Venus has a dense and hot atmosphere and a hot and dry surface; there is no liquid water and the amount of water vapour in the clouds is very low. These conditions are totally different from what we have on Earth. It is even more puzzling, regarding the common origin of Venus and Earth at times of formation of the solar system, where the composition of the planetary body and its atmosphere was supposedly very similar for both planets.
So if Venus had originally as much water as Earth, how could it disappear from the planet?

An answer to this fundamental question is now given by the Venus Express mission:

• In the magnetometer data of VEXMAG, first traces of planetary hydrogen can be found in the up-stream solar wind, still before this fast stream of charged solar particles is decelerated at the planet's bow shock and meets the outskirts of the atmosphere.
  In the outer layers of the exosphere intense solar UV-irradiation leads to ionisation of escaping hydrogen atoms, creating "new-born" protons. Interaction with the interplanetary magnetic field from the solar wind accelerates these protons and some of them can even leave the close neighbourhood of the planet. Their trace is now detected as characteristic waves in the measured magnetic field up to large distances from the planet.
  This is the first proof that Venus looses hydrogen directly to the solar wind, in a volume much large than expected. The upstream loss may have a significant contribution to the total water loss over the age of the solar system.
  
  
• The particle instrument ASPERA (IRF Kiruna, Sweden) measures the flux of charged particles in the close neighbourhood of the planet. Analysis of the ASPERA data together with the VEXMAG data shows that on the Venus night side a massive loss of hydrogen and oxygen ions takes place: the solar wind blows the ions over the poles into the plasma tail behind the planet and further into the depths of interplanetary space.
  The amounts of particle loss of hydrogen and oxygen have the ratio of 2:1, indicating that it is really water (H₂O) which is lost. This mechanism to strip off water from the planet on the night-side is supposed to be the main reason for the lack of water on present Venus.