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.
Advertisement
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.