Earth did life begin? Or, more precisely, how did life on Earth
begin? Theories abound, but all require the existence of basic building
blocks such as amino acids to construct the earliest forms of life.
One of the most exciting possible sources of these raw materials
may have been the comets and asteroids that rained heavily upon
Earth during its first billion years. Consequently, while life itself
likely arose on Earth, the building blocks of life may well have
had an extraterrestrial origin. If this cosmic origin of lifes
building blocks is correct, the hitchhiking organic molecules would
have had to withstand the extreme pressures and temperatures of
a fiery crash onto Earths surface.
answers to these questions about the origins of life are intimately
connected to questions related to what would happen to the organic
molecules of missile-borne chemical weapons if the attacking missile
were intercepted by another missile. Like so many seemingly disparate
subjects in science, these two are connected at a fundamental level,
in this case by questions concerning the fate of organic liquids
subjected to strong shock compression.
Past investigators have depended
on computer modeling to determine whether the molecules could survive
or be destroyed. But the limited success of these computational
and theoretical approaches has underscored the long-standing need
for experimental answers. Only recently have laboratory experiments
been able to explore the chemistry of the extreme shock regimes
associated with the delivery of amino acids to early Earth by comets
or the fate of missile-borne chemical weapon agents following missile
At Livermore, a team of scientists
led by physicistgeochemist Jennifer Blank is conducting a
series of shock experiments to explore the viability of extraterrestrial
delivery and to determine what happens to a chemical weapons
payload when intercepted by another missile. Blank says, These
experiments are the closest we can come in the laboratory to investigating
the role these primordial ice balls may have played in bringing
the building blocks of life to our planet or of testing the effects
of missile interdiction on an incoming chemical warheads payload.
The Livermore Connection
Blank and her team
are using Livermores 6.2-meter-long, two-stage, light-gas
gun to conduct their shock experiments on organic liquids. They
are focusing initially on cometary impacts. For decades, Livermore
has used its gas guns to perform shock experiments on solid materials.
Working with liquids in shock and recovery experiments is much more
difficult than working with solids and has not been done before.
Our high-pressure-shock organic chemistry experiments are
defining a new area of scientific exploration, Blank notes.
The teams experiments
generate impact velocities approaching 2 kilometers per second,
yield temperatures of 500 to 800°C, and produce a maximum impact
pressure of 40 gigapascals (about 400,000 times atmospheric pressure).
At Mach 6 speed, the gas guns impactor smashes into a small
metal capsule filled with mixtures of amino acids in water, mimicking
the supersonic collision of a comet with the rocky surface of Earth.
Right from the beginning,
we knew that something had happened, says Blank. The
solution that went into the capsule was clear, and what came out
was golden yellow. Through subsequent chemical analysis, the
team discovered that the initial amino acids in the mixture had
linked together to form peptides, from which proteins can be formed.
|A two-dimensional simulation
of an icy comet hitting the rocky Earth. This image represents
the distribution of kinetic energy 678 microseconds after impact,
sufficient time for the primary shock wave to pass completely
through the comet. The wide distribution of energy in the comet
indicates that the comets trailing portion should be coolerand
experience lower velocitythan the front end. Thus, the
rear part of the comet could arrive as a puddle of liquid water,
with a high concentration of organic compounds, ready to host
further chemical evolution. (Simulation by Greg Miller, Lawrence
Berkeley National Laboratory.)
Life as we know it requires three essential ingredients:
water, organic matter, and energy. An icy comet carrying organic
material that crashes into Earth could potentially supply all of
these ingredients in a single, tidy package.
An obvious question remains:
Are there amino acids in comets to begin with? Recent experiments
conducted at the National Aeronautics and Space Administrations
Ames Research Center in Mountain View, California, and at a number
of European research centers found that dehydrated amino acids are
easy to form in laboratory simulations of interstellar clouds. Scientists
do not yet know whether comets incorporate these molecules, but
spectroscopic studies of comets suggest that 20 percent of their
tails are organic material. Additionally, analysis reveals that
meteorites landing on Earth are replete with a variety of complex
organic compounds, including more than 70 different amino acids.
What has remained a mystery
is the manner in which cometary organic building blocks could survive
their journey to the early Earths surface.
The figure above shows a
model of a comet smashing obliquely into a rocky Earth. As the collision
occurs, the energy of the impact is distributed throughout the comet,
splashing jets of material in all directions. A backward splash
will have the lowest velocity relative to Earth, so this slower-moving
portion would have the greatest chance of surviving as an intact
puddle. The temperatures and pressures of these backward jets are
also those most readily achievable in laboratory experiments.
The high temperatures created
when a comet collides with Earth would certainly cause the breakdown
of organic compounds trapped in the collision. However, in the fraction
of the second of a strong shock impact, the extreme high pressures
can impede or even prevent molecules from breaking down. A predictive
theory of the organic chemistry under these extreme conditions does
not exist, so Blank and her team are exploring these uncharted regimes
in the laboratory.
|(a) A liquid sample is loaded
into a capsule, which in turn will be placed into the chamber
of Livermores 6.2-meter-long light-gas gun. (b) A schematic
of the gas gun in operation. A gunpowder explosion on the left
pushes the piston, which pushes helium gas to open the rupture
valve. The projectile then flies down the barrel of the gun
to hit the target inside the guns chamber.
Shock and Recovery
The proxy for a comet in
these experiments is a few drops of water and organic material contained
in a 2.5-centimeter-diameter stainless-steel capsule. The teams
biggest challenge has been to design a capsule capable of keeping
its liquid cargo intact during the high-pressure shock loading and
Another challenge was to
extract the material from the container after it has been smashed
by a supersonic projectile. After the still warm capsule is removed
from the gas-gun experiment tank, it is machined down on one side
to within about 15 micrometers of the liquid inside. The capsule
is then pierced with a special drill bit, and the contents are removed
with a syringe.
are characterized using liquid chromatography and mass spectrometry
(LCMS). A portion of the sample is pushed through a chromatographic
column, and as the liquid travels though the column, different compounds
separate out. Mass spectrometry is then used to determine the mass
of each component.
In the few dozen gas-gun
experiments performed to date, from 40 to 95 percent of the initial
amino acids survived. The fraction of survival depends on the impact
pressure, the projectile thickness, the starting concentration of
amino acids, and the structure of the amino acids.
An exciting and unanticipated
discovery was the nature of the reaction products created by the
impact. The dominant products are all possible pairs of the original
amino acids joined together by a peptide bond, which implies that
some of the energy of the impact has been harnessed to create larger
organic molecules. Peptide chains composed of more than two amino
acids are also produced in the experiments, though to a lesser degree.
To provide a comparison for
the experimental results, Livermore physicist Nick Winter is simulating
the pressure dependence of peptide formation on shock loading. Using
quantum chemistry methods, Winter is developing computational models
that will allow calculation of the pressure dependence of reaction
experimental capsules, one (left) filled with a solution of
approximately 125 microliters of liquid (amino acids in water)
and ready for a shock experiment, the other open to reveal the
hollow volume in the center to contain the liquid.
Three still images of the
supersonic impactor hitting the stationary test capsule. The
projectiles velocity is 0.81 kilometers per second,
which is about 2,000 miles per hour. The capsule is inside
the cylindrical pig.
the QuickTime movie.
a Chemical Weapon
allied series of experiments was performed to determine if missile-borne
chemical warfare agents might survive the impact from an intercepting
missile. During a missile intercept, portions of the liquid payload
of chemical agent would be subjected to strong shock waves. Currently,
no information is available about the chemical stability of these
compounds in such a scenario. Specifically, nothing is known of
the extent to which the payload might be altered as a consequence
of the intercept. A substantial portion of the lethal load could
be altered chemically to nontoxic compounds by the shock, reducing
the threat posed by the weapon, or the shock could increase the
toxicity of the chemicals. A precise knowledge of the chemistry
that would occur in an intercept is critical to the design of interception
strategies and technologies.
experiments with simulants of various chemical warfare agents are
the first of their kind. The goal is to provide laboratory evidence
of whether chemical agents might survive a missile intercept. Additionally,
unique reaction products from such experiments, detectable by remote
sensing methods, would constitute a simple means of determining
To extract a solution after
a gas-gun experiment involving chemical weapon simulants, analytical
chemists Armando Alcaraz and Pete Nunes of the Forensic Science
Center at Livermore use solid-phase microextraction (SPME) methods.
Using gas chromatographymass spectrometry to characterize
the solution before and after an experiment, the chemists have begun
to obtain results.
Again, we knew that
something had definitely happened, says Blank. The solution
that went in was pale yellow, and what came out looked like used
|Livermore scientists have
developed a new piercing device for extracting liquids under
pressure in metal capsules following gas-gun shock experiments.
(a) The piercing device consists of a clean glass chamber between
two metal plates. The top plate has a connection for a drill
bit and a silicon septum through which a needle can pass for
sample collection. (b) A shocked capsule is positioned in the
piercer under the drill. A Teflon sleeve around the drill bit
will capture any spray that occurs when the capsule is punctured.
(c) The capsule has been pierced and the drill retracted. Shocked
liquid has beaded on the surface of the capsule awaiting collection.
Challenges of Scale
how real are these laboratory simulations? Closer than one
might expect, says Blank. The impact conditions are
right on target for simulating defense scenarios. In contrast, the
impact temperature of a real ice ball hitting rock would be about
15 to 30 percent higher than the 500 to 800°C attained in our
current suite of gas-gun experiments. One might think that
the higher temperatures of an actual impact would destroy more of
the organic material. However, the temperatures achieved in the
laboratory experiments are already much higher than the thermal
stability limits of amino acids under ordinary atmospheric pressure.
Hence the high survival percentages of amino acids show clearly
that the destructive effects of high temperatures are buffered by
the accompanying high pressures in shock waves.
difference between laboratory shock experiments and an actual cometary
collision is the duration of the impact shocks. In the experiments,
materials are shocked for only a few microseconds, while a 1-kilometer-thick
ice ball hitting Earth would experience a shock wave lasting 1 second,
a time difference of six orders of magnitude. The experiments assume
that the comet is a single dense ball of ice. In reality, comets
are known to be aggregates of much smaller objects, so this apparent
discrepancy in time scales is much less severe.
||Examples of liquid chromatographymass
spectrometry spectra for a sample solution of two amino acids,
glycine and proline. Peaks corresponding to the major compounds
are plotted for the sample before and after it was shocked.
After the liquid was shocked, some glycine and proline persist
and four new compounds are present: glycineglycine, glycineproline,
prolineglycine, and prolineproline.
ultimate in experimental understanding will be to perform real-time
spectroscopic analysis of the liquids at 100-nanosecond intervals
during the shock process itself. Such measurements are a technical
challenge but would provide a more complete understanding of the
physical and chemical evolution of the different reaction pathways
from their onset. Obtaining such measurements is a goal that Blank
and her team hope to pursue in the future by relying on techniques
developed by Laboratory physicist Neil Holmes.
short time-scale measurements are also essential to more accurate
computer models of the chemistry of high-pressure, high-temperature
shock regimes. The quantity of data going into such simulations
is so enormous that an accurate representation can cover only a
brief period of time, typically a few nanoseconds. Thus, even the
supercomputers at Livermore need another generation or two of development
before these complex chemical and physical interactions over longer
time periods can be simulated. Until then and until the accompanying
computer codes can be developed, calibrated, and validated, scientists
knowledge and understanding depend on experiments such as those
being done by Blank and her team.
|This model of dipeptide production
shows that when (a) an aqueous solution bearing two amino acids,
glycine and proline, is shocked, (b) four dipeptide products
may result. The reaction of two amino acids to form a dipeptide
is accompanied by production of a water molecule, using a hydrogen
atom from one amino acid and an oxygenhydrogen group from
the other, hence the two possible forms of the glycineproline
dipeptide. (Model by Livermore physicist Nick Winter.)
the End Is the Beginning
Livermore research on shocked organic liquids has been much in the
news lately. After all, who doesnt
want to know how it all began on Earth? And in a postSeptember
11 world, a chemical weapon threat is an uncomfortably real possibility
that must be countered effectively.
is quick to make clear that the Livermore work depends on the talents
of her colleagues in engineering, analytical chemistry, biochemistry,
and computer modeling of hydrodynamics and quantum chemistry. Together,
this team is bringing a new understanding of how it all could have
started. Simultaneously, this research is providing critical information
that could help to counter the threat from missile-launched chemical
weapons of mass destruction.
Key Words: amino
acids, astrobiology, liquid chromatographymass spectrometry
(LCMS), origins of life, shock physics, solid-phase microextraction
For further information contact Jennifer Blank (925) 423-8566 (firstname.lastname@example.org).