Time Variability of the Microwave Structure of Saturn's Atmosphere
Molnar, L. A. (U. Iowa), Dunn, D. E. (U. Iowa)
We present Very Large Array maps of Saturn at two different epochs:
November 1995 and February 1997. In the first epoch 0.7, 2.0, 3.6, and
6.1 cm wavelength maps were made in the B array. The ring inclination
angle was 
, with the Saturn north pole tilted towards
Earth. In the second epoch 3.6, 6.1, 18, and 20 cm wavelength maps
were made in the A/B array, while 1.3 and 2.0 cm maps were made in the
B array. This time the ring inclination angle was 
, so that
the Saturnian south pole faces us now. In all cases both hemispheres
could be clearly mapped.
The first epoch maps are featureless at wavelengths less than or equal to 2.0 cm, but show latitude-dependent structure at longer wavelengths, as has been observed by others; and we interpret the behavior likewise: short wavelength emission arises high in the troposphere, where ammonia (the source of absorption) has mostly condensed out, with the remaining gas equally saturated at all latitudes. By contrast the longer wavelength emission arises from the region just below the clouds, where the ammonia abundance is very sensitive to any net vertical velocity. A downward velocity brings ammonia-deficient gas down, allowing a view of the warmer and brighter layers beneath.
The most prominent bright bands (seen at both 3.6 and 6.1 cm) were at
N and S latitude. The bright band extending from 20 to
N, observed by de Pater and Dickel (1991, Icarus 94, 474)
from 1981 through 1986, has either moved or disappered since that
time! We have therefore have found the first direct evidence of time
variation of global circulation patterns in Saturn's lower
troposphere.
When we compare our 1995 maps to contemporaneous optical images from the HST, we see that the boundaries of the belts near the equatorial zone are significantly offset from our observed features. We suggest the features are nonetheless related, but that the equatorial zone increases its latitude extent as it climbs into the stratosphere. The increased brightness of the 10 degree features may be correlated with the increased storm activity in the equatorial belt since 1990.