Other Comparisons between Venus and Earth
Most of Earth's carbon is locked up in carbonate minerals (limestone, etc). If it were all released to the atmosphere through extraordinary heating and volcanism, it would produce an atmosphere just like Venus. The abundance of Nitrogen on Venus appears to be quite close to that on Earth as well.
Water
However, the amount of water on Venus is much, much less than
that on Earth, by about 10,000 times! First, it's all in the
Venusian atmosphere (boiled away from the surface). But the
very low value of water today suggests Venus lost most of its
water to space. The steam in the Venusian atmosphere is
subjected to X-ray and UV radiation, which breaks up the
molecule into H and O, and Hydrogen easily escapes to space.
Why did Venus evolve differently?
Venus gets more heat from the Sun, because it is 30% closer.
This would naturally heat up the surface by about 30K. The
hotter atmosphere can hold more water, and more of the oceans
would evaporate. But additional water in the atmosphere will
only increase greenhouse heating! Before too long, the planet
created a run away greenhouse, whereby the oceans were
entirely boiled off, and then the atmospheric water destroyed
and lost to space.
When did Venus lose its Water?
It's hard to know when or how much water was lost from Venus.
The unusually high ratio of heavy Hydrogen (one proton, one neutron)
called Deuterium, to normal Hydrogen tells us it was indeed lost
to space.. But how much? Each time a new comet hits Venus, it
replenishes its water supply (a bit) with normal D/H ratios. For
this reason, we can't use current D/H ratios to estimate the
total water lost. However, it probably had at least 1/1,000
that of Earth, meaning it's lost >90% of its water.
Why doesn't Earth have a run away greenhouse?
Luckily, we're just a bit further from the Sun. The natural
heating from the Sun was not so great to put too much water into
the atmosphere. However, if you increase the Solar energy to
Earth by 40%, it will experience a runaway greenhouse.
This will happen in about 2 billion years, as our Sun runs out
of fuel and begins to grow into a red giant star.
Why does the Surface look Orange?
The Venera 13 images look orange because the thick cloud later
above never allows full sunlight to reach its surface.