Acknowledgement. We thank Science for their permission to use an excerpt from:

Hanel, R., et al. 1979. Infrared observations of the Jovian system from Voyager 2. Science 206 (4421), 952-956. (Excerpt from p. 955.)

Copyright AAAS, November 23, 1979.

Infrared Observations of the Jovian System
from Voyager 2

Jupiter's ring

. A set of 20 spectra was obtained during a radial scan of the ring. An average of this set, which showed no evident thermal signal, can be used to set an upper limit on the normal optical depth of the ring in the thermal infrared. For an optically thin emitting medium, the normal optical depth tau(nu) is given by

tau(nu) = I(nu)[f sec(theta) B(nu,T)]^(-1)

where I(nu) is the measured signal strength at wave number nu, f is the fraction of the instrument field of view filled by the medium, theta is the view angle measured from the normal, and B is the Planck function at the ring temperature T. Form images taken during the observation sequence (3), the bright portion of the ring in the IRIS field of view gives f=0.088; the viewing geometry gives theta=88.2 degrees; an isothermal black ring particle in thermal equilibrium with Jupiter and the sun will have a temperature of about 125 K. By using the instrument noise level as an upper limit to the ring signal, we obtain an upper limit on ring opacities (Table 2).

Table 2. Upper limit to normal optical depth of the Jovian ring; I(nu) is the upper limit of signal in the infrared spectrum of the ring.

(W cm(-2) sr(-1)/cm(-1)
250 1e-9 3e-4
400 1e-9 5e-4
600 2e-9 3e-3

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