NEPTUNE INNER SATELLITES AND RING-ARCS 

Christophe Dumas
Jet Propulsion Laboratory / Caltech)

Observations of the inner Neptunian system were until recently reserved
exclusively to the domain of space exploration programs. Voyager 
revealed in 1989 a complex system of small irregular satellites and
thin rings orbiting within the inner region of Neptune (2-5 R_Nep). 
Neptune's ring-arcs, which were discovered from star occultations 
prior to the Voyager flyby, were also imaged for the first time. Then, 
observations of the Neptune's inner system were stopped until 1998, when 
high-angular resolution imaging techniques made such programs possible 
from the ground (with AO) and space (with HST).

We will present astrometric and photometric measurements obtained with 
HST/NICMOS on Neptune's ring-arcs and its small satellites Proteus, 
Larissa, Galatea and Despina. Their surfaces display a reddish color in 
the visible/near-IR range and Proteus' albedo shows also a depression at 
2.2 micron that could be produced by C-H or CN bearing material present on 
the surface. Finally, precise astrometry of the inner moons and ring-arcs 
of Neptune revealed a mismatch between the semi-major axis of the ring-arcs 
and the location of the corotation inclined resonance created by Galatea. 
This result calls into question the role of this resonance to azimuthally 
confine the arcs.