Laser and ion irradiations of Ysson basalt to simulate space weathering on Phobos and Deimos

Introduction :  

The upcoming MMX (Martian Moon eXploration) mission [1] will carry the MIRS instrument (MMX Infrared Spectrometer, [2]) designed to investigate the surface composition of Mars’ moons, Phobos and Deimos. As airless bodies, these moons are subject to various space environment-induced alteration processes collectively referred to as space weathering. Space weathering encompasses a range of phenomena - including micrometeorite impacts and irradiation by solar wind and cosmic ray particles - that modify the optical, chemical, and mineralogical properties of surface materials. Gaining insight into how these processes alter the spectral signatures of airless bodies is essential not only for understanding the long-term evolution of these objects but also for accurately interpreting compositional data obtained through remote sensing. In this study, to support future MIRS observations, we investigate in the laboratory the effects of both micrometeorite bombardment and irradiation by charged particles on a basalt sample, analogous to the expected surface composition of Phobos and Deimos.

 

Sample :

The sample used is an unweathered fine-grained basalt from a lava flow of Pic d’Ysson (Massif Central, France). Originally used as a lunar simulant analog, this fresh tholeiitic basalt is fully crystallized and contains some phenocrysts of olivine (forsterite), pyroxene (diopside), and plagioclase (anorthite) along with magnetite [3]. It is slightly aqueously altered, as shown by its only ~1% water content [4]. The largest grains are composite polycrystalline regular blocks with a rough aspect and some visible shiny facets of phenocrysts. The basaltic rock Ysson was crushed and sieved to limit the fraction of the powder to grain sizes below 250 μm, and finally compacted to obtain several indurated pressed pellets (10 to 13 mm diameter).

 

Laser irradiation :

To simulate micrometeorite impacts, we carried out nanosecond pulse-laser experiments on a first pellet of Ysson basalt using the Laser-Induced Breakdown Spectroscopy (LIBS) suite of the SuperCam instrument spare [5, 6, 7] in Toulouse, France. We expect that the impact velocity of micrometeorites on the surface of Phobos can range from 8.5 to 15 km.s⁻¹ [8]. Micrometeoroid impact at ∼10 km.s⁻¹ of dust (1 μm in size) delivers an energy equivalent to 32 J.cm⁻², which is roughly equivalent to the energy provided by one laser shot with SuperCam (14mJ over 0.25mm²). The samples are located in a vacuum chamber, enabling to reach a pressure of around 10⁻³ mbar and approach the low oxygen fugacity conditions at the surface of the martian moons. Two grids of 10x10 points of one and three laser shots were realized. After laser irradiation, the sample was characterized using an Agilent micro-spectrometer equipped with a Focal Plan Array (128x128 pixels) detector installed at the SMIS beamline of the SOLEIL synchrotron in France [9], to study mid-IR spectral properties (2.5-12.5 μm). The projected pixel size on the focal plane was 5.5 μm and the field of view was 700 µm .

 

Ion irradiation :

We also irradiated an Ysson basalt pellet with ions to simulate other space weathering components. We used the INGMAR vacuum chamber allowing to monitor in situ the Visible to Near-InfraRed (VisNIR from 0.5 to 4 μm) reflectance spectroscopy during ion implantation on the SIDONIE ion separator (JANNuS-SCALP platform of IJCLab, Orsay). A first pellet of Ysson basalt was irradiated with He⁺ at 20 keV and ion flux of 10¹³ ions/(s.cm²) to simulate solar wind as in previous experiments within our team [10,11]. Several steps of irradiation were performed to reach a final ion fluence of 1.10¹⁷ ions/cm². A second pellet was irradiated with O⁺ at 30 keV (with same flux and fluencies as He⁺) to simulate implanted particles at the surface of Phobos that previously escaped from Mars’ atmosphere [12, 13] and may contribute to spectral modifications with time. The same mid-IR characterization at SOLEIL than for the lasered pellet was also performed.

 

Results and conclusion :  

Preliminary analysis of the data indicates that laser irradiation of the Ysson basalt (Figure 1) results in an overall decrease in reflectance within the 2.5–12.5 μm spectral range, along with a significant change in the Christiansen and Reststrahlen bands. Ion irradiation similarly causes a decrease in reflectance in the visible to near-infrared (VNIR) wavelength range. Further analyses are currently underway, including VNIR micro-spectroscopy on laser-irradiated samples. The resulting datasets will be compared to assess and distinguish the effects of these two space weathering processes within overlapping spectral domains, and will be presented during the meeting.

Figure 1 : Stereo microscope image of the first sample irradiated with a nanosecond pulse-laser (1 and 3 shots). The image on the right shows a zoomed-in version of the 3-shot grid (red square).

 

References :

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