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Vertical Structure and Optical Properties of Titans Aerosols from Radiance Measurements Made Inside and Outside the Atmosphere
Author and Affiliation:
Doose, Lyn R.(Arizona Univ., Tucson, AZ, United States);
Karkoschka, Erich(Arizona Univ., Tucson, AZ, United States);
Tomasko, Martin G.(Arizona Univ., Tucson, AZ, United States);
Anderson, Carrie M.(NASA Goddard Space Flight Center, Greenbelt, MD, United States)
Abstract: Prompted by the detection of stratospheric cloud layers by Cassini's Composite Infrared Spectrometer (CIRS; see Anderson, C.M., Samuelson, R.E. [2011]. Icarus 212, 762-778), we have re-examined the observations made by the Descent Imager/Spectral Radiometer (DISR) in the atmosphere of Titan together with two constraints from measurements made outside the atmosphere. No evidence of thin layers (<1 km) in the DISR image data sets is seen beyond the three previously reported layers at 21 km, 11 km, and 7 km by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M.G. [2009]. Icarus 199, 442-448). On the other hand, there is evidence of a thicker layer centered at about 55 km. A rise in radiance gradients in the Downward-Looking Visible Spectrometer (DLVS) data below 55 km indicates an increase in the volume extinction coefficient near this altitude. To fit the geometric albedo measured from outside the atmosphere the decrease in the single scattering albedo of Titan's aerosols at high altitudes, noted in earlier studies of DISR data, must continue to much higher altitudes. The altitude of Titan's limb as a function of wavelength requires that the scale height of the aerosols decrease with altitude from the 65 km value seen in the DISR observations below 140 km to the 45 km value at higher altitudes. We compared the variation of radiance with nadir angle observed in the DISR images to improve our aerosol model. Our new aerosol model fits the altitude and wavelength variations of the observations at small and intermediate nadir angles but not for large nadir angles, indicating an effect that is not reproduced by our radiative transfer model. The volume extinction profiles are modeled by continuous functions except near the enhancement level near 55 km altitude. The wavelength dependence of the extinction optical depth is similar to earlier results at wavelengths from 500 to 700 nm, but is smaller at shorter wavelengths and larger toward longer wavelengths. A Hapke-like model is used for the ground reflectivity, and the variation of the Hapke single scattering albedo with wavelength is given. Fits to the visible spectrometers looking upward and downward are achieved except in the methane bands longward of 720 nm. This is possibly due to uncertainties in extrapolation of laboratory measurements from 1 km-am paths to much longer paths at lower pressures. It could also be due to changes in the single scattering phase functions at low altitudes, which strongly affect the path length through methane that the photons travel. We demonstrate the effects on the model fits by varying each model parameter individually in order to illustrate the sensitivity of our determination of each model parameter.
Publication Date: Oct 09, 2017
Document ID:
20170002309
(Acquired Mar 17, 2017)
Subject Category: SPACE SCIENCES (GENERAL); LUNAR AND PLANETARY SCIENCE AND EXPLORATION
Report/Patent Number: GSFC-E-DAA-TN40032
Document Type: Journal Article
Publication Information: Icarus (ISSN 0019-1035); Volume 270; 355-375
Publisher Information: Elsevier Inc.
Contract/Grant/Task Num: NNX12AE82G; NNX10AF09G
Financial Sponsor: NASA Goddard Space Flight Center; Greenbelt, MD United States
Organization Source: NASA Goddard Space Flight Center; Greenbelt, MD United States
Description: 21p; In English
Distribution Limits: Unclassified; Publicly available; Unlimited
Rights: Copyright
NASA Terms: AEROSOLS; EXTINCTION; INFRARED SPECTROMETERS; OPTICAL PROPERTIES; OPTICAL THICKNESS; STRATOSPHERE; TITAN; TITAN ATMOSPHERE; VERTICAL DISTRIBUTION; ALBEDO; CLOUD COVER; DATA BASES; HIGH ALTITUDE; LOW ALTITUDE; PLANETARY RADIATION; SCATTERING FUNCTIONS; SPECTROMETERS
Other Descriptors: TITAN; ATMOSPHERE
Availability Source: Other Sources
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