The Solar Wind-Atmosphere Coupling

     Near-Mars space is strikingly different from Earth-space because of the absence of a substantial, intrinsic Martian magnet field.  Without the magnetic cavity of a magnetosphere to shield the upper atmosphere from the on-coming solar wind, Mars is subject to comet-like atmosphere erosion processes and solar-wind-induced current systems that have no terrestrial counterparts.  From previous missions to Mars and experience gained in orbit around similarly weakly magnetized Venus, we have developed ideas of how the Mars upper atmosphere and solar wind interact and the consequences for the planet.  In particular, we have observed that the scavenging of planetary ions may have resulted in the removal of ~1 m of surface water over 4.5 billion years.  More detailed studies which take into account the variability of the ionosphere through planetary history, give a much higher (~30 m) equivalent depth of water which has escaped due to the solar wind interaction process.

     The current atmospheric conditions on Mars indicate that water does not exist on the surface in any significant amount.  On the other hand, water associated with active volcanism existed in the past on the Martian surface.  The total amount of water released in the past together with volcanic material identified on the surface resulted in an equivalent water layer of 50 - 80 m.

     Recent models of the Mars-solar wind interaction have also suggested that solar wind absorption by the Martian atmosphere may be an important energy source for the Martian upper atmosphere.