Photography of Mercury in the visible light.© NASA
A special feature of planet Mercury, the closest planet to the Sun, is its magnetic field. Usually, a planetary magnetic field is generated by electric currents in the planetary interior, the liquid iron core, in a so-called dynamo process. In the past, it was assumed that Mercury should not possess a liquid core since it was believed to be completely solidified long ago. In constrast to this, NASA’s Mariner 10 mission confirmed at two short flybys a small but global magnetic field. So there is probably still a liquid core and a dynamo process in the interior of Mercury. An important requirement for the dynamo is that at least a portion of the core is liquid and is constantly stirred by convection. Using ground based radar measurements of the slightly uneven rotation, this liquid layer was detected. Nowadays, it is assumed that a light alloying material like sulfur keeps the core from freezing until the present.
In comparison to Earth, Mercury has a relatively large iron core. The core must be least partially molten. The existence and the extent of the inner solid core is still not constrained. © NASA/APL
But again, the Hermean magnetic field caused a headache for scientists. The question is: Why is the magnetic (dipole) field so weak? If you transfer the knowledge gathered from the terrestrial dynamo And other solar system bodies to the Hermean case, the magnetic field should be at least 10 times stronger according to the scaling laws. A number of researchers have speculated on how this conflict with dynamo theory could be resolved. Models that rely on a special interior structure have been developed and implemented in full computer simulations. These simulations are an important tool to look inside inside a planet. Now, the goal is to map the magnetic field to a higher degree in order to compare the measurements with the simulations. This is one of the reasons why ESA has initiated the BepiColombo mission to Mercury in collaboration with the Japanese Aerospace Exploration Agency.
The IGEP uses such a magnetometer for the BepiColombo mission (tri-axial fluxgate magnetometer). © IGEP
BepiColombo is going to be the first European Space mission to Mercury. The rocket launch is scheduled for 2015 from Korou in South America and only after 7 years of travel this mission will arrive. The mission consists of two sattelites:
How does convection look like inside the planetary core, that is responsible for the magnetic field? Here, the surface of the planet (grey) and the convective cells from a computer simulation (iso-volumes of equal z-vorticity) at the core-mantle boundary (blue and red) are shown. © Daniel Heyner
At the IGEP, we do not only develop and build magnetometers but also model planetary magnetic fields. Because of the weak planetary magnetic field only a small magnetosphere is formed. This is also highly dynamic compared to the terrestrial one. The problem is that due to these highly varying magnetic fields it is challenging to separate spatial and temporal changes in the magnetosphere. Which part of the magnetic field has its origin in the Planetary core and which stems from the magnetosphere? This issue is followed by the researchers at the IGEP. The two satellites promise a great amount of data the separation problem can be tackled with. The NASA mission MESSENGER currently in orbit around Mercury consists only of a single space craft. Thus, the dataset will be as large as it is expected for the BepiColombo mission.
Schematic model of the feedback mechanism. Mercury is in the center. The crust is grey, the mantle dark red, the fluid outer core yellow and the solid inner core dark grey. The soloar wind enters from the lower left and interacts with the planetary dipole field (one field line is shown in green). As a result the magnetopause is created (blue paraboloid) on whiche the magnetopause currents flow (indicated with a white arrow). These current cause another magnetic field (red), that is anti-parallel directed to the internal field at the core-mantle boundary. [Heyner (2011b)]
At the IGEP, also a special model explaining why Mercury possesses such a weak dipole field is developed. The magnetic field of the magnetosphere could also be the reason for the low magnetic field strength. Since the magnetic dipole field of the planet and the first degree magnetic field of the magnetosphere are inherently anti-parallel to each other – even after a polarity reversal – a negative feedback on the dynamo could arise. Researchers at the IGEP could demonstrate in a computer simulation that only a relatively weak external field from the magnetosphere is required to significantly alter the evolution of the dynamo. With the computer simulation, it was possible to derive a characteristic magnetic spectrum that can be compared to the BepiColombo data in the future.