Venus is the second planet from the Sun, with a nearly circular orbit having an
average radius of 0.7 A.U. This gives it an orbital period of 225 days. Venus
is peculiar in that its rotation is retrograde (in the opposite sense of the
Earth and all other planets except Uranus) and because it is very slow: a day
on Venus corresponds to 243 Earth days.
We know this because of DOPPLER SHIFTS:
Radio astronomers in 1962 bounced radio waves off of Venus and
the reflected wave came back partially Doppler shifter: the west
edge of Venus produced a blue shift, and the east edge a redshift.
This told them that Venus was rotating clockwise, and the data analysis
told them the period of 243 days.
At present, we have no solid
explanation for why this is so. The most plausible theories invoke the
collision of two large masses to form Venus in just such a way to cancel most
of the rotation for the two masses. Like Mercury, but unlike the other
planets, Venus has no moons.
Internal Structure/Absence of Magnetic field a Puzzle
The radius of Venus is almost exactly that of the Earth. Its average density
is 5.2 g/cc, which is slightly less than that of the Earth or Mercury, but
suggests a dense iron core and internal structure similar to that of the
Earth. Venus has an extremely weak
magnetic field, but that may be a consequence
of its having such a slow rotational velocity.
The weak field is puzzling also because it means Venus lost its
internal heat faster than Earth but we don't fully understand how.
Comment on Dynamos and Magnetic Fields
Note that magnetic fields are supported in planets by
molten conducting cores and it is thought that a magnetic field
signatures a molten core. This is because astrophysicists
think they understand how planetary fields are generated and why
they reverse periodically. This process is called the DYNAMO MECHANISM
and requires fluid highly conducting material. This is also
what is thought to be happening to sustain the field on the Sun.
Heres the idea: the magnetic
field in a good conductor cannot escape the material. When the material
is molten, liquid, or gaseous,
the field gets wound and twisted and is subject to the influence
of the flow. It turns out that gravity's influence on the flow is
such that fluid flows in rotating astronomical bodies
carry the field in such a way as to both amplify the field, and also allow
the global field to reverse periodically.
If there were no conductor, then the field would leave the material,
not be amplified, and just decay. If the field were a conductor but
not molten, the field would decay extremely slowly, but it would not
reverse. Thus the reversal and the absence of field decay
suggest the presence of a molten conductor.
While the logic works in one direction,
that is "presence of reversing, non-decaying magnetic field implies
fluid conductor where field is anchored"
people also like to argue the converse: "that conducting molten
cores also imply the presence of a strong magnetic field."
This is not necessarily true and so it still
may be that Venus has a molten core.
Venus is always covered by a thick layer of clouds that make it impossible to
see the surface for light in the visible part of the spectrum.
Light at radar wavelengths
penetrates the cloud deck and allows us to study the surface. A comparison of
the motion of the surface with that of the upper clouds indicates that while
the surface takes about 8 months to rotate, the clouds rotate all the way
around the planet in about 4 days. This indicates that there are very high
velocity winds in the upper part of the Venusian atmosphere.