FORGET the Restaurant at the End of the Universe鈥攖he Bar at the Centre
of the Galaxy is where it鈥檚 at. At a conservative estimate, the gas cloud
Sagittarius B2 contains 1027 (that鈥檚 a billion billion billion) litres of
alcohol at 200 per cent proof. Sadly, though, the alcohol is smeared throughout
an enormous region of space in the form of a super-tenuous gas. To get enough to
fill a whisky glass, you鈥檇 need to trawl a volume as big as the Earth
itself.
Sagittarius B2 is a vast cloud 150 light years across lying within 400 light
years of the giant black hole that lurks in the dark heart of the Milky Way. It
is one of the biggest of the molecular clouds that account, collectively, for
about 10 per cent of our Galaxy鈥檚 visible mass. And it鈥檚 a sobering (or not so
sobering) thought that there is considerably more alcohol in a cloud like
Sagittarius B2 than has been distilled in the entire history of the human
race.
Twenty years ago, molecular clouds seemed little more than amorphous clumps
of cold gas and dust floating between the stars. However, today鈥檚
high-resolution imaging has changed this picture. Molecular clouds turn out to
be very complex. 鈥淚f there is a typical one鈥攁nd they are a very varied
crowd鈥攊t consists of a relatively diffuse cloud of gas at about 20 degrees
above absolute zero studded with hundreds of hotter, denser regions at about 200
to 400 kelvin,鈥� says Lew Snyder, professor of astronomy and director of the
Laboratory for Astronomical Imaging at the University of Illinois,
Urbana-Champaign.
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Dubbed 鈥渉ot molecular cores鈥�, the hot dense regions are stellar nurseries.
They are also the places where molecules such as methyl alcohol (methanol) and
ethyl alcohol (ethanol) are formed.
Chemistry in these clouds would be mind-bogglingly slow if it had to wait for
the widely dispersed atoms or molecules flying through space to collide and
stick together. However, the clouds also contain cold dust grains, usually made
up of mantles of water ice and carbon compounds wrapped round silicon cores. The
dust speeds things up by transforming chemistry from three dimensions to two.
Molecular fragments collide and stick to the surfaces of the grains, then
鈥渕igrate鈥�, hopping around until they bump into a molecular mate. The energy to
make the fragments hop comes from the grains, which are heated by newborn stars
nearby.
Methanol and ethanol are among the 120 or so known interstellar molecules
manufactured in this way. According to current ideas, they are created in one of
two ways: they either form on grains of dust and then evaporate, or their
components form on the grains and evaporate, and the final assembly takes place
in space.
And it鈥檚 here, in space, that ethanol and methanol signal their presence.
Both molecules rotate like tiny tops, but only at certain distinctive
rates鈥攁n effect of quantum mechanics. Imagine a roundabout in a children鈥檚
playground that can only make a turn once a minute, twice a minute, three times
a minute, and so on, but is forbidden to turn at any other rates. The rotation
of molecules is restricted in a similar way.
If a molecule of alcohol collides with another molecule鈥攕ay, of
hydrogen, which is the most common molecule in space鈥攖he sudden jolt of
energy can cause its spin rate to jump from one value to another. When the
molecule relaxes back to a slower spin rate, it sheds the excess energy in the
form of a photon with a wavelength in the centimetre or millimetre range.
Since different molecules radiate at characteristic wavelengths, astronomers
have a foolproof means of distinguishing them. The great thing about light in
the millimetre and centimetre region of the spectrum is that it easily
penetrates the chokingly thick gas and dust in molecular clouds, providing a
unique window into regions of star formation.
Methanol emits light at a wavelength of about 30 centimetres. It was first
detected in 1970 when John Ball, Carl Gottlieb and A. E. Lilly of the
Harvard-Smithsonian Center for Astrophysics found it in both Sagittarius B2 and
a nearby cloud, Sagittarius A. Ethanol, on the other hand, emits light at about
3 millimetres. It was discovered in 1975 by a team led by Ben Zuckerman, then at
the University of Maryland, using the 36-foot National Radio Astronomy
Observatory dish at Kitt Peak in Arizona. The ethanol found by Zuckerman鈥檚 team
was trans-ethanol, one of the two possible conformations of the molecule. The
other, gauche-ethanol, was finally spotted in 1997 by John Pearson of the Jet
Propulsion Laboratory in Pasadena.
Alcohols are also highly concentrated within molecular clouds鈥攑erhaps
even more so than we realise. Unless astronomers can pick out the more
concentrated clumps in the clouds, they tend to assume that alcohols are evenly
spread. But it鈥檚 possible that there are many clumps that are too small to be
resolved by current millimetre-wave telescopes. If so, says Snyder, we could be
underestimating the concentration of the alcohol. It could even be 1000 times
more concentrated than we think it is.
In common with other interstellar molecules, alcohols have a key role to play
in star formation. A star is born when gravity causes a cloud of interstellar
gas to collapse. However, even the meagre heat in these clumps of cold gas can
generate enough pressure pushing outwards to oppose gravity. But the energy
radiated by molecules saps molecular clouds of their heat, enabling gravity to
gain the upper hand and start the collapse that leads to star birth.
But while alcohol was obviously important in the birth of the Sun and Earth,
the story doesn鈥檛 end there. Alcohol is common not only in interstellar space
but also in comets, the icy debris left over from the formation of the Solar
System 4.6 billion years ago. There was rather a lot of it, for instance, in
Comet Hale-Bopp.
The comet connection isn鈥檛 very surprising. Comets, like the Sun, congealed
out of a long-dead molecular cloud. 杏吧原创s believe that cometary collisions
with the newborn Earth supplied the planet with many of the molecules necessary
to jump-start biochemistry鈥攁nd alcohols are a prime example. They may well
have helped to form the large molecular chains that life needed, says Snyder.
鈥淲e suspect that alcohols from space played a key role in getting to amino acids
like glycine and alanine.鈥�
If he鈥檚 right, alcohol could lie at the root of all life. That鈥檚 something to
think about, next time you鈥檙e raising your glass.