Latest news with #dimethylsulfide
Yahoo
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.
Yahoo
25-05-2025
- Science
- Yahoo
Doubts over hope of alien life on exoplanet K2-18b as case for biosignatures weakens
A growing number of studies have cast doubt on earlier claims about signs of life on K2-18b, a planet 124 light-years away in the Leo constellation, concluding that the available data doesn't support such bold interpretations. Last month, scientists had announced that they might have identified what appeared to be the most promising signs of alien life discerned so far on the distant planet. However, latest research contends that there was not enough evidence to estimate alien presence. K2-18b orbits within the habitable zone of its star, making it a compelling target for the search of alien life due to the potential presence of liquid water. Astronomers working with the James Webb Space Telescope drew global attention in April by reporting possible traces of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in the planet's atmosphere. On Earth, these compounds are produced exclusively by living organisms such as marine algae, which is why they are considered potential biosignatures—chemical hints that life might exist elsewhere. Led by Cambridge University's Nikku Madhusudhan, the research team was careful to stress that the potential biosignature was far from conclusive. The signal reached a three-sigma level of statistical significance, meaning there remains a small but meaningful possibility that the result was a fluke. Two of Madhusudhan's former students, Luis Welbanks of Arizona State University and Matthew Nixon of the University of Maryland, were part of a team that re-examined the data behind the initial announcement. In a preprint study published online towards the end of last month, they reported that when using alternative statistical models, the signals originally interpreted as potential biosignatures no longer stand out. In one approach, the researchers broadened the pool of possible atmospheric chemicals from 20 to 90, significantly weakening the case for a unique biological explanation. Embracing the ongoing scientific discussion, Madhusudhan highlighted the importance of keeping an open mind throughout the research process. His team added to the conversation by releasing a new preprint study last week that expanded the list of potential atmospheric chemicals to 650. Among the top candidates identified as an indicator of alien life was dimethyl sulfide (DMS). Meanwhile, dimethyl disulfide (DMDS), which had featured prominently in their earlier announcement, was no longer considered a leading possibility. Astronomers observe distant exoplanets like K2-18b by tracking their passage across their host stars, which allows them to analyze how molecules in the planet's atmosphere absorb specific wavelengths of starlight. Earlier this week, a study led by postdoctoral researcher Rafael Luque at the University of Chicago combined Webb's observations of K2-18b in both near-infrared and mid-infrared wavelengths. The research found no statistically significant evidence for dimethyl sulfide (DMS) or dimethyl disulfide (DMDS). In addition, an earlier paper by Oxford astrophysicist Jake Taylor, using a basic statistical method, also reported no strong signs of biosignatures. However, Madhusudhan dismissed Taylor's paper noting the simple exercise used to draw conclusions was far from a robust method to observe physical phenomena. The scientist also highlighted that more data on K2-18b will be collected over the next year, which should help provide a clearer and more definitive picture.
