Near-infrared Reflectance of Rocks at high Temperature: Preliminary Results and Implications for Near-infrared Emissivity of Venus's Surface

Light emitted from Venus’s surface can be viewed through spectral “windows” in its atmosphere, in the nearinfrared
(NIR) around 1000 nanometer (nm) wavelengths. The NIR emissivity of Venus’s surface can constrain
rock types and their weathering state; emissivities can be measured directly or calculated from reflectances. We
measured the reflectances of igneous and sedimentary rocks at Venus’s surface temperature, 400 °C–500 °C at 850
and 950 nm; samples were heated in a box furnace in air, illuminated by light-emitting diodes (LEDs), and imaged
with a modified charge-coupled device (CCD) camera. Reflectances were also measured at 25 °C from 350 to
1400 nm. Rock reflectances at 850 and 950 nm and 400 °C–500 °C are nearly identical to those at 25 °C, except for
the effects of nanophase hematite forming on some surfaces. Fresh basalts have reflectances (high and low
temperatures) near 7.5%; a leucogranite similarly has reflectances near 50%. Pigmentary hematite has nearly
identical reflectances at high- and low-temperature at these wavelengths. Pigmentary hematite appears dark brown
400 °C–500 °C because its absorption edge has shifted to beyond the limit of human vision. These rock
reflectances imply that basalts should have emissivities near 0.9, and granite (and similar felsic rocks) should have
lower emissivities ∼0.5. Thus, basalt and felsic rock should be easily distinguished in NIR emissivity
measurements of Venus’s surface, such as are baselined in recent Venus mission proposals. Other sedimentary
rocks should have even lower emissivities: quartz sand at around ∼0.3, and anhydrite as low as 0.1.