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Planetary Magnetic Field and Magnetism of Meteorites
Bushra Mohamed Ibrahim
Ph.D. Student (Astronomy-Utah University, USA)
Introduction
Planetary magnetic field is composed of contributions from many
sources; Internal, crustal, and external. The internal field comes from the
core of the planet, this kind of field is called the main field. This field has
the most dominant of these fields. It is accounting for over 97% of the total
field observed at the Earth's surface, and ranging in intensity from about
30000 nT at the equator to about 60000 nT at the poles. In the crust, the iron
that exists associated with other magnetic minerals can have magnetization
that is induced by the present day ambient. This kind of magnetic field is
called anomalies magnetic field.
The external field is composed of the field of the ionosphere and
magnetosphere. At middle and low altitudes the heating by solar radiation on
the dayside and the cooling occurred on the night side of the atmosphere
generate tidal winds that move the ionosphere plasma throughout the main
field, and causes induced electric fields and currents in the dynamo region
between 100-140 km in height. This phenomena leads to terrestrial induced
magnetic field called the ionospheric field, its pattern depends mainly on
latitude, season, solar activity, and time of day. The interaction of the
atmospheric currents with the radiation belts near the planet, achieves a
magneto plaza closure called magnetosphere.
In this chapter, we will focus on the main magnetic fields of the Earth
and meteorites coming from the solar system planets.
Main Magnetic Field of Planets
A planet can have a main magnetic field if it is consisting of conducting
interior (like molten iron) rotating with relatively short period. Table (1)
shows the rotation periods of our solar planet, the total strength of the
magnetic field relative to the magnetic field strength of Earth, the magnetic
field type, and the core composition that is generates the magnetic field.
The table illustrates that the planets Venus, Mars, and Pluto have no
magnetic field. Venus's interior is still not well understood, some scientists
proposed that the core may be cooled and solidified. Others scientists
believed that the deep interior of Venus is probably like the Earth's, and have
a molten interior. Despite its molten interior the lack of magnetic field may
be due to its very slow rotation. Mars may have had a global magnetic field
in its past. Spacecrafts indicate that Mars have an incomplete magnetic field,
and it is too weak and cannot remotely measure. Its small size means that it
is cooled more rapidly than the Earth and had lost its molten interior. The
existence of inactive volcanoes on Mars surface indicates the existence of
molten interior, in the past. However, as a planet its continuous and fast
cool led to the loss of its molten interior and, consequently, to its main
magnetic field. Astronomers suggested that the interior of Pluto must be a
mix of water, ice, and rocks, which made Pluto does not have a magnetic
field.
The inclination of the magnetic field axis with respect to the rotation
axis of each planet determines the type of magnetic field. That’s mean,
when the inclination increase, the amount of dipole decreases, and mainly
it is associated with an increase in the quadruple field.
Table (1) The rotation period, magnetic field strength and type,
and core composition of each planet.
Magnetic Field
The
Planet
Rotation
Period
Total
Strength
(Earth's)
Mercury
58.81 day
0.006
Venus
243.69 day
0.00
Earth
23.9345 h
1
Mars
24.623 h
0.00
Jupiter
9.926 h
19519
Saturn
10.5 h
578
Uranus
17.24 h
47.9
Very inclined field
Neptune
16.11 h
27
Very inclined field
Pluto
6.405 day
0.00
-
Core Component
Type
Show Earth like
structure field
Approximately
Dipolar
Show Earth like
structure field
Highly axisymmetric
Dipolar Field
Partially molten
solidified
Molten Iron
Cooled & become
solidified
Metallic Hydrogen.
Metallic Hydrogen
Ionization in the
slushy ice layer.
Ionization in the
slushy ice layer.
Ice
In the following subsections brief descriptions of the magnetic field
and the interior structure of the Earth and meteorites coming from the solar
system planets.
Earth's Magnetic Field
The Earth's magnetic field is similar to that of a bar magnet, but this
similarity is superficial. The magnetic field of a bar magnet, or any other
type of permanent magnet, is created by coordinated spins of electrons and
nuclei within iron atoms. The Earth's core, however, is hotter than Curie
point temperature (i.e., 1043 K); at this temperature the orientations of spins
within iron become randomized. Such randomization causes the substance
loses its magnetic field.
Therefore, the main part of Earth's magnetic field is not caused by
magnetized iron deposits. It is created from the convection of molten iron,
within the outer liquid core, because of the planetary rotation that tends to
organize these electric currents in rolls aligned along the north-south polar
axis. When a conducting fluid flows across an existing magnetic field,
electric currents are induced, which in turn creates induced magnetic field,
this magnetic field reinforces the original magnetic field, and a dynamo is
created which sustains itself. The "Dynamo Theorem" explains how the
Earth's magnetic field is sustained; Figure (1) illustrates the internal structure
of the Earth.
Figure (1) The structure of the Earth
The geomagnetic poles (i.e., north and south) tilt with about 11.7° with
respect to the Earth's rotation axis (which defines geographic north and
south poles). Figure (2) illustrates the inclination of the geomagnetic poles
relative to the rotation axis. If Earth's magnetic field is perfectly dipolar, the
rotation axis and geomagnetic axis would coincide. There are significant non
dipolar terms that account approximately 3-5% from the total Earth's
magnetic field. Both the position and strength of Earth's magnetic poles
change slightly but measurable, from year to year. Earth's magnetic field
reverses at intervals ranging from tens of thousands to many millions of
years. There is no distinct theory interprets the occurrence of the
geomagnetic reversals. As an acceptable interpretation the changes in
geomagnetic field probably come from irregular motions of the molten iron
in the Earth's core].
Figure (2) The inclination of Earth's magnetic axis relative
to the rotation axis
Meteorites and Magnetism
The Magnetic field measurements used for the detection of meteorites, the
magnetic field for the selected region for study is given by the following
equation:
B=BE +BO
Where:
B is the magnetic field strength that measured by magnetometer
BE is the proposed earth’s magnetic field for the selected region
BO is the magnetic field for the remnant meteorite in this region
The proposed Earth’s magnetic field is determined by the magnetic maps
that drawn by using the international geomagnetic reference field associated
with the effect of the solar activity in that day of detection (where 95-97 %
of earth’s field is come from the Earth’s interior).Also, we must have a
magnetic anomalies map.
If the selected location have no remnant meteorite in it, then the magnetic
field strength that measured by magnetometer will be identical with the
magnetic field strength that proposed for the same region.
5. Magnetic field of Meteorite
All of the planets in the inner solar system are thought to have generated
dynamos at some point in their histories. In a dynamo, molten-hot iron flows
within the core, generating a magnetic field. As a result, the rocks on the
surface of a planetary body become magnetized, providing a record of a
planet’s early history. Scientists have attempted to characterize the
magnetization of meteorites in order to reconstruct asteroid evolution. But a
major challenge has been pinpointing the source of meteorites’
magnetization, which may be formed by any number of processes ‘such as
plasmas from a meteoroid impact, or more mundane causes, like passing a
magnet over a meteorite sample’. Determining that a meteorite’s magnetic
field is the result of an early dynamo is therefore a tricky problem. To solve
the problem we must be determine the magnetization and the age of a
meteorite sample, then to check that the observed magnetic field was, in fact,
due to an early dynamo.
In the first: examination the rock’s tiny crystals. When forming in a
magnetic field, a rock’s ferromagnetic crystals align in the direction of a
background field when the rock is heated, and measured the alignment of
these minerals, or the rock’s magnetic. The next step is must be determine
the age of the rock. Also, it must be analyzed the crystals in the meteorite to
determine the rock’s cooling history. While large impacts might create a
magnetic field, such impact-generated fields would only last a few tens of
minutes, and if a rock were to become magnetized in such a short period of
time, it would also cool equally quickly.