Is the seam around the middle maybe an image-stitching artifact?
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"The most unique, and perhaps most remarkable feature discovered on
Iapetus in Cassini images is a topographic ridge that coincides almost
exactly with the geographic equator. The ridge is conspicuous in the
picture as an approximately 20-kilometer wide (12 miles) band that
extends from the western (left) side of the disc almost to the
day/night boundary on the right. On the left horizon, the peak of the
ridge reaches at least 13 kilometers (8 miles) above the surrounding
terrain. Along the roughly 1,300 kilometer (800 mile) length over
which it can be traced in this picture, it remains almost exactly
parallel to the equator within a couple of degrees. The physical
origin of the ridge has yet to be explained. It is not yet clear
whether the ridge is a mountain belt that has folded upward, or an
extensional crack in the surface through which material from inside
Iapetus erupted onto the surface and accumulated locally, forming the
ridge. The origin of Cassini Regio is a long-standing debate among
scientists. One theory proposes that its dark material may have
erupted onto Iapetus's icy surface from the interior. Another theory
holds that the dark material represented accumulated debris ejected by
impact events on dark, outer satellites of Saturn. Details of this
Cassini image mosaic do not definitively rule out either of the
theories. However, they do provide important new insights and
constraints. "
...Jim Thompson
