Emissivity from the surface of Venus: the effects of alteration products calcite and anhydrite

The Venera and Vega landers (active during the 1960’s-80’s) provided the first compositional data from the surface of Venus. These landers showed that the volcanic plains on Venus are comprised of alkaline and tholeiitic basalts, very similar in composition to terrestrial rocks (e.g., Kargel et al., 1993; Surkov et al., 1984, 1986) and it is believed that much of Venus’ surface (approximately 80%) constitutes similar material. However, emissivity measurements in the near-infrared (NIR) from the Galileo flyby of Venus and the Venus Express mission showed two distinct regions where emissivity changed significantly (Gilmore et al., 2015; Hashimoto et al., 2008). Hashimoto et al. (2008) proposed that the lower-emissivity regions must be from a low Fe felsic material, potentially a granite, given that the NIR is sensitive to Fe content. Laboratory emissivity measurements in the NIR performed by Helbert et al. (2021) confirmed that lower Fe rocks do indeed have a significantly lower emissivity at Venus temperatures (~460°C) compared to higher Fe basalts, though the cause of the differences is as-yet unconfirmed. Either the lower-emissivity rocks are a different igneous rock type, or the change results from the presence of alteration minerals from gas-rock interactions affecting the emissivity spectra of a basalt.

Numerous alteration experiments and thermodynamic modelling studies have been completed under a variety of Venus surface conditions (~460°C, 90 bars, CO₂ and SO₂ gas atmosphere) on basaltic rocks, glasses, and mafic minerals. These studies have shown that basalt mainly alters to carbonates, sulphates and Fe-oxides (e.g., Reid et al., 2024). Two of the more common minerals among these experiments are calcite (CaCO₃) and anhydrite (CaSO₄). The growth of these alteration minerals and layer thickness is dependent on cation diffusion rates, such that coatings on the basalt could help indicate relative age/freshness of a lava flow. The emissivity of these two minerals at room temperature is significantly lower than that of a basalt, therefore, a basalt with alteration minerals should result in a lower emissivity than a fresh basalt. This has been shown using radiative transfer models and mixing equations for basalt and anhydrite/hematite (Dyar et al., 2021), however, there is currently a lack of laboratory emissivity data for this process at Venus temperatures. Hence, as a first step toward understanding the spectroscopic characteristics of this alteration process, this study presents emissivity spectra of intimate mixtures of basalt and its alteration phases, calcite and anhydrite, when observed under Venus temperature conditions and compares them to spectra of unaltered basalt and felsic rocks.

Natural samples of an alkaline basalt, anhydrite and calcite, and a glass synthesised from the basalt were prepared with grain sizes of 250-300 μm and 300–350 μm, and mixed with either 50 wt% or 10 wt% basalt/basalt glass, with the remainder comprising anhydrite and/or calcite. All mixtures were prepared to weigh ~10 g and dried for ~24 hours at 110 °C to remove excess H₂O. Additionally, a disc from the alkaline basalt was cut to produce a slab with a 5 cm diameter. Hemispherical reflectance and emissivity measurements were completed at the Planetary Spectroscopy Laboratory (PSL) at DLR, Berlin, in the VNIR range. Emissivity measurements were collected under vacuum (~0.7 mbar) with samples heated to temperatures ranging ~380 – 500°C using an induction system. Hemispherical reflectance measurements in a gold-coated hemispherical unit were also collected under vacuum and were completed prior to and after heating. The emissivity of a graphite slab was also collected at temperatures above and below those measured for the samples. The emissivity was then calibrated using the hemispherical reflectance measurements for corrections and the graphite slab as a black body.

The results of this work show that mixtures of basalt with calcite and/or anhydrite have a lower emissivity than basalt. Additionally, the emissivity values of these mixtures are distinct from felsic rocks which have a lower emissivity (~0.6-0.7; cf.  Helbert et al., 2021) than the 50 wt% basalt mixtures.  The basalt glass samples have a higher emissivity than their basalt counterpart; this may be due to the presence of phenocrysts in the basalt, scattering differences, or oxidation of the glass. The emissivity of the mixtures shows similar patterns and values, which may make it difficult to distinguish proportions of calcite versus anhydrite.

The upcoming missions to Venus, including VERITAS (NASA) and EnVision (ESA), will be equipped with NIR spectrometers to measure the surface. The results of this work highlight the need for emissivity to be collected at Venus temperature on a variety of minerals and mineral mixtures such that the measurements from these missions can be accurately interpreted.

 

References:

Dyar, M.D., et al. (2021) Icarus 358, 114139.

Gilmore, M.S., et al. (2015) Icarus 254, 350–361.

Hashimoto, G.L., et al. (2008) JGR Planets 113, 2008JE003134.

Helbert, J., et al. (2021) Science Advances 7, eaba9428.

Kargel, J.S., et al. (1993) Icarus 103, 253–275.

Reid, R.B., et al. (2024) JGR Planets 129, e2024JE008485. h

Surkov, Y.A., et al. (1984). JGR Solid Earth 89, B393-B402.

Surkov, Y.A., (1986) JGR Solid Earth 91, E215–E218.