Magnetic mystery on Mars
May 7, 1999
Planets in the inner solar system may have very similar early histories according to data from the Mars Global Surveyor. Previously planetary physicists believed that Mercury, Mars, Venus and the Earth all formed in different ways. Now Mars Global Surveyor has discovered magnetic 'fingerprints' on the Martian surface. These suggest that the planet once had active plate tectonics similar to the Earth and Venus, and a inner molten iron core. However, if plate tectonics did exist on Mars, they only lasted a few hundred million years before the iron-rich planetary core cooled and froze into a solid about four billion years ago (Science 284 790).
Geophysicists realised that the Earth's crust consisted of a series of tectonic plates when they noticed that as magma flowed out of the ocean ridges and cooled, iron in the magma magnetized in the direction of the Earth's magnetic field. This field changes direction roughly every 10000 years. The material from the ocean ridge gradually expands outwards along the ocean floor, creating a pattern of magnetic stripes on either side of the ridge.
The patterns seen on Mars are more strongly magnetised than those seen on Earth. "We had no idea we'd see anything of this magnitude. It was mind blowing, " said Jack Connerney of the NASA Goddard Space Flight Center in Maryland. One possible reason is the high iron concentration (15%) in the Martian crust. The magnetic strips are also longer and wider than those seen on Earth - which suggests that Martian magma flowed more quickly than the Earth's and the global magnetic field flipped less.
However, other physicists are perplexed by the results. In an article in the same issue of Science Sean Solomon of the Carnegie Institution in Washington writes that the magnetic footprints measured by Global Surveyor are too strong to be explained by plate tectonics and could therefore have been created by some other process such as chemical activity.