\documentstyle[paasms,psfig,pptwocol]{preprint}
\input pub.sty

\def \Otwo           {\ifmmode O_2\else O$_2$\fi}
\def \oz             {\ifmmode O_3\else O$_3$\fi}

\begin{document}

\title{DETECTION OF OZONE ON SATURN'S SATELLITES RHEA AND DIONE}

\author{K.~S.~Noll\\
\\
Space Telescope Science Institute\\
3700 San Martin Drive\\
Baltimore, MD 21218\\
\and 
T.~L.~Roush\/\thanks{Also at Dept.~Geosciences, San Francisco State 
University, San Francisco, CA 94132.} and D.~P.~Cruikshank\\
\\
NASA Ames Research Center\\
Moffett Field, CA 94035\\
\and
R.~E.~Johnson\\
\\
University of Virginia\\
Engineering Physics\\
Charlottesville, VA 22901\\
\and
Y.~J.~Pendleton\\
\\
NASA Ames Research Center\\
Moffett Field, CA 94035}

\pub{Nature}
\recacc{30 March 1997}{13 May 1997}
\maketitle

\abstract{
Rhea and Dione orbit Saturn within Saturn's magnetosphere where
they are exposed to particle irradiation from trapped ions.   Jupiter's
satellites Europa, Ganymede, and Callisto similarly reside within
Jupiter's radiation belts.  All of these objects have surfaces rich in
water ice$^{1,2}$.  Laboratory studies of the interaction of charged 
particle radiation with water ice predicted$^{3}$ the tenuous oxygen
atmospheres recently found on Europa$^{4}$ and Ganymede$^{5}$. 
Theoretical investigations did not foresee, however, the trapping of
significantly larger quantities of \Otwo\ in the surface ice$^{6}$. 
Also not predicted before being observed on Ganymede$^{7}$ was
the accumulation of detectable abundances of \oz\ produced by
ultraviolet or charged particle radiation acting on the \Otwo .   Here
we report spectra of Saturn's satellites Rhea and Dione  that show a
prominent absorption feature at 260 nanometers wavelength that we
identify as \oz .   Rather than requiring unusual circumstances as
might have been inferred so long as \oz\ was detected only on
Ganymede, the detection of \oz\ on Saturn's satellites indicates
instead that the process producing icy surfaces with trapped  \Otwo\
and \oz\ is more general.}