made for the uncertainty due to the poorly known successful Venus flyby sequences, providing us the tering of sunlight by atomic hydrogen. The CO2 pressure-induced coefficient. opportunity to collect the data reported here. The 26. D. D. Diner, J. Atmos. Sci. 35, 2536 (1979). preparation and processing of these data required the feature near 102.6 nm contains H Lyman-3 27. We thank the Galileo Project management and expertise of many people induding J. Anderson, T. (102.57 nm) formed by resonance scatter- engineering staff for their continuing support of Arakelian, K. Becker, V. Carrere, R Chang-Diaz, K. ing and may contain a contribution from 0 NIMS. We especially acknowledge the efforts of Edwards, E. Eliason, J. Gardner, M. Hemandez, S. the late C. Yeates, Galileo Science and Mission Lavoie, F. Leader, R. Lopez-Gautier, C. Mahoney, R. pumped by the solar Lyman-P line, as occurs Design Manager. We have lost a resourceful scien- Mehlman, M. Segura, J. Torson, L. Wainio, and J. in the dayglow of Earth (2). The EUV tist and a good friend in his untimely death. We are Yoshirnizu. We are gratefil to B. Bezard, D. Crisp, D. brightness ratio ofLyman-a to the feature at grateful to N. Ausman, Galileo Mission Director, Hunten, A. Lads, and W. Rossow for valuable discus- and the Orbiter Engineering Team for providing sion and comments. This work was supported under 102.6 nm on Venus is 170, compared to the crucial real-time commands to the spacecraft dur- NASA contract NAS 7-100 with the Jet Propulsion ratio of 624 measured by the EUV in the ing the Venus flyby. The Science Requirements Laboratory, California Institute ofTechnology. and Operation Planning staff (j. Dunne, K. Bux- interplanetary hydrogen background. Also baum, and V. Henderson) formulated the highly 23 May 1991; accepted 19 August 1991 visible are lines of He (58.4 nm) and O0 (83.4 nm), confirming earlier reported de- tections made with EUV instruments on Mariner 10 (3) and Venera 11 and 12 (4). The He line at 58.4 nm is due to resonance scattering of sunlight by atomic He. Several Galileo Ultraviolet Spectrometer Experiment: mechanisms can contribute to the formation Initial Venus and Interplanetary Cruise Results of O0 at 83.4 nm: direct photoionization excitation ofatomic oxygen, electron impact C. W. HORD, C. A. BARTH, L. W. Esposrro, W. E. MCCLINTOCK, ionization and excitation of atomic oxygen, W. R. PRYOR, K. E. SIMMONS, A. I. F. STEWART, G. E. THOMAS, and resonance scattering of sunlight by ox- J. M. AJELLO, A. L. LANE, R. W. WEST, B. R. SANDEL, ygen ions (5). A. L. BROADFOOT, D. M. HUNTEN, D. E. SHEMANSKY Other Venus emissions were observed in the region from 90 to 120 nm (Fig. 1); they The Galileo Extreme Ultraviolet Spectrometer obtained a spectrum of Venus atmo- are presumably an unresolved blend of sev- spheric emissions in the 55.0- to 125.0-nanometer (nm) wavelength region. Emissions eral atomic or molecular emissions or both. of helium (58.4 nm), ionized atomic oxygen (83.4 nm), and atomic hydrogen (121.6 The dayglow of Earth contains numerous nm), as well as a blended spectral feature of atomic hydrogen (Lyman-fl) and atomic oxygen and nitrogen emissions in this re- oxygen (102.5 nm), were observed at 3.5-nm resolution. During the Galileo spacecraft gion (5). The strongest unresolved line in on November 7, 2012 cruise from Venus to Earth, Lyman-a emission from solar system atomic hydrogen the Venus EUV spectrum is probably 0 (121.6 nm) was measured. The dominant source of the Lyman-a emission is atomic (98.9 nm), prominent in the dayglow of hydrogen from the interstellar medium. A model of Galileo observations at solar Earth, (6) where it is excited by photoelec- maximum indicates a decrease in the solar Lyman-a flux near the solar poles. A strong tron impact (5). Table 1 indicates the de- day-to-day variation also occurs with the 27-day periodicity ofthe rotation ofthe sun. rived brightnesses for the resolved and near- ly resolved lines and compares the values to O N 8 FEBRUARY 1990 THE EXTREME The EUV is mounted on the spinning ones obtained with EUV instruments on section ofthe spacecraft, and its field ofview Mariner 10 (3) and Venera 11 and 12 (4). Ultraviolet Spectrometer (EUV) www.sciencemag.org on the Galileo spacecraft obtained a sweeps out a great circle 0.87 wide on the Galileo brightness values represent the aver- spectrum of Venus in the wavelength range celestial sphere at about 3 rpm, passing ages over the portion of the sunlit disk of 55 to 125 nm. After this encounter, the through the north and south ecliptic poles in viewed by the instrument, whereas Mariner interplanetary cruise period from Venus in a plane perpendicular to the spacecraft spin and Venera brightness values represent the February 1990 to Earth in December 1990 axis. Commands to the EUV determine the largest value seen on the planet. Error esti- was used for observations of atomic hydro- fraction of the great circle observed and the mates for the Galileo data are based on the gen Lyman-a (121.6-nm) radiation from number of bins (called sectors) into which uncertainties in the absolute calibration of the interplanetary medium. The EUV (1) that fraction is divided. During the Venus the EUV in the laboratory. Downloaded from has a mechanical collimator and a single- encounter individual photon events, togeth- For the Lyman-a measurements made reflection Wadsworth optical system that er with a wavelength and a sky sector loca- during the interplanetary cruise period, the uses a concave grating to feed a 128-pixel, tion, were stored on the spacecraft tape great circle swept out by the EUV sampled bare microchannel plate detector. The re- recorder for later playback to Earth. During sulting spectra have 3.5-nm spectral resolu- the interplanetary cruise the photon events tion for objects that fill the 0.87 by 0.17 were integrated in the onboard memory of b4 . .. field of view of the instrument. The space- the EUV in a matrix of sector and wave- 4 O(I) craft spin axis maintains an orientation that length knowledge, which was periodically H(I) is within a few degrees ofthe sun while it is read out. 'S He(I) 102.6 H(1) 58.4 Ogi)8 .4 + 21.6 inside 1 astronomical unit (AU). During the Venus encounter EUV data - 3 + + 0(I) + were recorded for 75 min, 25 min of which 98.9 C. W. Hord, C. A. Barth, L. W. Esposito, W. E. contain the planet. The observed great circle 2 McClintock, W. R. Pryor, K. E. Simmons, A. I. F. was divided into 120 1.60 sectors: Venus 0.05 Stewart, G. E. Thomas, Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO filled 17 sky sectors as the spacecraft flew A1 80309. past at 22,000 to 29,000 km from the planet J. M. Ajello, A. L. Lane, R. W. West, Jet Propulsion 0. - ;_. .- Laboratory, California Institute of Technology, Pasade- center. The disk-integrated dayglow spec- 50 o;:60-, 70 80 90 100 110 12U 130 na, CA 91109. trum of Venus (Fig. 1) shows several well- Wavelength (nm) B. R. Sandel, A. L. Broadfoot, D. M. Hunten, D. E. Shemansky, Lunar and Planetary Laboratory, University resolved emission lines. The feature at 121.6 Fig. 1. The EUV spectrum of Venus. Error bars of Arizona, Tucson, AZ 85721. nm (H Lyman-a) is due to resonance scat- (± 1 r) have been placed on identified lines. 1548 SCIENCE, VOL. 253 latitudinally asymmetric, with the rate of ionization being 30% less over the solar poles than that in the ecliptic (9, 10). An important set of interplanetary Ly- c 750 man-a observations were made with the Galileo EUV in July 1990. The spacecraft 3750 CC500' was at a heliocentric distance of 1.2 AU and 500 Day 190 an ecliptic longitude of 3420. The spacecraft Day 193 -.--- 250- 250 Day 197- attitude was held in a fixed inertial position Day 200------ 0 I I I I from day 186 to day 200. During this O I ... North Ecliptic South Ecliptic North South ecliptic equator ecliptic equator period, the EUV scanned a complete great Ecliptic Ediptic pole (upwind ecliptic equator ecliptic equator pole (downwind circle that intercepted the ecliptic equator at pole (upwind pole (downwind direction) direction) direction) direction) ecliptic longitudes of 2520 and 72° near the Fig. 2. Interplanetary Lyman-a observations for a upwind and downwind directions of the Fig. 3. Interplanetary Lyman-a observations for great circle at an ecliptic longitude of2520 passing four different time periods, viewed from the same through the ecliptic poles. The observations are interplanetary atomic hydrogen cavity. The great circle as that in Fig. 2. marked with double pluses, which indicate the ± 1 EUV data were continuously integrated on- aF limits. The model results are shown by the solid board the spacecraft and then read out on line. days 190, 193, 197, and 200. The data from Figure 2 has a slight asymmetry between day 190 are shown in Fig. 2. The interplan- the data and the model, with the data lying the sky in 35 10° sectors that are 0.870 wide. etary Lyman-a intensity reaches a maximum above the model in the northern ecliptic As the spacecraft orbited the sun its spin axis in the upwind direction just north of the hemisphere and below the model in the was kept near the sun, so the EUV observa- ecliptic equator and a minimum in the southern ecliptic hemisphere.