SB5

The planetary Ice Laboratory has been developed at the University of Bern to measure various reflectance properties of analogues for small bodies and planetary surfaces with a special focus on icy surfaces. Its core facilities are a set of devices to produce well-characterized and reproducible analogue samples and a set of instruments designed to measure the reflectance properties of the samples over different spectral ranges, different geometrical configurations, either in total light or sorted by polarisation state.

We use the laboratory to work on a range of scientific questions, mostly related to current Solar System missions and the interpretation of remote-sensing data but also occasionally on telescopic observations of Solar System objects as well as distant exoplanets and planet-forming circum-stellar discs. The experimental datasets collected are then provided to public databases for use by the community.

Some of our most recent investigations include:

- Visible and near-infrared reflectance spectra of analogues for the icy moons of the outer Solar System. We have collected new data with salty ice samples (Fig. 1), before and after irradiation by electrons, which can be compared with reflectance data measured by previous missions. The dataset will also help with the preparation of the JUICE and Europa Clipper missions.

- Simulations of cometary activity, in the framework of the CoPhyLab project led by TU Braunschweig, with IWF Graz, MPS and DLR as partners. We develop quantitative methods to derive the ice-to-dust ratio from reflectance data in order to study the sublimation of dust ice mixtures prepared as cometary analogues. The results help interpreting the data from previous missions such as Rosetta and preparing future missions such as Comet Interceptor.

- Spectral reflectance properties of different types of icy and dry Martian analogues, most notably in relation with the quantitative analysis of images returned by the CaSSIS imager of Exomars TGO. These measurements are performed in a simulation chamber equipped to simulate the Martian conditions of composition, temperature and pressure and we look at the samples with an imager equipped with the spare bandpass filters from CaSSIS (Fig. 2).

- The linear polarisation properties of pure and mixed minerals as analogues for the surfaces of asteroids with implications for the interpretation of measured phase curves in terms of composition. (see also Spadaccia et al., this conference).

- Assessment of the potential of circular spectropolarimetry as a biosignature in reflected light for Solar System objects and exoplanets.

- Measurements of artificial samples for comparison with numerical simulations.

With this presentation, we will provide an overview of the main objectives of the overall project, detail the methodology used, and present some of the results from the most recent studies mentioned here.

Fig. 1: Blueish salty (NaCl) ice particles (~67µm) produced after the freezing in LN₂ of salty solutions. Particles freeze from the exterior to the interior, forming a thin mantle of pure crystalline ice around a core of amorphous hyper saline ice. After the LN₂ has evaporated, ice and salt in the core slowly crystallize as the temperature increases, generating Rayleigh scattering.

Fig. 2: The SCITEAS-2 simulation chamber configured to simulate the cold surface and atmosphere at the Martian poles and equipped with a microscope and a multispectral imager with the spare bandpass filters from CaSSIS.

How to cite: Pommerol, A., Patty, L., Spadaccia, S., Affolter, L., Becerra, P., Capelo, H., Cerubini, R., Cesar, C., Drozhzhova, T., Feller, C., Kipfer, K., Stoeckli, L., Valantinas, A., Yoldi, Z., and Thomas, N.: Reflectance properties of analogues for small bodies and planetary surfaces. Overview of experimental characterizations at the University of Bern, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-512, https://doi.org/10.5194/epsc2022-512, 2022.

Introduction

Solar System small bodies are presumed relics from the eve of the Solar System, as they were the first objects to accrete inside the protoplanetary disk. The P-/D-type asteroids are particularly interesting because of the similarity of their spectra, in the visible (Vis) and near infrared (NIR) wavelengths, with cometary nuclei, suggesting that they are the most primitive types of small bodies [1-3]. In the mid-infrared (MIR), emission spectra of both P-/D-type asteroids surfaces and cometary comae display a signature around 10 µm due to the fundamental mode of vibrations of Si-O in silicates [2, 4-5]. P-/D-type asteroids are among the low-albedo class, and are characterized by linear polarimetric phase curves in V-band with a minimum of polarization around 1.2 ± 0.4 % and an inversion angle around 8.5 ± 2.5° [6].

There are various indications that the surface of these primitive small bodies are covered by so-called “hyperfine” grains, with individual grain sizes smaller than the wavelength at which they are observed (< 1 µm) [7]. The spectral characteristics of these objects in the Vis-NIR and MIR have also been attributed to their peculiar micro-texture and grain size [4, 5]. Here, we investigate how the Vis-NIR spectra, MIR spectra and Vis polarimetric phase curves of surfaces made of hyperfine grains are influenced by the relative abundance of materials having strongly different optical indexes.

Methods

We used olivine and iron sulphide (a mixture of pyrrhotite and troilite; labelled as “FeS”) having contrasted optical indexes. Following a dedicated grinding protocol, we have produced grains of average diameter ranging from 0.3 to 0.6 µm, as imaged by electron microscopy [8]. Mixtures were produced by mixing the two powders in a mortar manually with the pestle for about 10 min. Reflectance spectra in the Vis-NIR range were obtained at IPAG with the SHADOWS instrument [9] (emergence e=30º, incidence i=0º).  MIR reflectance spectra were obtained using a Brucker Vertex 70V FT-IR spectrometer equipped with a reflectance kit A513/QA. Because small bodies MIR observations are emissivity spectra, we show on Figure 1b the experimental spectra as “1 - reflectance” to approximate their emissivity spectra according to the Kirchhoff’s law. Polarimetric phase curves were measured at the University of Bern with the POLICES instrument at 530 nm [10].

Results

Figure 1a presents the evolution of normalized reflectance spectra of olivine-FeS mixtures with decreasing volume concentration of olivine. We observe a general decrease of reflectance, associated to a modification of the spectral slope, as the concentration of FeS (opaque in the Vis) increases, with a blueing followed by a reddening of the spectra (Fig. 1a). The measurements reveal that mixtures of hyperfine grains made of two components with contrasted optical indexes have spectral and polarimetric properties which varies in strongly nonlinear way in the Vis-NIR (Fig. 1a,b, Fig. 2). In the MIR, while the spectra of the endmembers are relatively flat, the spectra of mixtures containing high concentrations of FeS exhibit the absorption bands of Si-O in the olivine around 10 µm.

Discussion

Spectra of mixtures of olivine and iron sulphide (or anthracite, not shown here) exhibit an emissivity feature in the MIR and various degree of bluing or reddening in the Vis-NIR, as observed on several small bodies (Fig. 1). Moreover, the same mixtures exhibiting the 10-µm feature also have a polarimetric phase curve similar to P-/D-type asteroids (Fig. 2).

Spectra of pure olivine, or mixtures with high concentration of olivine exhibit spectral features mainly due to reflectivity effects. As the concentration of FeS grains increases, these features are progressively replaced by the absorption bands of olivine material, maximum around 10 to 11 µm. In these cases, the olivine grains are well dispersed in a matrix of FeS grains which diffuse the light and enable photons to escape the sample after some absorption by olivine grains. A similar mechanism was pointed out with mixtures in KBr [5], but while mixtures with KBr are very reflective, mixtures with FeS have a much lower reflectance, compatible with the low reflectance and high emissivity values observed on small bodies.

The resemblance between mid-IR spectra of our hyperfine mixtures and P-/D-type asteroids emission feature (Fig. 1d) implies that elevated porosity is not a requirement for the presence of a silicate signatures at 10 µm. We show here that a relatively compact surface (porosity of the order of 50 %) exhibits similar mid-IR feature as cometary dust tails. An interpretation that can be proposed is that in both cases an optical separation of olivine grains occurred, whether by vacuum in the case of comae, or by optically featureless grains in the case of P-/D-type asteroids.

Finally, we note that some mixtures of hyperfine grains (such as olivine-FeS 10:90 vol%) exhibit altogether a red spectral slope in the Vis-NIR, a 10-µm feature in the MIR, and a V-band polarimetric phase curve similar to P-/D-type asteroids, reinforcing the hypothesize that these bodies are made of powdery mixtures of hyperfine grains.

Acknowledgments: We acknowledge funding from the European Research Council (ERC) (SOLARYS ERC-CoG2017_771691)

References:  [1] Capaccioni et al. (2015) Science 347, 6620. [2] Vernazza and Beck (2017) in Planetesimals, Cambridge Univ. Press. [3] Poch et al. (2020) Science 367, 6483. [4] Emery et al. (2006) Icarus 182, 496-512. [5] Vernazza et al. (2012) Icarus 221, 1162–1172. [6] Belskaya et al. (2017) Icarus 284, 30–42. [7] Levasseur-Regourd et al. (2018) Space Sci Rev 214. [8] Sultana et al. (2021) Icarus 35, 11412. [7] Mustard and Hays (1997) Icarus 125, 145-163. [9] Potin et al. (2018) App. Optics 57, 28. [10] Poch, (2018) JGR Planets, 123.

How to cite: Poch, O., Sultana, R., Beck, P., Spadaccia, S., Patty, L., Pommerol, A., Quirico, E., and Schmitt, B.: Hyperfine grains can explain altogether the Vis-NIR spectral slope, MIR emissivity, and V-band polarimetric phase curves of primitive small bodies, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-821, https://doi.org/10.5194/epsc2022-821, 2022.

Polarization phase curves of asteroids and other small airless bodies are influenced by the compositional and physical properties of their regolith. The mixing of minerals composing the regolith influences the negative polarization at small phase angles as it changes the multiple scattering properties of the medium.

In recent years, two new asteroid classes have been identified by their polarization phase curve at small phase angles [1][2]. The F-class shows an inversion angle (i.e. the phase angle at which the polarization becomes positive) relatively small to other asteroid families (about 14-16°), and the L-class (the so-called “Barbarians” after the prototype of this class (234) Barbara) show very high inversion angles, about 25-30°. The interpretation of these phase curves is challenging, and it has been proposed that the polarization properties of Barbarians are directly correlated to their surface mineralogy, in particular to the presence of spinel-bearing calcium-aluminum inclusions in a dark matrix [3][4]. The F-class peculiar negative polarization has been interpreted as the result of a homogeneous composition of the surface [1].

We know that the surface mineralogy is directly influencing the polarization phase curve of asteroids. In fact, asteroids tend to group in their respective spectral classes when their minimum of polarization is plotted versus  [5]. It has been experimentally demonstrated that a mixture of minerals with different albedos can deepen the negative polarization with respect to the polarization phase curves of the single minerals [6][7], and we call this effect the “mixing effect”. A systematic study on the “mixing effect”, however, is lacking in the literature. The mixing of bright ad dark minerals has been studied in polarization only for a few minerals, and it has been used to explain the polarization behavior of F-type asteroids and trans-Neptunian objects [8].

Our work aims to demonstrate experimentally how the “mixing effect” influences the polarization phase curve at small phase angles for different mineralogies relevant for asteroids and to determine how different aggregate sizes affect the negative polarization. We prepared a set of binary and ternary mixtures with different common minerals on asteroids and one set of the same mixture with different aggregate sizes. We measured their reflected light at 530 nm with full-Stokes polarimetry at phase angles ranging from 0.8° to 30°.

The mixing effect of those mixtures with both bright and dark minerals significantly changes the behavior of the phase curves in terms of minimum polarization, phase angle of the minimum, and inversion angle with respect to the mineral components that are mixed together (Fig. 1). Some of the binary mixtures show a deepening of the negative polarization when a low reflectance mineral is mixed with a high reflectance mineral. The inversion angle is affected similarly, increasing respect to the inversion angle of the single minerals. Interestingly, some binary mixtures with high and low reflectance minerals show no deepening of the negative polarization (e.g. spinel and graphite). In general, the polarization phase curve changes significantly every time that two or more minerals are mixed together. Furthermore, we find that the presence of aggregates up to cm-size does not affect the negative polarization.

Our binary mineral mixtures can explore large areas of the space (Fig. 2). Our results show that F-class asteroids could be not as homogeneous as previously thought, since similar reflectance,  and can be obtained by a mixture with 1:3 mass ratio of bright and dark minerals (forsterite and graphite).

Barbarians lay in a region that is not explored by our binary mixtures. Nonetheless, we observe that our binary mixtures have a higher inversion angle with increasing contrast of the two end-members (pure Mg-spinel excluded). Other studies found similar results for mixing bright and dark materials: [6] found that the inversion angle increases by 3° compared to pure fine silicates when adding 10% of 10 nm soot, and [7] found that a 1:1 mixture of sub-micron MgO and Fe₂O₃ shows a 9° higher inversion angle with respect to the highest inversion angle of the end-members (MgO). In our sample, the largest increase in inversion angle is given by a 1:1 mixture of silica and magnetite, with an excess of 1° compared to the inversion angle of pure silica. We expect that mixtures of very fine dark and bright particles could increase even more substantially the inversion angle compared with the single end-members. From this perspective, we propose that the polarimetric behavior of Barbarians is not merely related to the presence of a single mineral, but to a combination of very fine regolith having both bright  (CAIs) and dark components that are mixed finely together.

A good synergy between modeling, observations, and laboratory experiments has the potential to strongly aid in interpreting the surface properties of regolith when observing reflected polarized light. Both future sample-return missions and in-situ high sensitive polarimetric observations will greatly improve our
understanding of asteroid regolith properties, helping in the interpretation of astronomical measurements and constraining laboratory simulations to more realistic mineralogies.

[1] Belskaya, I. N., et al. "The F-type asteroids with small inversion angles of polarization." Icarus 178.1 (2005): 213-221

[2] Cellino, A., et al. "The strange polarimetric behavior of Asteroid (234) Barbara." Icarus 180.2 (2006): 565-567.

[3] Sunshine, J. M., et al. "Ancient asteroids enriched in refractory inclusions." Science 320.5875 (2008): 514-517.

[4] Devogèle, Maxime, et al. "New polarimetric and spectroscopic evidence of anomalous enrichment in spinel-bearing calcium-aluminium-rich inclusions among L-type asteroids." Icarus 304 (2018): 31-57.

[5] Belskaya, I. N., et al. "Refining the asteroid taxonomy by polarimetric observations." Icarus 284 (2017): 30-42.

[6] Zellner, B., T. Lebertre, and K. Day. " II-Laboratory polarimetry of dark carbon-bearing silicates." Lunar and Planetary Science Conference Proceedings. Vol. 8. 1977.

[7] Shkuratov, Yu G. "Negative Polarization of Sunlight Scattered from Celestial Bodies-Interpretation of the Wavelength Dependence." Soviet Astronomy Letters 13 (1987): 182.

[8] Bagnulo, Stefano, et al. "Exploring the surface properties of transneptunian objects and Centaurs with polarimetric FORS1/VLT observations." Astronomy & Astrophysics 450.3 (2006): 1239-1248.

How to cite: Spadaccia, S., Patty, L., Capelo, H. L., Thomas, N., and Pommerol, A.: Small phase angle polarization properties of regolith-like materials - the "Mixing Effect", Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-516, https://doi.org/10.5194/epsc2022-516, 2022.

Light scattering experiments have been carried out at visible light for two kinds of dust samples: narrow size distributions consisting of low-absorbing particles with sizes in the micron domain, plus millimeter-sized single grains. The polarimetric analysis of the scattered light has revealed as a very precise tool for size characterization of the samples of particles in the resonance regime, while it let us distinguish the compact/fluffy structure feature of particles much larger than the wavelength.

First, four narrow size distributions of forsterite particles, spanning a broad range of scattering sizes in the resonance regime, were measured. Forsterite is a Mg-rich olivine representative of low-absorbing cosmic dust particles that has been identified in several comets, e.g. 1P/Halley, Hale–Bopp, 9P/Tempel and 81P/Wild2 (see [1] and references therein). The maximum of the degree of linear polarization (DLP) decreases with the size of the particles. That maximum occurs around 90º in all cases, but the angle ranges from 75º to 140º, increasing with the grain size [2]. Furthermore, dust particles with size parameters from ∼ 6 to ∼ 20 seem to be responsible for the negative polarization branch (NBP). This is in agreement with the idea that the NBP might be caused by coherent back-scattering by wavelength-scaled features of the grains (see .e.g. [3]).

On the other hand, millimeter-sized particles were identified to reproduce the flux curve of Fomalhaut protoplanetary disk [4]. Light scattering was then
measured for the same kind of grains in an attempt to reproduce the comet 67P/Churyumov-Gerasimenko phase function obtained in situ by the OSIRIS
camera system onboard the ESA Rosetta mission. In that case it was found that large porous particles with organic inclusions can reproduce both, ground-based observations of the DLP and the OSIRIS phase functions [5].

References

1. Frattin, E., Muñoz, O., Moreno, F., Nava, J., Escobar-Cerezo, J., Gomez Martin, J.C., Guirado, D., Cellino, A., Coll, P., Raulin, F., Bertini, I., Cremonese, G., Lazzarin, M., Naletto, G., Forgia, F.L.: Experimental phase function and degree of linear polarization of cometary dust analogues. Monthly Notices of the Royal Astronomical Society 484, 2198–2211 (2019)

2. Muñoz, O., Frattin, E., Jardiel, T., Gómez-Martín, J.C., Moreno, F., Ramos, J.L., Guirado, D., Peiteado, M., Caballero, A.C., Milli, J., Ménard, F.: Retrieving dust grain sizes from photopolarimetry: An experimental approach. The Astrophysical Journal Supplement Series 256(17 (12pp)) (2021)

3. Muinonena, K., Penttilä, A., Videen, G.: Polarimetry of Stars and Planetary Systems, chap. Multiple Scattering of Light in Particulate Planetary Media. Cambridge University Press, Cambridge (2015)

4. Muñoz, O., Moreno, F., Vargas-Martín, F., Guirado, D., Escobar-Cerezo, J., Min, M., Hovenier, J.W.: Experimental phase functions of millimeter-sized cosmic dust grains. The Astrophysical Journal 846(85 (8pp)) (2017)

5. Muñoz, O., Moreno, F., Gómez-Martín, J.C., Vargas-Martín, F., Guirado, D., Ramos, J.L., Bustamante, I., Bertini, I., Frattin, E., Markannen, J., Tubiana, C., Fulle, M., Güttler, C., Sierks, H., Rotundi, A., Corte, V.D., Ivanovski, S., Zakharov, V.V., Bockelée-Morvan, D., Blum, J., Merouane, S., Levasseur-Regourd, A.C., Kolokolova, L., Jardiel, T., Caballero, A.C.: Experimental phase function and degree of linear polarization curves of millimetersized cosmic dust analogs. The Astrophysical Journal Supplement Series 247(19 (13pp)) (2020)

How to cite: Guirado, D., Muñoz, O., Frattin, E., Gómez Martín, J. C., Martikainen, J. A., Moreno, F., Jardiel, T., Peiteado, M., and Caballero, A. C.: Characterizing cometary dust grains with polarimetry, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-740, https://doi.org/10.5194/epsc2022-740, 2022.

Introduction: Airless bodies of the Solar System present surface evolution due to space weathering. Their regolith is exposed to thermal cycling, cosmic and solar-irradiation, solar wind sputtering and micrometeorites bombardment and associated vaporization. With time, these materials accumulate optically active opaque particles such as nanophase metallic iron on the surface or rims of dust grains or larger iron particles. The density increase in iron particles modifies the spectral properties of the material with a lowering of the albedo, and of the amplitude of absorption bands, generally associated with a global reddening of the spectral slope [1, 2].

Previous studies have shown that nanophase iron particles that induce spectral properties typical of space-weathering will accumulate at the surface of olivine and pyroxene minerals at temperature conditions corresponding to micrometeorite impacts [3]. The skin depth of weathered material depends on the weathering process and minerals.

Context for Phobos studies:  The moons of Mars, Phobos and Deimos, are some of the most enigmatic small bodies of the Solar System. Indeed, it is still debated in the community whether they originate from a large impact on Mars [4, 5], or if they are captured primitive asteroids [6]. The overall spectra of Phobos and Deimos have a strong red slope with no obvious diagnostic spectral features readily detected in the visible or near-infrared, similar to very primitive small bodies of the solar system. However, significant variations in the spectral slope indicate inhomogeneities of the material on the surface of Phobos, suggesting mixture of asteroidal and basaltic materials [7]. 

The Mars Moons Explorer (MMX) mission will bring back samples of Phobos to Earth in order to decipher the origin of the Martian Moons system [8]. The MMX InfraRed Spectrometer (MIRS) instrument on-board the mission is an infrared imaging spectrometer (0.9 to 3.6 μm) that will study the spectral properties of the surface of Phobos and Mars.

Laboratory analogue samples:  In this context, laboratory analogues were ‘weathered’ artificially by submitting basalt (such as the Pic d’Ysson basalt already used for laboratory studies on the lunar regolith optical and spectrophotometric behavior [9]) and meteorite samples to near fusion temperature (1000°C) in an controlled-atmosphere furnace under reducing conditions (partial pressure of CO/CO₂ of 96% corresponding to an oxygen fugacity of about 10^-17) for exposure times of a 2, 4 and 8 hours (Figure 1; [10]). 

Fig. 1: The controlled-atmosphere furnace at IRAP used for heating under reducing conditions [10]. 

Analysis of the weathered samples :  A first comparison between unaltered Pic d’Ysson basalt grains and grains subjected to 4 hours heating at 1100°C under reducing conditions (fugacity of 10^-17) is presented in Figure 2. Backscattered SEM images of basaltic grains before and after exposure are compared. The unaltered grain presents a typical crystalline structure with olivine, feldspars and ilmenite (bright minerals). Its bulk composition is given in Table 1.

Tab. 1: Elemental bulk composition of the Pic d’Ysson basalt unweathered (values given in wt.%).

When considering the altered basalt grain, the onset of alteration due to heating is clearly seen over a depth of about 30 microns (top left and left side of the images), with partial melting of the minerals and the appearance of nanophase metallic iron phases (presence of small white grains on the surface and inside the minerals).

Spectral analysis :  The near infrared reflectance spectra of the ‘unweathered’ (pristine) and ‘weathered’ (heat-treated) samples (1050°C for 4 hours) are then acquired with the SHADOWS instrument at IPAG [11]. The resulting spectra are illustrated in Figure 3 (unweathered in red, weathered in blue). The weathered samples (powdered or sintered) demonstrate a significant change of slope with a strong visible reddening of the spectrum. At the same time the crystal infrared absorption bands depths are generally reduced.

Fig. 2 : SEM analysis showing the onset of heat diffusion and iron metal formation left: crystalline basalt grain unaltered showing the presence of olivine and feldspar crystals with ilmenite (bright minerals). right: apparition of nanophase Fe over a 30 μm depth after 4 hrs heating at 1100°C under reducing conditions.

Fig. 3 : NIR analysis of samples heated at 1050°C for 4 hours  with SHADOWS [11] showing the reddening of the spectral slope between unaltered sample (red) and weathered samples (light blue: sintered; dark blue: powdered).

Conclusions and perspectives :  Laboratory simulations show that heat-treated samples under reducing conditions can recreate the conditions under which regolith may be weathered in space with surface alteration of grains and appearance of nanophase iron. The analysis of the grains’ spectral properties indicates significant reddening and decrease of the absorption band depths as seen on space weathered surfaces. For the moment, the slopes reached remain lower than the ones seen on the surface of Phobos and Deimos. Now that the weathering concept has been demonstrated, further experiments will be performed using Mars analogues and meteorite samples to better constrain the possible materials on the surface of Phobos. Grain size distribution effects will also be explored.  

Acknowledgment: this study was funded by CNES.

References:  [1] Hapke, B. (2001) JGR, 106(E5), 10039-10073. [2] Pieters, C.M., & Noble, S.K. (2016) JGR, 121(10), 1865-1884. [3] Weber, I., et al. (2020) EPSL, 530, 115884. [4] Canup, R., & Salmon, J. (2018) Science advances, 4(4), eaar6887. [5] Bagheri, A., et al. (2021) Nature Astronomy, 1-5. [6] Hansen, B. M. (2018) MNRAS, 475(2), 2452-2466. [7] Glotch, T. D., et al. (2018) JGR: Planets, 123(10), 2467-2484. [8] Kuramoto, K., et al. (2021) Earth, Planets and Space. Submitted. [9] Souchon, A.L., et al. (2011) Icarus, 215(1), 313-331. [10] Toplis, M.J., & Corgne, A. (2002) Contributions to Mineralogy and Petrology, 144(1), 22-37. [11] Potin, S., et al. (2018) Applied optics, 57(28), 8279-8296.

How to cite: Lasue, J., Pinet, P., Beck, P., Toplis, M., Munsch, P., and Oustric, M.: Spectral behavior of regolith analogues weathered by heating under reducing conditions. , Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-840, https://doi.org/10.5194/epsc2022-840, 2022.

1. Introduction
Over the past 35 years, our understanding of comets has been effectively revised by the different space missions, which have allowed to extensively characterise the coma and nuclei of seven comets [1]. While continuously supported by the results of ground observations, this understanding has been further im proved by the multiple laboratory experiments performed concurrently (e.g. [2, 3]).
As part of the efforts of the CoPhyLab project to further our knowledge of the physical processes occuring on comets [4, 5],
we will present the results obtained at the University of Bern on optical properties of silicon dioxide (SiO₂) and juniper charcoal (JChc), as well as of intimate mixtures made out of these two components, as detailled in [6].

2. Samples and experiments
We performed our characterization using the goniometer and spectral imager developped by the Planetary Imaging Group of the Bern university [7]. Measurements were acquired for samples of JChc and SiO₂powders but also for intimate mixtures of these two materials. Mixtures were prepared with a JChc fraction varying from 10% to 90% by mass, in 10% increments.

The goniometer PHIRE-2 [8] allowed us to acquire the bidirectional reflectance of the samples, across the incidence and emergence planes (from ±5° to ±80°), and for azimuths ranging from 0° to 180°. The measurements were performed using 6 broadband filters (centered at 450, 550, 650, 750, 905 and 1064 nm).
The spectral imager MoHIS [9] was used to retrieve the spectral properties of the considered samples across the visible and near-infrared domains (from 380 nm to 2.45 μm), with a spectral resolution of 15 nm in the visible and 6 nm in the near-infrared. The samples surface were imaged at phase angles lower than 5°, by a CCD array and a SWIR camera. Both channels' acquisitions were assembled into hyperspectral cubes.

3. Overview of the results
The measured spectra of the pure end-members, plotted alongside one of 67P/Churyumov-Gerasimenko’ spectrum, are presented here in Fig. 1. Phase curves of the samples in the principal plane and at 550 nm, are presented in Fig. 2.

The JChC and SiO₂ spectra distinguish themselves through their opposing behaviours (e.g. low vs. high reflectances, quasi-monotonic vs. slightly incurved profile). Additionally, the SiO₂ spectrum presents several dips in reflectance across the near-inrared domain, consistent with hydration and hydroxylation features [10]. Moreover, this spectrum’s overall reflectance decrease beyond 1.35 μm is consistent with a scattering regime change within the sub- and micrometric-sized grains [11].
On the other hand, the JChc spectrum presents a slight reflectance dip around 1.1 μm, which together with a 3% H/C ratio exposed by a CHN elemental analysis, hint at the presence of polyaromatic compounds, similarly to certain bitumens [12].
While the spectra of these end-members differ from that of 67P/Churyumov-Gerasimenko (Fig. 1), we found that differences in spectral slopes in the 535 nm – 880 nm range disappear in part when considering the phase reddening phenomenon.

Measured phases curves were modeled using the “Hapke” photometric model [13, 14]. At 550 nm, the best-fitting model parameters verify the mentionned dichotomy (w_SSA,JChc∼5.6% and p_v,JChc∼3.8%, w_SSA,SiO2∼97.5% and p_v,SiO2∼1).
All phase curves present a non-linear reflectance surge at low-phase angles, albeit with varying intensities (Fig.2). This observation is also reflected in the modelisation of the opposition effect (B_0,JChc∼1.4 and h_JChc∼0.15; B_0,SiO2∼0.6 and h_SiO2∼0.07). The photometric properties of the intimate mixtures were found to be driven by the JChc fraction, and the geometric and bidirectional albedoes were found to be best-fitted by an exponential function. 
Although neither the JChc or the SiO₂ samples present best-fitting parameters matching those obtained for surfaces of comet 67P [14, 15], some of the intimate mixtures were found to be partially comparable to these surfaces as well as to other planetary surfaces.

4. Perspectives
We have investigated the spectrophotometric properties of the samples considered for the CoPhyLab sublimation experiment. We will detail the results of this study, and how they compare to small bodies of the solar system. 

Acknowledgments
This work was carried out in the framework of the CoPhyLab project funded by the D-A-CH program (1620/3-1 and BL 298/26-1 / SNF 200021E 177964 / FWF I 3730-N36).

References
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[2] 10.1098/rsta.2016.0262
[3] 10.1016/j.icarus.2018.03.025
[4] 10.1063/5.0057030
[5] Lethuillier et al., 2021, Submitted.
[6] Feller et al., 2022, Submitted.
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[10] 10.1029/2007JE003069
[11] 10.1016/j.icarus.2017.10.015
[12] 10.1006/icar.1998.5955
[13] Hapke, 1993, 978-0-521-88349-8
[14] 10.1093/mnras/stw2511
[15] 10.1093/mnras/stx1834
[16] 10.1126/science.aag3161

How to cite: Feller, C., Pommerol, A., Lethuillier, A., and Gundlach, B.: Cometary Physics Laboratory: spectrophotometric experiments, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1082, https://doi.org/10.5194/epsc2022-1082, 2022.

The ICAPS setup was flown in 2019 on the Texus-56 sounding rocket and its successor, the Laplace facility, is planned for a usage on the ISS in 2023. Both experiments allow to investigate the very first stages of planet formation. The relevant growth processes require observation of many-particle processes consisting of millions of µm-sized particles in a low-density gas environments (typical a few 10s of Pa, such that Knudsen numbers of >>1 are achieved) for prolonged time. The latter requirement make it necessary to perform these investigations in a microgravity environment. The ICAPS and Laplace setups allow to actively protect the dust cloud against diffusion and other external residual forces and also allow concentration of the dust cloud by means of a unique actively-controlled thermophoretic trap acting on all three axes.

Simultaneously, the dust cloud is made available to a number of observational instruments. As a whole, the cloud is observed in 3D from two perpendicular overview cameras while also making a small central volume of 1 mm² cross-section accessible to a dedicated long-distance microscope with high-speed imaging to facilitate non-destructive in-situ analysis of the growing dust aggregates. An extinction sensor allows to measure the extinction throughout the cloud as a whole. Measurements of the electric charges of the individual particles can be conducted repeatedly by means of a controlled DC or AC fields in order to determine charge and charge evolution.

We will present the design and techniques of the ICAPS and Laplace setups, the current status of the Laplace facility and demonstrate its capabilities on a few select data from the Texus-56 flight of ICAPS.

How to cite: von Borstel, I., Blum, J., Schräpler, R., Aktas, C., Balapanov, D., Vedernikov, A., Brisset, J., Molinski, N., and Schubert, B.: The ICAPS & Laplace experiments to investigate the initial stage of planet formation, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-951, https://doi.org/10.5194/epsc2022-951, 2022.

Abstract

Understanding the composition of cometary nucleus will provide a direct link between present-day comets, KBOs, and Oort cloud objects and formation of cometesimals during the protoplanetary phase. We will understand whether cometary nucleus is primordial, and hence contain molecules of life. Our focus here is to derive compositional information of the nucleus of comet 67P-CG based on laboratory studies comparing with outgassing dichotomy of the comet, particularly that of CO₂ and H₂O and the corresponding minor volatiles.

Introduction

Interstellar ice grains and cometary outgassing have so much in common that it is hard to understand how such compositional similarity is preserved without the interstellar ice grains themselves preserved throughout the evolution of our solar system – from dense molecular clouds through protoplanetary disk to evolved solar system that has the cometary precursors – Kuiper Belt Objects and Oort Cloud.

While the link seems to be obvious, chemical evolution of ice grains in the protoplanetary phase is far from well understood. Particularly, if the ice particles were to cross the ice-line towards the protostar and sublimate leaving the refractory grains behind. It is hard to determine how similar composition is put together in the formation of cometary precursors and cometesimals. In addition to follow reaction pathways that could be used as tracers for the history of ice grain chemistry and transport, it is necessary to understand outgassing properties of long-period and short-period comets in order to correlate this outgassing to the physical and chemical composition of the nucleus through laboratory experiments and modeling. It is our goal to investigate and quantify the outgassing properties and dichotomy that is already noticed, but not yet fully understood.

Results

Here we present preliminary results from conventional temperature programmed desorption of ternary mixtures containing H₂O, CO₂, O₂, and CO (Figure 1). While the first two are the major species resulting in mutually exclusive outgassing, of the last two super volatiles, O2 is predominantly correlated with H₂O. Our laboratory data shows that excess CO₂, O₂, and CO that cannot be trapped into amorphous water ice, form separate ices at or below their individual condensation temperatures. Majority of this excess ice is sublimed at their respective pure ice sublimation temperatures. Some O₂ and CO are trapped in the CO₂ ice, which are seen to outgas along with CO₂ sublimation. Keep in mind that in the mass spectrometer presented in Figure 1, contribution of CO due to fragmentation of CO₂ is not removed. After completely outgassing aggregated ice other than H₂O ice, amorphous to crystalline transition occurs around 120-140 K, during which almost all trapped volatiles (CO₂, CO, and O₂) within amorphous ice are released. The entrapment of these volatiles is also dependent on the starting ice composition. For example, when CO₂ to H₂O ratio is 1:3 vs. 1:1, more CO₂ is trapped in H₂O ice (Figure 1). However, we expect upper limit of CO₂ trapping in H₂O ice. When crystalline H₂O ice sublimes, only a few percentage (< 3-5%) of the other volatiles are observed. Though the mixing ratio of our experiments is not exactly as it was observed by the Rosetta Mission, these data provide insight into the capacity of amorphous ice to trap other volatiles and sublimation temperatures of these volatiles forming pure ices. Our measurements are in qualitative agreement with earlier lab work (cf. Kouchi et al. 1994) though finer differences could be found, particularly around amorphous ice crystallization temperatures. Critical to the understanding of the ROSINA data from 67P-CG, is to use gas ratios in the lab comparable to the comet outgassing (Läuter et al. 2020) and vary the composition around this mixing ratio to determine the dependence of composition on sublimation properties. Keeping in mind that the ROSINA observed averaged outgassing of 67P-CG occurring from areas with very different illumination conditions, crystallization and sublimation seen in our experiments between 120 – 170 K seem to occur together in the observations. Future work includes further experimental work and comparison with observational data from Rosetta ROSINA instrument.

Figure 1: Laboratory data on outgassing properties of cometary ice analogs. It is evident that long-period comets originating from Oort cloud would have the significant outgassing of super volatiles, while short-period comets originating from KBOs and have gone through several orbits are depleted of these super volatiles, at least in the subsurface where the thermal equilibration could occur to temperatures beyond 80-100 K.

References

Kouchi, A., et al., A&A. 290, 1009-1018(1994)

Läuter, M., et al., MNRAS 498, 3995–4004 (2020)

Acknowledgements

This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, was under a contract with the National Aeronautics and Space Administration (NASA). Funding through NASA Discovery Data Analysis Program is gratefully acknowledged.

How to cite: Gudipati, M., Fleury, B., Rubin, M., and Altwegg, K.: Dichotomy of Cometary Outgassing, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-334, https://doi.org/10.5194/epsc2022-334, 2022.

Abstract

Comet Interceptor is a Fast mission of the European Space Agency (ESA) conceived to study a long period comet.

The mission concept includes a spinning probe venturing very near to the comet; on this probe, the Entire Visible Sky (EnVisS) camera will be mounted.

EnVisS is an intensity and polarimetric imager that will address several fields of cometary science by carrying out observations close to, and within a comet’s coma.

In this work, the instrument concept, design and scientific capabilities of EnVisS will be presented.

1. Introduction

The present Comet Interceptor mission configuration comprises a spacecraft and two probes. The spacecraft, called A, will make remote and in-situ observation of the target from afar. While the two probes, one provided by the Japan Aerospace Space Agency JAXA, called B1, and the other one provided by ESA, called B2, will venture near to the target performing a fly-by [1].

The B2 probe will be a spinning spacecraft, thus a rotational push-broom or push-frame imaging technique can be adopted for EnVisS to scan and image the whole scene around the spacecraft. Filtering strips mounted very near to the detector can be foreseen for studying the target in different wavelength ranges and performing polarimetric imaging.

EnVisS will map the intensity and the degree of linear polarization and polarization angle orientation of the light scattered by the dust particles in the comet coma with a full 180° phase angle coverage. The linear polarization depends on dust size distribution, morphology, porosity and composition [2]. The monitoring of linear polarization will provide some clues on how these parameters correlate.

2. Instrument acquisition concept

The EnVisS instrument has been conceived to work in the visible wavelength range from 550-800 nm. The extremely wide FoV of the instrument is designed to acquire the full sky.

EnVisS will feature a flexible push-broom/push-frame imaging technique, while the probe rotates (see Figure 1) slices of the sky are acquired that will be later stitched together on-ground to form a full sky image.

                        (a)                                                 (b)

Figure 1: In (a) illustration of EnVisS full-sky imaging scanning concept. In (b) schematics of the filters strips images on the 2kx2k detector.

The probe spin-axis will be pointing to the comet nucleus for most of the time, except at the closest approach when the comet nucleus will fall inside the camera FoV (see Figure 2 for the expected geometry of the fly-by and the EnVisS placement on the B2 probe).

                                            (a)                                                          (b)

Figure 2: In (a) expected B2 comet fly-by geometry in (b) placement of the EnVisS camera on the B2 probe.

3. EnVisS: Instrument Design

The EnVisS instrument is composed of different parts:

• a fish-eye lens, under study at the Leonardo S.p.A. company in Florence (IT);
• a commercial space-qualified detector from 3D-Plus [3], that will be equipped with a filter strip package;
• ad-hoc power and data handling units;
• software.

The design solution adopted for the filters in EnVisS allows for a compact, low mass and low complexity camera and avoids the use of a filter wheel. Each filter strip is covering a different area on the detector.

Three broad-band filters, all working in the same wavelength range, are foreseen for the camera:

• one broadband intensity filter positioned to be centered on the detector (see Figure 1b blue central strip I);
• two polarimetric filters with transmission axis angles oriented at 45° one to the other (see Figure 1b the red and yellow strips P1 and P2).

The EnVisS optical head will have a 3.45 mm focal length with an MTF of 70% at 45 lp/mm to match a 2x2 pixel resolution element. The full FoV of the instrument will be 180° in the direction of the spin axis, i.e. across-track direction, and 45° in the plane of rotation and motion of the scene, i.e. along-track direction. The full EnVisS characteristics are summarized in Table 1.

Table 1 Summary of EnVisS characteristics

The direction of apparent motion of the scene due to the S/C B2 rotation, i.e. along-track direction, is parallel to the vertical direction in Figure 1 b, while the horizontal direction, corresponds to the direction of the spin axis.

A flexible approach has been devised to allow obtaining the required SNR of 10 for the broadband images and 100 for each polarimetric image. Depending on the target object activity, the map of the coma will be taken with different spatial resolution, i.e. smearing and pixel binning.

Along track, the signal from the coma is not expected to change too much, high spatial resolution is not required and smearing can be tolerated. The spatial resolution is retained in the across-track direction and assures a sampling of the comet phase function every 0.2°. This strategy will also allow for an adjustment of the exposure time if the radiance of the coma is different from expected.

Further pixel binning on-board, or co-adding, on-ground, of the images over different rotations, could be considered if the signal happens to be extremely low.

Acknowledgements

This work has been supported by the Italian Space Agency (ASI) through a contract to the Istituto Nazionale di Astrofisica (2020-4-HH.0) and by the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa” award to the Instituto de Astrofısica de Andalucıa (SEV-2017-0709) and from project PGC2018-099425-B-I00 (MCI/AEI/FEDER, UE).

References

[1]  Snodgrass, C. and Jones, G. H., "The European Space Agency’s Comet Interceptor lies in wait", Nat. Commun. 10, 5418 (2019).

[2] Fulle, M., Levasseur-Regourd, A. C., McBride, N. and Hadamcik, E., "In situ dust measurements from within the coma of 1P/Halley, The Astronomical Journal, 119:1968-1977 (2000).

[3] https://www.3d-plus.com/

How to cite: Da Deppo, V., Della Corte, V., Zuppella, P., Nordera, S., Pernechele, C., Lara, L. M., Castro, J. M., Jiménez, J., Martinez, I., and Praks, J. and the EnVisS Team: The Entire Visible Sky (EnVisS) imager for the Comet Interceptor ESA mission, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-273, https://doi.org/10.5194/epsc2022-273, 2022.

Even if silicate glasses represent a major component in volcanic products, their spectral characteristics are not always considered when dealing with the interpretation of planetary and asteroidal surfaces and terrains. Space analogues described in spectral libraries are mostly constituted of crystalline materials rather than amorphous products. This happens because of the lack of spectral features of amorphous products, but this kind of blurred/featureless spectra are indeed observed on planetary surfaces, and, moreover, very recent studies have shown how amorphous phases can also deeply influence the spectral response of volcanic terrains [1]. For this reasons,  the Petro-Volcanology Research Group at the University of Perugia (PVRG) have started an extensive investigation of the spectral response of silicate glasses.

PVRG group has chosen to reproduce in the lab, using experimental petrology tools such as high-temperature furnaces, volcanic glasses with complex chemical compositions mimicking the chemical variability of possible natural magmatic bodies. PVRG have used different techniques to characterize such silicate glasses: reflectance in the Visible and Near-Infrared (VNIR) [2], again reflectance in the Mid-Infrared Region (Mid-IR) [3] and reflectance and emissivity in the Thermal-Infrared Region (TIR) [4].

We have observed how different spectral techniques allow us to retrieve interesting correlations between change in spectral characteristics and chemical/textural composition. As examples, we have observed promising correlations between spectral slope/albedo and iron content and speciation of the investigated samples in the VNIR (Fig. 1), but also between silica and alkaline content and spectral shift/shape in the Mid-Infrared Region (Fig. 2). Moreover, it is observed how evolution of spectral shape in the MIR region carries interpretable and useful information on granulometry of the investigated terrains.

These findings suggest that accounting for glassy phases for the interpretation of planetary volcanic terrains is useful because i) it permits to distinguish which parts of the spectra carry information about crystalline phases and which not, and ii) glasses spectra carry themselves important information (iron, silica and alkaline content) that can be used to classify volcanic rocks and therefore to define geological history of planetary bodies. For these reasons, we want to offer a tool where such dataset can easily be observed and investigated.

The ASI (Italian Space Agency) Space Science Data Center (SSDC) [5] is an infrastructure thought to archive open access data in the field of space sciences for different missions and purposes, and it operates using international standards such as those regarding Planetary Virtual Observatory [6]  to maintain and disseminate data applying the FAIR (Findable, Accessible, Interoperable, Reusable) principles. Within SSDC infrastructure, we have created an environment to provide open access to this kind of data by creating a spectral database in which raw spectral data can be found together with the relative peer-reviewed publications in which such data are discussed. Up to now (May 2022) the database is only reacheable through the SSDC pages and with access restricted to people inside the group, but it is already planned to made the catalog completely public in a very near future, not excluding the possibility of making it available as a VO resource in a more advanced phase. Such environment will be explorable in the “Catalogs” list of the SSDC website (https://www.ssdc.asi.it/) under the name Spectral Database for Silicate Glasses with Natural Composition (Fig 3), where it will be possible to visualize, compare, and filter-search spectra, making it a fundamental tool for planetary scientists to account spectra of amorphous phases within their analyses of planetary surfaces.

Acknowledgements

We acknowledge the support of ASI under the ASI-UniPG agreement 2019-2-HH.0.

References:

[1] Leight et al., 2022. Characterization of tephra deposits using VNIR and MIR spectroscopy: A comprehensive terrestrial tephra spectral library. Remote Sensing of Environment, 273, 112965.

[2] Pisello,  et al., 2022. Visible and near-InfraRed (VNIR) reflectance of silicate glasses: Characterization of a featureless spectrum and implications for planetary geology. Icarus, 374, 114801.

[3] Pisello,  et al., 2022. Reflectance of silicate glasses in the mid-infrared region (MIR): implications for planetary research. Icarus, under review.

[4] Pisello, et al., 2019. Retrieving magma composition from TIR spectra: implications for terrestrial planets investigations. Scientific reports, 9(1), 1-13.

[5] Zinzi et al., 2018. The SSDC contribution to the improvement of knowledge by means of 3D data projections of minor bodies. Advances in Space Research, 62(8), 2306-2316.

[6] Erard et al., 2019. Virtual european solar & planetary access (VESPA): a planetary science virtual observatory cornerstone. arXiv preprint arXiv:1907.06521.

How to cite: Pisello, A., Zinzi, A., Bisolfati, M., Porreca, M., and Perugini, D.: A new spectral database for silicate glasses: a fundamental resource to interpret characteristics of volcanic terrains on planetary bodies, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-539, https://doi.org/10.5194/epsc2022-539, 2022.

In astronomy emission spectroscopy is widely used technique to study various objects. Although emission spectra of individual compounds are quite well known so composition of the studied body can be analysed the information on processes active in distant object can be encoded in the determined spectra as well. To decode such information the individual elementary processes need to be well understood. For example, in case of comets which are simple leftovers from the beginning of the Solar System understanding the connection between the coma and the comet’s nucleus is critical because observations rarely detect the nucleus directly, and its properties must often be inferred from measurements of the coma. As measurements of the coma do not necessarily represent the characteristics of the nucleus due to spatial, temporal and chemical evolution of the emitted material, projecting the coma observations back to the nucleus requires understanding of the processes in the coma. Within the Rosetta various instruments were used for spectroscopy. OSIRIS was equipped with several band filters that are commonly used in ground-based studies (Farnham et al. 2000). The spectroscopic measurements done within the Rosetta mission led to conclusion that electron-molecule processes play much more significant role than it used to be suspected in the past (Bodewits et al. 2016). This fact can be utilized to remotely study the plasma-chemical reactions active in distant objects. For example, the formation of excited OH by electron impact on H2O has an electron temperature threshold of 9.1 eV; H Ly-a has a threshold of 15.4 eV; and the production of OI 130.4 nm emission requires electron temperatures above 23.5 eV (Beenakker et al. 1974; Morgan & Mentall 1974). At electron temperatures below 10 eV, collisions with water molecules result in ro-vibrational excitation and energy is emitted in the IR range (Itikawa & Mason 2005). Electron impact emission thus provides a sensitive diagnostic probe for studying coma (Bodewits et al. 2019). The presence of spectral structures depends on reaching the threshold electron energy for specific transition. Emission cross sections that have thresholds of 10-100 eV – typical for the coma (Broiles et al. 2016) – depend strongly on electron impact energy. Knowledge of emission spectra and cross sections for electron-molecule reactions is critical for evaluation of physical-chemical properties of cometary coma based on spectroscopic observations. For thorough analysis of such studies a reliable database of reference spectra and emission cross sections must be available. Especially the availability of cross section data the Achilles’ heel since most of the values were determined only for a few individual energies but for successful application in models of atmospheres or comas they need to be known for wider range of energies.

Such data can be provided by laboratory experiments studying individual electron-molecule interactions in various experimental configurations such as crossed beams of electrons and molecules. The example of the apparatus scheme aimed at studying electron induced excitation reactions is shown in the figure. The electron beam generated by trochoidal electron monochromator collides perpendicularly with molecular beam created by effusive capillary. Photons emitted by excited reaction products are guided out of the vacuum chamber into the spectrometer equipped by CCD or photomultiplier detectors. The details of the experimental system were described elsewhere (Orszagh et al. 2017).

Fig. 1. Scheme of the experimental apparatus. 

The main result of the experiment are the emission spectra (see second figure as an example) and emission cross section curves determined in precisely defined conditions. The shape of the spectrum given by population of individual ro-vibrational states and presence of individual spectral features depends strongly on excitation source and its energy (Bodewits et al. 2019).

Fig. 2. Emission spectrum of hydrogen induced by 50 eV electrons impact. 

From the relative cross section curves it is possible to determine the lowest kinetic energy (threshold energy) of the electrons necessary to create a specific excited product and after calibration to absolute values we get series of cross sections for individual electron impact energies. Below the threshold the spectral line or band will not be present in the spectrum. The third figure shows relative emission cross section of oxygen where two thresholds correspond to different processes generating excited oxygen atom.

Fig. 3. Relative emission cross section curve for oxygen atom. 

By putting spectral and relative cross sections information together by measuring emission spectra at several electron impact energies we can get a surface formed by emission cross section values within selected range of wavelengths and electron energies (see the last figure).

Fig. 4. 3D spectral map of emission induced by electron impact on pyridine. 

Such complex information on the kinetics of electron-molecule excitation reactions is unfortunately rare mostly due to the technically demanding experiment but they provide valuable reference for analysis of astronomical spectroscopic data. Electron-molecule reactions in environment such as cometary comas or atmospheres can serve as a remote probe for characterization of local conditions if reference laboratory data on their kinetics are available.

Acknowledgments

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 871149. This research was supported by Slovak grant agency VEGA within the projects nr. 1/0489/21 and 1/0553/22, by Slovak Research and Development Agency within project nr. APVV-19-0386, and by NASA ROSES project nr. NNH18ZDA001N-RDAP.

Beenakker, C.I.M. et al., 1974. Chemical Physics, 6(3), pp.445–454.

Bodewits, D. et al. 2016. Astronomical J., 152, 130.

Bodewits, D. et al., 2019. Astrophys. J., 885(167), p. 16.

Broiles, T.W. et al., 2016. Journal of Geophysical Research: Space Physics, 121 (8), pp.7407–7422.

Farnham, T.L., Schleicher, D.G. & A'Hearn, M.F., 2000. Icarus, 147, p.180.

Itikawa, Y. & Mason, N., 2005. Journal of Physical and Chemical Reference Data, 34(1), pp.1–22.

Morgan, H. D. & Mentall, J.E., 1974. Journal of Chemical Physics, Volume 60, Issue 12, pp. 4734-4739.

Orszagh, J. et al. 2017, Astrophys. J., 841(17).

How to cite: Orszagh, J., Stachova, B., Blasko, J., Matejcik, S., Bodewits, D., and Bromley, S.: Reference emission spectral data for astronomical observations, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-950, https://doi.org/10.5194/epsc2022-950, 2022.

Analysis of the photopolarimetric observational data obtained for the surfaces of the Solar System bodies requires understanding of the light scattering mechanisms that are responsible for the optical opposition phenomena such as the intensity surge (IS) and the negative polarization (NP) branch [1,2]. The width of the IS and the slope of the intensity curve, the inversion angle of polarization and the position and the depth of the NP feature depend on the physical properties upper layer of planetary regoliths. Measuring intensity and linear polarization as functions of the scattering θ or phase angle α (α = π - θ) one can characterize them and extract the information about the complex refractive index of the material, particles size, packing density and the surface microtopography [1-3]. This is possible if an adequate theoretical model is applied.

We numerically study light scattering from clusters of densely packed irregular particles [4,5] with dimensions much larger than the wavelength. Such structures with bulk packing densities of ρ = 0.5 and numbers of particles up to 5000 represent a realistic model for planetary regoliths (Fig. 1a). The size parameters of constituents are kr = 10 - 30. The Discontinuous Galerkin Time Domain (DGTD) method is applied to obtain a full wave solution of the electromagnetic problem (Fig. 1b). Thus, we avoid many approximations used in the popular radiative transfer models. With a numerically accurate solution one can correctly account for the single-scattering phase function, single-scattering polarization properties of particles and near-field effects. The input parameters like particle size, complex refractive index and packing density are clearly defined in this case as well.

The results of simulation for large clusters of non-absorbing irregular particles demonstrate the role of high packing density for light transport and the backscattering phenomena. Light propagation becomes highly localized in this case and mimics a percolation process that is controlled by the topology and degree of disorder of the medium. The topological differences in the structure of the considered clusters can be seen in the intensity and polarization scattering angle curves [6].

In the case of low-albedo highly absorbing layers the total optical response is determined by the contributions of single and mostly double scattering in the very upper layer. At high bulk packing density, the incident light does not penetrate deeper than a few particle sizes (Fig. 1b) [7].

We can distinguish two sources of NP for bright and dark samples. Single non-absorbing particles comparable with the wavelength can produce strong NP feature which is suppressed by multiple scattering in a dense multi-particle system [6,8]. At high absorption it is double scattering between close neighbour particles that produces NP as a result of coherent backscattering [7,9]. The single scattering component is positively polarized in this case. In Fig. 1c the simulation result for a thick absorbing layer is compared with the polarimetric data obtained for an F-type asteroid 419 Aurelia [10] and a boron carbide powder with particle size d = 7 μm [11].