Small bodies of planetary size cannot contract to the point of reaching fusion temperatures because of mechanical incompressibility. The planets closest to the sun, i.e. Mercury, Venus, Earth and Mars are essentially composed of silicon and iron. The more remote ones, Jupiter, Saturn, Uranus, Neptune, Pluto are large and formed of fairly light elements such as water, hydrogen, methane and ammonia in solidified, frozen form.
The contemporary average distance of the earth’s orbit from the sun is about 150 million kilometers. This is the astronomical unit. The earth, upon condensation from the stellar dust, had initially a diameter that was perhaps only as little as half the diameter it presents now. As a consequence, this very dense mass had a gravitational field sufficient to keep to a very large extent the initial atmosphere, mainly composed of hydrogen. We still experience today, at the upper levels of the stratosphere, a loss of hydrogen amounting to several thousand tons a year. Other planetary bodies, for example the moon, have lost all the hydrogen as well as more heavy elements.
1.8.1 The sun
Stars that are newly formed are richer in heavy elements than old stars like the sun, yet they are subject to violent events. The majority of them are formed near the galactic center and a cosmic galactic wind originating from the galactic center blows at such high speeds that these stars are stripped of their outer envelope. In addition, they appear in clusters that are reshuffled and subject to collisions. Planets that might evolve around these stars must live short lives and those that last are subject to a galactic wind of such magnitude that they would be unable to keep a protecting atmosphere that would shield them from bombardments with electrolyzed particles. This would prevent any attempt at evolution toward complex living organisms on their surface.
At the local level, the sun is an enormous fusion reactor that consumes every second about 600 million tons of hydrogen (H2). It produces thereby 596 million tons of helium (He). The 4 million tons that are missing every second represent the solar radiations, i.e. the mass converted in energy. The matter lost in one million years represents only 0.000007% of the total mass of the sun. The type of electromagnetic radiations that flood the earth is by no means unimportant and we will talk about it infra. Yet, the sun bombards the earth with more than photons. The heat of the sun’s core is such that it ejects at all times some matter. This matter is constituted of electrical particles and makes up the solar wind that hits the earth at the rate of 300 million particles per second, with a mean speed of 400 km per second. This solar activity is permanent and is doubled by an activity that oscillates every 11.1 years. Indeed, in addition to heat, the sun also produces a magnetic field. The lines of force of this field escape from the North Pole of the sun and reenter at the South Pole. This solar magnetic field extends to 2.5 billion km, beyond Pluto, and is constituted of violent currents of electrically charged particles. Yet, the sun rotates upon itself and the differential rotation of the inner plasma and the outer layers upsets the magnetic field within the sun itself. Every 11.1 years, the field rearranges itself and its polarity changes, with the concurrent production of violent solar eruptions that provoke magnetic storms. The speed of these storms, also constituted mainly of protons, is about 10,000 km/second. The seats of the magnetic activity of the sun’s surface appear as dark solar spots. They are dark because they are colder (about 4,500°C) than the general surface temperature of the sun (5,800°C). The solar surface temperature oscillates thus in accordance with the solar magnetic cycle that lasts 11.1 years, and this oscillation influences the temperature of the earth (fig. 1.10).
Figure 1.10. A. The length of the "growing season" at Eskdalemuir in Central England (5.5°N; 3°W). This is the number of days in the year during which the averaged temperature exceeds 5.6°C. This number is not uniform throughout the years but varies in a wave-like fashion.
B. Yearly mean sunspot numbers. The length of the "growing season" and the mean number of sunspots are in strong concordance.
The magnetic storms themselves provoke shock waves that upset and heat up the earth’s troposphere. The earth is protected from direct hits by ionized particles because it is constituted of a metallic core and because it rotates upon itself. It creates thereby a magnetic field (the Van Allen belts) that entraps the majority of the electrical bombardment. Those particles that manage to hit the earth do so mainly on the North and South Poles where they create boreal and austral auroras. Without this magnetic field, solar wind and storms would have blown the atmosphere of the earth away, letting deadly UV light attain its surface and would further, by direct hits, have destroyed all life on the planet.
The cosmic rays coming from outer space are composed of particles much heavier than those emitted by the sun. This indicates that the sun may have been formed after the explosion of a supernova and also that, in other parts of the galaxy, the eclosion of life would have been much more difficult than nearer the sun, because the direct hits due to cosmic rays would be unbearable for life.
Astronomers can’t agree where the most energetic particles in the universe come from. The fiercest of these travelers slam into earth’s atmosphere with more than 100 exa-electron volts (100EeV=1020 eV). Each of these tiny particles, thought to be protons or helium and carbon, pack as much energy as a fastball from a professional baseball pitcher. It has been calculated in 1966 that particles above 40 EeV collide with the microwaves remnant heat from the big bang, losing rapidly their energy. Over a few million light-years, any particle traveling faster than 40 EeV will have been battered down below it. So, the Ultrahigh–energy cosmic rays detected on earth must come from no farther than 100 million light-years. But so far nobody knows their origin.
1.8.2 The Earth-Moon System
126.96.36.199 The origin of the moon
The origin of the moon has long been mysterious. Two hypotheses were initially advanced: a splitting off from the earth (the "fission" model) and the co-accretion model which holds that Earth and Moon grew up together. But the composition, the age and the size of the moon are incompatible with these hypotheses. The third hypothesis, first proposed in 1975, is a giant impact.
The size of the moon is abnormal. It is 1,740 km in diameter, big compared to the moons of Mars, which are 27 and 15 km large, respectively. Venus and Mercury have no satellite at all. The moon, with 73.5 billion tons, represents 1.23% of the terrestrial mass. The moon is supposed to be 4.56 billion years old, which is a birth date almost identical to that of the earth. The sun was initially surrounded by a multitude of smaller planets that coalesced and aggregated into the 9 planets now existing. Initially, a large number of small bodies floated around and impacts among them were frequent. It seems almost certain that about 30 to 60 million years after the formation of the earth, a Mars-sized planet smashed into Earth. Far from being a chance encounter, such an impact was almost bound to occur during the first 100 million years. At that time, the size of the earth was only two thirds of what it now is. The impact liquefied Earth’s surface. Under the impact, the iron core of the object penetrated into the core of the earth and was assimilated, while some pieces of the mantle of both planets were ejected into space. Hence the absence of iron in the moon. The heat generated by the collision was such that the volatile elements of the dispersed matter vaporized and were lost, which would explain the moon’s deficit in sodium and potassium. For such an event to occur, the speed of collision must be below 14 km/second and the relative sizes of the colliding objects must be such that neither complete assimilation nor complete loss of the foreign body be possible. A planet slightly larger than Mars would be just about right. After the collision, the mantle pieces would be ejected at once at a distance of about 4 to 8 earth radii, resulting in an earth that has grown therewith considerably in size, provided itself with an oversized satellite revolving at a close distance, while the dual system lies at a fairly close distance from the sun.
Is the explanation here given for the formation of the moon and the earth nucleus the correct one? M. Toboul (Nature, 450:1206-1209, 2007) established that the impact occurred 50 million years after the birth of the solar system. This chronology questions the hypothesis that the earth nucleus formed as a consequence of a gigantic impact.
Strong interactions have taken place in the course of times and controlled the evolution of the dual system.
The tides raised by the planet earth on the moon -because tides are felt not only on oceans but also on solid bodies- have had the effect of bringing the spinning of the moon to such a state that the time of the satellite’s rotation upon itself is equal to its period of revolution around the planet. The moon will thus always present the same face to the earth, just like Mercury and Venus present always theirs to the sun since they also have been despun by the action of the sun. At the present distance from the earth, this despinning of the moon from a much faster initial rotation would have taken no longer than 10 million years.
The tides raised by the moon on the planet earth apply to solid bodies, to the atmosphere and most spectacularly to shallow waters. Tidal currents flow over the bottom of oceans and create a turbulent boundary layer. When ocean waves approach a sloping beach, an ever-diminishing depth of water carries their energy. In response, their amplitude augments to the point of formation of surf and sometimes also "tidal waves".
The mechanical energy of the movements of water is dissipated into heat, which is lost. Besides this dissipation of gravitational energy into heat, there takes place another phenomenon: the delay in response due to friction on solid bodies as well as on water (fig. 1.11). This friction again is dissipated by the production of heat.
Figure 1.11. Tidal bulges are lagging three degrees in phase because of friction. The lagging tides are carried ahead of the moon by the rotation of the earth. The view is from the pole.
The fact that the tidal bulges of the earth are not in line with the gravitational force of the moon has a twisting force on the satellite, which tends to orbit in a circular fashion. The twisting force also gives the moon an additional angular momentum thanks to which the lunar orbit is slowly expanding, by about 3 centimeters a year (fig. 1.12).
Figure 1.12. The moon’s orbit is an ellipse. The impulsive addition of angular momentum at the moon’ s closest approach to earth changes the orbit to a larger, more eccentric ellipse.
Since the orbit now expands, the moon must have been closer earlier in time. An analysis of the daily increments per yearly bands of growth ridges in fossil corals confirms that the earth had a day length of 22 hours, 380 million years ago. The length of the days thus increased during the last 380 million years by approximately 20 microseconds per year. The moon would have been in those days at a distance of 58 earth-radii instead of the present 60.25.
If we assume that the phase lag of 3° of an arc between gravitational force and tides has been constant in the course of time, then the moon was very near the earth less than 2 billion years ago. This fits with changes in the earth’s lithosphere that seems to have broken up into various continents around that time, while mountain building could be initiated. It is now believed that the moon was close to the earth, i.e. 10 earth-radii, about 4.5 billion years ago. At that time, the earth’s day had a length of about 2.6 to 5 hours. Tides several kilometers in height occurred. The energy dissipated by tidal friction exceeded, very probably, the solar heating of the earth. Much of the ocean water might have then been in the vapor state and even the mantle of the earth might have been partially molten. After a few thousand years, these extraordinary events would have subsided. This is only a hypothesis. It may very well be that there was no water on earth at that time, as I will expose infra.
In the future, the moon will continue to recede from the earth, until it reaches about 75 earth-radii. The length of the days on earth will increase, to reach a stage where the earth’s day and the lunar month will be equal.
Another very important influence of the moon is its effect on the atmosphere of the earth. Atmospheric gravity waves propagate upwards or downwards. A turbulence, wind or even nuclear detonations at the lowest level of the atmosphere can propagate upward. The density of the atmosphere decreases however continuously with an increase in height. As a result, the energy carried by an upward-moving wave is carried by fewer and fewer molecules, the higher the wave reaches into regions of different gas density. The fewer gas molecules carrying the energy can do so only if they oscillate with greater amplitude. Thus such atmospheric gravity waves tend, like surf, to grow stronger the further they rise above their source. The dissipation of atmospheric gravity waves leads to a heating of the atmosphere, which sometimes is at least comparable with the heating provided by solar radiation at ionospheric levels. A second not negligible effect is that the turbulence maintains the atmosphere in a chemically mixed state to heights of about 100 kilometers. With its cessation, the heavier species of molecules such as nitrogen rapidly diminish their concentration while the lighter species such as oxygen atoms increase in relative concentration. This point is very important for the achievement of life on earth, since it has allowed the creation of the ozone layer.
1.8.3 The Earth’s atmosphere
There are ways to analyze the composition of the primitive nebula that formed the sun, and to compare it with the actual atmosphere of the earth. They are different. This indicates that the probability for the atmosphere of the earth to derive from the nebula is small. The initial atmosphere of the earth was very dense and opaque. The density of this atmosphere must have impeded the dissipation of the terrestrial heat. In those days, the UV radiations and the solar wind originating from the sun were much more intense than now and they eliminated the first terrestrial gaseous envelope. Yet, the first, oldest sedimentary rocks (the metamorphic rocks of Isua, in Greenland) date from about 3.8 x 109 years. This indicates that, by that time, there existed oceans large enough to allow their formation. Since the earth originated 4.5 x 109 years ago, it must be assumed that, between these two dates, an intense and abrupt de-gassing occurred, whereby about 80% of the volatile matter entrapped in the terrestrial crust was released. The impact that formed the moon may have been the cause of it. The residual de-gassing still takes place by the activity of volcanoes. Since the Precambrian rocks and the rocks formed at later times are of fairly similar composition, one can deduct that the Precambrian oceans were almost identical to the oceans now existing. The conclusion is that the secondary atmosphere that occurred by an intense de-gassing that took place 500 million years after the earth formation did not differ greatly from what it is today. This abrupt de-gassing was composed of water (H2O), carbon dioxide (CO2), and nitrogen (N2) and was devoid of molecular oxygen (O2).
The solar bombardment at that time must also have been considerably attenuated since the earth was able to keep its secondary atmosphere. One way to explain this is that, at the time of the earth’s formation, 4.5 billion years ago, there existed no metallic core. But such a metallic nucleus (perhaps due to the impact that created the moon) existed 4 billion years ago, forming a magnetic field that could trap this solar wind. We have seen earlier that the Proto-Earth was supposed to have endured frequent impacts from small celestial bodies. On the basis of this hypothesis, the swift formation of oceans by an abrupt de-gassing may be disputed. It has been advanced that the primeval Earth was dry and devoid of free water. At the moment of its formation, the earth contained about 2% H2O. Most of this water may have been lost from earth since then, or else may still be stored in Earth’s deep interior. The inclusion of water in the mantle occurs at high pressure and high temperature with a global efficiency of 0.2%. When this capacity is integrated over the mass of the lower mantle, the total mass of water is roughly 5 times that of the oceans7.
The young planet was surrounded by a swarm of smaller bodies consisting mainly of comets. These would have had a mean diameter of 10 meters, traveling at about 30 kilometers per second at 5 to 10,000 kilometers above the earth. The continuous bombardment of the earth by these comets that were constituted mainly of ice would have contributed annually to about 0.4 mm of rainfall. Over the earth’s lifetime, this is enough to account for all the water in all the oceans. Water, according to this hypothesis, is a secondary acquisition of the earth, originating from Outer Space. Water would have accumulated slowly in the course of time and oceans would have formed gradually.
1.8.4 The earth’s surface temperature
The present surface temperature of the earth represents a balance between the sunlight that falls on the planet and the thermal emission that leaves it. The mean temperature of earth is now between 286°K and 288°K (about 14°C). Such a high temperature is due to a greenhouse effect: constituents of the atmosphere absorb the departing thermal emissions but do not interfere with the arrival of the sun’s rays.
The luminosity of the sun has increased by about 40% in geologic times. If one assumes conservatively that the sun’s luminosity was 30% less at the origin of the earth, then, also assuming that the composition of the earth’s atmosphere was identical to what it now is, the temperature of the earth would have been below seawater freezing point less than 2.3 billion years ago. This runs counter to geological and paleontological evidence, since sedimentary rocks and bacteria have been found existing at times earlier than 2 billion years. This cold temperature may have been counterbalanced by the action of the moon. Another parameter to consider is the history of hydrogen during the early times. A possible high concentration of about 1 bar (1 bar =106 Dyns/cm2 = 0.987 atmosphere) of pressure of this light element may have been present. Such a high concentration would have ended about 500 million years later by its dispersion into space. In this case, however, the retention of the thermal emission would have been of such magnitude that the temperature on the earth would have been above the normal boiling point of water until about 3.5 billion years ago. This is quite close to the time when bacteria appeared. In view of the thermostability of the prokaryotes, this is, however, not inconceivable.
An alternative to the hydrogen hypothesis is that the greenhouse effect was produced by the presence of small amounts of ammonia (NH3). Indeed, burgeoning life would be favored by the presence of this slightly reducing element that is a component of living matter. Also, this gas would protect the newly formed structures from dissociation by lethal UV light, in a way reminiscent of the action of the ozone layer now.
The burgeoning life as well as photodissociation would have then slowly consumed this ammonia, while the sun’s luminosity increased in the meantime. By this dual process, the atmosphere of the earth would have maintained at all times a temperature suitable for the appearance and evolution of life on earth. The subsequent total disappearance of ammonia is most likely due to oxidation by the oxygen produced abiotically by UV radiations and, later, by plant photosynthesis. This oxygen appeared in significant amounts about 2 billion years ago and resulted in a cooling-off of the earth that may have been very significant.
The postulate of high levels of atmospheric ammonia presents problems because the sunlight-sensitive ammonia would have required a methane haze for protection. That haze would, however, have cooled earth as much as the ammonia greenhouse could warm it. Another hypothesis is the Gaia hypothesis that a methane greenhouse was spawned by life itself: in the absence of oxygen, methane produced by ancient methanogenic bacteria could have reached levels 1000 times higher than today’s. By exuding a methane blanket with a little carbon dioxide, which would prevent the formation of a cooling methane haze, life itself could have warmed the frigid world to within a few degrees of its current temperature.
The three essential components of the atmosphere of the earth that have a direct influence on its surface temperature, are water, ozone and carbon dioxide. Changes are further introduced by the release of primitive gasses of volcanic origin.
1. Water. Water is a very peculiar element endowed with at least 3 characteristics not found with other similar compounds. Let us compare 3 hydrides of two similar atoms, oxygen and nitrogen: H2O (water), H2O2 (hydrogen peroxide) and H4N2 (hydrazine). All three freeze and boil at about the same temperature. However, whereas the density of these liquids continually increase with decrease in temperature, that of water reaches a maximum at 4°C and then decreases between 4°C and 0°C. This means that once the temperature reaches 4°C, further cooling causes the colder water to rise to the surface and eventually solidify as ice at the surface. This phenomenon has allowed life to exist at low temperatures in cold lakes and rivers. Up to now, only 3 other compounds besides water have been found to present the same behavior. A second abnormal property of water is its viscosity: usually, the more a liquid is compressed, the more viscous it becomes. This is not true for water compressed under 50°C: it becomes less viscous. This allows life to thrive in the depths of oceans. Finally, there is the strange fact that water may absorb a large amount of heat without showing much increase in temperature. This is not true for other liquids: in order to increase by 1°C the temperature of a gram of water, about twice as much more heat is necessary to raise the temperature of a gram of benzene. Also, water is the solvent par excellence of ionized substances such as salts and phosphates, and dissolving gases such as CO2 and O2, especially at cold temperatures. All these peculiar properties, restricted only to H2O, make the presence of this element on the planet an important factor in the further evolution of the elements towards life.
Water appears to be abundant. If spread uniformly on the earth’s surface it would provide a layer 3 km thick. Yet, considering the diameter of the earth, this represents in fact only a thin layer of moisture. It is believed that, in the early days, the diameter of the earth, which is now 12,740 km, was about 50% less. Considering that an increase in diameter of a sphere by 20 % already amounts to a twofold increase in volume, one may imagine how dense the primeval earth was. In those days, the layer of water was 8 km thick. That is, if water was present. At the outer limit, the splitting of water into its oxygen and hydrogen constituents, followed by the loss of the light hydrogen element into outer space, would have provided for a constantly slightly oxidant atmosphere. Yet, a reducing atmosphere was required for the development of life.
Water represents now 0.022 % of the total mass of the earth. More of it would have prevented the emergence of life from the primeval waters since no dry land would have been available, and less of it would have hindered the development of life itself. Water was probably discharged as a component of the secondary atmosphere, 500 million years after the formation of the earth or else was acquired from outer space. This lends credibility to the hypothesis that the first organic synthesis that led to the appearance of life was accomplished not in water but among clay particles.
2. Ozone. A second important element is ozone. As soon as free oxygen began to be produced from CO2, the solar ultraviolet radiations also began to produce ozone. Ozone is toxic, and even deadly, to primitive microorganisms. These have devised a mechanism for the destruction of ozone. However, when ozone is separated from the earth’s surface by the dense inferior atmosphere, as is the case today, the same ozone protects life from the intensity of UV solar radiations. It is not unlikely that, right from the beginning of ozone production, it was restricted to the upper atmosphere. The mutagenic effects of unfiltered solar radiations on the microorganisms from soil and oceans could very well endanger the survival of the whole of the biosphere. These mutagenic and carcinogenic effects increase proportionally two times with the decrease of ozone, so that a reduction of ozone by 3% increases skin cancers by 6 %. The totality of the atmospheric ozone, if compressed to the ground level, would amount to a 3 mm thick layer. Yet, it is now concentrated in a layer that is 25 km above earth, in the stratosphere.
3. Carbon dioxide (CO2). The complex equilibrium of ozone production and destruction is further influenced by CO2. Near the earth, in the troposphere, i.e. no higher than 18 km, CO2 has a greenhouse effect. Higher up, it has a cooling effect and protects ozone from destruction. The more fossil sources of energy are used, the more likely is the level of ozone to increase. After the destruction of ozone that occurred around 1960 due to nuclear experiments, ozone levels have increased up to 1972 and the level remained thereafter stationary i.e. in balance between destruction and production.
The CO2 emissions are ubiquitous. European estuaries release between 3.8 and 18 grams of CO2 per square meter per day. The surface of European estuaries is about 111,200 km2, excluding the Baltic Sea, and the emission of carbon by these estuaries is estimated to be between 30 and 60 million tons of carbon per year. The Amazon River and the Niger emit about 18 grams of CO2 per square meter per day. In developing countries, the overpopulation may be the cause of even larger emissions of organic carbon. Human activity produces each year about 7.1 x 1015 grams of carbon. Less than half of it stays in the atmosphere: atmospheric carbon dioxide increased at a rate of only 2.8 x 1015 grams of carbon per year, between 1988 and 1992. However, only about 2 x 1015 grams of the released carbon dioxide go to the oceans. About 2.2 x 1015 grams vanish into the land, probably taken up by plants during photosynthesis.
4. Volcanoes. However, not only man influences the almost perfect equilibrium ultimately reached between the earth, the air and life. Sometimes, an instantaneous injection of primitive gases (methane (CH4), sulfur dioxide (SO2), ammonia (NH3), and carbon dioxide (CO2)) perturbs the composition of the atmosphere now obtained (about 4/5 of nitrogen, 1/5 of oxygen, one percent of argon, a few thousands of carbon dioxide and a variable amount of water).
Explosive volcanoes significantly influence the climate. This is achieved not by the clouds of dust that dampen the sun’s rays but with invisible gases, among them sulfur dioxide. This gas is transformed in the high atmosphere into clouds of sulfuric acid, whose half-life is about one year. This acid aerosol cools the earth down and acidifies the waters.
These volcanic eruptions are by no means rare. In the last 85 years, there have occurred 1,700 eruptions. The explosive eruptions (of the St. Helen and Krakatoa type) are totally different to those that extrude liquid magma. The latter influence the climate only in a negligible way. The explosive eruptions influence the climate differently according to the location of the explosion: the quantity of volcanic aerosol reaching the stratosphere is twice as important in arctic regions than in temperate or equatorial ones. An eruption occurring in January in the Northern Hemisphere provokes a drop in temperature of -1.4°C within 2 months.
As the sun-earth system continues to evolve, the temperature of the earth will increase until a runaway greenhouse effect occurs. This will happen between 3 and 4.5 billion years from now, when there will occur an atmospheric pressure of 300 bars of steam. Of more immediate importance is the fact that we seem to be right now at the end of an interglaciary period (figure 1.13), although this assertion is disputable, as I will expose later.
Figure 1.13. An extrapolated temperature curve for the surface water of the central Caribbean Sea suggests that we are at the end of an interglaciary period. In the meantime, man is changing the composition of the atmosphere in a way that could work in synergy with natural causes. This may lead either to a runaway greenhouse effect, as we observe it today, or on the contrary lead to an accentuation of the coming cold wave. This could occur as early as within the next 200 to 300 years. This topic will be discussed later in this essay.
The appearance and evolution of life as we know it on earth is, however, dependent not only on temperature but also on the quality of the electro-magnetic radiations on which ride photons, that irradiate the planet.
1.8.5 Electromagnetic radiations on earth
The electro-magnetic radiations on which ride the photons range from gamma rays, whose wavelength can be as short as 0.001 angstroms (one tenth billion of a centimeter), to radiowaves that may have a wavelength of 1 kilometer or more. All these radiations are not equally suited for the performance of chemical reactions by photons. The work (E) a photon can accomplish is inversely proportional (h) to its wavelength (υ). This was discovered by Max Planck (E = hυ ). At long wavelengths, the chemical effectiveness of an electromagnetic radiation is simply nonexistent. All ordinary chemical reactions ("dark" reactions) need a radiation of wavelength comprised between 1,900 and 440 millimicrons (a millimicron is tenth million of a centimeter). The energy needed to activate these chemical reactions is acquired in collisions with other molecules, i.e. heat. Chemical reactions leading to the formation of elementary molecules such as water, ammonia, prussic acid, etc. are usually restricted to regions in space located near sources of infrared radiations.
In other reactions, however, called photochemical reactions, light supplies immediately the energy of activation. The wavelengths most useful in this photochemistry are comprised between 1,430 and 280 millimicrons. However, radiations shorter than 300 millimicrons, though very effective in photochemistry, are incompatible with higher forms of life. This is because life utilizes chemical structures that are extremely long, highly organized and very delicate, such as proteins and nucleic acids. These molecules usually function only when held in a very specific configuration that radiations of wavelengths shorter than 300 millimicrons, in their effectiveness, destroy. This has disastrous consequences for a living cell. The appearance of rudimentary forms of life thus requires a narrow band of electromagnetic radiations, and this band has still to be reduced further for the evolution of higher living forms.
Once higher forms of life were available on earth, the presence of light was exploited in two ways. The first was photosynthesis. The problem here is to use a minimum of photons to perform the task. Most green plants achieved this with chlorophyll. The second was vision, which hinges on differential sensitivity: on how to be able to see in glaring noonday light and also in starlight. Three animal phyla developed well-formed eyes based on carotenoids. These phyla are the arthropods (crabs, spiders, and insects), the mollusks (squid) and the vertebrates. The range of wavelengths allowing vision and photosynthesis is still narrower than the one encompassing photochemistry. It extends from 300 to 1,050 millimicrons (fig. 1.14).
Figure 1.14. The bulk of the wavelengths carrying the photons of the sun towards the earth are adequate for chemical reactions as well as for photochemistry.
The availability of the proper range of wavelengths is crucial in deciding whether living organisms can develop in useful ways. This is very probably as applicable everywhere in the Universe as on earth. A planet without a range of radiations between 300 and 1,100 millimicrons would virtually lack photochemistry and, a fortiori, photosynthesis because only this range is suitable for the performance of the tasks demanded. There cannot be a planet on which photosynthesis or vision can occur in the far infrared or the far ultra-violet because these radiations are inappropriate to perform these functions.
We live upon a very fortunate planet because the radiations that are useful in promoting orderly chemical reactions comprise about 75% of that of the sun. The longer wavelengths are sharply reduced by water vapor, CO2 and ozone in the atmosphere and even much more so by liquid water. The short wavelengths are reduced by the ozone layer of the atmosphere, which becomes opaque to wavelengths of 290 millimicrons and below.
Do extra-solar planets exist? Galileo Galilei discovered that Jupiter had a satellite because he could observe the planet with a telescope, instrument that his colleagues of former times did not have. He therewith showed that the earth is not the center of the Universe. Likewise, planets could not be observed until 1995 because neither the instruments needed to detect their presence nor the methods applicable to their detection (one must have an instrument able to measure the Doppler shifts in the spectra of light from the planets’ stars; one must also be aware that the Doppler shifts arise as the orbiting planets exert a gravitational pull on the stars and tug them in different directions) were available. In September AD 2002, the total number of known extra solar planets was 101. The methods used to detect the planets are such that only very heavy objects orbiting very close to their star are detectable. The lightest extra planet yet discovered is 30 times heavier than earth, orbiting around its star in 4 days, at a distance of 0.06 astronomic units. The astronomical unit is the average distance of the earth’s orbit from the sun, about 150 million kilometers. The closeness of these planets to their star precludes the apparition of life there. The existence of planets similar to earth is by no means excluded. Some were found in 2007.
Is there intelligent life on other planets? Sixty trillion observations with the radio telescope have been screened at Harvard for alien broadcasts. Only 11 signals were identified that could not be explained by natural sources or human interference. But nine of these did not come from space neighbors either, because they were only spotted once whereas an alien broadcast is expected to be continuous. Two of the signals have not yet been followed up, leaving a chance that one or both represent alien transmissions.