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People Behind the Science

Carl Edward Sagan (1934–1996)

arl Edward Sagan was an American (227°C/440°F) and 800 K (527°C/980°F)—
Castronomer and popularizer of astron- the range that would also be expected on
omy whose main research was on planetary the basis of emission rate.
atmospheres, including that of the primor- Sagan then turned his attention to the
dial Earth. His most remarkable achieve- early planetary atmosphere of Earth, with
ment was to provide valuable insights into regard to the origins of life. One way of
the origin of life on our planet. understanding how life began is to try to
Sagan was born on November 19, form the compounds essential to life in con-
1934, in New York City. Completing his ditions analogous to those of the primeval
education at the University of Chicago, he atmo sphere. Before Sagan, Stanley Miller
obtained his bachelor’s degree in 1955 and and Harold Urey had used a mixture of meth-
his doctorate in 1960. Then, for two years, ane, ammonia, water vapor, and hydrogen,
he was a research fellow at the University perhaps it was due to interactions between sparked by a corona discharge that simulated
of California in Berkeley before he trans- charged particles in Venus’s dense upper the effect of lightning, to produce amino and
ferred to the Smithsonian Astrophysical atmosphere; perhaps it was glow discharge hydroxy acids of the sort found in life-forms.
Observatory in Cambridge, Massachusetts, between positive and negative charges in Later experiments used ultraviolet light or
lecturing also at Harvard University, where the atmosphere; or perhaps emission was heat as sources of energy, and even these had
he became assistant professor. Finally, in due to a particular radiation from charged less energy than would have been available
1968, Sagan moved to Cornell University particles trapped in the Venusian equiva- in Earth’s primordial state. Sagan followed a
in Ithaca, New York, and took a position lent of a Van Allen Belt. Sagan showed that similar method and, by irradiating a mixture
as director of the Laboratory for Planetary each of these hypotheses was incompat- of methane, ammonia, water, and hydrogen
Studies; in 1970, he became professor of ible with other observed characteristics or sulfide, was able to produce amino acids—as
astronomy and space science there. He died with implications of these characteristics. well as glucose, fructose, and nucleic acids.
on December 20, 1996. The positive part of Sagan’s proposal was Sugars can be made from formaldehyde un-
In the early 1960s, Sagan’s first major to show that all the observed characteris- der alkaline conditions and in the presence
research was into the planetary surface and tics were compatible with the straightfor- of inorganic catalysts. These sugars include
atmosphere of Venus. At the time, although ward hypothesis that the surface of Venus five-carbon sugars, which are essential to
intense emission of radiation had shown was very hot. On the basis of radar and the formation of nucleic acids, glucose, and
that the dark-side temperature of Venus optical observations, the distance between fructose—all common metabolites found as
was nearly 600 K, it was thought that the surface and clouds was calculated to be be- constituents of present-day life-forms. Sa-
surface itself remained relatively cool— tween 44 km (27 mi) and 65 km (40 mi); gan’s simulated primordial atmosphere not
leaving open the possibility that there was given the cloud-top temperature and Sa- only showed the presence of those metabo-
some form of life on the planet. Various gan’s expectation of a “greenhouse effect” lites, but also contained traces of adenosine
hypotheses were put forward to account in the atmosphere, surface temperature on triphosphate (ATP)—the foremost agent
for the strong emission actually observed: Venus was computed to be between 500 K used by living cells to store energy.

Source: From the Hutchinson Dictionary of Scientific Biography. © Research Machines plc 2003. All Rights Reserved. Helicon Publishing is a division of Research Machines.

the motion of the Earth-Moon system (Figure 16.33). Water on
Earth’s surface is free to move, and the Moon’s gravitational at-
traction pulls the water to the tidal bulge on the side of Earth
facing the Moon. This tide-raising force directed toward the
Moon bulges the water in mid-ocean some 0.75 m (about 2.5 ft), This tidal
bulge is "left
but it also bulges the land, producing a land tide. Since land behind" as
is much more rigid than water, the land tide is much smaller Earth is pulled Gravitational Gravitational
at about 12 cm (about 4.5 in). Since all parts of the land bulge away. pull from the Moon pull from the
attracts Earth.
together, this movement is not evident without measurement by Moon attracts
tidal bulge.
sensitive instruments.
The tidal bulge on the side of Earth opposite the Moon
occurs as Earth is pulled away from the ocean by the Earth-
FIGURE 16.33 Gravitational attraction pulls on Earth’s waters
Moon gravitational interaction. Between the tidal bulges facing
on the side of Earth facing the Moon, producing a tidal bulge. A
the Moon and the tidal bulge on the opposite side, sea level is second tidal bulge on the side of Earth opposite the Moon is
depressed across the broad surface. The depression is called a produced when Earth, which is closer to the Moon, is pulled away
tidal trough, even though it does not actually have the shape of from the waters.

16-21 CHAPTER 16 Earth in Space 425