Now, the Venus Express probe, launched by the European Space Agency in
2005, has ventured beneath those clouds and found evidence that Venus
once had more
water than it does today. The probe also provided
detailed new measurements of the weather on Venus, proof of lightning
on the planet, and signs of a formerly unknown hot spot near its south
pole.
In nine papers appearing in the Nov. 29 Nature, researchers say these findings could be useful for understanding Earth''s
atmosphere too.
"Venus resembles the Earth in many, many ways,"
says Andy
Ingersoll of the California Institute of Technology in Pasadena. Not
only do Venus and Earth orbit the sun at similar distances, but the two
planets are similar in size, gravity, and composition. Though Venus''
atmosphere contains much more carbon dioxide than Earth''s, both have
water vapor.
Water''s role in Venus'' past, in particular whether there used
to be more of it, was one of the biggest questions about the planet,
says Ingersoll.
"There''s some water, but where''s the ocean on Venus?" he asks. "Venus Express has addressed that."
If Venus once had more water,
scientists figured, then vast
amounts of
hydrogen and oxygen must at some point have escaped Venus''
gravity. But some hydrogen comes in a heavy form, deuterium. It is
harder for the heavy form to escape gravity, so if lots of hydrogen
from water left Venus, the
ratio of
deuterium to hydrogen left behind
would rise.
A team of scientists led by Jean-Loup Bertaux of the Service
d''Aéronomie du CNRS in Verrières-le-Buisson, France, showed that the
deuterium-to-hydrogen ratio on Venus is, in fact, higher than that on
Earth.
If the water vapor in Venus'' atmosphere today were instead an
ocean, it would be 3 centimeters deep. Using the deuterium-hydrogen
ratio to estimate how much water has been lost, the scientists
extrapolated that there once would have been enough water to cover
Venus with at least 4.5 meters of water. (If all the water on Earth
were spread out, it would be 2.8 kilometers deep.)
Furthermore, in a separate paper, Stanislav Barabash of the
Swedish Institute of Space Physics in Kiruna showed that hydrogen and
oxygen ions are still escaping from Venus today.
"The surprising discovery was the escape of oxygen atoms and
hydrogen atoms keeps the same ratio as in a water molecule," says
Barabash.
Understanding how and why water leaves Venus has important
implications on Earth, Ingersoll says. When a climate heats up and
oceans evaporate, the increased water vapor in the atmosphere acts as a
greenhouse gas and can accelerate the warming of the oceans.
"If this runaway greenhouse effect could happen on Venus, could it happen on Earth too?" asks Ingersoll.
Insights into water on Venus weren''t the only surprise findings
from Venus Express. Scientists found a hot spot near the south pole
that''s 10°C warmer than the surrounding atmosphere. A hot spot of
similar shape and size had previously been discovered near Venus'' north
pole.
The probe also improved scientists'' understanding of weather
patterns on Venus. Radio signals sent through the clouds recorded a
difference in temperature between nighttime and daytime of 40°C, much
larger than anticipated. Other instruments showed a lightning rate
about half that on Earth.
Håkan Svedhem of the European Space Agency in Noordwijk, the
Netherlands, says that the Venus Express findings offer a much-needed
baseline for comparison with data from future missions.
"To follow all this and see how it evolves as a function of time will be interesting," he says.
More abstracts about the Sister Planet: Mission to Venus reveals watery past