Orbital images from the MESSENGER spacecraft show that ~27% of Mercury's surface is covered by smooth plains, the majority (>65%) of which are interpreted to be volcanic in origin. Most smooth plains share the spectral characteristics of Mercury's northern smooth plains, suggesting they also share their magnesian alkali‐basalt‐like composition. A smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope, suggesting more ultramafic compositions, an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation. The knobby and hummocky plains surrounding the Caloris basin, known as Odin‐type plains, occupy an additional 2% of Mercury's surface. The morphology of these plains and their color and stratigraphic relationships suggest that they formed as Caloris ejecta, although such an origin is in conflict with a straightforward interpretation of crater size–frequency distributions. If some fraction is volcanic, this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions. Smooth plains are widespread on Mercury, but they are more heavily concentrated in the north and in the hemisphere surrounding Caloris. No simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed. A likely volcanic origin for some older terrain on Mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large‐scale volcanic deposits rather than differences in crustal formational process.

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.

High‐resolution images from Chandrayaan‐1 Terrain Mapping Camera and Lunar Reconnaissance Orbiter Camera reveal landslides and gully formation on the interior wall of a 7 km‐diameter simple crater emplaced in Schrödinger basin on the farside of the Moon. These features occur on the steep upper crater wall, where the slope is ~35°. The gullies show a typical alcove‐channel‐fan morphology. Some gullies incise bedrock, where impact‐related faults are present. Slope failure along the concentric faults also led to formation of landslides. Dark slope streaks are abundant at the bright gully regions, especially near the fan and channel deposits. Spectral characteristics inferred from data obtained by Hyperspectral Imager and Moon Mineralogy Mapper on board Chandrayaan‐1 show that the gullies and landslides are characterized by high optical immaturity and devoid of prominent spectral absorption features related to water or hydroxyl molecules, suggesting youthful dry‐granular flows. Mass movements on the crater wall led to the formation of arcuate ridges and ponding of fine‐grained sediments on the crater floor. Runout flows from small impact craters on the slopes indicate that impact‐induced seismic shaking was responsible for the downslope mass movements. Crater size‐frequency distributions suggest a minimum age of 18–2 Ma for the gullies and 2 Ma for the landslides, while age of the host crater ejecta was inferred to be about 175 Ma. The gullies and landslides also occur on the interior wall of other impact craters elsewhere on the Moon and probably formed by similar processes.