Phase 1 Planetary Geology Investigation: Comparing Planet X to Earth

Introduction and Investigation Goals

This paper implements a Phase 1 Investigation to determine the fundamental geology and geophysics of an unknown planet, referred to here as Planet X. The investigation describes the critical features and characteristic ranges of the planet and attempts to ascertain the feasibility of life as well as the geological history of this world. To be successful in this mission, the investigative team must study the planet as a whole, with an emphasis on comparison and contrast with known bodies in our solar system.

Earth serves as the basis of this investigation because it is the planet we know best, and because it is a planet of extremes — featuring a highly active crust as well as an atmosphere rich in water vapor. By establishing Earth as a reference point, we can apply the principles of comparative planetology to develop hypotheses about Planet X.

Earth as a Geological Baseline

Earth is a terrestrial planet and, as such, has undergone differentiation, cratering, flooding by lava and water, and ongoing surface evolution. Differentiation is the separation of planetary material by density. We know Earth differentiated because seismic waves have revealed a core composed of metals — specifically nickel and iron — a thick mantle of dense rock, and a thin crust of low-density material. This structure includes both the lithosphere and the "plastic" asthenosphere beneath it.

The core is molten except at its very center. This conducting liquid generates the planet's magnetic field. The structure of Earth's interior, confirmed through seismology, provides a model that can be applied when analyzing the internal composition of other planets.

Tectonic Activity, Cratering, and Surface Processes

The plastic rock of the upper mantle deforms plates of crustal rock, making Earth's surface highly active through a process driven by mantle convection. The crust undergoes transformation, convergence, and separation. Mid-ocean rifts create new rock by allowing lava to seep up from below. Old crust is subducted into deep trenches at continental margins, where it is remelted — a process that causes volcanoes and earthquakes and continually recycles Earth's crust.

This active recycling explains why impact craters are rare on Earth compared to the Moon. The Moon is geologically inactive and lacks an atmosphere to mitigate damage from incoming meteorites, particularly those exceeding 350 tons. The contrast between the heavily cratered Moon and the relatively smooth surface of Earth illustrates how tectonic activity and atmospheric protection work together to erase the geological record of past impacts. Understanding plate tectonics on Earth therefore provides a critical framework for interpreting surface features on other planets.

Conclusion: Adjusting the Analysis by Planetary Type

Planet X may resemble Jupiter or Saturn in scale but lack the same interior structure, or it may share characteristics with Uranus and Neptune instead. Alternatively, if Planet X is very small — comparable to Pluto — the investigation would shift focus accordingly. A small number of impact craters on a tiny planet would suggest that the crust remains active. The next step would be to determine whether an atmosphere exists at all.

Using Pluto as a model, analysis of a small Planet X would initially point toward finding a thin atmosphere composed of nitrogen, carbon monoxide, and methane — assuming the planet is capable of capturing and retaining such a light atmospheric envelope. In all cases, the investigative methodology remains the same: use known planetary analogs, apply the tools of comparative planetology, and systematically characterize the geology and geophysics of Planet X from the outside in.