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Thursday, April 29, 1999 Published at 21:11 GMT 22:11 UK


Magnetic discovery on Mars

Magnetic stripes on Mars

By BBC News Online Science Editor Dr David Whitehouse

The rocks on Mars contain the magnetic imprint left by large-scale surface movements millions of years ago, according to a team of US and French researchers.

BBC Science Correspondent Sue Nelson: "Mars has more in common with Earth than ever before"
If confirmed, this would be the first direct evidence of the existence on another world other than Earth of what scientists call plate tectonics.

This describes how the planet's crust is divided into interlocking sections, or plates, that float on top of a partially molten mantle. Heat from the Earth's core drives a convection system within the mantle, causing the plates on the surface to move.

Barcode pattern

The plates separate, collide, and squeeze past each other. This explains a wide variety of events such as earthquakes, volcanic eruptions and mountain ranges.

The theory of plate tectonics on Earth was confirmed by the presence on the Atlantic sea floor of a series of magnetic stripes.

Scientists noticed that in each stripe, the crust's iron-bearing minerals aligned in the same direction, alternating between north and south. They saw that the same barcode-type pattern appeared on either side of a ridge running down the centre of the ocean basin.

Sea-floor spreading

The explanation was that the mid-ocean ridge is actually the site of sea-floor spreading, where two tectonic plates are forged by magma welling up in between them.

[ image: Mars pictured from Mars Global Surveyor]
Mars pictured from Mars Global Surveyor
As the magma cools to form new crust, its iron-bearing minerals magnetise and "fix" in the direction of the prevailing magnetic field. The field alternates from north to south every 10,000 years or so.

Data collected by the Mars Global Surveyor suggests similar stripes exist in Martian rock.

This suggests that many of the same forces that continue to form Earth's topography today have shaped Mars in the distant past.

Unlike the Earth, Mars's plate tectonic system appears to have faded about four billion years ago.

Similarly patterned stripes on Mars are much more strongly magnetised than those on Earth, possibly because the Martian crust is richer in iron. They are also much longer, some extending over 2,000km.

This may be because the spreading rate was much faster on Mars than on Earth, or because the magnetic field switched directions fewer times during the formation of new crust.

The idea that these magnetic patterns are relics of plate tectonics on Mars is likely to be controversial, and in the journal Science the researchers consider several possible alternative explanations for their findings.

[ image: How tectonic plates are formed]
How tectonic plates are formed

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