EVIDENCE FOR ANCIENT VOLCANISM ON MERCURY

Much of Mercury’s surface is comprised of smooth to gently rolling terrain that is more cratered than Mercury’s smooth plains but less cratered than the lunar highlands. This terrain, known as intercrater plains, is thought to be the result of widespread resurfacing that removed craters <40 km in diameter. The mechanism of this resurfacing is controversial and may have involved fluidized ejecta from ancient basin-forming impact events and/or effusive volcanic eruptions; little unambiguous evidence in favor of either origin was found in Mariner 10 data. Here we examine a broad region centered at 18°N, 20°E, that was not observed by Mariner 10. We confirm the presence of a large, ancient impact basin ~1500 km in diameter, similar to the Caloris basin (1550 km). Evidence for this basin’s presence includes preserved portions of the rim and basin sculpture, localized tectonic features, vents associated with the rim, and thinned crust. However, whereas the rim and basin sculpture are preserved along a northeastern segment, most primary features of the basin appear to have been buried by plains materials. A region of smooth plains ~400 km across is located at the center of the basin and embays intercrater plains, which cover the remainder of the basin floor and portions of the rim and exterior. These intercrater plains are intermediate in reflectance, but tens of superposed craters expose material of higher reflectance and with a steeper spectral slope, color properties that are common among deposits interpreted to be volcanic in origin and elsewhere shown by MESSENGER’s X-Ray Spectrometer to have elemental ratios consistent with a low-iron basalt-like composition. The color and stratigraphic relationships may indicate that either local and distant impacts produced a surficial cover of regolith that largely masks the original intercrater plains surface (analogous to lunar cryptomaria), or there is layering of volcanic units of different compositions. On the basis of these observations, we interpret this regional intercrater plains deposit as volcanic in origin, and its crater size–frequency distribution is consistent with an age of 4.0 Gy. Such early intercrater plains volcanism has important implications for the duration of Mercury’s volcanism, the planet’s interior thermal history, and the origin and evolution of the crust.