A NASA Discovery mission to conduct the first orbital study
of the innermost planet
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Mercury: The Key to Terrestrial Planet Evolution

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Question 3: What is the nature of Mercury's magnetic field?

Mercury's magnetosphere

The different components of Mercury's magnetosphere result from the complex and dynamic interactions between Mercury's magnetic field and the solar wind. Figure courtesy of J. A. Slavin, NASA Goddard Space Flight Center.

Mercury's magnetic field and the resulting magnetosphere, produced by the interaction of Mercury's magnetic field with the solar wind, are unique in many ways. Perhaps one of the most noteworthy observations about Mercury's magnetic field is the simple observation that the small planet has one. Mercury's magnetic field is similar in its "dipole" shape to Earth's magnetic field, which resembles the field that would be produced if there was a giant bar magnet at the center of the planet. In contrast, Venus, Mars, and the Moon do not show evidence for intrinsic dipolar magnetic fields, but the Moon and Mars have evidence for local magnetic fields centered on different rock deposits.

Earth's magnetosphere is very dynamic and constantly changing in response to activity of the Sun, including both solar storms and more modest changes in the solar wind and interplanetary magnetic field. We see the effects of these dynamics on the ground as they affect power grids and electronics, causing blackouts and interference with radios and telephones. Mercury's magnetosphere was shown by Mariner 10 to experience similar dynamics; understanding those variations will help us to understand the interaction of the Sun with planetary magnetospheres in general.

Although Mercury's magnetic field is thought to be a miniature version of Earth's, Mariner 10 didn't measure Mercury's field well enough to characterize it. There is even considerable uncertainty in the strength and source of the magnetic field. MESSENGER's Mercury flybys confirmed that there is a global magnetic field on Mercury, most likely arising from fluid motions in an outer liquid portion of Mercury's metal core, but there is uncertainty about the molten fraction of the core as well as whether the field is driven by compositional or thermal differences. However, the different ideas for the driving force behind Mercury's magnetic field predict slightly different field geometries, so careful measurements by spacecraft can potentially distinguish among current theories.

MESSENGER's magnetometer will characterize Mercury's magnetic field in detail from orbit over four Mercury years (each Mercury year equals 88 Earth days) to determine its precise strength and how that strength varies with position and altitude. The effects of the Sun on magnetosphere dynamics will be measured by the magnetometer and by the energetic particle and plasma spectrometer. MESSENGER's highly capable instruments and broad orbital coverage will greatly advance our understanding of both the origin of Mercury's magnetic field and the nature of its interaction with the solar wind.

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