EMISSIVITY SPECTRA OF VENUS-ANALOG MIXTURES OF BASALT AND ALTERATION PRODUCTS

Although nonlinear and linear mixing properties of visible-near-infrared (VNIR) reflectance and mid-infrared emissivity spectra, respectively, are well understood at room temperature for other bodies in the Solar System, the mixing properties of high-temperature emissivity spectra are less clear and virtually untested. In particular, 440C VNIR emissivity spectra are needed to interpret remote sensing data on Venus, where the dense CO₂-rich cloud cover permits observations only for a narrow range from ~0.80-1.20 µm. We present here results on spectra of mixtures of likely Venus phases to improve our understanding of mixing properties at this wavelength and assist with interpretation of VEM data.

This project uses a tholeiitic basalt “sand” from Husavik, Iceland (57% glass, 24% feldspar, 11% pyroxene, 4% Fe-Ti oxides, 3% olivine) as the basis for all mixtures. It was mixed with four likely Venus alteration products: anhydrite (CaSO₄), thenardite (Na₂SO₄), arcanite (K₂SO₄), and hematite (Fe₂O₃) in varying combinations. Bi-directional and hemispherical reflectance spectra were collected at 295K from 0.7-2.63 mm using a Bruker Vertex 80V spectrometer at the Planetary Spectroscopy Laboratory (DLR Berlin). Emissivity spectra were collected in an attached external simulation chamber with a high efficiency induction system to heat samples; spectra were measured at 400°, 440°, and 480°C from 0.85-1.20 µm.

Mixtures were studied using one of two methods. First, intimate mixtures of ca. 250 µm grains of the end-members were studied. Alternatively, mixtures were created on a large scale using custom-designed ceramic cups partitioned so they contain up to four different phases. Spectra from the intimate versus macroscopic mixtures allow the mixing properties of these materials to be compared for reflectance and emissivity.

Results show that bi-directional spectra, which are limited to a single phase angle, are not equivalent to hemispherical reflectance data. Although features occur at roughly comparable wavelengths, slopes and intensities are vastly different. Hemispherical spectra are comparable to 1 minus emissivity for single materials but this relationship breaks down when light and dark materials are mixed at any scale. Additional experiments to understand these phenomena are underway.