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R. G. French, C. Cardamone, J. Huff (Wellesley Coll.), A. Verbiscer (U. of Virginia)
Janus and Epimetheus are small coorbital satellites that lie just outside of the bright A ring of Saturn, making them challenging targets for detailed photometric studies. Approximate geometric albedos were determined from Voyager observations (Thomas et al., J. Geophys. Res., 88, 8743-8754, 1983). More recently, Poulet et al., J. Geophys. Res., 104, 24095-24110, 1999) made use of Hubble Space Telescope images taken in 1995 during the Saturn ring plane crossings to refine the albedos of these and other small Saturnian satellites, taking advantage of the reduced glare of the rings when they were seen edge-on. They concluded that Janus has a pronounced redness at wavelengths between 0.3 and 0.7 \mum, whereas Epimetheus is more nearly neutral. If these differences are real, they would suggest that the two satellites, which currently are intimately related dynamically by their coorbital nature, had different orgins. On the other hand, if Janus and Epimetheus were fragments of a common progenitor before becoming coorbitals, one would expect their colors to be similar to each other. We have begun a systematic photometric survey of Janus and Epimetheus, using a rich set of HST images taken between 1996 and 2002 with the WFPC2 camera over a wavelength range of 255 to 1042 nm. The absolute photometric accuracy and high spatial resolution of the WFPC2 have enabled us to obtain broadband spectra of these and other satellites whenever they are present in the full set of over 300 separate images. We find visible geometric albedos of Janus and Epimetheus consistent with those of Poulet et al. However, our preliminary results show that the two satellites have much more similar broadband colors than found by these investigators, consistent with a common origin. Refined results, making use of improved aperture corrections, should enable us to measure both the colors and possibly even the opposition surges of Janus and Epimetheus, using the HST observations. This work was supported in part by NASA PG&G grant NAG5--10197 and STSCI grant GO--08660.01A, and by NASA's Massachusetts Space Grant.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.