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3 days ago
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Does extraterrestrial life smell like the sea?
Dimethyl sulfide, also known as DMS, sounds like it could be a chemical compound you'd try to avoid on an ingredient label, or the poisonous ingredient in a murder mystery. But some scientists view this simple compound as a biosignature — a key indicator of life. So there was great excitement when DMS was discovered on a "sub-Neptune planet" far from our solar system – 124 light years away, or about 17 trillion miles, in the constellation Leo. 'We want to be a bit careful in claiming any evidence of life at this stage,' cautioned lead author Nikku Madhusudhan, of Cambridge University, about the findings he published last month in The Astrophysical Journal Letters, a publication of the American Astronomical Society, with other researchers from two American space institutes and two British physics and astronomy departments. 'We have to look at a lot more molecules, and we have, and we couldn't come up with a much better explanation,' Madhusudhan told Salon in a video interview. He admits he can't be 100% certain that dimethyl sulfide, or (CH3)2S, exists on the planet called K2-18 b. But it looks very likely, as last month's research built on a paper published in 2023 that also found suggestions of DMS on the same planet but relied on different evidence. But why would a random compound detected on a planet so far beyond our reach be a strong indicator of life? Well, let's consider the story of DMS on Earth, a story of the strange and poetic ways life appears and reappears in different guises — and with different scents. Dimethyl sulfide is the largest natural source of atmospheric sulfur on Earth, which means that it gets into the atmosphere and cycles around. But it starts its journey in the ocean. You're absolutely familiar with DMS, even if you've never heard of it before. It's the source of the smell of the sea, that sort of fishy, sort of eggy aroma that evokes deeply nostalgic reactions in, well, almost everyone. Interesting pushback came from Christophe Laudamiel, a master perfumer at Generation by Osmo. 'I have personally never used that ingredient for the smell of the sea,' he told Salon by email. 'It would be rather used for 'hot' smells and for ripe to overripe smells." He compared the odor of DMS to "fish that stayed too long in the sun," adding, quite understandably, that "we usually avoid" such associations "when we recreate the smell of the sea in perfumery." Rather than relying on those fish-rotting-in-sun odors to get ocean-smelling perfume, suggested Generation by Osmo founder and CEO Alex Wiltschow (also by email), "We combine aquatic notes with mineral wet stone notes, salty notes and clean air notes," along with, perhaps, "a touch of seaweed absolute as well or mossy top notes." Similarly environment-evoking are the substances geosmin and petrichor. Petrichor is the pleasant, earthy aroma of rain falling on dry soil, sometimes described more simply as the smell of rain. That word has almost become trendy. In fact its use appears to have skyrocketed in the past quarter-century, though it was coined in the journal Nature in 1964. Like geosmin, the substance that gives earth its characteristic "earthy" odor, petrichor remains close to the ground. Dimethyl sulfide, however, gets around. The DMS that cycles around our world is produced, for the most part, by marine organisms, most notably the microscopic plants known as phytoplankton that live in the nutrient-rich upper layer of the ocean. These tiny organism exist in abundance, which is why DMS is responsible for most of that smell we associate with the seaside. From the surface layer of the Earth's oceans, DMS, which is a volatile chemical, escapes into the air, joining the atmospheric cycling of sulfur. As one researcher describes this process, once in the atmosphere DMS "has other major effects, being the 'seed' that sets off cloud formation over the oceans. Indeed, the production of this molecule is on such a scale that it has major effects on the world's climate, thanks to its effect on the cloud cover over the oceans.' That quotation is nearly 20 years old, but scientists still don't know exactly to what extent DMS is responsible for seeding clouds, just that it's a significant factor. The tiny aerosol particles formed when DMS molecules are zapped by sunlight and other molecules in the atmosphere, which become the 'seeds' for clouds, also exert meaningful effects on our climate by reflecting sunlight back into space. In 2007, scientists at the University of East Anglia discovered that a single gene could produce dimethyl sulfide from dimethylsulfoniopropionate, or DMSP, the food that phytoplankton eat. As described in a paper in Science, you can take that gene, which has the catchy name dddD, from bacteria that live in the sea, or find it in other species of bacteria that hang out with plants instead but also produce DMS. Once you've found a bacterium with the dddD gene, you can clone it and stick it into an bacterium, which will then happily produce dimethyl sulfide. The aforementioned predecessor chemical DMSP is found, by the billions of tons, all over the world's oceans, seas and seashores. Marine plants and phytoplankton use it to protect themselves from the saltiness of seawater, literally as a buffer against stress. When these tiny plants die, some of their DMSP becomes available, as food for other bacteria. Terrestrial plants may also have symbiotic bacteria living in their root systems, which produce dimethyl sulfide from the DMSP released when their hosts die. This process — one kind of organism dies, offering sustenance to others — is how this cycle begins, at least on Earth. (If you can actually say that a cycle has a beginning or an end.) As one of the East Anglia scientists, Andrew Johnston, wrote in a 2007 project funding proposal, describing the role of DMS in seeding clouds, its importance has been known since 1971, "with some 30 million tons of it being liberated into the air, worldwide, every year.' Aquatic bird species such as sea petrels and shearwaters are attracted to the ripe-fish aroma, while Johnston later discovered that the Atlantic herring has strains of bacteria in its gut microbiome called Pseudomonas and Psychrobacter, which digest DMSP and break it down into, yes, dimethyl sulfide. How did those bacteria get inside a fish? Herring eat small plants known as mesozooplankton, which themselves eat the much smaller phytoplankton. This familiar ecological pattern — bigger creatures eating smaller creature — has internalized the production of this evocative and volatile to the food chain, it seems, the creation dimethyl sulfide can take place not just in the surface layer of the ocean, but inside herring guts as well. Herrings are vertebrates, in the greater evolutionary scheme not all that different from us. Does this mean that humans also have the potential to create sulfurous stinks from our own insides? Well, there's no evidence at this point that our microbiomes contain DMS-producing bacteria. But that's ok. As you may be aware, our species can produce our own glorious forms of stink. Dimethyl sulfide is an essential element in the characteristic odors of blood, serum, tissues, urine and breath in people (and rats). Not to mention the distinctive smell of feces and flatus, i.e., farts. Let's mention here that dimethyl sulfide is emitted during wildfires, and so contributes to a scent that has grown chillingly familiar in many parts of North America in recent years. It's also largely responsible for the smell of the delicately-named dead horse arum, a relative of the so-called corpse flower, or titan arum. Other flowers with unappetizing odors use different chemicals as their top notes, all with the purpose of attracting pollinators drawn to the aroma of their preferred type of rotting meat. Here for example is Wikipedia's almost lyrical rundown of the various sources of the corpse flower's scent: 'Analyses of chemicals released by the spadix show the stench includes dimethyl trisulfide (like limburger cheese), dimethyl disulfide (garlic), trimethylamine (rotting fish), isovaleric acid (sweaty socks), benzyl alcohol (sweet floral scent), phenol (like Chloraseptic), and indole (like feces).' Scientists comparing the molecules involved in producing the stench of dead horse arum with those produced by a rotting corpse found that dimethyl sulfide was associated with the middle stage of decomposition in actual corpses (to be clear, this involved dead mice, not dead horses or human cadavers). All this odoriferous research has convinced some scientists that DMS is intimately associated with life, making it an ideal biosignature if found hundreds of light years away on some lonely planet. Critics of Madhusudhan's findings point out, however, that dimethyl sulfide can exist without demonstrating life at all. For one thing, you can make it in a lab. As the perfumer Laudamiel told Salon, DMS is "often used in perfumery, but not for its low-tide, rotten egg facet.' The human nose can detect one part per million of DMS, as an unpleasant, cabbage-like smell used, for example, to add a warning signal to the poisonous gas carbon monoxide, which is otherwise odorless natural gas. DMS also results from kraft pulping, producing a ghastly, retch-inducing smell you'll have noticed if you've ever driven by a paper processing plant. It's produced naturally as bacteria do their work on dimethyl sulfoxide waste in sewers. When it's not saving us from asphyxiation or carrying out useful industrial processes, dimethyl sulfide also lends its "low-tide, rotten egg facet" as a nearly subconscious flavor in food and drinks, measured in a few parts per million. In brewing certain lagers, though, breweries may want that slightly funky flavor, and add enough DMS to cross the flavor threshold as a hint of the ocean (or of distant rotten eggs, or cabbage). The natural production of DMS is also medically useful. It turns out that as a kind of bacteria turns from existing peacefully in our mouths to causing colon cancer in our nether regions, it produces dimethyl sulfide. Worsening osteoporosis in older women may lead to exhaling DMS, as can the positive effects of a medication cocktail for children with cystic fibrosis. But how is it that the compound that gives us the glorious smell of the sea — and just perhaps, our first evidence of life on a distant planet — also provides the generally disagreeable fragrance of flatus, feces and flowers that smell like rotting meat? 'It works just like salt in a cake," explained Laudamiel. "In combination with other molecules, at low, unrecognizable dosages, it brings out the flavors of other facets." Unpleasant-sounding flavor notes such as "the overripe 'vomity' note found naturally in papaya ... the 'feet' note found in Parmigiano or the 'sweaty' note found naturally in dark chocolate" produce magical effects in combination with others and in just the right amount. Remove those notes, he concluded, and your papaya, cheese or chocolate will "taste much less yummy." Indeed, DMS, provided by nature at just the right dosage, is a component in the much coveted scent of truffles. Turning away from our planet with its stinky-feet cheese, vomity papayas and sweaty chocolate, and turning to the stars, DMS is used as an additive in rocket fuel, added to ethylene oxide to prevent exhaust nozzles getting dirty and stop carbon building up on firing-chamber surfaces. But no existing or planned spacecraft can get us anywhere near the next possible known source of dimethyl sulfide on K2-18 b, the planet where Madhusudhan and colleagues have found, thanks to the James Webb Space Telescope, what they think could well be this signature of life. Astronomers these days are really interested in sub-Neptune planets, meaning those with diameters larger than Earth but smaller than Neptune. It's an exotic niche that doesn't exist in our solar system, and could offer new possibilities for finding life. They're particularly interested in a newly-defined type of planet that could exist within that range: Hycean worlds, which would possess water-rich interiors, planet-spanning oceans and atmospheres rich in hydrogen gas. The Madhusudhan team's detection of methane and carbon dioxide gases on K2-18 b supports his argument that the planet might have surface water, as does the fact that they did not find ammonia, which is soluble in water — if that's detected in the atmosphere, there probably isn't an ocean. But while DMS is a biosignature here on Earth, other scientists point out that it could be cooked up by some other process elsewhere, just as it can be produced in a laboratory for industrial purposes. Some scientists have suggested other possible explanations for the signals found by Madhusudhan's team, including statistical noise. Two findings within the past year bolster these criticisms. One, described last October, is the presence of dimethyl sulfide in a comet named 67P/Churyumov-Gerasimenko, which no one would argue suggests biological activity. Madhusudhan says that does nothing to disprove his hypothesis; comets are known to be little laboratories that can cook up all sorts of unlikely things. 'The same comet also has molecular oxygen in it, right?' he countered. 'It also has methane and other molecules, including amino acids." Finding something in a comet, he said, "doesn't mean that it can't be a biosignature in a planetary atmosphere, because those are two very different environments." Another finding that may cast doubt on the idea that DMS equates to the presence of life is the discovery of dimethyl sulfide, which here on Earth makes the sea smell like the sea, drifting around in deep space between the stars. Reporting on the open science platform Arxiv in February, an international group of astronomers said they found DMS during an ultra-deep molecular line survey, which uses fancy telescopes to look at a spectrum of wavelengths in one particular stretch of outer space and then catalog its chemical composition and physical properties, such as temperature and density. In this case, they pointed their telescopes toward a Galactic Center molecular cloud named G+0.693-0.027. And there they found dimethyl sulfide, just vibing in the void.
News.com.au
11-05-2025
- Science
- News.com.au
New claim of ‘alien' life leaves space world in a bind
Life. But how would we know it? The world of astronomy is abuzz with excitement. Have we discovered the first-ever evidence of off-world life? Is the signal extracted from the interstellar noise a telltale of active biology? 'Astronomers have detected the most promising signs yet of a possible biosignature outside the solar system, although they remain cautious,' states a Cambridge University press release. What has them excited is the planet K2-18b. We only know it exists by the shadow it casts when it passes between its star and the Earth. But that shadow, like a stained-glass window, can tell a story. Across the gulf of space, the James Webb Space Telescope has managed to capture its light spectrum and atmospheric gases like methane, carbon dioxide, oxygen, and dimethyl sulphide also cast shadows when light strikes them. The challenge for astronomy is to catch enough of this distant light to dissect reliably and to interpret that data accurately. The Cambridge University astronomers have attempted to do just that. Scientists the world over have since been sciencing their results. How was the data collected? What data was collected? How was it analysed? Does it support the conclusion? 'While this discovery is intriguing, most astronomers – including the paper's authors – aren't ready to claim that it means extraterrestrial life exists,' says University of Arizona distinguished professor of astronomy Chris Impey. There simply isn't enough data. Yet. 'Searching for life beyond Earth is one of the great, profound pursuits of humankind. But any claim of life out there will require a thorough study by the scientific community as a whole before we have confidence in the results,' adds The Planetary Society 's chief scientist, Dr Bruce Betts. In a post-truth world, that confidence is more important than ever. 'Just like the boy that cried wolf, no one wants a series of false claims to further diminish society's trust in scientists,' Johns Hopkins University astrophysicist Kevin Stevenson argues. 'Context is important when it comes to science communication, particularly for a hot-button topic like the search for life beyond Earth, and we need to be responsible stewards in that respect.' Brave new worlds 'It is in no one's interest to claim prematurely that we have detected life,' Professor Nikku Madhusudhan, a University of Cambridge astronomer and lead author of the study, told media. But he still called his study a 'revolutionary moment'. 'It's the first time humanity has seen potential biosignatures on a habitable planet,' the professor explained. He added that the discovery was 'as big as it gets' for scientists. We don't know much about K2-18b. But we've guessed a lot. It was found in 2015 by the Kepler Space Telescope. It has a radius about 2.4 times larger than that of Earth. Its year is just 32.9 days and it orbits within the habitable 'Goldilocks' Zone (warm enough for liquid water) of a cool red dwarf star about 120 light-years away in the direction of the constellation of Leo. University of Cambridge astronomers believe it may have a thin hydrogen atmosphere over an oceanic surface. Thus the term 'Hycean' (hydrogen-ocean) world. But the opposite could also fit the limited known facts: K2-18b could have a thick atmosphere over a magma sea. One way to find out is to determine what that atmosphere is made o. That involves extracting a reliable spectrum from a shadow flickering within a pinprick of starlight. The cause of all the excitement is what the Cambridge University researchers believe is the presence of dimethyl sulphide. This molecule is only found on Earth as the by-product of plankton and bacterial life in our oceans. And that makes it a contender for the position of 'biomarker' – a chemical 'smoking gun' to the presence of life on other worlds. But has dimethyl sulphide really been found on K2-18b? 'Every detector has some noise from the random motion of electrons,' Professor Impey explained. 'The signal should be strong enough to have a low probability of arising by chance from this noise.' The planet was first photographed by the Hubble Space Telescope in 2016. Astrobiologists thought they saw indications of water vapour. Upon revision, this later turned out to be methane. The first look with the more powerful JWST was made by Professor Madhusudhan's team about two years ago. Analysing a different set of wavelengths than the most recent study, this initially seemed to show the presence of methane and carbon dioxide – and a hint of dimethyl sulphide. But the raw data they drew their conclusions from has since been re-examined by different scientists using different methods. They have not confirmed the presence of CO2 or dimethyl sulphide. The data from the most recent survey was released for general review on April 28. Does this raw data show what the Cambridge University researchers believe it does? To be science, independent scientists must be able to draw the same conclusions using various methods, and the premise has to be proven correct. Otherworldly chemistry 'We didn't know for sure whether the signal we saw last time was due to DMS (dimethyl sulphide), but just the hint of it was exciting enough for us to have another look with JWST using a different instrument,' Professor Madhusudhan explained. It's the second time the Cambridge University team has found the molecule. 'This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations. The signal came through strong and clear.' But K2-18b isn't Earth. Evidence has begun to emerge that dimethyl sulphide can be produced by non-biological means. Its signature spectrum has been spotted in the clouds of dust and gas that drift between stars. It may have been seen on an uninhabitable comet. Laboratory studies hint that it could be broken out of other molecules under UV light. Professor Madhusudhan concedes there could be previously unknown chemical processes at play. He and his team remain confident in the strength of their findings. 'Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach,' Professor Madhusudhan said. 'This could be the tipping point, where suddenly the fundamental question of whether we're alone in the universe is one we're capable of answering.' But, given the rarity of dimethyl sulphide, the Oxford University measurements have raised eyebrows. Especially as they indicate K2-18 b's atmosphere is awash with it in concentrations thousands of times greater than Earth's. 'The claim is intriguing, as these gases can be associated with biological activity,' a statement released by the Search for Extraterrestrial Intelligence (SETI) reads. 'On the other hand, the signal is modest, ambiguous, and potentially explainable as the result of noise or systematic error.' But, like all research, this study is just one step in a winding journey, filled with obstacles and dead-ends. 'This study is not a confirmation of life; not even a hypothesis that life is present on K2-18b, but a demonstration of where our methodological strengths and limitations lie, and what must come next to an unambiguous claim of life beyond Earth,' the SETI statement concludes. In science's famous last words: More research is needed. 'It'll take time to figure all this out,' adds The Planetary Society. Different teams will do their own investigations, collect more data, learn more about how DMS might be produced, and better understand how it might show up in a planet's atmosphere. Only then will we have a chance at the full story.
