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

Laraia, A. L. and Ingersoll, A. P. and Janssen, M. A. and Gulkis, S. and Oyafuso, F. and Allison, M. (2013) Analysis of Saturn’s thermal emission at 2.2-cm wavelength: Spatial distribution of ammonia vapor. Icarus, 226 (1). pp. 641-654. ISSN 0019-1035. doi:10.1016/j.icarus.2013.06.017.

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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 × 10^(−4) for the solar mixing ratio of ammonia vapor relative to H_2; 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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Ingersoll, A. P.0000-0002-2035-9198
Janssen, M. A.0000-0001-5476-731X
Gulkis, S.0000-0002-4480-3628
Oyafuso, F.0000-0002-8862-8737
Allison, M.0000-0001-9841-193X
Additional Information:© 2013 Elsevier Inc. Available online 27 June 2013. This research was conducted at the California Institute of Technology under contract with the National Aeronautics and Space Administration (NASA). It is partly based upon work supported by NASA under Grant No. 10-CDAP10-0051 issued through the Cassini Data Analysis and Participating Scientist (CDAPS) Program. A. Laraia was partially funded by a National Science Foundation (NSF) Graduate Research Fellowship Program (GRFP) fellowship. We acknowledge Sushil Atreya, Lena Adams, and Virgil Adumantroi for their invaluable contributions to the Juno atmospheric microwave radiative transfer (JAMRT) program. We would also like to acknowledge Kevin Baines for useful discussions on comparing RADAR and VIMS data.
Group:UNSPECIFIED, UNSPECIFIED, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
NSF Graduate Research FellowshipUNSPECIFIED
Subject Keywords:Saturn, Atmosphere Atmospheres, Structure Atmospheres, Composition Atmospheres, Dynamics Radio observations
Issue or Number:1
Record Number:CaltechAUTHORS:20131101-092901035
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Official Citation:A.L. Laraia, A.P. Ingersoll, M.A. Janssen, S. Gulkis, F. Oyafuso, M. Allison, Analysis of Saturn’s thermal emission at 2.2-cm wavelength: Spatial distribution of ammonia vapor, Icarus, Volume 226, Issue 1, September–October 2013, Pages 641-654, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42182
Deposited By: Ruth Sustaita
Deposited On:01 Nov 2013 16:46
Last Modified:10 Nov 2021 16:19

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