Three terrestrial radar telescopes allowed Russian and American astronomers to show that fluctuations of Mercury's rotation were common for a celestial body, having liquid core. Liquid ore hypothesis was born in 1974, after American interplanetary robot "Mariner-10" circumnavigated Earths neighbour.

Mercury is a difficult object for any kind of observations, since it can be seen only near horizon - either at dawn (in autumn), or at dusk (in spring). That is why this planet remains one of the most poorly studied objects of the Solar system. American interplanetary robot "Mariner-10" brought us the majority of data we now have about Mercury - duration of Mercurian day and year, photographs of Mercurian surface, data on atmosphere content, and a curious fact: the robot detected weak electromagnetic field of the planet. Said field was 100 weaker than Earth's magnetic field, however, such a small planet shouldn't have had any magnetosphere at all - the fact, which had no explanation at that time.

Think-tank of Jean-Luc Margot, which unites astronomers from Cornell University, USA, and scientists from other US and Russian institutions, used two brand new techniques (including speckle interferometry) to find a solution for this problem. Radar studies of Mercury lasted for five years and finally showed that fluctuations of planet's rotation were specific for a celestial body, having liquid core. Results of said studies are published in the last issue of Science magazine.

The work confirms the theory about Mercury generating its own magnetic field by means of dynamo effect - effect of convection within liquid electroconductive metal core (our Earth actually generates its magnetic field the same way). First, scientists measured small shifts of Mercury while revolving round its axis, and after they studied specific spotty speckle patterns, i.e. artifacts, appearing due to interference of rays, dissipated by object's discrete irregularities. "Parasite" fluctuations in fluorescence intensity of such structures appeared to have information on librations and object deformation.

Physicists have generalized received data and successfully detected periodic failures in Mercury rotation, caused by tidal interactions with the Sun, which has different effect on planets with liquid and solid cores. Margot's group used data of three telescopes: Robert C. Byrd Green Bank Telescope (West Virginia, USA), Arecibo Observatory (Puerto-Rico) and Goldstone Observatory (California).

Scientists plan to study "failures" and fluctuations of Venus's rotation - if any. Such studies became possible since unusual radar techniques, suggested by Russian astronomer Igor Kholin from Russian Institute of Space Research (co-author of article in Science), were improved.

Of course, not all mysteries of Mercury's magnetosphere are solved to date - the issue of core's emollescence is still a burning one. We hope that American "Messenger", which is scheduled for launch in next January, will shed light on this problem.

Source:
    Large Longitude Libration of Mercury Reveals a Molten Core//Science. 2007. V. 316. P. 710–714.
    Elementy.ru

Kizilova Anna