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R. Meier, B. A. Smith, T. C. Owen (IfA, UHawaii), R. J. Terrile (JPL)
In late fall 1997 we obtained high resolution images of Titan in the wavelength range between 0.97~\mum and 2.37~\mum using NICMOS (Near Infrared Camera and Multi-Object Spectrometer) aboard HST (Hubble Space Telescope). Pairs of observations, separated by one day, were recorded at three equally spaced intervals during Titan's 16-day orbit around Saturn. Our observing strategy allowed us to obtain a full longitudinal coverage of Titan's surface and atmosphere, while at the same time, the paired exposures also provided means to monitor the temporal evolution of the atmosphere over time scales of approximately one day. Filters selected corresponded to varying degrees of methane opacity, and therefore represented varying depths of penetration into Titan's thick and haze-loaded atmosphere, with some reaching all the way down to the surface. We present diffraction limited maps (0.14~arcsec FWHM or ~430~km at 1.6~\mum) of Titan's surface and/or near-surface features. The narrow point-spread-function of HST and the adopted dithering technique, which oversampled the 35~mas pixels by roughly a factor of two in x- and y-direction, led to images of high contrast and maximal resolution. The most prominent, continent-sized ``bright feature'' on Titan, which is commonly attributed to a weathered highland, can be seen at almost all wavelengths, i.e.\ even in filters that probe higher altitudes in the atmosphere. We exploited the good photometric qualities of NICMOS and attempted relative photometry on various characteristic surface features to constrain the nature of the highly differentiated surface. Although spectroscopic observations indicate that there may be significant cloud activity on Titan, we were unable to unambigously identify motions of large-scale cloud systems over time frames of days. To the contrary, we found a remarkable agreement with the surface features HST revealed in the visible and AO observations in the near IR.