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.

 2013 Elsevier Inc. All rights reserved.

Anomalous radar backscatter from Titan’s surface?

M.A. Janssen a,⇑, A. Le Gall a, L.C. Wyeb
a Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
b Stanford University, Electrical Engineering Department, Stanford, CA 94305, USA

Article history:
Received 21 April 2010
Revised 11 November 2010
Accepted 14 November 2010
Available online 26 November 2010

Satellites, Surfaces
Radio observations
Radar observations

a b s t r a c t

Since Cassini arrived at Saturn in 2004, its moon Titan has been thoroughly mapped by the RADAR instrument
at 2-cm wavelength, in both active and passive modes. Some regions on Titan, including Xanadu
and various bright hummocky bright terrains, contain surfaces that are among the most radar-bright
encountered in the Solar System. This high brightness has been generally attributed to volume scattering
processes in the inhomogeneous, low-loss medium expected for a cold, icy satellite surface. We can test
this assumption now that the emissivity has been obtained from the concurrent radiometric measurements
for nearly all the surface, with unprecedented accuracy (Janssen et al., and the Cassini RADAR
Team [2009]. Icarus 200, 222–239). Kirchhoff’s law of thermal radiation relates the radar and radiometric
properties in a way that has never been fully exploited. In this paper we examine here how this law may
be applied in this case to better understand the nature of Titan’s radar-bright regions. We develop a quantitative
model that, when compared to the observational data, allows us to conclude that either the
reflective characteristics of the putative volume scattering subsurface must be highly constrained, or,
more likely, organized structure on or in the surface is present that enhances the backscatter.
 2010 Elsevier Inc. All rights reserved.

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.

 2013 Elsevier Inc. All rights reserved.

Atmospheric Remote Sensing by Microwave Radiometry.pdf


To Glenys, Sandie, and Liza


The Earth's atmosphere is a puzzle and a concern. Its unpredictable weather has always been a force to be reckoned with, but until recent times, our trust was implicit in the robustness of the atmospheric system as the foundation of our biosphere. Now we are not so sure. Far from being robust, this system is better described as a balance among a number of natural forces that we barely understand. Our concern is deepened when we consider the threat to this balance from our burgeoning civilization and its effluents. What, for example, are the chlorofluorocarbons doing to the ozone layer, and how significantly and on what time scales will the increasing CO2 production change the climate? There will be no answers to such questions without a strong and steady program of research to understand fundamental atmospheric processes. Essential to this is the gathering of basic data such as temperature, pressure, wind, and the distribution of water vapor, clouds, and other active constituents. Such data enable us to test existing models for the atmosphere's energy balance, the depletion of the ozone layer, the hydrological cycle, climate trends, and other aspects of the atmospheric system that are of vital interest to us, and to formulate new and better models to guide us in the future. Remote sensing is central to this effort because it is the only way we can obtain the full spatial and temporal perspective needed to understand atmospheric processes. The strong conclusion is that the need for remote sensing will continue, and grow.


A comprehensive calibration and mapping of thethermal microwave emission from Titan’s surface is

reported based on radiometric data obtained at 2.18-cm wavelength by the passive radiometer included

in the Cassini RADAR instrument. Compared to previouswork, the present results incorporate the much

larger data set obtained in the approximately ten years following Saturn Orbit Insertion. Brightness

temperature data including polarization were accumulated by segments in Titan passes from Ta

(October 2004) through T98 (February 2014). The observational segments were analyzed to produce a

mosaic of effective dielectric constant based on the measurement of thermal polarization covering 76%

of the surface, and brightness temperature at normal incidence covering Titan’s entire surface. As part

of the mosaicking process we also solved for the seasonal variation of physical temperature with latitude,

which we found to be smaller by a factor of 0.87 ± 0.05 in relative amplitude compared to that reported in

the thermal infrared by Cassini’s Composite Infrared Spectrometer (CIRS). We used the equatorial temperature

obtained by the Huygens probe and the seasonal dependence with latitude from CIRS to convert

the brightness mosaic to absolute emissivity, from which we could infer global thermophysical properties

of the surface in combination with the dielectric mosaic. We see strong evidence for subsurface (volume)

scattering as a dominant cause of the radar reflectivity in bright regions, and elsewhere a surface

composition consistent with the slow deposition and processing of organic compounds from the

atmosphere. The presence of water ice in the near subsurface is strongly indicated by the high degree

of volume scattering observed in radar-bright regions (e.g., Hummocky/mountainous terrains)

constituting 10% of Titan’s surface. A thermal analysis allowed us to infer a mean 2.18-cm emission

depth in the range 40–100 cm for the dominant radar-dark terrains (the remainder of Titan’s surface)

at all latitudes of Titan, consistent with the deposition and possible processing and redistribution of

tholin-like atmospheric photochemical products.