Carl Sagan explained why Venus is a furnace

Person: Carl Sagan
Lived: 1934–1996
Field: Astronomy, planetary science
From: American

The story

Before the space age, Venus was often imagined as a warm, wet world, perhaps covered in swamps or oceans and even hospitable to life. Its thick clouds hid the surface completely, and in the early 1960s nobody knew for certain what conditions were like beneath them.

Carl Sagan had trained at the University of Chicago, where he learned planetary astronomy from Gerard Kuiper, who supervised his 1960 doctoral thesis, Physical Studies of the Planets. Kuiper taught him to test ideas with quick back-of-the-envelope calculations. Sagan listed the possibilities for Venus's surface, and his own view was that it was dry and very hot rather than the balmy paradise others imagined. After studying the radio waves coming from the planet, he concluded that the surface was around 500 °C (900 °F).

He argued that the dense carbon-dioxide atmosphere trapped heat through a powerful greenhouse effect. While a Miller Fellow at the University of California, Berkeley, from 1960 to 1962, he published an article on the atmosphere of Venus in the journal Science in 1961. As a visiting scientist at NASA's Jet Propulsion Laboratory he also contributed to the first Mariner missions to Venus, working on the design and management of the project. In 1962 the Mariner 2 probe confirmed his conclusions about the surface conditions.

The work launched his career. After the Science article, Harvard astronomers invited him to give a colloquium, and he joined the Harvard faculty as an assistant professor in 1963. The greenhouse explanation for Venus's heat has since been accepted by planetary scientists, and the Planetary Society lists the high temperature of Venus first among the planetary mysteries he helped solve.

Sagan kept returning to Venus as a lesson for Earth. In his television series Cosmos he described the planet's crushing, scorching surface as revealed by Soviet Venera landers and drew out its implications for Earth's own greenhouse effect. He later compared human-made climate change to the way Venus had become a hot, hostile world, and in 1985 he told the US Congress that the greenhouse effect would change Earth's climate.

Why it matters

Sagan's work on Venus helped found modern planetary science and gave the world its clearest example of how an atmosphere can trap heat, a lesson central to understanding Earth's climate.

About Carl Sagan

Brooklyn-born astronomer who explained the Venus greenhouse, helped send humanity’s messages on Pioneer and Voyager, and taught Cosmos viewers worldwide to look at the universe with wonder and doubt.

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Carl Sagan

1934–1996 · Astronomy, planetary science

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