IT鈥橲 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. 杏吧原创s working with NASA鈥檚 Mars
Global Surveyor say the planet鈥檚 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鈥檚 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鈥檚 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鈥檚 decline. 鈥淭he magnetic field is right at the heart of the
most interesting questions concerning Mars鈥攖he climate, the atmosphere,
even the life issue,鈥� said Bruce Jakosky of the University of Colorado at
Boulder.
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In 1999, David Mitchell of the University of California at Berkeley and Mario
Acuna of NASA鈥檚 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. 鈥淎 lot of theorists were surprised,鈥� 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鈥攖he charged portion of Mars鈥檚 residual atmosphere鈥攁nd any
remaining magnetic field.
Recently compiled maps show that in some areas of Mars鈥檚 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鈥檚 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鈥檚 colleague Janet
Luhmann that the solar wind stripped the atmosphere鈥檚 nitrogen, carbon, oxygen
and water during Mars鈥檚 first two billion years. This does not rule out the
possibility that some water is hidden underground. 鈥淭he planet is not completely
dead,鈥� says Victor Baker at the University of Arizona in Tucson.
So why did Mars鈥檚 magnetic field fail? Global fields are generated by the
dynamo effect of molten iron circulating in a planet鈥檚 core. David Stevenson of
Caltech thinks Mars鈥檚 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鈥檚 dynamo turning. 鈥淚 certainly think if plate tectonics ceased
on Earth, in 100 million years we wouldn鈥檛 have a dynamo,鈥� says Stevenson. And
with it might go our atmosphere.