Mercury turns faster than expected

Rotation of the innermost planet provides new insights about its core

Mercury False-Color Image Created by the MESSENGER Spacecraft © NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington
Read out

Nine seconds: That's how much the Mercury is heading for its previously assumed rotation. This is confirmed by data from the Mercury Probe MESSENGER. They also provide evidence that the core of Mercury is nearly three-quarters of its mass and has largely melted - which is very unusual for a planet of its rather small size. Why Mercury dances so out of line is still unclear.

Mercury is unusual in a number of ways: the planet closest to the Sun is smaller, denser and older than any other in the Solar System. He also has an extremely large core, which he may have owed to a primeval collision. And its surface is unusually dark and even carries remnants of ice, as the NASA spacecraft MESSENGER discovered.

Unique rotation

Unique in the solar system is the rotation of Mercury: Its approximately 59-day rotation is coupled to the 88-day orbit around the sun. He thus rotates exactly three times around his axis, in the same time, in which he revolves twice around the sun. The ratio between a rotation around the sun and the rotation time around its own axis is thus 3: 2, which is the second time in the solar system.

Thanks to new data from the MESSENGER probe, planetary scientists have now re-measured the rotation time of Mercury. They used accurate altitude measurements from the MLA Laser Altimeter aboard the spacecraft and terrain models to measure when certain landmarks reappear at exactly the same spot.

Landmarks on the surface help determine the rotation of Mercury more accurately. This model shows the height profiles of the laser altimeter on the MESSENGER spacecraft over the northern hemisphere of Mercury (red colors show heights, blue areas are deep). © NASA / JHUAPL / Carnegie Institution / DLR

Nine seconds too fast

The surprising result: The Mercury turns on its axis about nine seconds faster than expected. Nine seconds do not seem like much at first. But when it comes to the rotation of a planet around its own axis, nine seconds are not insignificant. On Mercury, this means that after four years you would not find a point on your equator where you suspected it, but moved it by 700 meters. display

"A possible explanation for the faster rotation of Mercury is that Jupiter is disrupting the railway of Mercury, " says Alexander Stark of the German Aerospace Center (DLR). Although the gravitational effect of the gas giant is very small measured by the powerful attraction of the near sun, it could be sufficient, according to the researchers, to influence the rotation of the planet. "This changes the distance of the sun and, as a consequence, the speed of rotation of Mercury, " says Stark.

So Mercury could be built inside CWitte / public domain

Mostly liquid "

And there's something else about the rotation of Mercury: it's egg. Because the planet is exposed to strong tidal forces from the sun, its rotational speed fluctuates slightly. This libration is even twice as large as is typical for a completely solid celestial body. At the equator, Mercury is thus hundreds of meters over 88 days. This periodic Torkel movement is an opportunity for planetary scientists because it leaves a lot to be learned about the inner life of the planet.

Similar to a raw or boiled egg spun on a tabletop, the libration says something about how solid or fluid the interior is. According to the new data, the libretto of Mercury has an amplitude of almost 39 arcseconds, the axis of rotation fluctuates by two arc minutes. This results in new knowledge about the core of Mercury. "The measurements confirm that Mercury has a large, partially melted core that accounts for more than half of the volume and over 70 percent of the planet's mass, " says Jörg Oberst from DLR Institute for Planetary Research.

Important dates for future missions

"By measuring the rotation speed and thus making it possible to return to the interior of Mercury, we have achieved one of the great mission goals of MESSENGER, " says DLR planetary scientist Alexander Stark. A correct rotation model for the planet is the basis for creating accurate maps that are also important in planning future missions to Mercury.

One of them is the space probe Bepi Colombo of the European space organization ESA, which is due to launch in 2017 to Mercury. She will continue exploring the surface and inner structure of the sunniest planet. (Geophysical Research Letters, 2015; doi: 10.1002 / 2015GL065152)

(German Aerospace Center (DLR), 10.09.2015 - NPO)