In the Lab with Venus: High-temperature emissivity spectroscopy for the interpretation of NIR data from Venus

Introduction: Venus  presents today a harsh environment characterized by extreme surface temperatures and pressures (approximately 740K and 95 bar, respectively), with an atmosphere predominantly composed of CO₂ and N₂ , along with small quantities of other gases, including sulfur compounds (such as SO₂ and H₂SO₄). The permanent cloud cover of Venus prohibits observation of the surface with traditional imaging techniques. Fortunately, Venus’ CO₂ atmosphere is transparent in small spectral windows near 1 μm allowing insights into the planet’s surface composition [1, 2]. Within the next decade, the ESA EnVision, and the NASA VERITAS and DAVINCI missions to Venus will carry instruments focused on the 1 μm spectral region to enhance our understanding of the planet's surface composition, geological history, and evolution. The Venus Emissivity Mapper (VEM) on VERITAS and VenSpec-M on EnVision [3], will play pivotal roles in mapping the surface of Venus, acquiring crucial orbital data in the NIR range. The Venus Descent Imager (VenDI) on DAVINCI, will acquire close images of the surface from beneath the clouds around 1 μm paired with emission measurements in the NIR from VISOR (Venus Imaging System for Observational Reconnaissance) [4].

Spectral laboratory work: Observations of the surface of Venus from orbit through the 1-μm windows can only be obtained on the nightside because reflected sunlight on the dayside hides any surface contribution [5]. Therefore, spectra of analog materials must be measured as emissivity, which is very challenging. In particular, thermal emission at these wavelengths is very low because it lies far below the main flank of the Planck curve for Venus surface temperatures, where the signal drops more than two orders of magnitude between 1.18 and 0.8 μm. In addition, dayside surface measurements performed by VenDI will need hemispherical reflectance measurements. To correctly interpret remote sensing data from Venus’ surface in the NIR range, laboratory measurements, replicating Venus' temperature, pressure, and chemical conditions, are essential.

Venus at PSL: At the Planetary Spectroscopy Laboratory (PSL) of the German Aerospace Center (DLR) in Berlin, we are operating a unique facility: it allows acquisition of spectra from solid and powdered materials, in air/vacuum, from low to very high T (-200° to 1000°C), over an extended spectral range (0.3 to > 100 µm) in a climate-controlled room. Currently, PSL operates three FTIR Spectrometers, featuring internal and external chambers for comprehensive spectral analyses. Emissivity, biconical, hemispherical reflectance, transmittance and Micro-FTIR reflectance measurements can all be performed.
A high-temperature emissivity setup (the Venus emissivity chamber, Figure 1), attached to a spectrometer allows routine measurements of NIR emissivity spectra of Venus' analogues at relevant Venus surface temperatures (400°C, 440°C, and 480°C) in vacuum (0.7 mbar) [6, 7]. Samples are heated in custom-made cups using a powerful induction system. 

Ongoing investigations: At PSL, emissivity measurements on Venus' analogs are routinely performed with the main goal of creating increasingly complex spectral libraries for the interpretation and calibration of NIR data from Venus. The experiments investigate the emissivity response of basalt vs felsic rocks, but  also of intermediate igneous compositions and mineral phases, mixtures of minerals and rock types, and potential Venus weathering products. Rock samples included in the calibration database are selected to represent a spread of compositions on the total alkali vs. silica (TAS) diagram for volcanic rocks and with a range of rock textures and grain sizes. Ongoing investigations include analysis of spectral mixtures; emissivity of weathered vs non-weathered samples; analysis on field campaign samples both on the field and in the laboratory [8, 9].

Spectral mixing effects: Using NIR emissivity spectra of mixtures at Venus' condition, we are investigating the best approaches for modeling this spectral region [10]. Measurements on mineral mixtures are used to test and simulate alteration of basalt in the Venus' atmosphere, using glass and mineral combinations that might occur within a weathered basalt, and to characterize intimate combinations of materials that might be found in the Venus regolith. Endmember minerals have been mixed with a tholeiitic basalt from Húsavík, Iceland [9] in 50% - 50% areal and intimate mixtures. The areal mixture measurements used custom-designed divided ceramic sample cups with each half containing material from one endmember. Preliminary results on the 50% - 50% mixtures show that as expected linear unmixing (LMM) [11] does not reproduce the NIR laboratory emissivity spectral behavior for some of the mixtures.

Weathering effects: Assessing how the rocks on the Venus' surface might chemically alter or weather due to surface-atmosphere interactions is crucial for interpreting data from the future missions to Venus. Joint work between PSL and the Hot Environments Laboratory (HEL) at NASA Goddard Space Flight Center (GSFC) is comparing the emissivity response of weathered vs non- weathered Venus’ surface analogs in the VenSpec-M/VEM and VenDI spectral range. For the weathering experiments, the samples are individually enclosed in the Small Venus Chamber (Lil’ VICI) of HEL (Figure 1) and heated to the average Venus surface temperature and pressure (740K & 95 bar). The samples are exposed to those conditions under a Venus simulated atmosphere (96% CO₂, 4%N₂, and 155ppm SO₂) for a minimum of 100hrs.

Conclusions: Measurements described here are part of the VenSpec-M/VEM data calibration and validation plan, in support of the creation of several increasingly complex spectral libraries of Venus emissivity analog measurements [12]. These will be used to train machine learning algorithms for the interpretation and validation of laboratory and remote-sensed data.

References: [1] Allen D. A. et al., (1984) Nature 307, 222–224 (1984). [2] Pollack J. B. et al. (1993) Icarus 103, 1–42. [3] Helbert J. et al. 2024, this meeting. [4] Garvin et al. (2024), LPSC. [5] Mueller N. et al. (2008), J. Geophys. Res. 113. [6] Helbert et al. (2023), LPSC, Abstract #1679. [7] Maturilli at al. (2023), LPSC, Abstract #2120. [8] Adeli S. et al., 2024, this meeting [9] Garland S. et al., 2024, this meeting; [10] Dyar et al. (2023) Geological Society of America, Vol. 55, No.#6 [11] Dalton J.B. (2009), JGR, Vol. 34. [12] Alemanno G. et al. (2023), SPIE, vol. 12686, doi: 10.1117/12.2678683.