A violent tail

The solar system’s innermost planet is even more mercurial than planetary scientists had thought, a trio of reports posted online July 15 in Science reveals.

When NASA’s MESSENGER spacecraft flew past Mercury for the third time in September 2009, it found evidence that magnetic storms are much more intense and rapid-fire there than on Earth.

One of the Science articles also provides new details about findings from the flyby, first announced last November, that suggest Mercury was volcanically active much more recently — as little as 1 billion years ago — than researchers had previously thought.

The September MESSENGER flyby is the first time scientists have documented the buildup of magnetic energy in Mercury’s magnetotail, the magnetic lines of force that form a region shaped like a comet’s tail on the planet’s night side. The magnetotail absorbed 10 times more magnetic energy from the sun than Earth’s magnetotail does. It then dumped that energy in just two to three minutes, compared to two to three hours for Earth’s field, report James Slavin of NASA’s Goddard Space Flight Center in Greenbelt, Md., and his colleagues.

It isn’t just Mercury’s proximity to the sun, but also the planet’s lack of an ionosphere to slow the interaction between the solar and planetary magnetic fields, that drives the rapid and intense energy buildup that leads to storms, says Slavin.

During the buildup, MESSENGER did not detect high-energy charged particles, which are often seen in conjunction with magnetic storms on Earth and were also reported by Mariner 10 during its Mercury flyby in 1974. The solution to that puzzle, which may not come until MESSENGER begins its yearlong orbit around Mercury next March, could shed light on how charged particles are revved up to high energies at Earth, Slavin says.

Signs of recent volcanism surprised researchers last fall when they first saw the images from September’s flyby. Most of the evidence for recent volcanoes comes from the discovery of a double-ringed impact basin, now called Rachmaninoff. The smooth plains material within the basin’s inner ring, which has a diameter of about 130 kilometers, has a different color than the outer material and its surface is marred by far fewer craters, Louise Prockter of Johns Hopkins University’s Applied Physics Laboratory in Laurel, Md., and her collaborators report.

The color and smoother surface within the inner ring indicate it is younger than the surrounding material and was deposited after the basin formed. The observations strongly suggest that volcanoes erupted on Mercury as recently as 1 billion years ago, more than 2 billion years later than previous estimates, Prockter says.

“The evidence that the smooth plains are young is very strong — you can clearly see a paucity of superposed impact craters on those plains compared with the surrounding basin terrain,” comments planetary scientist Paul Spudis of the Lunar and Planetary Institute in Houston. Volcanism is the best explanation for the origin of these deposits, he adds.

Previous deposits of smooth plains studied on Mercury have all been heavily cratered, which led to the prejudice that Mercury was geologically dead and more akin in thermal and volcanic history to Earth’s moon rather than to Mars, Spudis adds. “The new data require us to rethink that concept,” he says.

Prockter’s team has also found that a large volcanic vent northeast of
Rachmaninoff, discovered during the same flyby, is about twice as large as any other found on the planet.

The relatively recent volcanic activity suggests that Mercury's mantle doesn’t efficiently remove heat from the planet’s interior, Prockter says. If it had, “volcanic activity would have ceased earlier,” she adds.