Models of Aerosols at Continuum Wavelengths from Measurements Made inside the Atmosphere of Titan

Astronomy and Astrophysics – Astronomy

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Scientific paper

The Descent Imager/Spectral Radiometer (DISR) instrument on the Huygens Probe made spectral measurements of the upward and downward streaming sunlight in Titan's atmosphere from 140 km to the surface. These observations were supplemented by measurements of the solar aureole at 491 and 938 nm through horizontal and vertical linear polarizers at a variety of azimuths relative to the sun. The measurements constrain the vertical distribution, phase function, and single scattering albedos of Titan's aerosols.
We find that the aerosol opacity above 80 km altitude decreases with a scale height of 65 km. Between 80 and 30 km the cumulative aerosol opacity varies linearly with altitude. Below 30 km, the aerosol opacity again varies linearly with altitude, but with a different slope.
Above 80 km, the single scattering albedo is similar to values reported for some types of Tholin particles produced in the laboratory. At lower altitudes, the absorption in the aerosols particles is approximately half as great, possibly due to incorporation of ethane into the particles.
Below 30 km the wavelength dependence of the opacity is much smaller that at higher altitudes, implying a significant increase in the size of the particles, possibly due to incorporation of methane into the particles.
The degree of linear polarization near 90 degrees scattering angle is large in both wavelength channels of the Solar Aureole instrument, implying that the small dimension of the particles is less than 0.1 micron. The particles at all altitudes are strongly forward scattering. If approximated by Henyey-Greenstein phase functions, the forward scattering g is about 0.80. A small backscattering peak is also observed.
Comparisons of the single scattering cross sections, phase functions, and degree of linear polarizations with fractal aggregate particles are in progress. Recent results will be reported.

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