Why Study Magnetospheres?
The past 30 years of exploration have revealed that planetary atmospheres, exospheres, and magnetospheres (i.e., the planetary “space environment”) are an integral component of planetary systems, and also that magnetic fields and charged particles have played significant roles in the origin and evolution of the solar system. Particle detectors were originally added to the Pioneer 10 and 11 spacecrafts to measure the radiation environment to be encountered by these and subsequent spacecraft sent to image and traverse the Jovian system. As we began to explore the giant magnetosphere of Jupiter, it soon became clear that the space environment of the planet had an importance beyond the health of a spacecraft. It was quickly realized that the strong magnetic field of Jupiter links the planet to its moons and rings, and taps the rotation energy of the planet, accelerating charged particles, which then deposit their energy into the atmosphere of the planet, exciting intense aurora, or bombard the surfaces of satellites and rings. Below are some examples of the importance of understanding the space environment of solar system objects:
- Planetary magnetic fields tell us about internal structure.
Six planets and at least one moon are known to have internally generated magnetic fields. The factors controlling why some objects have magnetic fields and others do not put important constraints on the composition and thermal history of planetary interiors.
- Plasma wind erodes atmospheres.
Planetary atmospheres that are embedded in a flowing plasma are heated and eroded by the interaction. This removal of material has a significant impact on the composition and isotopic fractionation over evolutionary timescales (e.g., solar wind removal of atomic oxygen from Venus and Mars, ion tails of comets and probably Pluto, loss of atmospheric sodium and potassium from Mercury in substorms, magnetospheric interactions with the atmospheres of Io, Titan, Europa).
- Electrodynamic interactions reveal critical properties.
The electrodynamic interaction of a plasma with a solid object reveals bulk properties of its interior such as electrical conductivity and magnetization, which in turn, place important constraints on their composition and thermal history (e.g., electrical conductivity of the Galilean satellites, magnetization of Gaspra and Ida). Most specifically, the induction signature detected by the Galileo magnetometer is the only concrete evidence of a global liquid ocean at Europa.
- Comparison tests understanding.
The space environment of the magnetized planets is dominated by the planet’s magnetic field. There are many processes that these magnetospheres share in common, processes that control their large-scale structures, plasma sources, particle acceleration, and transport mechanisms, and there are important differences due to their disparate sizes, the characters of their moons, the distance from the Sun and the presence or absence of a thick atmosphere. Comparison of the wide range of magnetospheres, from the small magnetospheres of Mercury and Ganymede to the giant magnetospheres of the Jovian planets, allows us to understand these processes. Such results will have further interdisciplinary usefulness in interpreting extrasolar system astronomical objects.
- Auroral bombardment heats atmospheres.
The energy deposited in the auroral regions of the magnetized planets has dramatic atmospheric effects. For Earth, we know that the local changes in temperature, composition, and electrical properties of the upper atmosphere produced by auroral precipitation drive large-scale winds that affect the upper atmosphere. Similarly, changes and motion in the atmosphere are physically coupled back to the magnetosphere via electric fields generated by these winds and through the generation of energetic plasma scattered or accelerated back into the magnetosphere. Such coupling processes are probably commonplace and even stronger for the giant planets than for Earth, perhaps even dominating over the effects of solar radiation at times.
- Radiation modifies planetary surfaces.
When ices and rocks are bombarded by energetic particles, the molecular structures can be rearranged, chemical bonds changed, and material sputtered from the surface. Thus, surfaces of planetary materials have been altered over the history of the solar system by the radiation environment in which they are embedded. Such processes affect their spectral properties and even change the surface composition, perhaps playing an important role in prebiotic chemistry, generating organic materials on the surfaces of dust grains in the early solar nebula.
- Magnetic fields shield planets from energetic particles and cosmic rays.
Planetary magnetic fields deflect energetic particles originating from the Sun and the galaxy and reduce the particle fluxes hitting the planetary surface. Thus, the existence and variability of planetary magnetic fields can play an important role in accommodating or frustrating the development of life on planets.
- Electrodynamic forces transport dust.
Dust grains become electrically charged when embedded in a plasma. Electrodynamic forces on charged dust grains compete with gravity for control of the dynamics of dust grains. Recent studies of dusty plasmas show that these forces may be a critical consideration for understanding the role and behavior of dust in the solar nebula as well as in the current solar system.
- Planetary magnetospheres are laboratories.
Each planet provides a different a natural laboratory for the study of plasma interactions and electrodynamic phenomena. For example, the interaction of the satellite Io with Jupiter’s magnetosphere, which generates a million megawatts of power, strips about a ton of sulfur and oxygen atoms per second from Io’s atmosphere, drives powerful electron beams, and triggers intense radio and ultraviolet emissions.