Fundamental Astronomy

surrounding an elevated plain, the diameter of which
can be as big as several hundreds of kilometres. They
are possibly examples of local hot spots, mantle upwellings
that have expanded and formed bulges. When
the flow has stopped, the bulge has sunk and formed
a set of ring mountains.
In other places fluid lava flows have produced long,
sinuous channels extending for hundreds of kilometres.
Most of the Venusian impact craters are undeformed.
This indicates that the Venusian surface must be young
because erosion, volcanism and tectonic forces should
affect the craters, too. Resurfacing processes may frequently
cover the old craters, and all craters visible
are therefore young, presumably less than 500 million
years. There are no impact crates smaller than about
1.5–2 km because smaller meteoroids are burned in the
thick atmosphere.
The Earth and Venus are almost equal in size, and
their interiors are assumed to be similar. Venus has an
iron core about 3000 km in radius and a molten rocky
mantle covering the majority of the planet. Probably due
to its slow rotation, however, Venus has no magnetic
field. The analyses made by the Venera landers have
shown that the surface material of Venus is similar to
terrestrial granite and basalt (Fig. 7.26).
Venus has no satellites.
7.12 The Earth and the Moon
The third planet from the Sun, the Earth, and its satellite,
the Moon, form almost a double planet. The relative size
of the Moon is larger than that of any other satellite,
excluding the moon of Pluto. Usually satellites are much
smaller than their parent planets.
The Earth is a unique body, since a considerable
amount of free water is found on its surface. This is possible
only because the temperature is above the freezing
point and below the boiling point of water and the atmosphere
is thick enough. The Earth is also the only planet
where life is known to exist. (Whether it is intelligent or
not is yet to be resolved...). The moderate temperature
and the water are essential for terrestrial life, although
some life forms can be found in extreme conditions.
The diameter of the Earth is 12,000 km. At the centre,
there is an iron–nickel core where the temperature is
7.12 The Earth and the Moon
Fig. 7.27. Internal structure of the Earth. The speed of the
seismic waves, density, pressure, and temperature are shown
as a function of depth
5000 K, the pressure 3 × 10 11 Nm −2 and the density
12,000 kg m −3 (Fig. 7.27).
The core is divided into two layers, inner and outer
core. The inner core, below 5150 km comprises only of
1.7% of the mass of the Earth. It is solid because of
high pressure. The nonexistence of the seismic transverse
S waves below a depth of 2890 km indicates that
the outer core is molten. However, the speed of the longitudinal
P waves change rapidly at a depth of 5150 km
showing an obvious phase transition. It has been discovered
that the solid inner core rotates with respect to
the outer core and mantle.
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