Grain size effects on the infrared spectrum of mineral mixtures with dark components: New laboratory experiments to interpret low-albedo rocky planetary surfaces

¹ LESIA – Observatoire de Paris, Université PSL, CNRS, Université Paris Cité, Sorbonne Université, 5 place Jules Janssen, 92190 Meudon, France
² INAF – Astrophysical Observatory of Arcetri, largo E. Fermi no. 5, 50125 Firenze, Italy
e-mail: giovanni.poggiali@inaf.it
³ Dipartimento di Fisica, Università degli Studi di Firenze, Via Sansone no. 1, 50019 Sesto Fiorentino (FI), Italy
⁴ INAF – Astronomical Observatory of Padova, Vicolo dell'Osservatorio no. 5, 35122 Padova, Italy
⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ The Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
⁷ Department of Geophysics, Tohoku University, Sendai Miyagi, Japan
⁸ Institut Universitaire de France (IUF), 1 rue Descartes, F-75231 Paris Cedex 05, France
⁹ LATMOS, CNRS, Sorbonne Université, Université Versailles St-Quentin, Guyancourt, France

Received: 8 August 2023
Accepted: 22 December 2023

Abstract

Context. A number of bodies in the Solar System are characterized by dark surfaces, from carbonaceous asteroids to the enigmatic surface of Phobos and Deimos. Our understanding of the spectroscopic behavior of low-albedo surfaces remains incomplete. To improve the interpretation of remote sensing data, laboratory studies continue to serve as a pivotal tool for unveiling the physical state and composition of such surfaces.

Aims. Several processes can be simulated in the laboratory, however, the preparation and analysis of a complex mixing of analog material is one of the most fundamental among them, while also being one of the most complex when multiple components are used. In this work, we aim to study how dark material mixed with basaltic material at different grain sizes can affect the spectroscopic features from the near- to mid- infrared (1.25–25 µm).

Methods. Our sample set includes four series of basaltic mix (feldspar and pyroxene) at different grain sizes from <50 µm to 1000 µm, mixed with amorphous carbon at increasing weight percentages ranging from 1% to 50%. We analyzed several features on the spectrum of each mineral mixture. In particular, we investigated the behavior of the: (i) near-infrared slope; (ii) 2.7 µm OH-stretching band; (iii) Christiansen features; and (iv) Reststrahlen band and Transparency feature.

Results. The measurements presented in this work, which take into account a large wavelength range for the first time, point toward a critical effect of dark material, but with a different outcomes for each grain size. Some of the most interesting results involve the slope trend of modification with dark material and the variant behavior of the Reststrahlen band and Transparency feature.

Conclusions. This dataset will offer a key support in the interpretation of data collected on dark surfaces by past and future space missions. This knowledge will be also important in the context of linking analyses of returned samples with remote sensing data collected on planetary surfaces.