Being so near the Sun and having been visited by only one spacecraft (Mariner 10), Mercury is the least understood of the terrestrial planets. Even so, much has been learned from that one mission and from Earth-based telescopic studies. Although close-up pictures of Mercury are often mistaken for those of the Moon, being similarly highly cratered and mostly colorless, the two bodies have distinct and interesting differences.Mercury has three major terrain types: (1) heavily cratered, (2) intercrater plains, and (3) smooth plains. A minor but interesting type, the hilly/lineated terrain, was caused by the planet's interior focusing impact-generated seismic waves to a point at the opposite side of the planet from the event that formed the 1,300-km-diameter Caloris basin. Similar antipodal terrain's have been found on the Moon opposite the Imbrium and Orientale basins. Computer simulations indicate the focused waves generated vertical ground movements of more than 1 kilometer, creating the hilly terrain. The heavily cratered uplands record the late heavy bombardment period that ended about 3.8 billion years (b.y.) ago. The Caloris basin-forming impact may have been one of the last large impacts from this period. The intercrater plains are probably volcanic, and very old, about 4 b.y. The smooth plains are similar in mode of occurrence and morphology to the dark lunar maria, yet are much less obvious. These plains are found in old basins that formed during the late heavy bombardment and so must have formed around 3.8 b.y. ago. Volcanism mostly died out within the first 800 million years (m.y.) of its history.
Activity since then may have been limited to global compressive stress, probably caused by the planet shrinking an estimated 4 kilometers in diameter as it cooled (modeling of Mercury's thermal history suggests as much as 10 km shrinkage could have occurred). This compression is indicated by ridges (see figure) that cut the intercrater plains.
Some hints about the interior are given by Mercury's high uncompressed density (5.3 g/cm3 compared to Earth's 4.4 g/cm3), which indicates it has not only a higher iron content but also a core that makes up a much larger percentage of Mercury's volume than does Earth's core. Mercury's large core is a mystery. A proposed explanation is that a large impact during the final accretion period stripped off the lighter outer part of the planet. This large core may also account for Mercury being the only terrestrial planet other than Earth with a significant magnetic field. However, the field is so weak (1,000 times weaker than Earth's) that solar wind can reach the surface during high solar activity.