On a clear moonless night, take a look at the glowing band of the Milky Way
and you will see some dark patches that look almost like holes in the shining
milky band. With a bit of luck, you鈥檒l also spot a 鈥渟hooting star鈥� 鈥� a shining
meteor flashing across the sky. These two different sky-sights are both
examples of space dust
THE SPACE between the stars is not quite a perfect vacuum. A volume of
space the size of a small matchbox would contain 鈥� on average 鈥� half a dozen
atoms, mainly hydrogen. Spread much more thinly are the solid particles, which
astronomers call dust grains. The gas and dust is spread out between the stars
of our galaxy, the Milky Way, a vast Catherine wheel of 200 000 million stars
(including the Sun), about 100 000 light years across. (A light year is the
distance travelled by light in one year, 9.46 million million 鈥�
9.46 脳 1012 鈥� kilometres.)
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The dust between the stars is very different from house dust. The grains
are less than a thousandth of a millimetre across. A volume of space the size
of St Paul鈥檚 Cathedral in London would contain only one dust grain. Although
the dust grains are too small to see individually and they are spread so
thinly, the regions between the stars are so huge that the light from distant
stars has to pass an immense number of grains on its way to us.
A star lying 3000 light years away from us is dimmed to half its normal
brightness by the dust between the star and the Earth. For each additional
3000 light years of distance, a star鈥檚 light is cut by half again. As a
result, we can see hardly anything of what is going on at the centre of the
Galaxy, for example, about 30 000 light years away.
Not all the dust is spread out uniformly. In places we find interstellar
clouds. These are regions where the gas and dust in space concentrate into
denser clumps. The dense concentration of dust blocks out virtually all the
light from more distant stars, so the cloud appears as a dark silhouette.
People in countries near the Equator and in the southern hemisphere can see
the most famous dark cloud, the Coal Sack, as a black region in the Milky Way
right next to the Southern Cross. From the northern hemisphere, we can see a
whole string of dark clouds in the constellation Cygnus. They line up along
the middle of the Milky Way and appear to split it in half lengthwise. These
clouds stretch over several thousand light years with a million times as much
matter as the Sun.
Grains in close-up
Dimming the wavelengths
ALTHOUGH the grains in interstellar space are microscopic and lie hundreds
of light years away, astronomers have deduced a lot about them from their
sizes and shapes, and even their composition.
The first clue comes from examining the starlight that penetrates the
general dust in space. In the 1920s, a Swiss astronomer, Robert Trumpler,
discovered that stars lying behind interstellar dust not only appear dimmer,
but are also slightly redder in colour than we would expect. He concluded that
the dust grains were absorbing more of the shorter wavelengths (blue light)
than the longer (red) wavelengths. So particles in the Earth鈥檚 atmosphere make
the Sun look redder when it is low in the sky (its colour has nothing to do
with the shift to red caused by moving bodies, the Doppler effect).
This discovery tells us the size of the dust grains. They must be smaller
than the wavelength of red light (0.7 micrometres, where a micrometre is a
thousandth of a millimetre), because the dust has relatively little effect on
these wavelengths 鈥� just as an ocean wave is not affected if it encounters a
protruding rock that is smaller than the distance between the wave-crests. But
the grains must be comparable in size to the wavelength (0.4 micrometres) of
the blue light that the dust affects more seriously.
Astronomers can also deduce the shape of the tiny grains by observing the
light of distant stars that passes them. Normally the light from stars is
unpolarised: the electromagnetic waves making up the light can vibrate in all
directions. When the light has passed through dust in space, however, it
emerges as partially polarised radiation: the waves are more likely to vibrate
in one direction than in any other direction.
These results suggest that the dust grains are not spherical, but
elongated. There are weak magnetic fields in space (no one knows why
precisely) that can line up the grains so that they are all parallel (just as
you can use a magnet to line up iron filings). As light passes through the
lined up grains, it is easier for one direction of polarisation to pass than
the others 鈥� just as it is easier to wiggle a skipping rope up-and-down
through the vertical slats of a fence than it is to shake the rope from side
to side. Astronomers believe that interstellar dust grains are at least three
times as long as they are wide.
Rock chips and ices
Yellow stuff in the lab
THE DUST absorbs the radiation of distant stars well at particular
wavelengths. This group of wavelengths, or absorption spectrum, indicates what
the grains are made of. Cosmic dust, for example, absorbs radiation at the
wavelength of 9.7 micrometres, in the infrared part of the spectrum.
Experiments in the laboratory show that many rocks absorb light at this
wavelength, because they contain silicates 鈥� compounds of silicon and oxygen
with various metals, such as iron, magnesium and aluminium, and occasionally
hydrogen. So the cosmic grains consist partly of silicates: they are tiny
chips of rock.
This conclusion is not too surprising, as silicon and oxygen are among the
commonest elements in the Universe (see Inside Science No.29). Astronomers
suspect that the grains must also contain some of the other common elements,
such as carbon, nitrogen and hydrogen. Along with oxygen, these elements can
make up two kinds of molecule: organic molecules and ices.
The ices consist of ordinary water ice (frozen H2O), along with
solid ammonia (NH3), methane (CH4), carbon monoxide (CO)
and carbon dioxide (CO2). Astronomers have found that dust grains
in the densest clouds absorb infrared radiation at a wavelength of 3.1
micrometres. Laboratory experiments show that ordinary ice absorbs at this
wavelength, suggesting that ice covers rocky grains in the densest clouds.
Organic molecules contain many more atoms and consist of a backbone of
carbon atoms, with atoms of oxygen, nitrogen and hydrogen attached. In his
laboratory at Leiden University in the Netherlands, Mayo Greenberg has found
that ultraviolet radiation will start chemical reactions between the molecules
in a mixture of ices, and convert them into organic molecules. He ends up with
a complicated mix of organic molecules: because of its colour, he calls the
sticky mixture 鈥測ellow stuff鈥�.
So, the basic rocky chips of interstellar dust are probably coated with a
yellow sticky layer of organic matter, with 鈥� in the denser clouds 鈥� a
frosting of ices on top.
The interstellar dust also absorbs ultraviolet light at a wavelength of
0.22 micrometres. This shows the presence of carbon, in the form of graphite
(black 鈥減encil-lead鈥�), amorphous carbon (powdery specks of soot) or
hydrogenated amorphous carbon (fragments of carbon soot with hydrogen atoms
stuck all over the surface). The ultraviolet spectrum suggests that these
carbon grains are much smaller than the rocky grains, only 0.01 micrometres.
They make up only a few per cent of the total amount of dust in space.
As a complete alternative to this view of interstellar grains, the British
astronomers Fred Hoyle and Chandra Wickramasinghe have suggested that
interstellar grains are freeze-dried viruses and bacteria, perhaps even the
source of life on Earth and other planets. Few other scientists believe this
unorthodox theory: they regard it as an over-elaborate explanation of
phenomena that can be explained more simply.
Some dusty beginnings
Birth of the Solar System
ASTRONOMERS call the largest and densest interstellar clouds molecular
clouds because the gas atoms here can join up into simple molecules, such as
molecular hydrogen (H2), formaldehyde (H2CO) and ethanol
(C2H5OH).
The gas (and dust) is so dense that individual blobs can collapse under
their own gravity to form stars (see Inside Science No.11). Surrounding the
young stars is a swirling disc of gas and dust grains, that may be destined to
become a system of planets (see Inside Science No.30).
Our Solar System was born in this way, about 4.5 thousand million years
ago. Despite its dusty beginnings, however, the Solar System is now
comparatively free of dust grains. Most of the original interstellar grains
became incorporated into the planets. The rest were swept out of the Solar
System, when the young Sun expelled a high-speed 鈥渨ind鈥� of gases. Astronomers
call this stormy period the Sun鈥檚 T Tauri phase 鈥� named after a star born
recently some 500 light years away, and which is now living through a period
of violent activity.
Although the Sun is quieter now, a gentle solar wind still blows radially
outwards. The pressure of sunlight forces the larger dust particles outwards,
beyond the planets. For particles smaller than the wavelength of sunlight, the
situation is reversed: these tiny particles spiral inwards to disappear into
the Sun.
Nonetheless, the space between the planets is not completely empty. Shortly
after sunset, or before sunrise, you may spot a faint cone of light extending
upwards from the horizon. It passes through the constellations of the Zodiac.
Under exceptionally clear conditions, you can see the zodiacal light extend
right round the sky to the Gegenschein, a faint glow that lies exactly
opposite the Sun. The zodiacal light and the Gegenschein are caused by the Sun
lighting up a faint sprinkling of dust orbiting the Sun between the planets.
In 1983, Japanese researchers found a denser ring of dust circling close in
to the Sun, much closer than the planet Mercury. These dust particles are
heated to 1000 掳C, and glow brightly at infrared wavelengths. The
scientists picked up the dust ring when the Sun鈥檚 own intense infrared
radiation was blocked by the Moon during an eclipse. Soon afterwards, an
Earth-orbiting telescope called the Infrared Astronomical Satellite (IRAS)
found belts of dust in the asteroid belt between the orbit of Mars and
Jupiter.
Dusty comets and CHONs
New space missions
THE SUN cleans the dust away so effectively that it should have cleared out
the Solar System long ago. So somehow all this interplanetary dust must be
constantly replenished. Some of the dust comes from collisions between
asteroids, but most of the dust is debris shed by comets from the realm beyond
the planets.
When the Solar System was born, the icy dust grains in its distant outer
regions remained almost unchanged by the heat of the infant Sun. These
particles coagulated into icy bodies a few tens of kilometres across.
Astronomers often call these bodies 鈥渄irty snowballs鈥�; more technically, they
are cometary nuclei.
Occasionally, one of the cometary nuclei falls towards the inner reaches of
the Solar System. The Sun鈥檚 heat evaporates its ices, and the resulting
high-speed jets of steam and gas spurt out into space to form the familiar
coma (head) and tail of a comet. These jets also rip out the solid grains
from the cometary nucleus, to fill the coma and tail with recycled
interstellar dust.
In 1986, two Soviet and one European spacecraft flew through the coma of
Halley鈥檚 Comet to investigate a comet in close-up. The camera on Giotto, the
European craft, photographed the nucleus and its spewing jets in detail: the
comet was ejecting 12 tonnes of dust every second. Giotto ran into thousands
of dust particles, including one that knocked it sideways so that Giotto lost
radio contact with the Earth for a few minutes. But even the 鈥済iant鈥� grain
that disturbed Giotto鈥檚 path had a mass of only a thousandth of a gram, and
there were many more with masses down to a million-millionth of a gram.
All three craft carried instruments to analyse the elements in the dust
grains. They found three main kinds. Some of the grains are 鈥渞ocky鈥� silicates,
while the majority consist of a mixture of silicates with some organic
molecules. The third 鈥� and most enigmatic 鈥� type consist entirely of organic
molecules. Astronomers have called this third type the CHON particles, because
they consist only of carbon (C), hydrogen (H), oxygen (O) and nitrogen (N).
The CHON particles must consist of organic molecules that are less volatile
than the ices.
Astronomers hope that two further space missions will help us to unravel
the structure of the CHON particles. The Comet Rendezvous and Asteroid Flyby
mission is planned for around the year 2000. After flying past an asteroid,
the craft will meet up with a comet and keep it company as it rounds the Sun,
analysing the gas and dust it emits. Further in the future is the even more
ambitious Comet Nucleus Sample Return mission. This craft will scoop a sample
from the 鈥渄irty snowball鈥� nucleus of a comet.
Every August, some of the dust from a comet puts on a celestial display.
The Earth runs into a cloud of dust left behind by an old comet called Swift-
Tuttle, and the dust grains burn up in the Earth鈥檚 upper atmosphere as a
shower of shooting stars, or meteors.
For all its brilliance, the dust particle that causes a meteor is quite
small. It weighs roughly as much as a grain of sand, though it is fluffier in
texture. Even though the grains are individually small, they are so numerous
in interplanetary space that the Earth sweeps up some 100 000 tonnes of cosmic
dust every year. Because meteors burn up totally, they can tell us little
about space dust directly. So astronomers try to catch this space dust before
it hits the Earth鈥檚 atmosphere. From the experiments so far, scientists have
worked out how many grains there are of different masses, but they still have
little idea of what the grains are made of, because the impact on the
experimental equipment breaks up the grains.
Meteoritic falls
Asteroid impacts
VERY RARELY, a 鈥渇ireball鈥� flashes through the sky. To our eyes, it
resembles a brilliant meteor 鈥� brighter than any planet or star. Like a
meteor, a fireball marks the entry of a piece of space debris into the
atmosphere. But the object falling is a solid piece of stone or iron, often
weighing many kilograms. Much of the object will survive its fiery passage
through the atmosphere, to hit the ground as a meteorite.
Meteorites are not, in themselves, 鈥渃osmic dust鈥�. They are splinters of
rock or iron broken off in a collision between two asteroids 鈥� minor planets
in the region between Mars and Jupiter. The asteroids are the 鈥渕issing link鈥�
between space dust and the planets. When the Solar System formed, the dust
grains stuck together to form billions of objects resembling the asteroids.
Most of these accumulated into the nine major planets, so destroying all hints
of their original composition. In contrast, the material in the asteroids 鈥�
especially near the surface 鈥� has changed comparatively little since they
formed from an aggregate of cosmic dust.
Astronomers suspect that one particular type of meteorite, called a
carbonaceous chondrite, has come from the surface of largely unchanged
asteroids. It is a good place to look for grains of interstellar dust that may
have survived both the formation of the Solar System and the accumulation of
dust into an asteroid.
It takes months of painstaking work to sort out individual fragments and to
identify which ones are actually unaltered grains of interstellar dust because
the original dust grains are compressed and jumbled in the meteorite. One
vital clue has come from minute traces of gas contained within the tiny
fragments. The Sun, the planets and the other bodies born with the Solar
System have the same proportions of the various isotopes (atoms of the same
element but with different numbers of neutrons and thus of different atomic
mass) of elements such as neon and xenon. Researchers have found that some of
the fragments inside carbonaceous chondrites contain strange proportions of
these gases. This suggests that the fragments are interstellar grains that
have been preserved, more or less intact, from the time before the Solar
System was born. These fragments consist of various types of 鈥渞ocky鈥�, or
silicon-based, substances, and also carbon in various forms 鈥� including tiny
crystals of diamond.
By taking meteorites apart into their grains, researchers are building up a
collection of different types of interstellar dust. The unusual gases they
contain are specimens of the material of distant stars that we could never
hope to sample directly.
Making interstellar dust: sooty chimneys in space
SPACE DUST is formed by old dying stars. All stars are busy manufacturing
the elements of interstellar dust (silicon, oxygen, carbon and so on), as
nuclear reactions in their centres convert hydrogen and helium into more
complex elements. In a relatively young star, such as the Sun, these elements
stay well inside.
When a star becomes older, however, it begins to get smoky. The star
expands a hundredfold to become a red giant. Currents of gas in its huge
unstable atmosphere can dredge up the newly formed elements from the star鈥檚
interior to its surface. The temperature is now so low (below 3000 掳C) that
some of these gases condense into solids.
Depending on its life history, the surface of a red giant may be rich in
either carbon or oxygen. A carbon star produces a thick pall of soot because
the carbon atoms condense into large molecules just as carbon does in the
flames of a fire. The soot consists of carbon in various forms: flakes of
graphite and small formless lumps of amorphous carbon are probably present,
probably with hydrogen-coated molecules of 鈥渉ydrogenated amorphous carbon鈥�.
The soot may aso contain some spherical molecules composed of about 60 carbon
atoms enclosing a hollow space that resembles a football or two geodesic domes
stuck together. (This molecule is called buckminsterfullerene, after the
scientist Buckminster Fuller who invented the geodesic dome, which is made up
of identical triangles linked to form a dome.) The carbon grains form the
smaller component of interstellar dust.
If the star contains more oxygen than carbon, the oxygen atoms react with
silicon and any metal atoms are around, to form silicates, such as magnesium
silicate or iron silicate. Because oxygen-rich red giants are ten times more
common than carbon stars, they produce the major component of interstellar
dust 鈥� the comparatively large silicate grains.
When a star becomes sufficiently shrouded by newly condensed grains, the
dust particles can block out its light. One red giant star, R Coronae
Borealis, disappears entirely from view every few years at it belches out a
puff of interstellar dust. Other old stars are so enveloped in dust that we
cannot see them at all: only infrared telescopes can pick out the radiation
from the warm surrounding dust.
Exploding stars also eject gas than can condense into grains. In a nova
outburst, gas falls from a normal star onto a small white dwarf, and then
explodes. Gas condensing into dust grains can partially hide a nova several
weeks after it has exploded. A supernova is the entire disruption of a star
because of unstable nuclear reactions at its centre. A supernova produces
powerful shock waves that smash together grains of carbon soot with such force
that they change into tiny diamonds.
Catching dust on Earth: a fall of space dust
DURING the round-the-world expedition of the British scientific survey ship
Challenger in the 1870s, the scientists dredged up from the ocean floor some
microscopic spheres. They suggested that these spherules were tiny meteorites.
Astronomers now believe that most of the spherules are melted specks of
meteors burning up in the atmosphere, but a few are indeed particles of
genuine interplanetary dust that have drifted down to the Earth鈥檚 surface
unchanged. The best examples come from Antarctica, where they are preserved in
the ice. The spherules are made of a similar mix of elements to the
meteorites, and they may be 鈥渟parks鈥� struck off when asteroids have collided.
Inspired by the idea of dust drifting in from space, an American
researcher, Don Brownlee, has been trying to catch the particles as they fall
to Earth while they are still high in the atmosphere, above any likely sources
of terrestrial and artificial pollution. He has fished for the cosmic dust
with a converted U2 spy plane, which flies at an altitude of 18 000 metres.
The plane carries large plates, covered with oil to make them sticky to catch
the cosmic dust.
Brownlee鈥檚 technique has been highly successful. The flights have brought
back small fluffy pieces of dust that are almost certainly grains shed by
comets. A Brownlee particle is typically 10 micrometres in size, and is made
up of silicates and other rocks. It consists of hundreds of smaller fragments
stuck together 鈥� each possibly an individual grain of interstellar dust,
preserved in a comet and now thawed and captured for us to examine at leisure.
Further Reading
Comets: A Chronological History of Observation, Science Myth and Folklore,
by Donald K. Yeomans (Wiley, New York, $35); The Milky Way, fifth
edition, by Bart J. Bok and Priscilla F. Bok (Harvard University Press,
拢23.95).