Analysis of Saturn’s thermal emission at 2.2-cm wavelength: Spatial
distribution of ammonia vapor.

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

Article history:
Available online 27 June 2013
Saturn, Atmosphere
Atmospheres, Structure
Atmospheres, Composition
Atmospheres, Dynamics
Radio observations

a b s t r a c t

This work focuses on determining the latitudinal structure of ammonia vapor in Saturn’s cloud layer near
1.5 bars using the brightness temperature maps derived from the Cassini RADAR (Elachi et al. [2004],
Space Sci. Rev. 115, 71–110) instrument, which works in a passive mode to measure thermal emission
from Saturn at 2.2-cm wavelength. We perform an analysis of five brightness temperature maps that
span epochs from 2005 to 2011, which are presented in a companion paper by Janssen et al. (Janssen,
M.A., Ingersoll, A.P., Allison, M.D., Gulkis, S., Laraia, A.L., Baines, K., Edgington, S., Anderson, Y., Kelleher,
K., Oyafuso, F. [2013]. Icarus, this issue). The brightness temperature maps are representative of the spatial
distribution of ammonia vapor, since ammonia gas is the only effective opacity source in Saturn’s
atmosphere at 2.2-cm wavelength. Relatively high brightness temperatures indicate relatively low
ammonia relative humidity (RH), and vice versa. We compare the observed brightness temperatures to
brightness temperatures computed using the Juno atmospheric microwave radiative transfer (JAMRT)
program which includes both the means to calculate a tropospheric atmosphere model for Saturn and
the means to carry out radiative transfer calculations at microwave frequencies. The reference atmosphere
to which we compare has a 3 solar deep mixing ratio of ammonia (we use 1.352  104 for
the solar mixing ratio of ammonia vapor relative to H2; see Atreya [2010]. In: Galileo’s Medicean Moons
– Their Impact on 400 years of Discovery. Cambridge University Press, pp. 130–140 (Chapter 16)) and is
fully saturated above its cloud base. The maps are comprised of residual brightness temperatures—
observed brightness temperature minus the model brightness temperature of the saturated atmosphere.
The most prominent feature throughout all five maps is the high brightness temperature of Saturn’s
subtropical latitudes near ±9 (planetographic). These latitudes bracket the equator, which has some of
the lowest brightness temperatures observed on the planet. The observed high brightness temperatures
indicate that the atmosphere is sub-saturated, locally, with respect to fully saturated ammonia in the
cloud region. Saturn’s northern hemisphere storm was also captured in the March 20, 2011 map, and
is very bright, reaching brightness temperatures of 166 K compared to 148 K for the saturated atmosphere
model. We find that both the subtropical bands and the 2010–2011 northern storm require very
low ammonia RH below the ammonia cloud layer, which is located near 1.5 bars in the reference atmosphere,
in order to achieve the high brightness temperatures observed. The disturbances in the southern
hemisphere between 42 and 47 also require very low ammonia RH at levels below the ammonia
cloud base. Aside from these local and regional anomalies, we find that Saturn’s atmosphere has on average
70 ± 15% ammonia relative humidity in the cloud region. We present three options to explain the high
2.2-cm brightness temperatures. One is that the dryness, i.e., the low RH, is due to higher than average
atmospheric temperatures with constant ammonia mixing ratios. The second is that the bright subtropical
bands represent dry zones created by a meridionally overturning circulation, much like the Hadley
circulation on Earth. The last is that the drying in both the southern hemisphere storms and 2010–
2011 northern storm is an intrinsic property of convection in giant planet atmospheres. Some combination
of the latter two options is argued as the likely explanation.

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