Because it is so technically difficult to reach Mercury with spacecraft’s from Earth, the geology of Mercury is understood the least of all the terrestrial worlds. The main reason it is so hard to reach is due to how close Mercury is to the Sun. When a spacecraft moves down Sun’s gravitational potential well to get closer to Mercury, its potential energy is converted to kinetic energy. In order to not pass by Mercury quickly, the spacecraft must rely completely on rocket motors to enter a stable orbit or land. This is very fuel intensive; in fact, a trip to Mercury from Earth requires more fuel than the amount required to escape the Solar System. I thought this was a very interesting piece of information to illustrate the concept of a potential well and just how much Sun’s gravity can affect space travel as objects travel closer to the Sun.
Since Earth-based observation of Mercury is made difficult because of its proximity to the sun, most of the information we have about Mercury’s geology is space-observed. All of the space-observed information we have about the geology of Mercury comes from two NASA space probes, the Mariner 10 (Nov 1973 – Mar 1975) and the MESSENGER (Aug 2004 – Apr 2015). Less than 45% of the surface was mapped after the Mariner 10 completed 3 separate flybys, but more than 99% of the surface was mapped after the MESSENGER successfully entered Mercury’s orbit in 2011.
The surface of Mercury is mainly made up of plains and impact craters, and is overall very similar to the moon in appearance. These were created as a result of flood volcanism, which occurred fairly early in Mercury’s geological history. There are also vents located on Mercury’s surface, and they are thought to be the source of magma-carved valleys. Fault scarps showing thrust faulting are found inside craters at the poles of Mercury. Based on what we know about Mercury’s density, it is implied that it has a solid iron-rich core that takes up about 75% of its radius. Its magnetic equator is shifted to the north nearly 20% of the radius, and this is thought to be caused by one or more iron-rich molten layers around the core or uneven weathering and deposition by solar wind. There has been an observed possibility of ice on Mercury’s poles, but this claim is not confirmed. If the ice observations are correct, astronomers believe that it must have originated from external sources like impacting comets.