Three experiments were designed to analyze the chemical makeup of the atmosphere and clouds. Seen above, a radio-frequency mass spectrometer had a complex meter-long intake system to avoid becoming contaminated by cloud material as Venera-9's spectrometer had. Instead of admitting gas through a microscopic opening, it used an inlet valve that opened a large area for an extremely short time interval. And finally, it would not activate until far below the cloud and sub-cloud haze layer. A gas chromatograph measured chemical abundance in a completely different manner, by varying rates of diffusion through carefully chosen porous materials. The third chemical analysis experiment would collect cloud material in a filter and measure the spectrum of X-ray induced fluorescence. The spacecraft bus contained instruments geared for deep space and Venus flyby encounter: 30-166 nm Extreme UV Spectrometer Compound Plasma Spectrometer KONUS Gamma-Ray Burst Detector SNEG Gamma-Ray Burst Detector Magnetometer 4 Semiconductor Counters 2 Gas-Discharge Counters 4 Scintillation Counters Hemispherical Proton Telescope These included several hard-radiation sensors. The extreme-ultraviolet spectrometer was an extension of V.G. Kurt's work on atomic hydrogen. His new device, built in cooperation with French colleagues, could measure a complete spectrum that included the Lyman-α signatures of hydrogen, helium, oxygen and other elements. The KONUS gamma-ray detectors on Venera-11 and 12 worked in conjunction with a third detector on a Prognoz satellite in Earth orbit. These were used to triangulate the position of mysterious non-periodic blasts of radiation.

Flight Plan of Venera-11 Both weighing 4450 kilograms, Venera-11 and 12 were launched in 1978 on September 9 and 14. This launch window was a poor opportunity to orbit Venus, because the vehicles would arrive with three times the kinetic energy that Venera-9 and 10 did. Instead, hyperbolic trajectories and precise timing permitted sufficiently long radio links after landing. The Venera-12 arrived on December 21, and Venera-11 arrived on December 25. To make more room for cloud-analysis experiments, the parachute system was reduced to just a pilot chute, a supersonic braking chute, and a single descent chute which was jettisoned at the bottom of the cloud layer (49 km). As the atmosphere thickened with depth, the craft slowed from 50 meters/sec (112 mph) to a landing speed of 8 meters/sec (18 mph). Descent took 1 hour. Venera-12's landing raised a cloud of dust which darkened the sky for 20-30 seconds, until 1 meter/sec winds cleared it away. At Venera-11's site, no dust was observed. Venera-11 landed at 14° S 299° E. Conditions there were 92.6 atmospheres of pressure and a temperature of 452° C (846° F). It remained in radio contact for 95 minutes, until the bus vehicle moved out of range. Venera-12 landed about 850 km from there, at 7° S 294°, finding 93.6 atm and 468° C. It transmitted data to Earth for 110 minutes. Gas Mass Spectrometry Gas Chromatography Nitrogen 4.0±0.3 % 2.5±0.3 % Water 76, 130 ppm < 100 ppm Carbon Monoxide 28±7 ppm Oxygen < 20 ppm Sulphur Dioxide 130±35 ppm Neon 8.6±4 Argon 110±20 40±10 Krypton 0.6±0.2 The new atmospheric experiments provided information on important trace gases. The x-ray fluorospectrometer found chlorine to be ten times more common than sulphur in cloud material. This was interpreted to mean that the clouds contained a nonvolatile chlorine compound. Aluminum chloride was one speculation at the time. Unfortunately, the mass spectrometer on the Pioneer large probe became clogged by cloud droplets, but its gas chromatograph gave similar results to Venera, except for a much higher water content.

Burst of Electrical Activity Detected by Venera-12 The GROZA experiment detected bursts of low-frequency radio noise similar to those generated by lightening storms on Earth, but significantly more intense. There is some evidence that they originate in the deep atmosphere, far below the cloud layer. Lightning strikes around volcanic eruptions or some unknown atmospheric electrical phenomena could be responsible. It detected intense electrical activity during the descent of Venera-11, but much less from Venera-12, indicating that thunderstorms may be localized events on Venus as on Earth.