5 Organics on the Rocks: A Cosmic Origin for the Seeds of Life
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Although, thus far, amino acids have not been identified in the interstellar medium,
many organic compounds, some of them directly linked to our biochemistry, are routinely produced by cosmic chemistry, and are likely widespread throughout the Universe. Among them, two species with the peptide moiety, isocyanic acid (HNCO) and
formamide (HCONH 2 ) were detected [13, 14] just three years after radio astronomy
opened the way to the molecular Universe, with the discover of ammonia in 1968
[15]. This cannot be considered a fortuitous event, and in fact, these pretty simple
molecules are incredibly abundant in our Universe. Their detections together with
other complex organic species would have an added value if they occur in the gas
surrounding a young solar-type star, implying that their synthesis would be coeval
with the formation of planets in a protoplanetary system.
5.3 Is the Origin of Life Linked to Cosmic Chemistry?
The formation of a star is a violent and chaotic process in which the gas is flowing in
and ejected outwards at speeds up to hundreds of kilometres per second, as the gravitational infall is locally opposed by thermal, turbulent, and magnetic pressures, by
dynamical outflows, and—since the parent cloud is rotating—by angular momentum
effects. As a consequence of all such competing process a cloud contracting to form
a solar-type star forms a swirling disc. Initially, discs rapidly funnel material onto
the star but, as the surrounding molecular core is used up or otherwise disperses, the
accretion rate decreases, and only a small amount of the original material persists in
the disc. Such discs can be considered protoplanetary not only for the geometry of
the Solar System, but also for the high detection rate of exoplanets.
The physical conditions in a protoplanetary disc vary greatly, with hot and dense
regions of gas and dust near to the star and much colder material at greater distances from it, providing a suitable background in which cosmic chemistry may
be replicated. The Atacama Large Millimetre Array (ALMA) has allowed a detailed
observation of molecules in these regions. While CO, CO 2 , HCO, and H 2 CO are often
abundant species in the cold zones of the disc, CH 3 OH or CH 3 CN are only found in
a few regions, and more complex organic molecules are not observed [16–18]. Such
evidence is in striking contrast with the analysis of the debris of the planetary construction in the Solar System, where quantitative analyses confirm the existence of a
wide variety of organic species inside certain types of meteorites [19], and to a lesser
extent in comets [20]. In principle, these compounds can form during the assembly of
a protoplanetary system. The chemical inventory includes alcohols, amines, amides,
esters, amino acids, the building blocks of proteins, amphiphiles, the building blocks
of membranes, simple sugars such ribose, a crucial piece of the chemical machinery inside cells, and nucleobases, the building blocks of RNA and DNA. Some of
those amino acids [21] and sugar derivatives [22] contain significant excesses of
enantiomers having the same handedness of terrestrial biomolecules. Why is such a
wealth of complex organics not detected in the gas-phase of discs?
31
Although, thus far, amino acids have not been identified in the interstellar medium,
many organic compounds, some of them directly linked to our biochemistry, are routinely produced by cosmic chemistry, and are likely widespread throughout the Universe. Among them, two species with the peptide moiety, isocyanic acid (HNCO) and
formamide (HCONH 2 ) were detected [13, 14] just three years after radio astronomy
opened the way to the molecular Universe, with the discover of ammonia in 1968
[15]. This cannot be considered a fortuitous event, and in fact, these pretty simple
molecules are incredibly abundant in our Universe. Their detections together with
other complex organic species would have an added value if they occur in the gas
surrounding a young solar-type star, implying that their synthesis would be coeval
with the formation of planets in a protoplanetary system.
5.3 Is the Origin of Life Linked to Cosmic Chemistry?
The formation of a star is a violent and chaotic process in which the gas is flowing in
and ejected outwards at speeds up to hundreds of kilometres per second, as the gravitational infall is locally opposed by thermal, turbulent, and magnetic pressures, by
dynamical outflows, and—since the parent cloud is rotating—by angular momentum
effects. As a consequence of all such competing process a cloud contracting to form
a solar-type star forms a swirling disc. Initially, discs rapidly funnel material onto
the star but, as the surrounding molecular core is used up or otherwise disperses, the
accretion rate decreases, and only a small amount of the original material persists in
the disc. Such discs can be considered protoplanetary not only for the geometry of
the Solar System, but also for the high detection rate of exoplanets.
The physical conditions in a protoplanetary disc vary greatly, with hot and dense
regions of gas and dust near to the star and much colder material at greater distances from it, providing a suitable background in which cosmic chemistry may
be replicated. The Atacama Large Millimetre Array (ALMA) has allowed a detailed
observation of molecules in these regions. While CO, CO 2 , HCO, and H 2 CO are often
abundant species in the cold zones of the disc, CH 3 OH or CH 3 CN are only found in
a few regions, and more complex organic molecules are not observed [16–18]. Such
evidence is in striking contrast with the analysis of the debris of the planetary construction in the Solar System, where quantitative analyses confirm the existence of a
wide variety of organic species inside certain types of meteorites [19], and to a lesser
extent in comets [20]. In principle, these compounds can form during the assembly of
a protoplanetary system. The chemical inventory includes alcohols, amines, amides,
esters, amino acids, the building blocks of proteins, amphiphiles, the building blocks
of membranes, simple sugars such ribose, a crucial piece of the chemical machinery inside cells, and nucleobases, the building blocks of RNA and DNA. Some of
those amino acids [21] and sugar derivatives [22] contain significant excesses of
enantiomers having the same handedness of terrestrial biomolecules. Why is such a
wealth of complex organics not detected in the gas-phase of discs?