PIONEER SCIENCE INVESTIGATIONS
Project Scientist: J. H. Wolfe, NASA Ames
| Investigation | Principal Investigator | Primary Objectives |
|---|---|---|
| Magnetic fields | E. J. Smith, JPL | Measurement of the magnetic field of Jupiter and determination of the structure of the magnetosphere. |
| Magnetic fields (Pioneer 11 only) | N. F. Ness, NASA Goddard | Measurement of the magnetic field of Jupiter and determination of the structure of the magnetosphere. |
| Plasma analyzer | J. H. Wolfe, NASA Ames | Measurement of low-energy electrons and ions, determination of the structure of the magnetosphere. |
| Charged particle composition | J. A. Simpson, U. Chicago | Determination of the number, energy, and composition of energetic charged particles in the Jovian magnetosphere. |
| Cosmic ray energy spectra | F. B. McDonald, NASA Goddard | Measurement of number and energy of very high energy charged particles in space. |
| Jovian charged particles | J. A. Van Allen, U. Iowa | Measurement of number and energy distribution of energetic charged particles and determination of magnetospheric structure. |
| Jovian trapped radiation | R. Walker Fillius, UC San Diego | Measurement of number and energy distribution of energetic charged particles and determination of magnetospheric structure. |
| Asteroid-meteoroid astronomy | R. K. Soberman, General Electric | Observation of solid particles (dust and larger) in the vicinity of the spacecraft. |
| Meteoroid detection | W. H. Kinard, NASA Langley | Detection of very small solid particles that strike the spacecraft. |
| Celestial mechanics | J. D. Anderson, JPL | Measurement of the masses of Jupiter and the Galilean satellites with high precision. |
| Ultraviolet photometry | D. L. Judge, U. Southern California | Measurement of ultraviolet emissions of the Jovian atmosphere and from circumsatellite gas clouds. |
| Imaging photopolarimetry | T. Gehrels, U. Arizona | Reconnaissance imaging of Jupiter and its satellites; study of atmospheric dynamics. |
| Jovian infrared thermal structure | G. Münch, Caltech | Measurement of Jovian temperature and heat budget; determination of helium to hydrogen ratio. |
| S-Band occultation | A. J. Kliore, JPL | Probes of structure of Jovian atmosphere and ionosphere. |
Atmospheric Structure.
Several Pioneer investigations yielded information on the variation of atmospheric temperature and pressure in the regions above the ammonia clouds. Near the equator, at a level where the atmospheric pressure is the same as that on the surface of the Earth (1 bar), the temperature is -108° C. About 150 kilometers higher, where the pressure drops to 0.1 bar, is the minimum atmospheric temperature of about -165° C. Above this point the temperature rises again, reaching about -123° C near a pressure level of 0.03 bar. Presumably this temperature rise is due to absorption of sunlight by a thin haze of dust particles in the upper atmosphere of Jupiter.
Internal Structure.
The measurements of the amount of helium, of the gravitational field, and of the size of the internal heat source on Jupiter greatly clarified scientists’ understanding of the deep interior of the planet. Calculations showed that the core of Jupiter must be so hot that hydrogen cannot become solid, but must remain a fluid throughout the interior. Even at great depths, therefore, Jupiter does not have a solid surface. The theory that the Great Red Spot was the result of interactions with a surface feature below the clouds thus became untenable. Whatever its exact nature, the Red Spot must be a strictly atmospheric phenomenon.
Magnetic Field.
Pioneer data showed that the magnetic field of Jupiter has a dipolar nature, like that of the Earth, but 2000 times stronger. The calculated surface fields measured about 4 gauss, compared to a field of about 0.5 gauss on the Earth. The axis of the magnetic field was tilted 11 degrees with respect to the rotation axis, and it was offset by about 10 000 kilometers (0.1 RJ) from the center of the planet.
Pioneers 10 and 11 did not obtain very detailed pictures of the satellites of Jupiter. The best view was of Ganymede, which showed a surface of contrasting light and dark spots of unknown nature.