Mercury Was Once Bigger, Then It Shrank

As the planet cooled, it contracted and shut off the surface lava flows about 3.8 billion years ago

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury didn’t always look this way. NASA’s Messenger spacecraft has shown us the pockmarked face of the tiny barren planet, and astronomers suspect that ridges the running across the surface are actually wrinkles in the crust caused by a period of rapid cooling as the planet transitioned from molten to solid. The entire planet also shrunk—by about 6.8 miles in diameter—an action that seems to have "abruptly squeezed off volcanic activity, reports Marcus Woo for Wired.

Scientists knew that Mercury once had surface lava flows from volcanic activity—the Mariner 10 and Messenger spacecraft images told them that. But the new analysis of Messenger’s images tells us more, a group of planetary scientists led by Paul Byrne announced at the American Geophysical Union annual meeting this month.

The cratered surfaces of planets and other solar system bodies (such as moons or asteroids) tell a tale of the age of the surface. Older surfaces have had more time to get pummeled. Woo writes:

By counting the most recent craters that hit the southern lava plains after the lava cooled, Byrne and his colleagues found that the southern plains are no younger than the two main plains in the north. “You pretty much turned off the tap 3.8 billion years ago,” Byrne said.

That time period coincides with when Mercury was shrinking, suggesting the contraction may be what shut down the surface lava, Byrne says. Theoretical models have shown that planetary shrinkage would squeeze the outer layers of the planet, forming a tight seal that could prevent lava from reaching the surface.

Bryne and his colleagues’ work really helps "pin down the timing of when [volcanism] stopped," writes Woo.  It happened about 3.8 billion years ago. That lines up neatly with the last flows, a coincidence that suggests the shrinking was indeed responsible for turning off the lava tap, says Byrne.

But there’s another thing to consider. Woo reports:

Starting at about 4.2 billion years ago, asteroids and comets were swarming throughout the solar system, slamming into all of the planets during what’s called the period of Late Heavy Bombardment. But by 3.8 billion years ago, the impacts began to tail off—right when Mercury’s volcanism seems to have stopped.

It’s possible that the impacts played a role in volcanic activity on Mercury—by releasing pressure in the cooling crust and prolonging the period of volcanic activity. Computer simulations boosted by Messenger’s reconnaissance may help answer this question.

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