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WEBVTT
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Welcome to Sleepy Facts about the Universe. In
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the coldest, most vacant reaches of interstellar
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space, atoms are so sparse that they almost never
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meet. Yet astronomers detect a rich variety of
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molecules in these frigid clouds, from molecular
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hydrogen to formaldehyde and methanol. The question
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is how this chemistry proceeds when the conditions
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seem to forbid it. The answer lies not in the
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vast emptiness of the gas, but on the microscopic
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surfaces of interstellar dust grains. This episode
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examines why gas phase reactions fail in the
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extreme cold of a molecular cloud and explores
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the physical mechanisms by which dust grains
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solve the problem. A grain provides a solid surface
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where atoms can accumulate, wander and meet.
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It absorbs the excess energy of formation, allowing
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chemical bonds to stabilize rather than tear
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apart. The episode traces the sequential hydrogenation
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of carbon monoxide into formaldehyde and methanol,
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the dynamic chemical environment of icy mantles,
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and the various ways finished products dissolve
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back into the gas phase where telescopes can
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finally detect them. I wanted to understand why
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the universe is chemically rich when its emptiest
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places should be entirely dead. While working
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on this episode, I kept returning to one question.
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How do atoms reliably find each other in a vacuum
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so profound that a cubic centimeter holds only
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thousands of particles? Exploring the surface
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chemistry of cosmic dust reveals that these microscopic
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flecks are not passive debris. but active environments
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that make the molecular universe possible. In
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the coldest, most vacant reaches of the galaxy,
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chemistry somehow proceeds, building molecules
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that should not exist. Here, a cubic centimeter
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holds fewer atoms than a breath of air holds
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in trillions of trillions. The agent of this
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impossible synthesis is not a star, not a planet,
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but a fleck of dust smaller than smoke. Its frozen
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surface transforms near -perfect emptiness into
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the universe's most abundant chemical laboratory.
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Imagine a dust grain drifting through the cold
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darkness of an interstellar cloud. It is smaller
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than a particle of smoke, perhaps a tenth of
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a micrometer across. Around it stretches near
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-perfect emptiness. Temperatures fall to 10 or
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20 Kelvin. Densities are so low that a cubic
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centimeter contains only thousands of atoms.
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where Earth's atmosphere holds trillions of trillions.
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Yet astronomers detect molecular hydrogen, formaldehyde
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and methanol, these molecules that should barely
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exist in such sparse, frigid gas. The question
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this episode pursues is direct. How does chemistry
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proceed when atoms are so rare that they almost
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never meet? The answer lies on the surface of
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that dust grain. It is not a poetic suggestion,
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but a physical mechanism that transforms these
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microscopic particles into the dominant chemical
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laboratories of cold interstellar space to understand
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why the grain surface matters we must first grasp
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why the open gas fails in a cold molecular cloud
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hydrogen atoms drift at thermal speeds but the
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space between them is vast two atoms might collide
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and briefly form molecular hydrogen but the energy
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of formation would tear them apart again unless
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something carries it away. In dense terrestrial
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air, a third body, a nearby molecule, can absorb
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this excess energy. In interstellar gas, third
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bodies are themselves too rare. The reaction
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stalls. Similar problems afflict the formation
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of formaldehyde, methanol, and many other observed
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molecules. Gas phase reactions. while important
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for some species, cannot account for what telescopes
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actually detect. The grain enters as a physical
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solution. It provides what the gas cannot. It
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offers a surface where atoms and molecules accumulate.
Load full transcript
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It provides a solid lattice that absorbs energy.
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And it supplies an icy mantle that stores products
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for millions of years. Consider what happens
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when a hydrogen atom strikes this grain. It approaches
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at roughly a kilometer per second, driven by
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thermal motion in the cold cloud. Upon impact,
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it loses kinetic energy to the grain's lattice.
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This energy transfer is crucial. The grain, with
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its many atoms bound in solid structure, can
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absorb this energy without dramatic change. The
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hydrogen atom becomes trapped. It is bound to
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the surface by weak van der Waals forces, what
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chemists call physisorption occasionally it is
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held by stronger chemical bonds chemisorption
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the binding is modest pisisorption typically
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involves energies of 0 .1 to 0 .1 electron volts
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that is enough to hold the atom at 10 kelvin
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but not so strong that it becomes permanently
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fixed the surface itself demands attention these
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are not smooth planes Interstellar grains are
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irregular, porous, perhaps fractal in structure.
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They are composed of silicates like olivine or
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pyroxene, or of carbonaceous material including
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amorphous carbon and polycyclic aromatic hydrocarbons.
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Each grain presents a landscape of binding sites.
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There are shallow depressions, deeper wells,
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edges, corners and internal pores. An adsorbed
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atom sits in one such site, vibrating, until
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thermal energy or quantum tumbling allows it
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to hop to an adjacent site. At 10 Kelvin, this
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hopping is slow. A hydrogen atom might wait seconds
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or minutes between moves, but it is not frozen.
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The atom explores the surface, stepping between
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binding sites like someone crossing a rough floor
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in darkness, seeking another reactor. This mobility
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transforms the chemistry. In the gas phase, two
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hydrogen atoms must collide directly in the vast
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emptiness. That is a rare event and it produces
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an unstable intermediate. On the grain surface
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they accumulate independently, wander and eventually
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meet. The grain itself acts as the third body.
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It absorbs the 4 .5 eV released when two hydrogen
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atoms form molecular hydrogen. The product, H2,
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may then desorb, returning to the gas phase.
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It can be carried away by the reaction energy
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or by subsequent heating. Hama and Watanabe's
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laboratory experiments on amorphous silicate
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surfaces confirm that this process proceeds efficiently
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at interstellar temperatures. The grain has solved
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both problems of gas phase formation. It brings
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reactants together and it removes excess energy.
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The hydrogenation of carbon monoxide to formaldehyde
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and methanol illustrates how surface chemistry
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builds complexity through sequential steps. Carbon
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monoxide, CO, is abundant in interstellar gas.
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It is the most common carbon -bearing molecule
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after molecular hydrogen. When CO strikes a grain
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surface, it too can stick, particularly as ice
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mantles begin to form. A wandering hydrogen atom
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encounters this adsorbed CO and adds to it, forming
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HCO, the formal radical. Another hydrogen atom
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follows, producing formaldehyde, H2CO. Further
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hydrogen additions yield CH3O, then methanol
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CH3OH. Each step requires the same conditions.
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The reactants must be mobile enough to meet.
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The grain must absorb the energy of each addition.
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And the intermediate products must remain bound
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long enough for the next hydrogen to arrive.
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Garrett and Pauli's models of grain surface chemistry
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trace this hydrogenation sequence with specific
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rate constants. But we should note the uncertainty.
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These rates are inferred from laboratory analogues
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and theoretical models, not measured directly
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on interstellar grains. The binding energies
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of CO, HCO, 2CO and CH3OH on silicate versus
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carbonaceous surfaces differ. The diffusion barriers
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that control how
quickly species hop between
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sites vary with surface composition and structure.
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The models reproduce observed abundances reasonably
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well when these surface processes are included.
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But the exact rate constants remain uncertain
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by factors that can reach orders of magnitude.
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The ice mantle that forms around these reactions
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is itself a chemical environment, not merely
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inert storage. At temperatures below 20 Kelvin,
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water vapor, carbon monoxide, carbon dioxide,
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ammonia, and the products of surface hydrogenation
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freeze onto the grain, building layers of ice.
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Bugert, Gerakhin's and Wittit's review of infrared
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observations shows these mantles directly. Absorption
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features at specific wavelengths reveal solid
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H2O, CO, CO2, and CH3OH along lines of sight
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toward background stars and embedded protostars.
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The ice is real observed and chemically significant.
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Within this ice, radicals become trapped. These
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are species with unpaired electrons highly reactive.
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A hydrogen atom that fails to find a reaction
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partner may be buried in the growing ice, preserved
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for thousands or millions of years. When ultraviolet
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photons from nearby stars or cosmic ray impacts
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eventually break bonds in the ice, these radicals
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are liberated. They encounter other trapped species
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and initiate further chemistry. The mantle is
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thus not static but dynamic, a frozen reservoir
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of reactivity waiting for activation. The release
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of products back to the gas phase, where telescopes
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can detect them, occurs through several routes.
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Each root is dominant in different circumstances.
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Thermal desorption requires heating. Near a young
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star, as a protostar forms and warms its surroundings,
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grain temperatures rise above the sublimation
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points of various ices. Molecules return to the
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gas in sequence according to their binding strengths.
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Water ice, most strongly bound, persists longest.
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Carbon monoxide dissolves at lower temperatures.
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Photosorption driven by ultraviolet photons can
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eject molecules even in cold unheated regions.
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But the efficiency varies with wavelength and
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ice composition. Cosmic ray impacts, rare but
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energetic, can locally heat grains to temperatures
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sufficient for thermal desorption. This creates
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small pockets of released molecules. And the
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energy of reactions and cells, exothermic surface
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chemistry, can directly eject products. This
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is called reactive desorption or chemical desorption
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where the heat of formation provides the escape
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velocity. The physical properties of individual
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grains alter every rate in this chemistry. A
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porous grain with its labyrinthine internal surfaces
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offers more binding sites and longer diffusion
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paths than a compact sphere. The shape affects
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how gas particles strike and stick. Irregular
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grains present more surface area per unit mass.
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Composition matters profoundly. Silicate surfaces
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bind hydrogen differently than carbonaceous surfaces,
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changing diffusion barriers and reaction efficiencies.
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Binding strength determines how long species
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remain before dissolving, whether thermally or
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non -thermally. These variations mean that no
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single grain is representative. The chemistry
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of an interstellar cloud emerges from a distribution
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of grain properties modeled statistically but
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not individually resolved. The observational
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consequences are what connect this microscopic
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chemistry to astronomical practice. Radio telescopes,
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including the IRAM 30 -meter, ALMA, and the Green
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Bank Telescope, detect rotational transitions
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of molecules in the gas phase. The abundances
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of H2, H2CO, and CH3OH in cold clouds like TMC1
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or lone 134N exceed what gas phase ion molecule
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chemistry can produce. The discrepancy points
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to green surfaces as the hidden source. In hot
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cores, regions around massive
young stars where
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temperatures rise dramatically, the sudden release
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of grain mantle products creates a rich molecular
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soup complex organic molecules are detected in
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abundance these hot cores are understood as laboratories
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where the surface chemistry of cold preceding
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eras is suddenly exposed we should be careful
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not to overstate what is known the exact binding
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energies and diffusion barriers for hydrogen
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oxygen carbon and carbon monoxide on realistic
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interstellar grain surfaces remain uncertain.
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The relative importance of physorption versus
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chemisorption for H2 formation is debated. Some
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models emphasize weak physisorption with thermal
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or tunneling diffusion. Others suggest that stronger
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chemisorption sites may dominate under certain
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conditions. The role of polycyclic aromatic hydrocarbons
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Much smaller than the grains we have discussed,
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as alternative sites for H2 formation remains
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an active research question. The contribution
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of grain surface chemistry to truly complex organic
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molecules beyond formaldehyde and methanol is
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contested. Some researchers favor gas phase roots
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in the warm, dense conditions of hot cores. What
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can be said with confidence is this. Dust grains
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are not passive debris. They are active chemical
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surfaces that concentrate sparse reactants, absorb
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reaction energy, and store products in ice. Without
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them, the molecular richness of cold interstellar
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clouds would be inexplicable. The hydrogen molecules
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that permeate molecular clouds, the formaldehyde
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and methanol that trace denser regions, are largely
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the released products of surface chemistry on
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particles smaller than smoke. The mechanism is
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not industrial catalysis in the familiar sense.
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The grain does not necessarily lower activation
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energies through elaborate surface chemistry.
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Rather, it provides the physical conditions,
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concentration, energy absorption and storage
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that make reactions possible where they would
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otherwise be forbidden by the sparse, cold conditions
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of interstellar gas. It is catalysis by environment,
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by scale. by the simple fact that a solid surface
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in near -empty space becomes the only place where
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atoms reliably meet. When we detect methanol
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in a cold cloud or see the infrared signature
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of an ice mantle toward a distant star, we are
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observing the aftermath of processes that unfolded
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on countless microscopic surfaces over millions
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of years. Each grain is a laboratory. Each ice
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mantle is a record. Each released molecule is
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a messenger from the hidden chemistry of cosmic
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dust.