Resolve the conflict between solar abundances and helioseismology
In plain words
Measurements of the Sun's surface composition give less carbon, oxygen and other heavy elements than the Sun's internal sound waves imply.
Precise statement
Standard solar models using 3D non-LTE photospheric abundances (Asplund et al. 2009 and 2021, $Z/X \sim 0.018$) disagree with the helioseismic sound speed below the convection zone (deviations $\sim 1\,\text{percent}$), the convection-zone base $R_{\mathrm{cz}} = 0.713\,R_{\mathrm{sun}}$ and surface helium $Y_{s} \sim 0.248$, while higher-metallicity analyses (Magg et al. 2022, $Z/X = 0.0225$) restore much of the agreement. Determine the correct metal content and whether opacity revisions near $T \sim 2e6\,\mathrm{K}$ or other physics are needed. Answer: $Z/X$ with uncertainty consistent with helioseismology and the CNO solar neutrino flux.
What would settle it
A precise CNO neutrino flux measurement (fixing core C+N) together with laboratory opacity measurements at solar interior conditions.
Status in the literature
Borexino CNO neutrino fluxes (2020-2022) mildly favor the higher-metallicity composition; helioseismic opacity inversions suggest opacities above current tables near the convection-zone base.