quality:correction, structural:summarization
Scientific achievements

Former student David Morrison described Sagan as "an 'idea person' and a master of intuitive physical arguments and ' back of the envelope ' calculations", and Gerard Kuiper said that "Some persons work best in specializing on a major program in the laboratory; others are best in liaison between sciences. Dr. Sagan belongs in the latter group."

Sagan's contributions were central to the discovery of the high surface temperatures of the planet Venus. In the early 1960s no one knew for certain the basic conditions of Venus' surface, and Sagan listed the possibilities in a report later depicted for popularization in a Time Life book Planets. His own view was that Venus was dry and very hot as opposed to the balmy paradise others had imagined. He had investigated radio waves from Venus and concluded that there was a surface temperature of 500 °C (900 °F). As a visiting scientist to NASA's Jet Propulsion Laboratory, he contributed to the first Mariner missions to Venus, working on the design and management of the project. Mariner 2 confirmed his conclusions on the surface conditions of Venus in 1962.

Sagan was among the first to hypothesize that Saturn 's moon Titan might possess oceans of liquid compounds on its surface and that Jupiter 's moon Europa might possess subsurface oceans of water. This would make Europa potentially habitable. Europa's subsurface ocean of water was later indirectly confirmed by the spacecraft Galileo. The mystery of Titan's reddish haze was also solved with Sagan's help. The reddish haze was revealed to be due to complex organic molecules constantly raining down onto Titan's surface.

Sagan further contributed insights regarding the atmospheres of Venus and Jupiter, as well as seasonal changes on Mars. He also perceived global warming as a growing, man-made danger and likened it to the natural development of Venus into a hot, life-hostile planet through a kind of runaway greenhouse effect. He testified to the US Congress in 1985 that the greenhouse effect would change the Earth's climate system. Sagan and his Cornell colleague Edwin Ernest Salpeter speculated about life in Jupiter's clouds, given the planet's dense atmospheric composition rich in organic molecules. He studied the observed color variations on Mars' surface and concluded that they were not seasonal or vegetational changes as most believed, but shifts in surface dust caused by windstorms.

Sagan is also known for his research on the possibilities of extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by radiation.

He is also the 1994 recipient of the Public Welfare Medal, the highest award of the National Academy of Sciences for "distinguished contributions in the application of science to the public welfare." He was denied membership in the academy, reportedly because his media activities made him unpopular with many other scientists.

As of 2017, Sagan is the most cited SETI scientist and one of the most cited planetary scientists.