Topics

• "Is There Life on Earth?" would seem to be a frivolous question. We know there is life on Earth! Yet, suppose we try to answer the question while on Mars, or from any place far from Earth. Would we be able to determine if there is life on Earth? This is indeed a serious question, not easy to answer.

   

  

  Earth As Seen by Galileo After Second Flyby (Dec 8, 1992)

   

  One possible sign of (intelligent) life on Earth could be the surprising fact that Earth is unusually 'bright' in some regions of the radio spectrum, due to the leakage into space of our radio and tv broadcasts. In fact, at some frequencies, the Earth is probably the brightest object in the solar system, and its brightness varies in interesting and unusual ways through the day. See, for example, Earth's Twin. Notice however that the Earth's radio brightness is steadily decreasing thanks to the growing use of cable tv, optic fibers, artificial communication satellites, etc.
  More crucial, however, are the "abundance of molecular oxygen; the presence of chlorophyll, the pigment used by plants in combining carbon dioxide and water to form sugar; and a trace amount of methane (which is hard to maintain with all that oxygen around)."
  [ from   Remote Indicators of Life on Earth ]
• There is however a deeper and much more problematic question: what is life? See, for instance,   Is anybody out there? Define life. You can't? Neither can the scientists. Why?
  The question about the nature of life is inextricably related to the one about its origin(s):

  "How life begins remains a fundamental unsolved mystery. The origin of life on Earth is likely to represent only one pathway among many along which life can emerge. Thus the universal principles must be understood that underlie not only the origins of life on Earth, but also the possible origins of life elsewhere. These principles will be sought by determining what raw materials of life can be produced by chemical evolution in space and on planets. It should be understood how organic compounds are assembled into more complex molecular systems and the processes by which complex systems evolve those basic properties that are critical to life's origins. Such properties include capturing energy and nutrients from the environment, and manufacturing copies of key biomolecules. Clues from the biomolecular and fossil records, as well as from diverse microorganisms, should be explored in order to define better the fundamental properties of the living state."
  [ from Understand How Life Emerges from Cosmic and Planetary Precursors ]

  The main difficulty lies in the fact that, at this moment, we know only one form of life—ours. It is essentially impossible to define life when only one instance of it is available. The aim of science is to discover patterns, regularities, etc. in the data, and if the data consist of only one datum, not much can be done. A little—perhaps not so little—help may come from the discovery of 'primitive' bacteria in ancient ice, in the upper atmosphere, in rocks, etc. Read for example Microbes from 120,000-year-old Ice Sample Show Life's Tenacity. If not another form of life, these organisms can tell us much about the range of forms life on earth can take. An important step, but not a complete answer to the main question. That is why it is so important to keep looking for extraterrestrial life—if any is out there. But what does it mean to search for extraterrestrial life? There are two possibilities: extraterrestrial life is either similar to ours or different from it. In the first case, at least intuitively, we know what to look for.But if the second case applies, then what can we look for? We seem to be caught in a catch-22 dilemma.
  There are several ways we can attempt to overcome this impasse:
  1. Try to list what we consider to be the most defining characteristics of life as we know it, and then imagine such characteristics as particular cases of more general concepts.
  2. Try to understand, in theory and in experiment, the conditions of the pre-biotic Earth, and how they made our life possible. Then, again, we can study how they might be generalized.
  3. If some forms of life exist elsewhere, at least some of them may have evolved to the point of some form of intelligence. If such intelligent life is also interested in communication, we may expect some messages reaching us.
• Let's consider the first strategy. What do we consider defining characteristcs of life? Here is a plausible answer: [ from Studying Life: Biology Properties of Life ^[*] ]
  1. Hierarchical Order
     1. Levels of organization based on subunits.
     2. Each level builds on levels below it, but has emergent properties (properties not found within subunits).
     3. Extends from molecules to organisms and populations.
  2. Reproduction: production of own kind based on heredity.
  3. Growth and Development: increase in size and complexity based on programmed patterns contained in hereditary information (DNA).
  4. Energy Utilization: taking in energy to do work (move, grow, reproduce).
  5. Response to Stimuli: reaction to changes in surrounding environment.
  6. Homeostasis: Regulatory mechanisms that maintain stable internal environment despite fluctuating external environment.
  7. Evolutionary Adaptation: Emergent property of groups of organisms. Interaction between organisms and their environment. Individuals in later generations are better able to survive and reproduce in that environment.
  While this list does seem plausible, it is important to find out if it applies to other systems which we do not consider as alive. For example, "hierarchical order" applies also to chemistry in general, and "energy utilization" seems true of just about everything. These properties are too broad, if taken literally. So, let's dig a bit deeper. Consider "energy utilization." Our bodies do not simply utilize whatever energy happens to be available. For example, our body protects us from excessive thermal energy. In fact the body maintains a pretty stable internal temperature. Something similar can be said about our "response to stimuli:" we simply do not respond to many constant stimuli. The fact seems to be that the organism maintains an active interface with its surroundings. Such interface regulates our interactions with the rest of the world. Such highly selective behavior is not usually exhibited by inanimate systems.
  What about "reproduction" and "growth and development"? How can we generalize these features? It may seem pointless to notice that when we reproduce, our progeny will also be able to reproduce itself, and so on. Yet, this is essential. An organism does not merely reproduce itself, it also reproduces the … recipe for reproduction! And the recipe must always be able to undergo (evolutionary) changes. This is an important point, and we can try to generalize it as follows: living organisms incorporate the history of the changes (mutations) which their and other related species have undergone. But this history itself is not preserved in full. It is somewhat like a story that is passed on from person to person. The current version is not a simple sum of all its previous versions. Sometimes we can guess which word or sentence, say, a current word or sentence has replaced. Most of the times, however, we can not. This is another trait that life does not seem to share with non-life.
• The question then becomes, what does it mean to search for 'things' which interact actively and selectively with their environment? or 'things' which are capable of making (approximate) copies of themselves, which in turn are able to make copies of themselves, and so on? or 'things' which embody their own (approximate) history? These are difficult questions, for which no ready answers are available. Yet, it is possible to propose practical strategies for our search. Read the very clear section on the NASA's Astrobiology's program   Goals and Objectives.

 



Mars As Seen by the Global Surveyor Mars Orbiter Camera (August 20, 1997)

 

In many ways this is precisely what the two Viking missions to Mars attempted to do. See for example the description of the Biology experiment packages on Viking 1 and on Viking 2.
• Another important project searching for (intelligent) life is the SETI Project. "The mission of the SETI Institute is to explore, understand and explain the origin, nature and prevalence of life in the universe." One of the major projects

  "uses the world's largest telescopes (40 to 300 meters in diameter) to scrutinize the vicinities of nearby, sun-like stars. Stars are examined one by one over a portion of the microwave region of the electromagnetic spectrum for artificially produced signals. The Targeted Search System looks for signals in the range 1,000 MHz to 3,000 MHz, with a frequency resolution of 1 Hz."

  You can participate, if not in the collection of data, in their analysis. SeETI@home.

  "Most of the SETI programs in existence today, including those at UC Berkeley build large computers that analyze that data from the telescope in real time. None of these computers look very deeply at the data for weak signals nor do they look for a large class of signal types. [ … ] The reason for this is because they are limited by the amount of computer power available for data analysis. To tease out the weakest signals, a great amount of computer power is necessary. It would take a monstrous supercomputer to get the job done. SETI programs could never afford to build or buy that computing power. There is a trade-off that they can make. Rather than a huge computer to do the job, they could use a smaller computer but just take longer to do it. But then there would be lots of data piling up. What if they used lots of small computers, all working simultaneously on different parts of the analysis? Where can the SETI team possibly find thousands of computers they'd need to analyze the data continuously streaming from Arecibo?

  The UC Berkeley SETI team has discovered that there are already thousands of computers that might be available for use. Most of these computers sit around most of the time with toasters flying across their screens accomplishing absolutely nothing and wasting electricity to boot. This is where SETI@home (and you!) come into the picture. The SETI@home project hopes to convince you to allow us to borrow your computer when you aren't using it and to help us '…search out new life and new civilizations.' We'll do this with a screen saver that can go get a chunk of data from us over the internet, analyze that data, and then report the results back to us. When you need your computer back, our screen saver instantly gets out of the way and only continues it's analysis when you are finished with your work."

• Of course you may ask why, instead of listening for messages, don't we try to make our existence known by broadcasting a message of our own. The answer is simple: even the nearest star, Proxima Centauri, is more than four light years away. It would thus take more than four years to reach it. The situation worsens as we move further out. In any case a message was in fact sent by the Arecibo Radiotelescope in 1974. Here it is:

   

  

  The Arecibo Message

   

  Read Arecibo Broadcast. This experiment can help us also understand the following questions: how would we know that we received a message from some intelligent civilization? How would we decode it?

 
Readings, Resources and Questions

• ^[*] The corresponding link is broken.
• A very interesting and insightful book you should consider reading is L Margulis and D Sagan, What is Life? (U of California Press, 1995, 2000).
• Visit Is There Anybody Out There? , an excellent collection of clear and accessible articles on the search for extraterrestrial life. Visit also Astrobiology: The Search for Life, part of Exploratorium: The Museum of Science, Art and Human Perception.
  Another obligatory stop is NASA's Astrobiology.

  "Astrobiology is the study of the origins, evolution, distribution, and future of life in the universe. It requires fundamental concepts of life and habitable environments that will help us to recognize biospheres that might be quite different from our own." [ from Astrobiology and the Roadmap ]

• Scientists have tried to formulate educated guesses as to the likelihood of finding (intelligent) life in the universe. The first, and perhaps most famous of such attempts was the work of Frank Drake.

  "How can we estimate the number of technological civilizations that might exist among the stars? While working as a radio astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia, Dr Frank Drake (now Chairman of the Board of the SETI Institute) conceived an approach to bound the terms involved in estimating the number of technological civilizations that may exist in our galaxy. The Drake Equation, as it has become known, was first presented by Drake in 1961 and identifies specific factors thought to play a role in the development of such civilizations. Although there is no unique solution to this equation, it is a generally accepted tool used by the scientific community to examine these factors." [ from Drake Equation ]

  Here is the Drake Equation:

  N  =  R*  x  f_p  x  n_e  x  f_l  x  f_i  x  f_c  x  L

  where

  N   =  the number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable.
  R* =  the rate of formation of stars suitable for the development of intelligent life.
  f_p   =  the fraction of those stars with planetary systems.
  n_e  =  the number of planets, per solar system, with an environment suitable for life.
  f_l   =  the fraction of suitable planets on which life actually appears.
  f_i   =  the fraction of life bearing planets on which intelligent life emerges.
  f_c  =  the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
  L   =  the length of time such civilizations release detectable signals into space.

  You can try your hand at calculating N using PBS' On-line Calculator, where current estimates of these parameters, and the difficulties in calculating such estimates, are briefly discussed. Notice that some of these parameters (e.g. f_c) are very hard to even guess. However, "besides illuminating the factors involved in such a search, the Drake Equation is a simple, effective tool for stimulating intellectual curiosity about the universe around us, for helping us to understand that life as we know it is the end product of a natural, cosmic evolution, and for making us realize how much we are a part of that universe."
• Life is not the only complex system for which a definition still eludes us. Can you think of other complex systems for which this is also true?