[meteorite-list] MESSENGER: Measuring Mercury's Magnetic Field

[Ron Baalke]

http://messenger.jhuapl.edu/soc/highlights.html

Science Highlights from Mercury's Orbit
May 17, 2011

MESSENGER: Measuring Mercury's Magnetic Field

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.

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.

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.

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.