(abundance of something at/near the surface of a body)
The term surface abundance, the abundance of something
at something's surface, is often used for
stars, to indicate the abundance of some element or compound
at the surface, which is the part of the star producing
spectral lines, thus where the abundance of elements can be measured.
For example, the solar lithium problem is the issue
that the measured surface abundance of lithium on
the Sun is much lower than would be expected given
matter which is presumed to make up the Sun. Solutions
to the problem include theories that the lithium has
been mixed in the Sun in such a manner that the surface
abundance is lowered.