\documentstyle[psfig,pptwocol]{preprint}

\def \sqcm           {\ifmmode \hbox{cm}^{-2}\else cm$^{-2}$\fi}
\def \SOtwo          {\ifmmode SO_2\else SO$_2$\fi}

\input pub.sty  
\begin{document}

\title{DETECTION OF \SOtwo\ ON CALLISTO WITH\\
~\\
THE HUBBLE SPACE TELESCOPE}

\author{K.\ S.\ Noll\\
\\
Space Telescope Science Institute\\
3700 San Martin Drive\\
Baltimore, MD 21218\\
(email: noll@stsci.edu)\\
\and
R.\ E.\ Johnson\\
\\
Engineering Physics\\
University of Virginia\\
Charlottesville, VA 22903\\
(email: rej@uva.pcmail.virginia.edu)\\
\and
M.\ A.\ McGrath\\
\\
Space Telescope Science Institute\\
(email: mcgrath@stsci.edu)\\
\and
J.\ Caldwell\\
\\
York University\\
Dept.\ of Physics \& Astronomy\\
4700 Keele Street\\
North York, Ontario, M3J 1P3, Canada}

\pub{Geophysical Research Letters}
\recacc{13 January 1997}{19 March 1997}

\maketitle

\abstract{
We have detected \SOtwo\ in ultraviolet spectra of Callisto obtained
with the Hubble Space Telescope's Faint Object Spectrograph.  An
absorption band centered at 280~nm appears in the spectrum of
Callisto's leading hemisphere, but is not apparent in the spectrum
of the trailing hemisphere.  The band is similar to the \SOtwo\ band
on Europa's trailing hemisphere.  Callisto's leading hemisphere
spectrum can be well fit with models that include \SOtwo\ ice
absorption with N(\SOtwo )$ \ge 6 \x 10^{16}$ \sqcm. Callisto's
leading hemisphere is modified by impacts with micrometeorites; this
may directly or indirectly be a source of sulfur dioxide.}