Figure 1. The MESSENGER Magnetometer (MAG) is mounted behind the spacecraft at the end of a 3.6-m-long boom to ensure that the instrument is well-separated from fields created by the spacecraft electronics and moving parts.
May 17, 2011
Measuring Mercury's Magnetic Field
Figure 1. The MESSENGER Magnetometer (MAG) is mounted behind the spacecraft at the end of a 3.6-m-long boom to ensure that the instrument is well-separated from fields created by the spacecraft electronics and moving parts.
MESSENGER carries a sensitive Magnetometer that measures the vector magnetic field at the location of the spacecraft. The instrument is mounted on the end of a 3.6-m-long boom that extends away from the spacecraft in the direction opposite to the sunshade (Figure 1). The Magnetometer works like a three-axis compass that determines how strong the magnetic field is in all three directions and specifies the direction and strength of the magnetic field at every point in MESSENGER’s orbit around the planet. Since the first encounter of the Mariner 10 spacecraft with Mercury in 1974, we have known that Mercury has an internal magnetic field with a strength at the surface that is about 1% as strong as at Earth. The MESSENGER Magnetometer is a high-precision instrument that can sense fields only one millionth as strong as the field at the surface of the Earth, so magnetic signals that are only a tiny fraction of the maximum magnetic field at Mercury can be characterized. The global structure of Mercury’s magnetic field will be determined by combining data taken from all of MESSENGER’s orbits about the planet.
Figure 2. Schematic cross section of Mercury showing the solid inner core (yellow), molten outer core (orange), and solid crust/mantle (dark red). Mercury’s magnetic field is thought to arise from circulation of the molten outer core driven by heat released as the outer core cools and solidifies and the inner core slowly grows. Identifying how this circulation creates Mercury’s global magnetic field is a prime objective of the MESSENGER mission.
Planetary Magnetic Fields
Venus and Mars are the only planets in our solar system that do not have global planetary magnetic fields. Mercury is particularly interesting because its magnetic field is weak compared to those of the other planets. Planetary magnetic fields arise not because the planets contain giant permanent magnets, but because at least some portion of their interiors is fluid and electrically conductive. In Earth and in Mercury, that fluid is the molten iron of the planet’s outer core (Figure 2). As the core cools, molten material solidifies and heat is released. This heat can stir the remaining molten material to ciruclate much as boiling water circulates in a heated pot. The circulation of the molten outer cores amplifies any magnetic field present in the material and converts a small fraction of the energy of motion into a magnetic field, a process known as a magnetic dynamo. When the core cools sufficiently to become completely solid, or when the stirring action in the outer core becomes sufficiently weak, the dynamo stops, and the only remaining field is that of material in the planet’s outer crust that was permanently magnetized during the operation of the dynamo. Planetary magnetic fields therefore provide insight into past and current processes deep within the planet.
Why Does Mercury Have a Magnetic Field?
Of the rocky planets (Mercury, Venus, Earth, and Mars), Mercury is the smallest and Earth the largest. Because neither Venus nor Mars has a global magnetic field (although Mars has magnetized crust) it had been thought that Mercury would have no global field. Contrary to these expectations, Mariner 10 observations showed that Mercury indeed has a global field, albeit a weak one, and it has since been a challenge to understand how this field can have persisted over the lifetime of the planet. The leading hypothesis is that at least an outer shell of the core remains molten because it contains a lighter element as well as Iron, and the lighter element is present in sufficient abundance to lower the freezing point of the alloy, much as salt added to water lowers the freezing point of the mixture below that of pure water. Numerical simulations have shown that even a thin molten shell could support a dynamo and create the magnetic field seen today at Mercury, but many details of the process are uncertain. Deducing the origin of Mercury’s magnetic field is one of the central goals of the MESSENGER mission, and the Magnetometer is providing key data to address this question.
Figure 3. Artist’s sketch of the volume of space influenced by Mercury’s magnetic field, called the magnetosphere, in the presence of the solar wind and interplanetary magnetic field. As the solar wind impacts the planet’s magnetic field, it generates a shock wave that reaches within one Mercury diameter of the surface at the planet’s subsolar point, i.e., local noon. The boundary between this shocked solar wind and the magnetosphere (called the magnetopause) is very close to the planet, within one Mercury radius. Magnetic fields generated by the solar wind interaction complicate interpretation of the Magnetometer data, and a complete year of observations by MESSENGER will be required to identify the portion of those signals that arise from the magnetic field of Mercury itself.
Complications of a Weak Magnetic Field
As at Earth, Mercury’s magnetic field is immersed in the solar wind and the interplanetary magnetic field (Figure 3). At Earth the interactions between the magnetic field and the solar wind generate the spectacular aurora in the polar regions and are responsible for the Van Allen radiation belts. Because Earth’s magnetic field is comparatively strong, the solar wind does not change the magnetic field very much at ground level. It is for this reason that one can reliably use compasses for navigation. At Mercury however, the situation is quite different. Not only is the planetary magnetic field much weaker than Earth’s, but because Mercury is much closer to the Sun the solar wind is approximately ten times stronger. As a result the effect of the solar wind is about 1,000 times greater at Mercury and the volume over which Mercury’s magnetic field “shields” the planet, known as the magnetosphere, is comparably tiny. It extends only 40% of the planet’s radius toward the Sun, and the distortion of the magnetic field close to the surface is nearly as strong as the planet’s own magnetic field. To understand Mercury’s magnetic field, it is therefore essential to understand the interaction of that field with the solar wind.
Figure 4. MESSENGER’s one-year orbital mission will provide comprehensive coverage of the planet’s magnetic field and interplanetary environment. Orbital tracks of MESSENGER about Mercury are shown in black. The Sun is to the left, and the vertical axis shows distance away from the planet-Sun line. The inner and outer red curves are the magnetopause and solar wind shock-wave surfaces, respectively, in this plane (see Figure 3).
MESSENGER's Magnetic Mapping Program
Because the magnetic field carried by the solar wind that flows around Mercury’s magnetic field interacts strongly with the magnetic field of the planet, an extensive campaign in which we map out the magnetic field everywhere around the planet is required to separate the internal field of the planet from other fields. By taking data continuously, throughout the entire year of observations, the Magnetometer will collect more than 500 million measurements (Figure 4). Because the observations reach as close as 200 km from the surface – well within Mercury’s magnetosphere – and as far as 15,000 km – within the solar wind itself – the data will allow mapping of the magnetosphere’s boundaries, measurement of the currents along those boundaries, and separation of the internal magnetic field from these “external” sources to understand the dynamic processes that give rise to the planet’s magnetism.