Many worlds which form in a young solar system are neither very massive, nor composed of a great deal of heavy materials. thus, there is a sizable population of planets which are effectively dead, geologically speaking. Once the fires of their initial creation cool, the only internal geological events are small quakes powered by internal gases slowly seeping to the surface.
Such planets are literal fossils of the formation period of their solar systems. While they continue to accrue impact craters, the scars of their birth remain preserved for billions of years. In many cases, these take the form of maria, dark regions of basalt where vast seas of lava once surged, often times formed by massive impacts. Earth's Moon is marked by such regions.
These planets are quite often found as moons, having been formed through massive impacts with a parent body and a second, planet-sized interloper. The result is a new world formed from the lighter materials of both previous worlds. Indeed, to even form a planet in this manner requires a great deal of luck, for the impact must occur at the proper angle and velocity. However, Selenian planets can form by other means as well, and are often found in the inner regions of a solar system.
In most ways, Earth's Moon is a typical Selenian world. And although it is a moon, it is still representative of other Selenian planets.
From a fair distance above the surface, a Selenian world can appear rocky and ragged. From ground level, however, these worlds are often worn smooth by billions of years of meteoric impacts.
The Uses Of Selenian Worlds: While in a mineralogical sense Selenian worlds may not have much to offer, they do serve as excellent colonial sites. While an airless planet may not seem to make this likely, locked within the rocks can be found all that is needed: oxygen, water, even robust building material. Soil from these worlds, enriched with terrestrial elements, makes fine growing dirt for crops, even.
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