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An alternative theory to the methane/ammonia hypothesis, which also arose during the 1970s, focused on the possible role of carbon dioxide as a means of compensating for the dimness of the faint early sun. Carbon dioxide, like methane and ammonia, is capable of absorbing infrared energy being radiated from the surface of the Earth and, as such, is a greenhouse gas.

"For reasons closely related to the elimination of methane from the theoretical picture, carbon dioxide became a strong candidate for providing a means of compensating for the coolness of the faint early sun. More specifically, when methane is oxidized by the presence of [OH] radicals created through the ultraviolet photolysis of water vapor, carbon dioxide is a product.

"Thus, the oxidation of much of the methane in the early Archean era is considered by many researchers to be a good candidate for helping to generate a considerable amount of carbon dioxide. To this, one can add the substantial portions of volcanic emissions which consist of carbon dioxide.

"In modern times, currently active volcanoes have been estimated to release some 4 x 10¹⁰ kilograms of carbon per year. Most of this is in the form of CO₂.

"One reasonably could assume that the amount of carbon dioxide released through volcanic activity during the Archean era was, undoubtedly, far greater than is the case today. Nevertheless, almost any estimates one came up with in this regard would be both speculative and arbitrary to a large extent.

"Furthermore, how much of this outgassed carbon dioxide would have remained in the atmosphere during the Archean era depends on the amount of this material which would have entered into solution with the ocean, as well as on the amount of carbon dioxide which became incorporated into inorganic carbonate formations such as limestone. Unfortunately, knowing the amounts of carbon dioxide which are in any given form - gas, solid or liquid - at any given time is fairly difficult to pin down with any precision in the best of times, let alone some 4 billion years ago.

"Estimates of the amount of carbon dioxide in the atmosphere during the Archean era vary over a wide set of possibilities. Some people believe the amount of carbon dioxide in the prebiotic atmosphere rapidly decreased during the Archean era and remained at relatively low levels thereafter. Other researchers maintain the amount of carbon dioxide at the beginning of the Archean era was high and continued to remain relatively high for some time.

"Among those theorists who contend the amount of carbon dioxide in the atmosphere was fairly substantial, there are again differences in projected amounts. There are researchers who indicate there may have been as much as 100 bars, or 100 standard atmospheres, worth of carbon dioxide gas in the Archean atmosphere. Others suggest the amount of carbon dioxide in the ancient atmosphere may have been between 10 and 20 bars or standard atmospheres.

"A 100 bar atmosphere of carbon dioxide would result in surface temperatures of about 230 degrees Celsius. With a more modest 10 to 20 bars of atmosphere, the Earth's surface temperature is likely to have ranged between, say, 85 and 110 degrees Celsius.

"Both of these scenarios would create surface conditions capable of compensating for the coolness of the faint early sun, thereby eliminating the paradox created by the existence of sedimentary rocks and fossil evidence. Furthermore, even the 100 bar carbon dioxide atmosphere would not necessarily generate temperatures that automatically lead to a runaway greenhouse effect in which all of the surface waters would boil away and be present in the form of clouds or steam.

"The saturation water vapor pressure under such circumstances would be about 30 bars, or so. Consequently, when one adds this to the existing 100 bars of pressure of carbon dioxide, the temperature would have to be raised another 100 degrees before the ocean would start to boil under that kind of pressure."

"Didn't you indicate, Dr. Yardley, that the impact of a meteorite somewhat larger than one capable of vaporizing the photic zone of the ocean would generate a transient rise in temperature of 100 degrees?" asked Mr. Tappin.

"Yes," the professor confirmed.

"So," the defense lawyer suggested, "in the context of a 100 bar carbon dioxide atmosphere, the impact of a meteorite smaller than the one which created the Yucatan crater might be capable of triggering a runaway greenhouse effect?"

"Possibly," stated the professor. "The actual outcome might depend on a lot of different factors."

"Alright, Dr. Yardley, lets see if I have this right," Mr. Tappin said.

"Firstly, the early sun is thought to have generated 25-30 percent less luminosity than the sun of today. Under current circumstances, a sun this dim would have resulted in the freezing, among other things, of the oceans to a depth of 300 meters.

"Secondly, the 'big freeze' could have been avoided by an atmosphere with the right kind of compositional character. In other words, the faint early sun paradox could be avoided if the Earth's atmosphere contained enough greenhouse gases to be able to, first, absorb from the Earth, and, then, radiate back to the planet, sufficient levels of infrared energy to compensate for the 25-30 percent dimmer luminosity of the early sun.

"Thirdly, there are, in broad terms, two competing theories concerning the compositional make-up of the Archean era atmosphere. One theory champions methane and ammonia as the greenhouse gases of choice, while the alternative theory advocates carbon dioxide.

"Fourthly, in neither theory do we know, except in very broad terms, what the precise character of the composition, temperature or pressure of the Archean era atmosphere was. On the other hand, in both cases, there would have been enough infrared radiation absorbed and re-emitted by the respective gases of each theory to counter the cooling effects of the faint early sun.

"Finally, there are substantial arguments for, and against, each of the competing theories. Dr. Yardley, does my brief summary capture the gist of the matter vis-a-vis the faint early sun paradox?" inquired Mr. Tappin.

"Yes," acknowledged the professor, "I would say you have captured all of the highlights."

"Is there any preference among researchers between either of the two theories outlined by you, Dr. Yardley?" asked the defense lawyer.

"The early preference," noted the professor, "had been for the methane/ammonia hypothesis. Relatively, recently, however, the preference scales have been tipping rather heavily in the direction of the carbon dioxide perspective."

"Does anything rest on these preferences beyond resolving the faint early sun paradox issue?" wondered Mr. Tappin.

"Quite a bit, actually," stated the professor. "The methane/ammonia hypothesis is far more conducive to providing plausible accounts for the evolution of prebiotic systems than is the carbon dioxide hypothesis.

"The methane/ammonia atmosphere constitutes a reducing environment. Due to the way this kind of atmosphere provides an environment which is conducive to chemical reactions believed to be capable of leading to increasingly complex organic molecular forms, a methane/ammonia atmosphere lends fundamental support to the emergence, eventually, of a variety of biologically important complex hydrocarbons such as amino acids, purines, pyrimidines, ribose sugars and so on.

"On the other hand, a carbon dioxide atmosphere is, at best - and depending on what other molecules are considered to be present in such an atmosphere, only slightly reducing, and, therefore, much less conducive, and, perhaps, even antagonistic, to the gradual build up of the increasingly complex molecular forms required by evolutionary theory. In general, the more hydrogen gas there is postulated to be in a carbon dioxide dominated atmosphere, the greater will be, up to a point, the reducing character of that atmosphere. Alternatively, the more the ratio of [H₂] to [CO₂] falls away from 1, the less reducing will such an atmosphere be.

"Many researchers believe nitrogen, not hydrogen, was the most common gas next to carbon dioxide in the Archean era atmosphere. A nitrogen/carbon dioxide dominated atmosphere would have been either neutral or, possibly, according to some researchers, quite reactive with a propensity to break down , rather than build up, more complex hydrocarbons such as amino acids.

"As a matter of fact," pointed out Dr. Yardley, "this problematic dimension of a carbon dioxide dominated atmosphere inspired a couple of theorists, just a few years ago - 1994, I think - to develop another approach to the faint early sun paradox. In effect, these researchers seemed to feel there was no need to try to find ways of compensating for the cooling effect of a faint early sun.

"The starting point for their theory is to assume the Earth froze as a result of the early sun's 25-30 percent lower luminosity. The freezing would have created a 300 meter thick layer of ice near the ocean's surface.

"According to the architects of this theory, the layer of frozen ice would have served to protect chemical activity going on in the water below the frozen zone. In addition, the cold, but unfrozen, ocean water would have helped to preserve whatever organic molecules were formed since the decomposition of organic molecules is slower at these lower temperatures.

"In addition, these theorists allowed for the influx of large meteorites every million years or so. These large scale impacts would have melted the ice and helped stir things up, so to speak, in a variety of ways involving shock-synthesis of various hydrocarbons, mixing of organic materials, energy distribution and so on."

"How can one be sure," queried Mr. Tappin, "there would have been any ocean at all beneath the frozen zone, or if there were liquid ocean water below such a zone, how would one know how deep the water would be? If the chill caused by the faint early sun was present from the very beginning of the Archean era, then how would this affect the formation of the ocean?"

"The answer to your question," remarked Dr. Yardley, "would depend on a lot of different factors. For instance, scientists believe the process of core formation is likely to have raised the overall temperature of the planet to some 1500 degrees Celsius.

"Obviously, things would have to cool down considerably before lasting bodies of water could have begun to form on the surface. Before this point had been reached, there probably would be a time when the water being released into the atmosphere as a by-product of the core formation process would exceed the saturation level for water vapor in the atmosphere.

"The precise character of this saturation level would depend on things such as atmospheric temperature, pressure and composition. Once such a level was exceeded, then, for a time, there probably would have been a rapid precipitation and evaporation cycle in which water would not have collected on the surface, but humidity would have been quite pronounced.

"At some juncture, surface temperature, as well as atmospheric composition, temperature and pressure, along with water formation and precipitation would have collaborated to create conditions conducive to the generation of relatively stable bodies of water. How one factors a faint early sun into this process of ocean formation is rather difficult to say because so many of the variables being considered are uncertain.

"I'm sure a number of computer models concerning the nature of ocean formation in the Archean era have been developed. Depending on starting assumptions, different models likely would designate different depths of water as the point which would have to be reached before a frozen layer starts to form.

"Hydrothermal vents and volcanic activity also would have to be thrown into the mix since they both are capable of affecting water temperature, locally and, possibly, even globally. With each new variable which is added, the model becomes more complicated and, consequently, providing an answer to your question is less and less straightforward."

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