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How Were the Comets Made?
Explaining the composition of these 4.5 billion-year-old relics may require
scientists to revise their models of the primitive solar nebula
Joseph A. Nuth III
I
n traditional Hindu mythology, the
god Shiva is both creator and destroyer, bringing life and death to the
world. It is an odd pairing of talents
for a deity, and yet there may be a realworld counterpart in the heavens, an
avatar if you will, in the form of
comets. Some recent studies suggest
that a rain of comets in the very early
history of our planet, perhaps 4 billion
years ago, may have seeded the young
Earth with complex organic molecules
from space—key ingredients necessary
for life to arise. On the other hand, giant comet impacts may be responsible
for some of the major extinction events
in the history of life on Earth, including
the demise of the dinosaurs 65 million
years ago. Comets, it seems, may have
been both creators and destroyers in
our own history.
Yet the comets have a history of their
own, and the more we find out about
them the more enigmatic they seem.
Although it has been said that “a comet
is as close to being nothing as something can be”—in reference to the diffuse tail a comet emits near the sun—
that bit of “something” holds important
Joseph A. Nuth III has been head of the
Astrochemistry Branch at NASA’s Goddard Space
Flight Center since 1990. He often retreats to the
laboratory to conduct experiments designed to
further our understanding of the behavior of solids
in astrophysical environments. He obtained B.S.
degrees in astronomy and chemistry, an M.S. in
geochemistry and a Ph.D. in chemistry from the
University of Maryland at College Park.
Following several years as an NAS/NRC Resident
Research Associate at NASA Goddard and as an
NAS/NRC Research Management Associate at
NASA Headquarters, he joined the civil service at
Goddard in 1986 as an astrophysicist. Address:
Laboratory for Extraterrestrial Physics, Code 691,
NASA Goddard Space Flight Center, Greenbelt,
MD 20771. Internet: [email protected]
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American Scientist, Volume 89
clues for the planetary scientist. As best
as we can tell, comets are the most
primitive bodies in the solar system.
Some of the material inside a comet is
preserved in nearly the same state it
was in when the solar system was just
taking shape, before the Sun and the
planets were fully formed. Each comet
is effectively a “grab bag” sample of the
building blocks present in the nascent
solar system—the solar nebula—at the
time the comet was formed, about 4.5
billion years ago. Although the nebular
material may have undergone considerable processing before it was incorporated into the comet, very little has
been altered since. A comet is literally a
little piece of the past.
Although we can’t snatch a comet
from the sky and examine it in the laboratory, there are ways to get the next
best thing. We can measure the spectral properties of a comet as it swings
by the Earth, and we can examine
some of the particulate remains of
comets in the form of interplanetary
dust particles collected by special,
high-flying aircraft in the stratosphere.
On the basis of these studies we can
then make analogues in the laboratory,
and so understand something about
how a comet must be made. In this article I will review what such studies
have told us about the comets, and
what the comets tell us about the
processes that must have taken place
in the early history of our solar system.
A Snowball from Hell?
At a basic level, a comet is simply a collection of silicate dust and a smattering of organic molecules, coated with
ices made primarily of water. Some of
the ice-coated grains may have been
present in the giant molecular cloud
that partly collapsed to form the solar
nebula, but others must have formed
in the solar nebula itself. Part of the
task of understanding comet chemistry
is to determine when, where and how
the dust, the organics and the ices
came together.
In general, we believe that comets begin to form by an accreting “snowball”
effect in which the icy dust grains stick
together to form fractal-like aggregates
(Figure 2). This process begins at some
considerable distance from the center of
the solar nebula, perhaps as far as 100
astronomical units (AU) away. (For a
sense of scale, consider that the Earth is
merely one AU from the Sun, whereas
Pluto is 40 AU away.) At this stage, the
movements of the dust grains and the
small aggregates are coupled to the
movements of the ambient nebular gas.
Over time, however, as the aggregates
accumulate into compact, boulder-sized
snowballs, or cometesimals, they are
slowed down by drag in the ambient
gas, and they start to drift inward as
their orbits decay. As the cometesimals
fall closer to the center of the solar nebula, they continue to grow by the accretion of ice and dust grains, as well as by
merging with other aggregates in their
path. In due course this pile of rubble
becomes a comet, perhaps 10 to 20 kilometers across, which contains a collection of materials from a wide swath of
its orbital radius.
Estimates of how long it takes to
build a comet in this way depend partly on the size of the solar nebula in
which the comet forms. In one model,
Stuart Weidenschilling of the San Juan
Capistrano Institute has shown that a
good-sized comet could be made in
about 100,000 years. Weidenschilling’s
model assumes that the evolving solar
nebula had merely the minimum mass
needed to explain the composition of
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Tony & Daphne Hallas/Science Photo Library
Figure 1. Comets contain a grab-bag sample of materials that were collected during the early evolution of our solar system, about 4.5 billion
years ago. By examining the chemical composition of a comet, scientists can begin to understand some of the processes that must have taken
place inside the solar nebula before the Sun and the planets were fully formed. Here Comet Hale-Bopp displays long tails of ionized gas
(blue) and dust (white) as it makes a mad dash past the Earth in March 1997. The stars and the North America nebula (NGC 7000; red) in the
constellation Cygnus provide a graceful backdrop.
our solar system. A somewhat more
massive solar nebula could assemble a
comet much more quickly, perhaps in
as little as 10,000 years, since the gasinduced drag and gravitational instabilities are greater in the larger nebula.
The solar nebula itself had a limited
lifetime, from the moment it started to
collapse from the molecular cloud to the
point where the last of the gas had dissipated and the Sun and the planets had
formed. Current estimates for its duration range from about 100,000 years to
tens of millions of years. It was during
this period that the comets and most of
their organic constituents must have
been made. The relative time it takes to
build a comet and the duration of the
solar nebula have consequences for the
composition of the comets. If the time
scales are comparable, then all comets
should be fairly similar. If, however, the
lifetime of the solar nebula was much
greater than the time it takes to assemble a comet, then we could expect some
diversity among the comets. Since the
chemical composition of the nebula
changes with age, comets assembled
early on should be quite different from
those built late in the nebula’s life.
Attempts to model the composition
of the cometary volatiles—the ices and
the organics—have met with mixed
success. Bruce Fegley of Washington
University in St. Louis has been trying
to match the spectral properties of these
enigmatic bodies by resorting to a seemingly arbitrary mixture of interstellar
ices, volatiles from the solar nebula,
plus some volatile components that
must be synthesized at relatively high
temperatures and pressures. Such a
concoction is difficult to explain with
Weidenschilling’s model, in which the
cometary components are all formed far
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2001
May–June
229
a
interstellar dust grain
b
unprocessed dust grain
in solar nebula
c
loosely aggregated
fractal-like dust grain
organic
varnish
amorphous
silicate core
ice coating
0.25 micrometer
d
baseball-sized aggregate
10 centimeters
2–5 micrometers
0.5 micrometer
e
boulder-sized cometesimal
f
1 meter
comet nucleus
10–20 kilometers
Figure 2. Traditional views of comet formation hold that materials in the outer parts of the solar nebula aggregate into larger cometary components in a “one-way trip” as the material spirals in on its orbit around the protosun. The process begins with a grain of interstellar dust
(a)—made of an amorphous silicate core and a “varnish” of organic molecules—which is present in the molecular cloud that collapses to
form the solar nebula. Many of these dust grains acquire a coating of ices (b). Once inside the solar nebula, they form larger, fractal-like dust
particles (c) when the icy grains collide and stick together. Aggregation of the particles eventually produces cometesimals of ever-increasing
size (d) and (e) until a comet-sized object forms (f). Ultimately, the materials in a comet are collected from a wide swath of its orbital radius.
Recent research suggests that traditional models may be too simple.
from the high temperatures and pressures near the center of the solar nebula.
To get around this problem, Fegley
has suggested that the more complex
organics formed in the giant gaseous
subnebulae that have been proposed as
the first stage in the formation of the giant planets. These subnebulae would
have much higher temperatures and
pressures than other parts of the outer
solar nebula. As the giant, gaseous
protoplanets migrated inward to their
present locations, some of the gas from
the subnebulae escaped, providing a
source for the high-temperature, highpressure volatiles. In this view, variations among comets are due to different
proportions of materials arising from
various regions of the outer nebula.
Some recent work has further complicated scientists’ efforts to explain the
formation of comets. Observations of
infrared spectra from Comet Halley by
Humberto Campins, now at the University of Arizona, and Eileen Ryan,
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American Scientist, Volume 89
now at New Mexico Highlands University, indicate that some of its silicate
grains must consist of crystalline
olivine (Figure 4). This has been confirmed more recently by the Infrared
Space Observatory, which found that
Comets Hyakutake and Hale-Bopp
both contain magnesium-rich, crystalline olivine. The troubling aspect of
these observations is that crystalline
olivine has never been found in the
general interstellar medium or within
the giant molecular clouds that ultimately collapse to form new stars. It
stands to reason that the olivine crystals must be a product of processes that
took place as the solar nebula was
evolving.
What does it take to make a grain of
crystalline olivine? To answer this
question, my colleagues and I have
been attempting to create analogues in
the laboratory with properties much
like those of cometary grains (Figure 5).
By burning silane (SiH4) and magne-
sium-metal vapor in a stream of hydrogen gas at temperatures near 800
kelvins, and then heating (annealing)
the resulting “smokes” at higher temperatures in a vacuum, we have effectively been able to “cook up” grains of
crystalline olivine that bear a close resemblance to those formed in the solar
nebula. The trick, it turns out, is to anneal the ingredients at the right temperature for the right amount of time.
Starting with amorphous silicate
grains, much like those that would have
been present in the interstellar medium,
we found that crystalline olivine could
be produced in a matter of months at a
temperature of about 1,000 kelvins.
Raise the temperature a notch to about
1,100 kelvins and the same task can be
accomplished within a matter of minutes. However, if you raise the temperature as high as 1,600 kelvins, the grains
are vaporized. On the other hand, lowering the temperature to below 850
kelvins would require more than one
© 2001 Sigma Xi, The Scientific Research Society. Reproduction
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billion years to produce a crystal of
olivine from our smokes!
These experiments place a strong
constraint on the way in which the
comets must have been made. Because
no one believes the solar nebula could
have existed for a billion years, the crystalline olivine must have been annealed
at temperatures close to 1,000 kelvins.
But since such temperatures would
have destroyed the icy mantles that cover the silicate grains, we can conclude
that the “hot” and “cold” components
were made in separate regions of the
nebula, and then later mixed together.
This means that the standard scenario
of comet formation, involving a “oneway trip” of agglutinating cometesimals, cannot be the full story.
A Complex Solar Nebula
Scientists who study meteorites have
known for decades that a certain
amount of mixing must have taken
place in the primitive solar nebula.
Some meteorites contain highly processed materials that are inexplicably
embedded within a matrix of very
primitive materials. The processed materials include the CAIs (calcium-aluminum inclusions), which required
temperatures peaking near 2,200 kelvins
for their manufacture, and the chondrules, which contain less heat-resistant
minerals (such as olivine and plagioclase) that saw temperatures no higher
than 1,700 kelvins. The CAIs and the
chondrules are often embedded in a
matrix containing highly fragile carbonbased components (diamond, graphite
and silicon carbide grains), some of
which are only a few nanometers across,
and would be destroyed at temperatures
as low as 600 kelvins.
What could have brought these materials together? Some meteoriticists
suspected that lightning or magnetic
reconnection events might have provided localized regions of high temperature to form the chondrules while
preserving the more fragile materials
in the cooler, adjacent nebular regions.
This, however, doesn’t explain the CAIs,
which were formed at much higher temperatures and required a much longer
cooking time than is possible in a transient event such as lightning. Suitable
environments could be found closer to
the protosun, and some scientists suggested that turbulent convection might
have lofted the CAIs out to distances
of a few AU, where the asteroids are
currently found. (Asteroids are gener-
Figure 3. Potato-shaped nucleus of Comet Halley (measuring about 8 by 16 kilometers) was
imaged by the Giotto spacecraft in 1986. Giotto was one of many instruments, including
earthbound telescopes and other spacecraft, that were trained on the object as it swept
through the inner solar system. The information gathered during the comet’s passage has
caused scientists to revise their ideas of how comets are made. Here the bright rays emanating from the nucleus represent outgassing on the comet’s sunward side. (Image courtesy of
H. U. Keller, Halley Multicolour Camera, ESA and Giotto. Copyright Max-Planck-Institut für
Aeronomie.)
ally considered to be the parent bodies
of the meteorites.)
Pieces of the puzzle began to come
together in the mid-1990s when astronomers studying a superficially unrelated problem came up with a viable
mechanism for mixing materials in the
solar nebula. Frank Shu and his colleagues at the University of California,
Berkeley, were trying to understand
the dynamic interactions between
growing protostars and their nebular
accretion disks. According to their calculations, interactions between the disk
and the protostar could produce a
powerful wind that could account for
the bipolar outflows observed around
many young stars. Soon after proposing this “extraordinary wind” (or Xwind) model, Shu’s team realized that
the same violent interactions might be
responsible for producing both the
CAIs and the chondrules in the solar
nebula. The interface between the sur-
face of the protostar and the inner edge
of the accretion disk was just the right
temperature to produce these meteoritic inclusions. Moreover, these winds
could then toss the finished products
out to about 3 to 10 AU, where they
would be incorporated into accreting
planetesimals and become part of
some planet or asteroid.
The X-wind model of the solar nebula makes explicit predictions about the
temperatures, pressures and travel
time for materials ejected along specific trajectories from the protosun. To
date, tests have generally validated the
model. Kevin McKeegan of the University of California, Los Angeles, and
his colleagues have shown that the
measured isotopic ratios of beryllium
and boron in CAIs from the Allende
meteorite are consistent with radiation
fluxes expected from the X-wind model. They also noted that these same exposure histories would produce the ob-
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2001
May–June
231
8.35
9.0
9.77
10.87 11.30
11.87 12.22
flux
Comet
Halley
8.0
7.5
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
wavelength (micrometers)
Figure 4. Infrared spectrum of the dust in Comet Halley (top, red line) indicates the presence
of a crystalline silicate, olivine, which can be synthesized and reproduced in the laboratory
(bottom, red line; see also Figure 5). Since crystalline olivine requires high temperatures (on
the order of 1,000 kelvins) for its synthesis, the dust in Halley’s comet must have been fairly
close to the protosun in the early solar nebula before it was incorporated into the comet. This
implies that the “one-way trip” model of comet formation must be modified to accommodate
materials that were processed in the inner part of the solar nebula. Here the spectra are deconvolved (green in both spectra) to show the underlying similarities (matching peaks) of the
olivine in Halley and the laboratory sample. The two extra peaks (8.35 and 12.22) in the laboratory dust sample indicate a higher silica content than the dust in Comet Halley. The variation in the relative heights of the peaks indicates that Comet Halley was rich in magnesium.
served isotopic ratios of calcium-41
and manganese-53 seen in the meteorites. It now appears likely that some
fraction of the solids falling into the
protosun might have been ejected back
into the accretion disk after a period of
high-temperature processing.
silane (SiH 4 )
furnace
hydrogen
Although the X-wind model does
well in explaining the composition of
meteorites, it does not provide an easy
mechanism for annealing amorphous
silicates to produce the crystalline
grains seen in comets. Individual
grains exposed to the 1,600- to 2,200-
kelvin temperatures of the X-wind
near the inner edge of the accretion
disk would be vaporized rather than
crystallized. When the vapors later
cooled and recondensed, it is likely
that they would form amorphous silicates (such as those observed in circumstellar outflows around other
stars), rather than the crystalline grains
seen in comets.
All of this suggests that the theoreticians need to add yet another level of
complexity to the dynamic models of
the solar nebula. There must be a
mechanism that is capable of transporting grains that have been annealed
at temperatures near 1,000 kelvins, out
to regions where the ices of water and
hydrocarbons are stable and the cometesimals begin to accrete. One possibility is the presence of large-scale convective cells near the inner regions of
the disk that interact with material in
the X-wind in such a way that some of
the dust and gas becomes entrained
and transported outward (Figure 7).
Alternatively, Ronald Prinn of the
Massachusetts Institute of Technology
may have suggested an appropriate
mechanism nearly a decade ago. He
noted that most models of the solar
nebula simplified the equations used
to calculate the transfer of angular momentum in the system by dropping
some higher-order terms. It is ostensibly a harmless act that greatly reduces
the computational complexities involved. Prinn suggested, however, that
if the missing terms were included in
the computations, they would generate outflowing vortices that could mix
some of the gas and dust processed
near the inner nebula out to significant
distances. To date, no one has followed
up on Prinn’s suggestion.
dust collector
iron-magnesiumsilicate dust
dust removed
amorphous
silicate dust
helium
oxygen
iron carbonyl
(Fe(CO)5)
magnesium
metal
annealed
silicate dust
dust annealed at 1,000 kelvins
under vacuum
infrared
spectral analysis
Figure 5. Laboratory techniques can be used to create analogues of cometary dust. Silane (SiH4) and magnesium-metal vapor are burned in a
stream of hydrogen gas and oxygen at temperatures near 800 kelvins, producing “smokes” consisting of amorphous silicate dust. Annealing
the “smokes” at temperatures above 1,000 kelvins in a vacuum produces grains of crystalline olivine that bear a close resemblance to those
observed in comets with respect to their spectral properties (see Figure 4) and microscopic appearance (see Figure 6).
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American Scientist, Volume 89
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Clues from Other Stars
Fortunately, we needn’t rely solely on
theoretical models to understand the
processes that took place in the solar
nebula. At this very moment there are
countless numbers of protostars in various stages of birth within our Galaxy.
Although protostellar systems comparable in mass to the ancient solar nebula are too dim to observe, the behavior
of more massive protostars—Herbig
Ae and Be stars—can be studied to a
certain extent. Despite being more than
twice the mass of our Sun, these protostars are generally believed to behave
and evolve in a fashion similar to the
protosun.
Carol Grady of NASA’s Goddard
Space Flight Center and her colleagues
have shown that at least one Herbig
system (HD 163296) exhibits a collimated bipolar wind that is consistent
with the X-wind model, as well as an
uncollimated outflow that may lie parallel to the disk plane. This uncollimat-
Figure 6. Comet dust (left) and its laboratory analogue (right) are strikingly similar as
viewed in the scanning electron microscope. The comet dust is an interplanetary dust particle collected by an airplane in the Earth’s stratosphere. The laboratory analogue was
annealed for 4 hours at 1,000 kelvins in a vacuum. Both particles are about 15 micrometers
across. (Images courtesy of the author.)
Figure 7. Winds in the solar nebula may have been responsible for the mixing of “hot” and “cold” components found in both meteorites and comets. Meteorites contain calcium-aluminum-rich inclusions (CAIs, formed at about 2,000 kelvins) and chondrules (formed
at about 1,650 kelvins), which were created near the protosun and then blown (green arrows) several astronomical units away, into the
region of the asteroids between Mars and Jupiter, where they were embedded in a matrix of temperature-sensitive, carbon-based
“cold” components. The “hot” component in comets—tiny grains of annealed silicate dust (olivine)—is vaporized at about 1,600
kelvins, suggesting that it never reached the innermost region of the disk before it was transported (white arrows) out beyond the orbit
of Pluto, where it was mixed with ices and unheated silicate dust (“cold” components). Vigorous convection in the accretion disk may
have contributed to the transport of materials. This scenario is based on the X-wind model of the solar nebula developed by Frank Shu
and his colleagues at the University of California, Berkeley.
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2001
May–June
233
Figure 8. Protostellar systems such as HH30 (left) and IRAS 04302+2247 (right) offer clues to the appearance and processes in the early evolution of our solar system. Such images conform to the X-wind model of the solar nebula (see Figure 7), revealing the presence of a collimated bipolar outflow (left, green jet), wide-angle, uncollimated outflows on either side of the jet, infalling dust (white and yellow) and
thin, dark accretion disks, which bisect the images. The central stars are hidden within the dense accretion disks. Much of the dust surrounding these protostars is believed to come from infalling comets. Interestingly, the proportion of crystalline silicate dust increases as
the protostellar systems age, suggesting that the composition of the comets themselves may change as the nebulae evolve. (HH30 image
courtesy of Chris Burrows of STScI, the WFPC2 Science Team and NASA. IRAS 04302+2247 courtesy of D. Padgett of IPAC/Caltech, W.
Brandner of IPAC, K. Stapelfeldt of JPL and NASA.)
ed outflow may be the key to understanding how the dust grains annealed
near the protostar make their way to
the outer reaches of the nebula.
Unfortunately, because the closest of
these stars is about 300 light-years away,
detailed observations of these objects are
extremely difficult to make, even using
the state-of-the-art instruments aboard
the Hubble Space Telescope (Figure 8). It
is not yet possible to examine these objects with sufficient resolution to see the
inner portions of the disk or even its
geometry (the angle of the plane of the
disk with respect to the Earth). Without
the geometry of the system it is impossible to gauge the strength and extent of
the winds that might be emanating from
the inner regions of these systems. Nevertheless, Grady’s observations are tantalizing. If some form of uncollimated
wind recycles even a fraction of the material in the accretion disk, it would have
enormous consequences for the chemistry of the nebula.
Herbig stellar systems are interesting for another reason as well. Infrared
spectra of older Herbig Ae and Be stars
indicate that much of the dust surrounding the stars comes from infalling comets, which shed the particles
on their inward voyage. Collectively,
the infrared spectra of many Herbig
systems reveal a distinct trend: The
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American Scientist, Volume 89
dust around the youngest observable
stars is amorphous, but it becomes increasingly crystalline as the age of the
nebula increases. Dust grains are highly unstable over the period of the nebula’s lifetime. Small grains are blown
out of the system by photon pressure,
whereas larger grains spiral into the
protostar, where they are destroyed. In
order for us to see them in the older
systems, the dust grains must be released over time from cometesimals
and comets; otherwise they would
have been destroyed.
These observations provide further
evidence that processes inside protostellar nebulae are responsible for producing the crystalline grains. They
also suggest that the crystalline silicates are not created by a process such
as the impact shock associated with
amorphous dust falling onto the stellar accretion disk, otherwise even the
youngest stars would have crystalline
silicates. But it also suggests that as
the nebula ages, there is a steady accumulation of the crystalline grains
produced in the solar nebula that
must be transported to the region
where the comets are made.
Age-Dating Comets
The trend toward increasing crystallinity in the older nebulae has im-
portant implications for future studies
of the chemical evolution of the solar
nebula. Comets that contain only amorphous silicates must have formed early
in nebular history, whereas those containing a large fraction of crystalline
grains formed much later, perhaps as
the last gases of the nebula were dispersing into space. We might expect
other chemical components in comets
to show similar evolutionary trends
with age. For example, as the solar
nebula evolved, primitive species
(such as CO, CO2, N2) from the parent
molecular cloud would be increasingly
converted into complex organic molecules, such as hydrocarbons, alcohols
and amines. In this scenario, the fraction of complex organic molecules in a
comet should be positively correlated
with the fraction of crystalline silicate
dust it contains.
The possibility of placing comets
into a chronological order opens up
some interesting possibilities. We are
far from understanding how the simple organic ices found in the cores of
molecular clouds combined to form
the complex organic molecules present
in a comet. Modeling the chemistry of
a comet on the basis of telescopic observations of its dusty and gaseous emanations (in its coma, or “head”) is a
very complex affair. Molecules evapo-
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rate at various rates from a surface that
is far from homogeneous, and many of
the complex chemical species are destroyed by ultraviolet light from the
sun. More important, some of the most
interesting compounds—high-molecular-weight amino acids, sugars and
proteins—would not be present in sufficient quantities in the coma to be observed. The only feasible way to detect
these materials is by means of a sample-return mission to a passing comet.
However, in order to provide a complete record of the processes in the solar nebula we would want to select
comets that were formed at different
stages of nebular evolution. This is
where the spectral differences between
amorphous silicate grains and crystalline grains should come in handy. If
our proposed comet chronology is correct, we may be able to assess the relative age of a comet while it is still some
distance from the Earth merely by observing its infrared spectrum. In the
near future we could thus target specific comets for sample return missions
and so study various stages in the
chemical evolution of the solar nebula.
We would then begin to understand
the origin of the molecules that gave a
“jump-start” to life on Earth.
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