The Structure of the Solar Core
  Data from the Galileo mission to Jupiter and the Apollo mission to the Moon show isotopically distinct types of xenon in Jupiter and in the Sun and evidence of a mass separation process that selectively moves lighter elements and the lighter isotopes of each element to the solar surface. Understanding the source of solar luminosity, solar neutrinos, and mono-isotopic H-1 pouring from the solar surface depends on measurements of anti-neutrinos coming from neutron decay in the Sun and on the development of a better theoretical basis for the empirical evidence of a.) Repulsive interactions between like nucleons; b.) Clustering of nucleons; and c.) Neutron penetration of the gravitational barrier surrounding a neutron star. 
O. Manuel