The Effects of Transmission on Laboratory Emissivity Spectra – Preparation for Future ESA and NASA Missions to Venus

In the 2030s, several major missions are expected to take place to investigate the surface and interior of Venus including NASA’s VERITAS and ESA’s EnVision missions. While Venus is considered Earth’s twin and is our closest planetary neighbour in the Solar System, little is known about its surface composition. Due to its thick, permanent cloud cover, it was thought that only lander missions would be able to provide insights into the surface composition [1]. However, narrow atmospheric windows between 0.86 – 1.18µm in the CO2 cloud cover allow for the observation of its surface composition [1]. VERITAS’ VEM and Envision’s VenSpec-M will take advantage of these windows in order to provide the first near global surface composition maps of Venus, with the ability to distinguish between felsic and mafic rock types [1].

In preparation for these missions, extensive work is being carried out on the spectroscopy of Venus analogues in the DLR’s Planetary Spectroscopy Lab (PSL). The PSL is equipped with an emissivity chamber capable of analysing emissivity spectra under Venus temperatures (~460 °C) and near-vacuum conditions (~0.7 mbar), allowing analogue samples to be studied under similar conditions to Venus’s surface [2]. The particulate nature of these samples means that their grain structure and porosity may allow the measured emissivity to be influenced not just by the surface, but also by material at depth within the sample. In order to better understand the emissivity measurements being taken in the laboratory it is beneficial to constrain the potential effects that transmission may have on derived emissivity spectra.

The measurement of transmission through particulate material is particularly difficult, due to the inherent mechanical instability of the samples [3] and the extreme sensitivity of the signal to sample thickness [3]. Conventional transmission measurements of particulate materials are typically performed using KBr pellets or by vacuum dispersion and electrostatic spraying techniques [4]. However, these methods alter the original morphology of the material, including its porosity and grain structure, which may significantly influence its optical behaviour.

To preserve the natural morphology and porosity of the samples, a purpose-built transmission cell for use within a Bruker Vertex 80v FTIR spectrometer has been developed. The cell allows particulate samples to be measured while maintaining their original structure and enables sample thicknesses to be varied between 0.5–5 mm, in increments of 0.5 mm.

Basalt and granite, representing the possible mafic and felsic compositional extremes of the surface of Venus [5], were ground and sieved into grain size fractions of <25, 25-63, 63-125 and 125-250µm. Transmission measurements were then takento investigate the effects of grain size, compositional mixing, and porosity. Comparisons between grain-size fractions within each rock type constrained grain-size effects, while mixtures of basaltic and granitic material were used to investigate compositional and morphological effects. Mixtures containing varying grain-size distributions were analysed to assess the role of porosity.

Hemispherical reflectance measurements in the near-infrared were also acquired for each sample under vacuum conditions using a modified gold-coated integrating hemisphere attached to a Bruker Vertex 80v spectrometer. Emissivity measurements will subsequently be carried out at the PSL under Venus surface temperature conditions and near-vacuum pressures. Finally, bulk porosity was estimated from surface porosity measurements derived using 3D imaging and height classification with a Keyence VHX-7100 digital microscope, enabling the effects of porosity in transmission and emissivity to be assessed.

The ultimate goal of this work is to derive an empirical relationship describing transmission as a function of depth within particulate materials, allowing the potential influence of subsurface transmission on emissivity measurements to be assessed. Initial results indicate that in pure quartz particulate samples (125-250µm), transmission in the NIR region occurred through sample depths up to 3mm. While quartz itself is highly transparent in the NIR region, these results suggest that transmission through the sample may significantly influence the measured emissivity signal in quartz samples, indicating that material at depth may contribute to laboratory emissivity measurements.

 

[1] Smreker et al 2022. VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy): A Discovery Mission

[2] Maturilli et al 2019. The newly improved set-up at the Planetary Spectroscopy Laboratory (PSL)

[3] Gladimir et al 2019. Assessing the impact of porosity variations on the reflectance and transmittance of natural sands

[4] Kun 1993. Infrared-optical transmission and reflection measurements on loose powders

[5] Gilmore 2017. Venus Surface Composition Constrained by Observation and Experiment.