29 Apr 1999
Earth may not be the only planet with a surface forged in the same cauldron as Earth's ocean crust is formed today. Mars, it seems, once had active tectonic plates spreading away from long, narrow volcanic rifts, according to two reports in tomorrow's Science (pp. 790 and 794). Yet the shape and pattern of the telltale magnetic bands in the rock that hint at such tectonic activity on Mars are much different from Earth's, so geophysicists are reserving judgment for the moment.
Back in the 1960s, researchers realized that when magma inside Earth rises into the crest of a midocean ridge, cools, and solidifies, it records the magnetic field at the time. Magnetized crust continuously spreads away from a ridge in both directions, like tapes in a tape recorder; when Earth's magnetic field reverses, a new pair of stripes appears, one on each side of the ridge.
The Mars Global Surveyor (MGS) spacecraft first detected patches of magnetic field embedded in the crust apparently at random, like so many bar magnets strewn on the surface (Science, 10 October 1997, p. 215). They had apparently formed when blobs of magma near the surface solidified earlier in martian history, locking in bits of the magnetic field that existed at the time. That meant that although the interior of Mars has cooled and produces no magnetic field today, it once had enough heat to churn the planet's molten iron core into a magnetic dynamo.
After about 1000 orbits by MGS, some of the magnetic patches began to coalesce into a pattern, say Mario Acuña and John Connerney of NASA's Goddard Space Flight Center in Greenbelt, Maryland, members of the MGS magnetometer team. Across a huge swath of the southern hemisphere, wrapping a quarter of the way around the planet, irregular stripes about 100 kilometers wide and up to 2000 kilometers long appeared. That these martian stripes are the work of plate tectonics is "an eloquent hypothesis," says Frederick Vine, a professor emeritus at the University of East Anglia in Norwich, United Kingdom. Indeed, no one has a good idea what else could form such stripes.
But the pattern doesn't exactly match Earth's, so most researchers aren't ready to embrace martian plate tectonics. "You wouldn't say this is a dead ringer for the sea floor," notes paleomagnetist Robert Coe of the University of California, Santa Cruz. For example, the symmetrical pattern of stripes on either side of a spreading center, the clincher in the plate tectonics debate, has not been chronicled, as yet, on Mars.
--RICHARD A. KERR