Emissivity Measurements of Basalt, Calcite and Anhydrite mixtures to simulate weathered surface rocks on Venus

Abstract

The atmosphere of Venus is predominately composed of CO2, with minor amounts of N2, H2O and SO2 (e.g. Fegley 2014). This combination of gases, in conjunction with a high surface temperature (~460 °C), means that the weathering processes on Venus are very distinct from Earth. Information about the surface composition of Venus have been obtained thanks to the Venera and Vega missions (e.g. Surkov et al. 1986) which indicate that Venus is comprised of basaltic rocks very similar to terrestrial compositions (e.g. Treiman 2007). As such, numerous weathering experiments and modelling attempts of basalts under Venusian or near-Venusian atmospheric conditions have been conducted (see Filiberto and McCanta 2024) with multiple studies showing that calcite (CaCO3) and anhydrite (CaSO4) can form on the surface of basalts as a result of gas-rock interactions (e.g. Semprich et al. 2020, Reid et al. 2024; Renggli et al. 2020). The abundance of these minerals is dependent on time, thereby providing constraints on the timing of volcanic activity on Venus. Accurately identifying how recently Venus was volcanically active is important to understanding not only how Venus has evolved, but to put our own planet, the Earth, and Earth-like exoplanets in a global planetary context (e.g. Widemann et al. 2023). Analysing these weathering features and compositional differences on Venus will be achievable with the EnVision (ESA) and VERITAS (NASA) orbiters, which are set to launch in the 2030’s (Ghail et al. 2012; Smrekar et al. 2022). Both of these orbiters will be equipped with a suite of spectroscopic instruments (including VenSpec-M and the Venus Emissivity Mapper, respectively) that will be capable of analysing the surface within the near-infrared (NIR) spectral range between 0.86 and 1.18 μm (e.g. Smrekar et al. 2022). As such, the aim of this work is to understand how the proportions of calcite and anhydrite will affect the emissivity spectra of basalt under near-Venus surface conditions. This will be achieved through a series of mixtures that simulate the varying degrees of weathering observable on Venus by changing the total surface area coverage of each component. The Planetary Spectroscopy Laboratory (PSL, DLR in Berlin) is uniquely equipped with an emissivity chamber that can analyse emissivity spectra under Venus temperatures (~460 °C) and near-vacuum conditions (~0.7 mbar), thus allowing analogue samples to be studied under similar conditions that will be encountered by the EnVision and VERITAS orbiters.

Natural samples of tholeiitic basalt, anhydrite and calcite were crushed, manually or through the use of a Fritsch Disk Mill Pulverisette 13, and separated with a Retsch Vibratory Sieve Shaker AS 200 digit to a grain size of 300 – 350 μm at the Universität Münster; the bulk composition and purity of the natural samples was analysed with a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). Three pure end member mixtures were prepared for each natural sample along with two 1:1 mixtures of basalt with either calcite or anhydrite, and one mixture of 2:1:1 basalt, calcite and anhydrite (Figure 1); all mixtures were prepared to weigh ~10 g and dried for ~24 hours at 110 °C to remove excess H2O. Emissivity measurements were completed at PSL under near-vacuum conditions and at temperatures ranging ~400 – 480°C; samples were heated in custom-made ceramic sample holders using an induction system and the temperature was measured at the sample surface. Hemispherical reflectance measurements in the NIR were also collected at PSL under vacuum in a modified gold-coated hemispherical unit. These reflectance measurements were completed on the mixtures prior to and following heating as they are used in the calibration of the emissivity spectra.

Preliminary analysis of the measured spectra shows that the pure basalt emissivity measurements are distinguishable from mixtures of basalt with calcite, as the latter spectrum displays a downward slope; this suggests that identifying calcite on Venus using emissivity data is possible and could be used to imply if the source area has new or old lava flows. The emissivity spectra of mixtures of basalt with anhydrite, however, were less distinguishable from the pure basalt spectrum, suggesting it will be more difficult to identify the presence of anhydrite if it is not of significant quantity (i.e. more than 50% surface coverage).

Mineral spectral differences can be observed using emissivity measurements under Venus conditions. Future plans involve the acquisition of measurements on a larger range of suspected gas-rock reaction minerals, proportions of reaction minerals to surface rock, and with different surface rock compositions, in order to aide analysis for when the EnVision and VERITAS orbiters arrive at Venus.