by Nataliya GONTAREVA, Cand. Sc. (Phys. & Math.),
Yevgeniya KUZICHEVA, Dr. Sc. (Biol.), RAS Institute of Cytology
Is synthesis of bioactive compounds possible under the conditions of outer space on the surface of small bodies of the Solar system? Searches for an answer to this question are important for finding out whether delivery of organic matter to the surface of the primary Earth is possible in the period of absence of atmosphere and intense meteoric and comet bombardment.
Articles in this rubric reflect the opinion of the authors. - Ed.
The photo of Halley's comet is made by means of the telescope of Mount Wilson observatory (USA) during its appearance in 1986: to the left-general view of the comet tail, to the right-nucleus.
Orbit of Halley's comet and its assumed position in 2024, thirty-seven years before its next appearance in the Earth's environs, with regard to planetary orbits. The comet moves clockwise, while planets - contrariwise. Blue color is a part of orbits,stretching over ecliptic plane, the green one - under the plane. The Halley's comet orbit is almost completely under ecliptic plane.
Comets are celestial bodies turning around the Sun on stretched orbits and finding a "tail" as they approach it. Emerging from the depth of outer space, they look like red-hot balls with a train of gas and dust, sometimes tens of millions of kilometers long. Their nucleus (diameter from hundreds of meters up to 20 km) is a body of solid particles and ice, wrapped up in foggy cover - a coma reaching 80,000 km in diameter. Streams of sun rays kick out particles of gas from the coma and throw them back. Presumably, comets with a long period of revolution fly in to us from the Oort* cloud - millions of their nuclei rotate there. At present, approximately 200 comets, which regularly approach the Sun, have been discovered. Many of them are a part of the so-called families. For example, approximately 50 celestial wanderers (their full rotation around the Sun lasts for 3 - 10 years) create the family of Jupiter. The families of Saturn, Uranus and Neptune are a bit small. Namely, well-known Halley's comet relates to it. We will speak about it below.
It entered history of science as the first one for which the trajectory was calculated and repeated appearance was predicted. English astronomer and geophysicist Edmund Halley (1656 - 1742), who in the calculations based himself upon the evidence of observations over the position and movement of different comets in the sky stated in different sources, managed to do it. Having examined the data from 1337 to 1698, he came to the conclusion: "Quite many things make me think that the comet of 1531 which Apian had observed was similar to the comet of 1607 described by Kepler and Longomontan**, as well as to the one which I observed in 1682. All elements precisely coincide and only inequality of periods from which the first one is equal to 76 years and 2 months, and the second one is equal to 74 years and 10 months, apparently, conflicts with it, but a difference between them is not so great not to be able to ascribe it to some physical reasons. I explain inequality of the periods of the comet by similar reasons and, therefore, I make up my mind to predict return of this comet in 1758. If it comes back, there will be no reason to doubt that other comets shall come back too... However, many centuries will pass before we will learn the number of similar bodies revolving around their common center - the Sun...".
The astronomer's calculation was sensationally confirmed: in 1758, 16 years after his death the comet which had been named after him was again seen in terrestrial horizon. The next return of the "tailed star" relates to 1835; still another visit to the Sun was in 1910. Next 76 years passed, and in 1986 it again appeared in the distant neighborhood of the Earth. For the first time they started studying it directly in outer space by means of automatic
* The Oort cloud is a hypothetical cloud at the periphery of the Solar system, which serves as a source of observable comets. Dutch astrophysicist Yan Oort (foreign member of the USSR Academy of Sciences) developed the idea of its existence; the supposed space object is named after him. - Ed.
** Peter Apian (1495 - 1552) - a German astronomer, mathematician, cartographer; Johann Kepler (1571 - 1630) - a German astronomer, who discovered the laws of planetary motion; Christian Longomontan (1564 - 1647) - a Danish astronomer. - Ed.
flight vehicles. We'll tell about this in detail below. A new meeting is far away - the "wanderer" will come back to us only in 2061.
Studies of formation of the space guest is very important both for astronomy and science as a whole. According to a number of experts, Halley's comet has preserved a protoplanet matter in its structure, which is about 4.5 bin years old, which means that studying this celestial body, we will be able to get new data about physical and chemical processes which took place in the period of formation of the Solar system and its evolution.
However, the information relating to the problem of origin of life on our planet is no less valuable. It is quite probable that chemical transformations which had taken place inside the comet promoted synthesis of primary prebiological compounds, which later were brought to the Earth's atmosphere. This hypothesis was formulated, namely, by American scientists John Oro and Antonio Lascano in the 1970s, and afterwards it was confirmed by numerous experiments. However, Englishman Michael Walles adhered to the hypothesis put forward in the 1980s that life emerged in the ices of the comet nucleus. He paid attention to its strong radioactive heating, which, in its turn, promotes intense evaporation of moisture. As a volume of dissolved comet ice exceeds the one created in the process of water melting, appearance of warm cavities containing water and organic molecules in the nucleus is admissible. This situation meets the conditions required to form living matter. However, at the end of heating while celestial body moves away from the Sun, water is frozen again. Is this "dozing" life able to come out of anabiosis under the proper conditions, for example, after having fallen on the Earth?
Physicist and chemist Academician Vitaly Goldansky (1923 - 2001) stated another version of the panspermia theory in his works - importation of life to the Earth from the outside. He proposed a new approach to the problem based on the probability of creation of repeated units of proteins - amino acids - in outer space at ultralow temperatures. How is it possible? It is well known that, to combine atoms and molecules into larger units, it's necessary to overcome repulsive forces among particles. Only fast moving particles can "jump" over such a force barrier, for which a sufficiently high temperature is required. If it approaches absolute zero, their motion stops. In the latter case the so-called "tunnel effect"* saves the situation. The point is that a particle with a known degree of probability can pass below the required energy level without "jumping" over the barrier but kind of "crawling" under it. In accordance with the "tunnel effect" theory experimentalists obtained polymerization of amino acids at ultralow temperatures (A. Lascano, USA, 1990). However, in outer space these processes need inflow of energy, for example, in the form of ultraviolet radiation. Therefore, clusters of organic molecules "bathing" in sun beams can be found with a high probability on the surface of the comets passing not far from the Sun.
The substance of all the above-mentioned hypotheses is briefly as follows: life was brought to the Earth from the outside by celestial bodies, including comets. Proteins and their components - amino acids - are supposed to be in their structure. English astrophysicist Fred Hoyle (1915 - 2001) went further in his assumptions believing that in comet tails there could be not only prebiotic substances but viruses as well. When the Earth passes through such a gas-and-dust tail, it can take in different pathogenes. However, the presence of fully developed viruses in comet matter seems unlikely to us. Halley's comet, namely, does not pose a threat like that due to the strong rarefaction of
* Tunnel effect - in terms of quantum mechanics, a phenomenon when a microparticle whose energy is lower than a barrier height crosses a potential barrier. - Ed.
The Halley comet nucleus (A), photographed by means of JOTTO space vehicle in March 1986, and the comet structure (B).
its tail and its inverse orientation with respect to the Earth.
What makes scientists think that comets are carriers of organic matter? The progress of astronomy and, above of all of astrophysics has led to discoveries which have revolutionized the older ideas about chemical evolution in space. New radio astronomy data about the presence of bio-organic compounds in interstellar space, samples of lunar and Martian ground and cometary matter suggest extraterrestrial evolution of organic matter. The unity of elemental and structural composition of different space objects, speaks for the universality of physical and chemical laws in the Universe.
Now we know that interstellar space includes clouds of gas and dust where matter evolves. This fact has been verified by numerous radio astronomy observations, namely those at the Pulkovo observatory (in St. Petersburg). If the Earth, in the opinion of American scientist Cyril Ponnamperum as formulated in many works (International Conference of the Society on the Origin of Life, 1966), is an exemplary laboratory of processes which could have occurred times out of number in other planetary systems, interstellar space proves to be a gigantic laboratory of organic synthesis. Its chemical composition has surpassed the boldest expectations. Namely, hydrogen cyanide was detected in it - a unique combination in the context of the processes of chemical evolution on our planet, as not only sugars but pyrimidin and purine bases*, most probably, have originated on the Earth from its molecules, and formaldehyde, ammonia and water - before the beginnings of life on Earth. The next, more complex links in the chain of evolution have been discovered when studying the qualitative composition of meteoric and cometary matter. In meteorites amino acids, carbohydrates, and porphyrine - the simplest precursor of chlorophyll - were discovered.
A significant regularity results from a comparison of the composition of small cosmic bodies and Earth: the most common elements in the Universe (hydrogen, carbon, nitrogen and oxygen) form the basis of living matter. In the book Chemical Evolution: Origin of Life(1992) a group of the American scientists under Cyril Ponnamperum formulated a hypothesis that shock waves formed when planets collide with comets, serve as an efficient source of free energy for synthesis of organic substances. Heat, discharges of electricity, ultraviolet radiation, other similar natural sources are also contributing to this. Subsequently the substances thus obtained could enter into reactions on the surface of silicate, metallic and some other particles which form a part of cometary nuclei. Afterwards this hypothesis was confirmed experimentally in the laboratory of С Ponnamperum in the University of Maryland (USA).
There is still another reason in favor of the extraterrestrial origins of life brought in by comets. Judging by the isotopic composition, they are older than the Earth and contain the material from which planets came to be formed. Presumably, our planet has collided with comets something like one hundred thousand times in 4.5 bin years of its existence. During this time billions of tons of the "building blocks" of life could be brought to its surface. To what level did the process of creation of living matter evolve at the space stage?
Simulation experiments held at the orbital stations Salyut (1978, 1985) and Mir (1986, 1999) by the Group of Exobiology of the Institute of Cytology of the Russian Academy of Sciences**, showed that nucleotides and polypeptides with natural bonds could be formed in outer space. These experiments together with control ones made at ground stations revealed several fundamentally important regularities. First, they demonstrated resistance of
* Pyrimidin and purine bases are organic compounds taking part in the formation of nucleic acids and other biologically active substances. - Ed.
** See: N. Gontareva, Ye. Kuzicheva, "Life Coming From Cosmos?". Science in Russia, No. 5, 2006. - Ed.
biological molecules to the aggressive action of space environment and their ability to endure under arduous conditions. Second, it was shown that formation of more complex compounds is real, i.e. the assumption of the authors of the present article about evolutionary processes taking place in outer space was confirmed. Third, their hypothesis about the protective properties of meteoritic dust and lunar ground was confirmed. This again speaks for the relevance of the panspermia theory and a possibility of bio-organic molecules surviving within small bodies of the planetary system - comets and meteorites. Of course, this hypothesis needs additional proofs, both experimental and theoretical. Today the available scientific base allows to carry out complex experiments in orbit and approximate as much as possible their conditions to the real ones, for example, in the comet nucleus. Such works are planned on the Russian segment of the International Space Station (2012) and bio-satellites Bion (2010) with the participation of employees of the Institute of Cytology of the Russian Academy of Sciences.
We hope in future for more complete understanding of what happens in the midst of newcomers, occasionally visiting the environs of our planet. This synthesis requires, as it was already mentioned, inflow of ultraviolet radiation, heating, other sources of energy. And if we assume that similar compounds were created on the surface of comet or inside it under similar radiation conditions, then a chain reaction of chemical evolution could be launched as a result of even one and only contact with this celestial body.
Of course, not every "falling star" bears rudiments of life, and not every contact can become decisive. However, as a number of contacts with comets increases, a probability of successful delivery of biological material becomes more evident.
Now let's ask ourselves the question: is Halley's comet which periodically comes back to the Earth, one of the "progenitors" of life on our planet? The space mission Vega brought nearer the answer during its last (the thirtieth) appearance for all periods of observations of the "tailed star" in 1986. At that time the Soviet interplanetary automatic stations Vega-1 and Vega-2 were sent to meet it (later they both softly landed on the surface of Venus), and European Jotto and Japanese Sakikage and Suisei. As to national vehicles, they, with a small time interval, went through the coma surrounding the comet nucleus. Of course, a long contact is impossible at a relative speed of approach (about 80 km/sec) of the comet and the station, but even this short meeting (the minimum distance between Vega-1 and the comet made up 8.9 thous. km, between Vega-2 and the comet - 8 thous. km, and all analytic work in space took about 2 hours, as a whole) gave a lot of valuable information. The Jotto probe, which flew approximately 600 km from the "space guest", also successfully realized its mission.
What specifically have they found out? The nucleus of Halley's comet turned out to consist, mainly, of usual ice (with small inclusions of carbonate and methane ices), as well as dust particles. It's them that create a membrane of the "traveler". However, as it approaches the Sun, a part of them pass into the tail. The nucleus has an irregular shape, 14 km long and 7.5 km in diameter. It rotates about the axis which, according to German astronomer Friedrich Bessel (1784 - 1846), is almost perpendicular to the surface of the comet orbit. Orbiting period is 53 hours. In the course of examination of accumulated information, it was also found out that the nucleus has a porous structure and average density of about 0.3 g/cm. Studies of gaseous dust emissions from the nucleus showed that only about 10 percent of its area is active. The structures, which are seen on
the photos as ridges and cavities, appeared as a result of continuous changes of the surface morphology. The comet mass is approximately one billion times less than that of the Earth, while density of matter from its tail is practically equal to zero.
Chemical composition of gaseous emissions is different and depends on a distance from the nucleus. "Parent molecules" at the beginning of this chain are similar to that of the nucleus. As it was assumed, water molecules dominate (80 percent). Carbon monoxide (10 percent), carbon dioxide (2.5), methane (7), ammonia (0.1 percent), hydrogen cyanide and different hydrocarbons relate to other "parent" structures. Iron and natrium together with sulfur and hydrogen sulfide also form a part of the nucleus.
The comet gas is ionized by ultraviolet solar radiation by means of electrons and exchanging charges with solar wind plasma. There was discovered a large quantity of different ions including H3O+, H2O +, OH+, C+, CH+, O +, Na+, C2+, S+ and Fe+. Interplanetary station Jotto recorded emission of 19.8 t/s of gas and a little less of dust. In total, about 33 tons of material are released from the nucleus per second. All light elements, except for nitrogen, are available in the comet in the same ratio as in the Sun matter. However, as compared with the element composition of the Earth and meteorites, a number of significant deviations are observed. This speaks for the fact that Halley's comet consists of the more primitive material in the Universe which is known to us.
Studies of several thousands of dust particles have demonstrated their differences: in chemical compositions of some of them dominate light elements - hydrogen, carbon, nitrogen, oxygen, while in others prevail the elements, which form minerals: natrium, magnesium, silicon, calcium and iron. Identification of light particles at large distances from the nucleus proves their belonging to the splinters of the comet ice.
Let's summarize the obtained information in the light of the hypothesis on extraterrestrial origin of life. The fact that Halley's comet is older than the Earth and periodically returns to it, is very important. During the first visits, while our planet passed through the tail or coma, the comet matter (both organic and inorganic) could settle on the earth's surface. Even if at that time there were any organic compounds on the primeval Earth, even in this case their additional delivery could have played a crucial role. By the way, except for the comets, which occasionally come back like Halley's comet, organic matter could get to the planet in the course of single visits of other comets and meteorites. Later on, with the emergence of the atmosphere and water, new opportunities had opened for synthesis of more complicated compounds in liquid medium.
In a word, studies of Halley's comet has justified the hopes of scientists, confirmed the hypothesis on the mechanisms of appearance of the first organic compounds on our planet, which served as an impetus for further chemical evolution.
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