Cometary Physics Laboratory: spectrophotometric experiments

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
[1] 10.1146/annurev-astro-081710-1025
[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.
[7] 10.1007/s11214-019-0603-0
[8] 10.1016/j.pss.2011.07.009
[9] 10.1016/j.pss.2015.02.004
[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