  We use a fully 3-dimensional thermal evolution model to examine the effects
of a non-uniform surface albedo on the subsurface thermal structure of comets.
Surface albedo markings cast "thermal shadows", with strong lateral thermal
gradients. Corresponding compositional gradients can be strong, especially if
the crystallization of amorphous water ice is triggered in the hottest regions.
We show that the spatial extent of the structure depends mainly on the
obliquity, ther- mal conductivity and heliocentric distance. In some
circumstances, subsurface structure caused by the thermal shadows of surface
features can be maintained for more than 10 Myr, the median transport time from
the Kuiper Belt to the inner solar system. Non-uniform compositional structure
can be an evolutionary product and does not necessarily imply that comets
consist of building blocks accumulated in different regions of the
protoplanetary disk.
