The configuration of Saturn’s magnetic shield is not consistent with that expected from previous experience with the magnetospheres of Earth. Using data collected by NASA’s Cassini space probe, scientists found that the boundary where solar particles pour into Saturn’s magnetosphere has been persistently displaced from noontime location towards Saturn’s afternoon side. While the cusp of Earth is found around noon time, the Saturn cusp is believed to be mostly found at about 1 to 3 p.m. This unexpected finding comes from an analysis of 67 cusp events registered by the spacecraft over five years when it flew over the magnetopause the high-latitude border region of Saturn’s magnetic field in 2004-2010. This aspect is important because the cusp region serves as a signature for the magnetic field configuration of a planet and particle entry processes into a magnetosphere.
The planet makes the difference. The magnetosphere of Earth is strongly controlled by the interaction with the solar wind the flux of charged particles that emanates from the Sun. The magnetic system of Saturn has recently emerged as being driven primarily internally by the rotational movement of the planet as well as the ongoing input of matter originating from its rings and satellites. Saturn rotates once every 10.7 hours. In particular, a substantial portion of these charged particles is supplied by the jet streams of water molecules coming from the south polar region of Enceladus, Saturn’s tiny moon. Indeed, due to its influence, the small moon becomes a major factor in shaping Saturn’s magnetospheric environment.
In order to understand cusp distribution, it is necessary to have the corresponding magnetic environment. Computer simulation of Saturn’s magnetospheric environment based on in-situ measurements performed by the Cassini orbiter during five years 2004-2010 demonstrated that cusp distribution of Saturn was essentially different from the corresponding Earth’s. To be more specific, numerical simulation suggests that the morning hemisphere of Saturn is somewhat extended relative to its evening counterpart causing the cusp to shift towards afternoon and even late-afternoon periods of time. This result agrees with previous studies showing that the magnetic environment of quickly rotating gas giants is subject to a different set of physical processes compared to terrestrial worlds. Namely, on Saturn, the process of magnetic reconnection is mainly observed in high latitudes while it takes place mainly at noon for Earth.
These results carry implications that are relevant beyond Saturn’s orbit. First of all, it concerns the discipline of comparative planetology, where scientists study similarities and differences between various features in planetary environments in order to delineate common and peculiar mechanisms involved. In this case, the fact of existence of cusp shift on a fast-rotating gas giant planet suggests that there exist certain special features typical of the environment of all fast-rotating gas giants as a class, irrespective of particular properties of these planets. This can have implications concerning interpretation of magnetic environment of Jupiter, interactions of stellar winds with magnetospheres of exoplanets and role of satellites in the evolution of magnetic environment of their host planets.
These findings occur very timely. Saturn and, especially, its moon Enceladus are regarded as the most attractive candidates for astrobiological investigations within our solar system. Another mission to Saturn and its moons is going to be inevitable prior to getting the full understanding of biological implications of geophysical characteristics of the Saturn system. Andrew Coates explains it with these words: “A better understanding of Saturn’s environment is especially urgent now as plans for our return to Saturn and its moon Enceladus start to be developed.” In this sense, the skewed cusp region may become very important in mission planning. It will be crucial information for the planning of the next big mission to the outer planets, the Europa Jupiter System Mission.