Water synthesis in space

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Water is synthesized above the star Betelgeuse .

In the conditions of space , water can only be synthesized with difficulty. The necessary hydrogen and oxygen atoms only meet extremely rarely in a high vacuum. This makes chemical reactions very unlikely. In addition, the radiation from the stars would break down the molecules again. It ionizes atoms anyway, so that syntheses cannot take place in the first place.

However, many water resources have been discovered in space . They are found in interstellar space as fine ice particles or as water vapor . According to this, possibilities should exist with which water can be synthesized even far from planets and moons. Triatomic hydrogen ions, interstellar dust and interplanetary dust are of great importance . The triatomic hydrogen ions are created by ultraviolet radiation and then react with oxygen atoms, so that water molecules can form after further intermediate steps. Interstellar dust, on the other hand, effectively shields starlight and thus prevents molecules from decomposing. It also offers reaction surfaces that favor water synthesis. Finally, interplanetary dust forms water when hit by stellar winds . Here the hydrogen atomic nuclei of the stellar winds react with oxygen atoms from the dust.

In addition, water is synthesized in the atmospheres of stars as long as the ambient temperature is no hotter than about 3800K. The synthetic pathways that take place there have so far been largely unexplored.

Origin of hydrogen and oxygen

Water is made up of hydrogen and oxygen , the most common and third most common chemical element in the universe. Hydrogen formed in the course of the big bang . Oxygen was only created during certain nucleosynthesis inside stars. Later he was thrown into space by strong stellar winds from giant red stars and from supernovae . There it mingled with the rest of the interstellar matter.

The oldest known and still existing star has existed for 13.6 billion years and is called SMSS J031300.36-670839.3 . The star was formed just 220 million years after the Big Bang, which, according to the current interpretation of the data from the Planck Space Telescope , took place 13.82 billion years ago. The material from which SMSS J031300.36-670839.3 was formed already contained oxygen. That oxygen probably came from a very short-lived precursor star with 60 times the mass of the Sun and was mainly ejected during its supernova. Thus, both types of atoms for water synthesis have been present in space for at least 13.6 billion years.

Conditions in free space

Water molecules cannot normally be synthesized in free space. The void has a high vacuum with a density of one particle per cm³. The probability that hydrogen and oxygen atoms will come together to form water molecules is very low.

In addition, water synthesis takes place as an exothermic reaction . For example, in the oxyhydrogen reaction - the reaction of atomic oxygen with hydrogen molecules to form water vapor - 491 kilojoules per mole are converted into thermal energy. In a high vacuum, the energy cannot be transferred to other particles and remains in the water molecule in its entirety as vibration energy. This makes it tear apart.

However, both types of atoms could not react with each other anyway. They would lack the necessary valence electrons . This is due to the ionizing light that emanates from stars. Its short-wave ultraviolet light already supplies the 13.6 electron volts that are necessary for the ionization of hydrogen atoms. If a hydrogen atom is hit by ionizing radiation, the atomic nucleus and electron are separated. This means that hydrogen is often ionized in free space and can therefore not be used for chemical reactions.

Water synthesis in molecular clouds

In the BN-KL region within the Orion Nebula , water is synthesized ( infrared image ).

Interstellar nebulae lie in various places in space . One kind of interstellar nebula are the molecular clouds . They contain non-ionized atoms, the name-giving molecules and a lot of interstellar dust.

Interstellar dust particles are formed in supernovae and in the outer zones of red giant stars and red supergiant stars . This is why these dust particles have been present in the universe as long as the starting material of the star SMSS J031300.36-670839.3, i.e. for 13.6 billion years. In addition, almost as much dust has been recorded for the galaxy A1689-zD1 as is found within the Milky Way today. The light from that galaxy has a redshift of approximately 7.5. This means that it was 13.1 billion years to reach Earth. As a result, galaxies were able to collect large amounts of dust as early as 720 million years after the Big Bang. In the Milky Way, the molecular clouds - the gas-dust clouds - lie mainly within the galactic level on the inside of the spiral arms .

The dust particles help neutral atoms and molecules to form. Although it only makes up one percent of the cloud's mass, the dust acts as an effective radiation protection. He shields his cloud from most of the starlight coming from outside, including the ultraviolet radiation it contains. The dust particles absorb the radiation and convert it into long-wave infrared light . Infrared light has too little energy to ionize. The infrared light is emitted to the outside. The dust particles also absorb thermal energy from inside the cloud and also release it to the outside as infrared light. In this way, the interstellar dust acts as an effective cooling. The cloud loses thermal energy, so it can cool down to a temperature of 5 K. Correspondingly, the Brownian particle movement of the particles within the cloud becomes less. This makes the cloud denser and ultimately even opaque.

The interstellar dust generates a radiation-protected, cold and denser molecular cloud interior. Water molecules can be synthesized inside the cloud. Three different routes have been proposed for the synthesis.

Water synthesis with triatomic hydrogen ions

A neutral hydrogen molecule (H 2 ) drifts from inside the molecular cloud to its edge. There the hydrogen molecule is hit by ionizing radiation from nearby stars (or by cosmic gamma rays ). As a result, an electron is knocked out of the molecule. A simply ionized hydrogen molecule (H 2 + ) is created. This simply positively charged hydrogen molecule reacts with a neutral hydrogen atom called HI, which is common in molecular clouds. The reaction creates a single positively charged, three-atom hydrogen ion (H 3 + ).

The triatomic hydrogen ion reacts with a neutral oxygen atom. A simply positively charged oxonium ion (H 3 O + ) is created. Because particles are constantly being ionized at the edge of the molecular cloud, there are many electrons floating around freely. The oxonium ion captures such an electron. A neutral Rydberg radical (H 3 O) is formed. The addition of the electron is, however, a strongly exothermic process. The Rydberg radical begins to vibrate as strongly as if it had been heated to a temperature of 59727 K. The high vibration energy makes the Rydberg radical unstable. It disintegrates practically immediately after its synthesis. The Rydberg radical decays in three different ways:

  • There is a 71% probability that it will break down into two neutral hydrogen atoms (2 HI) and one hydroxyl particle (OH).
  • With a probability of 12.5% ​​it breaks down into a neutral hydrogen molecule (H 2 ) and a hydroxyl particle (OH).
  • With a probability of 16.5%, however, it breaks down to a neutral hydrogen atom (HI) and a molecule of water (H 2 O).

If the water molecule drifts into the radiation-protected interior of the molecular cloud, it will not disintegrate again due to ionizing radiation. In this way, water vapor can gradually collect there.

Water synthesis with interstellar dust

In the Orion Nebula, interstellar dust is involved in the synthesis of water.

Although the particle density inside molecular clouds is much higher than in free space, it is ultimately still very low. Therefore, even in the clouds, the probability would not be particularly high that atoms would meet as reaction partners. However, there is also interstellar dust in the clouds. Dust is of central importance in the following two water synthesis pathways.

Water synthesis with temporary storage on interstellar dust

As it drifts through the molecular cloud, an interstellar dust particle gradually collects other particles. This is because atoms and molecules stick to its cold surface. Neutral hydrogen atoms drifting past also freeze there.

On the surface of the dust particles, two hydrogen atoms react to form a neutral hydrogen molecule. The reaction is exothermic. The thermal energy detaches the hydrogen molecule from the surface of the dust particles. The hydrogen molecule drifts into the molecular cloud. There it can react with an oxygen atom in an exothermic reaction to form a water molecule.

If the new water molecule then hits another particle in the molecular cloud, it can pass on its high vibrational energy. Then it doesn't fall apart again immediately. It remains as an intact water molecule in the molecular cloud. Together with other water molecules, it forms thin water vapor. Most water molecules freeze again at some point on dust particles. They become part of the layers of ice that cover dust particles.

Water synthesis with constant storage on interstellar dust

In this synthesis route, too, atoms drifting past stick to the cold surfaces of the interstellar dust particles. Hydrogen atoms accumulate over time, but oxygen atoms also freeze. On the surface of the dust particles, two hydrogen atoms react with one oxygen atom to form a water molecule.

As always, the water synthesis reaction is exothermic. As a rule, however, the thermal energy released should not be sufficient to allow the new water molecule to drift into the free molecular cloud. Instead, the energy is simply absorbed by the dust particle. This is why the resulting water molecules often stick to the surface of the dust particle. There they form the water ice mantle over time.

Compared to the other two possibilities presented, this third water synthesis path is the most effective. For one , the chance that water molecules will form is not just 16.5%. On the other hand , the water molecules are not threatened by immediate disintegration, because they cannot pass on their vibrational energy with a low probability.

Water synthesis with interplanetary dust

Star winds can synthesize water in interplanetary dust particles ( EM uptake ).

Interplanetary dust particles can also be involved in water synthesis. This happens when dust particles are hit by stellar winds. Star winds consist largely of hydrogen atomic nuclei. Interplanetary dust, on the other hand, consists primarily of silicates . The hydrogen atomic nuclei of the stellar winds break down the mineral lattice of the silicates. As a result, oxygen atoms are released. An oxygen atom can then react with hydrogen to form a water molecule. The water molecules collect in the cortex of the interplanetary dust particles.

Through the same mechanism, water can also arise on the surface of celestial bodies that have no or only extremely thin atmospheres. For example, the regolith of the Earth's moon is enriched with water .

Water synthesis in stellar atmospheres

Water is also formed in the atmospheres of red giant stars and red supergiant stars . They contain water vapor outside the photosphere and chromosphere in a layer called the MOLsphere , which has temperatures of around 1500 K. The material for the MOLsphere is supplied from the star's surface and probably rises through giant convection cells and Alfvén waves . However, many questions about the origin and synthesis of water in MOLspheres still remain unanswered.

Furthermore, water vapor was discovered over red dwarf stars . Temperatures there are between 2800 K and 3800 K. It is also found over the sun's sunspots , where temperatures of around 3200 K prevail. The rest of the surface of this yellow dwarf star is about 5800 K too hot to allow water molecules to exist. Apart from these temperature readings, little is known about the synthesis of water via stars.

See also

literature

  • Henrik Buhr, Julia Stützel, Mario B. Mendes, O. Novotný, Dirk Schwalm, MH Berg, Dennis Bing, Manfred Grieser, Oded Heber, Claude Krantz, Sebastian Menk, Steffen Novotny, DA Orlov, Annemieke Petrignani, ML Rappaport, R. Repnow, D. Zajfman, Andreas Wolf: Hot water molecules from dissociative recombination of D 3 O + with cold electrons. In: Physical Review Letters. . No. 105, 2010, p 103,202th doi: 10.1103 / PhysRevLett.105.103202
  • Victoria Louise Frankland: Towards understanding the formation of water on interstellar dust grains . Edinburgh 2011. (online)
  • Jörn Müller, Harald Lesch: Where does the earth's water come from? In: Chemistry in Our Time. No. 37, 2003, pp. 242-246. doi: 10.1002 / ciuz.200300282
  • Andreas Wolf: Triatomic hydrogen in interstellar clouds and on Earth. In: Spectrum of Science. No. 07, 2012, pp. 12-14. (on-line)

Individual evidence

  1. ↑ Miguel Montargès, Pierre Kervella, Guy Perrin, Keiichi Ohnaka: Exploring the water and carbon monoxide shell around Betelgeuse with VLTI / AMBER. In: European Astronomical Society Publications Series. No. 60, 2013, p. 167 doi: 10.1051 / eas / 1360019
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  3. ↑ a b Jörn Müller, Harald Lesch: Where does the earth's water come from? In: Chemistry in Our Time. No. 37, 2003, p. 243 doi: 10.1002 / ciuz.200300282
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  21. ↑ Victoria Louise Frankland: Towards understanding the formation of water on interstellar dust grains . Edinburgh 2011, p. 6 (online)
  22. ↑ J. Mayo Greenberg: Cosmic Dust. In: Spectrum of Science Dossier. No. 4, 2003, pp. 56–57, 59 (online)
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