MARS has a magnetic field鈥攂ut it seems to be on its last legs. NASA鈥檚
Global Surveyor spacecraft has found that the strength of the Red Planet鈥檚 field
is less than 1/800th that of the Earth. The finding could explain why the planet
now appears inhospitable to life.
The strength of a planet鈥檚 magnetism provides an indirect measure of the
movement and temperature of its interior, says Mario Acuna of the NASA Goddard
Space Flight Center in Greenbelt, Maryland. The puny magnetic field on Mars
shows that the planet鈥檚 core must have cooled very quickly. 鈥淔or some reason it
aged prematurely,鈥� he says.
The Earth鈥檚 magnetic field is caused by the movement of the molten metal in
its core. The planet鈥檚 rotation spins the core, generating currents which create
the magnetic field. The molten core also provides the heat to power volcanoes
and the movement of continents.
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Although Mars has only one tenth the mass of the Earth, it spins just as
quickly and shows evidence of past volcanism. So scientists have long believed
that it must have a magnetic field鈥攅ven though previous spacecraft have
failed to detect anything
(This Week, 20 September, p 4).
In the coming weeks, Surveyor鈥檚 instruments will begin mapping the fine
structure of the magnetic field. From that information, says Acuna鈥檚 team may be
able to determine whether the planet鈥檚 core has any activity left in it at all,
or whether the remaining magnetic field is just a fossilised remnant of earlier
activity.
The results could further diminish the hopes of scientists hoping to find
life on Mars. If any organisms remain, they are most likely to live in
subterranean springs thawed by geothermal heat鈥攚hich depends on the
existence of a hot, active core.
But Acuna says it is difficult to explain why Mars cooled so much faster than
the Earth. 鈥淲e believe the two planets were formed at the same time from nearly
the same ingredients,鈥� he says. 鈥淚 guess the question is why we turned out to be
so lucky.鈥�