Laboratory Investigation of Venus Surface Conditions on Analogue Samples for the Interpretation of Venus Orbital Data

Plain Language Summary

Studying the surface of Venus is a challenge due to its high temperatures of approximately 460 °C, an atmospheric pressure of 93 bar, and its thick atmosphere composed mainly of carbon dioxide, nitrogen, and small amounts of corrosive compounds such as sulfur dioxide. These conditions make orbital remote sensing imaging techniques difficult. However, a few narrow spectral windows in the near-infrared, around 1 μm, allow the detection of signals coming from the surface. Future missions, including VERITAS, EnVision, and DAVINCI, will rely on these signals to study Venus’ surface emissivity and retrieve surface composition. To interpret these data, laboratory investigations that simulate the conditions of Venus’ surface are needed. This study examines two sets of mineral and rock samples: a first set weathered at the NASA Goddard Space Flight Center (GSFC) Hot Environments Laboratory (HEL), and a second set weathered at the NASA Glenn Extreme Environments Rig (GEER). All samples, fresh and altered, were then analyzed at the Planetary Spectroscopy Laboratory (PSL) at DLR for near-infrared reflectance and high-temperature emissivity. The microscopical and chemical characterization of the samples was performed at the Museum für Naturkunde (MfN, Berlin) and the University of Münster.

Introduction

The rocks on the surface of Venus are subject to a temperature and pressure of approximately 460 °C and 93 bar, respectively [1]. They are also exposed to a thick atmosphere composed mainly of CO₂ and N_2, with trace C-O-H-S species including SO₂, among other corrosive compounds [2]. Studying the surface composition of Venus is challenging due to the high spectral opacity of its atmosphere at almost all wavelengths [3]. However, in the near-infrared (NIR) range, around 1 μm, the atmosphere presents a few spectrally transparent windows allowing us to gain insights into the planet’s surface composition, geological history, and evolution [4,5]. Instruments like the Venus Emissivity Mapper (VEM) on VERITAS, VenSpec-M on EnVision, and the VISOR camera onboard DAVINCI will acquire data of Venus around the 1 μm region. The interpretation of these data requires emissivity measurements of Venus analogs acquired under Venus-like conditions [6,7].

Weathering is the main exchange mechanism between the solid surface of Venus and its atmosphere [8]. However, how, and to which extent, weathering might affect the spectroscopic signatures of surface rocks and minerals is still not well understood [9,10,11], limiting the knowledge and interpretation of the data. Laboratory measurements of samples under Venus’ surface conditions, including its temperature, pressure, and atmospheric composition, are therefore necessary for the accurate interpretation of the data obtained by remote sensing. The Planetary Spectroscopy Laboratory (PSL) at DLR is periodically collecting NIR spectra of experimentally weathered samples [12,13] to prepare for the interpretation of the remote sensing data from the VenSpec-M and VEM instruments.

Samples and Measurements

We present the spectral analysis of two main groups of altered samples. The first group includes a slab and grains (250–500 μm) of a biotite sample, which were exposed to simulated Venusian atmospheric conditions at the NASA Goddard Space Flight Center (GSFC) in the Small Venus Chamber (Lil’VICI) at the Hot Environments Laboratory (HEL). The second group consists of five samples, three basalts, a granodiorite, and the mineral aragonite from the collection presented in [14], comprising the fresh samples and their respective duplicate slabs weathered at the NASA Glenn Extreme Environments Rig (GEER) under Venus’ surface conditions for ten days. Both sample groups underwent the same suite of laboratory analyses at PSL. These include measurements of the spectra using Fourier Transform Infrared Spectroscopy (FTIR) for hemispherical reflectance at ambient temperature across the near-infrared spectral range and emissivity measurements at high temperatures simulating Venus’ surface temperatures (440–480°C). Chemical analyses were performed using a scanning electron microscope (SEM) and micro X-ray fluorescence (μXRF) spectrometer at the Museum für Naturkunde (MfN, Berlin). Additional SEM measurements were performed at the Institut für Mineralogie of the University of Münster. This work is part of a broader effort to compare results from samples that have been experimentally weathered in different facilities and assess how varying experimental conditions may influence emissivity and alteration processes.

References

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[14] Longo, A. Z. (2024). MSc thesis, University of North Carolina.