), ammonia (NH
), hydrogen sulfide (H
S), acetylene (CH), and ethane (CH
), as well as other trace constituents.A mass spectrometer in the Galileo Probe will directly and repeatedly sample Jupiter's atmospheric gases at different altitudes as the Probe descends.
Remote sensing instruments flown on four previous Jupiter flyby missions have provided valuable information on wind patterns and physical composition in the planet's upper atmosphere, but they have only "scratched the surface." Such instruments cannot precisely study the vertical mixing of gases. The Probe's mass spectrometer will provide a detailed analysis of the chemical composition of the atmosphere and aid in understanding the processes resulting in the complex, colorful clouds.
The Jovian atmosphere is star-like--some scientists believe its elemental abundances are virtually identical with those on the Sun. Comprised primarily of 90 percent hydrogen and 10 percent helium, it also includes minor amounts of the inert gases neon, argon, krypton, and xenon, and non-inert gases such as water, methane (CH), ammonia (NH), hydrogen sulfide (HS), acetylene (CH), and ethane (CH), as well as other trace constituents.
Unlike remote-sensing instruments, Probe instruments can make measurements in the lower atmosphere where clouds form. The Probe is expected to pass through at least two cloud layers composed of water and ammonia during its projected 60-minute lifetime.
Jupiter's wild colors originate deep in the atmosphere, and it may be possible to explore the role of sulfur in generating the colors.
The mass spectrometry measurements will also aid in understanding the processes involved in the formation of the solar system. The noble gases--helium, argon, krypton, and xenon--are chemically inert; that is, they do not combine with other elements to form other compounds. They do not settle out of a planetary atmosphere by liquifying or freezing, but remain in the gaseous state. Therefore, there should be the same abundances of them now as there was at the beginning of the solar system. Cosmic abundances of krypton and xenon are poorly defined, so the Probe's mass spectrometry offers the first opportunity to measure these elements in a single undisturbed reservoir. The results can be used to calibrate a very large mass of data on noble gas abundances in meteorites, the Earth, and the inner planets.
Voyager's discovery of lightning on Jupiter raises anew the possibility that organic compounds (the basis of life on Earth) may be formed in the Jovian troposphere. A classic laboratory experiment several decades ago showed that organic compounds can be formed when a spark is struck in a mixture of gases. Galileo will search for samples of organic compounds such as hydrogen cyanide and acetonitrile
Abundances of photochemically-produced gases and other trace constituents will also be evaluated.
The mass spectrometer identifies gases by measuring the mass of the ions produced when the gas is ionized by an electron beam. As gas is admitted to the ionization region of the ion source, it is ionized by an electron beam. The team energy can be varied. The ion beam is then directed into a quadrupole analyzer, a set of four hyperbolically-shaped rods 15 centimeters (6 inches) long. A radio frequency voltage is applied to the rods to filter the incoming ions. The voltage and frequency can be varied so that only ions of a chosen mass and charge can travel the length of the rods and be counted at the ion detector.
Atmospheric gases will enter the mass spectrometer through two inlet ports at the apex of the Probe. These ports will be sealed by metal-ceramic devices and kept under vacuum until the Probe enters the Jovian atmosphere. Pyrotechnic devices will then release the covers, allowing atmospheric gases to enter and be pumped to the test cells.
With its broad mass and sensitivity range, the instrument measures almost everything that enters it, making it ideal for this exploratory mission. The normal range of ion masses to be covered will be from 1 to 52 u (unified atomic mass unit) (4x10
to 190x10 pounds)with occasional sweeps from 1 to 150 u (up to 550x10 pounds) to search for heavier compounds.
A small fraction of the gas goes directly to the ionization region where the composition of the total sample is measured. One task of the spectrometer will be to separate the hydrogen from the gas samples to raise the relative abundances of the remaining gases in the sample. The instrument includes two "enrichment" cells and one "purification" cell. Some gas passes through the enrichment cell where substances called getters adsorb trace gases such as hydrogen sulfide, phosphine, and complex hydrocarbons until only the noble (inert) gases remain. The noble gases are admitted to the ion source for analysis. The enrichment cell is then heated, the adsorbed gases are desorbed, and the gases are admitted to the ionization region for analysis of the more complex compounds.
Galileo's mass spectrometer is a state-of-the-art instrument. Particularly difficult problems and choices in designing the instrument relate to the "plumbing": the inlet lines and the gas-handling, pressure-reducing, and pumping systems. A special problem was designing a pumping system that could efficiently remove one of the major components, helium (which is typically difficult to "pump"), without contaminating the spectrometer from the pumping system itself.
The instrument is being built at Goddard Space Flight Center. The engineering unit will be delivered August 1, 1982 and the flight unit is in fabrication now. The instrument weighs 11.8 kilograms (26 pounds) and consumes about 25 watts, about half of which is consumed in pumps and heaters in the pumping systems.
Dr. Hasso B. Niemann of NASA's Goddard Space Flight Center, Greenbelt, Maryland, heads a team of seven other investigators.
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