The Earth's core is solidifying from the inside out by forming solid metal at the center.
The solidification process on Mars may already have started, a process that takes billions of years.
"And if it has begun, it's a very, very long-term process," Stewart said.
The article will appear in tomorrow's issue of the journal Science
Missing Magnetic Field
The finding may aid planetary geologists in trying to figure out why Mars appears to have once had a magnetic field, but then lost it.
The Earth's magnetic field is created by currents in the spinning liquid layer outside the core.
One argument says that the Martian core had solidified, shutting off its once-active currents.
But if Mars has a liquid core, that explanation could not be valid.
Stewart speculates that the Martian magnetic field might have collapsed when the planet's surface locked into one big tectonic plate early in its history.
That might have slowed the rate at which heat escaped the interior, eliminating strong currents even in a liquid core.
The finding may also have applications to planets other than Mars, including Mercury as well as Mars-sized planets outside the solar system.
(Check out a virtual solar system.)
"It is a really interesting piece of work," said Raymond Jeanloz, an Earth and planetary geophysicist at the University of California, Berkeley, who was not associated with the study.
"It's oriented toward Mars, but its actually very general—about how a planetary core can evolve as the planet cools down."
Jeanloz was also impressed by the ability to do such a sophisticated study of Mars's metallic elements under such conditions.
"To be able to do this kind of high-end analytical work at these pressures is technically very remarkable," he said.
Meanwhile, lead author Stewart hopes that someday a probe will land on Mars carrying a seismometer that can use "Mars" quakes to read the structure of the Martian core.
"That would be exciting," he said.
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