UniverseTenth Edition

Chapter 27The Search for Extraterrestrial

Life

Roger Freedman • Robert Geller • William Kaufmann III

27-1 How comets and meteorites could have aided the origin of life on Earth

27-2 On which other worlds of our solar system life might have evolved

27-3 About the controversy over proposed fossil life from Mars

27-4 How astronomers estimate the number of other civilizations in the Galaxy

27-5 Why radio telescopes may be a useful tool for contacting alien civilizations

27-6 How astronomers hope to detect Earthlike planets orbiting other stars

By reading this chapter, you will learn

Complex Molecules and Carbon

27-1: The chemical building blocks of life are found in

space

Why Silicon is Unsuitable for Making Living Organisms

A Carbonaceous Chondrite

An Updated Miller-Uray Experiment

Extremophiles Can Take the Heat

27-2: Water and the potential for life

Extremophiles Can Take the Heat

Subsurface Ocean on Enceladus

Exploring Lake Vostok

Digging in the Martian Surface

A “Face” on Mars?

A “Face” on Mars?

A Meteorite from Mars

27-3: Meteorites from Mars have been scrutinized for

life forms

A Meteorite from Mars

Microscopic Tunnels In Martian Meteorite

Habitable Zones for Life

The Water Hole

27-5: Radio searches for alien civilizations are underway

27-6: Telescopes have been searching for Earthlike planets

The Infrared Spectrum of a Simulated Planet

Key Ideas• Organic Molecules in the Universe: All life on Earth, and

presumably on other worlds, depends on organic (carbon-based) molecules. These molecules occur naturally throughout interstellar space.

• The organic molecules needed for life to originate were probably brought to the young Earth by comets ,meteorites and interplanetary dust particles. Another likely source for organic molecules is chemical reactions in the Earth’s primitive atmosphere. Similar processes may occur on other worlds.

Key Ideas• Life in the Solar System: Besides Earth, other worlds in our solar

system—the planet Mars , Jupiter’s satellite Europa, and Saturn’s satellite Enceladus—may have had the right conditions for the origin of life.

• Europa appears to have extensive liquid water beneath its icy surface. Future missions may search for the presence of life there. Enceladus also appears to contain a large subsurface ocean, and some of this material is ejected through ice volcanoes for easier sampling.

• Mars once had liquid water on its surface, though it has none today. Life may have originated on Mars during the liquid water era.

Key Ideas• The Viking Lander spacecraft searched for

microorganisms on the Martian surface, but found no conclusive sign of their presence. The Mars Science Laboratory is designed to assess the past and present suitability of Mars for microbial life.

• An ancient Martian rock that came to Earth as a meteorite shows features that have been suggested as remnants of Martian life. Most of these features can be explained by geologic processes.

Key Ideas• Radio Searches for Extraterrestrial Intelligence: Astronomers

have carried out a number of searches for radio signals from other stars. No signs of intelligent life have yet been detected, but searches are continuing and using increasingly sophisticated techniques.

• The Drake equation is a tool for estimating the number of intelligent, communicative civilizations in our Galaxy.

• Telescope Searches for Earthlike Planets: A future generation of orbiting telescopes may be able to detect numerous terrestrial planets around nearby stars. If such planets are found, their infrared spectra may reveal the presence or absence of life.