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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.
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