•Different enhancement techniques for enhancing faint coma features are discussed.•Simulated coma images provide the ground truth for comparing techniques.•Pros and cons of enhancement techniques are ...investigated.•Do not rely on a single technique to reliably identify distinct features.
Many cometary coma features are only a few percent above the ambient coma (i.e., the background) and therefore coma enhancement techniques are needed to discern the morphological structures present in cometary comae. A range of image enhancement techniques widely used by cometary scientists is discussed by categorizing them and carrying out a comparative analysis. The enhancement techniques and the corresponding characteristics are described in detail and the respective mathematical representations are provided. As the comparative analyses presented in this paper make use of simulated images with known coma features, the feature identifications as well as the artifacts caused by enhancement provide an objective and definitive assessment of the various techniques. Examples are provided which highlight contrasting capabilities of different techniques to pick out qualitatively distinct features of widely different strengths and spatial scales. On account of this as well as serious image artifacts and spurious features associated with certain enhancement techniques, confirmation of the presence of coma features using qualitatively different techniques is strongly recommended.
The European Space Agency Rosetta/Philae mission to Comet 67P/Churyumov–Gerasimenko in 2014–2016 is the most complete and diverse investigation of a comet carried out thus far. Yet, many physical and ...chemical properties of the comet remain uncertain or unknown, and cometary activity is still not a well–understood phenomenon. We here attempt to place constraints on the nucleus abundances and sublimation front depths of H2O and CO2 ice, and to reconstruct how the nucleus evolved throughout the perihelion passage. We employ the thermophysical modelling code ‘Numerical Icy Minor Body evolUtion Simulator’, or NIMBUS, to search for conditions under which the observed H2O and CO2 production rates are simultaneously reproduced before and after perihelion. We find that the refractories to water–ice mass ratio of relatively pristine nucleus material is μ ≈ 1, that airfall material has μ ≈ 2, and that the molar abundance of CO2 relative H2O is near 30 per cent. The dust mantle thickness is typically ≲ 2 cm. The average CO2 sublimation front depths near aphelion were ~ 3.8 m and ~ 1.9 m on the northern and southern hemispheres, respectively, but varied substantially with time. We propose that airfall material is subjected to substantial fragmentation and pulverisation due to thermal fatigue during the aphelion passage. Sub–surface compaction of material due to CO2 activity near perihelion seems to have reduced the diffusivity in a measurable way.
Samarasinha & Mueller related changes of cometary rotation to other physical parameters for four Jupiter-family comets defining a parameter X, which is approximately constant within a factor of two ...irrespective of the active fraction of a comet. In this paper two additional comets are added to the original sample, and the claim of a nearly constant parameter X for these six comets is confirmed, albeit with a larger scatter. Taking the geometric mean of X for all of the comets above excluding 2P/Encke (as X for each comet was determined with respect to that of 2P/Encke), the expected changes in the rotation periods for a sample of 24 periodic comets are derived. From this sample we identify the comets that are most likely to show observationally detectable changes in their rotation periods. Using these 24 comets and including the six comets used to determine X, we find a correlation between the parameter ζ (i.e., the total water production per unit surface area per orbit approximated by that inside of 4 au) and the perihelion distance q; specifically, we derive ζ ∝ q−0.8 and provide a theoretical basis for this in the Appendix. This relationship between ζ and q enables ready comparisons of activity due to insolation between comets. Additionally, a relationship between the nuclear radius R and the rotation period P is found. We find that on average smaller nuclei have smaller rotation periods compared to the rotation periods of larger nuclei. This is consistent with expectations for the rotational evolution and spin-up of comet nuclei, providing strong observational evidence for sublimation-driven rotational changes in comets.
We present analysis of Hubble Space Telescope observations of Centaur 29P/Schwassmann-Wachmann 1 (SW1) while in outburst to characterize the outburst coma and place constraints on the nucleus' spin ...state. The observations consist of Wide Field and Planetary Camera 2 (WFPC2) images from Cycle 5, GO-5829 acquired on UT 1996 March 11.3 and 12.1, which serendipitously imaged the Centaur shortly after a major outburst. A multi-component coma was detected consisting of an expanding outburst dust coma with complex morphology possessing an east-west asymmetry and north-south symmetry contained within 5″ (∼19,000 km) of the nucleus, the residual dust shell of an earlier UT 1996 February outburst, and a nearly circular coma with underlying quiescent activity level detectable to ∼70″ (∼267,000 km) away from the nucleus. Photometry of the calibrated WFPC2 images resulted in an equivalent R-band magnitude of 12.86 0.02 for a measured 5″ radius aperture and an estimated (2.79 0.05) × 108 kg for the lower limit of dust material emitted during the outburst. No appreciable evolution of morphological features, indicating signatures of nuclear rotation, was detected between the two imaging epochs. The observations were modeled using a 3D Monte Carlo coma model to place constraints on the nucleus' rotation state. Modeling indicated that the morphology is representative of a non-isotropic ejection of dust emitted during a single outburst event with a duration of the order of hours from a single source region corresponding to ∼1% of the surface area. A spin period with lower limit of the order of days is suggested to reproduce the coma morphology seen in the observations.
We introduce a parameter, X, to predict the changes in the rotational period of a comet in terms of the rotational period itself, the nuclear radius, and the orbital characteristics. We show that X ...should be a constant if the bulk densities and shapes of nuclei are nearly identical and the activity patterns are similar for all comets. For four nuclei for which rotational changes are well documented, despite the nearly factor 30 variation observed among the effective active fractions of these comets, X is constant to within a factor two. We present an analysis for the sungrazing comet C/2012 S1 (ISON) to explore what rotational changes it could undergo during the upcoming perihelion passage where its perihelion distance will be ~2.7 solar radii. When close to the Sun, barring a catastrophic disruption of the nucleus, the activity of ISON will be sufficiently strong to put the nucleus into a non-principal-axis rotational state and observable changes to the rotational period should also occur. Additional causes for rotational state changes near perihelion for ISON are tidal torques caused by the Sun and the significant mass loss due to a number of mechanisms resulting in alterations to the moments of inertia of the nucleus.
We imaged Comet 252P/2000 G1 (LINEAR; hereafter 252P) with the Hubble Space Telescope and both 252P and P/2016 BA14 (PanSTARRS; hereafter BA14) with the Discovery Channel Telescope in 2016 March and ...April, surrounding its close encounter to Earth. The r′-band Af of 252P in a 0 2-radius aperture were 16.8 0.3 and 57 1 cm on March 14 and April 4, respectively, and its gas production rates were Q(OH) = (5.8 0.1) × 1027 s−1, and Q(CN) = (1.25 0.01) × 1025 s−1 on April 17. The r′-band upper limit Af of BA14 was 0.19 0.01 cm in a 19 2-radius aperture, and Q(CN) = (1.4 0.1) × 1022 s−1 on 2017 April 17. 252P shows a bright and narrow jet of a few hundred kilometers long in the sunward direction, changing its projected position angle in the sky with a periodicity consistent with 7.24 hr. However, its photometric light curve is consistent with a periodicity of 5.41 hr. We suggest that the nucleus of 252P is likely in a non-principal axis rotation. The nucleus radius of 252P is estimated to be about 0.3 0.03 km, indicating an active fraction of 40% to >100% in its 2016 apparition. Evidence implies a possible cloud of slow-moving grains surrounding the nucleus. The activity level of 252P in the 2016 apparition increased by two orders of magnitude from its previous apparitions, making this apparition unusual. On the other hand, the activity level of BA14 appears to be at least three orders of magnitude lower than that of 252P, despite its 10 times or larger surface area.
•Lightcurves were simulated for known non-principal-axis (NPA) rotations.•Lightcurves were analyzed to determine which periodicity signatures are present.•Component periods of NPA rotations in ...different coordinate conventions were linked.•These relations were used to interpret and rationalize lightcurve signatures.
Lightcurve observations of asteroids and bare cometary nuclei are the most widely used observational tool to derive the rotational parameters. Therefore, an in-depth understanding of how component periods of dynamically excited non-principal axis (NPA) rotators manifest in lightcurves is a crucial step in this process. We investigated this with the help of numerically generated lightcurves of NPA rotators with component periods known a priori. The component periods of NPA rotation were defined in terms of two widely used yet complementary conventions. We derive the relationships correlating the component rotation periods in the two conventions. These relationships were then used to interpret the periodicity signatures present in the simulated lightcurves and rationalize them in either convention.
•We compare two tested parametric models of sublimative torques to determine that cometary activity originates predominantly from regions of steep scarps.•We reason that mass wasting events (e.g., ...avalanches, landslides) are the primary mechanism for maintaining comet activity, and likely the reactivation of dormant comet nuclei.•This result suggests a new evolutionary paradigm for comet activity (quasi-equilibrium, episodic, quasi-dormant, and extinct), in which the frequency of mass wasting events, rather than activity or active fraction, is the metric determining a comet's evolutionary state.
Sublimative outgassing of comets produces torques that alter the rotation state of their nuclei. Recently, parameterized sublimative torque models have been developed to study rotation state changes of individual comet nuclei and populations of cometary bodies. However, these models simplify the interactions between the escaping gas and cometary surface into only a few parameters that hide the details of these complex interactions. Here we directly compare the X-parameter model (Samarasinha and Mueller, 2013) with the SYORP model (Steckloff and Jacobson, 2016) to tease out insights into the details of the gas-surface interactions driving sublimative torques. We find that, for both of these models to accurately model sublimative torques, the number of sublimating molecules that contribute to the net torque is largely independent of the detailed shape and activity of the nucleus, but rather depends primarily on the size of the nucleus and the effective heliocentric distance of the comet. We suggest that cometary activity must be largely restricted to regions of steep gravitational surface slopes (above the angle of repose), where mass wasting can refresh activity by shedding mantles of refractory materials and exposing fresh volatiles. We propose a new classification scheme for comets based on the frequency of this mass-wasting process (relative to the timescale of activity fading): quasi-equilibrium, episodic, quasi-dormant, and extinct.
The CN coma structure of the NASA EPOXI mission target, comet 103P/Hartley 2, was observed during 20 nights from 2010 September to December. These CN images probe the rotational state of the comet's ...nucleus and provide a ground-based observational context to complement the EPOXI observations. A dynamically excited cometary nucleus with a changing rotational rate is observed, a characteristic not seen in any comet in the past. The lack of rotational damping during the four-month observing interval places constraints on the interior structure of the nucleus.
•Five consecutive nights of observations of SW1 4 days after a major outburst were analyzed.•An expanding shell of material was measured to have a projected velocity of 0.11 ±0.02 km/s due to the ...outburst.•Modeling of the outburst coma suggest a slow (on the order of days) rotation period and/or a spin-pole directed towards Earth during observations.
We present analysis of five nights of R-band observations of Comet 29P/Schwassmann-Wachmann 1 (SW1) taken on September 2008 which show the comet undergoing an outburst. Coma morphology shows a projected asymmetric shell of material expanding radially and four linear features on the northern side of the coma at position angles 37°, 78°, 300°, and 353°. Using the measured projected radial outflow velocity of 0.11 ±0.02 km/s for the shell material, we calculate an outburst time of UT 2008-09-21.03 ±0.95 days. By tracking the inner and outer extent of the northern linear features, we found that the features are fully contained within the expanding shell of material. This suggested both shell and linear features originated during the same event and activity originating from different regions on the nuclear surface are not necessary to generate both types of morphological structure observed. A 3-D Monte Carlo coma model was used to model the outburst. Morphological features present in the observations were modeled allowing constraints to be placed on the spin state of SW1’s nucleus. The evolution of morphological features allows constraints on the rotation period P assuming an outburst duration Δt and the spin period constraints are expressed in terms of their ratio P/Δt. Since the spin-pole orientation could not be constrained, four spin-pole orientations were chosen for modeling the coma. Spin-period constraints for each assumed pole orientation are discussed. Overall, modeling suggested either a spin period on the order of days, a spin-pole orientation nearly along the sub-Earth direction, or a combination of both. To place an independent constraint on the outburst duration, radial surface-brightness profiles of the observations were compared with profiles from synthetic models, giving an upper-limit of Δt ≤ 1.5 days. Longer outbursts resulted in a higher number of dust grains in close proximity to the nucleus during the observations and a profile slope too steep to model observations. Lastly, from photometry of the five nights of observation, a lower limit of (1.8 ± 0.07) × 109 kg was estimated for the total amount of dust emitted during the outburst. Assuming the outburst was triggered by either the sublimation of pure CO or CO2 ice and a dust to gas ratio of ∼4 (Rosetta results for Comet 67P, Rotundi et al. 2015), a lower limit for the outburst duration on the order of hours was obtained.
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