|
The MAG/ER experiment aboard Lunar Prospector was designed with
two primary goals in mind: scanning the lunar crust for signs
of permanent magnetization and searching for electrical currents
flowing deep within the lunar interior -- the sign of a conductive
metallic lunar core. The two instruments combine to calibrate
the Moon's global magnetic field strength: the magnetometer measures
the field surrounding the spacecraft, and the electron reflectometer
surveys the lunar surface.
Currently, Ames Research Center is collecting data radioed through
NASA's Deep Space Network (DSN) over 380,000 km back from the
Prospector to Earth. Lunar magnetic field mapping is under way,
and once all the data has been collected and analyzed, scientists
will be able piece together the mysteries of the Moon. Though
it will take months before the data is completely analyzed, surprising
and useful information already has been gained.
Although it was previously believed that the Moon's magnetic
field was too weak to repel the charged particles of the solar
wind, an intriguing magnetic anomaly on the moon's surface has
been found that can stand off the solar wind, thus creating the
smallest known magnetosphere, magnetosheath and bow shock system
in the Solar System. While most planets' global fields create
a large encompassing magnetosphere around the entire body, the
moon contains magnetized rocks on its upper layers, some of which
are magnetized strongly enough to form small dipole magnetic fields
scattered on the lunar surface. These mini-magnetospheres, around
100 km in diameter (the Moon is approximately 3500 km in diameter),
can stand off the solar wind locally.
Another curious result of Prospector's preliminary data is the
presence of strong magnetic fields located diametrically opposite
young large impact basins on the lunar surface. There are two
components of the model illustrating how this odd field may have
formed. The first component of the model is that of the impacts'
physical effects at the antipodes (or, opposite side). When large
objects strike, seismic and surface waves are sent through the
lunar material. This results in unusual looking terrain at the
antipodes, where the rocks appear to have been temporarily fluidized
and then resolidified. In addition, when ejecta from the original
impacts are sent flying, secondary impacts occur where they land.
Most of this ejecta lands near the periphery of the basin, but
there is an increased amount found at the antipodes. The combination
of the primary and secondary impacts' physical effects cause a
shock, or pressure pulse, to be sent through the material of the
lunar crust. Microscopic metallic iron particles in the soil carry
this magnetization induced by this shock to the antipodal regions
causing an increased magnetic field. A second component of the
model involves the build up of ionized gas. Impacts at velocities
greater than 10 km/s will vaporize rock into hot gas, and this
hot gas is partly ionized into electrons and positive ions. The
ionized gas from the impact will expand around the moon and exclude
any ambient magnetic field from the ionized gas, forcing it around
until it converges at the antipodes, thus compressing and amplifying
the magnetic field at the antipodes.
This diagram shows the how the Earth's magnetic field is affected
by solar radiation-solar wind. The Earth's magnetic field is strong
enough to "stand off" the radiation, that is divert it around
the Earth much like a boulder in a running stream diverts water
around it. LP's Magnetometer/Electron reflectometer experiment
has revealed a tiny magnetic field on the Moon which is strong
enough to accomplish the same feat.
As for the question of resources in the surface layers, the solar
wind has been found to have nonuniformly (due to these magnetic
anomalies) implanted hydrogen in the crust. Strong anomalies deflect
the solar wind around the magnetic field, so we find concentrations
of hydrogen around the peripheries of magnetic aberrations, but
very little solar wind hydrogen is located directly at the center
of these regions. Around the edges, where hydrogen exists in local
concentrations, may be practical locations for future lunar bases.
This diagram shows the magnetic anomaly on the Moon which is
powerful enough to "stand off" the solar wind. This is the smallest
such magnetic shock front ever identified.
Of course, the process of completely mapping the lunar magnetic
fields is still in progress, and many more questions can be addressed
once the complete data set is analyzed. At that point, scientists
will be able to investigate the existence of a core and more accurately
determine its upper size limit. They can also determine the electrical
conductivity and postulate about the composition of the core.
In addition, mapping the direction of the magnetization, and therefore
determining the orientation of the field lines at the time of
magnetization, will help elucidate the origin of the lunar magnetic
field. Another enigma waiting to be solved is the unexpected correlation
between individual magnetic anomalies with unusual albedos markings
in the antipodal zones - the markings are lighter in color, and
therefore higher in albedo. Answers to these and many other questions
are anticipated as the mission develops.
|
