The researchers then used pairs of Earth-based radio telescopes located thousands of miles apart to monitor the returning patterns as the signals made their way across the surface.
The measurements told the team how quickly Mercury was spinning at various times.
Mercury's spin rate fluctuates slightly as the planet orbits the sun. Scientists have long known that, in theory, they could use those oscillations to determine whether the planet was completely solid.
Until now, though, the oscillations had never been measured.
The new data indicate that the spin rate oscillates about twice as much as it would if Mercury were solid. The planet is therefore at least partially liquid, Margot's team concludes.
Sean C. Solomon, a geophysicist at the Carnegie Institution in Washington, D.C., hailed the study's "spectacular results."
Scientists are now closer to resolving the mystery that arose some 30 years ago with the discovery that Mercury has a magnetic field, he said.
Among the other rocky planets, Venus has no magnetic field and Mars's field has long since become inactive.
(Related news: "Moon Has Iron Core, Lunar-Rock Study Says" [January 11, 2007].)
Like Earth's, Mercury's field may be a by-product of currents flowing through a molten outer core, Solomon said.
NASA's MESSENGER spacecraft, due to fly past Mercury next January, could eventually confirm that idea, he said.
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