Infrared imaging of Venus from IRTF/ProtoCAM observations in 1991

Infrared observations of Venusʹ night-side between 3 and 5 μm provide a valuable means to study the upper cloud structure (at approximately 68 km) and the thermal structure above the clouds. New observations between 3.67 and 5.08 μm, concerning spectral images of Venus obtained in October 1991 with the ProtoCAM/IRTF and a Circular Variable Filter, are presented. A cloud particle scale height for the upper clouds of 3.9±1 km is retrieved from limb darkening measurements, which is in good agreement with measurements from both space probes and Earth-based observatories. The observations show an increase in temperature from the centre towards the poles when sounding altitudes above 72 km. This, along with temperatures at cloud top levels, implies isothermal profiles between 68 and 74 km at high latitudes.