Among the six ivories in the group is a pair of saltcellars carved with images of starfish, birds and coiled snakes that were created by late 15th-century Sapi artists, the forebears of today's Bulom and Temne peoples in Sierra Leone.
The new images -- taken over several years -- reveal the channels which were carved out by flooding over an estimated 500 million year period -- a time when Mars was thought to be cold and arid.