In this talk, we will review the recent advances in photometric and polarimetric observations of asteroids.

For decades, photometry and polarimetry have been conventional methods employed in investigating asteroid surface properties. However, with the growing number of data from large surveys, we are now able to probe the diversity of the phase angle curves across different composition types and sizes of asteroids. In the recent years, a large number of polarimetric observations have also been obtained by various observatories across the globe, allowing to improve our understanding of the phase-polarization curve of asteroids and especially for near-Earth objects.

In photometry, the number of sky surveys has increased significantly. These surveys, serendipitously observing hundreds of small bodies (e.g [1, 2]), are obtaining photometric observations with consistent setups and photometric bands allowing to build reliable phase curves of hundreds of thousands of asteroids. 

As an example, [3] analysed the observations obtained by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey [1] composed of four 50 cm telescopes located at different observatories surveying the sky to search for potentially hazardous near-Earth objects. In 2022, ATLAS published its first version of the ATLAS Solar System Catalog (SSCAT) consisting of over 125 million observations of 580,000 asteroids and comets [4]. Using the ATLAS data [1] analysed 127,012 phase curves of 94,777 asteroids and determined their H magnitude and G1 and G2 parameters. They analysed the distribution of G1 and G2 parameters for the different taxonomic types and found a correlation between the G1 and G2 parameters and the taxonomic type of the asteroids mainly following their albedo distribution. They were able to differentiate the X-type objects into their P-, M-, and E- component which display relatively similar spectral shape, but differs significantly in terms of their albedoes. They also showed that interlopers in dynamical families can be identified by analysing their G1 and G2 parameters.

More recently [5] used the observations from the Zwicky Transient Facility (ZTF) survey to analysed 3,245,908 observations of 122,675 asteroids. They used both the H, G [6] and the H, G1, G2 [7]. As in the previous work from [3], they found strong correlation of the Gs parameters with taxonomy. But, the most important innovation from [4] is the introduction of a new term in the H, G1, G2 accounting for the variation of viewing aspect of the asteroids. As this new term is dependent on the spin axis orientation, they were not only able to better constrain the H and Gs but also to determine the pole orientation of the modelled objects. They found that the pole orientation obtained using the phase curved agree withing 20-30° in average with the pole orientation obtained using detailed shape modeling. 

In polarimetry, the Calern Asteroid Polarimetric Survey [8] recently published 2,100 single measurements for about 600 asteroids. This single survey significantly increased the number of polarimetric observations of asteroids as the largest catalog compiling observations obtained by many authors and many telescopes only contained 5,100 single measurements with observations starting since the early 1970s. 

Most of the polarimetric observations are usually obtained in the V band or in the R band. However, recently [9] analysed polarimetric observations of asteroid obtained in the near-infrared. [9] used the 200 inch Palomar telescope with the WIRC+Pol instrument to obtained observations in the J and H band. In [9] they present their results for the S- and C- complexes. They showed that C-complex asteroids almost show no variation of their phase-polarization curve as a function of wavelength and could be used as calibrator as their wavelength dependence is less that those of stars. On the other hand, S-type objects show large variation with respect to wavelength consistent with previous trend observed in different bands in the visible. In another work [10], they analysed the so-called Barbarian asteroids that are showing an unusually large value of the inversion angle. They showed that the Barbarians are showing a drastic change in their phase-polarization curve as a function of wavelength and that even the inversion angle is varying significantly. This trend was already observed in the case of (234) Barbara by [11] and is confirmed for with those new observations for other Barbarians and at larger wavelengths. 

In the recent years, a lot of focus has also been devoted to the observations of near-Earth objects (NEOs). NEOs can be observed at much larger phase angles than main-belt asteroids and thus display much larger value of the polarization. Asteroid like Phaethon, Ryugy or Bennu are showing polarization values as high as 40 to 50\%. On the other hands, high albedo objects like Nereus are showing polarization values as low as 1-2\% at similiar phase angles. This shows the high potential of using polarimetric observations at high phase angles to determine the albedo of near-Earth observations. Obtaining the albedo of NEOs is of high importance for planetary defense purposes as it allows to obtain an estimation of the size of the object which is of first importance to plan for mitigation strategies in case of the discovery of an impacting object.

[1] Tonry, J., Denneau, L., Heinze, A., et al. 2018, Publications of the Astronomical Society of the Pacific, 130, 064505
[2] Masci, F. J., Laher, R. R., Rusholme, B., et al. 2019, PASP, 131, 018003
[3] Mahlke, M., Carry, B., & Denneau, L. 2021, Icarus, 354, 114094
[4] https://astroportal.ifa.hawaii.edu/atlas/sscat/
[5] Carry, B., Peloton, J., Le Montagner, R., Mahlke, M., & Berthier, J. 2024, arXivpreprint arXiv:2403.20179
[6] Bowell, E., Hapke, B., Domingue, D., et al. 1989, Asteroids II, 524
[7] Muinonen, K., Belskaya, I. N., Cellino, A., et al. 2010, Icarus, 209, 542
[8] Bendjoya, P., Cellino, A., Rivet, J.-P., et al. 2022, Astronomy & Astrophysics, 665, A66
[9] Masiero, J. R., Tinyanont, S., & Millar-Blanchaer, M. A. 2022, The Planetary Science Journal, 3, 90
[10] Masiero, J. R., Devogèle, M., Macias, I., Jaimes, J. C., & Cellino, A. 2023, The Planetary Science Journal, 4, 93
[11] Devogèle, M., Tanga, P., Cellino, A., et al. 2018, Icarus, 304, 31

How to cite: Devogele, M.: Polarimetry and photometry  of small bodies in the Solar System, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-636, https://doi.org/10.5194/epsc2024-636, 2024.

INTRODUCTION

Dust is an fundamental component of the Solar System and plays a key role in the formation and evolution of the planetesimals in the protoplanetary disks. While cometary dust emitted during the activity is a remnant of the primordial material from which the solar system formed, asteroids processed surfaces provide clues about the dynamic processes involved (orbital evolution of the solar system). Understanding the nature of dust is essential for shedding light on the physical and chemical conditions involved in the early stages of planetary formation.

Comet Interceptor (ESA/JAXA) (CI) is aimed at closely observing a Dynamically New Comet (DNC) in its first approach to the inner Solar System [1]. Spacecraft B2, will carry a unique instrument, EnVisS—a fish-eye camera designed to perform in situ broadband imaging and polarimetric observations simultaneously for the first time. It will mark a significant milestone by providing the first highly resolved polarimetric measurements of cometary dust tail at phase angles beyond 120°, unreachable from ground [2].

In this framework, experimental data of the light scattered by well characterized cometary dust analogues provides a valuable benchmark to the in-situ observations.

MEASUREMENTS

In this talk I will present experimental Phase Fucntion (PF) and Degree of Linear Polarization (DLP) curves for a selected set of porous dust particles with sizes spanning from the micron- up to mm- size range. This experimental project is devoted to study the effect of composition, and aggregate size and porosity on the PF and main DLP curve features in the regions of maximum, inversion and minimum polarization. These measurements will provide a significant reference to the interpretation of the data obtained by EnVisS, the camera onboard the Comet interceptor ESA mission.

Special attention will be paid to the new measurements of randomly oriented mm-sized porous aggregates.

The measurements have been taken at the IAA Cosmic Dust Laboratory (CODULAB) [3] using an ultrasonic levitator to locate the dust aggregate at the scattering volume [4]. The measured PF and DLP curves are performed at 640 nm spanning from 4 to 177 degrees scattering angle range. As an example in Figure 1 I present preliminary PF and DLP curves for a porous particle in random orientation. Figure 2 shows the optical image of the studied grain of about 1 mm selected from a bulk JSC Martian dust analogue sample. The refractive index at the measured wavelength is n = 1.5 + i0.6*10^-3 [5]. The PF curve two well-defined regions: strong forward lobe in the 4° to ∼20° scattering angle range and an increase with decreasing phase angle from ∼20° to 177°.  The measured DLP curves shows the typical bell shape for irregular particles. The maximum of the DLP curve is placed at 50 degrees. i.e., it is shifted toward smaller scattering angles than in the case of a cloud of JSC Martian analog micron-sized particles. Further, the DLP shows a negative branch at backward direction with an inversion angle equal to 159 degrees.

This work is part of an ongoing experimental project devoted to understand photopolarimetric behavior of dust in asteroids and comets. All experimental data are freely available at the Granada-Amsterdam light scattering database scattering.iaa.es [6, 7, 8, 9].

Figure 1 . Preliminary PF and DLP curves for a porous particle in random orientation.

Figure 2. Optical image of the studied grain selected from a bulk JSC Martian dust analogue sample, the scale is equal to 100 microns.

References

[1] Jones G.H. et al., The Comet Interceptor Mission. SSR, Vol.220, 2024.

[2] Da Deppo V. et al. The Entire Visible Sky (EnVisS) imager for the Comet Interceptor ESA mission. EPSC2022-273.

[3] Muñoz et al., The IAA cosmic dust laboratory: Experimental scattering matrices of clay particles. Icarus, Vol. 211, pp. 894-900, 2011.

[4] Colin et al., An acoustic levitator for use in light scattering experiments at the IAA Cosmic Dust Laboratory. in prep

[5] Martikainen et al., Optical constants of Martian dust analogs at UV-visible-Near-infrared wavelengths. ApJS, vol. 268 (47), 2023.

[6] Muñoz et al., The Amsterdam-Granada Light Scattering Database, JQSRT, Vol.113, pp. 565-574, 2012.

[7] Gómez-Martín et al., On the application of scattering matrix measurements to detection

and identification of major types of airborne aerosol particles: Volcanic ash, desert dust and pollen JQSRT, Vol.271, 2021.

[8] Muñoz et al., Retrieving Dust Grain Sizes from Photopolarimetry: An Experimental Approach. ApJS, Vol. 256, pp.17, 2021.

[9] Frattin et al., Experimental phase function and degree of linear polarization curve of olivine and spinel and the origin of the Barbarian polarization behaviour. MNRAS, Vol. 517, pp. 5463-5472, 2022.

How to cite: Frattin, E.: Photopolarimetry of porous dust particles in preparation for the Comet Interceptor mission, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1028, https://doi.org/10.5194/epsc2024-1028, 2024.

Introduction

Planetary surfaces are typically composed of particles from micrometer scale up to decimeter-to-meter scale. Particle morphology plays a key role in the polarimetric properties of planetary surfaces with wavelength-scale particles and thus, in understanding polarimetric observations of planetary bodies. Laboratory-characterised dust particles provide a pool of realistic compact particle shapes applicable for boulders as well; however, the morphological properties of the laboratory-characterised shapes can be challenging to parameterise analytically. Due to the sensitivity of the polarimetric properties on the shape of the scatterer,  the role of specific shape features in the polarimetric properties can be difficult to assess. Ensemble-averaged scattering properties are needed for modelling, but exact laboratory measurements are slow and challenging. Therefore, it is useful to find a geometrically parameterizable shape, which is also consistent with the scattering properties of the laboratory-characterised particles. Various studies have been published on the modelling of scattering properties of atmospheric mineral dust, planetary regolith, or coma particles with a number of methods for the generation of compact particles with more morphological complexity than spheroids: some examples include Gaussian particles [1,2], concave-hull transformations [2], Poisson-Voronoi tessellation [3], and various compact agglomerates [4]. Recent decades have also provided various opportunities for in-situ investigations of planetary regolith, and thus, for improved morphological characterisation of regolith, which can be useful for finding the key morphological parameters [e.g., 5,6].

In this work, we utilise polyhedrons with surface roughness and investigate their polarimetric properties as well as macroscopic interfaces with rough interfaces composed of sub-wavelength particles. The long-term goal is to understand the polarimetric observations of planetary bodies at optical wavelengths as well as the microwave domain. Due to the large parameter space in multiwavelength studies including diverse shapes, here we focus on radar polarimetry at and near the backscattering direction and a few selected particle properties. Radar observations provide disk-resolved images of planetary bodies illuminated by fully polarised incident waves, which enables intriguing scattering scenarios that are not typically observable in optical polarimetry.

Figure 1. A polyhedral particle with 12 faces and a surface roughness standard deviation of 2% of the local vertex length, displayed in different orientations.

Particle morphology and numerical methods

The polyhedral particles with surface roughness are parameterised using any number of polygonal faces and small-scale surface roughness. The number of faces defines the sphericity statistically, as a sphere has essentially an infinite number of faces, whereas for example a hexahedron would have six faces. The particle is generated by Delaunay-triangularizing a unit sphere, then selecting a desired number of faces (here, 12-20) defined by randomly selected vertices (“seed vertices”), and scaling the length of the vertices based on their distance to the nearest seed vertex so that a flat face is formed. Finally, the surface roughness is applied using a Gaussian height distribution with a specified standard deviation. Figure 1 shows an example of one realisation of a rough polyhedral particle with 12 faces. We generated 20 realisations for each case to account for the stochasticity. The role of the particle sizes is investigated for size parameters (x=2πr_V/λ, where r_V is the radius of a volume-equivalent sphere and λ is the wavelength) from x=1 to at least x=10 or more depending on the refractive index; the computations are more time-intensive for greater refractive indices. In terms of the real part of the refractive indices, we considered values from 1.4 to 1.8 most relevant for the applications considered here, whereas for the imaginary part up to 0.01 was considered. The single-scattering properties were computed using the discrete-dipole approximation software ADDA [7].

Also polarimetric properties of rough interfaces are considered as in microwave applications the surface is composed primarily of densely-packed subwavelength-scale particles. For computations, we used an Improved Integral Equation Method Python code based on a Matlab code presented in [8].  

Preliminary results and discussion

We investigated the roles of particle size, shape, orientation, and material on the single-scattering properties. Also a variety of particle-size frequency distributions were applied as relevant for planetary remote-sensing applications. Due to the large parameter space, evaluating the role of specific particle properties on the polarimetric properties is challenging; however, we could observe that the size parameter, refractive index, and the shape and orientation play their own roles in an ambiguous way. The diagonal scattering matrix elements are extremely sensitive to the orientation, that is, the projected shape, but strongly correlate with each other for ensembles of particles at backscattering, as expected based on literature: P_1,1 – 2P_2,2 = P_4,4 [9], while P_2,2 = -P_3,3 [10], where P_i,j are scattering matrix elements at backscattering direction. 

The particle size is directly proportional to the polarisation ratios up to a certain size; however, the limiting size was found to vary based on other particle properties. The number of faces is a significant factor in determining how round the particles are statistically, and the roundness of the particles affects the amplitude of the fluctuations of the scattering matrix elements as a function of the scattering angle. For the case of 20 faces, two particles that were among the most elongated with a sharp edge on one side produced significantly different polarimetric properties more comparable to those produced by 12-face particles than the other 20-face particles. However, the role of the axis ratio was not self-evident, i.e., the axis ratio is only one factor among others. 

The presented scattering computations will be used for further computations to model scenarios where the individual particles form larger macroscopic structures and multiple scattering takes place.

References

[1] Muinonen et al. 1996, JQSRT 55(5):577; [2] Lindqvist et al. 2009, JQSRT 110:1398; [3] Ishimoto et al. 2010, JQSRT 111:2434; [4] Zubko et al. 2015, JQSRT 150:42; [5] Miyazaki et al. 2023, EPaS 75:171; [6] Wilkerson et al. 2024, Icarus 412, id. 115963; [7] Yurkin and Hoekstra, 2011, JQSRT 112:2234; [8] Ulaby and Long, 2014, Microwave Radar and Radiometric Remote Sensing, Artech House; [9] Mishchenko and Hovenier, 1996, Opt. Lett. 20:1356; [10] Van de Hulst, 1957, Light Scattering by Small Particles, Wiley.

How to cite: Virkki, A. and Turkki, M.: Modelling radar polarimetry of planetary surfaces, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-468, https://doi.org/10.5194/epsc2024-468, 2024.

Comet Interceptor (CI) is an ESA mission in cooperation with the Japanese Aerospace Exploration  Agency (JAXA). It aims to provide the first-ever in-situ (as opposed to ground-based observation) characterisation of a long period comet, which could be a dynamically-new comet or an interstellar object, and to perform the first simultaneous multi-point exploration of a cometary coma and nucleus.

The science of the mission encompasses two main themes: Comet Nucleus Science and Comet Environment Science. More specifically, the key questions to address are:

1) Comet Nucleus Science - What is the surface composition, shape, morphology, and structure of

the target object?

2) Comet Environment Science - What is the composition of the coma, its connection to the nucleus

(activity) and the nature of its interaction with the solar wind?

The CI mission consists of the main spacecraft (S/C A) and the two probes (named Probe B1 and Probe  B2, respectively).  The Comet Interceptor payload complement includes remote sensing and in situ measurement instruments accommodated on-board the main S/C and the two probes, so as to enable performing multi-point observations of the selected object during the fly-by.

Comet Interceptor will be launched together with the Ariel mission, to the Sun-Earth second Lagrange  point, L2. Following a waiting phase at L2, used to select the actual target object and to optimise the related transfer orbit, CI will cruise to the encounter and release the two probes shortly before performing the fly-by.  The duration of the waiting phase depends on the actual target and its maximum length is estimated to be four years. After their release, the two probes will perform autonomous operations, relaying the scientific data back to the main spacecraft. The maximum duration of the CI mission, from launch to the end of the post-encounter phase, is six years.

The payload for spacecraft A and B2 is provided by ESA Member States. Among these, on the B2 probe, the full sky camera EnVisS (Entire Visible Sky) will study and map the comet dust coma environment from an advantageous point inside the coma itself. EnVisS has been designed with a very wide 180° Field of View (FoV), which, by exploiting the B2 spacecraft rotation, enables the reconstruction of full-sky images. EnVisS adopts a flexible push-broom/push-frame imaging technique, wherein slices of the sky will be acquired while the probe rotates. Subsequently, on-ground, the acquired slices will be stitched together to form a full-sky image. The camera will be equipped with a 3-strip filter assembly transmitting the visible wavelength range 550-800 nm. One of the strips is a high-transmission broadband filter, while the others are linear polarization filters. Thanks to these filters, EnVisS will measure the intensity, the degree of linear polarization and polarization angle orientation of the light scattered by the dust particles in the comet coma, providing an unprecedented extended coverage of the phase angles.

How to cite: Lara, L. M., Gutiérrez, P., Castro Marin, J. M., Da Deppo, V., Della Corte, V., and Zuppella, P. and the EnVisS Team: EnVisS camera for the Comet Interceptor mission (ESA-JAXA): polarimetric observations of cometary dust. , Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1259, https://doi.org/10.5194/epsc2024-1259, 2024.

As one of the most thermally stable solvents, water is critical to grasping the material evolution of planetary systems. Ch-type asteroids show clear signs of water on their surfaces, anticipated to serve as a local water reservoir in today’s main asteroid belt region and also as the parent bodies of CM meteorites that are presumably the primary water source on early Earth. Aside from the composition of their building blocks, little is known about the physical makeup of dust particles, which is related to environmental information before they settled in their current orbits. Observations revealed polarimetric differences in Ch-type asteroids in the optical, indicating potential dust gradients among the bodies. In this presentation, we will report interim results we obtained from our new optical multi-band photometric and near-infrared polarimetric observations of large (>~80 km in diameter) Ch-type main-belt asteroids and discuss what insights we have gained into constraining the evolution of the bodies.

How to cite: Kwon, Y. and Masiero, J.: Surface Characterization of Hydrated Planetesimals inthe Main Asteroid Belt, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-661, https://doi.org/10.5194/epsc2024-661, 2024.

Polarimetric observations of comets reveal that sunlight scattered from their comae acquires partial linear polarization that can be quantified with the degree of linear polarization P = (F_⊥ – F)/(F_⊥ + F). Here F_⊥ and F correspond to fluxes of the scattered light measured with the polarizing filter (analyzer) oriented perpendicular to the scattering plane and parallel to the scattering plane, respectively (Zubko & Videen 2023 for review). P is dependent on the geometry of illumination and observation, which is described with the phase angle α. In comets, the phase-angle dependence of polarization reveals two stable features. At small phase angles (α < 25°), nearly all comets reveal the phenomenon of negative polarization (i.e., F_⊥ < F), and at larger phase angles, their polarization is positive (i.e., F_⊥ > F). In both domains, the angular dependence of polarization appears in the form of a bell-like branch. The amplitude of the negative-polarization branch is characterized with the minimum polarization P_min and phase angle where this minimum occurs α_min. Correspondingly, the positive polarization branch is described by P_max and α_max.

Observations of comets demonstrate a significant difference in their polarimetric responses. This dispersion appears most noticeably in the amplitude of the positive-polarization branch with P_max spanning the range from ~6% up to ~35% (Zubko & Videen 2023 for review). It is worth noting a disparity exists between the numbers of comets with high and low maximum of polarization P_max. While at least several dozens comets with P_max > 20% have been detected with confidence, only four comets having P_max < 10% have been reported. Interestingly, two comets with extremely low P_max, C/1975 N1 (Kobayashi–Berger–Milon) and 23P/Brorsen–Metcalf, were observed in 1975 and 1989, respectively (Chernova et al. 1993). Then, comets of this kind were not seen for almost thirty years, until Comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) (Zubko et al. 2020). The latest comet with extremely low P_max, C/2023 P1 (Nishimura) was the first one investigated by means of imaging polarimetry (Zheltobryukhov et al. 2024). The top panels in Figure 1 show the degree of linear polarization as a function of phase angle in the low-P_max comets observed in green-blue light (left) and in the red light (right). The bottom panels in Figure 1 show polarimetric images of Comet C/2023 P1 (Nishimura) as they were reported by Zheltobryukhov et al. (2024).

Öhman (1941) put forth a tentative explanation of the dispersion of P_max that is based on the depolarization effect of gaseous emissions. Within the Öhman’s concept, comets with low P_max were assumed to be rich in gas; whereas, high P_max was attributed to comets rich in dust. This hypothesis was adapted by Chernova et al (1993) and it was further pursued by Kolokolova et al. (2007). Nevertheless, at the time of publication by Chernova et al., there was evidence that the low-P_max comets are dust rich. Near-IR and mid-IR photometry demonstrated that C/1975 N1 (Kobayashi–Berger–Milon) and 27P/Crommelin are dust-rich comets, for instance (Ney 1982 and Krishna Swamy 1991, respectively). Zubko et al. (2016) demonstrated that the gaseous hypothesis cannot reproduce the location of the polarization maximum in the low-P_max comets. While observations reveal α_max = (80 ± 10)°, the gaseous emissions of any kind must yield α_max = 90°. Furthermore, the gaseous explanation cannot produce negative polarization (i.e., P_min = 0), whose absence has never been observed in comets (Zubko et al. 2016). The observations of Comet C/2023 P1 (Nishimura) are not compatible with the gaseous hypothesis; e.g., the aperture-average polarimetry and imaging polarimetry of this comet, both reveal polarization noticeably lower than what could emerge from gaseous emissions of any kind, i.e., 7.7% at α = 90° (see Figure 1).

An alternative explanation for dispersion of P_max in comets was suggested by Zubko et al. (2016) based on numerical simulation of light scattering by irregularly shaped agglomerated debris particles (see six examples of top in Figure 2). This approach suggests that the cometary coma is composed of at least two types of dust, whose refractive indices correspond to the Mg-rich silicates and carbonaceous material (organics and/or amorphous carbon). A mixture of these two components can satisfactorily reproduce the polarimetric observations of comets with different P_max simultaneously at several wavelengths by varying only the volume ratio of silicate and carbonaceous materials (see Zubko et al. 2016 for further details). As shown in the bottom of Figure 2, the same approach can fit the observations of the latest comets with extremely low P_max. Note, the radius of the agglomerated debris particles of both compositions spans the same range from r ≈ 0.08 μm up to 3.7 μm. Their light-scattering responses are averaged with a power-law size distribution with index n = 2.2. The relative abundance of the two components (by volume) is given in the legend of Figure 2. As one can see, the two-component model fits observations of the low-P_max comets suggesting domination of weakly absorbing Mg-rich silicate particles in their coma. This finding appears in good agreement with cold dust detected in C/1975 N1 (Kobayashi–Berger–Milon) by means of mid-IR photometry (Ney 1982).

References

Chernova G.P., et al., 1993, Icarus, 103, 144

Krishna Swamy K.S., 1991, Astron. Astrophys., 241, 260

Kolokolova L., et al., 2007, Astron. Astrophys., 463, 1189

Ney E.P. 1982, in L.L. Wilkening, ed., Comets. University of Arizona Press, Tucson, AZ, p. 323

Öhman Y., 1941, Stockholms Obs. Ann. 13, 11.1–11.20.

Zheltobryukhov M., et al., 2024, Mon. Not. Roy. Astron. Soc.: Lett., 528, L117

Zubko E., Videen G., 2023, in M.P. Mengüç and M. Francoeur, eds., Light, Plasmonics and Particles. Elsevier, Amsterdam, p. 327

Zubko E., et al., 2016, Planet. Space Sci., 123, 63

Zubko E., et al., 2020, Icarus, 336, 113453

How to cite: Zubko, E., Zheltobryukhov, M., Chornaya, E., Kochergin, A., and Videen, G.: Comets with extremely low positive polarization, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-343, https://doi.org/10.5194/epsc2024-343, 2024.

The interpretation of astronomical observations of comets, asteroids, and extrasolar objects such as protoplanetary and debris disks is essential for understanding the origin and evolution of planetary systems. Collecting electromagnetic radiation scattered or emitted by dust particles in these objects using powerful telescopes is often our only method to observe and characterize them. This work aims to enhance our understanding of the interaction between electromagnetic radiation and dust particles to improve the interpretation of remote observations of comets and data from future in situ missions like Comet Interceptor.

We present experimental scattering matrices for a set of well-characterized porous cm-sized cylindrical aggregates as regolith simulants. The regolith production process involves two steps:

• Monomer synthesis: Micron-sized particles were generated by milling of two bulk samples: olivine and the Mojave Martian Simulant (MMS-2). Narrow size distributions were achieved through sieving, dispersion in a liquid media, and repeated gravity settling at different intervals. The size distributions of the resulting powders were measured using a MasterSizer 2000 particle sizer.

• Consolidation: Centimeter-sized pellets of the micron-sized grains obtained in step 1 were produced through mechanical compaction and subsequent consolidation by thermal treatment. In addition, porous samples were obtained by mixing the powder of interest with ethylcellulose as pore former, which was removed during the thermal treatment.

This novel procedure allows us to obtain regolith sample sets in which only one physical property varies at a time, which is needed to disentangle the effect of different physical properties (grain size, surface roughness and composition) on the scattered light. The characterization of the samples includes optical microscopy, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) imaging. As examples, Figures 1 and 2, show SEM images for the two end members of the MMS2 regolith set as far as surface roughness is concerned: MMS2-6 (the flattest) and MeC60-4 (the roughest), respectively. The AFM images displayed in Figures 3 and 4 provide the corresponding surfaces height profile.

The experimental measurements of the scattered light by our regolith are performed at the IAA Cosmic Dust Laboratory at wavelengths of 488 and 640 nm. Due to the geometry of the regolith, the phase angle of the measured reflected light spans from 3 to 90 degrees. Preliminary results of the scattering matrix for the MMS2-6 and MeC60-4 regolith at the wavelength of 640 nm are shown in the Figure 5. The refractive index of the constituent particles at the measurement’s wavelength is equal to m=1.5 + i0.00035 (Martikainen et al 2023). In general, the increasing surface roughness affects all measured scattering matrix elements. That is specially the case for the F₂₂/F₁₁, F₃₃/F₁₁ and F₄₄/F₁₁ in the full measured phase angle range. It is interesting to note that in both cases the -F₁₂/F₁₁ ratio shows a shallow negative branch near the backward direction.

These preliminary results show the potential of our studies with this new kind of samples. Additionally, comparison with scattering measurements of other regolith samples will enhance our understanding of how surface roughness and composition affect scattered light by different surfaces.

Figure 1. SEM image of the surface of the sample MMS2-6.

Figure 2. SEM image of the surface of the sample MeC60-4.

Figure 3. AFM image about the height profile of the surface of the sample MMS2-6.

Figure 4. AFM image about the height profile of the surface of the sample MeC60-4.

Figure 5. Experimental scattering matrices of the surfaces of samples MMS2-6 (red) and MeC60-4 (green).

How to cite: García Izquierdo, F. J., Frattin, E., Martikainen, J., Muñoz, O., Gómez Martín, J. C., Jardiel, T., Peiteado, M., Caballero, A. C., Videen, G., Markkanen, J., Penttilä, A., and Muinonen, K.: Photopolarimetry of planetary regoliths: Effect of surface roughness and composition, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1053, https://doi.org/10.5194/epsc2024-1053, 2024.

On September 26, 2022, the NASA DART (Double Asteroid Redirection Test) spacecraft struck Dimorphos, the moonlet of Didymos, to test near-Earth object deflection through impact in the context of planetary defence (Daly et al. 2023). As well as causing a change in Dimorphos’ orbital period (Thomas et al. 2023), the impact caused a massive dust cloud to be ejected from the surface (e.g. Li et al. 2023; Opitom et al. 2023). By studying the characteristics and behaviour of the ejecta cloud, the DART mission provided a rare opportunity to peer under the surface of an asteroid and further our understanding of their global properties.

Didymos-Dimorphos was monitored in polarimetric mode before and after the impact (Bagnulo et al. 2023; Gray et al. 2024). The post-impact measurements revealed a significant drop in polarisation, suggesting differences between the material in the ejecta and that on the original regolith surface. Remarkably, even months after the impact, the polarisation remained persistently lower than pre-impact, despite photometric measurements showing that the system faded back to its “normal” brightness around 23 days post-impact (Graykowski et al. 2023). This suggests the presence of residual ejecta material still within the system months after the impact, either in orbit or deposited on the asteroid surface(s). This contrast in findings demonstrates the sensitivity of polarimetric measurements, revealing details often unattainable from traditional observations techniques.

In this presentation, we will be discussing these previous results, as well as presenting new polarimetric measurements obtained with VLT this year. In 2024, Didymos-Dimorphos is making another close approach to Earth, offering an opportunity to study the system around two years after the DART impact. Our goal with these observations is to establish whether the polarisation of the system has returned to the pre-impact level or remains at a lower level as observed post-impact and, thus, clarify if or how much ejecta material has remained in the system. This may ultimately benefit the Hera team (Michel et al. 2018) for planning the spacecraft trajectory to avoid damage by impacts of dust particles. Eventually, instruments on board Hera will reveal the spatial distribution of fresh material on the surfaces of the system. This unique opportunity will allow us to test and demonstrate the reliability and importance of polarimetric measurements of small solar system bodies.

Daly, R., et al. (2023), Natur, 616, 443; Thomas, C. A., et al. (2023), Natur, 616, 448; Li, J.-Y., et al. (2023), Natur, 616, 452; Opitom, C., et al. (2023), A&A, 671, L11; Bagnulo, S., et al. (2023), ApJL, 945, L38; Gray, Z., et al. (2024), PSJ, 5, 18; Graykowski, A., et al. (2024), Natur, 616, 452; Michel, P., et al. (2018), AdSpR, 62, 2261.

How to cite: Gray, Z.: Double Trouble: Polarimetric Insights into the Didymos-Dimorphos post-DART Evolution, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-458, https://doi.org/10.5194/epsc2024-458, 2024.