IAU Symposium 40, on Planetary Atmospheres, was held at Marfa, Texas, in the Paisano Hotel, October 26-31, 1969, hosted by The University of Texas at Austin and the McDonald Observatory of the University of Texas. The Organizing Committee consisted of: Dr. Tobias Owen (Illinois Institute of Technology), Dr. Carl Sagan (Cornell University), Dr. John Hall (Lowell Observa­ tory), Dr. Arvydas Kliore (Jet Propulsion Laboratory), Academician Georgi I. Petrov (Institute of Cosmic Research, Soviet Academy of Sciences), Dr. V. I. Moroz (Stern­ berg Institute), Dr. H. C. van de Hulst (Leiden University). Local arrangements were under the care of Dr. Harlan Smith (McDonald Observa­ tory), assisted especially by Dr. Ronald Schorn (Jet Propulsion Laboratory), Mr. Curtis Laughlin (McDonald Observatory), and Miss Virginia Church (University of Texas); also by Dr. Joseph Chamberlain (Kitt Peak National Observatory), Dr. Maurice Marin (McDonald Observatory), Mr. and Mrs. J. Bergstrahl and Mrs. K. MacFarlane (McDonald Observatory), and Dr. Donald Rea (NASA Headquarters). The scientific program was divided into three parts; Venus, Mars, and the outer planets. Detailed programs for these sections, and their subsequent editing, were respectively in the hands of the undersigned. It was possible to hold the Symposium because of generous sponsorship by the Executive Committee of the International Astronomical Union, including the provi­ sion of a travel grant for assisting younger astronomers to attend, and also including the active attention and assistance of the Assistant General Secretary, Dr. C. de Jager.

I / Venus.- The chemical composition of the atmosphere of Venus.- Exospheric temperature of Venus from Mariner 5.- Models of the Venus ionosphere.- Venus: Determination of atmospheric parameters from the microwave spectrum.- Peculiarities of mm and cm radiowave propagation in the Venus atmosphere.- Some consequences of critical refraction in the Venus atmosphere.- A two-parameter theory for Venus spectra.- A new short-wavelength carbon dioxide band in the spectrum of Venus.- Calculations of CO2 energy levels: The Ã1B2 state.- High spectral resolution interferometric planetary observations in the 7-25 ? region.- Limb darkening observations of Venus from 5 ? to 18 ?.- Water on Venus?.- Vegetative life on Venus ? Or investigations with algae which grow under pure CO2 in hot acid media and at elevated pressures.- Venus cloud contrasts.- On the nature of the Venus clouds.- Geochemical problems in the production of the Venus clouds.- The trouble with Venus.- II / Mars.- A. Optical Properties.- Colorimetry of Martian features by means of area scanning.- Longitudinal variations, the opposition effect, and monochromatic albedos for Mars.- Mie scattering and the Martian atmosphere.- The contribution of atmospheric aerosols to the Martian opposition effect.- Ultraviolet polarization measurements of Mars and the opacity of the Martian atmosphere.- Some problems of anisotropic scattering in planetary atmospheres.- B. CO2 Absorption.- An interpretation of the Mars spectrum taken by the Connes.- Observations of the Martian 1.2 ? CO2 bands.- Variations of the Martian CO2 abundance with Martian season.- Relative elevation differences revealed by near infrared CO2 bands on Mars.- Spectroscopic determination of surface pressure and elevation differences on Mars.- C. Water VaporAbsorption.- The spectroscopic search for water on Mars: A history.- The latitude variation of water vapor on Mars.- A report on Martian atmospheric water vapor near opposition, 1969.- High altitude interferometer spectra of Mars.- Mars: Occurrence of liquid water.- D. Mariner Results.- Mariner 6: Ultraviolet spectrum of Mars upper atmosphere.- Mariner Mars '69 celestial mechanics experiment.- Mariner 1969: Results of the infrared radiometer experiment.- Mariner 6 and 7 television pictures: Preliminary analysis.- E. Cloud Motions and Atmospheric Dynamics.- Estimates of boundary layer parameters in the atmospheres of the terrestrial planets.- The theory of similarity for large-scale motions in planetary atmospheres.- Cloud activity on Mars near the equinox: Comparison of the 1937 and 1969 oppositions.- Cloud motions on Mars.- F. Upper Atmospheres.- The effect of atmospheric dynamics on the upper atmosphere phenomena of Mars and Venus.- CO2+ dayglow on Mars and Venus.- III / Outer Planets.- Variations in the color of Jupiter.- The effective temperature of Jupiter's equatorial belt during the 1965 apparition.- Observations of Jupiter's cloud structure near 8.5 ?.- Phase behavior of light gas mixtures at high pressures.- Atmospheric depths of Jupiter, Saturn, and Uranus.- The spectral characteristics and probable structure of the cloud layer of Saturn.- Estimate of the H2 abundance in the atmosphere of Uranus from the pressure induced spectrum.- The spectrum of Uranus in the region 4800-7500 Å.- Upper limit of hydrogen and helium concentrations on Titan.- IV / Scientific Dedication of the 107-Inch Reflector.- Remarks made at the scientific dedication of the 107-inch reflector, October 30, 1969.