Long viewed as geologically dormant, evidence from Galileo and Cassini flybys and the Venus Express orbiter’s nighttime imagery in the 1.02-µm CO2 window shows that volcanic activity on Venus has occurred in geologically recent times (Smrekar et al, Science 328, 605-608, 2010). However, from above the clouds, the spatial resolution of the thermal 1-µm surface glow was restricted to >50 km due to intense scattering of the upwelling surface radiation by clouds: disappointingly poor to resolve features that could reveal how volcanic and other geologic processes work on the planet.
This is a compelling science driver for sending a balloon-borne “aerobot” to the sub-cloud region of Venus to obtain significantly higher spatial resolution, unhindered by cloud scattering. As it drifts along in the prevailing winds, long swaths—up to a hemisphere in extent during each 2.5-day nighttime pass—of surface spectral images could be obtained. All 5 of the surface-detecting CO2 windows between 0.85 and 1.2 µm could be used to constrain surface composition (eg, Baines et al, Icarus 148, 307-311, 2000).
However, observations from below the clouds still have to deal with imaging impediments from Rayleigh-scattering in Venus’ dense lower atmosphere. This motivates a radiative transfer investigation of surface image degradation for sensors located just below the clouds (47 km alt.) in the 0.85–1.2 µm range.
We use two complementary methods for quantifying image degradation by the intervening scattering medium in several windows between absorption bands. First, we compute the contrast ratio between the level of light coming straight from surface to sensor and the level of background light from multiple scatterings and reflections after surface emission. Here, Venus is special in that clouds above are far more reflective than the surface below. Clouds thus enhance significantly the ambient light, hence reduce contrast. The other detrimental effect of atmospheric scattering is blurring: gradual degradation of effective resolution of the sensor-atmosphere system with increasing (optical) distance from the scene. Here too, Venus’ below-cloud atmosphere is dominated by near-isotropic Rayleigh scattering rather than the familiar forward-peaked scattering for hazes on Earth.
Our analytical and numerical modeling confirms that key to high-resolution imaging is the light directly transmitted from surface to sensor. Everything else is a near-uniform background. Because the direct is not swamped by the background in the near-IR, features down to the camera pixel-scale (~10s of m) may be detectable after atmospheric correction.