5 Organics on the Rocks: A Cosmic Origin for the Seeds of Life
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make up the bulk of all atoms in the Universe. Of the remaining 0.1%, half comprises
oxygen atoms, and a quarter is carbon. All the other elements are compressed into
the left 0.025%. It is surprising then, if we look at the list of the approximately
200 molecules discovered in space, carbon-containing molecules dominate, making
organic chemistry the norm. In regions that are almost unimaginably cold and dark,
molecules are slowly forming, bit by bit, over literally millions of years. The key
feature of the chemistry going on in space is that it takes a long time. But still this is
not enough. Interstellar space is too cold for most chemical reactions to occur, as the
low temperature makes it hard for molecules drifting through space to acquire the
energy needed to break their bonds, and form new others to make larger molecules.
However, occasionally, the gas molecules collide with the dust grains. Some freeze
there to form molecular ices, and exposed to the harsh space environment can be
transformed by surface and bulk interactions into more complex molecules, before
being released again into the gas.
Astronomers have known for decades that there is a substantial amount of water
in space, and in fact water ice is the most abundant interstellar solid material. As a
dominant form of oxygen, the most abundant element in the Universe after hydrogen
and helium, water controls the chemistry of many other species. Water molecules
in the interstellar gas were detected very early in the history of astrochemistry. In
1969, Charles Townes and colleagues detected strong water emission in a line with
a wavelength of 1.38 cm, in the radio part of the spectrum [5]. Interstellar water ice,
as distinct from gaseous water molecules, was first identified by Gillett and Forrest
in 1973 [6], and is now widely detected in interstellar dark clouds.
Dust grains in cold astronomical regions build up appreciable ice mantles through
the accretion and subsequent surface chemistry of atoms and molecules from the gas.
The timescale for an atom or molecule to collide with a grain and stick to it (the socalled freeze-out time) is inversely proportional to the molecular number density;
in dense clouds it is of the order of 100,000 years or lower, generally smaller than
cloud lifetimes. Despite the fact hydrogen is a very weakly bound species, hydrogen
atoms have a residence time on dust grains at temperatures of 10–20 K long enough
to react with oxygen to form water molecules. The ice formed is not pure, but it may
contain CO, CO 2 , and other less abundant species. Such species may stick to a grain
directly from the gas-phase, or be formed in the same way as water, with carbon
combining with hydrogen to make methane, or nitrogen with hydrogen to produce
ammonia. These molecules are frozen over by a layer of ice, preserving them for
over millions of years. Such a favourable circumstance enables a dramatic chemical
transformation to take place in the interstellar space. Ices containing such simple
species can be “cooked” in various ways to produce more complex species. Such a
processing can be driven by fast charged particles, energetic radiation, in particular
ultraviolet photons and X-rays, as well as through the warming up of the ice. The
energy injection provides the activation of molecules initially present in the ices,
allowing the development of an important chemical reactivity.
What kind of chemical complexity can we expect? Chemistry in interstellar environments is in many ways very different from the chemistry in terrestrial chemical
laboratories that we are more accustomed to. Evidently, basic concepts remain the
29
make up the bulk of all atoms in the Universe. Of the remaining 0.1%, half comprises
oxygen atoms, and a quarter is carbon. All the other elements are compressed into
the left 0.025%. It is surprising then, if we look at the list of the approximately
200 molecules discovered in space, carbon-containing molecules dominate, making
organic chemistry the norm. In regions that are almost unimaginably cold and dark,
molecules are slowly forming, bit by bit, over literally millions of years. The key
feature of the chemistry going on in space is that it takes a long time. But still this is
not enough. Interstellar space is too cold for most chemical reactions to occur, as the
low temperature makes it hard for molecules drifting through space to acquire the
energy needed to break their bonds, and form new others to make larger molecules.
However, occasionally, the gas molecules collide with the dust grains. Some freeze
there to form molecular ices, and exposed to the harsh space environment can be
transformed by surface and bulk interactions into more complex molecules, before
being released again into the gas.
Astronomers have known for decades that there is a substantial amount of water
in space, and in fact water ice is the most abundant interstellar solid material. As a
dominant form of oxygen, the most abundant element in the Universe after hydrogen
and helium, water controls the chemistry of many other species. Water molecules
in the interstellar gas were detected very early in the history of astrochemistry. In
1969, Charles Townes and colleagues detected strong water emission in a line with
a wavelength of 1.38 cm, in the radio part of the spectrum [5]. Interstellar water ice,
as distinct from gaseous water molecules, was first identified by Gillett and Forrest
in 1973 [6], and is now widely detected in interstellar dark clouds.
Dust grains in cold astronomical regions build up appreciable ice mantles through
the accretion and subsequent surface chemistry of atoms and molecules from the gas.
The timescale for an atom or molecule to collide with a grain and stick to it (the socalled freeze-out time) is inversely proportional to the molecular number density;
in dense clouds it is of the order of 100,000 years or lower, generally smaller than
cloud lifetimes. Despite the fact hydrogen is a very weakly bound species, hydrogen
atoms have a residence time on dust grains at temperatures of 10–20 K long enough
to react with oxygen to form water molecules. The ice formed is not pure, but it may
contain CO, CO 2 , and other less abundant species. Such species may stick to a grain
directly from the gas-phase, or be formed in the same way as water, with carbon
combining with hydrogen to make methane, or nitrogen with hydrogen to produce
ammonia. These molecules are frozen over by a layer of ice, preserving them for
over millions of years. Such a favourable circumstance enables a dramatic chemical
transformation to take place in the interstellar space. Ices containing such simple
species can be “cooked” in various ways to produce more complex species. Such a
processing can be driven by fast charged particles, energetic radiation, in particular
ultraviolet photons and X-rays, as well as through the warming up of the ice. The
energy injection provides the activation of molecules initially present in the ices,
allowing the development of an important chemical reactivity.
What kind of chemical complexity can we expect? Chemistry in interstellar environments is in many ways very different from the chemistry in terrestrial chemical
laboratories that we are more accustomed to. Evidently, basic concepts remain the