Why is Venus the hottest planet even though Mercury is closer to the sun?
Mercury is closest to the sun, this is true. However, because of its proximity to
the sun it lacks an atmosphere. Mercury's gaseous molecules reached escape velocity
long ago and the atmosphere was 'burned off' leaving a vacant wasteland of rock
with a similar surface to the earth's moon. There is nothing to hold in solar
radiation, thus is flies out into space.
Venus is a different story. Its atmosphere is primarily composed of carbon dioxide
- the primary green house gas -, which acts like a one-way door. The solar heat
enters, but it cannot leave. Thus, an oven is created. This is why Venus has no
water. Pictures suggest that Venus once had an aqueous environment, but because of
climatic changes and intense heat.... it evaporated.
Also, because of an overactive core, Venus seems to recreate its continents every X
years. So, we do not expect to find fossils or many bacteria.
Joel Levy, State University of New York - Albany