Oral presentations and abstracts
Description: Remote sensing of Solar System objects such as planetary atmospheres, comets and asteroids, or the space between them, relies largely on the analysis of the solar light scattered by dust particles and clouds. Polarization is an invaluable tool for obtaining information on the physical properties (size, shape, and composition) and location of the scattering particles in the corresponding atmosphere.
The aim of this session is to bring together observers, modelers, and experimental physicists to discuss the most recent results and advances in the characterization of dust particles by their spectro-polarimetric pattern.
Long-periodic comet C/2011 KP36 (Spacewatch), defined by Bauer et al. 2013 (Aph. J. 773:22) as a Scattered Disc Object, has a period of around 238 years and perihelion distance q=4.88 au. Its semi-major axis (a=38.41 au) is larger than aphelion distance of Neptune (aN=30.07 au). Comprehensive observations of the comet were carried out at the 6-m BTA telescope of the Special Astrophysical Observatory (Russia) with the multimode focal reducer SCORPIO-2.
Long-slit spectra in the visible and photometric and linear polarimetric images with the g-sdss and r-sdss filters were obtained on November 25, 2016, after perihelion passage, when heliocentric and geocentric distances of the comet were 5.06 au and 4.47 au, respectively. The observations were done at the phase angle 9.57 deg. Two strong jet-like structures in solar and antisolar directions and two short and narrow jet features in a direction almost perpendicular to the Sun–comet direction were revealed in the coma. The cometary activity was characterized by Afρ values of 1065±11 cm in the g-sdss filter and 1264±17 in the r-sdss filter.
The spectrum of comet C/2011 KP36(Spacewatch) we observed shows emissions from the ion CO+. It is second comet, for which we detected emissions at large heliocentric distances more 4 au.
In the near-nucleus coma, the polarization is about 5% (in absolute value), then it decreases, reaching the minimal values about 2% in the region of 10000–20000 km. The polarization increases up to 3.7–5% at the distance of 25000–45000 km, and then the degree of polarization changes with wave-like variations up to 1–4%, depending on the cut direction, with increasing distance up to ~105 km. See Figure 1 for details.
On average, the (g–r) color index of the dust varies with increasing distance from the nucleus. In the near-nucleus region, the color index is about 0.7m. At distances from about 10000 to 20000 km from the photocenter, dust color is bluer (~0.33m) in comparison with the Sun color (color indices (g–r) for the Sun is 0.44m
To characterize the dust in this comet, we modeled both polarization and color in the directions of all jets as well as for the regular coma. We considered a variety of materials typical for cometary dust and Centaurs (including porous particles) and their mixtures and modeled the dust as ensembles of rough spheroids of a variety of obliquity and size. Our modeling allowed us to reproduce not only specific values of color and polarization but also their change with the distance from the nucleus using realistic ideas about possible changes in particle size and composition as they move through the coma and also to compare the dust in different cometary features.
Figure 1 A spatial distribution of polarization degree in comet C/2011KP36 (Spacewatch) in the r-sdss filter obtained at the phase angle 9.6 deg. The left image shows the polarization map for the whole coma with its associated scale bar in percentage on the top of the image, while the image on the right displays the coma area around the nucleus in an enlarged scale. The location of the optocenter is marked with a black cross, and jets with black lines. The arrows point in the direction to the Sun, North (N), East (E), and negative velocity vector of the comet as seen in the observer’s plane of sky (V). Negative distance is in the solar direction.
How to cite: Ivanova, O., Kolokolova, L., Luk’yanyk, I., Kreshenok, V., Rosenbush, V., Kiselev, N., Afanasiev, V., and Kirk, Z.: Scattering properties of dust in C/2011 KP36 (Spacewatch), Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-346, https://doi.org/10.5194/epsc2020-346, 2020.
We present detailed polarimetric analysis of pre-perihelion STEREO observations of Kreutz group comet C/2010 E6 (STEREO), focusing on the changes in polarisation with decreasing heliocentric distance in the pre-perihelion STEREO spacecraft hourly observations of the comet in March 2010. We utilise a bespoke image analysis method for the coronagraph data, finding the Stokes parameters for comet and its tail. The results show a clear variability in polarisation, which can be attributed to sublimation of refractory material.
Polarimetry is a useful tool for remote observations of comet refractory material, since it can provide constraints on its structure and composition. Near-Sun comets like C/2010 E6 (STEREO) are interesting partly for their frequency – Kreutz group comets represent the single largest comet family  – but mostly for the intense near-Sun environment they experience. The comet’s approach to perihelion is likely accompanied by sublimation of refractory material, therefore we can, by studying comet's changing polarimetric signature during its approach to the Sun, extract information about the composition of refractory material in cometary dust. The full analysis is presented in .
Comet C/2010 E6 (STEREO) belongs to the Kreutz family of sungrazing comets, which share very similar orbital parameters and are thought to be the remnants of a larger parent comet [1,3,4].
The comet was imaged by the twin STEREO (Solar Terrestrial Relations Observatory) spacecraft in March 2010; A (Ahead) and B (Behind). In this work only COR2 visible light (bandpass 650-750 nm) coronagraph imagery is used .
Permanently in the light path of COR2 is a linear polariser which can be set at three angles: 0º, 120º, and 240º relative to a reference position. A sequence (triplet) of images using the three different polariser angles in quick succession is taken once per hour.
The image analysis was conducted using a bespoke semi-automated routine fine-tuned for the use with STEREO COR2 data.
Firstly, the image triplets undergo standard pre-processing using the SECCHI_PREP routine from the SolarSoft library for IDL . Then the comet head is found interactively in the image triplets.
Orbital parameters are used to find the plane of the comet's orbit, in which the dust tail is assumed to lie; all data points in the images are then mapped onto this plane. From this, the phase angle - between the Sun, the comet, and the spacecraft – can be determined.
Comet tail is then traced in each separate image. Its transverse cross-sections first have their background removed using a cubic fit and are then truncated (integrated) to a single point for each longitudinal step.
The three resulting polarised intensity vectors from the image triplet are then aligned, and the polarimetric properties (Stokes parameters Q, U, and I) calculated. The method is similar to, but distinct from, the one used by [7,8].
We present two sets of plots of STEREO/SECCHI/COR2 observations of comet C/2010 E6 (STEREO): polarimetric (Figure 1) and photometric (Figure 2). All data is plotted against heliocentric distance. Since the data would contain considerable overlap otherwise, three different offsets are used to stagger the data.
The phase angle information is lost in these plots. STEREO-A phase angle observations range between ~35º and ~25º (right to left). The phase angle range in STEREO-B observations is ~135º to ~105º (right to left). The phase angle variation along the tail is small for each observation.
5. Discussion & Conclusions
Prior to perihelion, at which it likely disintegrated, comet C/2010 E6 (STEREO) exhibited a variety of peculiar behaviours - both photometric and polarimetric - which distinguish it from most other comet observations .
Most notable is the sudden drop in polarisation of the comet nucleus at ~20 R⨀, which then spread slowly along the comet tail. This is accompanied later (~15 R⨀) by a broadening of the intensity peak from the nucleus further down the tail. The broadening and brightening are consistent with those observed by e.g. .
The polarimetric signature of near-zero (STEREO-B, high phase angles) or negative (STEREO-A, low phase angles) polarisation spreading from the near-nucleus region along the tail is consistent with sublimation of silicate particles at local temperatures exceeding their sublimation point. This effect propagates slowly both through the ablating surface and through the dust tail, as sublimation acts as a source of cooling. Thus the observed negative polarisation at low phase angle may explained as the response of the as of yet unsublimated fluffy dust particles of primarily silicate composition.
We would like to thank M. Knight, K. Battams, and V. Andretta for their help and collaboration. This research is supported through the UK Science and Technology Facilities Council and has made use of data and/or services provided by the IAU’s Minor Planet Center.
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How to cite: Nezic, R., Bagnulo, S., and Jones, G.: Effects of heliocentric distance on polarisation of comet C/2010 E6 (STEREO), Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-817, https://doi.org/10.5194/epsc2020-817, 2020.
Linear polarization observations have suggested the presence of dust particles that scatter solar light within cometary comae and the interplanetary dust cloud. Recent progresses result from in-situ observations or remote observations from large telescopes, while avoiding contamination from atmospheric airglow. The interpretation of polarimetric observations, at given phase angles and wavelengths, is a powerful tool to better understand the composition and physical properties of these irregular dust particles and their formation processes. It now provides clues to the origin of the Solar System and to some characteristics of dust in stellar systems, as discussed in .
Along a line-of-sight within the interplanetary dust cloud, both solar distance (R) and phase angle (a) vary, meaning that the intensity and linear polarization (P) of solar light scattered by interplanetary dust (i.e. zodiacal light) need to be inverted to retrieve some local properties. In the near-ecliptic symmetry plane of the cloud, P(a) phase curves are smooth, with a slightly negative branch below 20° and a maximum by 90°. For a constant a=90°, P increases with R, at least between 0.5 au and 1.5 au. Numerical and experimental simulations (in the laboratory and under microgravity conditions) of such results suggest i) the presence in dust of low-absorbing minerals and absorbing complex organics, with a significant amount of fluffy aggregates, ii) some sublimation of semi-volatile organics with decreasing R and increasing temperatures [2; also 3-5 for reviews].
In situ observations have, up to now, only been done on board Giotto during its flybys, allowing us to suggest the presence of a fragment in the coma of 26P/Grigg-Skjellerup and to derive very low values for geometric albedos (about 3%) and densities (50 to 500 kg m−3) of 1P/Halley dust particles [6-7].
Dust whole-coma remote observations, which avoid contamination from gaseous emissions with narrow interference filters, have been used by various teams to obtain P(a) phase curves. As for the zodiacal light, they are smooth; also P (whenever positive) increases with wavelenght for a given a. Numerical and experimental simulations are consistent with a dust population of irregular and possibly fractal structure, with complex optical indices suggesting mixtures of minerals and more absorbing organics [8-10]. Polarimetric in situ observations and remote imaging of comae (at given phase and wavelenght) suggest that the properties of dust are quite different in dust jets than elsewhere in a dust coma, and in Oort-cloud comets than in periodic comets [9, 11-12]. Such trends fairly agree with the ground-truth provided by the Rosetta rendezvous with 67P/Churyumov-Gerasimenko [13 for a review]. Finally, experimental simulations of 67P pre-perihelion brightness phase curves provide clues to an important amount of organic compounds and to fluffy aggregates with sizes above 10 micrometres .
Back in time, polarimetric results on cometary dust contribute to a better understanding of the formation of dust particles, in external regions of the protosolar disk. They likely resulted from submicron-sized grains accreted at low collision velocities, with further addition of minerals processed near the proto-Sun [e.g. 15-16]. At the Late Heavy Bombardment epoch, enormous influxes of interplanetary dust might then have provided a massive delivery of cometary carbonaceous compounds on telluric planets [1; 17].
Beyond in space, properties of interplanetary dust may suggest improvements in the modelling of observations of protoplanetary and debris disks. Updating results on the modelling of the dust continuum emission of the protoplanetary disk around MWC 758 , with moderately porous spherical particles and the above-mentioned phase functions retrieved by Rosetta, provides a better agreement between synthetic and observed maps of emission at sub-millimetre wavelengths. Besides, observations of debris disks have now shown that light scattering properties of dust in debris disks are far from those predicted by Mie theory and in favour of irregular dust particles, as pointed out essentially by phase functions analysis [19-20]. These results can greatly benefit from in situ and remote cometary observations and from advanced modelling of cometary dust, and at the same time provide examples of dust properties in other stellar systems to put our own Solar System into context .
Polarimetric observations of dust clouds in the Solar System provide clues to the properties of dust particles within cometary comae and the zodiacal cloud. Such results are already noticed to improve the modelling of dust in protoplanetary and debris disks, through the use of irregular porous particles properties instead of more conventional compact spherical ones.
Comet Interceptor, ESA F-Class mission to an Oort-cloud comet, to be launched in 2028, should provide a better understanding of its local polarimetric properties with EnVisS experiment. Meanwhile, remote polarimetric observations of comets might contribute to the target selection.
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How to cite: Levasseur-Regourd, A.-C., Baruteau, C., Lasue, J., Milli, J., and Renard, J.-B.: Linear polarization as a tool to characterize interplanetary, cometary, and extrasolar dust particles , Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-391, https://doi.org/10.5194/epsc2020-391, 2020.
Debris discs represent the ultimate stage of planetary formation, after the initial gas-rich protoplanetary disc dissipated and potential giant planets have already formed in the system. Infrared observations have shown that at least 20% of main-sequence stars host a dusty debris disc , i.e. a massive analog to the Edgeworth-Kuiper belt in the solar system, typically detected at tens to hundreds of astronomical units from their parents stars. Considering the short lifetime of the dust particles compared to the system age, this dust population is believed to originate from collisions of a population of kilometre-sized parent bodies, called planetesimals, reminiscent of Kuiper belt objects or comets in our solar system. Collisions constantly replenish the disc and create smaller and smaller dust particles, until the smallest micron-sized particles leave the system expelled by the radiation pressure of the central star.
Polarimetry as a tool to study debris discs in scattered light
Extreme adaptive optics instruments working at optical / near-infrared wavelengths have revealed exquisite details on debris discs [2,3], allowing to extract the optical properties of the dust particles such as the phase function, the degree of polarisation and the spectral reflectance. These are three powerful diagnostic tools to understand the physical properties of the dust population : the size, shape and composition of the micron-sized dust particles.
An ideal laboratory: HR4796
It is however very rare to be able to combine all those three observables for the same system, because this requires different high-contrast imaging techniques to suppress the starlight and reveal the faint scattered light emission from the dust. Due to its properties, the narrow ring detected at 77 astronomical units around the A-type star HR4796 is a notable exception, with both unpolarised and polarised images covering the optical and near-infrared wavelengths . Two new studies have revealed the intriguing properties on this system. The first one  revisited optical polarised light images obtained earlier with SPHERE/Zimpol  (left figure) and proposed an original technique to retrieve with enhanced fidelity the scattering phase function of the dust in polarised light (inset of the right figure). This method more accurately accounts for projection effects that can enhance the surface brightness of a disc along the semi-major axis if the scale height of the ring is sufficiently large. The second study  extracted the degree of polarisation of the disc in the near-infrared from 20 to 120 degrees. Neither of those two studies can fully reconcile the degree of polarisation of the dust population, its scattering phase function and the particles blow-out size with a single scattering theory (right figure).
In this contribution, we propose to discuss the dust properties compatible with current and new observations of this disc. We will base the discussion on laboratory experiments and analogies with cometary dust particles, rather than relying on scattering theories assuming spherical dust particles. We show that this interdisciplinary approach moving away from the Mie theory enables to find interesting candidates to reconcile the optical properties and thermal behaviour of the dust particles in this system.
Figure: Left: VLT/SPHERE/Zimpol polarimetric observations of the HR4796 disc, in a broad (290nm-wide) optical filter centered at 735nm . North is up, East to the right, the West side of the disc is the forward-scattering side. Right: Extracted polarised scattering phase function (-S12) from 20 to 160 degrees described in  (inset graph) and Χ2 map of a grid of models parametrised by the porosity and minimum particle size, using the Distribution of Hollow Spheres  suggesting sub-micronic low porosity particles.
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How to cite: Milli, J., Baruteau, C., Lasue, J., Levasseur-Regroud, A.-C., Renard, J.-B., and Olofsson, J.: The polarimetric dust properties of the debris disc HR4796, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-249, https://doi.org/10.5194/epsc2020-249, 2020.
Introduction: In-situ and telescopic observations of icy moons during the last decades have improved our understanding of these bodies. One of the main features discovered is the production of pristine material at their surfaces by plumes, detected on Enceladus[1, 2] and possibly on Europa . In such dynamical environment, the ice particles could be produced by flash-freezing of aqueous solutions expulsed from the subsurface [4, and references therein].
At the University of Bern, the Setup for Production of Icy Planetary Analogues (SPIPA)  allows us to produce flash-frozen particles by pulverization of liquid salty solutions into liquid nitrogen. The freezing of the particles is dominated by the Leidenfrost effect  which generates a nitrogen outgassing that makes droplets levitating above it. The first seconds of crystallization are made at low (and rapidly decreasing) temperature, inducing a complex internal structure of the particles (mixed of amorphous and crystalline phases).
The characterization of icy surfaces in total light intensity can be complemented by the analysis of the degree of linear polarization of the light scattered by atmosphereless bodies . For many years, ground-based polarimetric observations have provided a consequent dataset to characterize icy surfaces [8, 9] allowing a better preparation for the upcoming space missions Europa Clipper (NASA) and JUICE (ESA).The polarized reflectance is highly sensitive to the morphology of grains (e.g. size, shape, structure) as much as the chemistry (e.g. composition and mixture) . The evolution of polarimetric properties of salty ice particles during warming-up, accompanied with the transition of the amorphous fraction into crystalline, gives insights on the morphology (mostly internal, but also external) of the grains, for the considered salt. Therefore, we have measured the evolution of the degree of linear polarization (Q/I) of salty ices with temperature raising until the eutectic point of the solutions (NaCl, MgCl2, Na2SO4 and MgSO4).
POLICES setup: The POLarimeter for ICE Samples has been developed at the University of Bern to measure, among others, the degree of linear polarization of icy samples  and its dependence to phase angle. The experiment is a goniometer with a fixed emergence angle. The incidence angle is defined by the position of the light source, fixed at the end of a rotating motorized arm. Light is either produced by LEDs at three different wavelengths (530, 625 and 810 nm), or in another configuration with a monochromator to select narrow spectral bandpasses. In both cases, depolarizers are used to ensure an unpolarized incident light.
To permit repeatable measurements of icy samples, a hermetic box, painted in black to mitigate stray light, has been added to the setup in order to control the relative humidity and prevent frost formation (Fig. 1A). As depicted in Fig. 1B, polarimetric phase curves can be measured with a phase angle ranging from 1.5° to 73° inside the box .
Figure 1: (A) POLarimeter for ICE Samples (POLICES). (B) 3D Back view with geometry of observation
Sample preparation and measurements: The degree of linear polarization is maximum at 90° of phase angle . This configuration is not reachable with our setup; 60° phase angle is a good compromise to record the evolution of linear polarization. We prepared salty ices made of spherical grains 67±31 µm large  from saturated aqueous solutions (30wt% of NaCl, 30wt% of MgSO4) and less concentrated solutions, with MgCl2 and Na2SO4.
The ice was placed inside POLICES on a copper plate precooled with liquid nitrogen. The Stokes coefficientswere continuously measured during the slow warming-up of the sample. A Pt-100 sensor inside the ice sample continuously measured the temperature of the ice.
Figure 2 shows the evolution of the Q/I values of the ices with increasing temperature. The evolution observed for a given salt seem to be rather independent on the studied wavelengths so far, with only small offsets of the Q/I values. The pristine material is, as far as understood for now, a mixture of amorphous phase of water and salt, as much as a partially crystallized water ice, in hexagonal and potentially cubic phase . The evolution with temperature corresponds to the reorganization of the sample, leading to a higher degree of crystallinity of both the ice and the salts.
The first steps of growth of these crystals (water ice and hydrated phases of salts) could be responsible for a minute fraction of Rayleigh scattering, associated to particles that are small compared to the wavelength. This mechanism is suspected to be responsible for the peaks of polarization at 175 K for the MgSO4 and centered around 195 K for NaCl (Fig. 2).
Figure 2 : Evolution of the degree of linear polarisation of salty ices. Here, for clarity, only MgSO4 is shown at 530 and 415 nm.
Summary and perspectives: The new series of measurements proposed in this work will complete the analyses of such brines in total light intensity. We intend to pursue this work with different amount of salts inside the initial brines, as well as other chemical components relevant for icy moons. In addition to measurements at a fixed phase angle, phase curves will be measured. Once validated, polarisation data will be distributed through the DACE and SSHADE platforms (https://dace.unige.ch/lossySearch/; https://www.sshade.eu/db/bypass).
Acknowledgments: The team from the University of Bern is supported by the Swiss National National Science Foundation, in part through the NCCR PlanetS.
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How to cite: Cerubini, R., Pommerol, A., Poch, O., Kipfer, K., and Nicolas, T.: Polarized reflectance of salty flash-frozen ice particles, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-580, https://doi.org/10.5194/epsc2020-580, 2020.