Ion Pickup at Comets: Comparison with Other Unmagnetized Objects

Comets provide a wonderful laboratory for studying the ion pickup process. As a comet nears the Sun, neutral atoms and molecules sublime from the nucleus and from ice grains in the coma, providing an extended water‐rich region. Photoionisation and charge exchange provide a source of new pickup ions, which immediately interact with the electric and magnetic field in the surrounding plasma. The missions to comets Halley, Giacobini‐Zinner, Grigg‐Skjellerup and Borrelly provided a wealth of data on pickup ions and their interaction with plasma waves, including observations of ring and bispherical shell distributions and mass loading over significant regions of space. At comet Grigg‐Skjellerup, non‐gyrotropic pickup ions were also seen. Here, we review the observations and interpretation of pickup ions at comets, illustrating what was learned from the cometary missions and following the evolution of the ion distribution. We also discuss the international Rosetta mission which will orbit rendezvous with and then orbit comet Churyumov‐Gerasimenko as it nears the Sun and as activity develops. The interaction will change significantly as this occurs in 2014–15. We also compare the pickup process at comets with that observed at other unmagnetized objects, including Titan, Enceladus, Venus and Mars, using data from Cassini, Venus Express and Mars Express.