Cometary Physics Laboratory: spectrophotometric experiments
Authors: Feller Clement , Pommerol Antoine , Lethuillier Anthony , Gundlach Bastian 
Published: September 2022

&lt;p&gt;&lt;strong&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;1. Introduction&lt;/span&gt;&lt;/strong&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;Over the past 35 years, our understanding of comets has been ef&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;fectively revised by the different space missions, which have al&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;lowed to extensively characterise the coma and nuclei of seven &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;comets [1].&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;While continuously supported by the results of &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;ground observations, this understanding has been further im &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;proved by the multiple laboratory experiments performed con&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;currently (e.g. [2, 3]).&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;As part of the efforts of the CoPhyLab project to further our &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;knowledge of the physical processes occuring on comets [4, 5],&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;we will present the results obtained at the University of Bern on &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;optical properties of silicon dioxide (SiO&lt;/span&gt;&lt;sub&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;) and juniper charcoal &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;(JChc), as well as of intimate mixtures made out of these two &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;components, as detailled in [6].&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;2. Samples and experiments&lt;/span&gt;&lt;/strong&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;We performed our characterization using the goniometer and &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;spectral imager developped by the Planetary Imaging Group &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;of the Bern university [7].&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;Measurements were acquired for &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;samples of JChc and SiO&lt;/span&gt;&lt;sub&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;2&lt;/span&gt;&lt;/sub&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;powders &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;but also for intimate mixtures of these two materials. Mixtures &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;were prepared with a JChc fraction varying from 10% to 90% by &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;mass, in 10% increments.&lt;br /&gt;&lt;br /&gt;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;The&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;goniometer&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;PHIRE-2&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[8]&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;allowed&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;us&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;to&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;acquire&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;the &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;bidirectional reflectance of the samples, across &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;the incidence and emergence &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;planes (from&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&amp;#177;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;5&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&amp;#176;&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;to&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&amp;#177;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;80&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&amp;#176;)&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;, and for azimuths ranging from&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt; 0&amp;#176;&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;to 180&amp;#176;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;.&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;The measurements were performed using 6 broadband filters &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;(centered at 450, &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;550, 650, 750, 905 and 1064 nm).&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;The spectral imager MoHIS [9] was used to retrieve the spectral &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;properties of the considered samples across the visible and &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;near-infrared domains (from 380 nm to 2.45&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&amp;#956;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;m), with a spectral resolution of 15 nm in the visible and 6 nm in the near-infrared. &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;The samples &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;surface were imaged at phase angles lower than 5&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&amp;#176;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;, by a CCD &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;array and a SWIR camera. Both channels' acquisitions were assembled into hyperspectral cubes.&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&lt;strong&gt;3. Overview of the results&lt;/strong&gt;&lt;br role=&quot;presentation&quot; /&gt;The measured spectra of the pure end-members, plotted alongside one of 67P/Churyumov-Gerasimenko&amp;#8217; 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.&lt;br /&gt;&lt;br role=&quot;presentation&quot; /&gt;The JChC and SiO&lt;sub&gt;2&lt;/sub&gt; spectra distinguish themselves through their opposing behaviours (e.g. low vs. high reflectances, quasi-monotonic vs. slightly incurved profile). Additionally, the SiO&lt;sub&gt;2&lt;/sub&gt; spectrum presents several dips in reflectance across the near-inrared domain, consistent with hydration and hydroxylation features [10]. Moreover, this spectrum&amp;#8217;s overall reflectance decrease beyond 1.35 &amp;#956;m is consistent with a scattering regime change within the sub- and micrometric-sized grains [11].&lt;br role=&quot;presentation&quot; /&gt;On the other hand, the JChc spectrum presents a slight reflectance dip around 1.1 &amp;#956;m, 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].&lt;br role=&quot;presentation&quot; /&gt;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 &amp;#8211; 880 nm range disappear in part when considering the phase reddening phenomenon.&lt;br /&gt;&lt;br role=&quot;presentation&quot; /&gt;Measured phases curves were modeled using the &amp;#8220;Hapke&amp;#8221; photometric model [13, 14]. At 550 nm, the best-fitting model parameters verify the mentionned dichotomy (w&lt;sub&gt;SSA,JChc&lt;/sub&gt;&amp;#8764;5.6% and p&lt;sub&gt;v,JChc&lt;/sub&gt;&amp;#8764;3.8%, w&lt;sub&gt;SSA,SiO2&lt;/sub&gt;&amp;#8764;97.5% and p&lt;sub&gt;v,SiO2&lt;/sub&gt;&amp;#8764;1).&lt;br role=&quot;presentation&quot; /&gt;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&lt;sub&gt;0,JChc&lt;/sub&gt;&amp;#8764;1.4 and h&lt;sub&gt;JChc&lt;/sub&gt;&amp;#8764;0.15; B&lt;sub&gt;0,SiO2&lt;/sub&gt;&amp;#8764;0.6 and h&lt;sub&gt;SiO2&lt;/sub&gt;&amp;#8764;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.&amp;#160;&lt;br role=&quot;presentation&quot; /&gt;Although neither the JChc or the SiO&lt;sub&gt;2&lt;/sub&gt; 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.&lt;br /&gt;&lt;br /&gt;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&lt;strong&gt;4. Perspectives&lt;/strong&gt;&lt;br role=&quot;presentation&quot; /&gt;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.&amp;#160;&lt;br role=&quot;presentation&quot; /&gt;&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&lt;img src=&quot;&quot; alt=&quot;Figure 1: Reflectance spectra of Juniper charcoal and SiO2 powders plotted alongside a spectrum from comet 67P&amp;#8217; surface presented in [16]. The bottom plot display the spectra normalised to their respective reflectance at 535 nm.&quot; width=&quot;1338&quot; height=&quot;1146&quot; /&gt;&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;Acknowledgments&lt;br role=&quot;presentation&quot; /&gt;&lt;/span&gt;&lt;/strong&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;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).&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;References&lt;/span&gt;&lt;/strong&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[1] 10.1146/annurev-astro-&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;081710-1025&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[2]&lt;/span&gt;&amp;#160;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;10.1098/rsta.2016.0262&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[3] &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;10.1016/j.icarus.2018.03.025&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[4] 10.1063/5.0057030&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[5] Lethuillier et al., 2021,&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;Submitted&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;.&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[6] Feller et al., 2022,&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;Submitted&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;.&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[7] 10.1007/s11214-019-&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;0603-0&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[8] 10.1016/j.pss.2011.07.009&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[9] 10.1016/j.pss.2015.02.004&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[10] &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;10.1029/2007JE003069&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[11] &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;10.1016/j.icarus.2017.10.015&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[12] &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;10.1006/icar.1998.5955&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[13] Hapke, 1993, 978-0-521-88349-8&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[14] &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;10.1093/mnras/stw2511&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[15] &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;10.1093/mnras/stx1834&lt;/span&gt;&lt;br role=&quot;presentation&quot; /&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;[16] &lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;10.1126/science.aag3161&lt;/span&gt;&lt;/p&gt;