Venus, long known as the hottest planet in the Solar System, may
have a layer of atmosphere so cold that carbon dioxide freezes and
falls as ice or snow.
A new paper describes this atmospheric layer, located 125
kilometres above Venus' surface. Scientists announced the finding 1
October after analysing five years of data from the orbiting Venus
Venus Express is a European Space Agency satellite that has been
circling Venus for the last six years, taking pictures and sending
back data from the planet.
The results will be published in the Journal of
"Since the temperature at some heights dips below the freezing
temperature of carbon dioxide, we suspect that carbon dioxide ice
might form there," stated Arnaud Mahieux, who is the study's lead
author and also a researcher at the Belgian Institute for Space
Scientists found the cold layer, believed to be at -175 degrees
Celsius, from watching sunlight filter through Venus'
The Sun's light showed the concentration of carbon dioxide gas
molecules along the terminator, which is the line (as seen from
orbit) between the night and day sides of a planet.
Researchers combined that information with measurements of
pressure at various altitudes in the atmosphere to determine the
The discovered cold layer appears to be in between two
layers of warmer atmospheric sections. More puzzling, it's colder
than any part of Earth's atmosphere, even though Venus is 40
million kilometres closer to the Sun.
Venus Express has seen bright patches in the atmosphere that
could be bits of ice or snow falling from this cold spot. Still,
Mahieux warned there could be other explanations for this
"They could also be caused by other atmospheric disturbances, so
we need to be cautious," he stated in a press release.
Venus' atmosphere is thick, and reflects so much light from the
sun, that the planet is frequently and easily visible as a bright
"evening" or "morning" star to stargazers on Earth.
Beneath that thick shield of cloud lies a surface baking at more
than 460 degrees Celsius, or hot enough to melt lead, according to
Scientists believe this is due to a runaway greenhouse effect
sparked when Venus' temperatures suddenly rose billions of years
ago. Water vapour and eventually, carbon dioxide from rocks rose
into the atmosphere, trapping heat close to the planet.
The environment is so severe that several Soviet spacecraft
landers, which reached Venus in the 1960s and 1970s, lasted just
minutes on the surface.
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