Polarimetry of Water Ice Particles Providing Insights on Grain Size and Degree of Sintering on Icy Planetary Surfaces

The polarimetry of the light scattered by planetary surfaces is a powerful tool to provide constraints on their microstructure. To improve the interpretation of polarimetric data from icy surfaces, we have developed the POLarimeter for ICE Samples (POLICES) complementing the measurement facilities of the Ice Laboratory at the University of Bern. The new setup uses a high precision Stokes polarimeter to measure the degree of polarization in the visible light scattered by surfaces at moderate phase angles (from 1.5 to 30{\deg}). We present the photometric and polarimetric phase curves measured on various surfaces made of pure water ice particles having well-controlled size and shape (spherical, crushed, frost). The results show how the amplitude and the shape of the negative polarization branch change with the particles sizes and the degree of metamorphism of the ice. We found that fresh frost formed by water condensation on cold surfaces has a phase curve characterized by resonances (Mie oscillations) indicating that frost embryos are transparent micrometer-sized particles with a narrow size distribution and spherical shape. Comparisons of these measurements with polarimetric observations of the icy satellites of the Solar System suggest that Europa is possibly covered by relatively coarser (~40-400 {\mu}m) and more sintered grains than Enceladus and Rhea, more likely covered by frost-like particles of few micrometers in average. The great sensitivity of polarization to grain size and degree of sintering makes it an ideal tool to detect hints of ongoing processes on icy planetary surfaces, such as cryovolcanism.Comment: 36 pages, 1 table, 11 figures, 2 data sets, accepted in Journal of Geophysical Research: Planet

Laboratory Experiments to Understand Comets

In order to understand the origin and evolution of comets, one must decipher the processes that formed and processed cometary ice and dust. Cometary materials have diverse physical and chemical properties and are mixed in various ways. Laboratory experiments are capable of producing simple to complex analogues of comet-like materials, measuring their properties, and simulating the processes by which their compositions and structures may evolve. The results of laboratory experiments are essential for the interpretations of comet observations and complement theoretical models. They are also necessary for planning future missions to comets. This chapter presents an overview of past and ongoing laboratory experiments exploring how comets were formed and transformed, from the nucleus interior and surface, to the coma. Throughout these sections, the pending questions are highlighted, and the perspectives and prospects for future experiments are discussed.Comment: 36 pages, 13 figures, Chapter accepted for publication on February 24th 2023, now in press for the book Comets III, edited by K. Meech, M. Combi, D. Bockelee-Morvan, S. Raymond and M. Zolensky, University of Arizona Pres

Experimental study of frost detectability on planetary surfaces using multicolor photometry and polarimetry

When the temperature and pressure conditions allow it, water ice can deposit as frost on the regolith of planetary surfaces. Frost is an important indicator of the surface physical conditions, and may trigger geological processes by its deposition and sublimation. This works aims to explore, experimentally, the possibility of detecting early stages of frost formation and to characterize its spectrophotometric and spectropolarimetric signatures in visible reflected light. We deposit ice on top of different regolith simulants, measuring the dust temperature, the thickness, and the morphology of the frost through a microscope, while measuring the reflected light at phase angles of 50° and 61°, and the linear polarization at phase angles of 5° and 16°, at three different wavelengths (450, 550, and 750 nm). We show that both the spectral slope (in particular between 450–550 nm), and the difference of polarization between 450 and 750 nm are efficient methods to detect frost layers with thicknesses as low as 10 to 20 μm. Furthermore, we find that the linear polarization at 16° relates to the temperature of the regolith i.e. the type of the deposited ice crystalline structure

Geometric calibration of Colour and Stereo Surface Imaging System of ESA's Trace Gas Orbiter

There are many geometric calibration methods for "standard" cameras. These methods, however, cannot be used for the calibration of telescopes with large focal lengths and complex off-axis optics. Moreover, specialized calibration methods for the telescopes are scarce in literature. We describe the calibration method that we developed for the Colour and Stereo Surface Imaging System (CaSSIS) telescope, on board of the ExoMars Trace Gas Orbiter (TGO). Although our method is described in the context of CaSSIS, with camera-specific experiments, it is general and can be applied to other telescopes. We further encourage re-use of the proposed method by making our calibration code and data available on-line.Comment: Submitted to Advances in Space Researc

VIS spectroscopy of NaCl-water ice mixtures irradiated with 1 and 5 keV electrons under Europa_s conditions: Formation of colour centres and Na colloids

Recent laboratory efforts and telescopic observations of Europa have shown the relevance of a yellow colouration of sodium chloride (NaCl) caused by crystal defects generated by irradiation. We further investigate this process by irradiating (with energetic electrons) different types of analogues where NaCl is associated in different ways to water ice. We produce two types of icy analogues: compact slabs and granular particles where we investigate two particle sizes (5 and 70 $\mu$m). We perform electron irradiation at cryogenic temperatures (100 K) and under high vacuum (10-7 mbar) conditions, with energies of 1 and 5 keV. We observe the formation of two different types of colour centres. The so-called F-centres (460 nm) were formed in every sample, but the intensity of the absorption band within the compact slabs surpassed any other icy analogues and was comparable to the intensity of the absorption band within pure NaCl grains. M-centres (720 nm) have not been detected at the surface of Europa so far, and were close to the detection limit during our irradiation of compact slabs. The slabs could be good analogues for Europa_s surface as they produce mainly F-centres. Other notable differences have been observed between compact slabs and granular samples, such as the presence of an absorption band at 580 nm attributed to colloids of Na, exclusively within granular samples. Such absorptions have not been reported in previous studies.Comment: 57 pages, 13 figure

Gas-solid carbonation as a possible source of carbonates in cold planetary environments

International audienceCarbonates are abundant sedimentary minerals at the surface and sub-surface of the Earth and they have been proposed as tracers of liquid water in extraterrestrial environments. Their formation mechanism is since generally associated with aqueous alteration processes. Recently, carbonate minerals have been discovered on Mars' surface by different orbital or rover missions. In particular, the phoenix mission has measured from 1 to 5% of calcium carbonate (calcite type) within the soil (Smith P.H. et al., 2009). These occurrences have been reported in area were the relative humidity is significantly high (Boynton et al., 2009). The small concentration of carbonates suggests an alternative process on mineral grain surfaces (as suggested by Shaheen et al., 2010) than carbonation in aqueous conditions. Such an observation could rather point toward a possible formation mechanism by dust-gas reaction under current Martian conditions. To understand the mechanism of carbonate formation under conditions relevant to current Martian atmosphere and surface, we designed an experimental setup consisting of an infrared microscope coupled to a cryogenic reaction cell (IR-CryoCell setup). Three different mineral precursors of carbonates (Ca and Mg hydroxides, and a hydrated Ca silicate formed from Ca2SiO4), low temperature (from -10 to +30°C), and reduced CO2 pressure (from 100 to 2000 mbar) were utilized to investigate the mechanism of gas-solid carbonation at mineral surfaces. These mineral materials are crucial precursors to form Ca and Mg carbonates in humid environments (0 < relative humidity < 100%) at dust-CO2 or dust-water ice-CO2 interfaces. Our results reveal a significant and fast carbonation process for Ca hydroxide and hydrated Ca silicate. Conversely, only a moderate carbonation is observed for the Mg hydroxide. These results suggest that gas-solid carbonation process or carbonate formation at the dust-water ice-CO2 interfaces could be a currently active Mars' surface process. To the best of our knowledge, we report for the first time that calcium carbonate can be formed at a negative temperature (-10°C) via gas-solid carbonation of Ca hydroxide. We note that the carbonation process at low temperature (<0°C) described in the present study could also have important implications on the dust-water ice-CO2 interactions in cold terrestrial environments (e.g. Antarctic)

Absolute calibration of the Colour and Stereo Surface Imaging System (CaSSIS)

The Colour and Stereo Surface Imaging System (CaSSIS) onboard the ExoMars Trace Gas Orbiter (TGO) has observed several standard stars and Jupiter with the aim of deriving the absolute calibration factors that convert observed signal into reflectance (”I over F”). The targets were observed through all four colour filters and hence a relative calibration between the filters was possible. In addition, observations of Phobos acquired during the TGO capture orbits in 2016 were analysed in support of the relative colour calibration. The results show that CaSSIS was around 13% less sensitive than predicted (normalised to the RED filter) - a value that is larger than the formal uncertainty and therefore significant. The relative colour calibration shows CaSSIS to be 4% more sensitive in the BLU than modelled which is close to the relative uncertainty. The text describes the analysis approach and methods for the various targets and newly recommended conversion factors are provided in table form

Seasonal southern circum-polar spots and araneiforms observed with the colour and stereo surface imaging system (CaSSIS)

The southern polar area of Mars is home to various seasonal activity commonly explained by the Kieffer model. During southern spring, the ice covering the polar area sublimates and leaves distinct features (spiders, spots, fans) observable from orbit. The Colour and Stereo Surface Imaging System (CaSSIS) onboard the ExoMars Trace Gas Orbiter (TGO), provides high-resolution multi-filter images of the Martian surface offering high sensitivity to colour contrasts. Its stereo capability is pivotal for momentary processes and offers a unique perspective for studying surface sublimation processes and their relation to atmospheric features. For the first time, we identify clouds well correlated with surface features (araneiforms and spots at southern circum-polar latitudes) hence motivating a new campaign to refine these observations over time periods where CO2 sublimation processes occur. We focus here on the structure of spot deposits and their evolution through time. We identify and describe seven structures: dark spot, bright-haloed spot, ringed spot, inverted spot, dark-haloed spot, banded spot, and bright spot. By morphological and spectral analyses, we hypothesize a new chronology of events that characterise the origin, formation and evolution of these features

Spectropolarimetry of life: airborne measurements from a hot air balloon

Does life exist outside our Solar System? A first step towards searching for life outside our Solar System is detecting life on Earth by using remote sensing applications. One powerful and unambiguous biosignature is the circular polarization resulting from the homochirality of biotic molecules and systems. We aim to investigate the possibility of identifying and characterizing life on Earth by using airborne spectropolarimetric observations from a hot air balloon during our field campaign in Switzerland, May 2022. In this work we present the optical-setup and the data obtained from aerial circular spectropolarimetric measurements of farmland, forests, lakes and urban sites. We make use of the well-calibrated FlyPol instrument that measures the fractionally induced circular polarization ($V/I$) of (reflected) light with a sensitivity of $<10^{-4}$. The instrument operates in the visible spectrum, ranging from 400 to 900 nm. We demonstrate the possibility to distinguish biotic from abiotic features using circular polarization spectra and additional broadband linear polarization information. We review the performance of our optical-setup and discuss potential improvements. This sets the requirements on how to perform future airborne spectropolarimetric measurements of the Earth's surface features from several elevations.Comment: 13 pages, 10 figures, to be submitted in SPIE Proceedings 12214-

A snapshot full-Stokes spectropolarimeter for detecting life on Earth

We present the design of a point-and-shoot non-imaging full-Stokes spectropolarimeter dedicated to detecting life on Earth from an orbiting platform like the ISS. We specifically aim to map circular polarization in the spectral features of chlorophyll and other biopigments for our planet as a whole. These non-zero circular polarization signatures are caused by homochirality of the molecular and supramolecular configurations of organic matter, and are considered the most unambiguous biomarker. To achieve a fully solid-state snapshot design, we implement a novel spatial modulation that completely separates the circular and linear polarization channels. The polarization modulator consists of a patterned liquid-crystal quarter-wave plate inside the spectrograph slit, which also constitutes the first optical element of the instrument. This configuration eliminates cross-talk between linear and circular polarization, which is crucial because linear polarization signals are generally much stronger than the circular polarization signals. This leads to a quite unorthodox optical concept for the spectrograph, in which the object and the pupil are switched. We discuss the general design requirements and trade-offs of LSDpol (Life Signature Detection polarimeter), a prototype instrument that is currently under development