Lab study indicates Mars has a molten core
16:53 31 May 2007
NewScientist.com news service
The long-dead magnetic field of Mars could eventually come back to life if the results of a new experiment are correct. The study, which suggests that Mars' core is mostly or completely liquid, may also help scientists unravel the mystery of why the planet’s magnetic field shut off billions of years ago.
It has been known since 2003 that at least part of Mars' interior is molten, based on how easily the Sun’s gravity distorts the planet’s shape, but no one knew whether it is completely liquid, or whether there is a solid inner core like Earth's.
Now a team of scientists, led by Andrew Stewart of the Swiss Federal Institute of Technology in Zurich, Switzerland, has succeeded in creating in the lab the high pressure and temperature expected in Mars' core.
Using chambers made of diamond, they compressed mixtures of iron, nickel and sulphur up to the maximum pressure expected in Mars' core, which is 40 gigapascals - 400,000 times the pressure of Earth's atmosphere at sea level.
The researchers found that, at the temperatures expected in the Martian core (upwards of 1500 Kelvin), the mixture should stay in liquid form. However, there could be some solidification in the outer part of the core if the sulphur content is only 10.6% - at the low end of estimates based on analysis of Martian meteorites.
"This observation indicates a complete absence of crystallisation in most, if not all, of the present-day Martian core," the team says in its research paper.
If they are right, it could explain why Mars' magnetic field turned off four billion years ago, whereas Earth’s field is still strong. In Earth, heat released by the gradual solidification of the inner core is thought to stir up the molten iron outer core, turning it into a dynamo that generates our magnetic field.
If there is no solid inner core to Mars, its early field must have had a different energy source. Scientists have speculated that the inner part of the Martian core might have been especially hot, enough to stir up the outer core and power a magnetic field. That heat source would soon have failed.
The team’s results also hint that the field could turn on again one day. There may already be some crystallisation in Mars' core, not as a solid lump in the middle but as iron "snow" higher up. If this snowing gets heavier, the energy released might eventually stir up the core again and turn it back into a magnetic dynamo.
"Due to crystallization within the core of Mars, convection and heat-flow could change dramatically and this could have knock-on effects on the rest of the planet," Stewart told New Scientist. "These effects could be quite broad and may include features such as a new magnetic dynamo being established, or even the start up of surface tectonics."
Sean Solomon, of the Carnegie Institution of Washington in Washington, DC, US, who is not a member of the team, says the new research shows that Mars' magnetic field was powered in a different way from Earth's field.
"This offers a strong laboratory based argument that the core of Mars has been completely molten through most of its history," he told New Scientist. "It removes the possibility that inner core solidification was also the energy source for the early dynamo on Mars."
But David Stevenson of Caltech in Pasadena, US, says it is too soon to draw firm conclusions about whether Mars has a solid inner core, since Stewart's team only considers interiors for Mars with 10% or more sulphur in them.
"This is a defensible choice, but not guaranteed," he says. It is difficult to extrapolate the overall sulphur content of Mars from looking at its abundance in meteorites, he says, and if the sulphur content is lower than Stewart's team assumed, the inner core could have solidified.
As for the magnetic field turning back on, he says it is "possible, but perhaps unlikely". Even if the core starts to solidify, the resulting heat flow might not be great enough to turn the magnetic field back on, he says.
Journal reference: Science (vol 316, p 1323)
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