[The text below is an already submitted research proposal aiming to study the origins of life utilizing the DECLIC experiment on board the spatial station.]
Early attempts at the problem.
According to Maynard Smith and Eörs Szathmary (1), the first serious proposal about the origin of life is due to A. I. Oparin (1924) and J. B. S. Haldane (1929). They argued that, if the primitive atmosphere lacked free oxygen, a wide range of organic compounds might be synthesized, using energy from ultraviolet light and lightning discharges.
In 1953, under the advice of Harold Urey, Stanley Miller tested the hypothesis by passing an electric discharge through a chamber containing water, methane and ammonia. A wide range of organic compounds were produced, including components of the nucleotides of which RNA and DNA are made.
However, some essential molecules were found to be absent or in very low concentration. More importantly, there was a lack of specificity in the reactions that take place, making difficult to understand how polymers, linked to specific chemical bonds, could have formed.
In a series of papers published between 1988 and 1992, Günter Wächtershäuser suggested that reactions may have taken place between ions bonded to a charged surface. Because opposite charges attract one another, ions in solution become attached to charged surfaces. They are free to move slowly accross the surface, while maintaining a constant orientation, greatly increasing both the speed and the specificity of the occuring chemical reactions.
Recently, researchers have demonstrated that confining molecules in small droplets can strongly enhance the rate of chemical reactions, suggesting application to prebiotic chemistry (2). These results point toward hydrothermal vents as a possible origin of life, but no mention is made of the critical point of water (3).
Self-organization and criticality
During the last 50 years, there has been growing evidence that self-organization processes take place when attraction forces are balanced against repulsive forces. They are of the same nature as the continuous phase transition one observes in a fluid in the state of critical opalescence at the so-called critical temperature. This analogy was first recognized by Per Bak et al. (4), in relation to the ubiquitous presence of the so-called 1/f noise. They called this process «self-organized criticality» (4).
A typical astrophysical example is star formation. The Jeans instability through which stars form is indeed similar to that which causes critical opalescence. In both cases, density fluctuations follow a power law (the so-called 1/f noise), as evidenced by the distribution of the initial mass function of new-born stars.
In his book entitled «The Self-Organizing Universe» (5) Erich Jantsh has shown that the whole universe self-organizes following similar sequences of events. A «macroevolution» during which large structures condense alternate with a «microevolution» during which new elementary components are formed. A summary of this process is shown on fig. 1. According to this scheme, star formation is part of the macroevolution. It triggers the formation of new atoms such as helium which are heavier than hydrogen. The formation of helium is part of the microevolution.
Fig. 1. The self-organization of the universe according to Eric Jantsch (1980).
Following Per Bak, we can view Jantsh’s macroevolution as a continuous phase transition and his microevolution as an abrupt phase transition, that is the evolution of the whole universe can be viewed as a process circling around a «critical point» (see Fig. 2).
Self-organization and energy dissipation
Ilya Prigogine has shown that self-organisation is a characteristic of dissipative structures, that is structures that spontaneously appear under a permanent flow of energy. Examples of dissipative structures include Bénard convective cells as well as living organisms.
Dissipative stuctures behave like heat engines: they use temperature differences to produce mechanical work. According to Carnot’s second principle of thermodynamics, this can only be achieved through cycles of transformations. Early heat engines used the transition of water from liquid to vapour to produce large volume variations.
Modern car engines are more efficient because they use much larger temperature differences to produce the same volume variations. However, much smaller temperature differences are sufficient to produce natural heat engines such as Bénard cells. This is especially true near a critical point where very small temperature differences produce very large volume variations.
The critical point of water
The critical pressure of water is 220 bars and its critical temperature is 374° C. In salted water, like the ocean, water becomes critical somewhat deeper than 2.200 m, whereas, in hydrothermal vents, the temperature easily reach and often exceeds 374° C.
Let us consider water in a deep hydrothermal vent well below 2.200m. It is heated somewhat above 374° C. Because its density is lower than that of the surrounging water, it forms convective plumes. As it moves upward, its pressure decreases below the critical pressure. Its temperature stays for a while above that of the environment before it cools down and sinks back toward the hydrothermal vents, closing the convective loop. At some point, the water becomes subcritical and condenses into droplets. Liquid water is then slowly and continuously converted back into gas without ever forming gas bubbles.
Fig. 2. The above surface shows the state of water around its critical point. The gray area shows the condensation zone.
Fig. 2 shows the state of water in a convective plume while it cirles around its critical point, as indicated by the arrow. Whereas the transition from liquid to gas is continuous, the transition from gas to liquid is abrupt. Periodically, water condenses forming very small droplets of liquid water. These droplets grow until water becomes entirely liquid. Then, it sinks toward a hydrothermal vent where it becomes supercritical. Later it is continuously transformed into gas, without ever forming gas bubbles.
The condensation of a gas into a liquid near its critical point is called critical opalescence. Very large density fluctuations are observed, a condition favorable to the formation of very small droplets. In the ocean, other molecules may condense as well. Polar molecules will keep the same orientation with respect to the surface of the droplet, thus favoring polar bonds. These conditions are particularly favourable to the formation of complex organic molecules.
A possible test for the origin of life
Although the above described conditions are suitable to the formation of complex organic molecules, the probability of occurence of such reactions remains quite small, unless the same situation repeatedly occurs during a very long amount of time.
One can roughly estimate the time for water to circle around a convective plume to be of the order of days, whereas the life time of an active submarine volcanoe may be of the order of a million year. Hence the same conditions may have repeatedly occured several million hundred times. Clearly, if one wants to reproduce this process in a laboratory, it must be considerably sped up.
The DECLIC experiment offers such an opportunity. DECLIC is an experiment on board of the international space station. One version is aimedare studying chemical reactions near the critical point of water. Its gravity-free environment allows the experiment to produce uniform critical conditions inside its whole volume within a three decimal accuracy. One should be able to set up these conditions so as to precisely circle around the critical point of water within a few seconds instead of a few days. Compared to the conditions at the origin of life, this would increase the speed of the process by at least a factor 105, probably much more because the whole experiment would constantly take place very close to the critical point.
By spectroscopically monitoring the chemical composition of the reaction chamber as a function of time, one should be able to reproduce in a few months and observe chemical reactions that took place over millions of years. We strongly suggest that such an experiment should be put on the DECLIC schedule.
- John Maynard Smith and Eörs Szathmary, The origins of life, Oxford (1999).
- Ali Fallah-Araghi et al. Enhanced Chemical Synthesis at Soft Interfaces: A Universal Reaction-Adsorption Mechanism in Microcompartments.
- K. Ruiz-Mirazo, C. Briones, and A. de la Escosura, Prebiotic Systems Chemistry: New perspectives of the origins of life, Chem. Rev. 114, 285 (2013).
- Per Bak, Chao Tang, and Kurt Wiesenfeld, Self-Organized Criticality: An Explanation of 1/f Noise, Phys. Rev. Letters 4, vol. 59 (1987)
- Erich Jantsch, The Self-Organizing Universe, Pergamon (1980).
[This proposal received the scientific approval of Roger Bonnet, former scientific director at ESA.]