S1977 E1 • The Planets: The Earth As A Planet

In his first Christmas Lecture, American astronomer and cosmologist Carl Sagan explores planet Earth and the place, scale and geometry of the “pale blue dot” in the Solar System. Sagan provides a unique insight into the history of our knowledge of the third planet from the Sun, formed 4.5 billion years ago. Using images and models of the planets in our Solar System, Sagan reveals how the heliocentric model of our universe, in which the Earth and planets revolve around the Sun, came to replace the earlier Aristotelian idea that our planet was at the centre and everything orbited around it. As the complexity of observational tools has developed from simple telescopes to complex spacecraft, so too has our understanding of the world we inhabit. Looking back on the evolution in space science in the years since Sagan’s lectures we have made huge advances in our understanding of our planet’s environment, climate, weather, geology and biology – as well as our relative place in the universe.

Première diffusion : 26 décembre 1977

S1977 E2 • The Planets: The Outer Solar System And Life

From ancient organisms to the plants and animals we see today, our planet showcases a spectacular array of life. But beneath such diversity lies an underlying unity. All life on Earth is based on two molecules (the proteins and the nucleic acids) and the origin of these molecules in the early stages of our planet’s development is inextricably linked to the origin of life. In his second Christmas Lecture, Carl Sagan travels beyond Earth to explore the possibility of life in outer space. To find the answer, he looks back to the early stages of the development of our atmosphere. The hydrogen from this atmosphere has since escaped to space from Earth, but not from bigger planets like Jupiter. When the hydrogen-rich gases of the early Earth are mixed together and supplied with energy, the essential molecular building blocks of the proteins and nucleic acids are formed. As Sagan suggests, although this process no longer occurs on Earth, such organic chemistry should be occurring in the outer solar system on Jupiter, and Titan, Saturn’s largest moon. The NASA twin spacecraft Voyager 1 and 2, launched a few months prior to these Lectures in 1977, were sent to space to explore this hypothesis.

Première diffusion : 27 décembre 1977

S1977 E3 • The Planets: The History Of Mars

Cold, arid, and tens of millions of miles away from Earth, Mars has intrigued scientists for centuries. The existence of liquid on its surface was confirmed by NASA’s flyby mission, Mariner 4, in 1965, but the question of whether life exists on our neighbouring planet has remained a subject of much speculation. In the 19th and 20th centuries, observers using only the naked eye and a telescope saw features on Mars which they interpreted as evidence for a dry but Earth-like climate, for vegetation which grew and decayed with the seasons, and for a great Martian canal network designed by a heroic but dying race of hydraulic engineers. In the third of his Christmas Lectures, Carl Sagan explores the mystery of the Red Planet. From its rocky craters to its polar ice caps, Sagan describes our understanding of the geology and chemistry of Mars, revealing the discovery of its two moons, Phobos and Deimos, in 1877, and the bizarre one-time suggestion that these moons were artificial satellites launched by an ancient but not extinct Martian civilisation.

Première diffusion : 28 décembre 1977

S1977 E4 • The Planets: Mars Before Viking

Observing the planets in our solar system from Earth provided limited scope for astronomers wishing to explore them in more detail. To get a better understanding of planets such as Mars, astronomers needed to get a closer look through the use of unmanned space probes which could beam data back remotely. In this Lecture, Carl Sagan explores the surprising discoveries made by Mariner 9, the first unmanned space probe to orbit another planet. This mission went on to provide scientists with a glimpse at Mars that was wildly different from their expectations. Sagan explores the features of Mars as uncovered by Mariner 9, including the formation of craters, presence of volcanoes, polar regions and the significance of its winding sinuous valleys and tributaries. These findings point towards a planet that has undergone considerable climate change, with early evidence that it might once have been conducive to terrestrial life.

Première diffusion : 29 décembre 1977

S1977 E5 • The Planets: Mars After Viking

In his fifth lecture, Carl Sagan takes a look at the design, launch and accomplishments of the Viking mission, a major chapter in the history of planetary exploration. After Mariner 9s remote orbital inspection of Mars, the Viking program finally allowed scientists to study the Martian surface in detail. The photography of the terrain, the chemical analysis of the soil and testing for microbiological life yielded stunning, yet enigmatic results. Results returned from the microbiology experiments gave signs consistent with life, however the search for organic molecules in the Martian soil turned up completely negative. The conflict between these results suggested either the presence of microbiological life or a non-biological, inorganic process occurring within the Martian soil. Four decades later – following the success of subsequent Mars rovers – the question concerning life on Mars is one that still remains unanswered.

Première diffusion : 30 décembre 1977

S1977 E6 • The Planets: Planetary Systems Beyond Our Sun

Theoretical work on the origin of solar systems suggests that planets are a frequent, if not invariable, accompaniment of stars. If there are billions of planets, if the origin of life occurs readily under general cosmic conditions, and if there are many worlds much older than the Earth for evolution to work upon, why shouldn’t the galaxy be brimming over with life? At the time Sagan delivered his Christmas Lectures in 1977, the only known planets were the ones in our own solar system. There was no evidence to suggest planets existed outside our solar system, or that there was a star other than our own sun producing planets. This was to change just over a decade later when the first evidence of planets orbiting a star was detected by radio astronomers in 1991. In the last of his six Christmas Lectures, Carl Sagan explores the concept of solar systems outside our own, and asks if this were the case, how similar they might be to ours. With more than 450 extra-solar planets discovered in the past twenty years, Sagan’s final lecture serves as a reminder of how far we’ve come in our understanding of what exists beyond Earth.

Première diffusion : 31 décembre 1977