A single spacecraft landing on the Moon can send methane molecules hopping across its surface at ballistic speeds. Within a week, more than half of that exhaust ends up frozen in the permanently shadowed craters near the poles.
Those same craters are among the most scientifically valuable places on the lunar surface. They may contain prebiotic organic molecules delivered by comets and asteroids billions of years ago, material that could hold clues to how life began on Earth. The new modelling study raises practical questions about how future missions should protect those records while continuing to explore.
Ballistic hops across an airless world
Without an atmosphere to slow them down, gas molecules from rocket exhaust follow simple ballistic trajectories. They bounce from point to point until they lose enough energy to stick in the coldest regions.
Their trajectories are basically ballistic. They just hop around from one point to another.
That is how Francisca Paiva, lead author of the study, describes the movement. Her computer model tracked thousands of individual methane molecules, factoring in the influence of solar wind and ultraviolet radiation. The simulation used the European Space Agency’s Argonaut lunar lander as a concrete example.
The results are striking. Methane released from a landing near the lunar South Pole can reach the North Pole in under two lunar days. Within seven lunar days, 54 percent of the exhaust becomes cold-trapped at the poles: 42 percent at the South Pole and 12 percent at the North.
Protecting sites of high scientific interest
Permanently shadowed regions at both poles act as natural cold traps. They preserve volatiles and organic compounds in a pristine state that cannot be found elsewhere. Contamination from human activity risks complicating the interpretation of samples that future missions hope to return.