Detection of frost formation on regolith-like materials - polarization and spectral variations

The Rosetta mission revealed the extraordinary complexity of the surface of comet 67P/Churyumov-Gerasimenko (67P) nucleus. The images and spectra from the OSIRIS and VIRTIS instruments showed a great variety of morphology structures as well as some variability of spectral features and slopes [1][2][3].

The study of water ice in comets is paramount for understanding the water content and evolution of those bodies. The surface of 67P is mainly covered by a mantle of organic-rich dust. When warmed by the solar thermal waves, the volatiles trapped in the subsurface layers start sublimating, and sometimes the upper dust mantle is disrupted, revealing the volatile-rich nature of the subsurface. These spots of exposed water ice show a flatter (“bluer”) spectral slope in the range 500-800 nm with respect to the average slope of the nucleus (11%/100 nm at 1.3° phase angle) [4][5]. The exposed ice spots can show diurnal variations or be stable over months [6], and the sublimation-triggered exposure mechanism has been demonstrated in the laboratory under comet-like conditions [5]. Observations showed that water vapor can also recondense on the shadowed regions, where the surface temperature is lower [7]. The recondensed frost becomes visible when the shadow retreats and disappears within minutes. Thermal modeling indicates that the ice thickness must be about 10 µm or lower [8], suggesting that the ice in these regions is most probably inter-mixed with dust particles.

We are investigating in the laboratory the detection of frost condensation on asteroid simulants (CR-type from [9]). We use the POLarimeter for ICE Samples (POLICES) at the University of Bern, installed inside a black airtight enclosure. The sample is in the middle of the enclosure, illuminated by an optical fiber connected to a monochromator and held by a motorized arm that illuminates the sample with different incidence angles. A camera is placed on the other side of the arm looking at the sample at an angle of 45°. Finally, a full-Stokes polarimeter (Dual PEM II/FS42-47) is placed on top of the enclosure above the sample, measuring the polarization state of the light scattered by the sample. The dust sample is actively kept at a temperature of approximately 150 K through liquid nitrogen circulation. At the beginning of the experiment, the dust layer is uncovered and air with 15% relative humidity is pushed into the enclosure while keeping the sample cold. The phase angle is fixed, and the sample is alternatively illuminated every 30 seconds with blue (450 nm) and red (750 nm) light. At each wavelength, the reflectance and polarization state of the sample are measured over time, while the frost grows on top of the cold grains.

In Fig. 1 we show an example of experimental results obtained following this procedure, with the sample illuminated with an incidence angle of 12°. In the first 20 minutes of condensation, the small particles of ice deposited on top of the sample are transparent and the overall reflectance increases by 0.4% in the blue and 0.2% in the red, while the linear polarization decreases from the initial value of -0.45% to a value of -1.15% in the blue and -0.67% in the red. The polarization continues to decrease until a minimum is reached in both wavelengths (after 45 and 65 minutes from the beginning of the experiment). At this point, the ice thickness makes it opaque and the reflectance increases more steeply, while the linear polarization decreases because of the increase in multiple scattering. The 450 – 750 nm spectral slope decreases by 1%/(100 nm) in the first 20 minutes. We would like to stress that a change in linear polarization of 1% at 450 nm is very significant, changing the shape of the whole polarization phase curve drastically. The polarization phase curve at small phase angles reflects important properties of the surface materials, and its sensitivity can be used to distinguish different classes of asteroids.

We simultaneously observed the ice deposition with a long-distance microscope (~10 µm resolution) and in the first 30 minutes of the experiment, no visible changes affected the sample, confirming that at the beginning the frost layer is smaller than 10 µm and mainly transparent.

We demonstrated that variations of linear polarization over time (especially at the shortest visible wavelengths) are a viable way to detect thin layers of frost depositing on top of dark dust layers when the spectral slope is still not affected significantly. Measuring the frost deposition with additional wavelengths and phase angles is a future extension of this work, and will allow us to compile the full spectropolarimetric properties of this process.

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[2] Thomas, N., et al. "The morphological diversity of comet 67P/Churyumov-Gerasimenko. Science 347: aaa0440." (2015).

[3] Capaccioni, Fabrizio, et al. "The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta." Science 347.6220 (2015): aaa0628.

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[5] Pommerol, Antoine, et al. "OSIRIS observations of meter-sized exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments." Astronomy & Astrophysics 583 (2015): A25.

[6] Filacchione, Gianrico, et al. "An orbital water-ice cycle on comet 67P from colour changes." Nature 578.7793 (2020): 49-52.

[7] De Sanctis, Maria Cristina, et al. "The diurnal cycle of water ice on comet 67P/Churyumov–Gerasimenko." Nature 525.7570 (2015): 500-503.

[8] Fornasier, Sonia, et al. "Rosetta’s comet 67P/Churyumov-Gerasimenko sheds its dusty mantle to reveal its icy nature." Science 354.6319 (2016): 1566-1570.

[9] Britt, Daniel T., et al. "Simulated asteroid materials based on carbonaceous chondrite mineralogies." Meteoritics & Planetary Science 54.9 (2019): 2067-2082.