Mariner 10 photograph of the surface of the planet Mercury, the closest planet to the Sun. The photo shows the edge of the Caloris Basin, the largest structural feature on Mercury. The basin is 1300 km in diameter, surrounded by concentric rings of mountains which rise about 2 km high. The inner basin is filled by plains, presumably of volcanic origin, that are highly ridged and fractured. Other smooth, less deformed plains extend for hundreds of kilometres eastward. The impact also influenced the formation of the unusually hilly terrain on the opposite side of the planet as a result of seismic wave activity.