How Mercury Retains an Atmosphere
Magnetic tornadoes were found in Mercury's magnetic field. The pink area represents the magnetopause. The tornadoes are technically known as "flux transfer events" (twisted lines) when they form at the magnetopause and "plasmoids" (yellow areas) when they form in the long magnetic "tail" extending from the night-side of Mercury. They act as open channels through which the solar wind can flow down to the surface of the planet and sputter neutral atoms into Mercury's atmosphere.
Credit: Image produced by NASA/Goddard Space Flight Center/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Image reproduced courtesy of Science/AAAS

Mercury should have trouble hanging on to its atmosphere. It is the closest planet to the sun , its searing daytime temperatures top 800 degrees Fahrenheit (450 degrees Celsius and its gravity is weak, only about 38 percent of Earth's. These conditions don't hold air.

New clues to why Mercury does have a thin atmosphere have been discovered by the MESSENGER spacecraft.

"Mercury's atmosphere is so thin, it would have vanished long ago unless something was replenishing it," James Slavin of NASA's Goddard Space Flight Center, Greenbelt, Md., said in a statement.

That something is the solar wind, charged particles that stream out from the sun and which have a tricky way of skirting the planet's protective magnetic field. The planet has tornado-like magnetic vortices that let charged particles from the sun pour in, Slavin and his colleagues found. The particles kick up atoms at the surface that replenish the planet's thin atmosphere.

Launching atoms

The solar wind is a thin plasma, a stream of electrically charged particles that race constantly from the surface of the sun at about 250 to 370 miles per second (400 to 600 kilometers per second).

Scientists didn't know how the particles got past Mercury's magnetic field. That's becoming clearer with analyses of information that the probe gathered during two close flybys of Mercury in 2008 together with older observations of the planet by the Mariner 10 probe in 1974 and 1975.

The results are detailed in the May 1 issue of the journal Science.

Mercury and Earth are both protected from solar radiation, at least to some degree, by each planet's magnetic field, or magnetosphere. The sun also has a magnetic field, which is carried throughout the solar system on the solar wind. Amidst the planets, the sun's field is called the Interplanetary Magnetic Field.

In its latest flyby, MESSENGER revealed that Mercury’s magnetosphere is leaky. Like huge tornadoes, twisted bundles of magnetic fields that are up to 500 miles wide or a third of the radius of the planet, can open where the magnetosphere interacts with the sun's Interplanetary Magnetic Field. When the two fields touch the event is called a reconnection. It opens a hole through Mercury's magnetosphere that allows solar winds to buffet its thin atmosphere and strike the planet's surface.

For some reason, there are more tornadoes than scientists had anticipated.

"Mercury's proximity to the sun only accounts for about a third of the reconnection rate we see," Slavin said. "It will be exciting to see what's special about Mercury to explain the rest."

Reconnections at Earth

Reconnections also happen in Earth's magnetosphere, but our planet's atmosphere is thick enough to protect the surface from cosmic radiation.

Venus and Mars do not have magnetospheres, but they do have atmospheres and they are exposed to solar winds. The winds carry off gases in their upper atmospheres, slowly eroding them down. Venus is more volcanic than Mars and its volcanoes can belch new gases into its atmosphere to restore what is stripped away. But Mars' atmosphere is slowly drifting off, borne on the solar wind. 

MESSENGER will fly by Mercury a third time on Sept. 29, and then it should begin to orbit the planet -- to intensify its scientific operations -- in March 2011.