Ch. 5: the Terrestrial Planets
We survey the four terrestrials first, then look
at details for Mercury, Venus, and Mars.
Terrestrial Planets’ Spin Axis and Rotation Rate
Terrestrial Planets’ Spin
Mercury’s sidereal rotation is tidally locked to 2/3 of an orbit.
Venus rotates very slowly backwards compared to the other
Planets, so that it is rotating clockwise (others are CCW).
Mercury and Venus both have almost no axial tilt, Earth and
Mars both have similar axial tilts and rotation rates.
Mercury’s Rotation is tidally locked to the Rotation period.
The solar day on Mercury is 176 days long,
which is two Mercury-years!
This tidal locking is different from that of the
Moon revolving around Earth because Mercury’s
orbit is elliptical and not nearly round like the Moon’s.
Atmospheres and Temperatures
Mercury has no atmosphere, just like our moon. Because
there is no atmosphere to trap heat, the night side of Mercury
gets very cold (100 K or -280oF). The day side of Mercury
gets up to 700 K (or 800oF).
The atmosphere of Venus is made up of carbon dioxide, with
clouds of sulfuric acid. The atmosphere is some 90 times
denser than Earth’s. The Greenhouse effect causes the
surface temperature of Venus to be close to 730K (or 860oF)
day or night.
Mars has a very thin atmosphere (less than 1% of Earth’s) of
mainly carbon dioxide. The surface temperature is
approximately 50 K (50oC) lower than Earth’s.
Mercury’s Surface
Mercury’s surface has
a large number of these
scarps or cliffs like giant
cracks in its surface.
Mercury never had plate
tectonics like the Earth.
When the crust of
Mercury cooled it shrank
causing the crust to crack.
Mercury’s Caloris Basin, due to a very large impact
Venus, Up Close
Because of Venus’s dense
cloud cover most of what
we know about Venus’s
surface and rotation comes
from using radar.
There have been only a few
spacecraft to land on Venus,
but each survived for only a
short time.
(This is an ultraviolet photo.)
Venus Radar Map taken by the Pioneer Venus spacecraft
Earth Radar Map, shown with scale similar to the Venus map
Venus map made by the Magellan spacecraft
In 1995 the Magellan
spacecraft was able
to make a much more
detailed radar map of
Venus.
Possibly active shield
volcanoes, craters,
and volcanic structures
called coronae were
seen by Magellan.
Venus’s Surface Features
including some
volcanic cones and “coronae”
A Venus Corona – a collapsed volcanic cone
Impact Craters on Venus – as seen with radar from above
Venus surface, taken with a camera on the Soviet “Venera”
probe (“Venera” just means “Venus” in Russian).
Atmospheres and Temperatures
Mercury has no atmosphere, just like our moon. Because
there is no atmosphere to trap heat, the night side of Mercury
gets very cold (100 K or -280oF). The day side of Mercury
gets up to 700 K (or 800oF).
The atmosphere of Venus is made up of carbon dioxide, with
clouds of sulfuric acid. The atmosphere is some 90 times
denser than Earth’s. The Greenhouse effect causes the
surface temperature of Venus to be close to 730K (or 860oF)
day or night. This is hot enough to melt lead or tin.
Mars has a very thin atmosphere (less than 1% of Earth’s) of
mainly carbon dioxide. The surface temperature is
approximately 50 K (50oC) lower than Earth’s.
Venus’s Atmosphere has an extreme Greenhouse effect
A Mars map shows the continent-sized features
and five landing sites of some early US spacecraft
A Martian Volcano, called Olympus Mons has clouds around it
Mars has the largest volcanoes in the Solar System, but all
are dormant. They are taller than any Earth volcano
because the low gravity on Mars allows them to grow tall.
Martian Ocean
Terrestrial Planet Interiors
Mercury’s mantle is solid (not semi-molten like the Earth’s) and
its iron core may have a solid center. There should be a liquid
iron layer which causes the magnetism of Mercury.
Not much is known about the interior of Venus – but it is thought
to be like a young Earth. (so we don’t have a figure for it above)
Mars is mostly solid and no longer geologically active in any way.
Earth’s Magnetosphere is due to it’s magnetism.
The Earth’s magnetic field is formed because of the
Earth’s fairly rapid rotation and because the Earth
has a molten iron outer core which can flow and
can have electric currents in it.
There is no Lunar magnetic field so it has little iron.
Terrestrial Planet Magnetic Fields
Mercury has a very weak magnetic field, partly due to its slow
rotation, even though it may have liquid iron in the core.
Venus has no measurable magnetic field, probably due to its
very slow rotation rate.
Mars has a magnetic field weaker than Mercury, meaning
that its core is either not liquid or not metallic.