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Crater size–frequency analyses have shown that the largest volcanic plains deposits on Mercury were emplaced around 3.7 Ga, as determined with recent model production function chronologies for impact crater formation on that planet. To test the hypothesis that all major smooth plains on Mercury were emplaced by about that time, we determined crater size–frequency distributions for the nine next‐largest deposits, which we interpret also as volcanic. Our crater density measurements are consistent with those of the largest areas of smooth plains on the planet. Model ages based on recent crater production rate estimates for Mercury imply that the main phase of plains volcanism on Mercury had ended by ~3.5 Ga, with only small‐scale volcanism enduring beyond that time. Cessation of widespread effusive volcanism is attributable to interior cooling and contraction of the innermost planet.

Vesta to the Core Vesta is one of the largest bodies in the main asteroid belt. Unlike most other asteroids, which are fragments of once larger bodies, Vesta is thought to have survived as a protoplanet since its formation at the beginning of the solar system (see the Perspective by Binzel , published online 20 September). Based on data obtained with the Gamma Ray and Neutron Detector aboard the Dawn spacecraft, Prettyman et al. (p. 242 , published online 20 September) show that Vesta's reputed volatile-poor regolith contains substantial amounts of hydrogen delivered by carbonaceous chondrite impactors. Observations of pitted terrain on Vesta obtained by Dawn's Framing Camera and analyzed by Denevi et al. (p. 246 , published online 20 September), provide evidence for degassing of volatiles and hence the presence of hydrated materials. Finally, paleomagnetic studies by Fu et al. (p. 238 ) on a meteorite originating from Vesta suggest that magnetic fields existed on the surface of the asteroid 3.7 billion years ago, supporting the past existence of a magnetic core dynamo.