Composition A sharp boundary divides the ancient dark terrain of Nicholson Regio from the younger, finely striated bright terrain of Harpagia Sulcus. The average density of Ganymede, 1.936 g /cm 3 , suggests a composition of approximately equal parts rocky material and water, which is mainly in the form of ice.  The mass fraction of ices is between 46–50%, slightly lower than that in Callisto.  Some additional volatile ices such as ammonia may also be present.   The exact composition of Ganymede's rock is not known, but is probably close to the composition of L / LL type ordinary chondrites , which are characterized by less total iron , less metallic iron and more iron oxide than H chondrites . The weight ratio of iron to silicon is 1.05–1.27 in Ganymede, whereas the solar ratio is around 1.8.  Ganymede's surface has an albedo of about 43%.  Water ice seems to be ubiquitous on the surface, with a mass fraction of 50–90%,  significantly more than in Ganymede as a whole.
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