11. D. E. Anderson, Jr., ibid. 81, 1213 (1976). 12. I. Ferrin, thesis, University of Colorado (1976); S. Kumar, D. M. Hunten, A. L. Broadfoot, Planet. Space Sci. 26, 1063 (1978). 13. V. I. Oyama, G. C. Carle, F. Woeller, J. B. Pol- lack, Science 203, 802 (1979). 14. This work was performed under NASA con- tracts NAS 2-8816 and NAS 2-9477. We ac- knowledge the contributions to this experiment pole itself. The Venus orbiter radiometric temper- ature experiment (VORTEX) on the Pio- neer orbiter is an infrared radiometer with ten spectral channels in the wave- length region from 0.2 to 60 ,tm, similar in concept to Earth weather satellite in- struments. Its principal attribute is the ability to measure thermal emission from the atmosphere at eight different, distinct height levels, and thus allow the vertical temperature structure to be recovered. Previous radiometers on U.S. and Soviet missions were simple cloud top mappers, without the ability to discriminate true atmospheric temperature structure from cloud morphology, and with little or no vertical coverage. The height range cov- ered by VORTEX, in contrast, is about 60 to 140 km. This includes the "Earth- like" range of temperatures and pres- sures; the region of the ultraviolet mark- ings, which exhibit much dynamical ac- tivity including the "4-day" circulation; the peak of the ionosphere, and the base of the exosphere. The temperature sounding channels all are in or near the v2 fundamental band of carbon dioxide. Two other channels measure reflected solar energy in the near infrared, and one is located in the far infrared near 50 ,mm. The instrument and its measurement ca- pabilities have been described (1, 2). Since orbital insertion on 4 December, the radiometer has been activated in all three of its operating modes: global map- ping, local imaging, and limb scanning. Excellent performance has been ob- tained in each case. The second and third of the modes listed require very high data rates and therefore can be used only infrequently without impacting the re- turn from the other science experiments. SCIENCE, VOL. 203, 23 FEBRUARY 1979 of the Pioneer Project Office, our colleagues on the Pioneer Venus Science Steering Group, and especially our colleagues and staff at the Univer- sity of Colorado's Laboratory for Atmospheric and Space Physics. The false-color image on the cover of this issue was made at Infomap, Inc., Boulder, Colo. 16 January 1979 Consequently, most of the VORTEX data so far are in the relatively low spa- tial resolution mapping mode (maximum spatial resolution approximately 20 km), and our report deals exclusively with this. The presentation of results from the solar reflectance channels is also de- ferred until a preliminary analysis has been performed. Data from orbit I are presented in sev- eral forms in three figures. Figure 1 90 E 0 80 c7 -i 160 180 200 220 240 260 28 TEMPERATURE, kelvins Fig. 1. About 4000 vertical temperature files were measured on orbit 1; these thre( typical of the equatorial and polar profil( the local times of day shown. The 11.S window temperature, corresponding appi mately to the temperature at unit op depth in the clouds, occurs at an altitud approximately 68 km on the dayside about 0.5 km lower on the nightside. The tude scale is derived, assuming a refer height of 52 km at 1 bar. [From NASA 8011 (1972)] shows temperature profile retrievals for selected regions representing dayside (afternoon) equatorial, nightside (pre- dawn) equatorial, and polar soundings. Figure 2 shows a cross section of the at- mosphere in the northern hemisphere, consisting of measurements of the tem- perature at six different height levels from equator to equator over the pole. This plot was made by selecting from the data set those soundings with near-nadir viewing geometry. Figure 3 is a rec- tangular coordinate map of Venus which has been enhanced in the image process- ing laboratory to bring out the structure of the cloud tops. The data shown repre- sent a ratio of the intensity in the 11.5- ,um window channel to that measured in the wing of the 15-,um CO2 band, which measures the atmospheric temperature above the clouds near 80 km altitude. This ratio process removes, to first or- der, horizontal atmospheric temperature differences and viewing geometry ef- fects, and leaves inhomogeneities in cloud opacity or height in the presence of a vertical temperature gradient as the principal source of contrast. The narrow strip to the bottom left is the data taken near periapsis, the width of the strip rep- resenting the distance between the hori- zons to either side of the spacecraft as it passes close to the planet. The principal 'conclusions so far from these and similar data from the first few orbits are as follows: 1) Over the limited range of longitudes observed so far, day-to-night temper- ature contrasts are very small. From about 65 to 80 km, the differences appear to be about 1 K on average at all heights. Between 80 and 100 km, about 5 K of contrast is observed, with the dayside X warmer. 2) Pressure modulator radiometer , measurements (1) near 100 km at the 3 equator show virtually no limb dark- 0o' ening. This implies temperatures con- stant to within I K over a vertical range of at least two scale heights (about 8 km) in this region. 3) The source function for cooling to space at altitudes near 125 km and above is extremely low, at least near the poles la where the orbital geometry allows us to measure it (1). pro- 4) The cloud top brightness temper- e are atures are about the same on average for es at measurements made near 5 a.m. and 4 roxi- p.m. Venus local time. This result should tical be compared with figure 8 of Ksanfoma- le of lity et al. (3), who interpreted Venera 9 and and 10 measurements in terms of 10 K alti- *ence SP- higher equatorial cloud temperatures all over the nightside, relative to the day. 5) Equator-to-pole temperature con- 0036-8075/79/0223-0779$00.50/0 Copyright 1979 AAAS Infrared Remote Sounding of the Middle Atmosphere of Venus from the Pioneer Orbiter Abstract. Orbiter infrared measurements of the Venus atmosphere in the 60- to 140-kilometer region show very small diurnal temperature differences near the cloud tops, increasing somewhat at higher levels. The seasonal (that is, equator to pole) contrasts are an order of magnitude larger, and the temperatures unexpectedly in- crease with increasing latitude below 80 kilometers. An isothermal layer at least two scale heights in vertical extent is found near the 100-kilometer altitude, where the temperature is about 175 K. Structure is present in the cloud temperature maps on a range of spatial scales. The most striking is at high latitude, where contrasts of nearly 50 K are observed between a cold circumpolar band and the region near the DAY SIDE EQUATOR I AL (4 PM LT) POLAR NIGHT SIDE EQUATORIAL (5 AM LT) 779