Mercury has a partially molten core
K.S. RAJGOPAL
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It was long believed that the core had solidified
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— PHOTO: AFP
CORE OF THE ISSUE: A molten core may make Mercury spin like a raw egg, says Jean-Luc Margot of Cornell University.
THOUGH MERCURY, closest planet to the sun, is different from earth in many aspects, it does have one property in common with earth — it has a magnetic field. Now, in its latest issue, Science reports the results of a study by a team of Cornell University researchers led by Jean-Luc Margot that provides evidence that Mercury really does have molten material inside it which gives it a magnetic field — though only 1 per cent of earth's in intensity. The deduction from the team's observations is that Mercury contains at least a partially liquefied core.
Now, in May 2007, comes the report of the team of researchers bouncing radar signals off of Mercury and recorded at pairs of radio antennas in California, West Virginia and Puerto Rico during 20 observation periods from 2002 through 2006.
Reflected signals
The signals reflected off of irregularities in Mercury's surface, Margot says, with patterns of reflected radio waves sweeping across the solar system. The faster the planet spun, the faster these patterns moved, so watching them allowed the scientists to monitor its rotation rate.
The sun's uneven gravitational pull makes the planet's spin rate fluctuate slightly over the course of its orbit.
The size of these fluctuations depends on whether Mercury is solid or liquid inside — just as eggs spin differently depending on whether they are raw or boiled, Margot says.
The researchers measured the fluctuations to be about 1 part in 10,000, which is larger than the expected rate for a completely solid planet, suggesting Mercury is partially molten inside. They suspect that the inner part of the core is solid but that its outer regions are still molten. The measurements were very carefully timed as Mercury and Earth are in the necessary alignment only for periods of 20 seconds at a time. "Everything has to happen within that 20-second time window," Margot said.
Earth's magnetic field is produced as it has a solid inner core of iron with a molten outer core and the convection motions in the molten iron-rich outer core produce a magnetic dynamo effect.
It was long believed that Mercury, with a mass 5 per cent that of earth would have cooled internally to the point where either the core had solidified or core convection no longer occurs thereby enabling conditions for non-existence of a magnetic field.
A surprise
Some thirty years ago, the detection of an internal magnetic field in Mercury by the Mariner 10 spacecraft, which flew by the planet, came as a surprise.
Although Mercury's high bulk density indicates that its dominantly iron central core is the largest fractional mass among the planets of the Solar system the detection of its magnetic field came as a surprise because Venus has no field and Mars and the Moon show evidence only for ancient global fields.
Why is the magnetic field weak?
ANOTHER QUESTION is why Mercury's magnetic field is so puny (just 1 per cent of earth's). If the dynamo theory is right, the planet's magnetism should be 30 times stronger than it is.
In 2006, Ulrich Christensen of the Max Planck Institute for Solar System Research in Katienburg-Lindau, Germany said in New Scientist that he believed the answer lay in the structure of Mercury's core.
The core's outer layers are `stably stratified', which means they are largely insulated from the heat of the swirling inner core. As a result, only the inner core rotates effectively to generate the magnetic field.
This braking effect is relatively important because Mercury has a very slow rotation, also affecting the dynamo's power.
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