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                                                Circular No. 4863
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM     EASYLINK 62794505

     G. Orton, Jet Propulsion Laboratory, communicates:  "Thermal
infrared images made at the NASA Infrared Telescope Facility by M.
Shure (Institute for Astronomy, University of Hawaii) on Aug. 4 UT,
and by G. Veeder and S. Lawson (JPL) on Aug. 15, indicate that the
bright appearance of Jupiter's South Equatorial Belt (SEB) at a
wavelength of 5 microns is considerably cooler than in its last
known observation, made for Orton, J. Friedson (JPL), and J.
Caldwell (York University) on Feb. 13 by IRTF staff as part of a
monitoring program.  At that time, and for several years prior, the
SEB and the North Equatorial Belt have been the brightest
axisymmetric features at 5 microns.  In August, the SEB appeared to
be as cool as the adjacent South Tropical just to the south.
Five-micron radiation from Jupiter is sensitive to the temperatures
of clouds at pressures near and greater than 600 mbar.  Orton
reports also that similar imaging observations at the IRTF by D.
Lynch and R. Russell (Aerospace Corporation) on Sept. 6 indicate
that the bright appearance of the SEB at 8.57 microns, a wavelength
primarily sensitive primarily only to clouds near the 600-mbar
pressure level, has also cooled relative to its last observed
appearance on 1988 Dec. 2, as observed by Orton, Friedson, and
Caldwell at the IRTF, which was fairly consistent with its
appearance for the past several years.  The observations indicate
that the visual change in the SEB albedo (cf. IAUC 4815, 4818, and
4819) is accompanied by an increase in the optical thickness of
the 600-mbar cloud, considered most likely to be composed of ammonia
ice condensate, either because of an increase in the number
of cloud particles or an increase in particle size.  Changes in
deeper cloud properties may also be possible.  Images taken by
Veeder and Lawson and later by Lynch and Russell at 18 microns
on the same dates cited above sensed temperatures near the 200-mbar
atmospheric level.  These indicate relatively minor changes since
the last similar observations by the IRTF staff for Orton, Friedson,
and Caldwell on 1989 Mar. 16.  Further changes in the SEB temperature
is considered possible, as the SEB was visually bright at the time
of the Pioneer 10 and 11 encounters in 1973 and 1974, respectively;
at that time, the 20-micron appearance of the SEB was quite cold."

     Visual magnitude estimates:  Aug. 6.16 UT, 14.9 (P. Sventek,
Houston, TX); Sept. 4.82, 14.1 (A. Boattini, Florence, Italy);
25.11, 13.3 (Sventek).

1989 September 25              (4863)             Daniel W. E. Green

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