Molecules in space

Foundations

The wrong kind of empty

Interstellar space is a poor place to do chemistry. It is cold, around ten degrees above absolute zero in the densest clouds. It is empty, with a density that would count as a perfect vacuum in any laboratory. And in the open it is bathed in ultraviolet starlight, which takes molecules apart as fast as they form.

Every one of those objections is true, and space is nonetheless full of molecules. More than three hundred different ones have been identified so far, from simple pairs of atoms to chains of a dozen carbons, sugars, alcohols and the components of amino acids. Working out how has taken sixty years, and the answer changed what we think the early Earth was handed.

Fingerprints on the radio

First, how does anyone know what molecules are out there? Lesson 2.5 read atoms by the light they absorb. Molecules are read by a different trick: they spin, and a spinning molecule can only spin at certain rates, set by its shape and mass. Each change between rates emits or absorbs radiation at a precise frequency, usually in the radio or millimetre band. A molecule therefore has a set of lines as individual as a fingerprint, and the same molecule can be measured in a laboratory on Earth to check.

Point a radio telescope at a dark cloud and the spectrum comes back crowded with lines. Since the first detections in the 1960s the count has grown steadily. Water, ammonia, formaldehyde, methanol, acetic acid, ethylene glycol, and long carbon chains that chemists on Earth would struggle to keep in a bottle.

The dust does the work

The puzzle was always the rate. Two atoms in a cold, thin gas essentially never meet, and if they do, the newly formed molecule carries so much energy from the reaction that it flies apart again, because there is no third body to take the energy away.

The solution is dust. About one percent of the mass between the stars is solid grains, roughly a ten-thousandth of a millimetre across, made of silicate and carbon and forged in the winds of dying stars. An atom that strikes a grain sticks to it. It then wanders across the surface until it meets another atom, and when they react, the grain absorbs the released energy. A dust grain is a workbench, a meeting room and a heat sink at once.

Molecular hydrogen, the most abundant molecule in the universe, is made this way and essentially no other way. So are much of the water, the methanol and the ammonia. In the coldest clouds, grains accumulate mantles of ice, layers of frozen water and carbon monoxide and methanol, and the chemistry continues inside the ice.

What ultraviolet does to ice

Then something counterintuitive happens. Ultraviolet light, the great destroyer of molecules in open space, becomes constructive inside an ice. It breaks bonds, but the fragments cannot escape; they are trapped in the frozen matrix, and they recombine into larger and more complicated molecules than they started as.

This can be tested. Take a vacuum chamber, cool a surface to ten kelvin, deposit a mixture of water, methanol, ammonia and carbon monoxide in the proportions seen in space, and shine ultraviolet on it. Warm it up and analyse what you have. In 2002 a group at NASA Ames did exactly this and found amino acids, the building blocks of proteins, made from nothing but the ingredients of an interstellar cloud and starlight.

The delivery

None of this would matter much if the molecules stayed in the clouds. They do not. A cloud collapses into a star and a disc, as in lesson 2.1, and the disc’s outer regions stay cold enough for the ice and organics to survive. They end up in comets and in the parent bodies of asteroids.

And then they arrive. On 28 September 1969, a meteorite broke up over Murchison in Victoria, Australia, and a hundred kilograms of it was collected quickly, before it could be contaminated. Analysis found amino acids, and crucially found many that terrestrial life does not use, in proportions no contamination could explain. The building blocks of proteins had arrived from space, ready-made.

Since then the list has grown. Murchison and similar meteorites contain amino acids, sugars including ribose, and nucleobases. The Rosetta spacecraft, orbiting a comet from 2014 to 2016, detected the amino acid glycine and phosphorus, an element central to DNA and to the energy currency of cells, in the gas around it.

What the Earth was given

By the time the Earth’s surface had cooled enough for liquid water, roughly a chapter from now in this course, it was not a sterile ball of rock that had to invent organic chemistry from scratch. It was being pelted with material that already contained water, amino acids, sugars and phosphorus, made in cold clouds and delivered by the leftovers of planet formation.

That does not explain the origin of life, which is chapter five’s problem and remains open. It removes one step from the problem. The ingredients were not the hard part.

One puzzle worth keeping

Life on Earth uses left-handed amino acids almost exclusively, though both mirror forms are chemically equivalent. The laboratory ice experiments produce equal amounts of both. Some meteorite samples show a small excess of the left-handed form, which might be an inherited bias from polarised starlight, and might be contamination or a local effect. Whether Earth’s handedness was delivered or chosen here is unresolved.

Deeper

Grain surface reactions

At ten kelvin, an atom striking a grain has a sticking probability near unity for most species. Hydrogen atoms then migrate by quantum tunnelling between surface sites, meeting on timescales of hours to days at typical dense-cloud fluxes. Recombination releases 4.5 electron volts, which is absorbed by the lattice or ejects the molecule from the surface; the balance between these determines whether molecular hydrogen returns to the gas. Heavier species remain frozen, building mantles tens of monolayers thick, dominated by water with roughly 20 to 30 percent carbon monoxide and a few percent each of methanol, ammonia and carbon dioxide. Hydrogenation on the ice converts carbon monoxide successively to formaldehyde and methanol, which is the observed route to complex organics.

The census

McGuire’s census records the molecule, the source, the telescope and the year for every secure detection. As of the 2021 edition the total exceeds 240 in the interstellar and circumstellar medium, with more in protoplanetary disks, exoplanet atmospheres and comets, and the total across all environments passing 300. The largest include cyanonaphthalene and other aromatic species, and the fullerenes C₆₀ and C₇₀. Roughly a third of all known interstellar molecules contain six or more atoms, and detections are accelerating as sensitive facilities such as ALMA and the Green Bank Telescope survey nearby clouds.

PAHs and the unidentified bands

A set of broad infrared emission features at 3.3, 6.2, 7.7, 8.6 and 11.3 micrometres appears almost everywhere in the galaxy where ultraviolet light meets gas. Since the 1980s these have been attributed to polycyclic aromatic hydrocarbons, flat fused-ring molecules of tens of carbon atoms, vibrating after absorbing ultraviolet photons. No single PAH has been unambiguously identified from these bands, though specific small aromatics have now been found by their radio lines. Collectively PAHs are estimated to carry of order ten percent of the interstellar carbon.

The water problem

Earth formed inside the snow line, where water ice could not condense, so its water must have been delivered. The deuterium-to-hydrogen ratio is the diagnostic, since ices formed at different temperatures and locations carry different ratios. Earth’s oceans have D/H near 1.5 × 10⁻⁴, matching carbonaceous chondrites; most measured Oort-cloud comets are about twice that, though comet 103P/Hartley 2 matched Earth and Rosetta’s 67P was substantially higher. The current consensus favours asteroidal delivery for the bulk, with comets contributing a minority, but the sample of measured comets is small.

The chirality question

Circularly polarised ultraviolet light, produced by scattering in star-forming regions, can preferentially destroy one enantiomer, and enantiomeric excesses of a few percent have been measured in some meteoritic amino acids, particularly those with no terrestrial counterpart, which rules out contamination for those cases. Whether such a small excess could be amplified into life’s near-total preference, and whether that amplification happened before or after life began, is unresolved.

Frontier

Building blocks and the origin of life

Finding amino acids, sugars and nucleobases in space and in meteorites is not the same as explaining how they were assembled into a self-replicating system. The gap between a rich prebiotic soup and the first cell is the subject of lesson 5.2, and it is genuinely open. What this lesson establishes is the starting inventory.

Molecules in disks and on other worlds

ALMA now maps molecules in the discs where planets are forming, showing which compounds survive and where. Webb measures ices along lines of sight through clouds and the atmospheres of exoplanets. The question of whether the chemistry that preceded Earth’s biology is universal, or an accident of our neighbourhood, is becoming answerable.

Transcript and shot list

Target runtime about 7 minutes. Built as: say why it should be impossible, find the workbench that makes it possible, then follow the products down to a meteorite you can hold. Shot types: ANIM is rendered from code with Manim, AI is a generated shot from Higgsfield with the shared style reference, REAL is agency imagery with credit, CAM is the narrator.

[00:00] S01 · REAL · A dark nebula silhouetted against a star field, such as the Horsehead or a Bok globule (ESO or NASA; confirm credit line).

Interstellar space is a terrible place to do chemistry. It’s ten degrees above absolute zero. It’s emptier than any vacuum we can make in a laboratory. And out in the open, ultraviolet starlight rips molecules apart as fast as they form. All of that is true. And space is full of molecules anyway. Over three hundred different ones, including sugars, alcohols, and the building blocks of proteins.

[00:31] S02 · ANIM · Cosmic calendar; a bar from about 8 January to now, labelled “this chemistry, continuously”.

This has been going on since the first dust formed around dying stars, and it’s happening in every molecular cloud in the galaxy tonight.

[00:42] S03 · ANIM · A molecule tumbling; its rotation quantised into steps; each step emits a radio line; a spectrum of lines becomes a fingerprint matched to a laboratory chart.

First, how does anyone know? Atoms are read by the light they absorb. Molecules are read by a different trick: they spin, and a spinning molecule can only spin at certain rates, set by its exact shape and mass. Each change between rates emits radiation at a precise radio frequency. So every molecule has a set of lines as individual as a fingerprint, and you can measure the same molecule in a lab on Earth to check. Point a radio telescope at a dark cloud and the spectrum comes back crowded.

[01:24] S04 · ANIM · Two atoms in a thin cold gas drifting past each other, never meeting; then meeting and flying apart because the energy has nowhere to go.

The puzzle was always the rate. In a gas that cold and that thin, two atoms essentially never meet. And if they do, the new molecule carries so much energy from the reaction that it immediately flies apart, because there’s no third body to absorb it.

[01:45] S05 · ANIM · A dust grain, a tenth of a micrometre across; atoms land and stick; they wander the surface, meet, react; the grain glows slightly, absorbing the energy; H₂ leaves.

The solution is dust. About one percent of the mass between the stars is solid grains, a ten-thousandth of a millimetre across, made of silicate and carbon, forged in the winds of dying stars. An atom that hits a grain sticks. It wanders the surface until it meets another atom, and when they react, the grain soaks up the energy. A workbench, a meeting room and a heat sink, all at once. Molecular hydrogen, the most abundant molecule in the universe, is made this way and essentially no other way.

[02:27] S06 · ANIM · A grain in a cold cloud accumulating ice layers: water, carbon monoxide, methanol, ammonia; chemistry continuing inside the ice.

In the coldest clouds, the grains grow mantles of ice. Water, carbon monoxide, methanol, ammonia, frozen on in layers. And the chemistry keeps going inside the ice.

[02:39] S07 · ANIM · Ultraviolet strikes the ice; bonds break; fragments cannot escape the frozen matrix; they recombine into larger molecules.

Then something counterintuitive. Ultraviolet light, the great destroyer of molecules in open space, turns constructive inside an ice. It breaks bonds, but the fragments can’t escape. They’re trapped, and they recombine into molecules bigger and more complicated than the ones they came from.

[02:59] S08 · ANIM · A laboratory: a vacuum chamber, a surface at 10 K, a deposited ice mixture, a UV lamp; the sample warms; a chromatogram shows amino acids.

And you can test this. Take a vacuum chamber. Cool a surface to ten kelvin. Deposit water, methanol, ammonia and carbon monoxide in the proportions seen in space. Shine ultraviolet on it. Warm it up, and analyse what you’ve got. In 2002 a group at NASA Ames did exactly that, and found amino acids. The building blocks of proteins, made from nothing but the ingredients of an interstellar cloud and starlight.

[03:32] S09 · ANIM · A cloud collapses into a star and disc; the cold outer disc preserves ices and organics; they end up in comets and asteroids.

None of which would matter if the molecules stayed in the clouds. They don’t. A cloud collapses into a star and a disc, and the disc’s cold outer regions preserve the ice and the organics. They end up in comets, and in the rocks that become asteroids.

[03:54] S10 · AI · A fireball over farmland at night, 1969; fragments on the ground. S11 · REAL · A fragment of the Murchison meteorite (museum or NASA image; confirm credit line).

And then they arrive. On the 28th of September 1969, a meteorite broke up over Murchison, in Victoria, Australia. A hundred kilograms were collected fast, before it could be contaminated. Analysis found amino acids. And crucially, it found many that life on Earth doesn’t use, in proportions no contamination could explain. The building blocks of proteins, arriving from space, ready-made.

[04:21] S12 · ANIM · A growing list: amino acids, ribose, nucleobases in meteorites; then Rosetta at comet 67P detecting glycine and phosphorus.

The list has kept growing. Murchison and meteorites like it contain amino acids, sugars including ribose, and nucleobases. And the Rosetta spacecraft, orbiting a comet for two years, detected glycine, an amino acid, and phosphorus, the element at the heart of DNA and of the energy currency of every cell.

[04:44] S13 · ANIM · The early Earth being pelted with organic-rich material; a caption: “not a sterile ball of rock”.

So by the time Earth’s surface had cooled enough for liquid water, it wasn’t a sterile ball of rock that had to invent organic chemistry from nothing. It was being pelted with material that already contained water, amino acids, sugars and phosphorus, made in cold clouds and delivered by the leftovers of planet-building. That doesn’t explain the origin of life. That’s chapter five’s problem, and it’s still open. What it does is remove one step. The ingredients were not the hard part.

[05:22] S14 · ANIM · Left- and right-handed amino acids as mirror images; life’s molecules almost all left-handed; laboratory ice makes both equally; a small excess in some meteorites; a question mark.

One puzzle worth keeping. Life on Earth uses left-handed amino acids almost exclusively, though both mirror forms are chemically identical in every other way. The ice experiments make both, in equal amounts. Some meteorite samples show a small excess of the left-handed form, which might be inherited from polarised starlight. Whether Earth’s handedness was delivered or decided here: unresolved.

[05:49] S15 · ANIM · Labels: ESTABLISHED for grain chemistry, the census and the meteorite organics; ACTIVE RESEARCH for water delivery and chirality.

The scorecard. That chemistry happens on dust grains, that hundreds of molecules exist in space, and that meteorites deliver amino acids and sugars: established. Where Earth’s water came from, and why life is left-handed: active research.

[06:06] S16 · AI into ANIM · The dark nebula from the opening; zoom to a single grain with its ice mantle; the calendar returns.

So: the coldest, emptiest place we know of is a chemistry laboratory, and its products end up in meteorites you can hold. One chapter of this course is now nearly done. Before we leave it, there’s a question worth answering properly, because the answer is usually given as a slogan. Are you actually made of stardust? Next lesson.

Shot list

Two AI shots, twelve code-rendered shots, two agency images. Estimated Higgsfield use: 3 to 6 generations including retries.