By analyzing data collected by NASA’s MESSENGER spacecraft, scientists have found that, on average, Mercury spins on its axis 9 seconds faster than had previously been anticipated.
This colorful view of Mercury was produced by using images from the color base map imaging campaign during MESSENGER’s primary mission. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up the planet’s surface. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington.
“Before the MESSENGER mission, we had insufficient information about Mercury, despite the three flybys of Mariner 10 spacecraft and measurements from Earth,” said Dr Alexander Stark from the German Aerospace Center’s Institute of Planetary Research, lead author of a paper published in the journal Geophysical Research Letters.
The precise measurement of the rotation allows planetary researchers to draw conclusions about Mercury’s internal structure and development. The strength of the periodic fluctuation of the rotational speed with respect to the average value was also measured.
“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,” said co-author Dr Jürgen Oberst, also from the Institute of Planetary Research.
Mercury occupies a special place in the Solar System; it is the closet planet to the Sun, at a distance of only 36 million miles (57.9 million km).
Due to this proximity, the planet is exposed to strong tidal forces. Its 59-day rotation period is coupled to its 88-day orbit around the Sun.
Mercury rotates about its axis exactly three times for every two orbital revolutions – the relationship between its orbit around the Sun and its rotation period is thus 3:2, which is not found elsewhere in the Solar System.
“One possible explanation for Mercury’s faster rotation is that Jupiter influences its orbit. As a result, its distance from the Sun varies, which in turn affects the planet’s rotation speed,” Dr Stark explained.
This small change was not detectable with previous measurement techniques.
“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,” Dr Stark said.
“A precise rotation model for the planet is the basis for generating accurate maps, which are important for planning future missions to Mercury.”
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Alexander Stark et al. First MESSENGER orbital observations of Mercury’s librations. Geophysical Research Letters, published online September 9, 2015; doi: 10.1002/2015GL065152
