Coma Of Comet Research Articles

A nearly terrestrial D/H for comet 67P/Churyumov-Gerasimenko.

Cometary comae are a mixture of gas and ice-covered dust. Processing on the surface and in the coma change the composition of ice on dust grains relative to that of the nucleus. As the ice on dust grains sublimates, the local coma composition changes. Rosetta observations of 67P/Churyumov-Gerasimenko previously reported one of the highest D/H values for a comet. However, reanalysis of more than 4000 water isotope measurements over the full mission shows that dust markedly increases local D/H. The isotope ratio measured at a distance from the nucleus where the gas is well mixed is close to terrestrial, like that of other Jupiter family comets. This lower D/H has implications for understanding comet formation and the role of comets in delivering water to Earth.

Tentative detection of cyclopropenylidene in comets

ABSTRACT We report the tentative detection of the ring molecule cyclopropenylidene (c-C$_3$H$_2$) in cometary comae. We observed three short-period comets: 73P/Schwassmann–Wachmann 3, 17P/Holmes, 103P Hartley 2, and two Oort Cloud comets: C/2007 N3 (Lulin) and C/2012 F6 (Lemmon) with the Arizona Radio Observatory Submillimeter Telescope (SMT) and the Atacama Large Millimeter/submillimeter Array (ALMA). We tentatively detected c-C$_3$H$_2$ in three of the comets, with molecule production rates in the range of $(0.2\!-\!3.8)\times 10^{25}$ molecules s$^{-1}$, and c-C$_3$H$_2$/H$_2$O mixing ratios of 0.01–0.19 per cent. Evidence from both the single-dish and interferometric data points to the presence of c-C$_3$H$_2$ being of a discrete transient nature and exhibiting an extended spatial distribution, indicating production from the decomposition of organic dust particles in the coma.

Dual Sources of S2 Observed in Comet 67P: Insights from Comparing ROSINA Measurements and Laboratory Simulations

Hydrogen sulfide (H2S) is the fifth most abundant molecule observed in the coma of comet 67P/Churyumov–Gerasimenko (67P). Prior to its incorporation into cometary materials, H2S likely underwent ultraviolet (UV) radiation exposure, which is thought to initiate a complex sulfur chemistry. We present an investigation into the UV photochemistry of H2S ices using infrared, Raman, and mass spectrometry techniques. Our study reveals the production of complex sulfur allotropes ranging from S2 to S6, alongside polysulfanes (H2S n , n = 2–3). Temperature-programmed desorption measurements postirradiation of H2S exhibit two peaks for S2 molecules: a broad peak between 80 and 140 K and a distinct peak at ∼245 K. Notably, larger allotropes S3–S5 exclusively display the 245 K peak. Furthermore, ROSINA measurements of the S2/H2S ratio during dust impact events and previously reported S2/H2S ratios in the undisturbed coma are compared to our laboratory-determined S2/H2S values. This analysis identifies two distinct sources of S2: a volatile S2 potentially sublimated directly from the comet’s surface and a secondary source likely resulting from fragmentation of larger sulfur chains during dust impacts. We determined the ratio of produced S2 to the initial H2S for both the volatile component and the refractory component at 245 K with both measurements conducted at an irradiation incident fluence of 2.25 × 1017 photons cm−2. These laboratory-derived S2/H2S ratios exhibit concordance with ROSINA measurements. When extrapolated to incident fluences anticipated in molecular clouds, this photoprocessing mechanism offers a plausible explanation for the measured S2/H2S ratio in comet 67P.

Performance assessment of an innovative light and compact dust shield for DISC onboard Comet Interceptor/ESA space probes

The dust ejected by cometary nuclei encodes valuable information on the formation and evolution of the early Solar System. Multiple short-period comets have been studied in situ, but several perihelion passages considerably modified their pristine condition. Comet Interceptor is the first space mission selected by the European Space Agency to study a pristine dynamically new comet in situ. During a fast flyby through the comet coma, hypervelocity impacts with dust particles will represent not only an important source of information, but also a serious hazard to the spacecraft and its payload. Here we discuss the assessment tests performed on the dust shield of the Dust Impact Sensor and Counter instrument (DISC), part of the Comet Interceptor payload, which will be directly exposed to the cometary dust flux. Using a Light-Gas Gun, we shot mm-sized particles at ∼5 km/s, transferring momenta and kinetic energies representative of those foreseen for the mission. The impact effects on the DISC breadboard were compared to theoretical predictions by a ballistic limit equation for hypervelocity impacts. We find that, with a simple improvement in the dust shield design, DISC is compatible with the expected cometary environment.

Activity of main-belt comet 324P/La Sagra

We study the activity evolution of the main-belt comet 324P/La Sagra over time and the properties of its emitted dust. We performed aperture photometry on images taken by a wide range of telescopes at optical and thermal infrared wavelengths between 2010 and 2021. We derived the combined scattering cross section of the nucleus and dust (when present) as a function of time, and we derived the thermal emission properties. Fitting an IAU H-G phase function to the data obtained when 324P was likely inactive, we derived an absolute nucleus magnitude $H_R = (18.4 0.5)$ mag using $G = 0.15 0.12$. The activity of 324P/La Sagra during the 2015 perihelion passage has significantly decreased compared to the previous perihelion passage in 2010, and it decreased even further during the 2021 perihelion passage. This decrease in activity may be attributed to mantling or to the depletion of volatile substances. The $Af profile analysis of the coma of the main-belt comet suggests a near-perihelion transition from a lower-activity pre-perihelion to a higher-activity post-perihelion steady state. We calculate a dust geometric albedo in the range of (2 - 45)<!PCT!>, which prevents us from constraining the spectral type of 324P/La Sagra, but we found an indication of dust superheating at 4.5 µm.

Infrared Spectra of Solid HCN Embedded in Various Molecular Environments for Comparison with the Data Obtained with JWST

Hydrogen cyanide (HCN) molecules serve as an important tracer for the chemical evolution of elemental nitrogen in the regions of star and planet formation. This is largely explained by the fact that N atoms and N2 molecules are poorly accessible for observation in the radio and infrared (IR) ranges. In turn, gas-phase HCN can be observed at various stages of star formation, including disks around young stars, cometary comas, and atmospheres of the planetary satellites. Despite the large geography of gas-phase observations, an identification of interstellar HCN ice is still lacking. In this work we present a series of IR spectroscopic measurements performed at the new ultrahigh vacuum cryogenic apparatus aiming to facilitate the search for interstellar HCN ice. A series of high-resolution laboratory IR spectra of HCN molecules embedded in the H2O, H2O:NH3, CO, CO2, and CH3OH ices at 10 K temperature is obtained. These interstellar ice analogues aim to simulate the surroundings of HCN molecules by the main constituents of the icy mantles on the surface of the interstellar grains. In addition, the spectra of HCN molecules embedded in the solid C6H6, C5H5N, and C6H5NH2 are obtained to somehow simulate the interaction of HCN molecules with carbonaceous material of the grains rich in polycyclic aromatic hydrocarbons. The acquired laboratory spectroscopic data are compared with the publicly available results of NIRSpec James Webb Space Telescope observations toward quiescent molecular clouds performed by the IceAge team.

Strongly Rotationally Dependent Lifetimes of C2 in the 23Σg- State: Implications for Predissociation in the Vacuum Ultraviolet Region.

The radiative and photodissociative properties of the dicarbon molecule, C2, in high-lying electronic states are of utmost importance for modeling the photochemical processes that occur in various astronomical environments. Despite extensive spectroscopic studies in the last two centuries, the photodissociation properties of C2 are still largely unknown, particularly for quantum states in the vacuum ultraviolet (VUV) region. Here, the lifetimes of C2 for each individual rovibrational level in the recently identified 23Σg- state are measured for the first time using a VUV-pump-UV-probe photoionization scheme. The lifetimes are found to be strongly dependent on the rotational and vibrational quantum levels in the 23Σg- state. The strongly rotationally dependent lifetimes observed here indicate that the 23Σg- state may mainly undergo a predissociation process through couplings with nearby repulsive electronic states. The current observation could have important applications in modeling the interstellar medium and cometary comae.

The strength of outgassed porous dust aggregates

Context. Outgassing of dust-ice aggregates plays an important role on the surfaces of cometary nuclei as well as for snow-line crossings in protoplanetary disks. Aims. To assess the stability of desiccated dust aggregates, we measured the tensile strength of silica dust samples over a wide range of volume filling factors. Methods. We produced these silica dust samples over a wide range of volume filling factors by gently evaporating dust-ice mixtures with various dust-to-ice mass ratios under vacuum conditions. The tensile strengths of these samples were then measured using the standardized Brazilian disk test. Experiments were performed in a vacuum and at room temperature but were also compared to measurements in air at room temperature and in a vacuum at elevated temperatures. Results. For spherical amorphous silica dust, we find no influence of the environmental conditions (air, vacuum, or heating) on the measured tensile strength. However, for angular crystalline silica dust we see a strong increase in tensile strength in a vacuum compared to air and an even higher increase when the samples are heated in a vacuum. For the spherical silica dust samples, we find a characteristic increase in the tensile strength with decreasing particle size. The tensile strength of samples with identical particle sizes increases strongly with an increasing volume filling factor. Extrapolation of our data to a volume filling factor of 0.1 (90% porosity) shows that a tensile strength as low as 1 Pa can be reached. Conclusions. Numerical simulations show that evaporating water ice in the subsurface layers of comets can reach gas pressures of ~1 Pa. Thus, a desiccated dust layer with a 10% volume filling factor should be detachable and released into the cometary coma. Using a relation between the tensile strength and the critical fragmentation energy, we predict the break-up speed of dust aggregates in mutual collisions as a function of the volume filling factor. Furthermore, we discuss the susceptibility of the aggregates to ram pressure. These values are relevant for protoplanetary disk research and for meteoroids entering planetary atmospheres.

Rotational state-to-state transition rate coefficients for H2O + H2O collisions at nonequilibrium conditions

The goal is to develop a database of rate coefficients for rotational state-to-state transitions in H$_ $O + H$_ $O collisions that is suitable for the modeling of energy transfer in nonequilibrium conditions, in which the distribution of rotational states of H$_ $O deviates from local thermodynamic equilibrium. A two-temperature model was employed that assumed that although there is no equilibrium between all possible degrees of freedom in the system, the translational and rotational degrees of freedom can be expected to achieve their own equilibria independently, and that they can be approximately characterized by Boltzmann distributions at two different temperatures, $T_ kin $ and $T_ rot Upon introducing our new parameterization of the collisional rates, taking into account their dependence on both $T_ kin $ and $T_ rot $, we find a change of up to 20<!PCT!> in the H$_ $O rotational level populations for both ortho and para-H$_ $O for the part of the cometary coma where the nonequilibrium regime occurs.

Coma and tail of Comet 67P/Churyumov–Gerasimenko during the 2021–2022 apparition

ABSTRACT We present results on the global activity of comet 67P/Churyumov–Gerasimenko (67P), the ROSETTA target, during its first perihelion passage after the mission and after an encounter with planet Jupiter. 67P was observed by broad-band filter imaging at 33 epochs during 2021 May 21 to 2022 June 1. The mean radial flux profiles suggest that ‘equilibrium’ conditions for the dust flow in the coma existed from around perihelion until about 100 d thereafter. We propose a scenario for the smaller and larger radial exponents, measured before and after the ‘equilibrium’ phase. Four coma fans with possible source regions at +40°, –10°, –50°, and –70° latitude on the nucleus are identified. The three fans at southern latitudes may be identical with fan sources seen during post-perihelion in 2015–2016. The rotation axis of 67P may not have changed at all or at least not much (<5°) from the orientation measured during the Rosetta mission at the comet. The dust streamers in the coma originated from observed coma fans, containing mostly dust emitted within days to weeks before observation. Two dust streamers, a long- and a short-lasting one, contained heavy dust grains from emission periods hundreds of days before perihelion. Similar emission periods are obtained for the dust seen in the tail region of the comet. Similarities and differences in the dust activity during the recent apparition with that of the Rosetta mission at the comet are found.

Deep Search for Phosphine in a Prestellar Core

Understanding in which chemical forms phosphorus exists in star- and planet-forming regions and how phosphorus is delivered to planets are of great interest from the viewpoint of the origin of life on Earth. Phosphine (PH3) is thought to be a key species to understanding phosphorus chemistry, but never has been detected in star- and planet-forming regions. We performed sensitive observations of the ortho-PH3 10 − 00 transition (266.944 GHz) toward the low-mass prestellar core L1544 with the Atacama Compact Array stand-alone mode of the Atacama Large Millimeter/submillimeter Array. The line was not detected down to 3σ levels in 0.07 km s−1 channels of 18 mK. The nondetection provides the upper limit to the gas-phase PH3 abundance of 5 × 10−12 with respect to H2 in the central part of the core. Based on the gas-ice astrochemical modeling, we find the scaling relationship between the gas-phase PH3 abundance and the volatile (gas and ice with larger volatility than water) P elemental abundance for given physical conditions. This characteristic and well-constrained physical properties of L1544 allow us to constrain the upper limit to the volatile P elemental abundance of 5 × 10−9, which is a factor of 60 lower than the overall P abundance in the interstellar medium. Then the majority of P should exist in refractory forms. The volatile P elemental abundance of L1544 is smaller than that in the coma of comet 67P/C-G, implying that the conversion of refractory phosphorus to volatile phosphorus could have occurred along the trail from the presolar core to the protosolar disk through, e.g., sputtering by accretion/outflow shocks.

Phase angle dependence of the dust cross section in a cometary coma

Context. Rosetta/OSIRIS took optical measurements of the intensity of scattered light from the coma of 67P/Churyumov-Gerasimenko over a wide range of phase angles. These data have been used to measure the phase angle dependent radiance profile of the dust coma. Aims. We want to provide information about the column area densities of the dust coma as seen from Rosetta. This information in combination with the measured OSIRIS phase function can then be used to determine the scattering phase function of the dust particles. Methods. We use a simple numerical model to calculate the dust density in the coma. For this we neglect all forces but solar gravitation and radiation pressure. As this cannot describe particles close to the surface of the comet, we assume starting conditions at a sufficient distance. We evaluate the column area density as observed from Rosetta/OSIRIS and compare the results for different spacecraft positions, dust sizes and surface activity distributions. Results. We find the phase angle dependence of the column area density to be largely independent of particle size and spacecraft positions. The determining factor is the activity distribution across the surface, especially the activity on the night side. For models with no night side activity, we find the column area density at high phase angles to be roughly two orders of magnitude larger than at low phase angles. Conclusions. The radiance profile measured from inside a cometary coma results from the combined effects of a phase angle dependent column area density and the scattering phase function. The radiance profile is therefore strongly dependent on the surface activity distribution, and - unless the dust emission is isotropic - any attempt to infer particle properties (as expressed through the scattering phase function) from such data must take into account and de-bias for this spatial variation of the dust column area density.

Product-specific reaction kinetics in continuous uniform supersonic flows probed by chirped-pulse microwave spectroscopy.

Experimental studies of the products of elementary gas-phase chemical reactions occurring at low temperatures (<50K) are very scarce, but of importance for fundamental studies of reaction dynamics, comparisons with high-level quantum dynamical calculations, and, in particular, for providing data for the modeling of cold astrophysical environments, such as dense interstellar clouds, the atmospheres of the outer planets, and cometary comae. This study describes the construction and testing of a new apparatus designed to measure product branching fractions of elementary bimolecular gas-phase reactions at low temperatures. It combines chirped-pulse Fourier transform millimeter wave spectroscopy with continuous uniform supersonic flows and high repetition rate laser photolysis. After a comprehensive description of the apparatus, the experimental procedures and data processing protocols used for signal recovery, the capabilities of the instrument are explored by the study of the photodissociation of acrylonitrile and the detection of two of its photoproducts, HC3N and HCN. A description is then given of a study of the reactions of the CN radical with C2H2 at 30K, detecting the HC3N product, and with C2H6 at 10K, detecting the HCN product. A calibration of these two products is finally attempted using the photodissociation of acrylonitrile as a reference process. The limitations and possible improvements in the instrument are discussed in conclusion.

Ejection and dynamics of aggregates in the coma of comet 67P/Churyumov-Gerasimenko

The process of gas-driven ejection of refractory materials from cometary surfaces continues to pose a challenging question in cometary science. The activity modeling of comet 67P/Churyumov-Gerasimenko, based on data from the Rosetta mission, has significantly enhanced our comprehension of cometary activity. But thermophysical models have difficulties in simultaneously explaining the production rates of various gas species and dust. It has been suggested that different gas species might be responsible for the ejection of refractory material in distinct size ranges. This work focuses on investigating the abundance and the ejection mechanisms of large aggregates (gtrsim 1 cm) from the comet nucleus. We aim to determine their properties and map the distribution of their source regions across the comet surface. This can place constraints on activity models for comets. We examined 189 images acquired at five epochs by the OSIRIS/NAC instrument on board the Rosetta spacecraft. Our goal was to identify bright tracks produced by individual aggregates as they traversed the camera field of view. In parallel, we generated synthetic images based on the output of dynamical simulations involving various types of aggregates. By comparing these synthetic images with the observations, we determined the characteristics of the simulated aggregates that most closely resemble the observations. We have identified over 30000 tracks present in the OSIRIS images, derived constraints on the characteristics of the aggregates, and mapped their origins on the nucleus surface. The aggregates have an average radius of $ cm and a bulk density consistent with that of the comet's nucleus. Due to their size, gas drag exerts only a minor influence on their dynamical behavior, so an initial velocity is needed to bring them into the camera field of view. The source regions of these aggregates are predominantly located near the boundaries of distinct terrains on the surface.

Dust-dust collisions in cometary comas: applications to comet 67P/Churyumov-Gerasimenko

Abstract Silica has emerged as a crucial component within inner comet comas. This work investigates silica dust aggregates and their interactions within cometary comas. We study the probability that aggregates in the size range 1-100 μm collide with each other in the coma and analyze the outcomes of such collisions by using the “Collision of Porous Aggregates” (CPA) Software, which incorporates mass, size, and porosity evolution of the dust population. Beginning with assumed initial distributions and physical properties for silica aggregates at the comet nucleus, we compute their collisional evolution from when they depart the nucleus until they traverse the coma. Using data of dust particles observed in the coma of comet 67P/Churyumov-Gerasimenko, we demonstrate that dust-dust collisions in cometary comas cannot be neglected. Our analysis yields final distributions in terms of mass, size, and porosity. To validate our findings, we compare them with in-situ measurements of 67P/Churyumov-Gerasimenko collected by the COSIMA (COmetary Secondary Ion Mass Analyser) instrument of the Rosetta mission. Our investigation reveals a notable agreement between our derived size distributions and the data acquired by COSIMA within the same size range. This study may be applied to any comet that presents a similar dust production as it approaches the Sun. The insights of this work may contribute to estimating other dust properties such as strength, absorption, reflectivity, and thermal conductivity and highlight the importance of considering dust-dust collisions when studying cometary comas and their evolution.

Cold electrons at a weakly outgassing comet

ABSTRACT Throughout the Rosetta mission, cold electrons (&amp;lt;1 eV) were measured in the coma of comet 67P/Churyumov–Gerasimenko. Cometary electrons are produced at ∼10 eV through photoionization or through electron-impact ionization collisions. The cold electron population is formed by cooling the warm population through inelastic electron–neutral collisions. Assuming radial electron outflow, electrons are collisional with the neutral gas coma below the electron exobase, which only formed above the comet surface in near-perihelion high-outgassing conditions (Q &amp;gt; 3 × 1027 s−1). However, the cold population was identified at low outgassing (Q &amp;lt; 1026 s−1), when the inner coma was not expected to be collisional. We examine cooling of electrons at a weakly outgassing comet, using a 3D collisional model of electrons at a comet. Electron paths are extended by trapping in an ambipolar electric field and by gyration around magnetic field lines. This increases the probability of electrons undergoing inelastic collisions with the coma and becoming cold. We demonstrate that a cold electron population can be formed and sustained, under weak outgassing conditions (Q = 1026 s−1), once 3D electron dynamics are accounted for. Cold electrons are produced in the inner coma through electron–neutral collisions and transported tailwards by an E × B drift We quantify the efficiency of trapping in driving electron cooling, with trajectories typically 100 times longer than expected from ballistic radial outflow. Based on collisional simulations, we define an estimate for a region where a cold electron population can form, bounded by an electron cooling exobase. This estimate agrees well with cold electron measurements from the Rosetta Plasma Consortium.

Parent Volatile Outgassing Associations in Cometary Nuclei: Synthesizing Rosetta Measurements and Ground-based Observations

Comets, as remnants of the solar system’s formation, vary in volatile-refractory content. In situ comet studies, such as the Rosetta mission to 67P/Churyumov–Gerasimenko, provide detailed volatile composition insights, while ground-based studies offer broader comet samples but in fewer species. Comparing 67P’s volatile correlations during the 2 yr Rosetta mission with those from remote sensing gives insights into volatile distribution in the nucleus and factors influencing their release. Our goal is to identify associations between volatiles seen from the ground and those in 67P. Given 67P’s seasonal variations, we segmented the Rosetta mission around 67P into six epochs, reflecting different insolation conditions. It has been suggested that there are at least two different ice matrices, H2O and CO2 ice, in which the minor species are embedded in different relative abundances within them. We employed various methodologies to establish associations among volatiles, such as volatile production rates, spatial distributions, patterns in mixing ratio, and local outgassing source locations. We note that different techniques of grouping molecules with respect to H2O and CO2 may yield different results. Earth’s atmosphere blocks CO2; however, due to observed differences between H2O and C2H6 from the ground and between H2O and CO2 from comet missions, C2H6 is suggested to be a CO2 proxy. Our study delves into cometary coma molecular correlations, highlighting their associations with H2O and CO2 matrices and advancing our understanding of the early solar system comet formation and evolution.

Impact of radial interplanetary magnetic fields on the inner coma of comet 67P/Churyumov-Gerasimenko

Context. The direction of the interplanetary magnetic field determines the nature of the interaction between a Solar System object and the solar wind. For comets, it affects the formation of both a bow shock and other plasma boundaries, as well as mass-loading. Around the nucleus of a comet, there is a diamagnetic cavity, where the magnetic field is negligible. Observations by the Rosetta spacecraft have shown that, most of the time, the diamagnetic cavity is located within a solar-wind ion cavity, which is devoid of solar wind ions. However, solar wind ions have been observed inside the diamagnetic cavity on several occasions. Understanding what determines whether or not the solar wind can reach the diamagnetic cavity also advances our understanding of comet–solar wind interaction in general. Aims. We aim to determine the influence of an interplanetary magnetic field directed radially out from the Sun – that is, parallel to the solar wind velocity – on the comet–solar wind interaction. In particular, we explore the possibility of solar wind protons entering the diamagnetic cavity under radial field conditions. Methods. We performed global hybrid simulations of comet 67P/Churyumov-Gerasimenko using the simulation code Amitis for two different interplanetary magnetic field configurations and compared the results to observations made by the Rosetta spacecraft. Results. We find that, when the magnetic field is parallel to the solar wind velocity, no bow shock forms and the solar wind ions are able to enter the diamagnetic cavity. A solar wind ion wake still forms further downstream in this case. Conclusions. The solar wind can enter the diamagnetic cavity if the interplanetary magnetic field is directed radially from the Sun, and this is in agreement with observations made by instruments on board the Rosetta spacecraft.

Inferring the CO2 Abundance in Comet 45P/Honda-Mrkos-Pajdušáková from [O i] Observations: Implications for the Source of Icy Grains in Cometary Comae

The study of cometary composition is important for understanding our solar system's early evolutionary processes. Carbon dioxide (CO2) is a common hypervolatile in comets that can drive activity but is more difficult to study than other hypervolatiles owing to severe telluric absorption. CO2 can only be directly observed from space-borne assets. Therefore, a proxy is needed to measure CO2 abundances in comets using ground-based observations. The flux ratio of the [O i] λ5577 line to the sum of the [O i] λ6300 and [O i] λ6364 lines (hereafter referred to as the [O i] line ratio) has, with some success, been used in the past as such a proxy. We present an [O i] line ratio analysis of comet 45P/Honda-Mrkos-Pajdušáková (HMP), using data obtained with the Tull Coudé Spectrograph on the 2.7 m Harlan J. Smith Telescope at McDonald Observatory, taken from UT 2017 February 21–23, when the comet was at heliocentric distances of 1.12–1.15 au. HMP is a hyperactive Jupiter-family comet (JFC). Icy grains driven out by CO2 sublimation have been proposed as a driver of hyperactivity, but the CO2 abundance of HMP has not been measured. From our [O i] line ratio measurements, we find a CO2/H2O ratio for HMP of 22.9% ± 1.4%. We compare the CO2/H2O ratios to the active fractions of the nine comets (including HMP) in the literature that have data for both values. We find no correlation. These findings imply that CO2 sublimation driving out icy grains is not the only factor influencing active fractions for cometary nuclei.

Dust color variations of comet C/2016 M1 (PanSTARRS)

Our research aims to study the dust environment of comets that are active beyond the snow line. The current work considers dust color variations in the coma of long-period comet C/2016 M1 (PanSTARRS). We observed the comet from July to October 2017 (12 epochs) using a 0.61-m telescope at the Skalnaté Pleso Observatory. The activity of the comet was stable and typical for other long-period comets. Our data show a slight bluing of the comet S'BR = −0.5 ± 0.9% per 1000 Å on August 26 and reddening to S'BR = 6.0 ± 1.4% per 1000 Å on August 28, while during the rest of the observation period, the comet mostly had red color that is typical of other comets; i.e., the average S'BR is equal to 9.5 ± 1.1% per 1000 Å. We have performed modeling of the observed color variations using model agglomerated debris particles. We could reproduce the dust color variations through changes in the dust composition; however, photometric data on their own do not allow us to make a precise conclusion. For instance, the bluest color could be induced by either water-ice, Mg-rich silicate, amorphous carbon, or their mixture. The reddest color is consistent with Mg-Fe silicates and organics. Nevertheless, further study of the comet in terms of polarimetry would yield a more robust conclusion.