THE MINERALOGY OF VENUS AND OUTSTANDING QUESTIONS: CURRENT VOLCANISM, PAST WATER, AND SURFACE WEATHERING

Is there recent volcanism? How recent? Does Venus have felsic, continent-like crust? What surface weathering reactions occur and how do they shape atmospheric chemistry? The mineralogy of Venus holds the key to these questions.

There have been no in-situ measurements of mineralogy on Venus. Decades ago the Soviet Union put a several landers on the surface that measured major elements, U, and Th, establishing a basaltic composition at least locally, with Earth-like concentrations of radiogenic material. RGB camera images suggest the presence of hematite.

Remote sensing of the surface is very challenging due to the dense atmosphere. The CO2 absorption window near 1 micron allowed the Galileo NIMS and Venus Express VIRTIS spectrometers to see the surface. Modeling of the VIRTIS surface brightness temperature for the southern hemisphere at 1.02 micron yield emissivity (after cloud and instrument corrections) anomalies both above and below the typical background value. The high emissivity anomalies are located on volcanic features interpreted as overlying mantle plumes based on the broad topographic highs and positive gravity anomalies. The emissivity difference between the background plains and these volcanic flows is consistent with the weathered and unweathered basalt. Collectively these observations suggest quite recent volcanism has occurred over mantle plumes features (or hotspots). The large low emissivity anomaly occurs at the one highland plateau in the southern hemisphere. This anomaly is consistent with the proposed felsic composition of these highly deformed plateaus (Glimore et al. 2016), called ‘tesserae’. However the data reduction relies on removing surface temperature as a function of elevation; elevation in the very rough tesserae has an uncertainty comparable to height difference needed to produce the low emissivity anomaly. The tesserae are compensated by crustal roots, and are relatively old, like Earth’s continents. On Earth, such massive quantities of felsic crust only occur when basalt melts in the presence of water; thus tesserae may have formed during an earlier, wetter time period.

Optimized remote sensing of surface emissivity in all 5 surface windows near 1 micron, along with laboratory spectra acquired at Venus T, would allow for definitive answers to these questions.