Cometary Nuclei Research Articles

СПЕКТРОФОТОМЕТРИЧНІ ДОСЛІДЖЕННЯ НАВКОЛОЯДЕРНОЇ ОБЛАСТІ КОМЕТИ 46P/WIRTANEN

The observations and research of the short-period comet 46P/Wirtanen by optical spectra with an average resolution (λ/Δλ ≈ 1400) are presented. Spectra in January 2008 at the Peak Terskol mountain observatory with the Zeiss-2000 telescope (D = 2.0 m; F2 = 16 m) were obtained. On the basis of the received spectral material, the identification of emission bands was carried out. The distribution of the total energy flux, reflected energy flux and the spectrophotometric gradient in the spectral range λλ = 4850–7650 ÅÅ are constructed. The quality of the received spectral material allowed estimating with high accuracy the gas productivity of the molecules C2, NH2 and the relative dust productivity in cometary filters GC, RC. The relative dust productivity (Afρ), the gas productivity (Q) for the molecular emissions of C2 and NH2, as well as some other physical parameters of the neutral gas cometary atmosphere were evaluated. The resulting values of physical quantities are characteristic for Jupiter family comets. The gas productivity of 46P/Wirtanen during the observation period QC2 = 3.97–4.12 × 1025 mol/s, QNH2 = 2.06–2.97 × 1025 mol/s. The obtained values of gas and dust productivity characterize the cometary coma of 46P/Wirtanen as dust-rich. In comparison with the data of 1998, a significant decrease in production in the atmosphere of the comet of the NH2 molecule was recorded. The spectrophotometric gradient obtained for comet Wirtanen is characteristic for other JFC and is 8.6 % per 1000 Å. The relative dust productivity (Afρ) at the time of observations in January 2008 was significantly higher than in previous passages of perihelion. The obtained results may be the consequence of evolution of the upper layers of the cometary nucleus and require further experimental confirmation.

Comet 67p/Churyumov–Gerasimenko, possible origin of the depression Hatmehit

On the nucleus of comet 67P/Churyumov-Gerasimenko numerous pits and depressions of different sizes are identified. In the present work the origin of depression Hatmehit is investigated. We propose a mechanism that includes recession of the surface due to sub-dust sublimation together with formation of cavities. Recession of the surface is calculated taking into account the sub-dust sublimation of water ice with volatile organic components, formation of a strengthened ice-dust crust due to sintering of ice grains and the increase of the gas pressure in pores due to release of gaseous CO. Our simulations indicate, that the formation of a 300 m deep depression due to the sub-dust sublimation of ice can take as little as 50 orbital periods assuming that the heliocentric distance at perihelion is close to its present value (Ip. et al., 2016). When amorphous water ice is present at small depth the release of gaseous CO may cause either compression of low-strength material beneath the strengthened crust, or ejection of the crust. In both cases a cavity is formed and grows due to the sublimation of ice from the wall and the floor.

Thermal infrared and optical photometry of Asteroidal Comet C/2002 CE10

C/2002 CE10 is an object in a retrograde elliptical orbit with Tisserand parameter −0.853 indicating a likely origin in the Oort Cloud. It appears to be a rather inactive comet since no coma and only a very weak tail was detected during the past perihelion passage. We present multi-color optical photometry, lightcurve and thermal mid-IR observations of the asteroidal comet. With the photometric analysis in BVRI, the surface color is found to be redder than asteroids, corresponding to cometary nuclei and TNOs/Centaurs. The time-resolved differential photometry supports a rotation period of 8.19 ± 0.05 h. The effective diameter and the geometric albedo are 17.9 ± 0.9 km and 0.03 ± 0.01, respectively, indicating a very dark reflectance of the surface. The dark and redder surface color of C/2002 CE10 may be attribute to devolatilized material by surface aging suffered from the irradiation by cosmic rays or from impact by dust particles in the Oort Cloud. Alternatively, C/2002 CE10 was formed of very dark refractory material originally like a rocky planetesimal. In both cases, this object lacks ices (on the surface at least). The dynamical and known physical characteristics of C/2002 CE10 are best compatible with those of the Damocloids population in the Solar System, that appear to be exhaust cometary nucleus in Halley-type orbits. The study of physical properties of rocky Oort cloud objects may give us a key for the formation of the Oort cloud and the solar system.

Thermophysics of fractures on comet 67P/Churyumov-Gerasimenko

Context. The camera OSIRIS on board Rosetta obtained high-resolution images of the nucleus of comet 67P/Churyumov-Gerasimenko (67P). Great parts of the nucleus surface are composed of fractured terrain. Aims. Fracture formation, evolution, and their potential relationship to physical processes that drive activity are not yet fully understood. Observed temperatures and gas production rates can be explained or interpreted with the presence of fractures by applying appropriate modelling methods. Methods. We followed a transient thermophysical model approach that includes radiative, conductive, and water-ice sublimation fluxes by considering a variety of heliocentric distances, illumination conditions, and thermophysical properties for a set of characteristic fracture geometries on the nucleus of 67P. We computed diurnal temperatures, heat fluxes, and outgassing behaviour in order to derive and distinguish the influence of the mentioned parameters on fractured terrain. Results. Our analysis confirms that fractures, as already indicated by former studies about concavities, deviate from flat-terrain topographies with equivalent properties, mostly through the effect of self-heating. Compared to flat terrain, illuminated cometary fractures are generally warmer, with smaller diurnal temperature fluctuations. Maximum sublimation rates reach higher peaks, and dust mantle quenching effects on sublimation rates are weaker. Consequently, the rough structure of the fractured terrain leads to significantly higher inferred surface thermal inertia values than for flat areas with identical physical properties, which might explain the range of measured thermal inertia on 67P. Conclusions. At 3.5 AU heliocentric distance, sublimation heat sinks in fractures converge to maximum values >50 W / m2 and trigger dust activity that can be related mainly to H2O. Fractures are likely to grow through the erosive interplay of alternating sublimation and thermal fatigue.

Radar backscattering from a large-grain cometary coma: numerical simulation

We numerically simulate the circular polarization ratio of the radar signal backscattered from a large-grain cometary coma and compare the simulation results with the radar measurements for seven comets. We apply the discrete dipole approximation method and a model of random irregular particles. Our results confirm water ice composition of the cm-sized chunks detected by the NASA Deep Impact space probe in the vicinity of the nucleus of Comet 103P/Hartley 2. The index of the power-law size distribution in this case can be constrained to the range n ≈ 3.3–4.3. For the other considered comets the circular polarization ratio can be reproduced with variations of the power index between 2 and 5.

Collapse-driven formation of depressions on comet 67P/Churyumov–Gerasimenko

The extremely diverse surface of comet 67P/Churyumov–Gerasimenko contains a large number of depressions or craters of very different scales. Among the most prominent are two large roughly circular depressions, each with radii of several hundred meters. In this work a model for the formation of the depressions is proposed. It is based on the theory of the deformation of a thin circular elastic plate under its own weight. The plate covers a circular cavity with a given radius. The resilience of the plate diminishes over time as a result of its thinning which is itself a consequence of sublimation. When the stress limit is achieved, a gravitational collapse occurs: the plate cracks and the remnants fall into the cavity bottom. A formula that links the radius of the plate corresponding to collapse with the plate thickness has been derived. The formula was discussed for the large intervals of the values of parameters that characterize surface layers of cometary nuclei. It was found that the surface above large cavities collapses sooner than one of a similar thickness that covers a smaller cavity. So, if the collapse mechanism theory works, that larger depressions are therefore older than smaller ones.

Normal Gravity Fields and Equipotential Ellipsoids of Small Objects in the Solar System: A Closed-form Solution in Ellipsoidal Harmonics up to the Second Degree

Abstract We propose a definition for the normal gravity fields and normal figures of small objects in the solar system, such as asteroids, cometary nuclei, and planetary moons. Their gravity fields are represented as series of ellipsoidal harmonics, ensuring more robust field evaluation in the proximity of an arbitrary, convex shape than using spherical harmonics. The normal gravity field, approximate to the actual field, can be described by a finite series of three terms, that is, degree zero, and the zonal and sectoral harmonics of degree two. The normal gravity is that of an equipotential ellipsoid, defined as the normal ellipsoid of the body. The normal ellipsoid may be distinct from the actual figure. We present a rationale for specifying and a numerical method for determining the parameters of the normal ellipsoid. The definition presented here generalizes the convention of the normal spheroid of a large, hydrostatically equilibrated planet, such as Earth. Modeling the normal gravity and the normal ellipsoid is relevant to studying the formation of the “rubble pile” objects, which may have been accreted, or reorganized after disruption, under self-gravitation. While the proposed methodology applies to convex, approximately ellipsoidal objects, those bi-lobed objects can be treated as contact binaries comprising individual convex subunits. We study an exemplary case of the nearly ellipsoidal Martian moon, Phobos, subject to strong tidal influence in its present orbit around Mars. The results allude to the formation of Phobos via gravitational accretion at some further distance from Mars.

Thermal Modeling of Comet-like Objects from AKARI Observation

Abstract We investigated the physical properties of the comet-like objects 107P/(4015) Wilson–Harrington (4015WH) and P/2006 HR30 (Siding Spring; HR30) by applying a simple thermophysical model to the near-infrared spectroscopy and broadband observation data obtained by the AKARIsatellite of JAXA when they showed no detectable comet-like activity. We selected these two targets because the tendency of thermal inertia to decrease with the size of an asteroid, which has been demonstrated in recent studies, has not been confirmed for comet-like objects. It was found that 4015WH, which was originally discovered as a comet but has not shown comet-like activity after its discovery, has effective size D = 3.74–4.39 km and geometric albedo with thermal inertia J m−2 K−1 . The corresponding grain size is estimated as 1–3 mm. We also found that HR30, which was observed as a bare cometary nucleus at the time of our observation, has km and with J m−2 K−1 . We conjecture the pole latitude . The results for both targets are consistent with previous studies. Based on the results, we propose that comet-like objects are not clearly distinguishable from their asteroidal counterparts on the D–Γ plane.

Hamiltonian Formulation and Perturbations for Dust Motion Around Cometary Nuclei

In this paper we analyze the dynamical behavior of large dust grains in the vicinity of a cometary nucleus. To this end we consider the gravitational field of the irregularly shaped body, as well as its electric and magnetic fields. Without considering the effect of gas friction and solar radiation, we find that there exist grains which are static relative to the cometary nucleus; the positions of these grains are the stable equibria. There also exist grains in the stable periodic orbits close to the cometary nucleus. The grains in the stable equibria or the stable periodic orbits will not escape or impact on the surface of the cometary nucleus. The results are applicable for large charge dusts with small area-mass ratio which are near the cometary nucleus and far from the Solar. It is found that the resonant periodic orbit can be stable, and there exist stable non-resonant periodic orbits, stable resonant periodic orbits and unstable resonant periodic orbits in the potential field of cometary nuclei. The comet gravity force, solar gravity force, electric froce, magnetic force, solar radiation pressure, as well as the gas drag force are all considered to analyze the order of magnitude of these forces acting on the grains with different parameters. Let the distance of the dust grain relative to the mass centre of the cometary nucleus, the charge and the mass of the dust grain vary, respectively, fix other parameters, we clacluated the strengths of different forces.

Chaotic dynamics around cometary nuclei

We apply a generalized Kepler map theory to describe the qualitative chaotic dynamics around cometary nuclei, based on accessible observational data for five comets whose nuclei are well-documented to resemble dumb-bells. The sizes of chaotic zones around the nuclei and the Lyapunov times of the motion inside these zones are estimated. In the case of Comet 1P/Halley, the circumnuclear chaotic zone seems to engulf an essential part of the Hill sphere, at least for orbits of moderate to high eccentricity.

Infrared Spectroscopy of HR 4796A's Bright Outer Cometary Ring + Tenuous Inner Hot Dust Cloud

Abstract We have obtained new NASA/IRTF SpeX spectra of the HR 4796A debris ring system. We find a unique red excess flux that extends out to ∼9 μm in Spitzer IRS spectra, where thermal emission from cold, ∼100 K dust from the system’s ring at ∼75 au takes over. Matching imaging ring photometry, we find the excess consists of NIR reflectance from the ring, which is as red as that of old, processed comet nuclei, plus a tenuous thermal emission component from close-in, T ∼ 850 K circumstellar material evincing an organic/silicate emission feature complex at 7–13 μm. Unusual, emission-like features due to atomic Si, S, Ca, and Sr were found at 0.96–1.07 μm, likely sourced by rocky dust evaporating in the 850 K component. An empirical cometary dust phase function can reproduce the scattered light excess and 1:5 balance of scattered versus thermal energy for the ring with optical depth in an 8 au wide belt of 4 au vertical height and M dust > 0.1–0.7 M Mars. Our results are consistent with HR 4796A, consisting of a narrow shepherded ring of devolatilized cometary material associated with multiple rocky planetesimal subcores and a small steady stream of dust inflowing from this belt to a rock sublimation zone at ∼1 au from the primary. These subcores were built from comets that have been actively emitting large, reddish dust for >0.4 Myr at ∼100 K, the temperature at which cometary activity onset is seen in our solar system.

Meter-scale thermal contraction crack polygons on the nucleus of comet 67P/Churyumov-Gerasimenko

We report on the detection and characterization of more than 6300 polygons on the surface of the nucleus of comet 67P/Churyumov-Gerasimenko, using images acquired by the OSIRIS camera onboard Rosetta between August 2014 and March 2015. They are found in consolidated terrains and grouped in localized networks. They are present at all latitudes (from North to South) and longitudes (head, neck, and body), sometimes on pit walls or following lineaments. About 1.5% of the observed surface is covered by polygons. Polygons have an homogeneous size across the nucleus, with 90% of them in the size range 1 – 5 m and a mean size of 3.0 ± 1.4 m. They show different morphologies, depending on the width and depth of their trough. They are found in networks with 3- or 4-crack intersection nodes. The polygons observed on 67P are consistent with thermal contraction crack polygons formed by the diurnal or seasonal temperature variations in a hard (MPa) and consolidated sintered layer of water ice, located a few centimeters below the surface. Our thermal analysis shows an evolution of thermal contraction crack polygons according to the local thermal environment, with more evolved polygons (i.e. deeper and larger troughs) where the temperature and the diurnal and seasonal temperature range are the highest. Thermal contraction crack polygons are young surface morphologies that probably formed after the injection of 67P in the inner solar system, typically 100,000 years ago, and could be as young as a few orbital periods, following the decreasing of its perihelion distance in 1959 from 2.7 to 1.3 a.u. Meter scale thermal contraction crack polygons should be common features on the nucleus of Jupiter family comets.

Bidirectional reflectance of laboratory cometary analogues to interpret the spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko

In this work we provide measurements of the bidirectional reflectance in the VIS-NIR spectral range for a selection of laboratory samples to deduce parameters such as their visible spectrum, phase curve, hemispheric albedo and phase reddening. The aim is to simulate the reflectance of the comet nucleus in order to aid the interpretation of data from the surface of comet 67P/Churyumov-Gerasimenko (67P) acquired by the Rosetta OSIRIS imaging system. We produced a broad variety of well characterised and reproducible samples containing water ice, carbonaceous compounds and complex organic molecules. We were able to reproduce the individual global spectrophotometric parameters (albedo, spectrum, phase reddening, phase curve) of 67P by using mixtures of activated charcoal, tholins and water ice. However, no mixture was able to fit all parameters simultaneously. The samples with highest porosities best match the phase curve of 67P. Spectral considerations from our experiments show that the particle size of the darkening agent on the cometary surface should have a lower limit of a few hundred nanometres to several micrometres and cannot be dominated by particles in the range of some tens of nanometres. Furthermore, our findings indicate that the bright ice patches observed by OSIRIS and VIRTIS should be relatively dust free at small scale.

Cliffs versus plains: Can ROSINA/COPS and OSIRIS data of comet 67P/Churyumov-Gerasimenko in autumn 2014 constrain inhomogeneous outgassing?

Context. This paper describes the modelling of gas and dust data acquired in the period August to October 2014 from the European Space Agency’s Rosetta spacecraft when it was in close proximity to the nucleus of comet 67P/Churyumov-Gerasimenko.Aims. With our 3D gas and dust comae models this work attempts to test the hypothesis that cliff activity on comet 67P/Churyumov-Gerasimenko can solely account for the local gas density data observed by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the dust brightnesses seen by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) in the considered time span.Methods. The model uses a previously developed shape model of the nucleus. From this, the water sublimation rates and gas temperatures at the surface are computed. The gas expansion is modelled with a 3D Direct Simulation Monte Carlo algorithm. A dust drag algorithm is then used to compute dust volume number densities in the coma, which are then converted to brightnesses using Mie theory and a line-of-sight integration. Furthermore we have studied the impact of topographic re-radiation on the models.Results. We show that gas activity from only cliff areas produces a fit to the ROSINA/COPS data that is as statistically good as a purely insolation-driven model. In contrast, pure cliff activity does not reproduce the dust brightness observed by OSIRIS and can thus be ruled out. On the other hand, gas activity from the Hapi region in addition to cliff activity produces a statistically better fit to the ROSINA/COPS data than purely insolation-driven outgassing and also fits the OSIRIS observations rather well. We found that topographic re-radiation does not contribute significantly to the sublimation behaviour of H2 O but plays an important role in how the gas flux interacts with the irregular shape of the nucleus.Conclusions. We demonstrate that fits to the observations are non-unique. We can conclude however that gas and dust activity from cliffs and the Hapi region are consistent with the ROSINA/COPS and OSIRIS data sets for the considered time span and are thus a plausible solution. Models with activity from low gravitational slopes alone provide a statistically inferior solution.

Formation of cometary nuclei during low-velocity collisions of planetesimals

An investigation of the properties of the 67P/CG nucleus shows convincingly that some of the cometary nuclei resulted from low-velocity collisions of planetesimals. The hypothesis about the origination of a cometary nucleus from colliding bodies has long been known, but it is comet 67P/CG that has imparted topicality to it. In this paper we briefly formulate the combining or disruption criteria and show that in several cases the geometry of the cometary nucleus retains some information about the collision process and allows the impactor momentum to be found.

A binary main-belt comet.

Asteroids are primitive Solar System bodies that evolve both collisionally and through disruptions arising from rapid rotation. These processes can lead to the formation of binary asteroids and to the release of dust, both directly and, in some cases, through uncovering frozen volatiles. In a subset of the asteroids called main-belt comets, the sublimation of excavated volatiles causes transient comet-like activity. Torques exerted by sublimation measurably influence the spin rates of active comets and might lead to the splitting of bilobate comet nuclei. The kilometre-sized main-belt asteroid 288P (300163) showed activity for several months around its perihelion 2011 (ref. 11), suspected to be sustained by the sublimation of water ice and supported by rapid rotation, while at least one component rotates slowly with a period of 16 hours (ref. 14). The object 288P is part of a young family of at least 11 asteroids that formed from a precursor about 10 kilometres in diameter during a shattering collision 7.5 million years ago. Here we report that 288P is a binary main-belt comet. It is different from the known asteroid binaries in its combination of wide separation, near-equal component size, high eccentricity and comet-like activity. The observations also provide strong support for sublimation as the driver of activity in 288P and show that sublimation torques may play an important part in binary orbit evolution.

Development and characteristics of Mechanical Porous Ambient Comet Simulants as comet surface analogs

Comets are icy remnants of the Solar System formation, and as such contain some of the most primitive volatiles and organic materials. Sampling the surface of a comet is a high priority for the New Frontiers program. Planetary simulants are crucial to the development of adequate in situ instruments and sample acquisition systems. A high-fidelity comet surface simulant has been developed to support hardware design and development for one Comet Surface Sample Return tool, the BiBlade Comet Sampler. Mechanical Porous Ambient Comet Simulants (MPACS) can be manufactured to cover a wide range of desired physical properties, such as density and cone penetration resistance, and exhibit a brittle fracture mode. The structure of the MPACS materials is an aggregated composite structure of weakly-bonded grains of very small size (diameter ≤ 40 μm) that are most relevant to the structure of the surface of a comet nucleus.

Solar-insolation-induced changes in the coma morphology of comet 67P/Churyumov-Gerasimenko

Context. 67P/Churyumov-Gerasimenko (67P/C-G) is a short-period Jupiter family comet with an orbital period of 6.55 years. Being the target comet of ESA’s Rosetta mission, 67P/C-G has become one of the most intensively studied minor bodies of the Solar System. The Rosetta Orbiter and the Philae Lander have brought us unique information about the structure and activity of the comet nucleus, as well as its activity along the orbit, composition of gas, and dust particles emitted into the coma. However, as Rosetta stayed in very close proximity to the cometary nucleus (less than 500 km with a few short excursions reaching up to 1500 km), it could not see the global picture of a coma at the scales reachable by telescopic observations (103 - 105 km). Aims. In this work we aim to connect in-situ observations made by Rosetta with the morphological evolution of the coma structures monitored by the ground-based observations. In particular, we concentrate on causal relationships between the coma morphology and evolution observed with the Nordic Optical Telescope (NOT) in the Canary Islands, and the seasonal changes of the insolation and the activity of the comet observed by the Rosetta instruments. Methods. Comet 67P/C-G was monitored with the NOT in imaging mode in two colors. Imaging optical observations were performed roughly on a weekly basis, which provides good coverage of short- and long-term variability. With the three dimensional modeling of the coma produced by active regions on the Southern Hemisphere, we aim to qualify the observed morphology by connecting it to the activity observed by Rosetta. Results. During our monitoring program, we detected major changes in the coma morphology of comet 67P/C-G. These were longterm and long-lasting changes. They do not represent any sudden outburst or short transient event, but are connected to seasonal changes of the surface insolation and the emergence of new active regions on the irregular shaped comet nucleus. We have also found significant deviations in morphological changes from the prediction models based on previous apparitions of 67P/C-G, like the time delay of the morphology changes and the reduced activity in the Northern Hemisphere. According to our modeling of coma structures and geometry of observations, the changes are clearly connected with the activity in the Southern Hemisphere observed by the Rosetta spacecraft.

On the reflectance of dust in comets

Reflectance of cometary dust is a key parameter used in the characterization of comets. In the literature, the reflectance of single-scattering cometary dust particles is widely assumed to be the same as that of the cometary nucleus. We discuss this assumption and demonstrate its inconsistency with photometric observations of comets, laboratory optical measurements, and numerical simulation of light scattering from single-scattering dust particles and particulate surfaces composed of the same particles. We estimate the reflectance of cometary dust particles using a comprehensive physical model of polarization measured in comets over wide range of phase angle and at different wavelengths in the visible. The model predicts that the reflectance of dust in comets inversely correlates with their maximum of positive polarization Pmax. We find that even the darkest dust particles appearing in comets with the highest Pmax, reflect considerably more incident solar-radiation energy, up to 200%, compared to what is thought for cometary nuclei. We also find that the reflectance retrieved from polarimetry in the visible appears in good quantitative accordance with previous estimations from infrared observations of comets. Our findings suggest that the dust production of comets is currently overestimated and may require revision.

Long-term survival of surface water ice on comet 67P

Numerous water-ice-rich deposits surviving more than several months on comet 67P/Churyumov-Gerasimenko were observed during the Rosetta mission. We announce the first-time detection of water-ice features surviving up to 2 yr since their first observation via OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) NAC (narrow angle camera). Their existence on the nucleus of comet 67P at the arrival of the Rosetta spacecraft suggests that they were exposed to the surface during the comet’s previous orbit. We investigated the temporal variation of large water-ice patches to understand the long-term sustainability of water ice on cometary nuclei on time-scales of months and years. Large clusters are stable over typical periods of 0.5 yr and reduce their size significantly around the comet’s perihelion passage, while small exposures disappear. We characterized the temporal variation of their multispectral signatures. In large clusters, dust jets were detected, whereas in large isolated ones no associated activity was detected. Our thermal analysis shows that the long-term sustainability of water-ice-rich features can be explained by the scarce energy input available at their locations over the first half year. However, the situation reverses for the period lasting several months around perihelion passage. Our two end-member mixing analysis estimates a pure water-ice equivalent thickness up to 15 cm within one isolated patch, and up to 2 m for the one still observable through the end of the mission. Our spectral modelling estimates up to 48 per cent water-ice content for one of the large isolated feature, and up to 25 per cent water ice on the large boulders located within clusters.