Abstract The surface of Venus is in contact with a hot (~470 °C), high pressure (92 bars), and caustic (CO2 with S, but little H2O) atmosphere, which should cause progressive alteration of the crust in the form of sulfate and iron-oxide coatings; however, the exact rate of alteration and mineral species are not well constrained. Different experimental approaches, each with its own limitations, are currently being used to constrain mineralogy and alteration rates. One note is that no experimental approach has been able to fully replicate the necessary conditions and sustain them for a significant length of time. Furthermore, geochemical modeling studies can also constrain surface alteration mineralogy, again with different assumptions and limitations. Here, we review recent geochemical modeling and experimental studies to constrain the state of the art for alteration mineralogy, rate of alteration, open questions about the surface mineralogy of Venus, and what can be constrained before the fleet of missions arrives later this decade. Combining the new results confirms that basalt on the surface of Venus should react quickly and form coatings of sulfates and iron-oxides; however, the mineralogy and rate of alteration are dependent on physical properties of the protolith (including bulk composition, mineralogy, and crystallinity), as well as atmospheric composition, and surface temperature. Importantly, the geochemical modeling results show that the mineralogy is largely controlled by atmospheric oxygen fugacity, which is not well constrained for the near-surface environment on Venus. Therefore, alteration experiments run over a range of oxygen and sulfur fugacities are needed across a wide range of Venus analog materials with varying mineralogy and crystallinity.

In addition to being much smaller, they are composed primarily of rocks and metals. These, in turn, are made of elements that are less common in the universe as a whole. The most abundant rocks, called silicates, are made of silicon and oxygen, and the most common metal is iron. We can tell from their densities (see Table 7.2) that Mercury has the greatest proportion of metals (which are denser) and the Moon has the lowest. Earth, Venus, and Mars all have roughly similar bulk compositions: about one third of their mass consists of iron-nickel or iron-sulfur combinations; two thirds is made of silicates. Because these planets are largely composed of oxygen compounds (such as the silicate minerals of their crusts), their chemistry is said to be oxidized. When we look at the internal structure of each of the terrestrial planets, we find that the densest metals are in a central core, with the lighter silicates near the surface. If these planets were liquid, like the giant planets, we could understand this effect as the result the sinking of heavier elements due to the pull of gravity.

Venus’ surface can be viewed in emission through the relatively opaque atmosphere via a few spectral ‘windows’ in the near-infrared (NIR, most near 1 μm). Venus’ surface appears to show emissivities that correlate with surface geology, and these emissivity variations are interpreted as differences in surface rock type (mafic vs. silicic) and/or extent of weathering (Fe(2+) silicates vs. Fe(3+)-oxide-coated). To understand and quantify the observed variations in NIR emissivity, laboratories are measuring high-T NIR emissivity directly. For example, the measured emissivities of basalts in the wavelength range 0.85 – 1.2 μm are ~0.9. This value can be tested by measurement of reflectance, because Kirchoff’s Law holds that emissivity (e) = 1 – reflectance (r). The r of basalt in the NIR is ~0.1 so its e should be ~0.9. However, high-T NIR e’s of silicic igneous rocks (granitic, rhyolite) are reported to be 0.8-0.9, which is inconsistent with r values of 0.3-0.7 of such rocks at room-T. For both datasets to be correct, the r values of silicic igneous rock would have to decrease precipitously between room and Venus surface temperatures. This seems unlikely.