Session 17. Rings II
Contributed Oral Parallel Session, Tuesday, October 13, 1998, 10:30-11:30am, Madison Ballroom C

## [17.06] First Infrared Imaging of the Neptune Ring Arcs: HST/Nicmos Results

R. J. Terrile, C. Dumas (JPL), B. A. Smith (Univ. Hawaii), M. Rieke, G. Schneider, R. Thompson (Univ. Arizona), E. Becklin (UCLA), D. Koerner (Univ. Pennsylvania)

Neptune's ring arcs have not been imaged since the Voyager 2 flyby in 1989 (Smith {\it et al.} 1989, {\it Science} {\bf 246}, 1422-1449). We used the HST and its near-infrared camera NICMOS to obtain the first detection at infrared wavelengths of the Neptune system of ring arcs. The faint inner satellites Proteus and Larissa were also detected. Scattered light coming from the giant planet was reduced considerably during the observations by positioning Neptune partially outside of the field of view of the camera. In addition to this, we used a filter whose bandpass is centered at 1.87\mum. This wavelength corresponds to a strong absorption by methane in Neptune's atmosphere.\\

Two different values for the mean motion of the ring arcs (Nicholson {\it et al.} 1995, {\it Icarus} {\bf 113}, 295-330) fit the ground-based and Voyager data sets. Initial results from this partially competed program indicate that the smaller value of 820.1118 deg/day for the arc motion is a better match to the data than the previously adopted value of 820.1194 deg/day. If this result is confirmed, it will call into question the close match between the semi-major axis of the arcs and the location of the 42:43 corotation resonance with Galatea (Porco 1991, {\it Science} {\bf 253}, 995-1001). From the measurements made at visible wavelengths with Voyager (Thomas and Veverka 1991, {\it JGR Supp.} {\bf 96}, 19253-19259) and at 1.87\mum with HST/Nicmos -- and assuming negligeable changes in the ring arcs color since 1989 -- we will be able to determine the visible/infrared color-index for the arcs and the two faint satellites detected and compare it to other primitive bodies of the outer solar system.