Experimental Investigation of Oxidation of Pyroxene and Basalt: Implications for Spectroscopic Analyses of the Surface of Venus and the Ages of Lava Flows

Alteration of basalt on the surface of Venus should be dominated by nanophase hematite and sulfate coatings, but the timescale of oxidation and how that affects spectroscopic measurements has not been extensively studied. The oxidation rate and the effect on reflectance spectroscopy are needed to constrain the age of lava flows with high emissivity that were previously measured by the European Space Agency's Venus Express Mission. Here we investigate the effects of oxidation on the mineralogy and visible to near-infrared (VNIR) spectroscopy of augite, diopside, pyroxenite, and an alkali basalt. The experimental and spectroscopic data are used to constrain the surface basaltic weathering rates for Venus. Further, we use the spectroscopic measurements to constrain how quickly igneous signatures would degrade during oxidation of the surface. Our results show that basalt and olivine oxidize within weeks to months on the surface of Venus and their VNIR results are dominated by hematite. Pyroxenes take longer to alter with minor hematite forming on the surface—largely in cracks. Instead, Fe 3+ is predominantly forming in the crystal structure. Our work suggests that if basalt on the surface of Venus contains olivine and/or glass, the high emissivity lavas previously measured would be only a few years old. If, instead, those high emissivity lavas are fully crystalline and contain no olivine or glass (which is unlikely), they would still only be decades to hundreds of years old. Therefore, this is consistent with recent work suggesting that Venus is volcanically active today.