From analysis of the orbiter atmospheric drag (OAD) data obtained from the orbital decay of the Pioneer Venus orbiter from December 9, 1978, to August 7, 1979, atmospheric densities have been determined and tabulated near 16 ¿N latitude between 140 and 190 km for all times of day. Maximum daytime densities on Venus are approximately 8¿10-13 g cm-3 at an altitude of 150 km (dropping by a factor of 24 at night) and 7¿10-14 g cm-3 at 170 km (dropping by a factor of 82 at night). Comparative atmospheric densities on earth at 150 km are a factor of 3.5 higher during the day with only a 1% diurnal variation. An atmospheric composition, temperature, and density model based on OAD vertical structure measurements is presented. The inferred exospheric temperature and atomic oxygen concentrations are surprisingly insensitive to model assumptions. This model indicates that atomic oxygen is the major species in the Venus atmosphere above 145 km at night and above 160 km during the day with mixing ratios near 140 km in excess of 0.1. Atomic oxygen concentrations determined from drag measurements near 170 km range from 1¿109 cm-3 during the day to 3¿107 cm-3 at night. Exospheric temperatures inferred from OAD measurements are found to vary from 100 ¿K at night to approximately 300 ¿K during the day. The very low exospheric temperatures discovered at night lower than temperatures near 100 km, are inconsistent with the concept of a planetary thermosphere, in which temperature increases with altitude. We have adopted the term 'cryosphere' to define this region of cooling where temperature decreases with altitude. The phase of the diurnal temperature variation is consistent with theoretical models of an atosphere with mean motion to the west more rapid than planetary rotation superimposed on solar-antisolar motion. They very low nighttime temperatures and our observation of an absence of the predicted nighttime atomic oxygen bulge suggest substantial downward eddy transport of heat as well as of atomic oxygen. Finally, the OAD data indicate the neutral upper atmosphere of Venus is unexpectedly insensitive to both solar extreme ultraviolet variations and variations in the soalr wind. Thus, the Venus upper atmosphere may not be controlled by the major heating mechanisms generally assumed for planetary thermospheres.