New Scientist, vol 169 issue 2277, 10/02/2001, page 4

Eugenie Samuel
The day the dynamo died

With no magnetic field, Mars lost its atmosphere and its chance of life

IT'S ONE of the central mysteries of the Solar System: why does a
sizeable planet like Mars not have an atmosphere that could nurture and
sustain life? That question may now have been answered. Scientists
working with NASA's Mars Global Surveyor say the planet's atmosphere
was blown away by the solar wind, following the demise of its magnetic
field four billion years ago. This happened so soon after Mars's formation
that it is unlikely complex life would have had time to evolve.

Every planet in the Solar System is buffeted by the solar wind, a stream
of charged particles flowing from the Sun. Earth's magnetic field acts as a
shield, deflecting the solar wind around and behind the planet.
Researchers have long known that Mars does not have such a global magnetic field, but
recent studies have indicated that the planet did once have one, and that
its disappearance spelled the end for the atmosphere.

Last week, members of the Mars Global Surveyor team met in La Jolla,
California, and one of their aims was to try and pin down the timing of
the stages in the planet's decline. "The magnetic field is right at the heart
of the most interesting questions concerning Mars-the climate, the
atmosphere, even the life issue," said Bruce Jakosky of the University of
Colorado at Boulder.

In 1999, David Mitchell of the University of California at Berkeley and
Mario Acuna of NASA's Goddard Space Flight Center in Greenbelt,
Maryland, found that the oldest terrain on Mars showed signs of having
been magnetised by an ancient global field. But the terrain inside nearby
impact craters was not magnetised, so the magnetic field must have
switched off before the time of the impacts. The impacts date to about
four billion years ago, only about half a billion years after Mars formed. "A
lot of theorists were surprised," says Mitchell.

But did the disappearance of the field mean doom for the atmosphere?
Since 1999, the Mars Global Surveyor has been taking all sorts of
measurements, including the strength of the solar wind, the density of the
ionosphere-the charged portion of Mars's residual atmosphere-and any
remaining magnetic field.

Recently compiled maps show that in some areas of Mars's southern
hemisphere the ionosphere extends above the 400-kilometre altitude of the
probe, while elsewhere the solar wind has beaten it down closer to the
surface. According to Mitchell, the areas where there is more ionosphere
coincide with areas on the ground where remnants of Mars's global
magnetic field are preserved in magnetised rocks. These rocks seem to be
magnetised strongly enough to protect pockets of ionosphere from the
solar wind.

This information confirms calculations made by Mitchell's colleague Janet
Luhmann that the solar wind stripped the atmosphere's nitrogen, carbon,
oxygen and water during Mars's first two billion years. This does not rule
out the possibility that some water is hidden underground. "The planet is
not completely dead," says Victor Baker at the University of Arizona in
Tucson.

So why did Mars's magnetic field fail? Global fields are generated by the
dynamo effect of molten iron circulating in a planet's core. David
Stevenson of Caltech thinks Mars's lack of plate tectonics may be key. On
Earth, plate tectonics cool the mantle relative to the core, increasing
the convection that keeps our planet's dynamo turning. "I certainly think if
plate tectonics ceased on Earth, in 100 million years we wouldn't have a
dynamo," says Stevenson. And with it might go our atmosphere.

(c) Copyright New Scientist, RBI Ltd 2000

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