spaceSpace and Physics

Mercury Spins Nine Seconds Faster Than We Thought It Did


Jonathan O'Callaghan

Senior Staff Writer

2307 Mercury Spins Nine Seconds Faster Than We Thought It Did
Sorry we ever underestimated you, Mercury. NASA/JHUAPL/Carnegie Institution of Washington/DLR

Break out the calculators: Mercury has been found to spin nine seconds faster than expected. That might not sound impressive, but this latest research suggests that how Mercury spins and orbits the Sun is more complicated than thought – and it also helps to confirm it has a molten core.

The data on Mercury’s movement was gathered by NASA’s MESSENGER spacecraft, which orbited Mercury from March 2011 to April 2015, when it was purposefully crashed into the surface. The innermost planet of our Solar System, at a distance of 60 million kilometers (37 million miles) from the Sun, rotates once every 59 days, and completes an orbit around the Sun every 88 days; a ratio of exactly 3:2. It is the only planet in the Solar System with such a ratio. The research was published in Geophysical Research Letters.


The orbit of Mercury – or indeed any planet – is not circular, though. It is a very slightly oblong-shaped elliptical orbit. This means that the pull of the Sun varies slightly throughout the year. However, the discovery of a nine-second speed boost over previous studies was a surprise. Such is the offset that a particular point on the equator would shift by about 700 meters (2,300 feet) over four years compared to previous estimates of Mercury’s rotation.

"It is not a huge difference, parts per million, but it is unexpected," said Jean-Luc Margot, a planetary scientist with the University of California, Los Angeles, and a co-author of the new study, in a statement.

The scientists think the reason might not be the Sun, but Jupiter. The gas giant completes an orbit around the Sun every 12 years, and it’s thought it may be causing a slight libration – or wobble – in Mercury’s rotation, with a 12-year cycle. This is just one theory though; it’s hoped that ESA’s upcoming BepiColombo mission, due to launch in 2017, could shed further light on what’s happening.

But the libration is important, because it points to Mercury having a liquid outer core, based on how unsteady it is. "This confirms that Mercury has a large, partially molten core, which accounts for more than half of the volume and approximately 70% of the mass of the planet," says Jürgen Oberst, from the German Aerospace Center (DLR) Institute of Planetary Research, in a statement. If Mercury were entirely solid, its libration would be twice as big.


Using the information, more accurate maps of Mercury can be created, which are important when planning future missions like BepiColombo. "With the measurement of the rotational speed and the resulting conclusions regarding the interior of Mercury, we have accomplished one of the major objectives of the MESSENGER mission," said lead author Alexander Stark, also from DLR.

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