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Science 4 July 1986:
Vol. 233. no. 4759, pp. 74 - 79
DOI: 10.1126/science.233.4759.74
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Articles

Ultraviolet Spectrometer Observations of Uranus

A. L. BROADFOOT 1, F. HERBERT 1, J. B. HOLBERG 1, D. M. HUNTEN 1, S. KUMAR 1, B. R. SANDEL 1, D. E. SHEMANSKY 1, G. R. SMITH 1, R. V. YELLE 1, D. F. STROBEL 2, H. W. MOOS 2, T. M. DONAHUE 3, S. K. ATREYA 3, J. L. BERTAUX 4, J. E. BLAMONT 4, J. C. MCCONNELL 5, A. J. DESSLER 6, S. LINICK 7, and R. SPRINGER 7

1 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721.
2 Department of Physics, Johns Hopkins University, Baltimore, MD 21218.
3 Department of Atmospheric and Oceanic Science, University of Michigan, Ann Arbor, MI 48109.
4 Service d'Aeronomie du Centre National de la Recherche Scientifique, B.P.N. 3 91370 Verrieres le Buisson, France.
5 Department of Earth and Atmospheric Sciences, York University, Downsview, Toronto, M3J 1P3 Canada.
6 Space Science Laboratory, Marshall Space Flight Center, Huntsville, AL 35812.
7 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109.

Data from solar and stellar occultations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are kmown as electroglow emissions. The energy source for these emissions is unknown, but the spectrum implies excitation by low-energy electrons (modeled with a 3-electron-volt Maxwellian energy distribution). The major energy sink for the electrons is dissociation of molecular hydrogen, producing hydrogen atoms at a rate of 1029 per second. Approximately half the atoms have energies higher than the escape energy. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. This corona is augmented by the nonthermal hydrogen atoms associated with the electroglow. An aurora near the magnetic pole in the dark hemisphere arises from excitation of molecular hydrogen at the level where its vertical column abundance is about 1020 per square centimeter with input power comparable to that of the sunlit electroglow (approximately 2x1011 watts). An initial estimate of the acetylene volume mixing ratio, as judged from measurements of the far ultraviolet albedo, is about 2 x 10-7 at a vertical column abundance of molecular hydrogen of 1023 per square centimeter (pressure, approximately 0.3 millibar). Carbon emissions from the Uranian atmosphere were also detected.

Submitted on March 28, 1986
Accepted on May 5, 1986


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