ESA - Venus Express - Caught in the wind from the Sun

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Caught in the wind from the Sun

28 November 2007

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Magnetometers can detect tiny amounts of hydrogen originating from Venus in the solar wind by sensing small variations in the magnetic field caused by the presence of these particles in the undisturbed solar wind. Analysing these variations, or plasma waves, one can calculate how much of the hydrogen in the solar wind originates from Venus. Since all of these particles escape the planet, one can calculate how much hydrogen is lost outside the plasma wake.

The plasma analyser ASPERA is able to measure hydrogen (H+) originating from Venus in the wake. One can also calculate how much is lost through the wake. This number is much higher than what is lost in the solar wind, therefore: Venus loses its hydrogen through the wake!

Credits: Foreground: ESA / ASPERA-4 and MAG team; background artist’s impression: ESA (Image by C. Carreau)

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Measurements from the Analyser of Space Plasmas and Energetic Atoms (ASPERA-4) onboard ESA’s Venus Express show that Venus loses two hydrogen ions for each ion of oxygen. In other words, Venus loses water.

The atmosphere loses its gases very slowly, but the relative amount of hydrogen and oxygen in the atmosphere does not change.

Credits: Foreground: ESA / ASPERA-4 team; background artist’s impression: ESA (Image by C. Carreau)

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This figure is based on data from the magnetometer (MAG) on board ESA’s Venus Express. MAG has allowed the clear determination of the position of Venus’ bow shock – the boundary region which separates the undisturbed solar wind from the solar wind affected by the planet’s presence. This is a very sharp boundary, and is present at all planets.

After having encountered the bow shock, the solar wind flows around the planet. Then, due to interaction with the ionised upper atmosphere, it is deviated. The location of the bow shock is expected to be where there is a complete deflection of the solar wind by the magnetised ionosphere (upper atmospheric layer).

Before Venus Express, we had no knowledge of whether or not solar wind would reach the atmosphere when solar activity is at maximum in its 11- year cycle. MAG measurements showed that it almost entirely deviated and little or no solar wind enters Venus’ atmosphere even at solar maximum. So, the atmosphere is shielded by the magnetic field induced by the solar wind itself.

The high-resolution magnetic field data from MAG has also allowed the identification of the outer boundary of the magnetosphere induced by the interaction with the solar wind: a ‘magnetopause’ that deflects and stops the solar wind.

Credits: ESA / MAG team

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The plasma package called the Analyser of Space Plasmas and Energetic Atoms (ASPERA-4) on board ESA’s Venus Express has established, for the first time, the composition of escaping plasma on Venus.

Venus loses mainly oxygen, hydrogen, and helium via its atmosphere’s interaction with the solar wind. The hydrogen that escapes, originates from the small amounts of water present in the atmosphere. Water molecules in the atmosphere, or water vapour, break down under the action of the Sun’s ultraviolet radiation to produce one oxygen atom and two hydrogen atoms. Solar ultraviolet light ionises the atoms that are then picked up by the solar wind and accelerated. They eventually escape the planet. Oxygen can also originate from breaking carbon dioxide molecules.

Credits: ESA / ASPERA-4 team

Looking at Venus

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