The truth about Mercury and volcanoes explains a lot about how rocky planets form in general.

A fascinating new study finds that volcanic activity on Mercury ended a long, long time ago — about 3.5 billion years ago to be exact, adding to scientists’ understanding of the geological evolution of Mercury. It also expands scientists’ understanding of what happens when rocky planets cool and contract.

Scientists at the North Carolina State University were able to determine when most of Mercury’s volcanoes that were responsible for forming much of its crust ended by taking photographs of the surface with NASA’s MESSENGER mission, according to an NCSU statement. Obviously, we don’t have any physical samples to conduct radiomatric dating, sor esearchers were forced to use something called crater size-frequency analysis, which allows them to estimate the age based on the number and size of craters on the surface of the planet, using mathematical models to determine just how old the volcanic deposits are.

That is where they got to their figure of 3.5 billion years, which is a lot different to other planets in our solar system, including our own.

“There is a huge geological difference between Mercury and Earth, Mars or Venus,” NC State assistant professor and planetary geologist Paul Byrne said in the statement. “Mercury has a much smaller mantle, where radioactive decay produces heat, than those other planets, and so it lost its heat much earlier. As a result, Mercury began to contract, and the crust essentially sealed off any conduits by which magma could reach the surface.

“These new results validate 40-year-old predictions about global cooling and contraction shutting off volcanism,” e continued. “Now that we can account for observations of the volcanic and tectonic properties of Mercury, we have a consistent story for its geological formation and evolution, as well as new insight into what happens when planetary bodies cool and contract.”