LIFE IN THE UNIVERSE

Is there any life in the Universe beyond Earth? There's no evidence that there is, but it seems likely--there's nothing special about the Earth.

What conditions led to the origin of life on Earth, and how likely is it those conditions have occurred elsewhere in the Universe?

WHAT IS LIFE?

Common features:

1. reacts to environment
2. metabolism (draws energy from environment)
3. reproduction

LIFE ON EARTH

building blocks: relatively small compounds containing carbon, oxygen, hydrogen and nitrogen (and sometimes sulfur).

• amino acids combine to form proteins.
• nucleotide bases combine to make DNA

Chemical evolution. Amino acids and nucleotides must have formed before life.

• found in meteorites, so they may have come from space.
• may have formed on the early Earth: Urey-Miller experiment, 1953, makes amino acids from water, methane, carbon dioxide, ammonia, heat and electricity

Fossil records of single-celled life go back 3.5 billion years and perhaps earlier. Must have started earlier, but step from chemical to biological evolution has left no record.

Other chemical bases for life

It does not seem likely that life can be based on significantly different chemistry.

• Silicon is in some ways similar to carbon in its chemistry, but the chemical bonds are weaker, and silicon cannot make molecules as complex as carbon.
• Water is the universal solvent. Liquid ammonia has been proposed, but its low temperature may prevent necessary chemical reactions from taking place

LIFE ON OTHER PLANETS IN THE SOLAR SYSTEM

Moon, Mercury lack liquid water, protective atmospheres

Venus too hot, lacks water

Titan has methane/ammonia/nitrogen atmosphere, but is probably too cold

Europa, Ganymede and Callisto may have liquid water--perhaps most likely site for other life in Solar System

Mars too cold and lacks liquid water, but formerly had more atmosphere, higher temperatures and running water. Perhaps fossils of early life (microorganisms) will be found there; it's conceivable some primitive forms of life evolved to survive changing conditions.

• 1996 claim of fossils in Martian meteorite found in Antarctica
• Viking lander tests appeared to indicate presence of life but it was later shown that the reactions detected may have been produced by chemical processes. No organic compounds detected. Inconclusive.

GREEN BANK (or DRAKE) EQUATION

Estimated number of technological civilizations present in Milky Way presently =

average rate of star formation (10 / year)
x fraction of stars having planetary systems
x average number of planets within life zone for various types of star and star system
x fraction of habitable planets that develop life
x fraction of life-bearing planets on which intelligence appears
x fraction of intelligent life forms that develop technology
x average lifetime of a technological civilization

= 100-1 billion ?

THE EXTINCTION OF THE DINOSAURS

Possibly cause: impact of large meteorite results in "nuclear winter,"

Evidence:

• iridium layer found in fossil record at about time of extinction of dinosaurs (iridium is generally rare in Earth's surface)
• large numbers of species (including others besides dinosaurs) become extinct at about the same time
• Chixulub crater in Yucatan may be the one

Extinctions show some evidence of being periodic. Are they caused by some distant companion of the Sun, Nemesis, that passes through the Oort cloud every 26 million years, showering Earth with meteorites? Or by the Sun passing through a spiral arm?

HUMAN POPULATION GROWTH

current rate of growth of world population: about 80 million people per year. Population passes 6 billion in 1999; some predict levelling off at about 12 billion.

Source: United Nations Population Division, Department for Economic and Social Information and Policy Analysis

Can Earth support this population?

COMMUNICATION AND SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE (SETI)

If average lifetime=1 million years then the average distance between civilizations in the galaxy is 150 ly, or 300 years for messages to go back and forth

If they exist, should we contact them?

Communications via radio signal

Earth has been broadcasting in the radio frequency range for most of this century (Marconi, 1895-1901; invention of vacuum tube, Edison, Fleming 1904, De Forest, 1906; first transatlantic contact, 1901).

FM broadcasts since 1936 (Edwin Armstrong). A characteristic of terrestrial radio signals as seen from other solar systems is that they are strongest in the plane tangent to the Earth's surface, and so they would vary with time as the Earth rotates (strongest when eastern US and western Europe are "rising" and "setting"). Earth is "brighter" than the Sun at radio frequencies!

SETI search is ongoing

Previous section

your instructor