Day 96: Mercury fly-by reveals volcanic history, magnesium and a whoppin’ crater

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institute of Washington

A major research collaboration unmasked Mercury today, revealing more than people have ever known about the closest planet to the Sun.

NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft flew by Mercury last fall, and researchers have been mining its data and images to fill in maps for nearly half the planet that were previously blank.

The new data also reveal a history of volcanoes that makes Mercury more like Mars than the Moon, and a modern surface that contains some very familiar materials — magnesium, for one, and possibly iron and titanium. 

The findings were unveiled in a press conference today and appear in four separate papers in the journal Science.

This new image was compiled from the views of several cameras aboard the mission, in different wavelengths, as it departed Mercury.

Impact craters blasted recently by asteroids, such as the yellow crater called Kuiper near the center, have contrasting ejecta and rays because they excavated fresh material. Kuiper is 38 miles (62 km) across.

Other impact craters, such as those with ejecta that appear blue, have color contrasts because they excavated material from below the planet’s surface.

Lermontov crater, 94 miles or 152 km across, near the top left, appears orange and is thought to contain pyroclastic deposits from explosive past volcanic activity.

And then, there’s Rembrandt.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institute of Washington

Rembrandt is more than 700 kilometers (430 miles) in diameter. If the Rembrandt basin had formed on the east coast of the United States, it would span the distance between Washington, D.C., and Boston.

Researchers believe the Rembrandt basin formed about 3.9 billion years ago, near the end of the period of heavy bombardment of the inner Solar System. Although it’s ancient, the Rembrandt basin is younger than most other known impact basins on Mercury.

Thomas Watters of the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum, lead author on one of the papers, said this is the first time scientists have seen terrain exposed on the floor of an impact basin on Mercury that is preserved from when it formed.

“Terrain like this is usually completely buried by volcanic flows,” he said.

The landforms in the basin differ from anything seen on Mars or the Moon, suggesting multiple stages of volcanic and tectonic activity. The most recent tectonic event in Rembrandt was the formation of a large thrust-fault scarp more than 620 miles (1,000 kilometers) long.

Watters said the fault is “on the scale of the San Andreas fault in California,” a consequence of interior cooling and global contraction of the planet.

Up until last year, globes of Mercury were blank on one side. The Mariner 10 spacecraft explored the small planet in three flybys (1974-1975), but since no more than half was ever seen it remained the least understood of the four terrestrial planets – Mercury, Venus, Earth and Mars.

Through MESSENGER, researchers have not only constructed a nearly complete globe, they have also started teasing out the composition of the planet’s crust and chronicling its origin and evolution.

With its ancient craters and smooth plains both covered in a fine-grained gray soil, the surface of Mercury superficially resembles the surface of the Moon. 

But while the highlands on the Moon are thought to have formed as the result of a global magma ocean, evidence now points to a mercurian crust that formed more like Mars’ crust did.

Research led by Brett Denevi, a postdoc at Arizona State University, confirms that volcanism on Mercury was widespread, visible across nearly the entire planet, and that much of the crust may have formed in repeated volcanic eruptions. 

Some dark units seen on the crust are consistent with high concentrations of iron- and titanium-bearing oxide minerals being excavated from below the surface, but that link hasn’t been confirmed.

Researchers from the University of Colorado at Boulder, however, are confident about the magnesium. 

“Detecting magnesium was not too surprising, but seeing it in the amounts and distribution we recorded was unexpected,” said William McClintock, a senior research associate in CU Boulder’s Laboratory for Atmospheric and Space Physics. 

The CU-Boulder instrument also measured other elements in the exosphere during the Oct. 6 flyby, including calcium and sodium. “Since calcium and magnesium are chemically similar, we might expect them to have a similar distribution in Mercury’s exosphere,” McClintock said. “But they don’t, and we don’t yet understand why.”

MESSENGER might help clear that up  – and reveal more mysteries — when it makes its third and final flyby of the planet this fall, before setting into orbit around it.

Sources: press releases from NASA, Arizona State University (via Eurekalert) and CU Boulder

See also: MESSENGER page