Low-temperature hyper-spectral acquisitions of slabs with water ice and Martian simulant MGS-1.

Introduction:

Several space missions have confirmed the presence of ice in our Solar System, including on the surface and subsurface of Mars. The North Polar Cap on Mars shows stratified scarps made of water ice with a minor content of inclusions. These stratified sequences constitute the North Polar Layered Deposits (NPLD). Inclusions vary in terms of compositions - such as lithic materials and dry ice - and in terms of quantity because of climate changes due to astronomical parameters variations [1]. Variations among polar layers are so evident that they can be detected from orbital instruments, such as the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the NASA Mars Reconnaissance Orbiter (MRO). This space spectrometer operates in the VNIR-SWIR range (400-4000 nm), with a spectral sampling of 6.55 nm/channel and a spatial resolution of 18.4 m/px, enabling the detection of the surface composition on Mars [2].

Our project aims to produce synthetic icy analogs with similar spectral characteristics to the uppermost part of the North Polar Cap of Mars. Spectra of these analogs will be comparable to the North Polar Layered Deposits (NPLD) spectra collected by CRISM to better understand the composition of the dust inclusions into the polar layers.

Methodology:

 Martian Simulants

After a complete characterization of three commercially available Martian simulants (Figure 1) [3], which are Mars Global High-Fidelity Martian Dirt Simulant (MGS-1) [4], Mojave Mars Simulant (MMS-1) and Enhanced Mars Simulant (MMS-2) [5], we selected the most suitable simulant for our project. Indeed, the MGS-1 simulant, in particular its finest component (0-32 µm), fit pretty well the spectrum of the atmospheric dust, that could be entrapped in the North Polar Layered Deposits [6]. Moreover, the 0-32 µm grainsize reflects the grainsize that the Martian wind could raise and maintain in atmosphere [7].  

Figure 1. Three Martian commercially available simulants analyzed in this work.

Laboratory set-up:

We used the Headwall Photonics Nano Hyperspec VNIR imaging camera and the Micro Hyperspec SWIR imaging camera and their accommodation stage. The accommodations stage was modified to allow spectral acquisition of icy sample at low temperatures: cooling system for the sample-holder, thermocouple, glove-box filled with nitrogen to prevent the water condensation over the samples (Figure 2).

Figure 2. Hyperspectral camera and its modified stage to acquire spectra of icy slabs.

Icy slabs

Mixtures containing different quantities of the finest grains of MGS-1 and deionized water were frozen in narrow slabs to prevent the separation of the two components. The freezing was performed using the followed two methods:

• at -80°C simulating the summer temperature at the Martian North Pole [8] to achieve a heterogeneous distribution of the dust into the ice (slow-cooling slab);
• instantaneously in liquid nitrogen for a homogeneous distribution (fast-cooling slab).

We acquired hyperspectral data using the set-up previously described, varying not only the dust content into the icy slabs but also the sample temperature during the acquisitions (Figure 3).

Figure 3. Example of a slab slowly cooled at 193K with 25% dust.

Preliminary results:

Variations in dust amount.

Both slow-cooling and fast-cooling slabs display absorption bands at 500 nm due to the iron charge transfer and at 1500 and 2000 nm associated with water ice. Increasing the inclusion percentage in the mixtures resulted in a deepening of the 500 nm band and a weakening of 1500 and 2000 nm bands.

Figure 4. VNIR and SWIR spectra of fast-cooling slabs varying the dust content.

Variations in temperature.

Considering that surface temperatures on the North Polar Cap varies from 148 K to 203 K, in winter and in summer respectively [8], we performed experiments within this range.

The major spectral features keep their positions unchanged in both typologies of slabs. We recorded a general upward shift of the whole reflectance and the weakening of the absorption band at 1650  nm, with the increasing of the sample temperature. 

Figure 5. SWIR spectra of slow-cooling slab with 25% dust, varying the sample temperature.

Variations due to different cooling methods.

The spectra of the fast-cooling slabs have more marked spectral features in the whole wavelength range than the slow-cooling slabs spectra. This is probably due to the different procedures of sample preparation and cooling, that cause different crystal grainsize.

Figure 6. SWIR spectra of fast-cooling and slow-cooling slabs with 15% dust at -90°C.

Conclusions:

The laboratory set-up presented in this work enables the imaging hyper-spectral acquisition of icy slabs at low temperatures. Moreover, icy slabs are probably more representative than granular ice of the exposed compact ice along the walls of the Martian North Polar Layered Deposits, as well as of the icy crust of small bodies in the outer Solar System. Additionally, the icy slabs allow us to incorporate up to 35% dust into the ice whereas granular ice preparation can not exceed 5% of dust content.

Finally, we are now improving the laboratory set-up with the building of a cryo-genic cell, which allows us to reach even lower temperature and have a better control of the temperature and atmospheric environment during the experiments.

References:

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