Geophysical Research Letters
Supporting Information for
VIS-NIR reflectance of water ice/regolith analogue mixtures and implications for
the detectability of ice mixed within planetary regoliths.
Zuriñe Yoldi1,*, Antoine Pommerol1, Bernhard Jost1, Olivier Poch1,2, Julien Gouman1 and
Nicolas Thomas1
1Physikalisches
2Center
Institut, Universität Bern, Bern, Switzerland.
for Space and Habitability, Bern, Switzerland.
Contents of this file
Text S1 to S2
Figures S1 to S2
Introduction
This supporting information provides details about our laboratory procedures.
S1. Sample Preparation
Both fine and coarse-grained ice particles were produced using the SPIPA (Setup
for the Preparation of Icy Planetary Analogues) in the LOSSy laboratory at the University
of Bern (Pommerol et al., 2011, 2015).
Fine-grained ice was produced from ultrapure distilled water using a large
ultrasonic nebulizer. The water is nebulized in form of water droplets –ranging from 0.47
to 6 µm-, which are conducted through a plastic pipe inside a freezer. The extremity of
this pipe is held a few centimeters above a stainless steel vessel, which is in direct
contact with a piece of copper plunging into liquid nitrogen. As the water droplets exit the
pipe, they freeze instantaneously on contact with the air at 213K, and then sediment into
the vessel. The temperature of the bottom of the vessel is stable around 150K, which
limits the thermal metamorphism. Observations of fresh ice particles under a cryo-SEM
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(scanning electron microscope) reveal the almost perfectly spherical shape of the
particles and their smooth surface. Quantitative analyses of cryo-SEM images of the
samples lead to the determination of the size distribution of the samples. A mean
diameter of 4.54 µm was derived, with a standard deviation of 2.54 µm, regarding the
size distribution, and a volume moment mean of 9.45µm.
Coarse-grained ice is produced using another type of nebulizer. In this case, the
distilled water is held in a spray bottle, connected with a 2 mm diameter tube to the
ultrasonic device, placed inside the freezer. The liquid is pumped into the sonotrode and
spreads out as a thin film on the nozzle surface. Because of the ultrasonic vibrations, the
water film is disintegrated into micro-droplets. Those droplets fall directly into a stainless
steel vessel filled with liquid nitrogen and freeze instantly. As for the fine-grained ice,
cryo-SEM images were analyzed to derive the size distribution of the particles. The
mean diameter is 67µm with a standard deviation of 31µm. The particles are perfectly
spherical with smooth surfaces.
We have checked using cryo-SEM pictures (Fig. 1), the final diameter of the
droplets produced by these nebulizers. With the help of an imaging software we have
measured the diameter of the ice particles and computed the average result.
S2. Measurements
S2.1 Choice of wavelength
As explained in the paper, there are two major advantages of working with a 750
nm wavelength: the better SNR of the instrument and the shorter time needed to make
the measurements. In order to compare our results with 1064 nm reflectance results, we
have studied the reflectance of our two end-members at both wavelengths.
In the case of JSC1-AF, its REFF shows almost no difference when measured at
both wavelengths (Fig S1). This response is expected for relatively dark samples for
which shadow hiding dominates the opposition effect, whereas coherent backscattering which introduces a dependence of the opposition peak with wavelength-, is negligible.
Nevertheless, pure water ice shows coherent backscattering. In addition, ice shows an
absorption band centered around 1.1µm. Fig S1 shows the increase of absorption at
1064 nm in the case of coarse-grained ice. The mean difference of reflectance between
both wavelengths is 15%. Nevertheless, as already stated by Hapke (1993): “the fine
particles can have an effect [on the reflectance] all out of proportion to their mass
fraction”. We see indeed a significant darkening of the sample with only a few wt% of
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JSC-1AF in water ice. This means that as soon as JSC-1AF is added into the ice, the
effect of coherent backscattering is lowered, and consequently, the wavelength
dependence.
S2.2 Calibration
The standard calibration procedure for the PHIRE-2 instrument is detailed by
Pommerol et al., 2011. It consists essentially in a normalization of the raw voltage signal
measured with the sample by the raw voltage signal measured with a white Spectralon
reference (99% hemispheric reflectance). However, since the installation of a beam
splitter head on the instrument to permit BDRF measurements at low and null phase
angles, two other steps of calibration are necessary to calibrate reflectance
measurement acquired at phase angles lower than 10°; a geometric configuration in
which the beam is transmitted/reflected two times through the beam splitter.
First, the replacement of the mirror by a beam splitter implies a loss of the energy
reaching the detector, by a factor of approximately two (the average beam splitting ratio
of the beam splitter, which is slightly wavelength-dependent). The calibration of all data
acquired with the beam splitter is performed by comparison with the measurements
acquired with the mirror head. This mirror head blocks the incident beam for phase
angles lower than 3°. The width of the beam splitter is such that it affects the incident
beam for phase angles lower than 5°. This leaves a range of 2° over which the
measurements performed with both heads can be compared, allowing us to calibrate the
effects of the beam splitter on the measured signal.
Second, a small fraction of the incident light that reaches the beam splitter is
scattered in all directions and part of it can reach the detector fiber. Although the fraction
of the incident light scattered on the optical surface is very minor, the proximity of the
beam splitter to the detector fiber result in a non-negligible contribution of this straylight
to the measured signal. We characterize the total straylight by acquiring reflectance
measurements of a specular neutral density filter in a non-specular geometry. In this
configuration, the amount of light reflected by the filter is not measureable and the entire
signal consists of straylight. The signal values measured at this step are then
substracted from all signal values measured with a sample.
Although these two calibration steps should in theory be performed only once, we
have observed variations of the calibration constants derived from both steps over time.
We were able to link the variation of these calibration constants to the slow deposition of
dust on the upper surface of the beam splitter. Therefore, in order to mitigate these
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effects, calibration constants have been systematically re-measured every day, after the
acquisition of the measurements with samples.
As explained by Pommerol et al. (2013), the application of the reciprocity principle
(i.e. permuting the positions of the light source and the detector should not change the
value if reflectance measured) to PHIRE-2 measurements can be used to derive
accurate estimates of the measurements relative uncertainties. By performing
permutations for a large number of combinations of incidence and emission angle and
systematically comparing the results, we derive relative uncertainties of the order of 2%.
S2.3 Choice of the Incidence
As explained in the article, the PHIRE-2 instrument permits measurements at
normal incidence, but measuring at low but non-zero incidence results in more accurate
measurements because of the higher mechanical accuracy of the goniometer. Fig. S2
shows both measurements at 0° and 20° incidence angles, indicating that reflectance
differences are minor, so that both results can be compared.
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Figure S1. REFF of JSC1-AF (diamonds) and coarse grained ice (triangles) at 750
(grey) and 1064 (yellow) nm. No REFF differences are found for the basalt, and a mean
of 15% of difference is computed for the ice. For 750 nm two measurements have been
averaged. For 1064 nm ten measurements have been averaged, and even thought a
degraded SNR is observed. Relative errors are estimated to ~2%.
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Figure S2. REFF for the same sample of JSC-1AF and fine grained ice at 0° (left) and
20° (right) incidence angle at 750 nm. Relative uncertainties of ~2%.
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