  Helioseismology has shown that the chemical composition of the Sun has
changed over its lifetime. The surface abundance of helium and heavy elements
is believed to have decreased by up to 10% relative to their initial values.
However, this reduction is too small to be tested by direct observations of the
photospheric chemical composition. Here, we compare the predicted variations in
the solar photospheric composition with precise measurements of abundances in
meteorites and the solar wind composition. Although elemental composition
ratios can vary by roughly a percent (e. g. for Ca/Mg and Ca/Fe) over the Sun's
lifetime, their measurements are rife with uncertainties related to
uncertainties in the interpretation of meteoritic measurements, photospheric
determinations, and the complex fractionation processes occurring between the
upper photosphere and lower chromosphere and the corona. On the other hand,
isotopic ratios can be measured much more accurately and are not expected to be
affected as much by extrasolar processes, although more work is required to
quantify their effect. As the isotopic ratios evolve in the Sun proportionally
to the mass ratios of the isotopes, light elements yield the highest variations
in isotopic ratios. They are predicted to reach as high as 0.6% for
$^{18}$O/$^{16}$O and are only slightly lower in the cases of
$^{26}$Mg/$^{24}$Mg and $^{30}$Si/$^{28}$Si. Such a value should be well within
the sensitivity of new missions such as Genesis.
