A new search for surface features on the dark leading side of Iapetus

Abstract

The most unusual property of the Saturnian satellite Iapetus is the extreme difference in brightness between the leading and trailing/ polar hemispheres (albedo 0.05 and 0.5,respectively). While individual Voyager images of the bright side show many features like craters or scarps, the dark Cassini regio area appears featureless. The main reason for this lack is the low signal-to-noise ratio of the raw-image data. Depending on the used filter and exposure time, the dark side is (on average) onyl about 2-3 % brighter than the dark sky. Another problem is the low resolution of the Cassini regio frames (23 km/pixel or worse). The S/N ratio can be improved using subpixel registration and the summation of different frames from one image sequence. This allows the S/N ratio to be improved bya factor of greater than three. Preliminary results indicate the features appears within Cassini regio at a higher signal level. One example is a circular structure with a diameter of about 400 km in the northern part of the center of the leading side. A description of these features is presented.