A less detailed image of Europa clearly reveals the illuminated crescent but supplies little information about the surface of this ice-covered satellite.
Satellite Atmospheres.
Two experiments yielded exciting new information on possible atmospheres of the Galilean satellites. First was an occultation, in which the Pioneer 10 spacecraft was targeted to pass behind Io as seen from Earth. At the moments just before the spacecraft disappeared, and just after reemergence from behind the satellite, the radio signal was influenced by a thin layer of ionized gas, in which the electrons had been stripped from the atoms by absorbed sunlight or by other processes. The ionosphere thus discovered had a peak density of about 60 000 electrons per cubic centimeter. In addition, a very extended far-ultraviolet glow, probably due to atomic hydrogen, was found near the orbit of Io by the ultraviolet photometer.
Masses of Jupiter and Its Satellites.
Precise radio tracking of the Pioneers as they coasted past Jupiter and its satellites revealed that Jupiter is about one percent heavier than had been anticipated, and several satellites were found to have masses that differ by more than ten percent from values determined previously. These improvements in knowledge of the masses were required to achieve the close satellite flybys being planned for later missions.
The Pioneer spacecraft carried this plaque on the journey beyond the solar system, bearing data that tell where and when the human species lived and that convey details of our biological form. When Pioneer 10 flew by Jupiter it acquired sufficient kinetic energy to carry it completely out of the solar system. Some time between one and ten billion years from now, the probe may pass through the planetary system of a remote stellar neighbor, one of whose planets may have evolved intelligent life. If the spacecraft is detected and then inspected, Pioneer’s message will reach across the eons to communicate its greeting.
The Inner Magnetosphere.
A great deal of the scientific emphasis of the Pioneer missions was directed at characterizing the particles and fields in the inner magnetosphere, the region in which charged particles are trapped in stable orbits. The Pioneers found that it extended to about 25 RJ, well beyond the orbit of Callisto. Within this region, instruments on the spacecraft recorded the numbers and energy of electrons, protons, and ions. The electrons reached a maximum concentration near 3 RJ, and their numbers remained almost constant from there in toward the planet. The maximum concentration of protons observed by Pioneer 10 was at 3.4 RJ, a little inside the orbit of Io. Pioneer 11 penetrated deeper and found another maximum, about twenty times higher, at 1.9 RJ; at this distance, 10 million energetic protons hit each square centimeter of the spacecraft every second. It was believed that the gap between these two peaks was due to tiny Amalthea, the innermost satellite, which orbits at 2.5 RJ. Apparently this satellite sweeps up the particles as it circles Jupiter. Another large dip in the proton distribution was attributed to sweeping by Io, with smaller effects seen near the orbits of Europa and Ganymede. There was an additional small effect at 1.8 RJ, later found by Voyager to be due to Jupiter’s ring and its fourteenth satellite.