40Ar retention in the terrestrial planets. 
The solid Earth is widely believed to have lost its original gases through a combination of early catastrophic release and regulated output over geologic time.  In principle, the abundance of 40Ar in the atmosphere represents the time-integrated loss of gases from the interior, thought to occur through partial melting in the mantle followed by melt ascent to the surface and gas exsolution.  Here we present data that reveal two major difficulties with this simple magmatic degassing scenario--argon seems to be compatible in the major phases of the terrestrial planets, and argon diffusion in these phases is slow at upper-mantle conditions.  These results challenge the common belief that the upper mantle is nearly degassed of 40Ar, and they call into question the suitability of 40Ar as a monitor of planetary degassing.  An alternative to magmatism is needed to release argon to the atmosphere, with one possibility being hydration of oceanic lithosphere consisting of relatively argon-rich olivine and orthopyroxene.