Is titan the cradle of life?
Good day, dear geek community! I submit to your court my first translation and the first post on Geektimes - “Titan - the cradle of life?” (In the original - “Titan - The Abode of Life”). Original from NASA here . The paper considers the physicochemical properties of Titan from the point of view of the possibility of the occurrence of methanogenic life forms on it. Chancery and other delights tried to minimize, but so far, in my opinion, not too successfully. In any case, I ask those interested under the cat.
Currently, the search for life on other planets is inextricably linked with the search for water on them. Water is a necessary component for a variety of processes and phenomena that support life on Earth, including the water cycle in nature and the familiar biochemistry of carbon in a liquid aquatic environment. We take these factors as a given, associating their development with the appearance of liquid water. The role of water as a universal solvent is closely related to its chemical and physical properties. Considering other liquids as the basis of life, it is necessary to take into account their physicochemical properties in terms of meeting the needs of life.
Titan - the only celestial body, which proved the existence of a liquid - methane and ethane - on the surface. Being the largest satellite of Saturn, Titan is quite small compared to Earth: the force of gravity on it is about 1/7 of the earth. Atmospheric pressure on the surface of Titan is 1.5 times higher than Earth's, and its average temperature is 95 K. Nitrogen (95%), methane (5%) and hydrogen (0.1%) prevail in the atmosphere, and traces of complex organic compounds are present in it. In the lower layers of the atmosphere of Titan, there is an active circulation of liquid methane with seasonal cloud formation and precipitation. Titan is in synchronous orbit near Saturn with an orbital period of 16 days (the cycle of day and night). The slope of the axis of rotation of Saturn in ~ 27˝ provides a change of seasons in the northern and southern hemispheres of Titan with a cycle of 30 years.
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The lower layers of the atmosphere of Titan are too dense to react to the 16-day cycle, but their changes during the 30-year cycle are quite noticeable. In summer, dense clouds accumulate in the polar regions of Titan, where many photochemical reactions take place. Dissociation of atmospheric nitrogen and methane leads to the appearance in the upper atmosphere of Titan of the haze of organic molecules, some of which settle to the surface. One of the main products of the photochemical reaction is ethane, which is collected on the surface and is miscible with methane. Hydrocarbon lakes formed on Titan reach ~ 1000 km in diameter. The hydration of Titan’s soil with methane and ethane was proven by a Huygens landing probe.
Can we assume that in the conditions of Titan, life originated on the basis of liquid hydrocarbons? The purpose of this article is to try to characterize Titan as a possible cradle of life, considering the properties of the hydrocarbon habitat, the biochemistry of carbon in it and the ecosystems that form on its basis.
The possibility of life in the environment is determined by the physicochemical interactions of liquids (water in the case of the Earth) and the presence of solid particles in it. Sunlight, volcanism and other physical processes create the most important conditions for life on Earth, such as:
Are there such conditions on Titan?
The possibility of the existence of chemical energy sources on Titan has been well studied. It has been proven that organic products of photochemical reactions in the atmosphere of Titan are capable of releasing energy when interacting with atmospheric hydrogen.
Hydrogenation of ethine (acetylene) is the most effective reaction, releasing 334 kJ of energy per mole of C 2 H 2 , which is comparable to the energy required to trigger the growth of methanogens on Earth (40 kJ), or with the reaction of methane and oxygen, as a result of which ~ 900 kJ / mol. The reactions presented in the table are exothermic, but under the conditions of Titan they are kinetically inhibited, which is ideal from the point of view of biology. So, for example, in terrestrial conditions, the reaction of O 2 and CH 4 is kinetically inhibited, but methanotrophs, catalyzing the reaction, receive energy as a result of its flow.
If redox couples (for example, C 2 H 2 , 3H 2 ) are formed in the atmosphere, widely distributed on Titan and readily soluble in liquid ethane and methane, we can assume that the photosynthesis of supposed life is not required. Nevertheless, it is very interesting to consider the possibility of its occurrence. The models of the distribution of light in the atmosphere of Titan and direct measurements of the level of illumination by the Huygens descent vehicle give us a very complete picture of the penetration of sunlight to the surface through the atmosphere. Due to the remoteness of Titan from the Sun (10 a.e) and dense haze in the atmosphere, the maximum illumination on its surface does not exceed 0.1% of the Earth's. Nevertheless, these conditions are more than suitable for photosynthesis, which in terrestrial conditions continues even when the solar flux is limited to 10 6 times as compared with solar noon. Thus, photosynthesis on the surface of Titan is possible with the use of pigments similar to those of the earth. It is worth noting that even taking into account all the above factors, the amount of energy produced by the surface of Titan from the Sun is orders of magnitude greater than what can be obtained as a result of chemical reactions. Therefore, Titan’s biosphere would most efficiently consume solar energy directly. On Earth, in the process of photosynthesis, mainly CO 2 and H 2 O are used, while on Titan, CH 4 could take their place, and H 2 would be a byproduct of the reaction.
Organic substances, including nitrogen, are found on Titanium quite often. Thus, carbon, hydrogen, and nitrogen are present in a variety of compounds. There is water ice on the surface of Titan, but no other compounds with oxygen have yet been found. Because of this, the estimated life on Titan can have a very limited set of elements used as nutrients, compared to earth life.
A poor set of nutrients may affect the level of development of life forms. The table below shows the organic compounds readily soluble in methane and ethane and present on Titan.
The most serious problem that life on Titan can face is access to inorganic elements (Fe, Cu, Mn, Zn, Ni, S, Ca, Na, K, etc.) that are soluble in water. Of particular interest is the use of metals as the main components of enzymes. In such a situation, two approaches are possible:
The biogeochemical cycle of phosphorus in the Earth’s biosphere is an example of the use of a hard-to-reach substance, which is not in a ready-to-eat form and is not part of the natural circulation of substances. Something similar may occur on Titan for inorganic chemical elements. Possible sources of such substances may be meteor showers and comets. It is worth noting that it is in this way that the level of CO, CO 2 and H 2 O in the atmosphere is maintained on Titan. Thus, a small, but sufficient for life forms stream of inorganic substances gets together with organic haze to the surface, where it can be recycled and used.
An alternative to the above may be the complete abandonment of life forms from the use of inorganic elements in principle. If life on Titan does not require photosynthesis and does not need to fix nitrogen from N 2 (free nitrogen is present in organic substances synthesized during photochemical reactions), then two main reasons for which you need to use metal-based catalysts are to exclude. In addition, it is assumed that water molecules on Titan can be used in the same role as metals in enzymes on Earth. The use of hydrogen bonds for the construction of supramolecular structures in water is impossible; however, it is assumed that under the conditions of Titan their forces will be acceptable for holding complex structures at low temperatures. The hydrogen bond (5-30 kJ / mol) is stronger than van der Waals forces, but weaker than covalent (~ 300 kJ / mol) or ionic (20-30 kJ / mol) forces. Thermal energy (RT) on Titan at a temperature of 95 K is ~ 1 kJ / mol. Water, being one of the few polar molecules on Titan, is well suited for the formation of hydrogen bonds. Individual H2O molecules or their small clusters held in hydrocarbon “cells” can play the role of catalysts in structures based on hydrogen bonds.
On Earth, life is common because habitable liquid water is common. Even in the driest terrestrial desert of Atacama in Chile there is liquid water. Liquid on Titan, like terrestrial water, is also widespread: many large lakes in the northern and at least one large lake in the southern hemisphere have been discovered. Huygens' landing gear data showed that the soil in the equatorial region of Titan was moistened with methane and ethane. Observations from orbit suggest that most of the soils in low latitudes are wetted. It is possible that there are also small lakes near the equator. The nature of their origin, filling and distribution is still not exactly known. Most of the major bodies of water, with the exception of Lake Ontario, are located in the northern hemisphere, with 97% of the lakes located in a region 900x1800 km in size (about 2% of the surface area of Titan).
All reservoirs on Titan can be divided into two large groups. Large lakes (several hundred kilometers wide) reach depths of up to several hundred meters. Their coastline is uneven, they are connected to river channels (for example, Sea of Liegei). Small lakes, by contrast, are smaller, and their coastline is more even. Empty hollows, very similar to small lakes, are located 250 meters above the lakes themselves. This may indicate the presence of aquifers and a subsurface network of channels that establish a certain level of fluid.
Lakes on Titan. Map of the northern hemisphere of Titan in artificial colors. blue is lakes, brown is land. The map is based on Cassini radar data. The Kraken Sea, the largest lake on Titan, is located just below the pole. Top right - the second largest Sea Ligei.
Radar image of the Sea of Ligei, showing the complex coastline and the connection of the lake with the rivers. Lake width - about 400 km
It is assumed that the lakes on Titan were formed like karst on Earth as a result of the dissolution of solid organic substances by liquid methane and ethane. Spectral images from orbit allow us to distinguish five types of land on Titan:
According to radar data, in the soil around polar and equatorial lakes a high content of hydrocarbons and nitriles is recorded, but the complete absence of water ice. However, the radiation spectrum of the surface at the landing point of “Huygens” indicates a layer of granular water ice covered with moist soil. Extrapolating the data of the Huygens spectrometer to other equatorial regions, we can assume that the soil at low latitudes is constantly moistened. This may be caused either by the existence of a wet subsurface layer, or by systematic rains, the precipitation of which in these latitudes is confirmed. It is assumed that the soil can remain wetted from 5 to 50 days after precipitation. Thus, if life can exist in liquid CH 4 and C 2 H 6 , then it should be extended to Titan.
The surface of Titan at the landing site of Huygens, 10.2 ° S, 192.4 ° W. The image is numbered 8 visible stones, the sizes of two of them are indicated side by side. The distance from the landing module is indicated in blue. The stones are supposed to consist of water ice covered with solid organic matter. The rounded shape of the stones indicates the effect of fluid on them.
On Earth, water is a mixture of the H 2 O molecules themselves with solid particles — salts. Dissolved air in it can be neglected. Evaporation leads to the separation of fresh water from salts, resulting in the formation of two different liquid habitats: fresh (lakes and rivers) and salt (seas and oceans). Due to the fact that most of the surface of the Earth is covered with water, the processes of evaporation and precipitation are a continuous cycle.
As noted above, liquid sediments have been recorded in several equatorial regions of Titan. In addition, clouds form in the summer in the polar regions and in the middle latitudes. As a rule, the amount of precipitation significantly exceeds evaporation in latitudes> 60 °, while in low and middle latitudes the evaporation volume is higher, which is consistent with the absence of water bodies in these areas.
Unlike the Earth, liquids on the surface of Titan consist of three main components: methane, ethane and dissolved atmospheric nitrogen (solubility of N 2 in methane and ethane reaches 20%). Ethane is not volatile compared to methane and nitrogen, as a result of which it remains on the surface when the liquid evaporates. Thus, nitrogen and methane (and, to a much lesser extent, ethane), while in the atmosphere, can interact with fluids on the surface. Rain on Titan is a mixture of these gases.
Studies have shown that a three-component liquid behaves differently when evaporating and condensing than a one-component liquid, due to the different volatility of its constituent compounds. In particular, the density of the liquid increases with increasing temperature. Thus, the fluid in the polar regions is less dense than at the equator. It is also known that the density of liquid on Titan is inversely related to pressure, which is fundamentally different from the properties of water on Earth. All of the above causes a more complex system of circulation of fluid on Titan as compared with the earth.
The above-mentioned differences are reflected in the composition of the lakes of Titan: the northern ones consist of methane, while the southern ones consist of ethane. It is possible that Lake Ontario is part of the once larger evaporated reservoir. By this it resembles the terrestrial Dead Sea. However, in contrast to the Earth, it is not known whether the reservoir becomes less suitable for life when it is saturated with a less volatile liquid. Ethane is a much stronger solvent of organic molecules than methane (~ 20 times more advantage) and nitrogen, and therefore ethane lakes may be more suitable for the emergence and development of life forms.
If the lakes on Titan are karst, then their age can be comparable to the age of the youngest forms of the Moon’s relief (less than ~ 100,000 years). In addition, it is assumed that the rate of formation of karst structures in the middle northern latitudes is three times higher than that in the southern ones. This is explained by the fact that, according to the predictions of the climate model, more intense, but less frequent rains fall in southern latitudes.
Thus, methane and ethane on Titan are part of an active and complex circulation of fluids, including precipitation, evaporation, formation of lakes and soil moisture. Such cycles should be acceptable for the transport of nutrients and waste, without which life cannot exist.
Earth life is based on the chemical activity of carbon-containing compounds in liquid water. The basis of the possible life on Titan should be the chemical reactions of carbon in a liquid mixture of ethane and methane. The key structures used by earthly life forms (for example, the lipid bilayer as part of the cell membrane; amino acids; DNA) can work only with a water-like solvent. On Earth, carbon biochemistry provides:
Can these processes take place under the conditions of Titan?
It is assumed that a necessary factor in the emergence of life is the appearance of a shell between the inner part of the living system and the external environment. On Earth, the cell membrane is based on a lipid bilayer, and it acts through the interaction of bipolar lipids with liquid water. As a result of recent research, a model of the membrane, called a nitrogenosome , capable of functioning in liquid methane at low temperatures was proposed. This membrane consists of small organic nitrogen-containing compounds, such as acrylonitrile. The structural integrity of the membrane is based on the attraction between the polar “heads” of nitrogen-rich molecules and their adhesion to nitrogen and hydrogen atoms. And although the synthesis of azotosoma in the laboratory is extremely difficult, the structure itself is a completely viable model under Titan conditions.
It was found that any information carrier molecule (such as DNA) should not change its shape depending on the information encoded in it. On this major trait, for example, DNA molecules differ from proteins. Replacing even one amino acid in the sequence will lead to a radical change in the shape of the protein, while DNA is immune to such changes. Therefore, DNA is a suitable molecule for storing information, but proteins are not.
Recent studies have shown that esters that were considered candidates for the role of information carrier molecule on Titan are practically insoluble at temperatures below 170 K (and on Titan, we recall ~ 95 K), and solubility of biopolymers is a necessary condition for the development of life. Water is a good solvent only because it is in a liquid state at high temperatures and its molecules are polar. Thus, the main factors preventing solubility on Titan are the low temperature and non-polar character of methane and ethane molecules. The search for a molecule soluble under these conditions is still ongoing.
If such a molecule is found, then the bond between its parts will probably be hydrogen. One option could be a hydrogen bond with polar oxygen- and nitrogen-containing molecules. In addition to them, information can be stored by electrically conducting polymers - polypyrrole or polyaniline. They consist of carbon, nitrogen and hydrogen and can make the transition between stable redox states, which can be the basis for coding information.
For terrestrial life, protein has become the main structural molecule. Using only ~ 20 essential amino acids, life forms synthesize a huge number of different proteins. Separate proteins are “packaged” into more complex forms due to both the interactions between themselves and, mainly, their hydrophobic and hydrophilic properties.
In liquids on Titan, analogs of proteins can be hydrocarbon chains, aromatic-based structures, carbon nanostructures (including graphene), and various types of fullerenes. Adding nitrogen to these compounds can markedly increase their diversity.
It is well known that most life forms on Earth live in groups. Within such groups, a more efficient exchange of substances and genetic information occurs. According to the latest data, microbial communities tolerate harsh conditions much better than single individuals. Liquid terrestrial water allows organisms to establish physical contact; in addition, it transports substances secreted by the cell.
If life on Titan is based on biomolecules that exist in liquid methane and ethane, then it is likely that similar terrestrial ecosystems can form there. Signaling molecules for life forms can be low-molecular-weight hydrocarbons that are mobile in a methane-ethane liquid medium. If genetic material on Titan is stored in soluble polymers, then they can also be mobile in a liquid medium on its surface. It is even possible that there is a similarity of terrestrial viruses with hydrocarbon envelopes and genetic material inside.
Given the enormous difference between the alleged forms of life on Titan from the earth, it is necessary to develop a strategy for finding life on this satellite of Saturn. Its basic principles, however, have already been worked out.
One of the main properties of life is its selectivity with respect to the molecules used. On Titan, various variations of chemically similar substances may be present, and life forms will have to make a choice between them. Thus, in the presence of life, on Titan there should be a significant difference in the concentration of different molecules, while in an abiotic environment, the drops will be less dramatic.
The most striking example of the biological selectivity of life is chirality. Life on earth uses only L-amino acids, not their D-analogues. Finding homochirality on Titan would be a serious testimony to the existence of life. The simplest example of chirality is an atom with four groups attached to it in such a way that, when it is superimposed on it and its mirror reflection, they do not coincide. The formation of chirality centers is possible by adding nitrogen to hydrocarbons.
The existence of life can not but affect the composition of the environment. So, most of the O 2 , CO 2 , CH 4 and even N 2 in the earth's atmosphere is produced by living organisms. The study of the atmosphere of Titan seems to be much simpler than collecting samples of the soil from its surface, so they cannot be neglected. It is believed that the most accurate indicator of biological activity on Titan can serve as H 2 . The consumption of atmospheric hydrogen by life forms will noticeably affect its content in the troposphere, provided that its consumption exceeds 10 9 cm -2 * s -1 . As a result of photochemical reactions in the atmosphere of Titan, from 0.32 to 1.2 x 10 9 cm -2 * s -1 C 2 H 2 and from 1.2 to 15 x 10 9 cm -2 * s -1 C 2 H 6 are formed . If we assume that methanogens consume ~ 20% of this volume, then the hydrogen content at the surface of Titan will become approximately constant. Otherwise, its quantity will gradually increase with the rise upwards.
Schematic distribution of hydrogen at the surface of Titan in the presence of (solid line) and the absence of methanogenic life forms
Another sign of life is the level of acetylene and ethane. It is confirmed that the surface of Titan is several orders of magnitude less ethane than it should be according to the models. The latter predicted that ethane should be enough to cover the surface of Titan with a layer several meters thick. “” . , , , . “”. , .
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Content:
- Introduction
- Habitat
- Energy sources
- Nutrients
- Fluid habitat
- Fluid Cycle and Transport of Substances
- Carbon biochemistry
- Compartmentalization as a factor of autonomy
- Molecules-carriers of information and methods for their replication
- Structural molecules and methods for their synthesis
- Ecosystems
- Search for life
- findings
Introduction
Currently, the search for life on other planets is inextricably linked with the search for water on them. Water is a necessary component for a variety of processes and phenomena that support life on Earth, including the water cycle in nature and the familiar biochemistry of carbon in a liquid aquatic environment. We take these factors as a given, associating their development with the appearance of liquid water. The role of water as a universal solvent is closely related to its chemical and physical properties. Considering other liquids as the basis of life, it is necessary to take into account their physicochemical properties in terms of meeting the needs of life.
Titan - the only celestial body, which proved the existence of a liquid - methane and ethane - on the surface. Being the largest satellite of Saturn, Titan is quite small compared to Earth: the force of gravity on it is about 1/7 of the earth. Atmospheric pressure on the surface of Titan is 1.5 times higher than Earth's, and its average temperature is 95 K. Nitrogen (95%), methane (5%) and hydrogen (0.1%) prevail in the atmosphere, and traces of complex organic compounds are present in it. In the lower layers of the atmosphere of Titan, there is an active circulation of liquid methane with seasonal cloud formation and precipitation. Titan is in synchronous orbit near Saturn with an orbital period of 16 days (the cycle of day and night). The slope of the axis of rotation of Saturn in ~ 27˝ provides a change of seasons in the northern and southern hemispheres of Titan with a cycle of 30 years.
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The lower layers of the atmosphere of Titan are too dense to react to the 16-day cycle, but their changes during the 30-year cycle are quite noticeable. In summer, dense clouds accumulate in the polar regions of Titan, where many photochemical reactions take place. Dissociation of atmospheric nitrogen and methane leads to the appearance in the upper atmosphere of Titan of the haze of organic molecules, some of which settle to the surface. One of the main products of the photochemical reaction is ethane, which is collected on the surface and is miscible with methane. Hydrocarbon lakes formed on Titan reach ~ 1000 km in diameter. The hydration of Titan’s soil with methane and ethane was proven by a Huygens landing probe.
Can we assume that in the conditions of Titan, life originated on the basis of liquid hydrocarbons? The purpose of this article is to try to characterize Titan as a possible cradle of life, considering the properties of the hydrocarbon habitat, the biochemistry of carbon in it and the ecosystems that form on its basis.
Habitat
The possibility of life in the environment is determined by the physicochemical interactions of liquids (water in the case of the Earth) and the presence of solid particles in it. Sunlight, volcanism and other physical processes create the most important conditions for life on Earth, such as:
- Sources of chemical or light energy
- Nutrients
- Liquid habitat
- Fluid Cycle for Nutrient and Waste Transportation
Are there such conditions on Titan?
Energy sources
The possibility of the existence of chemical energy sources on Titan has been well studied. It has been proven that organic products of photochemical reactions in the atmosphere of Titan are capable of releasing energy when interacting with atmospheric hydrogen.
Hydrogenation of ethine (acetylene) is the most effective reaction, releasing 334 kJ of energy per mole of C 2 H 2 , which is comparable to the energy required to trigger the growth of methanogens on Earth (40 kJ), or with the reaction of methane and oxygen, as a result of which ~ 900 kJ / mol. The reactions presented in the table are exothermic, but under the conditions of Titan they are kinetically inhibited, which is ideal from the point of view of biology. So, for example, in terrestrial conditions, the reaction of O 2 and CH 4 is kinetically inhibited, but methanotrophs, catalyzing the reaction, receive energy as a result of its flow.
If redox couples (for example, C 2 H 2 , 3H 2 ) are formed in the atmosphere, widely distributed on Titan and readily soluble in liquid ethane and methane, we can assume that the photosynthesis of supposed life is not required. Nevertheless, it is very interesting to consider the possibility of its occurrence. The models of the distribution of light in the atmosphere of Titan and direct measurements of the level of illumination by the Huygens descent vehicle give us a very complete picture of the penetration of sunlight to the surface through the atmosphere. Due to the remoteness of Titan from the Sun (10 a.e) and dense haze in the atmosphere, the maximum illumination on its surface does not exceed 0.1% of the Earth's. Nevertheless, these conditions are more than suitable for photosynthesis, which in terrestrial conditions continues even when the solar flux is limited to 10 6 times as compared with solar noon. Thus, photosynthesis on the surface of Titan is possible with the use of pigments similar to those of the earth. It is worth noting that even taking into account all the above factors, the amount of energy produced by the surface of Titan from the Sun is orders of magnitude greater than what can be obtained as a result of chemical reactions. Therefore, Titan’s biosphere would most efficiently consume solar energy directly. On Earth, in the process of photosynthesis, mainly CO 2 and H 2 O are used, while on Titan, CH 4 could take their place, and H 2 would be a byproduct of the reaction.
Nutrients
Organic substances, including nitrogen, are found on Titanium quite often. Thus, carbon, hydrogen, and nitrogen are present in a variety of compounds. There is water ice on the surface of Titan, but no other compounds with oxygen have yet been found. Because of this, the estimated life on Titan can have a very limited set of elements used as nutrients, compared to earth life.
A poor set of nutrients may affect the level of development of life forms. The table below shows the organic compounds readily soluble in methane and ethane and present on Titan.
The most serious problem that life on Titan can face is access to inorganic elements (Fe, Cu, Mn, Zn, Ni, S, Ca, Na, K, etc.) that are soluble in water. Of particular interest is the use of metals as the main components of enzymes. In such a situation, two approaches are possible:
- Reduced (compared to earth) use of [conservative use] of hard-to-reach items
- Use of H 2 O as a substitute for inorganic substances mentioned
The biogeochemical cycle of phosphorus in the Earth’s biosphere is an example of the use of a hard-to-reach substance, which is not in a ready-to-eat form and is not part of the natural circulation of substances. Something similar may occur on Titan for inorganic chemical elements. Possible sources of such substances may be meteor showers and comets. It is worth noting that it is in this way that the level of CO, CO 2 and H 2 O in the atmosphere is maintained on Titan. Thus, a small, but sufficient for life forms stream of inorganic substances gets together with organic haze to the surface, where it can be recycled and used.
An alternative to the above may be the complete abandonment of life forms from the use of inorganic elements in principle. If life on Titan does not require photosynthesis and does not need to fix nitrogen from N 2 (free nitrogen is present in organic substances synthesized during photochemical reactions), then two main reasons for which you need to use metal-based catalysts are to exclude. In addition, it is assumed that water molecules on Titan can be used in the same role as metals in enzymes on Earth. The use of hydrogen bonds for the construction of supramolecular structures in water is impossible; however, it is assumed that under the conditions of Titan their forces will be acceptable for holding complex structures at low temperatures. The hydrogen bond (5-30 kJ / mol) is stronger than van der Waals forces, but weaker than covalent (~ 300 kJ / mol) or ionic (20-30 kJ / mol) forces. Thermal energy (RT) on Titan at a temperature of 95 K is ~ 1 kJ / mol. Water, being one of the few polar molecules on Titan, is well suited for the formation of hydrogen bonds. Individual H2O molecules or their small clusters held in hydrocarbon “cells” can play the role of catalysts in structures based on hydrogen bonds.
Fluid habitat
On Earth, life is common because habitable liquid water is common. Even in the driest terrestrial desert of Atacama in Chile there is liquid water. Liquid on Titan, like terrestrial water, is also widespread: many large lakes in the northern and at least one large lake in the southern hemisphere have been discovered. Huygens' landing gear data showed that the soil in the equatorial region of Titan was moistened with methane and ethane. Observations from orbit suggest that most of the soils in low latitudes are wetted. It is possible that there are also small lakes near the equator. The nature of their origin, filling and distribution is still not exactly known. Most of the major bodies of water, with the exception of Lake Ontario, are located in the northern hemisphere, with 97% of the lakes located in a region 900x1800 km in size (about 2% of the surface area of Titan).
All reservoirs on Titan can be divided into two large groups. Large lakes (several hundred kilometers wide) reach depths of up to several hundred meters. Their coastline is uneven, they are connected to river channels (for example, Sea of Liegei). Small lakes, by contrast, are smaller, and their coastline is more even. Empty hollows, very similar to small lakes, are located 250 meters above the lakes themselves. This may indicate the presence of aquifers and a subsurface network of channels that establish a certain level of fluid.
Lakes on Titan. Map of the northern hemisphere of Titan in artificial colors. blue is lakes, brown is land. The map is based on Cassini radar data. The Kraken Sea, the largest lake on Titan, is located just below the pole. Top right - the second largest Sea Ligei.
Radar image of the Sea of Ligei, showing the complex coastline and the connection of the lake with the rivers. Lake width - about 400 km
It is assumed that the lakes on Titan were formed like karst on Earth as a result of the dissolution of solid organic substances by liquid methane and ethane. Spectral images from orbit allow us to distinguish five types of land on Titan:
- Bright terrain
- Dark Equatorial Dunes
- Blue areas
- Areas that emit at a wavelength of 5 μm
- Dark lakes
According to radar data, in the soil around polar and equatorial lakes a high content of hydrocarbons and nitriles is recorded, but the complete absence of water ice. However, the radiation spectrum of the surface at the landing point of “Huygens” indicates a layer of granular water ice covered with moist soil. Extrapolating the data of the Huygens spectrometer to other equatorial regions, we can assume that the soil at low latitudes is constantly moistened. This may be caused either by the existence of a wet subsurface layer, or by systematic rains, the precipitation of which in these latitudes is confirmed. It is assumed that the soil can remain wetted from 5 to 50 days after precipitation. Thus, if life can exist in liquid CH 4 and C 2 H 6 , then it should be extended to Titan.
The surface of Titan at the landing site of Huygens, 10.2 ° S, 192.4 ° W. The image is numbered 8 visible stones, the sizes of two of them are indicated side by side. The distance from the landing module is indicated in blue. The stones are supposed to consist of water ice covered with solid organic matter. The rounded shape of the stones indicates the effect of fluid on them.
Fluid Cycle and Transport of Substances
On Earth, water is a mixture of the H 2 O molecules themselves with solid particles — salts. Dissolved air in it can be neglected. Evaporation leads to the separation of fresh water from salts, resulting in the formation of two different liquid habitats: fresh (lakes and rivers) and salt (seas and oceans). Due to the fact that most of the surface of the Earth is covered with water, the processes of evaporation and precipitation are a continuous cycle.
As noted above, liquid sediments have been recorded in several equatorial regions of Titan. In addition, clouds form in the summer in the polar regions and in the middle latitudes. As a rule, the amount of precipitation significantly exceeds evaporation in latitudes> 60 °, while in low and middle latitudes the evaporation volume is higher, which is consistent with the absence of water bodies in these areas.
Unlike the Earth, liquids on the surface of Titan consist of three main components: methane, ethane and dissolved atmospheric nitrogen (solubility of N 2 in methane and ethane reaches 20%). Ethane is not volatile compared to methane and nitrogen, as a result of which it remains on the surface when the liquid evaporates. Thus, nitrogen and methane (and, to a much lesser extent, ethane), while in the atmosphere, can interact with fluids on the surface. Rain on Titan is a mixture of these gases.
Studies have shown that a three-component liquid behaves differently when evaporating and condensing than a one-component liquid, due to the different volatility of its constituent compounds. In particular, the density of the liquid increases with increasing temperature. Thus, the fluid in the polar regions is less dense than at the equator. It is also known that the density of liquid on Titan is inversely related to pressure, which is fundamentally different from the properties of water on Earth. All of the above causes a more complex system of circulation of fluid on Titan as compared with the earth.
The above-mentioned differences are reflected in the composition of the lakes of Titan: the northern ones consist of methane, while the southern ones consist of ethane. It is possible that Lake Ontario is part of the once larger evaporated reservoir. By this it resembles the terrestrial Dead Sea. However, in contrast to the Earth, it is not known whether the reservoir becomes less suitable for life when it is saturated with a less volatile liquid. Ethane is a much stronger solvent of organic molecules than methane (~ 20 times more advantage) and nitrogen, and therefore ethane lakes may be more suitable for the emergence and development of life forms.
If the lakes on Titan are karst, then their age can be comparable to the age of the youngest forms of the Moon’s relief (less than ~ 100,000 years). In addition, it is assumed that the rate of formation of karst structures in the middle northern latitudes is three times higher than that in the southern ones. This is explained by the fact that, according to the predictions of the climate model, more intense, but less frequent rains fall in southern latitudes.
Thus, methane and ethane on Titan are part of an active and complex circulation of fluids, including precipitation, evaporation, formation of lakes and soil moisture. Such cycles should be acceptable for the transport of nutrients and waste, without which life cannot exist.
Carbon Biochemistry on Titan
Earth life is based on the chemical activity of carbon-containing compounds in liquid water. The basis of the possible life on Titan should be the chemical reactions of carbon in a liquid mixture of ethane and methane. The key structures used by earthly life forms (for example, the lipid bilayer as part of the cell membrane; amino acids; DNA) can work only with a water-like solvent. On Earth, carbon biochemistry provides:
- Principle of compartmentalization
- The existence and ability to copy molecules that store and transmit information
- The existence of structural molecules and methods for their synthesis
Can these processes take place under the conditions of Titan?
Compartmentalization as a factor of autonomy
It is assumed that a necessary factor in the emergence of life is the appearance of a shell between the inner part of the living system and the external environment. On Earth, the cell membrane is based on a lipid bilayer, and it acts through the interaction of bipolar lipids with liquid water. As a result of recent research, a model of the membrane, called a nitrogenosome , capable of functioning in liquid methane at low temperatures was proposed. This membrane consists of small organic nitrogen-containing compounds, such as acrylonitrile. The structural integrity of the membrane is based on the attraction between the polar “heads” of nitrogen-rich molecules and their adhesion to nitrogen and hydrogen atoms. And although the synthesis of azotosoma in the laboratory is extremely difficult, the structure itself is a completely viable model under Titan conditions.
Molecules-carriers of information and ways to double them
It was found that any information carrier molecule (such as DNA) should not change its shape depending on the information encoded in it. On this major trait, for example, DNA molecules differ from proteins. Replacing even one amino acid in the sequence will lead to a radical change in the shape of the protein, while DNA is immune to such changes. Therefore, DNA is a suitable molecule for storing information, but proteins are not.
Recent studies have shown that esters that were considered candidates for the role of information carrier molecule on Titan are practically insoluble at temperatures below 170 K (and on Titan, we recall ~ 95 K), and solubility of biopolymers is a necessary condition for the development of life. Water is a good solvent only because it is in a liquid state at high temperatures and its molecules are polar. Thus, the main factors preventing solubility on Titan are the low temperature and non-polar character of methane and ethane molecules. The search for a molecule soluble under these conditions is still ongoing.
If such a molecule is found, then the bond between its parts will probably be hydrogen. One option could be a hydrogen bond with polar oxygen- and nitrogen-containing molecules. In addition to them, information can be stored by electrically conducting polymers - polypyrrole or polyaniline. They consist of carbon, nitrogen and hydrogen and can make the transition between stable redox states, which can be the basis for coding information.
Structural molecules and methods for their synthesis
For terrestrial life, protein has become the main structural molecule. Using only ~ 20 essential amino acids, life forms synthesize a huge number of different proteins. Separate proteins are “packaged” into more complex forms due to both the interactions between themselves and, mainly, their hydrophobic and hydrophilic properties.
In liquids on Titan, analogs of proteins can be hydrocarbon chains, aromatic-based structures, carbon nanostructures (including graphene), and various types of fullerenes. Adding nitrogen to these compounds can markedly increase their diversity.
Ecosystems
It is well known that most life forms on Earth live in groups. Within such groups, a more efficient exchange of substances and genetic information occurs. According to the latest data, microbial communities tolerate harsh conditions much better than single individuals. Liquid terrestrial water allows organisms to establish physical contact; in addition, it transports substances secreted by the cell.
If life on Titan is based on biomolecules that exist in liquid methane and ethane, then it is likely that similar terrestrial ecosystems can form there. Signaling molecules for life forms can be low-molecular-weight hydrocarbons that are mobile in a methane-ethane liquid medium. If genetic material on Titan is stored in soluble polymers, then they can also be mobile in a liquid medium on its surface. It is even possible that there is a similarity of terrestrial viruses with hydrocarbon envelopes and genetic material inside.
Search for life
Given the enormous difference between the alleged forms of life on Titan from the earth, it is necessary to develop a strategy for finding life on this satellite of Saturn. Its basic principles, however, have already been worked out.
One of the main properties of life is its selectivity with respect to the molecules used. On Titan, various variations of chemically similar substances may be present, and life forms will have to make a choice between them. Thus, in the presence of life, on Titan there should be a significant difference in the concentration of different molecules, while in an abiotic environment, the drops will be less dramatic.
The most striking example of the biological selectivity of life is chirality. Life on earth uses only L-amino acids, not their D-analogues. Finding homochirality on Titan would be a serious testimony to the existence of life. The simplest example of chirality is an atom with four groups attached to it in such a way that, when it is superimposed on it and its mirror reflection, they do not coincide. The formation of chirality centers is possible by adding nitrogen to hydrocarbons.
The existence of life can not but affect the composition of the environment. So, most of the O 2 , CO 2 , CH 4 and even N 2 in the earth's atmosphere is produced by living organisms. The study of the atmosphere of Titan seems to be much simpler than collecting samples of the soil from its surface, so they cannot be neglected. It is believed that the most accurate indicator of biological activity on Titan can serve as H 2 . The consumption of atmospheric hydrogen by life forms will noticeably affect its content in the troposphere, provided that its consumption exceeds 10 9 cm -2 * s -1 . As a result of photochemical reactions in the atmosphere of Titan, from 0.32 to 1.2 x 10 9 cm -2 * s -1 C 2 H 2 and from 1.2 to 15 x 10 9 cm -2 * s -1 C 2 H 6 are formed . If we assume that methanogens consume ~ 20% of this volume, then the hydrogen content at the surface of Titan will become approximately constant. Otherwise, its quantity will gradually increase with the rise upwards.
Schematic distribution of hydrogen at the surface of Titan in the presence of (solid line) and the absence of methanogenic life formsAnother sign of life is the level of acetylene and ethane. It is confirmed that the surface of Titan is several orders of magnitude less ethane than it should be according to the models. The latter predicted that ethane should be enough to cover the surface of Titan with a layer several meters thick. “” . , , , . “”. , .
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Source: https://habr.com/ru/post/396915/
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