Saturn’s thermal emission at 2.2-cm wavelength as imaged
by the Cassini RADAR radiometer

M.A. Janssen a,⇑, A.P. Ingersoll b, M.D. Allison c, S. Gulkis a, A.L. Laraia b, K.H. Baines a, S.G. Edgington a,
Y.Z. Anderson a, K. Kelleher a, F.A. Oyafuso a
a Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109, United States
b California Institute of Technology, Pasadena, CA, 91125, United States
c NASA Goddard Institute for Space Studies, New York, NY, 10025, United States

Article history:
Available online 21 June 2013
Saturn, Atmosphere
Atmospheres, Structure
Atmospheres, Composition
Radiative transfer
Radio observations

a b s t r a c t

We present well-calibrated, high-resolution maps of Saturn’s thermal emission at 2.2-cm wavelength
obtained by the Cassini RADAR radiometer through the Prime and Equinox Cassini missions, a period covering
approximately 6 years. The absolute brightness temperature calibration of 2% achieved is more than
twice better than for all previous microwave observations reported for Saturn, and the spatial resolution
and sensitivity achieved each represent nearly an order of magnitude improvement. The brightness temperature
of Saturn in the microwave region depends on the distribution of ammonia, which our radiative
transfer modeling shows is the only significant source of absorption in Saturn’s atmosphere at 2.2-cm
wavelength. At this wavelength the thermal emission comes from just below and within the ammonia
cloud-forming region, and yields information about atmospheric circulations and ammonia cloud-forming
processes. The maps are presented as residuals compared to a fully saturated model atmosphere in
hydrostatic equilibrium. Bright regions in these maps are readily interpreted as due to depletion of
ammonia vapor in, and, for very bright regions, below the ammonia saturation region. Features seen
include the following: a narrow equatorial band near full saturation surrounded by bands out to about
10 planetographic latitude that demonstrate highly variable ammonia depletion in longitude; narrow
bands of depletion at 35 latitude; occasional large oval features with depleted ammonia around
45 latitude; and the 2010–2011 storm, with extensive saturated and depleted areas as it stretched
halfway around the planet in the northern hemisphere. Comparison of the maps over time indicates a
high degree of stability outside a few latitudes that contain active regions.

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