30
C. Cecchi-Pestellini
same, but the physical conditions are such that some of the usual hypotheses are no
longer valid. The exotic reaction methodologies offered to synthetic organic chemists
to forge new chemical bonds are of course not available in space, and the synthesis
must occur through processes that a modern chemist undertaking routine synthesis
in a laboratory would consider insufferable or archaic. The basic idea is that radicals trapped in the iced mantles acquire mobility and react forming complex organic
molecules as the dust temperature reaches ∼ 30 K [7]. The origin and quantity of the
trapped radicals is uncertain; they could be e.g., pieces of iced species broken by the
ultraviolet photons or, the results of the incomplete hydrogenation of the simple parent species. Generally, their nature should reflect the ambient physical and chemical
conditions, and the radiation environment.
The complex species arising into the ice may be released to the gas-phase, where
they are eventually observed, through a variety of processes. Thermal desorption
seems a possible cause; it becomes sufficiently rapid in interstellar conditions when
the ice temperature rises to about 100 K, but is totally negligible for typical ice
temperatures of 10 K. Species on or near the surface of a dust grain can be desorbed
by photons of the interstellar radiation field. Where extinction is high so that the
interstellar radiation field plays no significant role, photodesorption may be driven
by the weak radiation field created when cosmic rays ionize hydrogen atoms and
molecules. The subsequent recombination spectrum of ions and electrons generates
an ultraviolet field [8]. As noted first by Léger and collaborators in 1985 [9] the
passage of heavy cosmic rays through dust grains deposits heat in the grains. On large
grains, the heat is deposited in a cylindrical volume along the track of the cosmic
ray, and thermal desorption of weakly bound molecules such as CO can occur from
the heated areas of the surface at the intersection of the cylinder and the surface.
Where the grains are small enough, the cylinder encompasses the entire grain, so
desorption occurs from the entire surface area of small grains. Other mechanisms
involve desorption driven by surface chemistry, grain explosions, and grain-grain
impacts. The efficiency of a process typically depends on the involved molecular
species, and it is mediated by the energy transfer at the molecular level (see e.g.,
[10]).
The organic molecules discovered in space until few years ago consisted of
a backbone of carbon atoms arranged in a single and roughly straight chain. In
2014, astronomers announced to have for the first time detected towards the galactic
centre—in the Sgr B2(N) molecular cloud—a carbon-containing molecule called isopropyl cyanide, in which the carbon structure branches off in a separate strand [11].
Two years later, another complex organic molecule, propylene oxide was found again
in Sgr B2(N) [12]. It is a chiral molecule, meaning that it exists in non-superimposable
forms that are mirror images of one another. Both of the characteristic traits of these
molecules are essential features of terrestrial biochemistry, and reflect the evidence
that organic matter can be naturally synthesized in space. The discovery that such
kind of molecules exist well outside our Solar System, long before a planetary surface is created, suggests that the potentially prebiotic chemistry traced by asteroids
and comets in our planetary system could be replicated elsewhere in the cosmos,
supplying with these life-bearing elements newly born planets.
C. Cecchi-Pestellini
same, but the physical conditions are such that some of the usual hypotheses are no
longer valid. The exotic reaction methodologies offered to synthetic organic chemists
to forge new chemical bonds are of course not available in space, and the synthesis
must occur through processes that a modern chemist undertaking routine synthesis
in a laboratory would consider insufferable or archaic. The basic idea is that radicals trapped in the iced mantles acquire mobility and react forming complex organic
molecules as the dust temperature reaches ∼ 30 K [7]. The origin and quantity of the
trapped radicals is uncertain; they could be e.g., pieces of iced species broken by the
ultraviolet photons or, the results of the incomplete hydrogenation of the simple parent species. Generally, their nature should reflect the ambient physical and chemical
conditions, and the radiation environment.
The complex species arising into the ice may be released to the gas-phase, where
they are eventually observed, through a variety of processes. Thermal desorption
seems a possible cause; it becomes sufficiently rapid in interstellar conditions when
the ice temperature rises to about 100 K, but is totally negligible for typical ice
temperatures of 10 K. Species on or near the surface of a dust grain can be desorbed
by photons of the interstellar radiation field. Where extinction is high so that the
interstellar radiation field plays no significant role, photodesorption may be driven
by the weak radiation field created when cosmic rays ionize hydrogen atoms and
molecules. The subsequent recombination spectrum of ions and electrons generates
an ultraviolet field [8]. As noted first by Léger and collaborators in 1985 [9] the
passage of heavy cosmic rays through dust grains deposits heat in the grains. On large
grains, the heat is deposited in a cylindrical volume along the track of the cosmic
ray, and thermal desorption of weakly bound molecules such as CO can occur from
the heated areas of the surface at the intersection of the cylinder and the surface.
Where the grains are small enough, the cylinder encompasses the entire grain, so
desorption occurs from the entire surface area of small grains. Other mechanisms
involve desorption driven by surface chemistry, grain explosions, and grain-grain
impacts. The efficiency of a process typically depends on the involved molecular
species, and it is mediated by the energy transfer at the molecular level (see e.g.,
[10]).
The organic molecules discovered in space until few years ago consisted of
a backbone of carbon atoms arranged in a single and roughly straight chain. In
2014, astronomers announced to have for the first time detected towards the galactic
centre—in the Sgr B2(N) molecular cloud—a carbon-containing molecule called isopropyl cyanide, in which the carbon structure branches off in a separate strand [11].
Two years later, another complex organic molecule, propylene oxide was found again
in Sgr B2(N) [12]. It is a chiral molecule, meaning that it exists in non-superimposable
forms that are mirror images of one another. Both of the characteristic traits of these
molecules are essential features of terrestrial biochemistry, and reflect the evidence
that organic matter can be naturally synthesized in space. The discovery that such
kind of molecules exist well outside our Solar System, long before a planetary surface is created, suggests that the potentially prebiotic chemistry traced by asteroids
and comets in our planetary system could be replicated elsewhere in the cosmos,
supplying with these life-bearing elements newly born planets.