The molecule is called ethyl formate. Its chemical formula is C2H5OCHO, with eleven atoms arranged as an ester. On Earth, it is one of the compounds responsible for the flavour of raspberries, and it carries a faint rum-like scent. It is also used industrially in the manufacture of safety glass and as a solvent in flavour and fragrance production. There is nothing exotic about ethyl formate at the molecular level. What was exotic, in 2009, was finding it twenty-six thousand light years from Earth.
The detection was made by a team of astronomers from the Max Planck Institute for Radio Astronomy in Bonn, Cornell University, and the University of Cologne. The lead author was Arnaud Belloche. The team was using the IRAM thirty-metre radio telescope at Pico Veleta in southern Spain to survey a giant cloud of gas and dust near the centre of the Milky Way called Sagittarius B2. They were not looking for raspberries.
They were looking for the building blocks of life.
What was actually being searched for
Sagittarius B2 is one of the largest molecular clouds in the galaxy, approximately one hundred and fifty light years across, sitting roughly four hundred light years from the supermassive black hole at the galactic centre. For more than four decades, it has been the most productive single location for discovering new organic molecules in interstellar space. The cloud is so consistently the source of new molecular discoveries that astrochemists in the 1990s gave its densest core a nickname, the “Large Molecule Heimat” (German for “Large Molecule Home”), because so many of the most complex molecules detected in space had been first found there.
The reason astronomers have been searching Sagittarius B2 for decades is that they want to know whether the chemistry of life can occur without life. The amino acids that form the proteins in every living organism on Earth are relatively simple molecules. The smallest, glycine, has only ten atoms. If amino acids can form in interstellar space, then the chemistry of biology is not unique to planetary environments, which has implications for how life might originate elsewhere and how it may have originated on Earth. The question has been one of the central questions of astrobiology since the 1970s.
The standard approach to answering it is to point a radio telescope at a dense interstellar cloud and look for the spectral signatures of progressively more complex organic molecules. Each kind of molecule, when it rotates in space, emits a distinct set of radio-frequency signals at predictable wavelengths. By matching these signals to laboratory measurements of known molecules, astronomers can identify what is present in the cloud without ever sending a probe to it.
Sagittarius B2 has yielded most of the complex organic molecules detected in space to date. Among them are ethanol, vinyl alcohol, methanol, methyl formate, formic acid, dimethyl ether, glycolaldehyde, acetic acid, and acetamide. Each of these is a molecule of significant biological relevance. Each of them was detected before ethyl formate was.
What Belloche actually found
In their 2009 paper, published in the journal Astronomy and Astrophysics, the Belloche team reported the detection of two new molecules in Sagittarius B2(N). The first was ethyl formate, the eleven-atom ester that gives raspberries part of their flavour. The second was n-propyl cyanide, a twelve-atom molecule that is, in concentrated form on Earth, highly toxic. Both molecules were detected at the same time, in the same cloud, in roughly comparable abundances.
The detection of ethyl formate was significant. The detection of n-propyl cyanide was scientifically more significant. At the time, twelve atoms was the largest single molecule ever detected in interstellar space outside of certain elongated carbon chains, and the molecule’s branched structure was particularly interesting because amino acids also have branched structures. The detection of n-propyl cyanide demonstrated that the kind of structural complexity required for amino acids could in fact be assembled in the cold, low-density environment of an interstellar cloud.
The science press, when it covered the finding, led almost entirely with the ethyl formate detection and the raspberry flavour. The propyl cyanide detection, which was the harder scientific result and the more significant one for astrobiology, was a footnote in most coverage.
The Guardian’s 2009 article on the discovery, written by science correspondent Ian Sample under the headline “Galaxy’s centre tastes of raspberries and smells of rum, say astronomers,” was the source that most subsequent coverage drew from. It quoted Belloche directly on the limitation of the raspberry-flavour framing. Belloche told the paper that ethyl formate “does happen to give raspberries their flavour, but there are many other molecules that are needed to make space raspberries.”
The qualification was largely lost in subsequent retellings. The popular framing of the story, which has now been repeated for more than fifteen years, settled on the claim that space tastes like raspberries and smells like rum.
Why the popular claim is incomplete
Several things are worth saying clearly about what the Belloche detection does and does not show.
Ethyl formate is one of approximately fifty molecules the survey detected in Sagittarius B2(N). The cloud contains, among many other compounds, ethanol, methanol, vinyl alcohol, hydrogen cyanide, carbon monoxide, water, dimethyl ether, and the same n-propyl cyanide that was reported alongside the ethyl formate. The cloud does not, on the chemical evidence, predominantly smell or taste like anything human sensory experience has a category for. It contains a complex mixture of organic compounds, only one of which happens to produce a flavour familiar to humans.
The raspberry flavour itself is also chemically more complicated than the popular framing suggests. Ethyl formate contributes to raspberry flavour, but it is not the dominant compound. The dominant compound is raspberry ketone, technically 4-(4-hydroxyphenyl)butan-2-one, which has not been detected in Sagittarius B2 or anywhere else in the interstellar medium. A full raspberry flavour also requires alpha-ionone, beta-ionone, and a number of other terpene-derived compounds. The simplification that ethyl formate alone “tastes like raspberries” is the kind of thing that survives in popular science writing because it is easy to remember, not because it is chemically accurate.
The cloud is also far too thin to taste. The density of even the densest parts of Sagittarius B2 is approximately one trillion times less than the density of the Earth’s atmosphere at sea level. The molecules are real and the spectral signatures are unambiguous, but the cloud is, by any human sensory standard, a near-vacuum. An astronaut floating through the densest part of Sagittarius B2 would experience approximately as much taste sensation as an astronaut floating through any other part of empty space, which is to say none.
And the claim that “space” tastes like raspberries reduces a finding about one specific dust cloud at the centre of the Milky Way to a claim about space in general. Most of the universe, by volume, is the intergalactic medium between galaxies, which contains approximately one hydrogen atom per cubic metre. The interstellar medium within galaxies contains a wider range of molecules, but the chemistry varies enormously from one location to another. The composition of Sagittarius B2 is not representative of the composition of space.
What space actually smells like, according to astronauts
The closest thing humans have to direct sensory data about the chemistry of space comes from astronauts returning from spacewalks. When the airlock of the International Space Station is repressurised after an extra-vehicular activity, the air that has been in contact with the spacesuits, the helmets, the gloves, and the tools brings with it a distinctive smell. Multiple astronauts who have done spacewalks have described it in similar terms.
Don Pettit, who flew long-duration missions on the International Space Station, described the smell in detail in a 2003 NASA blog post that has been the most-cited single source on the subject. Pettit wrote that the smell was hard to describe but that “the best description I can come up with is metallic; a rather pleasant sweet metallic sensation.” He went on to compare it specifically to his memory of working as an arc welder during college summers, repairing logging equipment in the heat of an open workshop. Other astronauts have described the same smell in similar terms. Thomas Jones, who flew on multiple Space Shuttle missions, described it as resembling seared steak. Peggy Whitson described it as burnt metal. The composite picture from multiple astronaut testimonies is of a sharp, acrid, metallic odour that clings to spacesuit fabric and equipment after every spacewalk.
The chemistry behind this is not fully understood. The current best explanation is that the smell is produced by ionised oxygen atoms from the upper atmosphere bonding to materials on the exterior of spacesuits, producing free radicals that, when warmed by the airlock pressurisation, release the characteristic odour. The compounds involved include various nitrogen-oxygen and carbon-oxygen species that are produced by the interaction of solar ultraviolet radiation with the trace atmospheric gases at orbital altitude.
None of these compounds smell like raspberries or rum. The actual smell of low-earth orbit, by the testimony of every astronaut who has been there, is closer to a welding shop than a fruit bowl.
What the finding actually means
The Belloche detection of ethyl formate and n-propyl cyanide in Sagittarius B2(N) was scientifically important for a reason that has very little to do with raspberries. It demonstrated, with high confidence, that complex organic molecules with the kind of branched structures characteristic of biological compounds can form in the interstellar medium under cold, low-density conditions. The team’s chemical modelling, which they included in the same paper, indicated that these molecules form on the surfaces of tiny dust grains in the cloud, where smaller molecular radicals can combine in stages to build progressively more complex structures.
The implication is that the chemistry of life is not unique to planetary surfaces. The kinds of molecules that life on Earth uses to build itself can be assembled by the slow physics of interstellar space, given enough time and enough dust. Sagittarius B2 has been doing this kind of chemistry for approximately ten million years, which is a short time on cosmological scales but a long time for organic chemistry.
The next stage of complexity, which Belloche and others continue to search for, is the detection of the amino acids themselves. In 2009, the same year as the Belloche paper, NASA researchers led by Jamie Elsila at the Goddard Space Flight Center reported the first confirmed detection of an amino acid in a comet, with glycine identified in samples returned by the Stardust mission from comet Wild 2. The Stardust glycine detection was confirmed by carbon isotope analysis, which showed the molecule was extraterrestrial rather than terrestrial contamination. A confirmed detection of an amino acid in an interstellar cloud, rather than in cometary material returned to Earth, has not yet been made. If it is made, it will be the strongest single piece of evidence that the chemistry of life can begin before planets even form.
That is the actual story behind the popular claim. The science press shortened it into raspberries because raspberries make a more memorable headline than prebiotic chemistry. The shortening is forgivable, but it obscures what was actually interesting about the finding.
What is at stake
The question of whether the chemistry of life can occur in interstellar space is one of the most important open questions in astrobiology. If amino acids and other biological precursors are routinely produced in molecular clouds before stars form, then every star that forms in such a cloud, including the Sun, would have inherited a starting inventory of organic chemistry from the cloud it formed in. This would substantially change the picture of how Earth got its first organic molecules and how common life-relevant chemistry might be in other planetary systems.
The Belloche detection of ethyl formate and n-propyl cyanide is one step in answering that question. Each detection of a new and more complex molecule in Sagittarius B2 narrows the gap between the chemistry that interstellar space has been confirmed to produce and the chemistry that biological life requires.
The narrowing is still incomplete. The detection of unambiguous amino acids in interstellar space has not yet been made. The signatures of more complex prebiotic molecules, including sugars and the precursors of nucleotides, have been searched for and not yet confirmed.
The chemistry is moving in the right direction. The popular version of the story has lost most of the actual scientific content along the way, but the underlying research continues at the same telescopes, in the same molecular cloud, with the same patient methods.
It tastes, on the best current evidence, like more than just raspberries.
