comet in the form of icy grains which sublimated and fragmented within the first hour after the impact. The light curve of the comet after the impact and the amount of material leaving the comet ( of water ice and a presumably larger amount of dust) suggest that the impact ejecta were quickly accelerated by collisions with gas molecules. Therefore, the motion of the bulk of the ejecta cannot be described by ballistic trajectories, and the validity of determinations of the density and tensile strength of the nucleus of Tempel 1 with models using ballistic ejection of particles is uncertain.     

Dust coma morphology in the Deep Impact images of Comet 9P/Tempel 1

We present an overview of the dust coma observations of Comet Tempel 1 that were obtained during the approach and encounter phases of the Deep Impact mission. We use these observations to set constraints on the pre-impact activity of the comet and discuss some preliminary results. The temporal and spatial changes that were observed during approach reveal three distinct jets rotating with a 1.7-day periodicity. The brightest jet produces an arcuate feature that expands outward with a projected velocity of about 12 m s⁻¹, suggesting that the ambient dust coma is dominated by millimeter-sized dust grains. As the spatial resolution improves, more jets and fans are revealed. We use stereo pairs of high-resolution images to put some crude constraints on the source locations of some of the brightest features. We also present a number of interesting coma features that were observed, including surface jets detected at the limb of the nucleus when the exposed ice patches are passing over the horizon, and features that appear to be jets emanating from unilluminated sources near the negative pole. We also provide a list of 10 outbursts of various sizes that were observed in the near-continuous monitoring during the approach phase.     

Chandra observations of Comet 9P/Tempel 1 during the Deep Impact campaign

We present results from the Chandra X-ray Observatory's extensive campaign studying Comet 9P/Tempel 1 (T1) in support of NASA's Deep Impact (DI) mission. T1 was observed for ∼295 ks between 30th June and 24th July 2005, and continuously for ∼64 ks on July 4th during the impact event. X-ray emission qualitatively similar to that observed for the collisionally thin Comet 2P/Encke system [Lisse, C.M., Christian, D.J., Dennerl, K., Wolk, S.J., Bodewits, D., Hoekstra, R., Combi, M.R., Mäkinen, T., Dryer, M., Fry, C.D., Weaver, H., 2005b. Astrophys. J. 635 (2005) 1329-1347] was found, with emission morphology centered on the nucleus and emission lines due to C, N, O, and Ne solar wind minor ions. The comet was relatively faint on July 4th, and the total increase in X-ray flux due to the Deep Impact event was small, ∼20% of the immediate pre-impact value, consistent with estimates that the total coma neutral gas release due to the impact was 5×¹⁰⁶ kg (∼10 h of normal emission). No obvious prompt X-ray flash due to the impact was seen. Extension of the emission in the direction of outflow of the ejecta was observed, suggesting the presence of continued outgassing of this material. Variable spectral features due to changing solar wind flux densities and charge states were clearly seen. Two peaks, much stronger than the man-made increase due to Deep Impact, were found in the observed X-rays on June 30th and July 8th, 2005, and are coincident with increases in the solar wind flux arriving at the comet. Modeling of the Chandra data using observed gas production rates and ACE solar wind ion fluxes with a CXE mechanism for the emission is consistent, overall, 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, with intermittent impulsive events due to solar flares and coronal mass ejections.     

Nuclear spin temperature of ammonia in Comet 9P/Tempel 1 before and after the Deep Impact event

The Deep Impact mission succeeded in excavating inner materials from the nucleus of Comet 9P/Tempel 1 on 2005 July 04 (at 05:52 UT). Comet 9P/Tempel 1 is one of Jupiter family short period comets, which might originate in the Kuiper belt region in the solar nebula. In order to characterize the comet and to support the mission from the ground-based observatory, optical high-dispersion spectroscopic observations were carried out with the echelle spectrograph (UVES) mounted on the 8-m telescope VLT (UT2) before and after the Deep Impact event. Ortho-to-para abundance ratios (OPRs) of cometary ammonia were determined from the NH₂ emission spectra. The OPRs of ammonia on July 3.996 UT and 4.997 UT were derived to be 1.28±0.07 (nuclear spin temperature: Tspin=24±2 K) and 1.26±0.08 (Tspin=25±2 K), respectively. There is no significant change between before and after the impact. Actually, most materials ejected from the impact site could have moved away from the nucleus on July 4.997 UT, about 17 h after the impact. However, a small fraction of the ejected materials might remain in the slit of UVES instrument at that time because an excess of about 20% in the NH₂ emission flux is observed above the normal activity level was found [Manfroid, J., Hutsemékers, D., Jehin, E., Cochran, A.L., Arpigny, C., Jackson, W.M., Meech, K.J., Schulz, R., Zucconi, J.-M., 2007. Icarus. This issue]. If the excess of NH₂ on July 04.997 UT was produced from icy materials excavated by the Deep Impact, then an upper-limit of the ammonia OPR would be 1.75 (Tspin>17 K) for those materials. On the other hand, the OPR of ammonia produced from the quiescent sources was similar to that of the Oort cloud comets observed so far. This fact may imply that physical conditions where cometary ices formed were similar between Comet 9P/Tempel 1 and the Oort cloud comets.     

Asymmetries in the distribution of H2O and CO2 in the inner coma of Comet 9P/Tempel 1 as observed by Deep Impact

Prior to the impact event, Deep Impact monitored the ambient inner coma of Comet 9P/Tempel 1 at high spatial resolution in July 2005. Gaseous H₂O and CO₂ are unambiguously detected in the infrared spectra collected with the HRI-IR spectrometer aboard Deep Impact. Detailed distribution maps of these volatiles in the inner coma, within 60 km from the nucleus, are produced from the integrated emission bands of H₂O (2.66 μm) and CO₂ (4.26 μm). Uncorrelated asymmetries are determined in the spatial distribution of both species indicating chemical heterogeneities within the nucleus. Although present at some abundance surrounding the entire nucleus, H₂O has a pronounced enhancement in abundance in the sunward direction rotational phases, evidence that the dominant process of subliming water ice from the nucleus is solar heating. In contrast, CO₂ is enhanced in the regions near the negative rotational pole of the nucleus, suggesting localized outgassing there. Both species show an increase in radiance above the limb of the nucleus toward Ecliptic North. The distribution maps also suggest that the process of dust removal from the nucleus is strongly connected to the outgassing of volatiles. Detailed study of these coma asymmetries gives insight to the relative abundances of the dominant molecular components of the inner coma, source regions of the native volatiles, anisotropic outgassing of the nucleus, and the formation and evolution of the nucleus. A quiescent water production rate for Tempel 1 on July 4, 2005, is estimated to be .     

Evolution of the Deep Impact flash: Implications for the nucleus surface based on laboratory experiments

The Deep Impact flyby spacecraft obtained high-speed images of the evolving impact event. Multiple exposures captured a self-luminous impact flash, caused by the heating and vaporization of the cometary surface. Laboratory investigations show that target conditions affect the photometric and spatial evolutions of the impact flash; thus, the flash can be used to constrain the state of the target if the other initial impact conditions are known. Through comparisons of DI flash observations to laboratory impact experiments, the impact flash evolution can be used to determine the type of impact that occurred and to interpret the nature of the impacted Tempel 1 surface. The Deep Impact flash was of relatively long duration, though its luminous efficiency was an order of magnitude lower than expectations. Both uprange and downrange self-luminous plumes were observed. Comparisons of the DI observations with the results of laboratory experiments suggest that the surface of Tempel 1 contains silicates, volatiles, and carbon compounds, and is a highly-porous substrate.     

Radio OH observations of 9P/Tempel 1 before and after Deep Impact

We observed 18-cm OH emission in Comet 9P/Tempel 1 before and after Deep Impact. Observations using the Arecibo Observatory 305 m telescope took place between 8 April and 9 June, 2005, followed by post-impact observations using the National Radio Astronomy Observatory 100 m Green Bank Telescope 4-12 July, 2005. The resulting spectra were analyzed with a kinematic Monte Carlo model which allows estimation of the OH production rate, neutral gas outflow velocity, and distribution of the out-gassing from the nucleus. We detected typically 24% variability from the overall OH production rate trend in the two months leading up to the impact, and no dramatic increase in OH production in the days post-impact. Generally, the coma is well-described, within uncertainties, by a symmetric model with OH production rates from 1.6 to , and mean water outflow velocity of . At these low production rates, collisional quenching is expected to occur only within 20,000 km of the nucleus. However, our best-fit average quenching radius is 64,200 ± 22,000 km in April and May.     

The internal structure of Jupiter family cometary nuclei from Deep Impact observations: The “talps” or “layered pile” model

We consider the hypothesis that the layering observed on the surface of Comet 9P/Tempel 1 from the Deep Impact spacecraft and identified on other comet nuclei imaged by spacecraft (i.e., 19P/Borrelly and 81P/Wild 2) is ubiquitous on Jupiter family cometary nuclei and is an essential element of their internal structure. The observational characteristics of the layers on 9P/Tempel 1 are detailed and considered in the context of current theories of the accumulation and dynamical evolution of cometary nuclei. The works of Donn [Donn, B.D., 1990. Astron. Astrophys. 235, 441-446], Sirono and Greenberg [Sirono, S.-I., Greenberg, J.M., 2000. Icarus 145, 230-238] and the experiments of Wurm et al. [Wurm, G., Paraskov, G., Krauss, O., 2005. Icarus 178, 253-263] on the collision physics of porous aggregate bodies are used as basis for a conceptual model of the formation of layers. Our hypothesis is found to have implications for the place of origin of the JFCs and their subsequent dynamical history. Models of fragmentation and rubble pile building in the Kuiper belt in a period of collisional activity (e.g., [Kenyon, S.J., Luu, J.X., 1998. Astron. J. 115, 2136-2160; 1999a. Astron. J. 118, 1101-1119; 1999b. Astrophys. J. 526, 465-470; Farinella, P., Davis, D.R., Stern, S.A., 2000. In: Mannings, V., Boss, A.P., Russell, S.S. (Eds.), Protostars and Planets IV. Univ. of Arizona Press, Tucson, pp. 1255-1282; Durda, D.D., Stern, S.J., 2000. Icarus 145, 220-229]) following the formation of Neptune appear to be in conflict with the observed properties of the layers and irreconcilable with the hypothesis. Long-term residence in the scattered disk [Duncan, M.J., Levison, H.F., 1997. Science 276, 1670-1672; Duncan, M., Levison, H., Dones, L., 2004. In: Festou, M., Keller, H.U., Weaver, H.A. (Eds.), Comets II. Univ. of Arizona Press, Tucson, pp. 193-204] and/or a change in fragmentation outcome modeling may explain the long-term persistence of primordial layers. In any event, the existence of layers places constraints on the environment seen by the population of objects from which the Jupiter family comets originated. If correct, our hypothesis implies that the nuclei of Jupiter family comets are primordial remnants of the early agglomeration phase and that the physical structure of their interiors, except for the possible effects of compositional phase changes, is largely as it was when they were formed. We propose a new model for the interiors of Jupiter family cometary nuclei, called the talps or “layered pile” model, in which the interior consists of a core overlain by a pile of randomly stacked layers. We discuss how several cometary characteristics—layers, surface texture, indications of flow, compositional inhomogeneity, low bulk density low strength, propensity to split, etc., might be explained in terms of this model. Finally, we make some observational predictions and suggest goals for future space observations of these objects.     

A ballistics analysis of the Deep Impact ejecta plume: Determining Comet Tempel 1's gravity, mass, and density

In July of 2005, the Deep Impact mission collided a 366 kg impactor with the nucleus of Comet 9P/Tempel 1, at a closing speed of 10.2 km s⁻¹. In this work, we develop a first-order, three-dimensional, forward model of the ejecta plume behavior resulting from this cratering event, and then adjust the model parameters to match the flyby-spacecraft observations of the actual ejecta plume, image by image. This modeling exercise indicates Deep Impact to have been a reasonably “well-behaved” oblique impact, in which the impactor-spacecraft apparently struck a small, westward-facing slope of roughly 1/3-1/2 the size of the final crater produced (determined from initial ejecta plume geometry), and possessing an effective strength of not more than . The resulting ejecta plume followed well-established scaling relationships for cratering in a medium-to-high porosity target, consistent with a transient crater of not more than 85-140 m diameter, formed in not more than 250-550 s, for the case of (gravity-dominated cratering); and not less than 22-26 m diameter, formed in not less than 1-3 s, for the case of (strength-dominated cratering). At , an upper limit to the total ejected mass of 1.8×¹⁰⁷ kg (1.5-2.2×¹⁰⁷ kg) is consistent with measurements made via long-range remote sensing, after taking into account that 90% of this mass would have stayed close to the surface and then landed within 45 min of the impact. However, at , a lower limit to the total ejected mass of 2.3×¹⁰⁵ kg (1.5-2.9×¹⁰⁵ kg) is also consistent with these measurements. The expansion rate of the ejecta plume imaged during the look-back phase of observations leads to an estimate of the comet's mean surface gravity of (0.17-0.90 mm s⁻²), which corresponds to a comet mass of mt=4.5×¹⁰¹³ kg (2.3-12.0×¹⁰¹³ kg) and a bulk density of (200-1000 kg m⁻³), where the large high-end error is due to uncertainties in the magnitude of coma gas pressure effects on the ejecta particles in flight.