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Radio interferometric investigations of Saturn's rings at 3.71- and 1.30-CM wavelengths

Citation

Schloerb, F. Peter (1978) Radio interferometric investigations of Saturn's rings at 3.71- and 1.30-CM wavelengths. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-10292008-160948

Abstract

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Interferometric observations of Saturn and the rings have been obtained at 3.71 and 1.30 cm wavelengths. The observations have been analyzed by both model fitting and aperture synthesis techniques. They show that the rings have a very low brightness temperature, but attenuate the thermal emission from the planet significantly where they cross in front of it. The latter effect, when combined with the estimate of the ring brightness temperature, permits the optical depths of the rings to be estimated. The fits of the interferometric data to Saturn models in which the A and B rings are combined into a single ring are given in Table (1). The 3.71 cm observations were made at two epochs and the relative areas of the A and B rings that obscured the planet were different. The apparently inconsistent optical depth results of the two 3.71 cm data sets, then, indicate that the A ring optical depth is significantly less than that of the B ring. The relative areas of the A and B rings are the same for the 1976 1.30 cm and 3.71 cm observations and their optical depth results may be directly compared. They indicate that the ring optical depths are the same at the two wavelengths. These optical depths are quite similar to those estimated at visible wavelengths. The ring brightness temperatures, which are shown in the table normalized by the brightness temperature of the planet to remove any errors in the absolute calibration of the data, are also the same at the two wavelengths. No variation in the brightness temperature of the rings with tilt angle (B) was detected. A significant amount of radiation from the C ring was detected by the 3.71 cm observations, and the ring was also found to attenuate the planetary emission significantly. Unfortunately, the 1.30 cm observations were not sensitive enough to detect the C ring. The brightness temperature and optical depth results for the individual rings that are implied by all of the 3.71 cm observations are given in Table (ii). Limb-darkening of the planetary emission was simulated by solving for the best fitting planetary radius. No limb-darkening was detected at 3.71 cm, but an apparently significant amount was detected at 1.30 cm. The results at the two wavelengths are significantly different and indicate that the planet is more limb-dark at 1.30 cm than at 3.71 cm. This finding is interesting since it is contrary to what was predicted by atmospheric models which fit the Saturn microwave spectrum. The aperture synthesis analysis is independent of the model fitting and can be used to confirm its results and search for new features not included in the models. The aperture synthesis maps confirm the model fitting results and require no new brightness structures. In particular, no azimuthal variations of the brightness temperature of the rings were detected. The aperture synthesis maps also indicated that the true position of Saturn may be offset from the values given in the American Ephemeris and Nautical Almanac by about 0.3 arcsec. Consideration of simple physical models of the rings has shown that the radiation from the rings at centimeter wavelengths is almost entirely thermal emission from the planet that is scattered to the Earth by the ring particles. The models indicate that the ring particles are very good scatterers and very poor emitters at microwave wavelengths, and this conclusion sets constraints upon the size and composition of the ring particles. The similarity between the optical depths of the rings at visible and centimeter wavelengths probably indicates that the particles are much larger (≥ 1 meter) than the centimeter wavelengths. The large sizes and excellent scattering properties of the particles indicate that they are composed of either a highly reflective or transparent material. At this time water ice is the most likely candidate, since it has been detected in the rings spectroscopically and is known to be highly transparent to microwaves at the low temperatures found at Saturn's rings.

TABLE (i)- SUMMARY OF COMBINED A AND B RING RESULTS: [...] + only the B ring blocked the planet. * includes effect of offset in planetary position from AENA value.

TABLE (ii)- SUMMARY OF 3.71 CM RESULTS FOR INDIVIDUAL RINGS:

Ring [...] Optical Depth

A 0.030 ± 0.012 0.2 ± 0.2 B 0.050 ± 0.010 0.9 ± 0.2 C 0.040 ± 0.014 0.1 ± 0.1

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geological and Planetary Sciences
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Muhleman, Duane Owen
Thesis Committee:
  • Unknown, Unknown
Defense Date:14 December 1977
Record Number:CaltechETD:etd-10292008-160948
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-10292008-160948
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4309
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:19 Dec 2008
Last Modified:26 Dec 2012 03:07

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