Mercury More Exciting Than Mars

Mercury was once seen as a cold, dead little world, spinning around the sun unchanged for the past 4 billion years.

No longer: Observations from the Messenger spacecraft say it’s anything but.

NASA’s orbiter is sending back evidence of massive volcanism, strange impact craters and magnetic tornadoes that funnel plasma directly from the sun to the planet’s surface.

“It’s definitely not this picture of an ancient world where everything that happened to it happened billions of years ago and nothing happened since then,” said Tom Watters of the Smithsonian Institution in Washington, DC. “We’re seeing a very dynamic planet that has a lot going on today.”

A slew of papers in Science report what Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging) saw on its Oct. 6 flyby of the small rocky planet. The spacecraft (whose admittedly contrived name was chosen because Mercury was the messenger to the gods in Roman mythology) launched in 2004, and will fly by Mercury a total of three times before settling into orbit in 2011.

us_satellite_rembrandt_basin_cropCombined with mapping by Mariner 10 in the ’70s, we now know what 90 percent of the surface looks like.

And it looks weird. Superficially, Mercury looks a lot like the moon: small, grayish-brown and pockmarked with craters. Some scientists assumed that Mercury’s surface formed the same way the moon’s did, with lighter rocks rising to the surface of a magma ocean and congealing into a brittle crust early on. But the new observations reveal that 40 percent of the surface was formed by volcanoes.

“Up until before Messenger’s arrival, we weren’t even sure that volcanism existed on Mercury,” said Brett Denevi of Arizona State University, lead author on a paper describing the evolution of Mercury’s crust. “Now we’re seeing it’s very widespread across the surface.”

Most were probably effusive volcanoes, in which molten muck bubbles up from cracks in the surface and spreads, filling in crater basins and smoothing the landscape. But a few were explosive volcanoes, what Denevi described as “big fire fountains of magma” whose ejected material turned to glass before hitting the ground.

Messenger also spotted an impact basin that is “unlike anything I have ever seen anywhere in the solar system,” said Watters, the lead author on a paper focusing on this funny feature. The basin, dubbed Rembrandt, is one of the biggest and youngest craters on Mercury. If it were on Earth, its edges would extend from Boston to Washington, D.C. But for Rembrandt, size doesn’t matter. It’s what’s inside that counts.

Most impact basins — craters more than 186 miles in diameter — are formed by impacts so forceful that they crack the planet’s crust all the way to a liquid or molten layer underneath. The molten rock oozes into the basin and fills it up. The basin sags under the weight of all this extra material, and deforms the ground around it. The interior of the basin crinkles up into ridges, and the edges stretch into troughs.

Rembrandt is only partially filled with volcanic material. Even stranger is its pattern of ridges and troughs, which lie side by side in a radial pattern from the center, rather than being confined to either the rim or the middle.

“That pattern is like nothing we’ve ever seen before,” Watters said. “We’re going to have to go back and rethink the way that we generate these stresses in these basins.”

Observations from Messenger might help solve a few mysteries, in addition to creating new ones. Scientists wondered if Mercury had a stable magnetic field fueled by a liquid core like Earth, or a ghostly magnetic field left over from a once-liquid core that’s since frozen, like Mars. The Messenger flyby clinched it in favor of an active magnetic field — and though it’s weak, it’s really active.

Earth has an atmosphere and a strong magnetic field to protect it from the constant stream of ions ejected from the sun at supersonic speeds known as the solar wind. Mercury is not so lucky. Its magnetic field is just 1 percent the strength of Earth’s, and it has no atmosphere to speak of. What atmosphere it has is tenuous and extremely variable, made up mostly of sodium one day and mostly calcium the next.

This leaves its surface vulnerable to being ravaged by the solar wind at the best of times. Messenger flew by at a particularly bad moment, and ran right through a magnetic tornado.

Magnetic tornadoes form when the magnetic field in the solar wind links up to the field generated by a planet, a process called magnetic reconnection. Bundles of magnetic field lines connect the surface of the planet directly to the surface of the sun, and as the solar wind pushes them away from the sun, they twist and whirl like cyclones. On Earth, these cyclones (technically called “flux transfer events”) dance on the ionosphere, creating the Northern Lights and messing up GPS systems.

On Mercury, though, the twisters were 10 times as strong as any magnetic cyclones observed on Earth. With so little atmosphere to interfere, Mercury’s magnetic tornadoes are great spinning chutes that ionized gas can slide down.

“They act as magnetic channels or open windows that allow solar wind plasma from the sun, very fast and very hot, to come right down those field lines and impacts the surface,” said Jim Slavin of NASA Goddard Spaceflight Center. When the gas hits the surface, it knocks off neutrally-charged atoms and sends them on a loop high into the sky.



Slavin thinks that could explain Mercury’s inconsistent atmosphere. “People had hypothesized that it was because of day-to-day changes in the solar wind, and how much is able to reach the surface and sputter off neutral atoms,” he said. “We may have discovered the answer to that longstanding question, why is the atmosphere so variable: Because reconnection, when it occurs, is just so intense.”