The formation of Mercury's smooth plains

There has been extensive debate about whether Mercury's smooth plains are volcanic features or impact ejecta deposits. We present new indirect evidence which supports a volcanic origin for two different smooth plains units. In Borealis Planitia, stratigraphic relations indicate at least two distinct stages of smooth plains formation. At least one of these stages must have had a volcanic origin. In the Hilly and Lineated Terrain, Petrarch and several other anomalously shallow craters apparently have been volcanically filled. Areally extensive smooth plains volcanism evidently occurred at these two widely separated areas on Mercury. These results, combined with work by other researchers on the circum-Caloris plains and the Tolstoi basin, show that smooth plains volcanism was a global process on Mercury. Present data suggest to us that the smooth and intercrater plains may represent two distinct episodes of volcanic activity on Mercury and that smooth plains volcanism may have been triggered by the Caloris impact. High-resolution and multispectral imaging from a future Mercury spacecraft could resolve many of the present uncertainties in our understanding of plains formation on Mercury.