ARE STANDARD SOLAR MODELS RELIABLE?Yes, says John Bahcall of the Institute for Advanced Study. Fewer than expected solar neutrinos register in terrestrial detectors. This solar neutrino problem is usually attributed to the hypothetical transmutation of neutrinos from one type into another en route from sun to earth. One alternative is to propose that helium-3 from the cooler outer layer of the sun sinks to the lower, warmer depths, thus moderating neutrino production (Update 295). This modification in the model of the sun is undesirable, Bahcall believes (Bahcall et al.,Physical Review Letters, 13 Jan. 1997). He cites recent helioseismic measurements of the velocity of sound waves inside the sun; the velocity in turn depends on the ratio of the temperature to the mean molecular weight at any particular depth inside the sun. Bahcall's analysis finds a very good agreement (for all depths in the sun) between the measured values of sound velocity and the values predicted using the standard solar model, and a much poorer agreement for models including helium-mixing. (Physics Today, March 1997.)