The solar wind has a much bigger impact on Mars than previously thought according to the first results from the ASPERA-3 instrument on Mars Express. Rickard Lundin of the Swedish Institute of Space Physics and an international team of co-workers have found that the solar wind -- a supersonic plasma of charged particles that flows from the Sun -- can penetrate deep into the atmosphere of Mars. One consequence of this is that water and other volatile molecules could escape from the planet (R Lundin et al. 2004 Science 305 1933).
Many theories have been put forward to explain how Mars changed from being a warm, wet planet to a cold, dry one. Recently it was estimated that a volume of water equivalent to a planet-wide ocean with a depth of between 14 and 34 metres could have escaped from the red planet during the past 3.5 billion years. Unlike the Earth, Mars does not have a magnetic shield to protect it from the solar wind, so particles from the Sun may have played a crucial role in shaping the Martian atmosphere.
Lundin and colleagues made in situ measurements of the solar wind flowing towards Mars, and the “planetary wind” flowing away from the planet. The planetary wind consists of volatile materials that are energised, ionised and accelerated by the solar wind as it penetrates Mars’ atmosphere.
They found that the solar wind can penetrate as deep as 270 kilometres above the Martian surface. Moreover, they found that positively charged hydrogen and oxygen ions flowing away from the planet can have energies as high as several keV at low altitudes, which means that they have enough energy to escape. According to Lundin and co-workers, the combined escape of hydrogen and oxygen ions might be evidence for a slow dehydration of Mars.