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Abstract: We derive the spin state of the nucleus of Comet 103P/Hartley 2, its orientation in space, and its short-term temporal evolution from a mixture of observations taken from the DIXI (Deep Impact Extended Investigation) spacecraft and radar observations. The nucleus is found to spin in an excited long-axis mode (LAM) with its rotational angular momentum per unit mass, M, and rotational energy per unit mass, E, slowly decreasing while the degree of excitation in the spin increases through perihelion passage. M is directed toward (RA, Dec; J2000)=8±4°, 54±1° (obliquity=48±1°). This direction is likely changing, but the change is probably <6° on the sky over the ∼81.6days of the DIXI encounter. The magnitudes of M and E at closest approach (JD 2455505.0831866 2011-11-04 13:59:47.310) are 30.0±0.2m2/s and (1.56±0.02)×10−3 m2/s2 respectively. The period of rotation about the instantaneous spin vector, which points in the direction (RA, Dec; J2000)=300±3.2°, 67±1.3° at the time of closest approach, was 14.1±0.3h. The instantaneous spin vector circulates around M, inclined at an average angle of 33.2±1.3°, with an average period of 18.40±0.13h at the time of closest approach. The period of roll around the principal axis of minimum inertia (“long” axis) at that time is 26.72±0.06h. The long axis is inclined to M by ∼81.2±0.6° on average, slowly decreasing through encounter. We infer that there is a periodic nodding motion of the long axis with half the roll period, i.e., 13.36±0.03h, with amplitude of ∼1° again decreasing through encounter. The periodic variability in the circulation and roll rates during a cycle was at the 2% and 10–14% level respectively. During the encounter there was a secular lengthening of the circulation period of the long axis by 1.3±0.2min/d, in agreement with ground-based estimates, while the period of roll around the long axis changed by ∼−4.4min/d at perihelion. M decreased at a rate of −0.038 (m2/s) per day in a roughly linear fashion. Assuming a bulk density between 230–300kg/m3 and a total volume for the nucleus of 8.09×108 m3, the net torque acting on the nucleus was in the range 0.8–1.1×105 kgm2/s2. In order to bring the spacecraft photometric and imaging data into alignment on the direction of M, the directions of the intermediate and short principal axes of inertia had to be adjusted by 33° (on the sky) from the values indicated by the shape model with an assumed homogeneous interior. The adjusted direction of the intermediate axis is RA, Dec=302°, −16.5°. The morning and evening terminators in the images are identified, and the variation of the insolation at three regions on the nucleus associated with active areas calculated. The plume of water vapor observed in the inner coma is found to be directed close to the direction of local gravity over the sub-solar region for a range of reasonable bulk densities. The plume does not follow the projected normal to the surface at the sub-solar point. [Copyright &y& Elsevier]

Abstract: The properties of 50 jet and jet-filament outflows from 27 active areas observed on the four comet nuclei that have been visited by spacecraft (1P/Halley, 19P/Borrelly, 81P/Wild 2, and 9P/Tempel 1) are investigated and we propose a taxonomic categorization in which there are three types of active areas: Type I that is dominated by the sublimation of H2O through the porous mantle; Type II that is controlled by the localized and persistent effusion of super-volatiles from the interior; and Type III that is characterized by episodic releases of super-volatiles. The zonally averaged distribution of active area locations associated with Type II outflows over the four comets is calculated and we find that they are distributed randomly in latitude. In longitude, the distribution shows a marginal tendency for active areas to occur more frequently in the region near the ends of the long axis or, alternatively, a tendency to avoid the region close to the ends of the intermediate axis. Combining observations of filamentary structure with exploratory hydrodynamic calculations we find that Type II outflows are likely to be relatively cold laminar flows (Re<1000) of a mixture of CO2, CO and H2O that are highly collimated (6–10° full-cone angle) during the daytime as a result of being constrained by the ambient H2O atmosphere. We propose that they become visible as a result of the turbulent momentum flux at the base of the filamentary structure that causes the friable surface to release dust at a higher rate than in surrounding areas. We present evidence that indicates that geophysical flows occur on cometary nuclei other than 9P/Tempel 1 and discuss a possible scenario for the long-term evolution of cometary surfaces near the Sun. We conclude with an exposition of a cometary activity paradigm brought up-to-date with discoveries made with recent space missions, associated Earth-based investigations, and the results of this work. [Copyright &y& Elsevier]

Abstract: Data on the UT 2005 June 14 mini-outburst of Comet 9P/Tempel 1 taken from different viewpoints have been examined for morphological differences and parallax. The data were taken with the Hubble Space Telescope (HST), from the Deep Impact (DI) spacecraft, and from the Calar Alto Observatory, Spain. The mini-outburst source region was found to be located near 218±6E, 6±5N on the Deep Impact nucleus shape model. The mini-outburst occurred at local solar time. The distribution of light in the mini-outburst is similar to that expected for an ejecta curtain. The method and software used to determine the surface location was checked using position angles of the impact ejecta plume as seen from DI and HST. The general region of impact was recovered and a downrange tilt of the ejecta curtain axis of 10.2 deg from the surface normal was found. We computed tracks of possible source regions for nine other mini-outbursts seen from DI. Five of these tracks converge on the 2005 June 14 event location. Three of the tracks converge at a second location near (60E, 20S), well separated from the first. Multiple mini-outbursts arise at each location either from a single source or from a few sources in close proximity. The mini-outbursts occur both at night and during the day indicating at most weak, if any, control by direct sunlight. The times of outburst are non-random with a preference for early afternoon, dusk and midnight. None of the mini-outbursts occurred near dawn. They occur at low latitudes (between ) near the points where the principal axis of minimum moment of inertia cuts the surface. These regions are furthest from the center of figure and have the lowest effective surface gravity. We use these results to develop a conceptual model of the mini-outburst process and make comparisons with the theoretical calculations. We find that the tensile strength of the sub-surface material must be very low (e.g., ) and, on the basis of features imaged on the western facet of the nucleus, suggest that inflation of the sub-surface may be occurring. Our model makes specific predictions about the kind of surface morphology that should result from mini-outburst activity. We show that one of the isolated rimless depressions and the close-packed depressions found in the Deep Impact images have the properties needed and identify them as possible sites of past and current mini-outburst activity. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: We have studied the photometric properties of the nucleus of a hyperactive comet, 103P/Hartley 2, at visible wavelengths using the DIXI flyby images with both disk-integrated and disk-resolved analyses. The disk-integrated phase function of the nucleus has a linear slope of 0.046±0.002mag/deg and an absolute magnitude of 18.4±0.1 at V-band. The nucleus displays an overall linear, featureless spectrum between 400nm and 850nm. The linear spectral slope is 7.6±3.6% per 100nm, corresponding to broadband solar-illuminated color indices B–V of 0.75±0.05 and V–R of 0.43±0.04. Disk-resolved photometric analysis with a Hapke model returns a best-fit single-scattering albedo of 0.036±0.006, an asymmetry factor of the Henyey–Greenstein single-particle phase function of −0.46±0.06, and a photometric roughness of 15±10°. The model yields a geometric albedo of 0.045±0.009 and a Bond albedo of 0.012±0.002. The overall photometric variations of the nucleus are small, with an equivalent albedo variation of 15% FWHM, and a color variation of 12% FWHM. Some areas near the terminator visible in the inbound images show an albedo of more than twice the global average value, and a much bluer color than the average nucleus. The overall photometric properties and variations of the nucleus of Hartley 2 are similar to those of the nuclei of Comets Wild 2 and Tempel 1 as studied from previous spacecraft flyby missions at similar resolutions. [Copyright &y& Elsevier]

Abstract: 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 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 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. [Copyright &y& Elsevier]

Abstract: Images returned from the Deep Space 1 (DS-1) spacecraft during its encounter with Comet 19P/Borrelly are used to study its disk-integrated and disk-resolved photometry and its thermal properties. A disk-integrated phase function was constructed from a combination of DS-1 images and ground-based observations, giving a geometric albedo of and a phase slope of 0.043 mag deg−1. The shape model of Borrelly [Kirk, R.L., Howington-Kraus, E., Soderblom, L.A., Giese, B., Oberst, J., 2004a. Icarus 167, 54–69] and the ephemerides of DS-1 were used to analyze the disk-resolved photometric data with Hapke''s theoretical model. It was found that the surface of Borrelly displays large photometric heterogeneities in its photometric parameters. The single-scattering albedo, w, varies by a factor of 2.5 with an average of ; the asymmetry factor, g, ranges from almost isotropic (−0.1) to strongly backscattering (−0.7) with an average of ; the roughness parameter, , is less than 35° for most parts of surface but ranges up to 55° in some areas. Its average is . The observed 1-D temperature profile is modeled well by the standard thermal model (STM) for inactive regions and is found to be consistent with a very low thermal inertia. Water sublimation in the source region of the fan jet is observed to decrease the surface temperature from the STM predictions by 20–40 K. The source areas of two collimated jets could not be determined from either photometric model or thermal model. It is evident that the fan jet activity occurring on Borrelly''s surface can be correlated to areas of relatively high albedo, weak backscattering, and high roughness. [Copyright &y& Elsevier]