Hofstadter, Mark David (1992) Microwave observations of Uranus. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:09222011-102938775
This thesis explores the atmosphere of Uranus using microwave observations at wavelengths from 1 to 20 cm, with primary emphasis on high resolution VLA data at wavelengths of 2 and 6 cm. While radio maps of Uranus have been published previously, this is the first detailed analysis and interpretation of such observations. Atmospheric structures are mapped to depths greater than has been seen on any giant planet. Several features of the data are immediately clear. First, there are strong horizontal and vertical gradients in the atmospheric properties that control the radio brightness. Polar regions are much brighter than lower latitudes, and the deep troposphere (pressures greater than a few tens of bars) appears much dimmer than would be expected based on the upper troposphere. (Both these results had been postulated in previous works, but older observations lacked the resolution to confirm them.) A second important feature of the data is that the intrinsic latitudinal brightness variations determined in this work at 2 cm and 6 cm are highly correlated with each other and with Voyager infrared measurements, suggesting a common cause. Because these data sets probe different altitudes between 50 and 0.1 bar, the cause must be acting over this altitude range of about 250 km. Another immediate result, independent of atmospheric modeling, is that the radio brightness features have not changed significantly in the 8 years between 1981 and 1989.
Since radio brightness is a function of temperature and composition, the observations can be used to map these properties as a function of latitude and height. Arguments are presented that indicate compositional gradients are the dominant factor controlling the brightness variations, and these compositional changes are used as a tracer to infer the general circulation and some of the chemical processes of the atmosphere. The most likely interpretation of the data is that the Southern Hemisphere is dominated by a single meridional circulation cell, with an upwelling centered near -25° latitude that brings absorber rich air parcels from 50 bars up to the 0.1 bar region. As parcels rise, the absorber mixing ratio drops by a factor of about 100 between 25 and 10 bars, and then a further factor of 2 at higher altitudes. These depletions are probably due to condensation. The absorber depleted parcels then move poleward and descend, dominating the atmospheric composition over the pole down to 50 bars, but not deeper. This circulation is consistent with the zonal winds and upper atmospheric temperatures observed by Voyager in the context of a simple, linear, dynamical model. The model suggests that the forcing driving these motions occurs within the upper few hundred bars of the atmosphere. The species most likely to be responsible for microwave absorption in the atmosphere is NH_3, and at depth it appears to have a molar mixing ratio within an order of magnitude of 1.4 x 10^(-4), the solar value. The formation of an NH_4SH cloud above 30 bars can account for the primary depletion of NH_3, while NH_3 ice condensation at 5 bars accounts for the rest. Most of the results discussed here, however, are independent of what the absorbing species actually is.
Superimposed on the large scale brightness pattern are smaller brightness oscillations, less than about 15° wide in latitude. These long lasting features are reminiscent of the zones and belts of Jupiter, and could be the result of variations in either cloud altitudes or the depth of penetration of subsiding air parcels. A more extensive analysis is needed, however, to understand these small scale structures. The final point addressed in this work is the seasonal variability of the atmosphere. While no variations exist in the current high resolution data set, which covers about 10 years of the mid-summer season, it is expected that detectable changes will occur over 20 to 40 year time scales (each season on Uranus lasts 21 years). The magnitude of the variations, however, cannot be determined from the available data.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Subject Keywords:||Planetary Science and Electrical Engineering|
|Degree Grantor:||California Institute of Technology|
|Division:||Geological and Planetary Sciences|
|Major Option:||Planetary Science|
|Minor Option:||Electrical Engineering|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||1 October 1991|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||22 Sep 2011 18:03|
|Last Modified:||19 Apr 2016 22:57|
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