TP8

1 Introduction
The current understanding of space-weathering is that spectral changes of space-weathered airless planetary bodies are mainly caused by the gradual formation of submicroscopic-sized iron particles (smFe⁰) in the upper layer of the regolith. The resulting spectral changes include the darkening and reddening of the spectrum as well as the obstruction of diagnostic features [1,2,3]. Previous research also indicates that very small particles (nanophase iron <10 nm) mainly account for reddening and larger iron particles cause spectral darkening without reddening [3]. This theory has been adopted in a variety of studies such as [3,4,5]. However, this interpretation does not come without issues. The estimated sizes of large particles may deviate from the experimental particle size [3]. Recently, Denevi et al. [6] suggested that agglutinates might be the main cause for substantial reddening even though they are abundant in large iron particles. We add another aspect to the discussion: Previous mathematical treatments of small iron particles implicitly assumed that the distances between particles are large and no interactions occur. However, TEM imagery of lunar soil and laser irradiated samples [1] suggests that cases exist in which the iron particles are close together and form layers or clusters. We apply advanced light scattering theory [7] to simulate clusters and layers of very small iron particles with varying geometries and densities. We find that interparticle interactions between nanophase iron particles can alter the spectral slope and counteract the reddening effect.

2 Methods
To investigate the spectral behaviour of nanophase iron particle layers or clusters, we carry out three steps.
(1) First, we define our baseline. We utilise Mie scattering to simulate the spectral effects that non-interacting smFe⁰ (radius = 5nm) has on a fresh lunar soil spectrum as described by [4]. A fresh highland spectrum from calibrated data of the Moon-Mineralogy-Mapper (M³) [https://pds-imaging.jpl.nasa.gov/volumes/m3.html] is converted to single scattering albedo via the Hapke model [8]. Mie modelling yields the single scattering albedo w, the phase function p(g) and the extinction/scattering efficiencies Q_ext and Q_sca of smFe⁰ [9]. We use the optical constants of iron from [10]. The soil albedo and the iron albedo as well as the phase functions are combined via Hapke’s mixing equation [8]. The resulting albedo w_{soil, smFe0} is subsequently fed into the Hapke model to generate computationally space-weathered soil spectra.
(2) Secondly, we assume that not all iron particles are present in the form of single particles but many of them are somewhat closely packed and form layers. Now, we simulate these layers. The particle radii are drawn from a normal distribution with μ = 22nm and σ = 10nm and populate a box with dimensions 700nm x 700nm x 100nm (figure 1b). The dimensions of the box represent a small layer of particles that accumulate at the grain rim. The depth (100nm) is largely inspired by TEM imagery that suggest depths of roughly 10–100nm [1]. We utilise the T-matrix method [7] using the CELES framework [11] and compute the interaction between adjacent particles. Again, the resulting quantities (albedo, phase function, efficiencies) are fed into the Hapke model to simulate a computationally weathered spectrum.
(3) Finally, we compare the results of steps (1) and (2).

Figure 1a: TEM image of small iron particles (small dots) occurring in the melt and vapor phases of the soil grain (image from [1]).

Figure 1b: Artificially created particle geometry with varying radius. The size is also colour coded with red particles being the largest, blue the smallest, and other sizes in between.

3 Results
To evaluate the influence of layered smFe⁰, we chose a fresh highland spectrum near crater Krasovskiy [4], see figure 2a (blue solid line). The spectrum is red-sloped, has no strong features and a significant hydroxyl/water absorption band above 2.8μm. We simulate the spectral behaviour of single isolated iron particles according to (1) and obtain a spectrum with considerable reddening and some darkening (red line in figure 2a). If a mixture of single non-interacting particles and interacting layered particles is included, the spectral slope becomes flatter in the NIR region, i.e., exhibits less reddening as seen in figure 2a (black line). For the sake of comparison, we tuned the abundances of the components such that the weathered spectra have similar reflectance values near 2.5μm.

Figure 2a: Simulated spectra from the fresh spectrum (blue) using non-interacting particles (red) and layered particles (black)

Figure 2b: Normalised spectra at 1μm

4 Conclusion
In figure 2a and 2b one can clearly see that interacting particles organised in layers counteract the reddening effect. Nanophase iron alone can thus mimic the behaviour of microphase iron which has previously been believed to be solely responsible for this effect [3,4]. This results in the following consequences: The distinction between nanophase iron and microphase iron become less clear. Not only the particle size matters but also the geometric alignment and the inter-particle interactions are important if the particles are somewhat dense. To arrive at a comprehensive understanding of space weathering and its spectral characteristics, the geometry, density, and distribution and interaction of smFe⁰ should be taken into consideration. Further simulation campaigns are necessary to characterise the dependency of the spectral behaviour on these geometric parameters.

[1] Carle M. Pieters and Sarah K. Noble, 2016, JGR Planets, 121(10):1865–1884
[2] Bruce Hapke, 2001, JGR Planets, 106(E5):10039–10073
[3] Paul G.Lucey and Miriam A.Riner, 2011, Icarus, 212(2):451–462
[4] Kay S. Wohlfarth, et al., 2019, The Astronomical Journal, 158(2):80
[5] Antti Penttilä, et al., 2020, Icarus 345:113727
[6] Brett W. Denevi, et al., 2021, LPSC2021
[7] Michael I.Mishchenko, 2006, Cambridge University Press
[8] Bruce Hapke, 2012, Cambridge University Press
[9] Christian Wöhler, et al., 2019, Science Advances, 3(9):1701286
[10] M. R. Querry, 1985, Optical constants
[11] Amos Egel, et al., 2017, eprint.

How to cite: Arnaut, M., Wohlfarth, K., and Wöhler, C.: The interaction between multiple nanophase iron particles changes the slope of lunar reflectance spectra, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-770, https://doi.org/10.5194/epsc2021-770, 2021.

We present a multi-scale light-scattering model that is capable of simulating the reflectance spectra of a regolith layer. In particular, the model can be applied to a case where the regolith grains have varying amounts of nanophase inclusions due to space weathering of the material. As different simulation tools are employed for different size scales of the target geometry (roughly, nano-, micro-, and millimeter scales), the particle size effects, the surface reflections, and the volume scattering can all be properly accounted for. Our results with olivine grains and nanophase iron inclusions verify the role of the nanoinclusions in the reflectance spectra of space-weathered materials. Together with the simulation results, we give simplified explanations for the space-weathering effects based on light scattering, namely the decrease of albedo, the general increase of the red spectral slope, and the dampening of the spectral bands. We also consider the so-called ultraviolet bluing effect and show how the change in the spectral slope over the ultraviolet-visual wavelengths is due to the decrease of reflectance in the visual wavelengths rather than the increase of reflectance in the ultraviolet part.

How to cite: Penttilä, A., Väisänen, T., Markkanen, J., Martikainen, J., Kohout, T., Videen, G., and Muinonen, K.: Rigorous light-scattering simulations of space-weathering effects on reflectance spectra, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-642, https://doi.org/10.5194/epsc2021-642, 2021.

Introduction:

Atmosphereless bodies of the Solar System present surface evolution under the harsh conditions of space weathering. Their regolith are exposed to thermal cycling, cosmic and solar rays irradiation, solar wind sputtering and micrometeorites bombardment and vaporization. With time these materials accumulate optically active opaque particles such as nanophase metallic iron particles on the surface or rims of dust grains or larger iron particles. The density increase in iron particles modifies the spectral properties of the material with a lowering of the particles albedo, and of the absorption bands amplitude generally associated with a global reddening of the spectral slope [1, 2].

Previous studies have shown that nanophase iron particles inducing weathering spectral properties will accumulate at the surface of olivine and pyroxene minerals under heating conditions simulating micrometeorite impacts [3]. The skin depth of weathered material depends on the weathering process and minerals.

Context for Phobos studies:

The moons of Mars, Phobos and Deimos, are some of the most enigmatic small bodies of the Solar System. Indeed, it is still debated in the community whether they originate from a large impact on Mars [4, 5], or if they are captured primitive asteroids [6]. The overall spectra of Phobos and Deimos are very red with no obvious diagnostic spectral features readily detected in the visible or near-infrared, similar to very primitive small bodies of the solar system. However, significant variations in the spectral slope indicates inhomogeneities of the material on the surface of Phobos. Comparison with analogues such as Tagish Lake meteorite and basalts suggests a probable mixture of asteroidal primitive material with basaltic materials such as the one originating from the nearby surface of Mars [7]. 

The Mars Moons Explorer (MMX) mission should attempt to bring back samples of the surface of Phobos to Earth in order to decipher the origin and dynamics of the Martian Moons system [8]. The MMX InfraRed Spectrometer (MIRS) instrument on-board the mission is an infrared imaging spectrometer in the range 0.9 to 3.6 μm that will extensively study the spectral properties of the surface of Phobos and provide context for the sample collection region.  

Method:

In this context, we are preparing laboratory weathered analogues samples by submitting basalt (such as the Pic d’Ysson basalt already used for laboratory studies on the lunar regolith optical and spectrophotometric behavior [9]) and meteorite samples to near melting temperature (1000-1200°C) in an atmosphere controlled oven under reducing conditions (partial pressure of CO/CO₂ of 96% corresponding to an oxygen fugacity of about 10^-17) mimicking the space environment alteration effects for different exposure times (Fig. 1). We then explore the process of development and formation of submicroscopic metallic Fe, also referred to as nanophase iron particles (npFe⁰) [10] on the surfaces of the weathered samples.

Results:

A first comparison between unaltered Pic d’Ysson basalt grains and grains subjected to 4 hours heating at 1100°C under reducing conditions (fugacity of 10^-17) is presented in Figure 2. The unaltered basalt grain present typical crystals of olivine, feldspar and ilmenite on the left. On the right, the onset of heat alteration is clearly seen over a depth of about 30 microns, with the melting of crystals and the apparition of nanophase metallic iron phases (presence of small white grains on the surface and inside the crystals).

Conclusion:

Laboratory simulations by heating samples under reducing conditions can recreate the conditions under which regolith may be weathered in space with surface alteration of grains and apparition of nanophase iron on the altered depth. The effect of the changes on the grain spectral properties depending on the experimental conditions will be further studied for applications to airless bodies in the Solar System. The samples will be analyzed with SEM at IRAP and their reflectance spectra acquired with the SHADOWS instrument at IPAG [12] for future comparisons with the spectra of Phobos and Deimos.

Fig. 1: The atmosphere controlled oven at IRAP for heating under reducing conditions [11].

Fig. 2: First results of the SEM analysis showing the onset of heat diffusion and iron metal formation on the surface of the basaltic grains. left: crystalline basalt grain unaltered showing the presence of olivine, feldspar crystals and ilmenite. right: onset of alteration with apparition of nanophase iron over a 30 micrometer depth after 4 hours heating at 1100°C under reducing conditions.

Acknowledgements

This study was funded by CNES.

References:

[1] Hapke, B. (2001) JGR, 106(E5), 10039-10073. [2] Pieters, C.M., & Noble, S.K. (2016) JGR, 121(10), 1865-1884. [3] Weber, I., et al. (2020) EPSL, 530, 115884. [4] Canup, R., & Salmon, J. (2018) Science advances, 4(4), eaar6887. [5] Bagheri, A., et al. (2021) Nature Astronomy, 1-5. [6] Hansen, B. M. (2018) MNRAS, 475(2), 2452-2466. [7] Glotch, T. D., et al. (2018) JGR: Planets, 123(10), 2467-2484. [8] Kuramoto, K., et al. (2021) Earth, Planets and Space. Submitted. [9] Souchon, A.L., et al. (2011) Icarus, 215(1), 313-331. [10] Pieters, C.M., et al. (2000) Meteor. And Planet. Sci., 35, 1101 [11] Toplis, M.J., & Corgne, A. (2002) Contributions to Mineralogy and Petrology, 144(1), 22-37. [12] Potin, S., et al. (2018) Applied optics, 57(28), 8279-8296.

How to cite: Lasue, J., Pinet, P., Toplis, M., Beck, P., Cao, L., and Munsch, P.: Regolith weathering through heating samples under reducing conditions for Phobos surface studies, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-172, https://doi.org/10.5194/epsc2021-172, 2021.

Introduction: Solar wind ions and impacts of micrometeoroids are the leading processes that weather the surface of airless planetary bodies in the solar system. As a result, key diagnostic features of their spectra get altered. The most prominent changes in the silicate-rich bodies in the visible (VIS) and near-infrared (NIR) wavelengths are increase in the spectral slope, reduction of the albedo, and subduction of the mineral absorption bands (see, for example, Hapke 2001).

Our work aims at understanding what are the similarities and differences between the effect of the solar wind ions and micrometeoroid impacts on the final spectra of the silicate-rich bodies. Our study is based on laboratory simulations using planetary analogue materials.

Methods: We have used two different terrestrial minerals, olivine and pyroxene. Each material was ground and dry-sieved to sizes smaller than 106 μm. Subsequently, we created pressed pellets from potassium bromide, which served as a base, and 100 mg of the mineral, which created the top layer of the pellet.

Ion irradiations were conducted at two different laboratories. Hydrogen irradiations proceeded at the Accelerator Laboratory of the University of Helsinki, using 5 keV ions with varying fluences from 10¹⁴ to 10¹⁸ ions/cm². Subsequent spectral measurements were done out of the vacuum chamber after surface passivation of the samples. Helium and argon irradiations were done using the INGMAR set-up (IAS-CSNSM, Orsay) with 20 and 40 keV ions and with fluences from 10¹⁵ to 10¹⁷ ions/cm². Spectra were measured in the vacuum chamber, where we irradiated the samples.

Individual 100-fs laser pulses were shot into a square grid on the pellets’ surface to simulate the micrometeoroid impacts (as in Fazio et al. 2018). Various densities of the pulses per cm² simulated different weathering stages. Spectral measurements were done outside of the vacuum chamber.

The spectral measurements covered, in all the set-ups, wavelengths from 0.54 to 13 μm, i.e. VIS to mid-infrared wavelengths. After the measurements, we evaluated the evolution of the spectral parameters estimated using the Modified Gaussian Model (Sunshine et al. 1990, 1999).

Results: Variation of the spectra in the VIS range was similar for H⁺- and laser-irradiated samples, but we have identified a difference in the NIR wavelength range. Laser irradiation caused greater changes in NIR than any of the ions we used, see Fig. 1. The reason for such difference in behaviour may be the different penetration depth of the irradiating ions and laser pulses. While laser penetrates approximately 100 μm under the surface of the pellet, our ions did not penetrate deeper than 150 nm. Spectra of the laser-irradiated samples thus bear information solely from the irradiated material, while spectra of the ion-irradiated samples are a mixture of the top-most altered layers and the unaltered underlying layers. The relative contribution of the irradiated material is then smaller in the ion case.

Otherwise, we found that the original mineralogy of the pellet is more determinative to the evolution of the spectral parameters than the space weathering agent (ions or laser pulses). While olivine and pyroxene showed albedo variations of a similar order, the evolution of pyroxene’s spectral slope was negligible when compared to olivine, see Fig. 2.