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European Space Agency

Press Release

Greenhouse effects... also on other planets
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14 February 2003

Our planet is warming up, and experts warn that the consequences will
be serious. To see precisely how the process works, scientists need as 
much information as possible and from many different sources. 

There are valuable clues out in space. ESA's missions to Venus, Mars,
and Saturn's moon Titan will soon provide useful information to
understand how our own planet's climate is regulated.

The Earth's climate seems to be changing much more quickly than it
used to do. We are altering one of our planet's natural 'temperature
regulators' - the greenhouse effect. The greenhouse effect determines
the flow of energy arriving at and leaving from Earth.

The Sun's energy heats the Earth's surface and the planet radiates
energy back into space. However, certain atmospheric gases trap some
of the outgoing energy, retaining heat. Without this natural
phenomenon, average temperatures on Earth would be 30 degrees lower
than the current 15 degrees Celsius. Unfortunately, fossil-fuel
combustion and deforestation release large amounts of greenhouse gases 
to the atmosphere, strengthening the greenhouse effect. Global
temperatures have increased more than half a degree in the last
century as a result. 

For a really strong greenhouse effect, we should look at Venus. Venus
is similar to Earth in terms of size and mass, but its surface
temperature is about 460 degrees Celsius. This is hot enough to melt
lead! The Venusian atmosphere is mainly made up of carbon dioxide, a
greenhouse gas. On Earth, carbon dioxide makes up only a tiny fraction 
of the atmosphere.

However, man-made emissions have caused carbon dioxide concentrations
here to increase by about 30% since pre-industrial times. Why is there 
so much carbon dioxide in the Venusian atmosphere? What made Venus 
evolve so differently from Earth? "Good questions. That is precisely 
one of the things we want to find out" says Hakan Svedhem, Project 
Scientist for ESA's mission Venus Express, due for launch in 2005.

Is Venus a mirror that reflects how the Earth will be if global
warming continues at its current speed? "Venus will help us understand 
what happens when the greenhouse effect is really extreme.  However, 
it's not a good example of what will happen to Earth due to human 
activities. Life on Earth would disappear due to the extreme
temperatures much before reaching even half of the concentrations of
carbon dioxide on Venus!" says Svedhem.

As a complete contrast to Venus, there is Mars. The Red Planet
displays hardly any greenhouse effect. Mars does have some atmospheric 
carbon dioxide, but almost no atmosphere! The existing atmosphere is 
so thin that it cannot retain energy from the Sun.  There are 
therefore extreme temperature contrasts between day and night and sun 
or shade. However, most scientists agree that Mars was much warmer in 
the past and even had oceans, which means that the atmosphere was also 
very different. About 3600 million years ago, something happened and 
the planet evolved towards its current state.  What could have 
triggered such a huge change in climate? "We will answer this question 
with Mars Express" explains Agustin Chicarro, Project Scientist of 
ESA's mission to Mars, due for launch in May 2003.

Finally, if we look at Titan, Saturn's largest moon, we see a moderate 
greenhouse effect mostly due to the large concentrations of methane, 
another greenhouse gas, in its atmosphere. Astronomers have compared 
Titan with the early Earth. It would be a suitable place for life if 
it were not so cold: its surface is extremely cold at about - 180 
degrees Celsius. Understanding which factors influence Titan's climate 
would be enormously helpful to us, on Earth. "What we learn on Titan 
will certainly be useful to understand the other planets", confirms 
Jean-Pierre Lebreton, Project Scientist of Huygens, ESA's probe to 
Titan. 

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