May 3, 1999 Washington DC - Mars may once have maintained a plate tectonics system, according to a team of U.S. and French scientists who have analyzed new magnetic information about the planet's crust. The team reports its findings, which may be the first direct evidence that plate tectonics are not unique to Earth, in two papers in the 30 April issue of Science.
"When the possibility of there once having been a plate tectonics system on Mars appeared in the data we were extremely excited. This interpretation has very interesting implications for our understanding of Mars and the other planets," said Henri Reme, of the Centre d'Étude Spatiale des Rayonnements at CNRS, in Toulouse, France, who is a member of the team.
Plate tectonics leave a distinctive magnetic signature in the Earth's crust, and it is this type of signature that scientists working with data from the Mars Global Surveyor spacecraft think they see in the oldest regions of crust on Mars. If confirmed, this would mean that many of the same forces that continue to form Earth's topography today may once have shaped Mars, before grinding to a halt.
When it emerged in the 1960s, the theory of plate tectonics revolutionized geologists' understanding of Earth just as the theory of natural selection illuminated evolution and the big bang theory transformed cosmology. Geologists began to agree that Earth's crust is divided into interlocking sections, or plates, that float atop the partially molten mantle. Heat from deep inside Earth's core drives a convection system within the surrounding mantle, causing the plates on the surface to shift like pieces of a restless jigsaw puzzle. As the plates separate, collide, and squeeze past each other, these motions form the basis for a unified explanation for a wide variety of geologic questions such as why earthquakes and volcanic eruptions occur, why and how mountain ranges form, and why ancient fossils and rocks from a seemingly common location have been found on disparate continents separated by miles of ocean.
What clinched the theory of plate tectonics on Earth was the presence on the Atlantic sea floor of a bar-code-like 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. Intriguingly, these stripes formed symmetrical mirror images on either side of a ridge running down the center of the ocean basin. The now-standard explanation for this curious pattern is that the mid-ocean ridge is actually the site of sea-floor spreading, in which two tectonic plates diverge and allow magma to well up. As the magma cools to form new crust, its iron-bearing minerals magnetize and "freeze" in the direction of the prevailing magnetic field, which flip-flops north to south every 10,000 years or so. Such alternating, mirror-image magnetic stripes have become the definitive signature of plate tectonics on Earth.
The similarly patterned stripes on Mars are much more strongly magnetized than those on Earth, possibly because the martian crust is richer in iron. They are also much longer, some extending over 2,000 km in length, and about ten times wider, possibly 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.
"We had no idea we'd see anything of this magnitude. It was mind blowing, really," said research team member Jack Connerney of NASA Goddard Space Flight Center in Maryland.
The researchers were also able to closely explore many other patches of magnetism elsewhere on Mars, in addition to the magnetic stripes that occur mostly in the southern hemisphere. They describe these features in a second paper in the same issue of Science.
Scientists have long been fascinated by the striking difference between Mars' two hemispheres. The northern hemisphere lies below Mars' mean surface level and is much younger than the elevated "highlands" of the southern hemisphere, which are heavily scarred with impact craters and wide channels. The team's findings suggest that the highlands might be remnants of early crust that was formed between two spreading plates. In some places the magnetic imprint may have been preserved, and in others it may have been demolished by later impacts and heating events such as volcanism.
The idea that these magnetic patterns are relics of plate tectonics on Mars is likely to be controversial, and in their paper Connerney and his colleagues consider several possible alternative explanations for their findings. For example, although it would be difficult to explain the alternating magnetic orientation of the stripes in this scenario, lava flows from volcanic eruptions can create a linear shape. When interpreting information from other planets, "we have to be careful not to stick rigidly to our terrestrial experiences," cautioned Connerney. "At least we can start the discussion here."
That the information was collected at all was the serendipitous consequence of an effort to use less rocket fuel. The Mars Global Surveyor spacecraft used a low-power technique known as aerobraking that took advantage of the drag exerted by Mars' atmosphere to slow the spacecraft down and reign in its orbit close to the planet. During the aerobraking process, the spacecraft made many passes around the planet and frequently dipped to low altitudes, allowing the magnetometer on board to take surprisingly extensive, detailed readings of the planet's surface. In addition, a problem with one of the solar panels on board forced the spacecraft to proceed with aerobraking more slowly than usual-allowing even more information to be collected.
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