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Aerobot Navigation in Venus’ Clouds: Can the Solar Disc be Tracked in Longer-Than-Visible Wavelengths?

Presentation #406.02 in the session Venus (Poster + Lightning Talk)

Published onOct 23, 2023
Aerobot Navigation in Venus’ Clouds: Can the Solar Disc be Tracked in Longer-Than-Visible Wavelengths?

There is considerable scientific interest in deploying an autonomous aerobot in Venus’ opaque clouds. The most tantalizing questions include: determination of cloud characteristics above the aerobot; absorption of solar radiation in the cloud layers; measurement of local concentration of the unknown UV absorber. However, there are considerable technical challenges to address before such a mission becomes feasible. The predominant issues are: - characterization of solar and thermal environments in Venus clouds, and their impact on balloon performance as well as gondola thermal control; - spectral analysis of solar radiation at depth in the clouds, and its impact on solar power generation; - demonstration of the feasibility of determining the solar azimuth in the clouds using IR measurements. In this study, we investigate that last question starting with cloud particle optics and radiative fluxes obtained by Knollenberg and Hunten (1980) and Tomasko et al. (1980), respectively from the Pioneer Venus cloud particle size spectrometer (LCPS) and solar flux radiometer. The ceiling for the aerobot is 62 km, but it is desirable for it to operate at much larger depths. We focus on an altitude of 56.7 km. The first task is to use the available particle size distributions (PSDs), which are invariably bimodal (~0.4 and ~2 um), on a layer-by-layer basis (down to 56.7 km) to drive Mie scattering computations at both the 0.6 um wavelength used by the LCPS and at the thermal IR wavelengths of interest. Two notional broadband imagers were considered: 8-13 and 5-50 um. The Mie code predicts the PSD-averaged scattering and absorption cross-sections, along with the scattering phase function, given the known complex refractive index of sulfuric acid across wavelengths. The 0.6 um optical properties are used to calibrate the optical depth profile in the thermal wavelengths based on the known visible data. To a first approximation, detectability of the solar disc depends on the contrast between the directly transmitted light and the diffuse background light caused by scattered sunlight as well as blackbody radiation emitted by the clouds (due to the non-negligible absorption in the thermal IR). The 2nd task is therefore to compute these directly and diffusely transmitted radiances along the slant path to the solar source. Our estimates show that there should be enough contrast for a basic image processing code to find the sun in the background thanks to the concentration of the rays into the solar disc. A rough estimate of the broadband absorption by the dominant gas (CO2) does not change that outcome.

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