There is a theory that is currently in vogue in astrochemistry called “Assembly Theory”. He postulates that highly complex molecules, many acids, for example, could only come from living things. Molecules are part of living things, or are things that intelligent living things make.
If assembly theory holds up, we could use it to search for aliens, scanning distant planets and moons for complex molecules that ought be evidence of living things. That’s the latest idea from the creator of the Assembly Theory, the chemist Leroy Cronin of the University of Glasgow. “This is a radical new approach,” Cronin told The Daily Beast.
But not all experts agree that it would work, at least not anytime soon. To take chemical readings of distant planets, scientists rely on spectroscopy. This is the process of interpreting a planet’s color palette to assess the possible mix of molecules in its atmosphere, land, and oceans.
Spectroscopy is not an exact science. That might leave alien-hunting astrochemists and astrobiologists guessing, for now. “There are a lot of uncertainties,” Dirk Schulze-Makuch, an astronomer at the Technical University of Berlin, told The Daily Beast.
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Scientists have been actively looking for signs of extraterrestrial life for at least a century. The search for extraterrestrial intelligence (SETI) accelerated in the 1950s and 1960s, with the advent of radio-based SETI. In SETI radio, scientists point sensitive radio receivers at the sky and listen for faint signals that could have originated with some alien civilization.
In the decades after the success of SETI radio, astronomers broadened their search. Increasingly powerful telescopes allowed them to capture colorful spectroscopic images of planets and moons, and then interpret those colors to make guesses about the chemical composition of the atmosphere. Certain elements could be prerequisites for life. Many astrobiologists agree that a planet must have carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorous just to have a chance to support biological evolution.
Once life has evolved on some distant exoplanet, it could paint the planet into complex molecules by mixing these and other elements. There could be chlorophyll, the substance that allows plants to absorb energy from light. It is made up of a family of molecules that combine carbon, hydrogen, oxygen, and magnesium that together give it a molecular mass of nearly 900.
But chlorophyll isn’t the only complex molecule that could be a marker of life. According to a new peer-reviewed study by Cronin and his British and Spanish colleagues, most molecules with a molecular mass of at least 300 could be evidence of extraterrestrial microbes, or even intelligent aliens.
Cronin and his team came to this conclusion after analyzing 10,000 chemicals that are present here on Earth. “Most molecules larger than [a] molecular weight [or mass] out of 300 [are] connected to the existence of life on Earth,” they wrote.
These complex molecules make up our bodies, the waste products of our bodies, and even the chemicals we make. Pharmaceuticals, for example. “This is because complex molecules…are too complex to form by chance in any detectable abundance and can therefore only be generated by the complex biochemical pathways found in biological cells,” Cronin and PhD wrote. coauthors of it.
In other words, if you find complex molecules on some distant planet or moon, then you probably found life, Cronin and company said.
That’s an exciting prospect for scientists, but there’s a problem: Not everyone agrees on what “complex” means. Yes, a molecular mass of at least 300 or so it correlates with Cronin’s notion of complexity. But there are too many possible exceptions, including forms of chlorophyll, for mass to be the standard. “There are many competing notions about chemical complexity,” Cronin and his team admitted.
Cronin’s Assembly Theory addresses this problem. The theory “estimates the complexity of a molecule by quantifying the minimal constraints required to build an object from the building blocks.” In simple language, the theory asks how many times, at least, a simple molecule would need to add an element or copy part of itself to achieve a given structure.
Any molecule that takes 15 steps must reach a molecular mass of 300 or more and qualify as “complex,” according to Cronin. And if Earth’s chemicals are any guide, the widespread presence of such a complex molecule on an alien planet or moon is a strong signal from nearby living things.
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Niels Ligterink, a physicist at the University of Bern in Switzerland, told The Daily Beast that he agrees with Cronin’s thinking. “In general, I would say that chemical complexity, in this case determined with Assembly Theory, is a good additional tool for looking for life.”
Assemblage Theory helps sidestep a big question in astrobiology, Ligterink added. Life on Earth has DNA or RNA, the nucleic acids that carry genes. It’s not safe to assume that extraterrestrial life would share this basic structure, Ligterink said. “But we can be pretty sure that extraterrestrial life is also chemically complex.”
But applying Cronin’s theory to the everyday search for alien life is easier said than done. How can a scientist take samples of molecules on an “exoplanet” that is light years away? They just can’t, not with today’s technology, anyway. The best they can do is inspect an exoplanet with a powerful telescope, the new James Webb Space Telescope or the Vera Rubin telescope in Chile, to name two, and analyze the color palette through spectroscopy.
See, each element absorbs certain wavelengths of light and reflects others. Carbon absorbs a little violet and blue and a lot of orange, leaving a lot of greens, reds and yellows. Nitrogen has practically the opposite light absorption pattern. Spectroscopy looks at those wavelengths and helps identify what type of chemistry they correspond to. Certain mixtures of colors could point to complex molecules that combine various elements in intricate ways.
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The challenge with spectroscopy is precision. Imagine the light pattern of each element like a fingerprint. Now imagine a million smudged fingerprints on top of each other. “The spectral signature… can rarely be attributed solely to a specific molecule,” Schulze-Makuch said.
We may need much better telescopes or probes to make Cronin Assemblage Theory work as an alien-hunting strategy between distant exoplanets and their moons. That could take a while.
But it’s possible that the same theory could help scientists find evidence of extraterrestrial life in existing data from the closest planets and moons. There are reams of data from various missions to Mars since NASA’s Viking probes first landed on the Red Planet in 1976.
The two Viking probes collected soil samples, boiled them and analyzed the escaping gases. The probes transmitted the data to NASA. Crunching the numbers, scientists from the Gil Levin agency concluded that the probes had found the first chemical evidence of extraterrestrial microbes.
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Levin was ready to announce to the entire world that we would be making first contact with microbial ET. But his NASA colleagues insisted he had misread the data, a position the space agency has held for 47 years. Levin did not respond to a request for comment.
The Viking data, as well as data from other previous space missions, is worth reconsidering, Cronin said. If there is evidence of complex molecules, perhaps there are signs of life that scientists have missed. “It’s possible,” Cronin said.
In that way, assembly theory could help us make sense of past search for alien life long before it helps with future searches.
Read more at The Daily Beast.
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