ESA Science & Technology - Major Discoveries by Venus Express: 2006-2014

7. Water loss

Venus and Earth are similar in size and density, so these close neighbours almost certainly formed at the same time and from the same constituents. Yet today Earth is a water-rich world, whereas Venus is extremely dry. What happened to all of the water that presumably existed on the infant Venus?

Venus Express has confirmed that a large quantity of water has been lost into space over billions of years. The spacecraft's magnetometer instrument (MAG) made the first ever detection of atmospheric loss on Venus's day side. Then, in 2007, the Analyser of Space Plasma and Energetic Atoms (ASPERA) instrument discovered a rapid loss of hydrogen and oxygen from the night side.

This depletion occurs because water molecules in its upper atmosphere are split by incoming ultraviolet radiation from the Sun. This process creates two hydrogen atoms and one oxygen atom for each dissociated molecule.

The solar wind – a stream of charged particles from the Sun – then strikes the upper atmosphere and carries the hydrogen and oxygen atoms into space. Unlike Earth, Venus does not generate a magnetic field which can protect its atmosphere from the solar wind.

Venus Express has measured the rate of this escape and confirmed that roughly twice as much hydrogen as oxygen is escaping, confirmation that water (H₂O) is the source of these escaping ions.

It has also shown that a heavy form of hydrogen, called deuterium, is progressively enriched in the upper regions of Venus's atmosphere because the heavier gas finds it less easy to escape the planet's gravitational grip.

This is one of the major discoveries made by Venus Express during the last eight years. Read more on the following pages: