Why don some planets and moons (satellites have geologic activity?
A geologically active globe would have a geoDynamo in its core. The core may be molten liquid consisting of iron/nickel and be geologically active, or it may have cooled and solidified, and is therefore inactive. A geodynamo creates the magnetic field required to repel the solar wind and hold down an atmosphere. Keep in mind that it is the magnetic field of Earth which repels the solar wind and protects our water molecules from being photodissociated by solar ultraviolet. Mars once had flowing rivers, but lost all of it due to photodissociation by solar ultraviolet. There are 166 moons in our solar system and it will be a daunting task to profile them to enable us to terraform them over time.
