442 research outputs found
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
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
Surface charging of thick porous water ice layers relevant for ion sputtering experiments
We use a laboratory facility to study the sputtering properties of
centimeter-thick porous water ice subjected to the bombardment of ions and
electrons to better understand the formation of exospheres of the icy moons of
Jupiter. Our ice samples are as similar as possible to the expected moon
surfaces but surface charging of the samples during ion irradiation may distort
the experimental results. We therefore monitor the time scales for charging and
dis- charging of the samples when subjected to a beam of ions. These
experiments allow us to derive an electric conductivity of deep porous ice
layers. The results imply that electron irradiation and sputtering play a
non-negligible role for certain plasma conditions at the icy moons of Jupiter.
The observed ion sputtering yields from our ice samples are similar to previous
experiments where compact ice films were sputtered off a micro-balance.Comment: arXiv admin note: text overlap with arXiv:1509.0400
Near-infrared reflectance spectroscopy of sublimating salty ice analogues. Implications for icy moons
The composition of the surface of the Galilean icy moons has been debated since the Galileo mission. Several chemistries have been proposed to explain the composition of the non-icy component of the moon's surfaces, notably, sulphuric acid hydrates and magnesium and sodium sulphates. More recently, magnesium and sodium chlorides have been proposed to explain features observed in ground-based observations. We have considered four salts (NaCl, Na2SO4, MgSO4 and MgCl2) with various concentrations, to produce salty ice analogues. Granular particles were produced by a flash-freezing procedure. Additionally, compact slabs of salty ices were produced by a slow crystallisation of salty liquid solution. These two methods mimic the end-members (plumes and slow ice block formation) for producing hydrated salty ices on the surface of icy moons such as Europa and Ganymede. We have monitored the near-infrared (NIR) evolution of our salty ices during sublimation, revealing differences between the granular particles and the slabs. The slabs formed a higher amount of hydrates and the most highly hydrated compounds. Granular ices must be formed from a more concentrated salty solution to increase the amount of hydrates within the ice particles. The sublimation of salty ices removed all excess water ice efficiently, but the dehydration of the salts was not observed. The final spectra of the slabs were most flattened around 1.5 and 2.0 ÎĽm, especially for the Na2SO4, MgCl2 and MgSO4, suggesting the presence of stable, highly hydrated compounds. We find that Na2SO4, MgCl2 and MgSO4 are most compatible with the non-icy component at the surface of the icy moons as observed by the NIMS instrument on Galileo and by ground-based observations
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
The evaluation of French major highway projects : recommendations based on American experience
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1995.Includes bibliographical references (p. 104-106).by Alexis de Pommerol.M.S
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 ÎĽm. 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
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 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
- …