| Autor: |
Antony, Tinu, Suresh Raju, C., Mohan, Nithin, Swarup, Govind, Oberoi, Divya, Krishna Moorthy, K. |
| Předmět: |
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| Zdroj: |
Journal of Geophysical Research. Planets; Jan2022, Vol. 127 Issue 1, p1-14, 14p |
| Abstrakt: |
Radiometric observations of Venus have revealed a monotonic decrease of brightness temperature (Tb) in the decimeter wavelength regime. Such a decrease has also been independently reported by the observations using interferometric radio‐telescopes, like the Very Large Array (VLA) in the USA and the Giant Metrewave Radio Telescope (GMRT) in India. In this work, we have carried out microwave radiative transfer (RT) simulations of thermal emission from the Venusian surface at decimeter wavelengths to examine the role of subsurface properties of Venusian regolith in the continuous reduction of Tb at the microwave‐radiowave spectral domain. These simulations are compared against spectral microwave measurements by GMRT over a wide decimeter wavelength regime ranging from ∼23–128 cm. Good agreements are obtained for simulations that consider a two‐layer Venusian surface (a low‐loss medium overlaid over a reflecting/lossy medium) a situation that would arise if the subsurface layer has high dielectric properties due to the presence of semiconducting mineral assemblages such as pyrites, ferroelectric minerals, magnetite–hematite, magnetite–pyrite or magnetite–hematite–pyrite equilibrium assemblages. The thickness of the top layer, the possibilities of the formation of such layers and their geological and chemical evolutions are also discussed. Plain Language Summary: Millimeter to decimeter radiometry measures thermal emission emanating from the planetary atmosphere, surface and subsurface. Radiometric observations of Venus showed a peculiar trend in the spectral variation of thermal emission (brightness temperature ‐Tb), which increases up to a wavelength of ∼6 cm and then decreases with increasing wavelength. The increase in thermal emission at shorter wavelengths (mm to cm), primarily contributed by the atmosphere, is well understood. However, a satisfactory explanation for the reduction of emission at the longer wavelengths in the decimeter regime is still lacking. We have provided an explanation to this problem by using radiative transfer (RT) simulations and Tb observations of Venus at decimeter wavelengths using the Giant Metrewave Radio Telescope (GMRT) located in Pune, India. Our studies suggest that Venus has a two‐layer surface structure with a low loss basaltic surface medium overlaid over a denser reflecting or semiconducting subsurface due to the presence of mineral assemblages, the rocks with iron‐rich mineralogical compounds formed by cooling and solidification of lava or magma. The evolution of the Venusian surface to the current state through volcanism, global resurfacing and related chemical weathering processes that support our argument are explained. Key Points: Investigated Venus' decreasing brightness with increasing decimeter wavelength using Radiative Transfer simulation and GMRT observationHypothesize possible existence of two‐layer surface, a layer of anhydrite‐basalt mixture overlaid on dense reflecting/conducting subsurfaceReflecting/conducting subsurface layer reduces Venus' Tb by reflecting back thermal radiations from deeper depths at decimeter wavelengths [ABSTRACT FROM AUTHOR] |
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