Nine seconds is not a lot – those who are nine seconds late for an appointment are, so to speak, on time. But when it comes to the rotation of a planet around its own axis, nine seconds is not insignificant. On Mercury, this means that a spot at the equator would, in four years, not be where one would expect it to be; it would have shifted by 700 metres. By comparing precise altitude measurements conducted by the laser altimeter MLA on board NASA’s MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft with terrain models, which were derived using camera data from the spacecraft, scientists under the leadership of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have found that, on average, Mercury rotates around its own axis approximately nine seconds faster than had been anticipated.
“Before the MESSENGER mission, we had insufficient information about Mercury, despite the three flybys of Mariner 10 spacecraft and measurements from Earth,” explains Alexander Stark from the DLR Institute of Planetary Research. The precise measurement of the rotation allows scientists 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 percent of the mass of the planet,” says Jürgen Oberst, also from the DLR Institute of Planetary Research.
First measurements from orbit
The MESSENGER spacecraft reached its target on 18 March 2011, where it performed 3308 orbits until the mission ended on 30 April 2015. “With MESSENGER, we were ‘on site’,” says Stark, who studied the rotation of Mercury, together with DLR’s Frank Preusker and Jürgen Oberst and a US team. The result has now been published in ‘Geophysical Research Letters’ of the American Geophysical Union (AGU).
Mercury occupies a special place in the Solar System; it is the closet planet to the Sun, at a distance of only about 60 million kilometres. Due to this proximity, it 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.
Planet with unsteady movement
“One possible explanation for Mercury’s faster rotation is that Jupiter influences its orbit,” says Stark. “As a result, its distance from the Sun varies, which in turn affects the planet’s rotation speed.” This small change was not detectable with previous measurement techniques. From the periodic unsteadiness of Mercury’s motion on its orbit, its internal structure can be deduced – in much the same way, to give a simple example, that one can tell if an egg is raw or hard-boiled by spinning it on a table top. In particular, the proportions of solid and liquid materials can be determined. In the case of Mercury, with the help of gravitational field measurements, the size and density of the core could be deduced.
Knowledge for the next Mercury mission
“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,” says Stark. A precise rotation model for the planet is the basis for generating accurate maps, which are important for planning future missions to Mercury. The European Space Agency BepiColombo mission to Mercury, scheduled for launch in 2017, will explore the surface and internal structure of the planet closest to the Sun. Then, DLR will be back in flight around Mercury; among the 11 scientific instruments on board the spacecraft are the laser altimeter BELA and the spectrometer MERTIS, both a contribution of DLR together with its partners.