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Aerosol acidity largely regulates the chemistry of atmospheric particles, and resolving the drivers of aerosol pH is key to understanding their environmental effects. We find that an individual buffering agent can adopt different buffer pH values in aerosols and that aerosol pH levels in populated continental regions are widely buffered by the conjugate acid-base pair NH4 +/NH3 (ammonium/ ammonia). We propose a multiphase buffer theory to explain these large shifts of buffer pH, and we show that aerosol water content and mass concentration play a more important role in determining aerosol pH in ammonia-buffered regions than variations in particle chemical composition. Our results imply that aerosol pH and atmospheric multiphase chemistry are strongly affected by the pervasive human influence on ammonia emissions and the nitrogen cycle in the Anthropocene. [ABSTRACT FROM AUTHOR]

High-resolution gravity data from the Gravity Recovery and Interior Laboratory spacecraft have clarified the origin of lunar mass concentrations (mascons). Free-air gravity anomalies over lunar impact basins display bull's-eye patterns consisting of a central positive (mascon) anomaly, a surrounding negative collar, and a positive outer annulus. We show that this pattern results from impact basin excavation and collapse followed by isostatic adjustment and cooling and contraction of a voluminous melt pool. We used a hydrocode to simulate the impact and a self-consistent finite-element model to simulate the subsequent viscoelastic relaxation and cooling. The primary parameters controlling the modeled gravity signatures of mascon basins are the impactor energy, the lunar thermal gradient at the time of impact, the crustal thickness, and the extent of volcanic fill. [ABSTRACT FROM AUTHOR]

The author comments on a report within the issue by H. J. Melosh and team on the modeling of mass concentrations, or mascons, in lunar surface meteorite impact mare basalt basins, which create gravitational anomalies. The researchers suggest that mascons develop in impact craters by a process of mantle flow from the crust annulus and uplifted mantle, which create density anomalies following crustal surface cooling. Also explored is the potential use of mascons to estimate prior surface heat flux on the Moon or the planets Mars, and Mercury.

The article discusses research presented at the 2013 Lunar and Planetary Science Conference in Woodlands, Texas from March 18-22 that helps explain the origin of hidden extra mass under many lunar basins, making them basins with mass concentrations, or mascon basins. An overview of two developments in planetary science that helped generate a solution to the mascon mystery is presented including data from the U.S. National Aeronautics and Space Administration's (NASA's) twin-spacecraft Gravity Recovery and Interior Laboratory (GRAIL) mission, which offered a detailed picture of mascons, and advances in computer modeling that allowed scientist to match simulated mascon formation to the GRAIL gravity map.