The primary constituents of Jupiter have long been suspected to be hydrogen and helium, the two simplest and lightest atoms. However, it has proved impossible to derive accurate measurements of the abundance of these two elements from astronomical observations. On the basis of a rather simple infrared measurement, Pioneer investigators found He/H₂ = 0.14 ± 0.08. On Voyager, IRIS was able to obtain much improved infrared spectra, yielding an initial value of He/H₂ = 0.11 ± 0.3. Voyager scientists expect that further analysis will reduce the uncertainty to about ± 0.01. The ratio of 0.11 is in excellent agreement with the solar value of about 0.12, supporting the idea that Jupiter and the Sun have similar elemental compositions. Astronomers have known for a long time that, in addition to hydrogen and helium, the compounds methane (CH₄) and ammonia (NH₃) are present in the visible atmosphere of Jupiter. In the 1970s, additional spectra in the infrared resulted in the discovery of water (H₂O), ethane (C₂H₆), germane (GeH₄), acetylene (C₂H₂), phosphine (PH₃), carbon monoxide (CO), hydrogen cyanide (HCN), and carbon dioxide (CO₂). All these are trace constituents, with two of them, ethane and acetylene, apparently formed at high altitudes by the action of sunlight on methane. A total of approximately 100 000 infrared spectra, many of small regions on the disk, were obtained by IRIS. These spectra generally show hydrogen, helium, methane, ammonia, phosphine, ethane, and acetylene. In addition, excellent spectra were obtained in “hot spots,” regions in which breaks in the upper clouds permit radiation from deeper layers to escape. (The hot spots generally correspond to dark brown regions on photographs of the planet.) IRIS measured temperatures in the hot spots up to -13° C but no higher; apparently this temperature corresponds to the top of a deeper cloud deck. Spectral features indicative of the presence of water vapor and germane were clearly seen in the hot spots. Further analysis of the IRIS spectra will be required to derive the abundances of the gases detected. However, even the preliminary data showed how variable Jupiter can be, especially in its upper atmosphere. The two hydrocarbons, ethane and acetylene, vary in relative abundance with latitude; there is less acetylene near the poles. In addition to this planetwide trend, smaller variations were seen from place to place and between the observations in March and July. All the variations will eventually provide information on the processes of formation, transportation, and destruction of hydrocarbons in the upper atmosphere. ELEMENTS DETECTED IN THE JOVIAN MAGNETOSPHERE Element Atomic Instruments Number