Geology of Mercury

Of all the terrestrial planets in the Solar System, the geology of Mercury is the least understood. This stems largely from Mercury's proximity to the Sun which makes reaching it with spacecraft technically challenging and Earth-based observations difficult.

Mercury's surface is dominated by impact craters, basaltic rock and smooth plains, many of them a result of flood volcanism, similar in some respects to the lunar maria. Other notable features include vents which appear to be the source of magma-carved valleys, often-grouped irregular-shaped depressions termed "hollows" that are believed to be the result of collapsed magma chambers, scarps indicative of thrust faulting and mineral deposits (possibly ice) inside craters at the poles. Long thought to be geologically inactive, new evidence suggests there may still be some level of activity.

Mercury's density implies a solid iron-rich core that accounts for about 60% of its volume (75% of its radius). Mercury's magnetic equator is shifted nearly 20% of the planet's radius towards the north, the largest ratio of all planets. This shift lends to there being one or more iron-rich molten layers surrounding the core producing a dynamo effect similar to that of Earth. Additionally, the offset magnetic dipole may result in uneven surface weathering by the solar wind, knocking more surface particles up into the southern exosphere and transporting them for deposit in the north. Scientists are gathering telemetry to determine if such is the case.

After having completed the first solar day of its mission in September 2011, more than 99% of Mercury's surface had been mapped by NASA's MESSENGER probe in both color and monochrome with such detail that scientists' understanding of Mercury's geology has eclipsed the level achieved following the Mariner 10 flybys of the 1970s.