Comets, Carbonaceous Meteorites discovered in the meteorites recovered from the Earth have contributed largely to the research regarding the origin of the biosphere. The origin of life on Earth was and remained the question that has invaded the human mind since antiquity. Scientists have always tried to develop theories and build models to find the answer. Several disciplines have made their imprint to explain the origin of life on Earth. First, astronomers point out that the famous Big Bang gave birth to the Universe 13.5 billion years ago, reactions and events are produced, and thus the formation of the Earth 4.5 billion years ago.
Quest for the Origin of Life
Theories on the birth of the Earth have described it as the result of a collision in the solar system. In those early years of birth, the Earth was not ready to receive the first traces of life. During the azoic eras, the bombardment of meteorites and comets, the ocean of Magma invading the Earth, prepared this planet for the appearance of life. Water being the first source and form of all life, constitutes the essential part of our globe, in the oceans, is also a significant sign of the existence of life.
Organic substances have also played a vital role in the development of life on Earth. Theories have been proposed, related to a clean origin within the Earth’s interior and other extraterrestrials. For the latter, it was the comets and meteorites that bombarded the Earth, brought water and organic compounds to the Earth in quantities that should not be negligible.
Comets and Carbonaceous meteors continue to visit our blue planet, prompting scientists to collect them and analyze them in laboratories.
The extraterrestrial bodies may have been the spark of life on Earth; they brought water to Earth in the form of ice reflected in their compositions. The analysis of comets and carbonaceous meteorites found high levels of organic compounds in these bodies. It opened a great debate on the significant impact of carbonaceous comets and meteors on the origin of life on Earth and its development into today’s biosphere.
Introduction to the Universe
According to the famous Big Bang Theory, the Universe dates back to 13.8 billion years (Perlmutter et al., 1998), which includes galaxies, solar systems, stars, planets, and an infinity of celestial bodies. All of them were responsible for the birth of each entity or even element, including the first traces of life on planet Earth until it evolves to its current state. To know how life appeared on Earth, it is, therefore, necessary to illustrate the first elements formed in the Universe.
Nucleosynthesis
Nucleosynthesis was the first physical process leading to the creation of atomic nuclei, followed by primordial nucleosynthesis. At that moment, the elements that appeared dominant in the Universe were Hydrogen and Helium.
After billions, the composition of the Universe will change slightly. At the heart of the most massive stars, the most complex atoms (carbon, oxygen, nitrogen, metals) will be formed. The meeting of these atoms on the surface of asteroids formed the first ”organic” molecules containing carbon (Sephton, 2002).
Our young Earth is formed in a flat disk of ”stardust,” also including the Sun and its eight planets (Javoy, 1998). Water and organic compounds formed by hydrogen, oxygen, and carbon, will be the first signs of life on Earth.
Constantly bombarded with asteroids and meteorites, the Earth receives organic molecules that are synthesized in ocean water, on coastal sands, or around deep hydrothermal springs. After a primitive Earth passing through dramatic phenomena, life finally appears. Among the hypotheses relating to the appearance of life on Earth are comets and carbonaceous meteorites, which are the aim of this work.
From Primitive Earth to the Biosphere
Until 3.9 billion years ago, collisions in the solar system were commonplace (Cellino et Bendjoya, 1998). One of them, 4.5 billion years ago, with a body the size of Mars, was so violent that much of the Earth’s outer shell was torn off, orbited, and formed the Moon. On this occasion, our planet lost its first atmosphere and its eventual oceans, finding itself covered by an ocean of Magma a thousand kilometers deep (Righter et al., 1997). These giant impacts will have consequences on the atmosphere that will be formed (Yin et al. 2002), and the young Earth evolved into its present state: Biosphere.
The Biosphere
The biosphere is the set of living things on the surface of the Earth, has continuously changed over geological time. This evolution has been going on for four billion years, but most of the knowledge in this phenomenon relates to the last 650 million years (Lethiers, 1998). The word ”biosphere” first appeared in 1875 in the book The Formation of the Alps by geologist Eduard Suess.
Elementary bricks of Life- Origin of water on Earth
First of all, for a planet to be ”habitable” by living beings like ours, it must be rocky; there must be liquid water (Brack, 1993). As water is a significant constituent of everybody, it is closely associated with the history of life on the planet. It has been found interesting to identify elements of synthesis to account for the origin of water on Earth. First, there is the hypothesis that attributes an original stock of water to the Earth at the time of its formation (Chyba, 1987).
The stock that is found in its mantle that would have primarily degassed when the volcanic activity of the planet was much higher than today, more than 4 billion years ago in Hadean. The gases from volcanoes contain a lot of water vapor (Kasting, 1979).
The Hypothesis
According to the dominant theory, the Earth was formed by the collision of small celestial bodies, the planetesimals. Since they are deficient in water, the water must have been brought either by a larger planetesimal or by a rain of smaller objects from space. It is the hypothesis of an extraterrestrial origin, a water supply by a bombardment of comets and water-rich asteroids, or even by the flow of micrometeorites (Kaula, 1979).
These two hypotheses are not independent since the Earth comes from the accretion of rocky bodies and also probably from comets. The origin of the water is, in both cases, extraterrestrial, dating back to the conditions of the formation of the bodies of the Solar System in the initial protoplanetary disk (Engrand et al., 1999).
Organic Compounds
Life is all about the cell. For its formation, it needs complex organic macromolecules, polymers such as proteins, nucleic acids; but above all, we must make these already complex molecules.
This first step in the history of life, the oldest, therefore consists in synthesizing amino acids, nitrogenous bases from simple and omnipresent molecules containing the main building blocks of life: carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) (Kuriyan et al., 2012).
In 1920, a theory was developed postulating the formation of organic molecules in the ”primordial soup” of the oceans, under an atmosphere then very poor in oxygen (Hartmann et al., 2000).
In 1950, the American chemist Miller confirmed in the laboratory the possibility of these prebiotic syntheses and the ”bricks” of life were formed: proteins, fats, and sugars. It is assumed that ”protocells” surrounded by membranes are then born. In these confined spaces, RNA macromolecules appear.
RNA can replicate itself autonomously: a first RNA living world is established, populated by viruses, and formed of these protocells that can divide (Robertson et al., 2012).
Later, some of the RNA is transformed into DNA, so we have real cells with genomic DNA and several functional RNA. Chlorophyll appears in some cells (bacterial and then vegetal), allowing the fixation of atmospheric carbon by photosynthesis resulting in the oxygen into the atmosphere. Pluricellular beings finally appear, by the assembly of primitive cells, and all evolve (Joyce, 1989).
It appears we cannot neglect the contribution of extraterrestrial materials at the time of the origin of life. Indeed, the contributions of water, carbon, nitrogen, and phosphorus, just after the time of the ” high bombardment,” may have given the top start of life on Earth.
Extraterrestrial Origins of Life on Earth
Comets are objects less than ten kilometers in size, made up mainly of ice and dust, which move in heliocentric orbits. Indeed, when a ”dirty snowball” approaches the Sun, some of its matter sublimates, and the star develops a tail of dust several million kilometers long. This material reflects the Sun’s light, making the body visible from the Earth (Detay, 2004).
The impact of comets that hit the Earth 4 billion years ago may have played an essential role in the appearance of life on the blue planet (Sugahara et Mimura, 2015). To explain the arrival of water on Earth, comets, with their ice cores, seem an ideal candidate, but analyses have shown that their water differs from that of our oceans (Engrand et al., 1999).
Recently, however, a published study based in particular on measurements of comet 46P/Wirtanen by SOFIA, NASA’s Stratospheric Observatory for Infrared Astronomy, has established that a family of comets contains water similar to Earth’s water: hyperactive comets (Lis et al., 2019).
To find the source of the Earth’s water, researchers are studying isotope ratios, in particular the deuterium to hydrogen (D/H) ratio of water, as deuterium is a more substantial form of hydrogen.
The Presence of Water
The ice of a comet sublimates (Lis et al., 2019), as it approaches the Sun, producing an atmosphere of water vapor. However, the D/H ratios of comets measured so far have generally been between two and three times that of oceans, which would mean that comets have brought only about 10% of the water on Earth.
During its near-Earth passage in December 2018, comet 46P/Wirtanen could be analyzed thanks to the SOFIA observatory. It exhibited the same D/H as terrestrial water. It belongs to the class of hyperactive comets, which, when approaching the Sun, release more water than the surface of their nucleus should allow.
The result is an inverse correlation between this activity fraction and the D/H of water vapor: the more a comet tends towards hyperactivity, the more its D/H decreases and approaches the terrestrial D/H (Bockelée-Morvan et al., 2019). Hyperactive comets, whose water vapor comes partly from grains ejected into their atmosphere, therefore have a D/H equivalent to that of terrestrial water, unlike those whose halo gas is produced only by surface ice (Detay, 2004).
Other Organic Molecules
On the other hand, simple organic molecules are detectable in the tails of comets (Ehrenfreund et Charnley, 2000). The Rosetta-Philae (2014-2016) and Giotto Probe (1986) missions have confirmed the presence of organic molecules in comets. Wisteria has moreover been detected in the tail of the comet Tchouri. It has been possible due to the Rosina instrument installed on the Rosetta probe.
These celestial objects were able to bring vast quantities of extraterrestrial organic matter and provide a significant fraction of the Earth’s water (GOUNON-PESQUET, 2015). Observations made from Earth and thanks to the Vega and Giotto probes have indeed shown that Halley’s comet, like those of Hyakutake and Hale-Bopp, moreover, was rich in organic material (Crovisier, 1998).
After capturing comet dust with the Stardust probe, it was discovered that this water was different from that found on Earth (Wozniakiewicz et al., 2015). A new theory assumes that it was meteorites from the asteroid belt that bombarded the planet and brought its water and organic compounds.
Carbonaceous Meteorites
Around 3.7 and 4.5 billion years ago, the planet was bombarded by meteorites at a rate eight to eleven times greater than current activity. Each year, the Earth’s surface receives nearly 100 tons of carbonaceous meteorites. This is very little compared to the approximately 20,000 tonnes of micrometeorites.
The interplanetary grains related to the most primitive meteorites – those of the carbonaceous chondrite group, which vary in diameter from 50 to 500 microns, are very rich in carbon, hence their name.
It is estimated that these micrometeorites carry an estimated 100 tonnes of carbon to Earth each year. This quantity was probably close to 30,000 billion tons during the active phase of the terrestrial bombardment (Maurette et al., 1995).
The flow of micrometeorites was then a thousand times more intense than today. Numerous samples were taken from Greenland, and Antarctic ice sheets have revealed the presence of amino acids, metallic sulfides, oxides, clays, etc. in their interior. Thus, the amino acids, which are used in the construction of automaton parts, could be of extraterrestrial origin (Lawless et al., 1979).
The study of meteorites on Earth – mostly from the main asteroid belt – has established that, on average, the D/H ratio was of the order of 140 ppm.
In contrast, this ratio, when determined in some comets, was between 150 and 300 ppm. As on Earth, it is about 150 pm, at the very least, comets would not be, by far, the primary source of water on Earth (Greenwood et al., 2011).
The primitive Earth would have known significant levels of extraterrestrial material, in particular, highly reactive phosphates, released at the surface in the form of phosphides, much more soluble and reactive than the orthophosphates previously considered, and allowing the realization of new synthesis routes, much more probable, of organic phosphate molecules (Pasek, 2008). These reduced phosphate compounds come from the oxidation of Schreibersite in rocks of extraterrestrial origin (meteorites) 3.8 billion years ago.
The oxidation of this mineral in water leads to a set of P resources that can be mobilized for prebiotic synthesis (Karki et al., 2017). the mid-twenties
Carbonaceous chondrites are the most primitive meteorites in the solar system. Their elemental composition is very close to that of the Sun, which represents most of the matter that made up the solar system (Hayatsu et al., 1979). The presence of organic matter in meteorites and, in particular, carbonaceous chondrites have been known since the mid-twenties the work of Berzelius (Mason, 1963).
The Orgueil Meteorite
The fall of the Orgueil meteorite in Tarn-et-Garonne in 1864 led to the discovery of an organic macromolecular substance close to terrestrial coals, according to Berthelot (Mason, 1963). The first amino acids were then identified. But very quickly, it turns out that they are terrestrial contaminants, acquired after the fall of the meteorite. However, some significant discoveries would show that not all organic constituents were contaminants.
First, the work of Miller and Urey in 1953 showed that abiotic synthesis of amino acids was possible under certain favorable conditions. Then the fall of Murchison in Australia in 1969 led to discoveries. Very quickly, it was shown that this meteorite contains amino acids, some common on Earth, others unknown in the Earth’s natural environment (Kvenvolden et al., 1971). Structural and isotopic arguments confirm that they are indeed extraterrestrial.
The Characteristics of Carbonaceous Chondrites
Carbonaceous chondrites are characterized by their richness in carbon, water and, volatiles compared to other chondrites. This carbon is found in various forms: carbonates, silicon carbide grains, diamonds, graphite grains, and organic matter. Organic matter accounts for most of the carbon. In the following years, many teams described the organic content of carbonaceous chondrites, consisting of a soluble and an insoluble fraction (Komiya et al., 1993).
Water-soluble compounds and organic solvents in meteorites were mainly described for Murchison found in Australia in 1969, due to contamination problems and lower abundance in other carbonaceous chondrites (Swart et al., 1983).
A multitude of soluble compounds of extraterrestrial origin can be extracted, and several families of carboxylic acids, linear and aromatic hydrocarbons, alcohols, ketones and, nitrogen compounds such as amines, amino acids or nitrogen heterocycles have been identified (Sephton, 2002). Because they contain many organic compounds, some carbonaceous meteorites are also likely to have carried life on Earth. For example, the Murchison meteorite contains nearly 70 different amino acids, including eight protein amino acids (Kvenvolden et al., 1971).
RNA Like Entities
Compartmentalization molecules or amphiphilic molecules that are composed of a hydrophilic head and hydrophobic carbon chains come together to form the cell membrane and thus guarantee its cohesion.
Partitioned structures similar to cell membranes have indeed been obtained by immersing various organic substances extracted from the Murchison and Allende meteorites in an aqueous medium (Lerner, 1995). Because they are considered to be older, ribose nucleic acids (RNA) are reconstituted in the laboratory: the synthesis of sugars is carried out from formaldehyde; the purine bases A, G are obtained from hydrocyanic acid; and finally, phosphorus is extracted from terrestrial magmatic rocks.
This element is also present in carbonaceous meteorites, in the form of chloroacetate. Formaldehyde and hydrocyanic acid are found in comets (Kissel et al., 1987).
PAHs have been discovered in carbonaceous chondrites [ (Orgueil CI1), Murchison (CM2) and, Allende (CV3)] and in carbonaceous micrometeorites found in Antarctica (AMMs). The study of the AMMs collected by Maurettes M. in Antarctica showed that 80% of the grains contained organic matter (2% carbon) and did not fuse when they crossed the atmospheric layer. Amino acids were detected there.
The amount of carbon brought by these grains to the Earth’s surface can be estimated at about 30 to 50 trillion tonnes. They would have produced the spark of life. A scientific study conducted by researchers at Canadian McMaster University says that life on Earth may have started after meteorites fell into burning puddles (McSween et al., 1999).
Conclusion
It is estimated that 4.5 to 4 billion years ago, around 1015 kgs of organic molecules had arrived on the Earth. The past meteorite fluxes and the carbon content of the meteorites and cometary fragments that fell on the Earth have been taken into consideration for the estimation. U.V photons and cosmic rays irradiated the H2O content of the comets and carbonaceous chondrites, resulting in the formation of complex molecules.
The example of Orgueil Meteorite that fell in 1864 proves that the meteorites have brought the elementary bricks of life on the Earth.
Image Credits:
Cover Image- Michael_Hiraeth@ Pixabay
Research Scholar Geology, Morocco
Geo-sciences, Soil Sciences, Remote Sensing
M.Sc Geology
