Time for a new neural code?
by vaporization, cannot be used as building blocks of the planets. So these papers provide no support for popular planetary building modelss7. Incidentally, they also dispose of the old idea8 that the depletion of sodium and potassium in lunar basalts was due to evaporation as the basaltic lavas were extruded on to the hard vacuum and low temperatures of the Moon's surface. The low abundance of the alkali elements in the Moon (less than 20% of the Earth's already depleted budget) is now shown to be an inherent property of the material that formed the Moon. Evaporative loss of potassium during the formation of chondrules and refractory inclusions is also ruled out, so that the depletion of volatile elements in chondrules can no longer be ascribed to loss during the flash-melting episode that melted the precursor silicate dust balls. Chondrules are major components of the chondrites, dated at 4,563 million years old9; the refractory inclusions give slightly earlier ages of 4,566 million years9. So it seems that the depletion of volatile elements occurred before the formation of the chondrules, or the refractory inclusions, and thus before 4,566 million years ago. Because these are the earliest available samples of the Solar System that we can measure, we must retreat to a remoter epoch in our search for the mechanism for depletion. Although Humayun and Clayton2 incline to condensation from an initially hot nebula as the cause, considerable problems remain. The observed oxygen isotopic heterogeneity in meteorites seems unlikely to have survived in such a hightemperature environment, while condensation might be expected to affect the isotopicratios. It is tempting to ascribe the depletion to early violent solar activity that swept water and other volatiles out to a 'snow line' at five astronomical units from the Sun, where condensation enabled the rapid, early growth of Jupiter. All of these problems provide rich pickings for further investigation. About 20 years ago, oxygen isotope data from Clayton's laboratory'0 clarified the relationships among the meteorite