Thursday, December 21, 2006
Most planets have a magnetic field, like Earth, but with different strength or structure. It shields the planet from the energetic particles of the solar wind. The origin of the magnetic field is explained by a dynamo process, driven by the convective flow of an electrically conducting fluid in the planet's core. Computer models based on the dynamo theory reproduce the properties of Earth's magnetic field well, but the weakness of Mercury's field is a puzzle. Theory predicts that it should have 30% of Earth's strength, but only 1% has been observed.
Ulrich Christensen (info) of the Max-Planck-Institute for Solar System Research, Katlenburg-Lindau, Germany, presents a new model, in which the outer part of Mercury's iron core does not convect and a dynamo operates only in its deep part. Computer simulations show that here a strong but fluctuating magnetic field is generated. Similar to the skin effect in high frequency technology, where rapidly varying currents and magnetic fields hardly penetrate to the centre of a wire, only a small fraction of the dynamo-generated field can escape through the stagnant part of the iron core.
NASA's MESSENGER spacecraft*, being on its way to Mercury, and the ESA mission BepiColombo**, planned for the next decade, will test the model predictions for Mercury's field. If they are confirmed, remaining doubts about the general validity of the dynamo theory for the generation of planetary magnetic fields will be removed.
Press Release via "Mercury's Magnetic Field Explained?" [Astronomy]
Based on the journal Nature paper:
"A deep dynamo generating Mercury's magnetic field"
by Ulrich R. Christensen
Mercury has a global magnetic field of internal origin and it is thought that a dynamo operating in the fluid part of Mercury's large iron core is the most probable cause. However, the low intensity of Mercury's magnetic field - about 1% the strength of the Earth's field - cannot be reconciled with an Earth-like dynamo. With the common assumption that Coriolis and Lorentz forces balance in planetary dynamos1, a field thirty times stronger is expected. Here I present a numerical model of a dynamo driven by thermo-compositional convection associated with inner core solidification. The thermal gradient at the core-mantle boundary is subadiabatic and hence the outer region of the liquid core is stably stratified with the dynamo operating only at depth, where a strong field is generated. Because of the planet's slow rotation the resulting magnetic field is dominated by small-scale components that fluctuate rapidly with time. The dynamo field diffuses through the stable conducting region, where rapidly varying parts are strongly attenuated by the skin effect, while the slowly varying dipole and quadrupole components pass to some degree. The model explains the observed structure and strength of Mercury's surface magnetic field and makes predictions that are testable with space missions both presently flying and planned.
*Info on Nasa's MESSENGER can be found at the mission website:
The MESSENGER core team consists of individuals from across all aspects of the mission, managing how each of the sub teams work together, influence each other, and affect the mission as a whole.
The Science Team is composed of scientists from different institutions and with different areas of expertise to form a comprehensive, diverse team that is well qualified to address the scientific objectives of the mission.
Each of the MESSENGER instruments has a team devoted to its development, calibrations, and operations during the mission, working to ensure that the returned scientific data will provide unprecedented insights into Mercury.
Each subsystem of the spacecraft has a team of highly qualified engineers involved in the design, testing, and operations of the mission, committed to seeing MESSENGER become the first spacecraft to orbit Mercury.
MESSENGER participation can be found all across the country, with some international contributions too. An interactive map shows the many involvements that all come together to create the mission. (abridged)
**About the BepiColombo mission:
BepiColombo, an ESA mission in cooperation with Japan, will explore Mercury, the planet closest to the Sun. Europe's space scientists have identified the mission as one of the most challenging long-term planetary projects, largely because Mercury's orbit so close to the Sun makes the planet difficult for a spacecraft to reach and difficult to observe from a distance. Scientists want to study Mercury because of the valuable clues it will provide in understanding how planets form.
Only one probe has visited Mercury so far, NASA's Mariner 10 which flew past three times in 1974-5 and returned the only close-up images of the planet so far. The information gleaned when BepiColombo arrives will throw light not only on the composition and history of Mercury, but also on the history and formation of the inner planets in general, including the Earth.
The mission will consist of two separate spacecraft that will orbit the planet. ESA is building one of the main spacecraft, the Mercury Planetary Orbiter (MPO), and the Japanese space agency ISAS/JAXA will contribute the other, the Mercury Magnetospheric Orbiter (MMO).
The MPO will study the surface and internal composition of the planet, and the MMO will study Mercury's magnetosphere, that is the region of space around the planet that is dominated by its magnetic field.