Development of Near-IR Spectroscopic Techniques for Mineral Detection at the Surface of Venus, 15-R8128
Principal Investigators
Mark A. Bullock
Victoria Hamilton
Constantine Tsang
Erik Wilkinson
Inclusive Dates: 01/01/10 – 07/01/11
Background — The clouds and dense atmosphere of Venus prevent the kind of infrared remote sensing required to identify minerals at its surface. Furthermore, the high temperature and pressure of Venus' surface alters the interaction of visible and infrared light with mineral crystals. Therefore, the infrared signatures of common rock forming minerals are unknown under Venus surface conditions. Below the clouds, visible and infrared light is absorbed at most wavelengths by CO2 and H2O vapor absorption bands. In addition, scattering of light by atmospheric molecules is much more vigorous in Venus' atmosphere than in Earth's since Venus' is 60 times denser. This kind of scattering is highly dependent upon wavelength, so it is possible to calculate spectral regions that are optimal tradeoffs between atmospheric scattering and atmospheric transparency. In addition to reflecting sunlight during the day, the hot surface of Venus glows at near-infrared wavelengths. Calculations of the scattering and absorption of solar radiation, as well as the surface emission, are necessary to model the appearance of the surface within the best spectral windows. These calculations drive the design of a camera system optimized for acquiring images on descent through the atmosphere of Venus.
Approach — SwRI researchers adapted a computer package originally developed by the project manager for calculating the radiative energy balance of the Venus atmosphere. This 1-D climate model primarily calculated fluxes and heating rates in the Venus atmosphere. Researchers used the HITRAN and HITEMP spectral databases to calculate the line-by-line absorption coefficients for the nine known absorbing gases in Venus' atmosphere from 0.6 to 2.5 μm. Researchers then calculated the upward and downward radiation in the five most transparent bands at 0.65, 0.85, 1.02, 1.10, and 1.18 μm. This was done by solving the radiative transfer equation at 80 levels in the atmosphere, including absorption, scattering, and emission by both the atmosphere and surface. The theoretical results compared favorably with atmospheric in situ data from the Soviet Venera 13 and 14 spectrophotometers. In addition to the optical challenges, the exposure time constraints imposed by motion of a descent vehicle through the atmosphere of Venus were investigated.
Accomplishments — Even within the spectral windows, calculations show that light scattering by atmospheric molecules – Rayleigh scattering – swamps the signal from the surface at short wavelengths. Venus' sky is very bright, in part because light within the transparent windows is reflected several times between the surface and the bottom of the clouds. Just beneath the clouds, the sky is 200,000 times brighter than the surface at 0.65 μm. However, at 1.02 μm, the sky is only 38 times brighter than the surface. The exposure time is tightly constrained by descent vehicle motion. Venus descent probes have exhibited rotations as fast as 20 °/s. To ensure sharp images from a high-resolution descent camera, exposures must be shorter than 10 msec. To observe the Venus surface on descent, therefore, a long wavelength-enhanced fast CCD camera with a deep well depth and high dynamic range (24 bits) will be necessary.
