Mercury is a mysterious little world. Many features on its surface show evidence that since its formation, cooling of its core has caused the entire planet to shrink by between about 5 and 10 kilometres in radius, but for that shrinking to occur, heat must have somehow escaped. A series of simulations now show that it may have travelled from Mercury’s core through its mantle via heat pipes.

Many of our current models for the early history of Mercury fail to explain all of the features we see now. For example, the stretch marks left behind by the planet’s contraction suggest that most of the shrinking happened early on, within the first 500 million years after Mercury formed, before continuing on at a slower rate.

Georgia Peterson at the University of British Columbia in Canada and her colleagues analysed images of Mercury’s surface sent back from the Messenger and Mariner 10 missions and ran 2430 simulations of Mercury’s evolution – changing various parameters including the initial temperatures of the core and mantle and the types of materials making up those layers – to determine how the planet’s features may have formed. They presented the work at the virtual Lunar and Planetary Science Conference on 17 March.

“Everything about Mercury is a little bit strange,” says Lauren Jozwiak at Johns Hopkins University in Maryland. “There are all these parts that don’t quite fit, and this work takes the data and tries to go back and make the models explain it.”

Magnetic rocks in the crust demonstrate that the planet had a global magnetic field around the same time as the period of rapid contraction, but we don’t know how that magnetic field was sustained. Planetary magnetic fields are generally formed by churning in their molten cores, which creates a phenomenon called a magnetic dynamo. Without taking into account volcanism, Mercury’s early core wouldn’t have churned enough to maintain a magnetic field, said Peterson.

Read more: Mercury once had a graphite crust floating on a sea of magma

But if magma travelled from the planet’s core through its mantle, finally solidifying to form a crust on the surface, that could have stirred up the core. Usually, volcanoes are fuelled by many small veins of magma called dikes, but Mercury may have been cooled by melted rock flowing through larger tubes called heat pipes.

“There is so much melt being generated that the inside of the planet streamlines it into a sort of mega dike which is really efficient in getting heat out of the interior,” says Jozwiak.

Peterson’s team found that magma flowing to Mercury’s surface in heat pipes could have cooled down the mantle quickly, explaining why the planet shrank so rapidly in its early years.

“By including heat pipes, their model of how Mercury’s heat changes over time does a good job of predicting features we observe on Mercury’s surface,” says Kayla Iacovino at NASA’s Johnson Space Center in Florida.

Cooling the mantle would have created temperature gradients inside the planet that would enhance the churning of the core. “Early mantle cooling associated with volcanic resurfacing is great for maintaining a dynamo,” said Peterson. Understanding how Mercury’s early magnetic field worked could help us explain what keeps its modern-day magnetic field going.