Complex organic molecules are the basic components of life. We know from many astronomical observations that they are not exclusively owned by the Earth. There are lots of natural formations of organic biomolecules in the interstellar medium near the stars, comets, and even many planetary satellites in the solar system.
However, the formation of these molecules is a subject that scientists are still studying. Although we have not yet found traces of life in space, we have detected the diversity and abundance of these molecules through a variety of detection equipment (highly sensitive radio or optical telescopes, or space probes in the solar system). It seems to imply that they can be synthesized in space.
Of these complex organic molecules, glycine (glycine: H2N-CH2-COOH), which has been found in interstellar media, comets, and meteorites, is the simplest amino acid in proteins and is essential for terrestrial organisms. . The current hypothesis is that this complex organic molecule may have been produced through the radiation and thermal processes of various astrophysical ice. They can be agglomerated around dust particles in dense and cold (10-20 K) molecular clouds in the interstellar medium, or on the surfaces of comets, planets, and their icing objects in the solar system. In all of these cases, ice consisting of simple molecules such as water, carbon dioxide, ammonia, methane, etc. is exposed to a variety of radiation fields (ultraviolet rays, X-rays, or gamma rays, high-energy electrons, protons, cosmic rays). .
To date, most of the astrochemical studies carried out in the laboratory have focused on the chemical reactions induced by ultraviolet radiation. This is because many stars embedded in the interstellar medium emit a large amount of ultraviolet rays, and high-energy cosmic rays also generate ultraviolet radiation from abundant hydrogen molecules. The resulting free radicals such as H· and CH 3O then promote ice formation in interstellar medium. Subsequent chemical reactions of the particles.
In addition, it is widely recognized that when a series of high-energy radiation such as cosmic ray particles, stellar wind particles, X-rays, and gamma rays interact with condensate, a large amount of secondary non-thermal electrons are derived. Most of them are low-energy electrons with energy below 100 eV. Compared with photons, they can excite very different electronic states and can exhibit larger interaction cross sections.
Therefore, people are increasingly interested in the role of low-energy electrons in the radiation process of astrophysical ice. In the laboratory, researchers can irradiate nanometer-thickness ice samples in ultra-high vacuum at ultra-low temperatures to simulate the interaction of secondary low-energy electrons with astrophysical ice.
In a simulation experiment, researchers at the University of Sherbrooke in Canada used electron guns to irradiate a thin layer of ice covered with basic molecules of methane, ammonia, and carbon dioxide. Using mass spectrometry, known as temperature-programmed desorption, they demonstrated that the use of low-energy electrons to irradiate aggregates containing carbon dioxide, ammonia, and methane molecules in a 1:1:1 mixture can form a complete glycine. The researchers published this finding in the recent Journal of Chemical Physics.
Prior to this, they used similar mass spectrometry methods to produce ethanol from the two components of methane and oxygen. Although these are just simple molecules, they are far less complex than the macromolecules in the organism. But the appearance of glycine proved that low-energy electrons can convert simple molecules into more complex forms, showing how the basic components of life are formed in space, and then delivered to Earth through the impact of comets or meteorites.
○ The low-energy electrons produced in matter through space radiation (such as galaxy cosmic rays) can induce the formation of glycine in astrophysical molecular ice. In the experiments, mixed condensates of ammonia, methane, and carbon dioxide under low temperature (20K) and ultra-vacuum environments were used to simulate ice particles in the interstellar medium and were irradiated with low-energy electrons with intensities of 0 to 70 eV. | Image credit: NASA/Hubble/STScI
The researchers found that under such experimental conditions, for low-energy electrons with an average energy of 70 eV, one strike of the sample ice layer per 260 electrons can form a glycine molecule. This figure appears to be small. According to calculations, if the proportion of mixed agglomerates of carbon dioxide, methane, and ammonia in the interstellar medium is 1:1:1, then the secondary low-energy electrons per second through a square centimeter of ice crystal surface energy. The number of glycine molecules is 60. In this case, it takes 9.2 x 1012 seconds, or 2.9 x 105 years, to produce a glycine monolayer containing 5.5 x 1014 molecules per square centimeter.
To investigate how realistic the formation rate is in space, the researchers calculated the probability that a carbon dioxide molecule would encounter both methane molecules and ammonia molecules, as well as the intensity of the radiation they encountered. The mixed ice crystals used in the experiment are extremely unlikely to exist in astrophysical or planetary ice. According to data, ice in interstellar space may contain about 20% of carbon dioxide, about 2% of methane, and about 10% of ammonia.
Therefore, they calculated that the probability of a carbon dioxide molecule meeting with methane and ammonia molecules is approximately 0.0528. This means that a layer of glycine monolayer molecules must be formed from dense clouds and it takes 5.5 × 106 years. For ice that has a similar composition and exposure to more powerful Jupiter satellites, the formation of such a monolayer of glycine molecules takes about 30 days.
The author, Huels, said: “In space, the most important thing is time. We do this to understand the possibility of this output, that is, is it realistic or totally unreasonable? And our conclusion is that For the rate of formation of glycine or similar biomolecules, this result is still quite possible.”