Chapter 8 Looking for Life Beyond Earth

Organic Molecules in the Universe Complex molecules and carbon may be the basis for life. Atoms that can bond to only two other atoms, like the atoms denoted X shown here, can form a chain of atoms called a linear molecule. The chain stops where we introduce an atom, such as those labeled Y and Z, that can bond to only one other atom. A carbon atom (denoted C) can bond with up to four other atoms and can form more complex, nonlinear molecules like glucose.

Evidence of Carbon from the Early Solar System If life is based on organic molecules, then these molecules must be present on a planet’s surface in order for life to arise from nonliving matter. Evidence for this comes from ancient meteorites that date from the formation of the solar system and that are often found to contain a variety of carbon-based molecules.

The Miller-Urey Experiment Demonstrated that simple chemicals can combine to form the chemical building blocks of life. In a closed container, they prepared a sample of the most common molecules in the solar system: hydrogen (H2) ammonia (NH3) methane (CH4) water vapor (H2O) They then exposed this mixture of gases to an electric arc (to simulate lightning) for a week. The inside of the container had become coated with a reddish-brown substance rich in amino acids and other compounds essential to life.

Finding Liquid Water Is Essential Water has been found on both Europa and the Moon.

Searching for Life on Mars In 1976 Viking 1 and Viking 2 were sent to Mars to search for water and signs of life. Results were inconclusive.

New Evidence for Martian Water Visible-light images show the south polar cap of Mars, but do not indicate its chemical composition. By using a camera tuned to different wavelengths of infrared light, the Mars Express spacecraft was able to identify the distinctive reflections of an upper layer of carbon dioxide ice and a deeper layer of water ice.

New Evidence for Martian Water In this formation, some layers are made of dust deposited by the Martian winds; others were laid down by minerals that precipitated out of standing water. The bluish color shows the presence of millimeter-sized spheres of gray hematite, which forms in water-soaked deposits. (False-Color)

Meteorites from Mars 12+ meteorites formed on Mars have managed to make their way to Earth. These SNC meteorites are named after the first three examples found (Shergotty, Nakhla, and Chassigny). SNC meteorites are identified by the chemical composition of trace amounts of gas trapped within them. This composition is very different from that of the Earth’s atmosphere, but is a nearly perfect match to the composition of the Martian atmosphere found by the Viking Landers.

The Drake Equation The number of technologically advanced civilizations in the galaxy may be estimated by a single mathematical sentence. N = number of technologically advanced civilizations in the galaxy whose messages we might be able to detect R* = the rate at which solar-type stars form in the galaxy fp = the fraction of stars that have planets ne = the number of planets per solar system that are Earthlike (i.e., suitable for life) fl = the fraction of those Earthlike planets on which life actually arises fi = the fraction of those life-forms that evolve into intelligent species fc = the fraction of those species that develop adequate technology and then choose to send messages out into space L = the lifetime of a technologically advanced civilization

Searching for Planets Using “Wobbles” A planet and its star both orbit around their common center of mass, always staying on opposite sides of this point. Even if the planet cannot be seen, its presence can be inferred if the star’s motion can be detected.

Searching Using Images and Spectra A Sun-like star, and orbiting planets The simulated infrared spectrum of one of the planets, showing absorption lines of water vapor, ozone, and carbon dioxide The infrared spectrum of such planets will make it possible to identify worlds on which life may have evolved.

Radio Searches for Alien Civilizations The Water Hole: a range of radio frequencies from about 103 to 104 megahertz (MHz) in which there is little noise and little absorption by the Earth’s atmosphere. This noise-free region may be well suited for interstellar communication.