Evidence from seismology, heat flow at the surface, and mineral physics is combined

with the Earth's mass and moment of inertia to infer models of the Earth's interior
– its composition, density, temperature, pressure. For example, the Earth's mean
specific gravity (5.515) is far higher than the typical specific gravity of rocks
at the surface (2.7–3.3), implying that the deeper material is denser. This is also
implied by its low moment of inertia ( 0.33 M R2, compared to 0.4 M R2 for a sphere
of constant density). However, some of the density increase is compression under
the enormous pressures inside the Earth. The effect of pressure can be calculated
using the Adams–Williamson equation. The conclusion is that pressure alone cannot
account for the increase in density. Instead, we know that the Earth's core is
composed of an alloy of iron and other minerals.[10]

Reconstructions of seismic waves in the deep interior of the Earth show that there
are no S-waves in the outer core. This indicates that the outer core is liquid,
because liquids cannot support shear. The outer core is liquid, and the motion of
this highly conductive fluid generates the Earth's field. Earth's inner core,
however, is solid because of the enormous pressure.[12]

Reconstruction of seismic reflections in the deep interior indicates some major

discontinuities in seismic velocities that demarcate the major zones of the Earth:
inner core, outer core, mantle, lithosphere and crust. The mantle itself is divided
into the upper mantle, transition zone, lower mantle and D′′ layer. Between the
crust and the mantle is the Mohorovičić discontinuity.[12]

The seismic model of the Earth does not by itself determine the composition of the
layers. For a complete model of the Earth, mineral physics is needed to interpret
seismic velocities in terms of composition. The mineral properties are temperature-
dependent, so the geotherm must also be determined. This requires physical theory
for thermal conduction and convection and the heat contribution of radioactive
elements. The main model for the radial structure of the interior of the Earth is
the preliminary reference Earth model (PREM). Some parts of this model have been
updated by recent findings in mineral physics (see post-perovskite) and
supplemented by seismic tomography. The mantle is mainly composed of silicates, and
the boundaries between layers of the mantle are consistent with phase transitions.
[10]

The mantle acts as a solid for seismic waves, but under high pressures and
temperatures, it deforms so that over millions of years it acts like a liquid. This
makes plate tectonics possible.

Magnetosphere
Main article: Magnetosphere
Diagram with colored surfaces and lines.
Schematic of Earth's magnetosphere. The solar wind flows from left to right.
If a planet's magnetic field is strong enough, its interaction with the solar wind
forms a magnetosphere. Early space probes mapped out the gross dimensions of the
Earth's magnetic field, which extends about 10 Earth radii towards the Sun. The
solar wind, a stream of charged particles, streams out and around the terrestrial
magnetic field, and continues behind the magnetic tail, hundreds of Earth radii
downstream. Inside the magnetosphere, there are relatively dense regions of solar
wind particles called the Van Allen radiation belts.[26]

Methods
Geodesy
Main article: Geodesy
Geophysical measurements are generally at a particular time and place. Accurate
measurements of position, along with earth deformation and gravity, are the
province of geodesy. While geodesy and geophysics are separate fields, the two are
so closely connected that many scientific organizations such as the American
Geophysical Union, the Canadian Geophysical Union and the International Union of
Geodesy and Geophysics encompass both.[39]

Absolute positions are most frequently determined using the global positioning
system (GPS). A three-dimensional position is calculated using messages from four
or more visible satellites and referred to the 1980 Geodetic Reference System. An
alternative, optical astronomy, combines astronomical coordinates and the local
gravity vector to get geodetic coordinates. This method only provides the position
in two coordinates and is more difficult to use than GPS. However, it is useful for
measuring motions of the Earth such as nutation and Chandler wobble. Relative
positions of two or more points can be determined using very-long-baseline
interferometry.[39][40][41]

Gravity measurements became part of geodesy because they were needed to related
measurements at the surface of the Earth to the reference coordinate system.
Gravity measurements on land can be made using gravimeters deployed either on the
surface or in helicopter flyovers. Since the 1960s, the Earth's gravity field has
been measured by analyzing the motion of satellites. Sea level can also be measured
by satellites using radar altimetry, contributing to a more accurate geoid.[39] In
2002, NASA launched the Gravity Recovery and Climate Experiment (GRACE), wherein
two twin satellites map variations in Earth's gravity field by making measurements
of the distance between the two satellites using GPS and a microwave ranging
system. Gravity variations detected by GRACE include those caused by changes in
ocean currents; runoff and ground water depletion; melting ice sheets and glaciers.
[42]