Venera 4 made the first in-place chemical analysis of Venus’s atmosphere.
The Soviet Venera 4 probe arrived at Venus in 1967 and sent measurements directly from the planet’s atmosphere during its descent. Its instruments found that Venus’s atmosphere was dominated by carbon dioxide, with smaller amounts of nitrogen and very little oxygen and water vapor. This was the first successful in-situ atmospheric analysis performed at another planet.
Venera 4 also transformed scientists’ understanding of Venus’s physical environment. It measured a high temperature and atmospheric pressure far greater than many earlier estimates. The probe survived entry into another planet’s atmosphere, deployed a parachute, and transmitted data to Earth through its carrier spacecraft.
The mission is sometimes confused with Venera 7, which achieved the first successful transmission from the surface of Venus in 1970. Pioneer Venus later made extensive orbital and atmospheric studies, while Mariner 2 performed an earlier Venus flyby without making an in-place chemical analysis. Venera 4’s data helped establish Venus as a hot, dense, hostile world rather than an Earth-like planet hidden beneath clouds.