WHEN Mars was young it had shifting tectonic plates on its surface, new data
from the Mars Global Surveyor suggest. The Martian data reveal a pattern of
alternating magnetic polarisations in the planet鈥檚 crust just like those that
form on Earth as the seafloor spreads.
On Earth, magma emerging from mid-ocean rifts is magnetised by the Earth鈥檚
field. As the magma cools, this field direction 鈥渇reezes鈥� into the rock.
Occasional reversals in the direction of the Earth鈥檚 field every half million
years or so on average mean that as the magma spreads, it forms strips
magnetised with alternating polarities. That pattern of magnetic fields in the
seafloor clinched the case for terrestrial plate tectonics.
Mars seemed too small to retain enough internal heat to support the process,
but telltale features that have turned up in data from the Mars Global Surveyor
are forcing scientists to rethink their ideas. The data show there are parallel
strips of crust about 200 kilometres across and up to 2000 kilometres long on
the ancient southern highlands that are magnetised in alternating
directions.
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鈥淭he most natural explanation we can think of is the same process we see
working on Earth,鈥� says Jack Connorney of NASA鈥檚 Goddard Space Flight Center
near Washington DC. The strength of magnetic fields in the crust also turned out
to be surprising, he says. 鈥淥n Mars, the crustal field is more than ten times
the magnitude of any crustal signature you鈥檇 measure for terrestrial rocks.鈥�
Although Mars now has no magnetic field, Connorney says that an internal
dynamo must have generated the field that magnetised the surface rocks when the
planet was young. As on Earth, the field must have flipped direction
occasionally. Connorney suspects that heat from within Mars powered
plate-tectonic motion for at least half a billion years. At that point, the
Martian dynamo died, so rocks formed since then are not magnetised.
Connorney and his colleagues report the observations in two papers in last
week鈥檚 Science (vol 284, p 790 and p 794). He thinks the Martian rocks
have preserved the fields because they contain iron-rich minerals. 鈥淏ut we
probably won鈥檛 know what the magnetic minerals are until somebody goes there
with a rock hammer,鈥� he admits.
Maria Zuber, a geophysicist at the Massachusetts Institute of Technology,
agrees that the new observations make a convincing case for a strong dynamo on
the youthful Mars. But she would like to see more data to confirm that moving
tectonic plates created the magnetic patterns. 鈥淚t鈥檚 going to take more rigorous
testing to verify that,鈥� she says.