Jupiter Results
The scientific results of the Pioneer flybys of Jupiter were many and varied. As is always the case, some old questions were answered and new problems were raised by the spacecraft data. Highlights of these results are summarized below.
Photographs of Jupiter.
The line-scan imaging systems of Pioneer returned some remarkable pictures of the planet during the two encounters, showing individual features as small as 500 kilometers across. In addition, the Pioneers were able to look at Jupiter from angles never observable from Earth.
One of the discoveries made from these pictures was the great variety of cloud structures near the boundaries between the light zones and dark belts. Many individual cloud patterns suggested rising and falling air. The convoluted swirls evident in these regions appeared to be the result of dynamic motions; unfortunately, with only the few “snapshots” obtained during the hours of the flyby, the actual motions of these clouds could not be followed.
Thermal Emission.
In the year the Pioneer Project was begun, astronomers on Earth had measured that Jupiter emitted more heat than it absorbed from the Sun. From the Earth these measurements could be made only of the sunlit part of the planet; neither the night side nor the poles could be seen. One of the main objectives of the Pioneer flybys was to determine the heat budget accurately from temperature measurements at many points on both the sunlit and the night sides.
The Pioneer data confirmed the presence of a heat source in Jupiter and supplied a quantitative estimate of its magnitude. The global effective temperature was found to be -148° C, to a precision of ±3 degrees. This temperature implies that Jupiter radiates 1.9 times as much heat as it receives from the Sun. The corresponding internal heat source is 10¹⁷ watts. Surprisingly, the poles were as warm as the equator; apparently, the atmosphere is very efficient at transferring solar heat absorbed near the equator up to high latitudes, or perhaps the internal component of the heat comes preferentially from the polar regions.
Helium in the Atmosphere.
The Pioneer infrared experiment made the first measurement of the amount of helium on Jupiter. The ratio of the number of helium atoms to the number of hydrogen atoms was found to be He/H₂ = 0.14 ± 0.08. This is consistent with the known solar ratio of He/H₂ = 0.11. Measurements of helium in the upper atmosphere were also made by the ultraviolet experiment.
One of the best Pioneer images of Jupiter was obtained at a range of 545 000 kilometers by Pioneer 11. Structure within the Great Red Spot and the surrounding belts and zones can be seen. There was much less turbulent cloud activity round the spot at the time of the Pioneer flybys than was seen five years later by the Voyager cameras.
Pioneer 10 confirmed theoretical models of Jupiter that suggest the planet is nearly all liquid, with a very small core and an extremely deep atmosphere. The liquid interior seethes with internal heat energy, which is transferred from deep within the planet to its outer regions. The temperature at the center may be 30 000 K. Since the temperature at the cloud tops is around -123° C, there is a large range of temperatures within the planet.
Distance (km) Visible clouds Hydrogen gas Cloud tops Ammonia crystals Ammonium hydrosulfide crystals Ice crystals Water droplets -123° C Transparent atmosphere Hydrogen/Helium gas 70 000 Transition zone 1980° C 60 000 Liquid hydrogen 50 000 11 000° C Transition zone 3 million atmospheres pressure 40 000 Liquid metallic hydrogen 30 000 20 000 Possible “sea” of helium 10 000 30 000 K Possible solid core 0
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.
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.
The Outer Magnetosphere.
From about 25 RJ outward to its boundary near 100 RJ, the Jovian magnetosphere is a complex and dynamic place. Beyond about 60 RJ, both Pioneers found the boundary to be highly unstable, apparently blown in and out by variations in the pressure of the solar wind, which consists of charged particles flowing outward from the Sun. In this region concentrations of Jovian particles are sometimes seen that rival the inner magnetosphere in intensity. Between 60 RJ and 25 RJ, a region sometimes called the middle magnetosphere, the particle motions are more ordered, and for the most part electrons and protons are carried along with the planet’s rotation by its magnetic field. Near the equatorial plane, the flow of these particles produces an electric current circling the planet, and this current in turn generates its own magnetic field, which approaches in strength that of Jupiter itself. Occasionally the outer magnetosphere collapses down to about 60 RJ, and energetic particles are squirted from the middle region into space; these bursts of Jovian particles can sometimes be detected as far away as Earth.
At the same time that Pioneer scientists were analyzing their results and developing new concepts of the Jupiter system, a new team of investigators had been selected for the next mission to Jupiter. From about 1975 on, attention shifted from Pioneer to its successor—Voyager.
The Voyager spacecraft are among the most sophisticated, automatic, and independent robots ever sent to explore the planets. Each craft has a mass of one ton and is dominated by the 3.7-meter-diameter white antenna used for radio communication with Earth. Here Voyager undergoes final tests in a space simulator chamber. [373-7162AC]