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 Research 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).