Water in the Universe: 368 (Astrophysics and Space Science Library)

From this table we see that water has a high heat capacity. The latent heat is the amount of energy in the form of heat released or absorbed by a substance during a change of phase e.

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Values for the specific latent heat for different substances and phase transitions are given in Table 1. The enthalpy of condensation is exactly equal to the enthalpy of vaporization, but has the opposite sign. Why do have substances different enthalpy? From the molecular point of view the enthalpy is the energy that is needed to overcome the Van der Waals force between the atoms of a substance. These forces are very weak e. The enthalpy of condensation is the heat that is released to the surroundings to compensate for the drop in entropy.

As a gas condenses to a liquid the entropy drops; if Tb denotes the boiling point, then this change of entropy may be written as: The triple point denotes the pressure and temperature where water can coexist in all three states. Whenever a water molecule has enough energy to leave the surface of water, we speak of evaporation. It is also evident, that by this process, since the evaporating molecule takes the energy with it, the remaining water becomes cooled—there is less energy left there. This effect is known as evaporative cooling and a well known process is perspiration.

Such a process can be also found on the dwarf planet Pluto. Infrared measurements made with the Submillimeter Array in Hawaii have shown that Pluto has about 10 degrees lower surface temperature than its satellite Charon. Sunlight causes the nitrogen ice on the surface of Pluto to sublimate phase transition from frozen to gas which causes a cooling. The dew point is the temperature to which air must be cooled for water vapor to condense into water. The condensed water is then called dew, the dew point is a saturation point.

When the dew point falls below freezing, the water vapor creates frost frost point.

Water in the Universe (Astrophysics and Space Science Library)

Humidity is the amount of water in air. Relative humidity is the ratio of the partial pressure of water vapor in a parcel of air to the saturated vapor pressure of water vapor at a given temperature. Absolute humidity is the quantity of water in a particular volume of air and it changes as air pressure changes. When water vapor condenses onto a surface, this surface will be warmed and the surrounding air cooled. In the atmosphere, condensation produces clouds, fog and precipitation.

Water vapor will condensate also on surfaces, when the temperature on that surface is below the dew point temperature of the atmosphere. Water vapor is lighter or less dense than dry air and therefore it is buoyant with respect to dry air. The molecular mass of water is This shows at which temperatures and pressures water is found in which of the three states: If the pressure in the Martian atmosphere doubles to e. This type of water ice is also called Ih. Frost is a deposit from a vapour with no intervening liquid phase.

Light reflecting from ice can appear blue, because ice absorbs more of the red frequencies than the blue ones. Ice appears in nature in forms as varied as snowflakes and hail, icicles, glaciers, pack ice, and entire polar ice caps. Water becomes less dense at the transition to ice because the water molecules begin to form hexagonal crystals of ice. Ice is the only known non-metallic substance to expand when it freezes. The density of ice increases slightly when the temperature decreases. Ice can also be superheated beyond its melting point. The ice we know from everyday live also snow has a hexagonal structure.

At higher temperatures and pressures ice can also form a cubic structure Ic. The difference between these forms is their crystalline structure. The predominant form of ice found on Earth is hexagonal crystalline ice. The ice found on extraterrestrial objects e. If hexagonal ice is found on some other planet or satellite of a planet its formation is explained by volcanic action.

In history ice played also an important role for cooling. Let us give few examples: Until recently, the Hungarian Parliament building used ice harvested in the winter from Lake Balaton for air conditioning. Icehouses were used to store ice from winter. In BC in Persia ice was brought in during the winters from nearby mountains in bulk amounts, and stored in specially designed, naturally cooled refrigerators, called yakchal. Amorphous Ice Amorphous ice is lacking a crystal structure.

There are three different forms of amorphous ice: Ice Ih Normal hexagonal crystalline ice. Virtually all ice in the biosphere is of this type. Ice Ic A metastable cubic crystalline variant of ice. The oxygen atoms are arranged in a diamond structure. It is produced at temperatures between — K, and is stable for up to K, when it transforms into ice Ih. Occasionally this type of ice is found in the upper atmosphere.

For properties of the other forms of ice see the relevant textbooks. The formation process is quite complex, basically there exist two mechanisms. One oxygen atom and two hydrogen atoms combine, they are frozen and then they start to evaporate. Both reactions occur in star forming regions. These reaction take place in star and planet forming gas irradiated by far UV or X-rays.

The forth form of water has been detected outside the solar system by the satellite mission Herschel Space Observatory with the HIFI instrument a high resolution spectrometer for the far IR. When ions with opposite charges form a compound there occurs electrical attraction holding the ions together and this is called ionic bond. When atoms form bonds by sharing electrons we speak of a covalent bond. An example of a covalent bond is molecular hydrogen, H2.

Carbon can form covalent bonds simultaneously with four other atoms and therefore complex molecules such as sugars, proteins and others are created. When an atom gives up electrons, we say it is oxidized, when an atom gains electrons it is reduced. We gain energy from food by oxidation of sugar and starch molecules. Forming bonds requires energy, breaking bonds generally releases energy some activation energy is needed.

Acids in the stomach dissolve food, acids in soil help make nutrients available to growing plants. The strength of an acid and base is given by the pH value. Pure water has a pH of 7. The sulfuric acid is the major component of the clouds of the atmosphere of Venus. The water molecules are either bound to a metal center or crystallized with the metal complex. An example of such a structure is NiCl2 H2 O 6. Salts are compounds composed of a metal ion plus a non metal or polyatomic ion, e.

Hydrated salts or Hydrates are salts which have a definite amount of water chemically combined. Some common hydrates are: The dot indicates an attractive force between the polar water molecules and the positively charged metal ion. The water released on heating is called the water of hydration. Spectral Signatures Water absorbs longer wavelength stronger than shorter wavelengths. The reflectance of shorter wavelengths is higher and this is the reason why water looks blue or bluegreen in the visible and darker when observed at IR wavelengths.

Since water in nature is not pure, there are always some suspended particles in the upper layers of a water body. The reflectivity of water increases and it appears brighter the color being shifted slightly towards longer wavelengths. Chlorophyll in algae absorbs more of the blue wavelengths and reflects the green, making the water appear more green in color when algae is present.

The topography of the water surface rough, smooth, floating materials, etc. The reflectance of clear water is generally low. However, the reflectance is maximum at the blue end of the spectrum and decreases as wavelength increases. Hence, clear water appears dark-bluish. Turbid water has some sediment suspension which increases the reflectance in the red end of the spectrum, accounting for its brownish appearance. The appearance of terrestrial water lines in the spectra of stars is demonstrated in Fig.

Let us consider this balance in more detail: The evaporation from soil, streams, rivers and lakes is less than a tenth of this value: Transpiration from vegetation is higher than the latter value: Again precipitation over ocean is much higher than over land. Thus about one-tenth of water evaporated from oceans falls over land.

This water is recycled through terrestrial systems and drains back to the oceans in rivers. About 40 km3 are carried back to the oceans each year. Therefore 90 percent of the water evaporated from the ocean falls back on the ocean as rain. But considering the numbers given above we see that there is a surplus of water on land. Oceans store heat and release it slowly. Wind currents distribute heat in the latent energy of water vapor. In the tropics warm, humid air rises and is transported to cooler latitudes. It is seen that only 2.

Note that the water of crystallization in rocks is far larger than the amount of liquid water! Oceans contain also 90 percent of the living biomass on Earth. On the average, an individual molecule spends about years in the ocean before it evaporates and starts through the hydrologic cycle again.

In deep ocean trenches there is almost no exchange between water molecules and therefore they may remain undisturbed for tens of thousands of years. As it has been stressed already, oceans strongly influence the climate by storing heat. Moreover, there are also currents that transport warm water from the equator to higher latitudes and cold water from the poles to the equator. This current carries times more water than all rivers on earth put together.

In tropical seas the water is warmed by the sun, diluted by rainwater and aerated by waves. In higher latitudes surface waters are cold and more dense. Those dense waters sink to the bottom to the ocean floors flowing toward the equator, warm water however is less dense and floats on top of this cold water. Also different salinity plays an important role in these processes.

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Glaciers, ice caps and snowfields tie up 90 percent of the fresh water which makes 2. During the last ice age, 18 years ago, about one-third of the continental landmass was covered by ice sheets and since then most 24 1 Water on Earth, Properties of Water of this ice has melted. The largest remnant is in Antarctica, here the ice sheets can be as much as 2 km thick and 85 percent of all ice on earth is stored there.

The smaller ice sheet on Greenland and the floating ice around the North Pole makes 10 percent of the ice and the mountain snow peaks and glaciers constitute the remaining 5 percent. Glaciers are in fact rivers of ice sliding very slowly downhill. Polar ice sheets and alpine glaciers contain more than three times as much fresh water as all the lakes, ponds, streams, and rivers. These anthropogenic aerosols enhance scattering and absorption of solar radiation. Constraints on future changes in climate and the hydrologic cycle were studied by Allen and Ingram, [5]. Climate change is expected to accelerate water cycles and thereby increase the available renewable freshwater resources, therefore this would slow down the increase of people living under water stress currently more than 2 billion people, see Oki and Kanae, [].

In Yang et al. As a consequence, in a warmer climate forced by increasing CO2 the intensity of the hydrological cycle can be either more or less intense depending upon the degree of surface warming. For a recent review on that topic see also Wild and Liepert, [] where further references can be found. Chapter 2 Life and Water Water is an essential element for life.

Life, especially extraterrestrial life is discussed in many textbooks. Astrophysical and astrochemical insights into the origin of life were reviewed by Ehrenfreund et al. In this chapter we will outline how life can be defined and has evolved on Earth and the role of water for this process. The tissue fluid and the plasma are in a steady state with the fluid inside the cells.

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There is a strict balance between water intake and water losses homeostasis. This question appears very simple, however it is not. A philosophical review on that question is given by Gayon, A. Aristotle defined life as animation, life can be also defined as mechanism or organization Kant. Until very recently, viruses were not considered in discussions on the origin and definition of life.

This situation is rapidly changing, and it has been recognized that viruses have played and still play a major innovative role in the evolution of cellular organisms Forterre, []. Life scientists and chemists have not come to a conclusive definition of life. There are several characteristics for life as we know it from Earth: Cells mainly consist of cytoplasm bound by a very thin membrane. This membrane serves also as a protection against the environment. There exist basically two types of cells: The prokaryotic cells are simpler than the eukaryotic, they contain no nucleus.

The first cells appeared on Earth up to 3. Cells must be able to divide. There are two mechanisms, the mitosis and the meiosis.

For example, the human body loses about 50 million cells each seconds that must be replaced. In the mitosis genetic information is equally distributed to two daughter nuclei. Each new cell gets one of the two daughter nuclei. In the process of meiosis which is essential for sexual reproduction and might have appeared first 1. Daughter cells contain half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.

Plant responses to stimuli are generally much slower than those of animals. New cytoplasm is produced, damage repaired and normal cells are maintained. Metabolism includes photosynthesis, respiration, digestion and assimilation. All these include complex chemical processes. Leaves of sensitive plants e. Mimosa pudica fold within a few seconds after being disturbed or subjected to sudden environmental changes.

The molecules are organized into compartments, membranes and other structures in the cell. Bacteria are considered to have the simplest cells known, yet such a cell contains at least different kinds of proteins and other substances. Other organisms are more complex. Adaption to the environment: Natural selection leads to adaption to their environment. Today, many species are threatened with extinction because they are not able to adapt fast enough to the changing environment. There exist many examples of self-organization in biology: Molecular self-assembly is crucial to the function of cells: Molecular self assembly seems to be the key for the transition from non-living to living substances.

The question is, however, what are the conditions of the environment temperature, pressure, atmosphere, solvents that are required that such a self assembly occurs. Urey1 carried out a famous experiment at the University of Chicago. They simulated the primitive Earth atmosphere and ran continuous electric currents simulating lightning storms, which were very common on the early Earth, to this environment. After one week, 10—15 amino acids were found in this primordial soup.

Such a layer can be found in the atmosphere of Titan, the largest satellite of Saturn and is produced by methane photolysis in the presence of nitrogen. An organic haze layer would preferentially absorb ultraviolet light, thereby allowing ammonia and methane to persist in the atmosphere. In the s black smokers Fig. These are chimney-like structures above hydrothermal vents. At the mixture of the hot mineral rich water with cold water, these sulfides are precipitated and the vent water therefore appears black in color. Thus a thermophilic origin of life seems plausible.

Summarizing there are basically three different theories about where life has evolved on our planet: The primordial soup theory seems to be no longer acceptable without strong mod2 Sometimes 3 Hydrogen also called underwater geyser. In any case water played an important role by acting as a solvent for the different molecules.

In the early twentieth century S. Arrheniusdeveloped his panspermia hypothesis. According to this theory life might have originated somewhere in the universe and would have spread out automatically. The origin of life seems to be strongly connected to the presence of water for another reason.

The Sun emits also UV radiation which is extremely hostile to life. Today we are protected against this radiation by the Ozone layer. This layer was slowly formed when the plants enriched the atmosphere with free oxygen. At the time when the first cells developed about 3. However water, liquid or as ice, provides a good protection against this radiation.

Therefore, it seems logical that life originated in such an environment. Because the early Sun was less luminous, the temperatures could have been lower on Earth. A global frozen ocean several m thick could have provided an ideal shield against UV radiation. A frozen Earth could also be the result of a close encounter of the solar system with a nearby passing star, when the Earth is expelled out of the solar system or at least to larger distances from the Sun. Adams and Laughlin, [3] showed that for some time due to radiogenic heating life would still be possible around hydrothermal vents.

Nisbet and Sleep, [], discuss early hyperthermophile life near hydrothermal systems. The development of anoxygenic and then oxygenic photosynthesis would have allowed life to escape the hydrothermal setting. By about 3 million years ago, most of the principal biochemical pathways that sustain the modern biosphere had evolved. The construction of the basic building blocks for life monomers is easy to explain extraterrestrial origin, Urey-Miller experiment, black smokers.

However, it is much more difficult to explain the formation of polymers out of monomers. For the process of polymer production water plays also a negative role in an aequous environment, hydrolysis transforms the polymers into their constituent monomers. In this context we also mention autocatalytic reactions. In such reactions, one or more of the products are the same as one or more of the reactants. The non linearity can lead to the spontaneous creation of order out of chaos.

In nature there are many examples for such processes. Out of the random motion of air molecules a hurricane can be formed where the molecules all follow a vortex motion. This does not stand in contradiction with the second law of thermodynamics. The order created by living systems on Earth is produced at the expense of the increasing entropy inside the Sun which provides 30 2 Life and Water the necessary energy.

Even habitability is defined very often as the zone around a star in which, on a hypothetical planet, water can exist in liquid form. However, there are organisms that live under extreme physical or geochemical conditions, the extremophiles. Most of them are microbes including representatives of all three domains Bacteria, Archaea, and Eucarya. Some types of extremophiles are listed in Table 2. The hyperthermophiles were discovered in the s in hot springs in Yellowstone National Park.

It consists of hyperthermostable proteins. Autotrophs produce food from inorganic compounds, the heterotrophs use organic molecules as food. Some bacteria and some archaea have this ability. But note that all these lifeforms consist of cells with cytoplasm, where water is an important part. The cytoplasm is the part of a cell that is enclosed within the plasma membrane. For example halophilic bacteria produce energy to exclude salt from their cytoplasm to avoid protein aggregation.

The detection of life under such extreme conditions makes us hope to find life also under the harsh conditions on our neighbor planet Mars. Cavicchioli, [64] provided a review on extremophiles and the search for extraterrestrial life. Their main properties are: From Being to Becoming: Time and Complexity in the Physical Sciences. At Home in the Universe: To be useful, any solvent must remain liquid within a large range of temperatures. Otherwise variations in conditions on a planet or satellite of a planet will freeze or boil the solvent and living organisms will be destroyed.

As the external pressure decreases e. For regulating the temperature, the heat capacity is the relevant parameter. Water has a high heat of vaporization; a living cell can respond to a temperature increase by 32 2 Life and Water vaporizing just a small amount of water. The heat capacity of water is 4. Thus water seems to be ideal for temperature regulation. Mammals have a complex brain and precise temperature regulation allows to function this complex brain and cell systems properly.

In order to form aggregates of organic compounds, the surface tension of a solvent is relevant and water has a high surface tension. Thus water is much more appropriate for life regarding its heat capacity, heat of vaporization, surface tension and dissolving capability than other substances. Another important property of water is that it extends as it freezes. The cells of organisms that freeze will rupture. Putting the crew of an interstellar spaceship on ice would not help since humans consist mainly of water and the cells are destroyed by freezing. Ammonia does not expand upon freezing.

Another aspect has to be considered: Water molecules will be dissociated under intense UV radiation and the free oxygen contributes to ozone production. A planet with oceans consisting of ammonia does not produce such a shield. Life based on ammonia or methyl alcohol will be in some respects more robust and better protected against cosmic catastrophes than life based on water. However, it seems that complex life cannot be based on these because an effective regulation of temperature of the organisms is needed. Let us speculate further.

There is one advantage for Astronauts whose life chemistry depend on ammonia as solvent: The two laws of thermodynamics are fundamental: Energy may be transformed e. The entropy tends to increase in all natural systems. These two laws can be applied to biological systems. Organisms are highly organized. This organization can only be maintained by a constant supply of energy. This energy is used by the cell to do work, some of that energy is lost as heat, we call this dissipation. ATP Adenosine triphosphate transports chemical energy within cells for metabolism.

The standard amount of energy released from hydrolysis of ATP is: Prokaryotes7 use many compounds to obtain energy in the form of ATP and this enables them to habitate a wide variety of environmental habitats. The process to obtain energy in non photosynthetic cells is as follows: An oxidation is the following reaction: Organisms that derive energy from sunlight are called phototrophs. The chemotrophs are further divided into: Most are unicellular; they are divided into bacteria and archaea.

Let us consider for example the methanogens: These are produced by chemoorganotrophs using fermentation of organic material. This is essential for all organisms most of which can survive only within a narrow temperature range. The organisms stop to grow and reproduce. Two important factors help to moderate and maintain temperatures on Earth: At the top of our atmosphere 1.

More than half of the incoming sunlight may be reflected or absorbed by clouds, dust and gases. Short wavelength radiation is filtered out e. UV by ozone and cannot reach the surface. The distribution of the incoming solar radiation is illustrated in Fig. Incoming solar radiation is absorbed by land and sea and reflected into space by water, snow and also land surfaces.

However, this small percentage is the energy base for any life in the biosphere. Photosynthesis use blue and red light; most planets reflect green wavelengths, therefore, they appear green Fig. The chlorophyll molecules absorb light energy and create chemical bonds that serve as the fuel for all subsequent cellular metabolism.

There occur two types of reactions: Enzymes split water molecules and release molecular oxygen, O2. This is the source for all the oxygen in our atmosphere on which all animals depend for life. During the lightdependent reaction two types of mobile, high-energy molecules are created: These provide energy for the next process.

These glucose molecules provide the building blocks for larger, more complex organic compounds. The photosynthetic reactions can be summarized as see also Fig. Glucose, C6 H12 O6 is an energy-rich compound. Other enzymes release the energy in these compounds and other complex molecules such as lipids, proteins, nucleic acids are formed, or movement of ions across membranes, changes in cellular structure etc. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea.

The amount of energy trapped by photosynthesis is approximately terawatts per year. From the sugar molecule, carbon dioxide and water is released. Animals and humans eat plants or other animals that have eaten plants. Organic molecules in the food are broken through cellular respiration to obtain energy. Note that in both, photosynthesis and respiration, water plays an important role. Photosynthesis as an energy source is limited to at most the top few hundred meters of water bodies, and to the land surface.

Chapter 3 Water on Planets and Dwarf Planets In this chapter the planets and dwarf planets of the solar system are discussed. First we give a short description about their most relevant physical properties and structure and then the occurrence of water in their atmospheres and surfaces is reviewed. The European Space Agency, ESA, approved this program and the main goals are to study the evolution and origin of water in Mars, the outer planets, Titan, Enceladus and comets.

The Sun contains All other objects in the solar system orbit the Sun. Five of them can be seen with the naked eye and were known by the ancients Mercury, Venus, Mars, Jupiter and Saturn. Furthermore, their orbits have not been totally cleaned up. Their shape is almost spherical. All categories of objects described above appear at specific locations in the solar system. The inner solar system contains the terrestrial planets and the Main Belt of asteroids. In the middle region there are the giant planets with their satellites and the centaurs.

The solar system is not unique in the universe, in the beginning of about extrasolar planetary systems were known. The formation of such systems seems to be a normal process. Some important data of the planets are given in Table 3. In this table D denotes the distance from the Sun, P the period of revolution about the Sun, R the radius of the planet and PRot the rotation period. The minus sign in the column for the rotation period indicates rotation in the sense opposite to the revolution about the Sun. Note the strong carbon dioxide absorption in the spectra of Venus and Mars.

Our Earth is the largest of the terrestrial planets. The mass is 5. Over the whole evolution, the mass of land has increased steadily, during the past two billion years, the total area of the continents has doubled. Like the other terrestrial planets, the interior of the Earth can be divided into layers: The geothermal gradient is the rate of increase in temperature per unit depth in the Earth.

This internal heat is produced by radioactive decay of 40 K, U, Th. Alps up to km. The Earth is the only known object in the solar system where water can be found in liquid state on its surface. If this is a prerequisite for life, then the chances to find life on our neighbor planets Venus and Mars are very low. This extremely high temperature contrast is a consequence of lacking an atmosphere. In , Butler, Slade and Muhleman [53] studied Mercury using a radar system consisting of a meter foot dish antenna at Goldstone, CA. The beam of 8. This reflection resembles the strong radar echo seen from the ice-rich polar caps of Mars.

This led to the assumptions that ice could persist inside deep craters near the poles of Mercury. Normal ice absorbs radar signals, but ice at extremely low temperatures reflects them. Each antenna is 25 meters in diameter. Radar observations of Mercury have revealed the presence of anomalous radar reflectivity and polarization features near its north and south poles where it never gets warmer than K. Therefore it is argued that maximum surface temperatures in shaded cratered regions near the poles are below K and at a temperature lower than K water ice could be stable to evaporation for several billion years see also Paige et al.

Where does this ice on Mercury come from? Continual micrometeoritic bombardment of Mercury over the last 3. The spacecraft was launched on August 4, and performed two Venus flybys in and and made its first Mercury flyby on Jan 14, Mercury orbit insertion is planned in Its surface is hidden by dense clouds Fig.

Observations made from ground by radar and from satellite missions and even successful landings revealed an extremely dry surface resembling the surface of Mercury or the Moon, so the conditions for life are not very promising. It is interesting to note, that this high surface temperature can be found on any place on the surface of Venus, there are no big differences between e. Therefore, contrary to Mercury, it is unlikely to find ice or water near the poles of Venus. Venus rotates in Earth days in the direction opposite to its orbital motion and one revolution about the Sun lasts The surface of Venus shows craters and ancient volcanoes Fig.

Meat Mons, an 8 km high volcano. Magellan synthetic aperture radar data is combined with radar altimetry to develop a three-dimensional map of the surface. The region from 60 to km is called mesosphere. At the top of the thick clouds there is a four day superrotation, above km there is a solarantisolar circulation. Most of the H2 O is found below the cloud base at about 47 km. Its signature can be observed in a spectral window between 0. For H2 O abundance determinations the following windows are most often used: Let us compare the water content in the atmosphere of Venus with that in the terrestrial atmosphere: Another technique to measure the water content in a planetary atmosphere is using solar occultation in the IR.

Such experiments can be made during sunrise or sunset. When molecules start to absorb the radiation, structures appear in the spectrum. Those structures are characteristic of a specific molecule and their amplitude or depth are in direct relation to the quantity of this species present in the sounded atmosphere. Vertical distributions of the molecular density and mixing ratios of H2 O and HDO in the Venus mesosphere were measured by Fedorova et al. The atmosphere was sounded during solar occultation in the range of altitudes from 65 to km. An enrichment of D to hydrogen indicates the escape of water from Venus.

The mission operations will be extended up to One of the main tasks of Venus Express is to answer the question where the water on Venus has gone. For the first time it was possible to measure directly the water loss of Venus which is about molecules per second Delva et al. Using 6 Parts 7 For per million by volume. Roughly twice as many hydrogen atoms as oxygen atoms were escaping.

Because water is made of two hydrogen atoms and one oxygen atom, the observed escape indicates that water is being broken up in the atmosphere of Venus. This water escape can be explained by the fact that Venus does not have a protective magnetic field like the Earth. Therefore, the energetic solar wind particles strike the upper atmosphere and carry off particles into space. Thus the interaction between the unprotected Venus atmosphere and the solar wind can explain the present hydrogen and oxygen loss. Atmospheric and water loss from early Venus was studied by Kulikov et al. During the accretion phase of the solar system, the protosun was already formed and tiny particles started to grow rapidly to form planetesimals which then collided to form the planets.

In the inner part of the solar system metals and rock condensed, in the outer parts ice. After the accretion phase, an additional unknown amount of H2 O was supplied to Venus by cometary impacts. During its early evolution, Venus has lost most of its H2 O because of the very active young Sun: The thermal emission from this window orignates from heights 0—15 km. The evolution of Venus Earth and Mars will be compared in a separate chapter.

Moreover, because of its elliptic orbit, the closest distance to Earth can range from about 56 Million to more than Million km. Schiaparelli observed Mars during its approach to Earth in He claimed to have found on the surface of Mars a network of channels which he called canali. Later, it was suggested by Lowell and Flammarion that these canali were constructed by intelligent Martians to distribute water on the dry planet.

From that time on beginning of the twentieth century the legend of little green men on Mars was born. One of the polar caps is clearly seen as well as some surface features and clouds in its atmosphere. HST Many attempts were made to investigate the red planet with satellite missions, however, until only 18 of 37 launch attempts to reach Mars have been successful. The first images of the surface of Mars from a satellite mission were obtained in by Mariner 4 US. In for the first time a satellite could be brought into orbit around a planet with exception of Earth of course.

The results were disappointing because the surface of Mars appears more like that of the Moon and there are no signs of the canali claimed to have been observed. There are channel like structures8 on its surface which are a hint that this planet underwent large climatic variations in the past with episodes of liquid water on its surface.

For liquid water on the martian surface, the atmosphere must become much denser see also Chap. Water can only sublimate on the surface of Mars i. The permanent portion of the north polar cap consists almost entirely of water ice. In the northern hemisphere winter an additional coating of frozen carbon dioxide about one meter thick is deposited. The south polar cap also acquires a thin frozen carbon dioxide coating in the southern hemisphere winter. Beneath this is the perennial south polar cap, which is in two layers.

The top layer consists of frozen carbon dioxide and about 8 meters thick. The bottom layer is very much deeper and is made of water ice. This can be regarded as a sign of long-term increase in solar irradiance that affects both the Earth and Mars. Also albedo variations on Mars may play a role Fenton, Geissler and Haberle, []. Geomorphologic evidence for liquid water on Mars indicating a hydrologic cycle in the past was mentioned by Masson et al. These follow form the many channels and canyons observed on the surface of Mars.

The mechanism of gully formation is still under debate. There are stronger water signatures water absorption bands at the gully-exposed sites than in the surrounding areas. Therefore the formation of the gullies can be explained by water and gully formation is still locally active on the Martian surface in the present time Fan et al.

More about methods of detection of water on Mars can be found in Chap. Such measurements are important input factors for understanding the annual water cycle on Mars. For example Fedorova et al. Measurements of Martian water column abundances obtained by ground-based high-resolving-power spectroscopy in the very near IR Also the influence of long lasting dust storms on the water content can be seen. The Sun formed by a collapse of an interstellar dust and gas cloud about 4. The formation of the Sun was a fast process, it took only several years to reach the main sequence in the Hertzsprung-Russell diagram see Chap.

A main sequence star is characterized by hydrostatic equilibrium throughout its interior. Hydrostatic equilibrium is reached when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. Depending on its mass, a star may remain on this main sequence see Chap. However, the early sun was quite different from the sun we know. It rotated much faster. Stellar rotation is an important parameter to trigger stellar activity. The present day sun shows an activity cycle with a period of about 11 years.

The number of sunspots varies but also the number of flare occurrence these are energetic outbursts caused by a reconnection 9 , During such outbursts, energetic particles and radiation UV, X-ray are emitted.

Such interactions can produce storms in the magnetosphere of the Earth. The solar wind consists mostly of electrons and protons with energies of about 1 keV. Due to the high temperature in the solar corona and due to some acceleration process these particles can escape from the gravity field of the Sun. The early sun was less luminous but the energetic outbursts were of higher amplitude and occurred more frequent than presently. The UV emission was about 10, the extreme UV and the X-ray emission times the present values.

The lower luminosity of the early Sun would imply lower surface temperatures on the terrestrial planets and e. From geology we know that Earth was never frozen totally. This paradox is called the faint young Sun problem. Thermal and various nonthermal atmospheric escape processes influenced the evolution and isotope fractionation of the atmospheres and water inventories of the terrestrial planets efficiently. Another suggestion to the early faint Sun paradox was made by Shaviv, [].

Cosmic rays are supposed to have a cooling effect on the global climate on Earth. The stronger the flux of cosmic rays, the stronger the cooling. This could also explain the global temperature decrease during the Maunder Minimum which lasted from —, a period where no solar activity was recorded. High solar activity means a high solar wind flux and a strong heliosphere that protects the planetary system from cosmic ray particles, hence less cosmic ray particles penetrate to the solar system during phases of high solar activity.

The faint young Sun was much more active, therefore a stronger solar wind flux could be assumed. Consequently, during this faint young Sun evolution, the flux of energetic cosmic ray particles was lower than today, therefore, the climate was warmer on Earth. One important criterion is of course the surface temperature. On Mercury, the temperatures are high because of its close position to the Sun and there cannot exist water besides, possibly, near the poles. It is reasonable to assume that Venus and Earth and even Mars accumulated similar amounts of primordial water during their accretion phase.

So why did Venus lose its water? Theoretically, Venus could have quite favorable conditions for liquid water to exist on its surface. But there are two effects to consider. The present dense atmosphere of Venus with the greenhouse gas carbon dioxide causes too high surface temperatures. However, these unfavorable conditions might have been different in the early evolution of Venus.

Therefore, on early Venus, water oceans could have existed. But the Sun became more luminous and due to its closer distance compared with Earth, Venus received more radiation from the Sun. Especially, the UV part of radiation has to be mentioned here. It causes a splitting of the water molecules which led to a runaway greenhouse effect.

The lighter hydrogen escaped from the atmosphere, the heavier oxygen was bound with surface minerals. Such a scenario was described by Kasting, []. This has to be seen in connection with plate tectonics.

On Earth, gases like carbon dioxide are released but they are also absorbed by its active plate tectonics. On Venus, there are no signs of plate tectonics in recent time so its atmosphere became enriched with that greenhouse gas. The problem of a wet early Venus was studied by several authors. During evaporation, the lighter hydrogen preferentially escapes leaving the heavier deuterium.

Therefore, additional proxies for water content on a planet must be investigated. A study of the surface mineralogy of Venus could provide another proof for the existence of large amounts of water during its early evolution phase. Under the presence of water, hydrous minerals formed on or below its surface.

However, these minerals are unstable under the extreme temperature and pressure conditions on the surface Zolotov et al. Johnson and Fegley, [], argued that the mineral tremolite might indicate a higher water content on early Venus. On Earth, tremolite is found in basic rocks in contact with siliceous carbonate rocks. One possible reaction for the formation is: Tremolite can survive decomposition on Venus over geologic time scales at current and at higher surface temperatures. At present there exist no mineralogical data of the Venusian surface.

Abundances of Mg and heavier elements were measured by spectrometers on Venera 13 and Venera 14 and Vega 2 landers. Unlike the Moon and Mars, there are no known meteorite samples from the Venusian surface. Thus, if future spacecraft can detect tremolite, this discovery could serve as ultimate evidence of a wetter Venusian history.

The detailed loss of hydrogen by splitting up of water molecules in connection with less solar luminosity but higher amplitudes of activity up to a factor of in X-ray domain was studied by Kulikov et al. The H2 O content of early Venus can be estimated to about 0. However, during the first million years of the evolution of Venus, there might have been even much more water because of the intense bombardment phase—the number of impacts of cometary like bodies which mainly consist of water was considerably higher than today.

The solar luminosity was less and therefore the hydrogen escape occurred also at a lower rate. During the active phase of the young Sun, the exospheric temperatures were considerably higher than today about K which lead to enhanced hydrogen diffusion. Therefore, the oxygen related to water that was picked up during this phase strongly depends on the short wavelength radiative input from the Sun UV and X-ray. Besides the influence of short wavelength radiation on the water loss also solar wind effects have to be taken into account Bauer, [22].

Another important isotope for studying the water-history of Venus is 40 Ar. This may imply that Venus is not thoroughly degassed and its interior has not been dried over time. A dry interior of Venus can be explained by a near headon collision of two large planetary embryos. Such a collision would be sufficiently large to melt totally and briefly vaporize a significant proportion of both bodies. This would allow much of the released water to react rapidly with iron Davies, [95].

Gullies such as these may have formed by runoff of liquid water. The problem is that the temperature is normally too cold and the atmosphere too thin to sustain liquid water on Mars. However, water could burst out from underground layers and remain liquid long enough to erode the gullies. Another explanation of the gullies involve carbon dioxide. In the first unmanned spacecrafts landed on the martian surface Viking 1 and Viking 2.

Mars appeared as a dry, rocky desert with no signs of water. In the s however, that picture changed again. Many features on the martian surface were detected that could have been only formed by water as an example see Fig. Estimates of the amount of water outgassed from Mars, based on the composition of the atmosphere, range from 6 to m, as compared with 3 km for the Earth. On the other hand, large flood features, valley networks, and several indicators of ground ice suggest that at least m of water have outgassed.

This contradiction can be explained if early in its history, Mars lost part of its atmosphere by impact erosion and hydrodynamic escape Carr, [58]. The later one shows the appearance of new, light-colored deposit, possibly due to running water. Comparing images of the side of a crater taken in and , it was seen that the second showed gullies apparently caused by water bursting out of the crater wall Fig. Another example is shown in Fig. Gully Formation from Laboratory Simulations were compared with features observed on Mars Conway et al.

Flows on the martian surface are erosive, flow faster and further than on Earth, and produce unique sedimentary features see Fig. Mariner 9 images showed equatorial sinuous channels on Mars. Sagan, Toon and Gierasch, [] mentioned two possible stable climates: The triggers for a transition from one state to another are changes in the obliquity, solar luminosity and albedo variations of the polar caps.

Climate changes on Mars have been studied by various authors. Evidence for climate change on Mars was investigated by Lewis and Read, []. They identified four different climate regimes, depending on the obliquity of the spin axis of Mars. Global circulation models on Mars predict tropical ice accumulations at times of high obliquity Haberle, [].

Groups of layers cut across each other. Nakamura and Tajika , [] studied obliquity changes on terrestrial like planets that possess a CO2 atmosphere. Obliquity changes result into drastic climate changes because of a runaway sublimation of permanent CO2 ice. Their simulation led to ring-like structures of CO2 ice at mid latitudes. Yokohata, Odaka, and Kuramoto, [] studied the albedo feedback mechanisms of H2 O and CO2 ices in the atmosphere-ice cap system. High atmospheric pressure presumed for past Mars would be unstabilized if H2 O ice widely prevailed.

As a result, a cold climate state might have been achieved by the condensation of atmospheric CO2 onto ice caps. On the other hand, the low atmospheric pressure, which is buffered by the CO2 ice cap and likely close to the present pressure, would be unstabilized if the CO2 ice albedo decreased. They contribute to the martian global albedo, a larger value for the albedo means that more incoming solar radiation is reflected back into space and not absorbed in the martian surface, thus the planet is cooled.

There seems to be a lot of ground ice in the martian soil. The decade of this century has noticeable a renewed curiosity within the dynamics and physics of the small our bodies of the sunlight approach, Asteroids, Comets and Meteors. New observational evidences resembling the invention of the Edgeworth-Kuiper belt, sophisticated numerical instruments comparable to the symplectic integrators, analytical instruments akin to semi-numerical perturbation algorithms and as a rule a greater realizing of the dynamics of Hamiltonian platforms, most of these components have converged to make attainable and important the learn, over long time spans, of those "minor" items.

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