Natural conditions for life

The conditions for the creation of life-ordering processes out of disorder (or, more specifically, chemical order by formation of complex organic molecules and organisms from inorganic matter) can now be deduced from the First, Second and Third Laws of Thermodynamics:

1. It is necessary that the system be open (or at least non-isolated) to exchange energy (as well as mass) with its environment;

2. An influx of low-entropy energy that can do work is necessary;

3. An outflow of high-entropy energy (heat produced by transformation of work to heat) is necessary (this means that the temperature of the system must inevitably be greater than 2.726 K);

4. Entropy production accompanying the transformation of energy (work) to heat in the system is a necessary cost of maintaining the order; and

5. Mass transport processes at a not too low rate are necessary (a prerequisite).

This implies that the liquid or gaseous phase must be anticipated. A higher temperature will imply a better mass transfer, but also a higher reaction rate. An increased temperature also means a faster breakdown of macromolecules, and therefore a shift towards catabolism. A temperature approximately in the range of 260-340 K must therefore be anticipated for carbon-based life.

The rates of biochemical reactions on the molecular level are determined by the temperature of the system and the exergy (energy that can do work; see Chapter 5) supply to the system. Hierarchical organisation ensures that the reactions and the exergy available on the molecular level can be utilised on the next level, the cell level, and so on throughout the entire hierarchy: molecules ! cells ! organs ! organisms ! populations ! ecosystems. The maintenance of each level is dependent on its openness to exchange energy and matter. The rates in the higher levels are dependent on the sum of many processes on the molecular level. They are furthermore dependent on the slowest processes in the chain: supply of energy and matter to the unit ! the metabolic processes ! excretion of waste heat and waste material. The first and last of these three steps limit the rates and are determined by the extent of openness, measured by the area available for exchange between the unit and its environment relative to the volume. These considerations are based on allometric principles (Section 3.6; Peters, 1983; Straskraba et al., 1997).

In addition to the five conditions given above, it is necessary to add a few biochemically determined conditions. The carbon-based life on Earth requires first of all an abundant presence of water to deliver the two important elements, hydrogen and oxygen, as solvent for compounds containing the other needed elements (see below), as a compound which is liquid at a suitable temperature with a suitable diffusion coefficient, a suitable specific heat capacity to buffer temperature fluctuations and a suitable vapour pressure to ensure a suitable cycling (purification) rate of these crucial chemical compounds.

Life on Earth is characterised by about 25 elements. Some of these elements are used by life processes in micro amounts, and it cannot be excluded that other elements could have replaced these elements on other planets somewhere else in the Universe. Several metal ions are, for instance, used as coenzymes and are often important parts of high molecular organic complexes. Other ions may be able to play similar roles for biochemical processes and complexes. It is, on the other hand, difficult to imagine carbon-based life without at least most of the elements used in macro amounts, such as nitrogen for amino acids (proteins—the enzymes) and amino bases, phosphorus for ATP and phosphorous esters in general and sulphur for formation of some of the essential amino acids.

The biochemically determined conditions can therefore be summarised in the following two points:

6. Abundant presence of the unique solvent water is a prerequisite for the formation of life forms similar to the life forms as we know from Earth.

7. The presence of nitrogen, phosphorus, and sulphur and some metal ions seems absolutely necessary for the formation of carbon-based life.

A last and eighth condition should be added: the seven other conditions should be maintained within reasonable ranges for a very long period of time. The genes may ensure that, if an advantageous property of an organism has been developed, the property can be hereditary and the following generations will be able to maintain the advantageous property. The probability to create (complex) life spontaneously is so low that even the time from 'the big bang' would not have been sufficient. It is therefore necessary that the development toward life is made step-wise with conservation of each achieved progress to allow further development to ride on the shoulders of the progress already made. Many mechanisms are probably involved in the emergence of a progressive property on the first hand, but indisputable random processes based on trial-and-error are also important in the emergence of progressive properties. This implies that carbon life is not formed overnight. The history of evolution on Earth shows that, after a suitable temperature was achieved and water was abundant, probably of the order of 108 years or more (Nielsen, 1999) were needed to form the first living cells with some type of primitive genes to ensure a continuous development (evolution) from inorganic components dissolved in water. Phytoplankton fossils have recently been found at Isua, Greenland by Minik Rosing (Nielsen, 1999). The age of the fossils was determined to be 3.8 billion years old, or about 100 million years after the termination of the massive bombardment of meteors that characterised the first 600-700 million of years after Earth was born. Numerous theories have been published to explain how this development may have happened, probably in many steps: inorganic matter formed organic molecules by a through-flow of low-entropy energy, organic molecules formed high molecular organic compounds, self-catalytic processes occurred, complex organic molecules were brought randomly in contact by adsorption on clay particles, and many other processes are mentioned in these presented theories. Which theory is right is not important in this context. The focal point is that the seven above-mentioned conditions must be fulfilled for a sufficiently long period of time, which leads to the eighth condition:

8. As the formation of life from inorganic matter requires a very long time, probably of the order of 108 years or more, the seven conditions have to be maintained in the right ranges for a very long time, which probably exceeds about 108 years.

After the Mars Pathfinder mission, it has been discussed whether Mars hosts or has hosted life. Clearly, conditions 1 -7 are not met on Mars today. The climate is too harsh and water is far from being present in the amount needed for the planet to bear life. There are, however, many signs of a warmer and wetter climate at an earlier stage. It therefore looks as if the seven conditions may have been valid and the question is: ".. .have they also prevailed for a sufficient period of time?" If later missions to Mars will show that it is the case, the next obvious question is: "Will life inevitably be the result of self-organising processes if the eight conditions (the eighth condition about sufficient time should of course be included) are fulfilled? It should be expected that primitive life has been present on Mars at an early stage, provided that the warmer and wetter conditions have prevailed for sufficient time. The further evolution from unicellular organisms (maybe prokaryotic) to more and more complex organisms such as we know from Earth could not be realised on Mars because the climate changed and the water disappeared. Latest investigations of Mars-originated meteorites have made it almost certain that there has previously been microbiological life on Mars. The latest geological investigation has furthermore shown that there has previously been plenty of water on Mars, which also points toward a prior existence of life on Mars. It is, of course, still an open question how long a time this microbiological life has been present. The Mars Pathfinder mission will be able to answer this question.

Another possibility for life in our solar system exists on Europa, one of the moons of Jupiter (Sweinsdottir, 1997). Europa is characterised by a coverage of ice. It implies that there is plenty of water on Europa, which means that one of the important conditions for life is fulfilled. Some researchers (Sweinsdottir, 1997) suggest that the chance to find life on Europa is higher than on Mars. Europa has of course much less sunlight and the surface temperature is probably too low, but volcanic activity in the deeper parts of the oceans on Europa is very probable, and it could provide the needed low-entropy energy for the formation and maintenance of life.

The probable number of civilisations, N, which could be detected in our Galaxy is expressed by the Drakes equation:

What do all these factors mean? ne is the number of planets that have an appropriate distance from the star to ensure a suitable temperature, i.e. that water is mainly in the liquid form (see also Section 3.7). Astronomers believe that this number is about 1.0. The value of a relative number of stars with planets fp < 0.5. The probabilityf that life is formed under these temperature conditions in accordance with our discussion above is estimated as 1.0. This is because life is a result of low-entropy energy flow and the right temperature and the presence of the elements that are important for life. The value offi is the probability that the right conditions have been maintained a sufficiently long time to allow the entire evolution from the most simple life form to man. It is hardly possible to estimate this number very accurately, but we may estimate that 1 out of 100 planets would be able to maintain approximate life conditions for a very long period of time. fc is the probability that radiotelescopes are developed to receive signals from other intelligent lifeforms (it is estimated to be 0.1). R is the number of stars formed per year, and L is the lifetime for advanced civilisations. R is about 1-10 according to astronomers; let us say 1.0. These estimations of the factors give that N = 0.0005L. There should, in other words, be a reasonable probability to find other civilisations in our Galaxy.

The Drakes equation is maybe too simple and does not consider other factors that are important for the evolution of life. For instance, it cannot be excluded that the size and distance of our moon from Earth are of utmost importance for the rate of evolution. The changes on Earth caused by the gravity of the moon including the tide may be the disturbances that are needed to ensure a good mixing of the oceans and provoke challenges for the organisms to evolve. It cannot be excluded that the probability to have a moon of a suitable size and at a suitable distance, fm, is as low as 0.02.

Another important factor for evolution could be the catastrophes that have probably occurred 3-4 times during the last 500 million years due to a collision between Earth and an asteroid or a comet. From time to time—not too frequent and not too rare—it may be beneficial to start from a radically different situation to give new emergent life forms a chance to take over and come up with new and better solutions to survival. This is probably what happened (see also Section 11.8) when the dinosaurs were replaced by the mammals about 65 million years ago, when probably an asteroid collided with Earth and created a very difficult situation for the survival of the various life forms present on Earth at that time. The probability that a planet has a suitable frequency of catastrophes is dependent on the formation of the planet system. We do not know to what extent other sun systems have asteroids and comets in approximately the same amount as our sun system, fca, to provide a suitable frequency of catastrophes. It could therefore be necessary to estimate that it has only a probability of 0.1, although this estimation is very uncertain.

With this expansion of the Drakes equation:

we get N = 10~6 L, and the probability to find civilisations is reduced considerably, which may explain why we have still not got a radio signal from other civilisations. If L = 10,000 years to come up with a guess that is between very pessimistic and very optimistic guesses, the probability is as low as 10 2 to receive a radio signal—one that may even be from a civilisation very far away.

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