Early Bombardment History and Volcanism of Mercury Reveals Planet's Age

Sun-scored Mercury is the closest planet to the sun, whipping around the star once every 88 days. With its pocked surface, this planet is known for the constant bombardment it endures from space debris. Yet in addition to its craters, there are portions of smooth plains. Now, NASA has discovered a little bit more about Mercury. Scientists have studied the planet's surface to understand whether these plains were formed by volcanic flows or composed of material ejected from the planet's giant impact basins.

In order to get a better glimpse of this planet's surface, researchers examined images from NASA's MESSENGER spacecraft. While the pictures showed that the younger plains resulted from vigorous volcanic activity, though, the scientists needed to delve a bit further to determine how far in the past this volcanic activity may have occurred.

The researchers measured the sizes and numbers of craters on the most heavily cratered terrains using images obtained by MESSENGER. They then extrapolated to Mercury a model that was originally developed for comparing the moon's crater distribution to a chronology based on the ages of rock samples gathered during the Apollo missions.

In the end, the scientists found that the oldest visible terrains on Mercury are between 4 and 4.1 billion years old. The first 400 to 500 million years of the planet's evolution are, in fact, not recorded on its surface. This means that volcanic activity was likely present.

"By comparing the measured craters to the number and spatial distributing of large impact basins on Mercury, we found that they started to accumulate at about the same time, suggesting that the resetting of Mercury's surface was global and likely due to volcanism," said Simone Marchi, the lead author of the study, in a news release.

The findings not only reveal Mercury's age, but also show that the oldest terrains on the planet are contemporary with the so-called Late Heavy Bombardment (LHB). This period of intense asteroid and comet impacts is recorded in lunar and asteroid rocks and by the numerous craters on the Moon, Earth, Mars and Mercury. This could mean that while some of the resurfacing was due to volcanism, part of it could have also been due to the impact of large projectiles hitting Mercury's thin solid crust.

The findings are published in the journal Nature.