The great escape of water from Venus

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 19 December, 2008

Venus Express has provided new clues that suggest how the planet lost its water, which could have once been as abundant as it is on Earth today.

Water is essential for life as we know it, and with Earth and Venus approximately the same size and having formed at the same time, astronomers think it inevitable that both planets began with similar amounts of the precious liquid. Today, however, things couldn’t be more different, with Earth's atmosphere and oceans containing 100,000 times the total amount of water on Venus.

The solar wind strips Venus's atmosphere of its charged particles, and could be responsible for our 'twin' planet's loss of water. Credit: ESA/C.Carreau.

Last year, the Analyser of Space Plasma and Energetic Atoms (ASPERA) on board Venus Express showed that there was a great loss of hydrogen and oxygen on the night-side of Venus, and roughly twice as many hydrogen atoms as oxygen atoms were escaping into space. Because water is made of two hydrogen atoms and one oxygen atom, the observed escape indicates that water is being broken up in the atmosphere of Venus.

Now the spacecraft has detected the unmistakable signature of hydrogen gas being stripped from the day-side, too. "This is a process that was believed to be happening at Venus but this is the first time we measured it," says Magda Delva, who lead the investigation. In spite of the low concentration of water on Venus some 2 x 10^24 hydrogen nuclei were being lost every second from Venus's day-side.

The solar wind, a stream of charged particles that ‘blows’ through space past the planets carrying electrical and magnetic fields throughout the Solar System is the likely culprit for stripping Venus of its atoms since Venus does not generate a magnetic field, which protects the Earth's atmosphere from the solar wind. Instead, at Venus, the solar wind directly impinges on the planet’s upper atmosphere and carries off particles into space. Planetary scientists think that the planet has lost part of its water inventory in this way since the planet was born.

The discovery takes scientists a step towards understanding the
details, but it does not provide the last piece of the puzzle. To be certain that the hydrogen is coming from water, the team must also detect the loss of oxygen atoms on the day-side and verify that there are approximately half as many leaving Venus as hydrogen. So far, this has not been possible. "I keep looking at the magnetometer data but so far I can't see the signature of oxygen escaping on the day-side," says Delva.

This also highlights a new mystery. "These results show that there could be at least twice as much hydrogen in the upper atmosphere of Venus than we thought," says Delva. The detected hydrogen ions could exist in atmospheric regions high above the surface of the planet but the source of these regions is unknown.

Venus Express has been in orbit around Venus for around two years and is answering questions regarding the complex dynamics of the planet's atmosphere, its local and regional weather systems, and its interaction with the solar wind, in order to better understand the evolution of the planet, and how it has become so different to the Earth.