Mercury
The closest planet to the Sun is Mercury. It is also the smallest planet in the Solar System. The first American spacecraft mission to Mercury was in 1974-1975 by Mariner 10. the spacecraft flew by the planet and sent pictures back to Earth. Mariner 10 photographed only 45 percent of Mercury's surface, so scientists do not know what the other 55 percent looks like. What they do kknow is that the surface of Mercury has many craters and looks much like Earth's Moon. I also has cliffs as high as 3 kilometers on its surface. These cliffs may have formed at a time when Mercury apparently shrank in diameter.
Why would Mercury have shrunk? Mariner 10 detected a weak magnetic field around Mercury. This indicates that the planet has an iron core. Some scientists hypothesize that the crust of Mercury solidified while the iron core was still hot and molten.
As the core cooled and solidified, it contracted. The cliffs might have resulted from breaks in the crust cause by this contraction.
Why would Mercury have shrunk? Mariner 10 detected a weak magnetic field around Mercury. This indicates that the planet has an iron core. Some scientists hypothesize that the crust of Mercury solidified while the iron core was still hot and molten.
As the core cooled and solidified, it contracted. The cliffs might have resulted from breaks in the crust cause by this contraction.