The latest findings, reported in this week's issue of the journal Nature, support this theory.
An instrument on the ESA spacecraft known as a plasma analyzer showed that the main ions escaping Venus's atmosphere are oxygen, helium, and hydrogen.
Hydrogen and oxygen ions were shown to be escaping in the same proportions as they are found in water, H2O—providing a likely mechanism for how water has been leaving Venus.
"If you break up an H2O molecule, the hydrogen and oxygen escape at a proportional rate, so that seems to be where the water's going," Grinspoon said today at a press conference at ESA headquarters in Paris, France.
Further evidence for Venus's vanished water comes from high levels of atmospheric heavy hydrogen, or deuterium, the study team said.
"As hydrogen escapes, it leaves behind the heavier deuterium, so the amount of deuterium residue is an important clue to the amount of water that's been lost," Grinspoon said.
Estimates based on Venus's levels of deuterium suggest that the planet has lost anywhere from 13 feet (4 meters) of surface water up to "an Earth's ocean's worth," the scientist added.
Analysis of newly gathered data on the amounts of deuterium found at different levels of Venus's atmosphere should lead to much better estimates of exactly how much water has disappeared into space, Grinspoon said.
The Venus Express team also found unexpected evidence for a warm layer of air 56 to 75 miles (90 to 120 kilometers) above the night side of the planet.
Previously the upper atmosphere of Venus's night side was thought to be so cold that it was named the cryosphere.
Violent winds were also detected in the planet's upper atmosphere. At 43 miles (70 kilometers) above Venus's surface, wind speeds reached 225 miles (360 kilometers) an hour.
In addition, the mission team found that Venus, like Earth, has huge areas of circulating air over both its polar regions.
But Venus's mass of swirling polar clouds forms a double vortex with temperatures much higher than the surrounding area. The warm air may be evidence of a planetary circulation system, with hot air flowing to the poles from equatorial regions.
The ESA spacecraft also provided the first solid evidence for lightning on Venus, according to the study team.
"It's quite possible there's as much lightning on Venus as on Earth," team member Grinspoon said.
This finding is highly surprising, according to planetary scientist Andrew Ingersoll with the California Institute of Technology in Pasadena, California.
"There shouldn't be any [lightning] on Venus, whose clouds are like [Earth's] smog clouds, which do not produce lightning," Ingersoll wrote in a commentary also appearing in Nature.
"Perhaps we have simply not thought of all the ways electricity can be generated in a planetary atmosphere," he added.
Venus Express mission scientist Fred Taylor from the University of Oxford, England, said that while Venus's climate is very different from Earth's, there are many common processes at work.
"On Venus, these have worked to virtually eliminate water from the planet while maintaining high levels of carbon dioxide, while Earth has retained much of its water and lost most of its atmospheric carbon dioxide," he said.
"In the light of the new data, it is possible to construct a scenario in which the climates on Venus and Earth were very similar when they started out, and then evolved to the state we see now, like twins separated at birth," Taylor said.
"Billions of years ago there is even the possibility that Venus would have been habitable."
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