Final Mercury Flyby Reveals Huge Magnetic "Power Surges"

Things happening on Mercury that "we clearly do not understand," expert says.

An artist's impression of the MESSENGER spacecraft in orbit around Mercury.


The space weather report for Mercury: stormy, with a chance of power surges.

New data from the third and final flyby of the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft have revealed surprisingly intense electromagnetic storms in Mercury's magnetic "tail," part of the planet's magnetic field.

Such tails form when the solar wind—charged particles streaming from the sun—pushes on a planet's magnetic field. The deformed field flows around the planet in a windsock shape, like river water flowing around a rock.

All eight planets in the solar system except Mars and Venus have magnetic fields and tails, although Mercury's field is the smallest and weakest.

But during a September 29, 2009, flyby of the tiny planet, MESSENGER watched as Mercury's magnetic tail collected enormous amounts of energy from the solar wind.

In just 90 seconds, the tail increased magnetic field power by 200 percent during an event known as a magnetic substorm. The tail then snapped back to normal, dissipating the energy over the next minute and a half.

On Earth, a similar process—called tail loading—takes an hour and increases the magnetic field's energy by only about 10 percent.

"This is all very curious," said Jim Slavin, a solar physicist at NASA Goddard Spaceflight Center and lead author of a new paper describing the finding.

"We have very weak solar wind conditions, yet we're seeing more tail loading than what we see on Earth. What's going to happen when the [solar] wind conditions pick up?"

MESSENGER may have a chance to find out: The space probe will settle into a stable orbit around Mercury in 2011, just in time for a predicted peak in solar activity in 2012 or 2013.

No Hyper-Auroras on Mercury

On Earth, substorms and tail loading are responsible for creating auroras. We see the northern and southern lights when charged particles from the storms leak into Earth's atmosphere, energizing air molecules and causing them to emit light.

The magnetic storms have also been linked to bursts of radiation that can interfere with Earth-orbiting satellites and even be harmful to astronauts.

When the Mariner 10 probe visited Mercury in 1974, the spacecraft detected the radiation but no substorms. When MESSENGER saw the recent substorms, it detected no radiation bursts.

"There're some things here we clearly do not understand," Slavin said.

The scientist added that, because Mercury has almost no atmosphere, it's not likely the supercharged substorms would create uberauroras. (See a picture of massive auroras on Jupiter.)

Still, whether the tiny planet could have some version of an aurora is "a question we discuss a lot over drinks," Slavin said.

The substorm research is part of a set of papers based on MESSENGER data that will appear in tomorrow's issue of the journal Science.