Volcanoes Smoothed Over Mercury's Surface 500 Million Years After Formation

Mercury had more than a few wrinkles erased when its surface was flooded with lava some 500 million years after its formation, according to a new study published in the journal Nature.

Despite being a rocky planet itself, the surface of Mercury is significantly different from those of other well-known rocky bodies, such as the Moon and Mars. First documented by the space probe Mariner 10, the inner-most planet is covered by smooth as well as cratered plains, the origins of which long eluded scientists.

In order to solve this mystery, Simone Marchi of the NASA Lunar Science Institute at the Southwest Research Institute (SwRI) in Boulder, Colo., collaborated with NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) team to discover whether the plains were formed by volcanic flows or composed of material ejected from the planet's giant impact basins.

MESSENGER, the first spacecraft to orbit Mercury, provided new insights showing that the younger plains at least resulted from vigorous volcanic activity. Still, scientists were unable to establish limits on how far into the past this volcanic activity may have occurred, or to what degree the planet's surface may have been resurfaced by very old volcanic plains.

Now, a team of scientists has concluded that the oldest visible terrains on Mercury are between 4 billion to 4.1 billion years old, and that the first 400 to 500 million years of the planet's evolution are not etched on its surface.

In order to come to this conclusion, the team measured the sizes and numbers of craters on the most heavily cratered terrains using images obtained by the MESSENGER spacecraft during its first year in orbit around Mercury.

The researchers then extrapolated to Mercury a model originally developed for comparing the Moon's crater distribution to a chronology based on the ages of rock samples gathered during the Apollo missions.

"By comparing the measured craters to the number and spatial distribution 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," Marchi, who has a joint appointment between two of NASA's Lunar Science Institutes - one at the SwRI in Boulder and another at the Lunar and Planetary Institute in Houston - said in a press release.

Those results set the age boundary for the oldest terrains on Mercury back to the same point in time as the Late Heavy Bombardment (LHB) period, an age of intense cosmic bumper cars recorded in lunar and asteroidal rocks and by the numerous craters on the Moon, Earth and Mars in addition to Mercury.

Meanwhile, the researchers found that the youngest and broadest volcanic provinces dated back roughly 3.6 billion to 3.8 billion years, just after the end of the LHB, leading the researchers to hypothesize that these large projectiles puncturing the planet's thin surface may have caused increased volcanism that ultimately led, according to Chapman, to a "global resurfacing."