  Much of our understanding of the internal structure of the Sun derives from
so-called standard theoretical solar models. Unfortunately, none of those
models agrees completely with observation. The discrepancy is commonly
associated with chemical abundance, and has led to what is now called the solar
abundance problem, the resolution of which has previously been out of sight.
But now the Borexino Collaboration, who recently announced measurements of the
pp-chain solar neutrinos, are optimistic that %in future they will be able to
measure the flux $\Phi_{\rm CNO}$ of the neutrinos emitted by the relatively
weak CNO cycle. Since C, N and O constitute the majority of the heavy elements,
that measurement will permit a crucial determination of the heavy-element
abundance $Z_{\rm c}$ in the Sun's energy-generating core, thereby shedding
important light on the problem. To accomplish that determination, a robust
relation between $Z_{\rm c}$ and $\Phi_{\rm CNO}$ will be required. That
relation is $Z_{\rm c}= 0.400\, \Phi_{\rm CNO}$, where $\Phi_{\rm CNO}$ is in
units of $10^{10} {\rm cm}^{-2}{\rm s}^{-1}$.
