A. Perspective

• There are billions of stars in each galaxy and about 100 billion galaxies within reach of our telescopes. (See Study Guide 2.)
  The total number of stars in the observable universe is of order 10,000,000,000,000,000,000,000 (or 10²²).

• Therefore:
  • If you believe Earth and humanity are special, uniquely fortunate, or specially created (the "pre-Copernican" view), then we are ALONE in this unimaginably vast cosmos.

  • If you believe we are average (the "Copernican" view), then the universe TEEMS with billions of advanced lifeforms.

A revolution in prospects for astrobiology

1. 1995+: The discovery of planetary systems around nearby stars (see Guide 11). We have discovered hundreds of other planetary systems in the last 18 years. Apparently, many stars like the Sun have planets. Despite the technical challenges, we have already identified several Earth-size candidate planets. Most astronomers are confident that Earth-like planets will be found soon in the "habitable zones" around other stars.

2. 1996: Evidence suggesting the presence of fossil microorganisms on Mars (see Guide 17). This claim is highly controversial. But right or wrong, it has stimulated the development of vigorous research in the rapidly growing field of "Astrobiology."

3. The discovery of water-rich environments on Mars (in the past) and under the surfaces of Europa and Enceladus. And the unique but promising hydrocarbon-rich surface of Titan.

B. Life on Earth

Age: Life on Earth is ancient. It has thrived for at least 3 billion years, according to the fossil record, and the character of lifeforms has changed radically throughout that time.
Functional basis: random chemical interactions, governed by natural physical principles.
No special "vital force" is required. The laws of physics and chemistry that govern the structure and functioning of organic systems are exactly the same as those that govern inorganic systems.

Unity: Despite extraordinary diversity, there is only one type of terrestrial life at the molecular level
• A small set of chemical building blocks (large molecules constructed from abundant atoms like H,C,N,O, a few others)

• The key constituent is carbon, which offers 4 chemical bonds as the basis for forming complex molecules

• Nucleic acids (DNA, RNA): provide an enormous capacity for information storage and chemical control for molecular reproduction

  
  

350 million years of evolution on Earth: characteristic terrestrial lifeforms
from the Cambrian (500 Myr ago, left) to the Jurassic (150 Myr ago, right) Periods

C. Evolution

The basic principle of evolution is very simple: those organisms that are best adapted by their genetic makeup to a given environment thrive and pass those favorable genetics on to subsequent generations. Dispersion in genetic material in each generation is guaranteed by chemical mutations; mutations that are accidentally favorable are amplified by natural selection and propagate forward into modified species. On Earth, there have been hundreds of millions of generations preceding the familiar lifeforms we see around us. The consequent adaptation to an immense variety of environmental niches is profound.

E.g.: the fossil record is now fabulously rich (250,000 species)---vastly more so than in Darwin's day---and clearly shows the radical change of lifeforms on Earth through time. Rich fossil collections exist for ages between 10,000 and 500,000,000 years.
E.g.: geological age-dating by radioactive isotopes is now highly accurate and graphically reveals the enormous stretch of time over which evolution on Earth has been working.
E.g.: genetics, the mechanism by which inherited characteristics are passed from one generation to the next, was mysterious in Darwin's time but is now thoroughly understood, as are the causes of the mutations that engender changes in species.
E.g.: DNA mapping has recently demonstrated astonishing chemical similarities between humans and other higher lifeforms. We share 98.8% of our genes with chimpanzees.
It is now clear that humans are part of a continuum of life on Earth, not a special class.
This is probably the most stringest test of evolutionary biology ever made, and it passed with flying colors. Darwin, Huxley, and the other leading biologists up to 50 years ago could not have imagined how precise or conclusive such chemical tests of the evolutionary principle could be.

E.g.: detailed measurements of the brightnesses & colors of stars in star clusters permit us to age-date their formation times. These range from about 10 million years for clusters in active star-forming regions to 13 billion years in the case of "globular clusters" like M80.
E.g.: by using powerful telescopes to study the Hubble Deep Field and other distant regions, astronomers can observe the universe as it was billions of years ago. No other field of science is able to make such direct observations of the distant past. Not only can we see the past, we also can determine how different the contents of the universe were at earlier times and trace how they change with "lookback time." There is no doubt that the universe has evolved.
E.g.: spacecraft observations of the cosmic background radiation from the Big Bang have recently determined the age of the universe to be 13.7 billion years.

They are not important among active scientists, who accept evolution as a foundation of modern science. Scientists involve themselves in these controversies only insofar as they try to protect the integrity of their disciplines and of science education from political intrusion.
In these debates, evolution is often mistakenly conflated with the question of the origin of life on Earth. Scientists do not presently have a good understanding of how (or even whether) life originated on Earth. That discussion is hypothetical. The evidence for evolution (that is, for a change over time) of lifeforms on Earth, however, is entirely independent of the question of life's origins. You do not have to know where a sapling came from in order to know that a tree is growing.

D. Origin of Life on Earth?

• Probably in oceans; energy sources: solar radiation, volcanic vents, lightning....

• Early molecular evolution was simulated in the classic Miller-Urey experiment. Sugars, amino acids, and DNA bases were generated in only a few days from pre-biotic ingredients. This demonstrated how easy it is to start a chemical synthesis sequence that can lead to organisms.

• Note that organic molecules thrive even in ostensibly hostile environments: meteorites, interstellar clouds. Bio-evolution could have begun elsewhere than on Earth.

• Also note that once it begins, molecular evolution is subject through natural selection to the same kind of acceleration toward survivable and proliferating forms as are biological organisms.

• Discoveries of "extremophiles" (organisms thriving under unexpectedly harsh conditions of temperature, pressure, or acidity) demonstrate how robust simple life forms can be.

• For 3/4 of its history, life on Earth consisted only of very simple organisms, entirely unlike the profusion of advanced types seen today.

The notion of panspermia simply pushes the question of how life originated back one level. Panspermia has occasionally been very controversial, but at present there is no way to decide whether life originated on Earth itself or came here from elsewhere.
An obvious way for lifeforms to propagate in a given planetary system is through comets or meteoroids, as the "SNC" meteorites from Mars vividly demonstrate. Meteoroids could have spread Mars life to Earth or vice versa.
For more information on panspermia, click here.

Size of the "habitable zone" for Earthlike planets surrounding four different types of stars;
"F" stars are more massive than the Sun, "K" and "M" are less massive

E. Life Elsewhere in Our Solar System

Requirements

• Raw materials;
  
• Dense medium (preferably water, but other liquids/dense gases are possible);
  
• Protected environment, maintained in the appropriate temperature/pressure range;
  
• Energy source; primarily sunlight, but volcanic vents, lightning, etc. are alternatives;
  
• Sufficient time (100 Myr-1 Byr?)

Habitable Zone

For a particular type of parent body (terrestrial planet, Jovian planet, comet), we can define a habitable zone as those distances from the parent star for which that type can offer a comfortable biosphere, neither too hot nor too cold, for life to develop.
In the solar system, the habitable zone (see above) for Earthlike planets ranges from about 0.95 AU to 1.5 AU, covering (of course) the Earth's orbit but not reaching to either Venus or Mars. For Jovian planets, it would be much larger---but we aren't sure that life can thrive on Jovian planets.

Possible biospheres on the outer satellites: (Left) The icy surface of Europa (pseudocolor, Galileo mission); (Middle) The water vapor geysers of Enceladus (pseudocolor, Cassini mission); (Right) The hydrocarbon-rich surface of Titan, as viewed by the descending Huygens probe.

Possibilities

1. Venus: no! High temperature and corrosive atmosphere sufficient to sterilize surface of all Earth-like life.

2. Mars: plausible evidence for biosphere > 1 Byr ago with abundant water; SNC meteorites provide some evidence for microorganisms. Too cold and dry now for life? Absence of ozone in atmosphere allows damaging solar UV flux at surface.

3. Jupiter, Saturn atmospheres? Results from the Galileo probe (1995), which sampled the outermost layers of Jupiter's atmosphere, were not promising but don't exclude a biosphere.

4. Europa (J) and Enceladus (S), each with evidence for a liquid water reservoir/ocean lying beneath the visible crust of ice. Probably the most promising sites for bio-exploration after Mars.

5. Titan's (S) extraordinary hydrocarbon-rich atmosphere is a possible biosphere. The Cassini mission has demonstrated the presence of liquid hydrocarbons (methane, ethane) on Titan, and some scientists think "methanogenic" lifeforms might exist there. Most believe the very low temperatures (-180 degrees C) would preclude living organisms on the surface. As in the case of Europa and Enceladus, however, deep reservoirs with more favorable temperatures are possible. Here is a prospectus for exobiology on Titan.

6. Comet nuclei: ice, often with an apparent organic molecule coating; temperatures low; could act as "portable reservoirs" of organisms

F. Intelligent Life Elsewhere

Warning

There is also the danger of carbon or planetary chauvinism. E.g. intelligent life need not be confined to planets or even planetary systems. Why not an sentient interstellar gas cloud? This idea was explored in the famous science fiction novel The Black Cloud by astronomer Fred Hoyle.

The Drake Equation

1. There are 100 billion stars in our Galaxy

2. Assume 0.001% - 5% of the stars have Earth-like planets.
   Support? Recent detection of extra-solar planets. Even though most of these are Jupiter-class, rather than Earth-size, most astronomers expect that the fraction of stars with Earth-like planets will prove to be of order 5% or larger. This is at the high end of the range considered plausible over the last 50 years.

3. Assume all develop life leading to advanced civilizations (i.e. assume Earth is average)
   "Earth is average" is the Copernican assumption, which has proved so successful in studies of the structure of our inorganic universe. However, we have very little intuition here, and some biologists would argue that the chances of developing technological species are small.

4. Assume the communication phase lasts 10,000 years.
   Note: we have only recently entered this phase. It has been 80 years since we developed commercial radio stations that could be detected over interplanetary distances. Our artificially generated EM radiation is the most definitive marker of advanced lifeforms on this planet. Human EM signals are now propagating out through the Galaxy, with the most distant signals being about 80 light years away.

5. Combine all above. ===> 10 - 10,000 communicating civilizations in our Galaxy

6. ===> Distance to nearest: 10,000 - 1000 light years

Interstellar migration/exploration:

Here's a relevant, even amazing, fact: We human beings managed to launch four interstellar spacecraft in the first 20 years of the space age! The four Pioneer and Voyager spacecraft, sent to study the Jovian planets, have already left the Solar System.
The most distant manmade object (Voyager 1) is now 11.2 billion miles (121 Astronomical Units) from Earth.

There is a growing community of scientists and engineers making preliminary plans for interstellar travel. At right is a painting of a popular starship design, a "Bussard interstellar ramjet."

A famous question, now called the "Fermi Paradox" because it was first raised by physicist Enrico Fermi in the 1950's, is therefore: "Why aren't they here?"
If interstellar expansion is possible for thousands of Galactic species, there ought to be aliens cluttering up the Solar System. We don't see them, so is the estimate for advanced civilizations badly wrong?

All answers to the question are sheer speculation, but I think the best is: "They are here, but we don't recognize them."

SETI = "Search for Extraterrestrial Intelligence"

G. UFO'S, Alien Artifacts

• UFO's?
  See Guide 18 and links therein. There is no credible evidence that an intelligent species (other than our own) is involved in the UFO phenomenon. An overwhelmingly strong media influence governs the number & similarity of reports. Publicity can even provoke mild mass hysteria (e.g. alien abduction claims).

• Alien artifacts?
  • There have been a number of claims of physical evidence for an alien presence. One of the most interesting is the face on Mars, a partially-illuminated feature in the Cydonia region of Mars imaged (poorly) by the Viking spacecraft (1970's) and looking like a carved human head. One of the original Viking images is shown below (left panel). This elicited speculation about civilizations on Mars, some of which is discussed here.

  • In April 1998, the Mars Global Surveyor was retargeted to image the same region at much higher resolution. Several good images were returned, showing the fully illuminated "face" to be an unambiguously natural feature. See the right panel below. (Some of the interesting features in the original data, like the "headdress," were actually data drop-outs.) Yet better images have been more recently obtained by the Mars Express orbiter. No other candidates for alien artifacts are credible either.

H. The Recognition Chasm

• The mean age difference between two Galactic species is likely to be 100's of millions of years.

• Given favorable conditions, successful advanced lifeforms may have continued evolution for a large fraction of that time. (Once they reach a certain level of development, they become immune to the more serious astronomical hazards for lifeforms, such as asteroid impacts and stellar evolution.)

• The age separation is much more important than the spatial separation
  ====> us : them ~ goldfish : us

• Since recognition and communication is possible only for cultures in close intellectual proximity,
  ====> Aliens would appear to be natural phenomena.

Study Guide 23
Bennett textbook, Chapter 24
Optional Reading:
Are We Alone? by J. Trefil & R. Rood (Clemons Lib: QB 54.R55)
The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science by Steven J. Dick (SciEngr Lib: QB 54.D47 1996).
Rare Earth: Why Complex Life is Uncommon in the Universe by Peter Douglas Ward & Donald Brownlee. (QB54.W336.2000).
Lonely Planets: The Natural Philosophy of Alien Life by David Grinspoon

Talk.Origins (a thorough web discussion of evolution vs. creationism)
Science, Evolution, and Creationism (2008 report from the National Academy of Sciences)
Essay on Intelligent Design Creationism (Donald Simanek)
Good summaries of development of life on Earth:
Origin & Unicellular Organisms
Multicellular Organisms
The Age of Animals

NASA's Astrobiology Program
The Astrobiology Web and its Exobiology page.
Encyclopedia of Astrobiology, Astronomy, & Spaceflight
Mars Global Surveyor Press Releases on "Face on Mars"
The SETI Institute
The Planetary Society SETI Page
Centauri Dreams (site devoted to the potential of interstellar exploration)
Institute for Interstellar Studies (similar)
"The Great Silence" (about the Fermi Paradox, by David Brin)
Astronomy 3420: Life Beyond the Earth
Guide 18: Life on Mars, War of the Worlds, and the UFO phenomenon (O'Connell)