Science

If it's silicon-based life, will we ever find aliens?

If it's silicon-based life, will we ever find aliens?If it's silicon-based life, will we ever find aliens?

In the search for extraterrestrial life, astrobiologists are always looking for the simplest and most robust forms of life that have the best chance of surviving in harsh extraterrestrial environments. However, the chemicals associated with simple organisms can often be produced through abiotic pathways as well. So, scientists sometimes believe that alien life has been discovered, but lack conclusive evidence.

Not only that, but extraterrestrial life may be very different from life on Earth. For example, "silicon-based life" is a regular feature of science fiction, and we assume that their long chains of molecules are composed mainly of silicon rather than carbon, so their chemical composition would be very different from the life we are familiar with. If this is the case, how do we find extraterrestrial life? Is there something special about "life" itself that would allow us to know exactly where it exists?

Found and not found

In the mid-1970s, two NASA Viking probes flew to Mars in search of life, but came to controversial conclusions. Evidence that life may exist on Mars came from an isotope labeling experiment: food required by microbes was labeled with carbon 14 and added to a sample of Martian soil; if any microbes ingested the labeled food, those microbes released radioactive carbon dioxide, which was detected by the instrument.

As a result of the experiment, both Viking landers, which are 6500 km apart on Mars, detected radioactive carbon dioxide in the soil of the experimental group, while the heat-sterilized control soil did not, suggesting that microbial metabolism occurs in the Martian soil. However, other life-detection experiments conducted by the two Viking ships have not found any signs of life.

If it's silicon-based life, will we ever find aliens?

Image of Mars taken by the Pirate 2 lander (image credit: NASA/Wikipedia)

In 1996, scientists discovered a Martian meteorite in Antarctica that contained microfossil (microscopic fossils) suspected of containing microorganisms, seemingly adding to the evidence for the existence of life on Mars. However, subsequent studies have pointed out that several abiotic pathways could also easily produce traces of so-called microfossils.

Recently, some scientists claim to have found large amounts of phosphine in Venus' atmosphere, whereas on Earth, phosphine is mainly produced by microorganisms. But other scientists question this result, arguing that even if phosphine is present in Venus' atmosphere, it may come from some bizarre form of volcanic activity on Venus, rather than from life activity.

These stories of the search for extraterrestrial life follow a similar developmental pattern: initial clues are exciting, then suspicious, and finally the hypothesis of life is rejected. Again and again, astrobiologists seem to find only biosignature, but frustratingly, biosignatures cannot be used as conclusive evidence for the existence of extraterrestrial life. Are there any indicators that would allow us to be sure that extraterrestrial life has been found?

New ideas from "complexity"

A study published in Nature Communications proposes a new idea called "assembly theory". Instead of focusing on simple biological traces, assembly theory focuses on the complexity of the nature of life. It is based on the idea that any form of life in the universe encodes life information in complex combinations of molecules, and that this complexity is very different from that of inanimate matter.

In the field of astrobiology, calls for attention to "complexity" have been going on for some time, and NASA defined "life" as complex in 1994: life is a self-sustaining chemical system capable of Darwinian evolution. The problem is that this definition contains key concepts that are inherently complex, difficult to test and difficult to quantify. As NASA's chief scientist Jim Green put it, "I can't build machines that can look for 'evolution,' 'reproduction' or 'metabolism. ' machines for these processes."

Assembly theory, on the other hand, provides a clearer and more universal definition of life. Assembly theory assumes that for any object (object) in any environment, the likelihood that it originates from a living activity increases as its abundance (abundance) and complexity increase. Abundance refers to the frequency with which the object occurs in the environment, while complexity can be measured by estimating the steps required to assemble such an object.

Study co-author Sara Walker, a biophysicist at Arizona State University, believes that assembly theory is a milestone in the field of astrobiology because it presents the first operational complexity measure that gives theories about the nature of life the opportunity to be combined with experimental observations.

Complex molecules

Although assembly theory applies to objects at multiple scales, researchers have focused on its application at the molecular level. This is because molecules are the most fundamental component of biology, both in the laboratory and in the universe. To measure the complexity of molecules, the team defined a "mass assembly number" (MA), which is assigned to different molecules by an algorithm.

MA refers to the number of steps required to build a molecule in the ideal case. We know that a molecule can usually be synthesized in multiple ways, and MA corresponds to the shortest assembly path among them. It considers only valence rules, no other constraints including chemical reaction conditions, and the object created at each step can be reused in subsequent steps. Therefore, molecules with fewer types of chemical bonds and higher symmetry have lower MA values, and vice versa.

If it's silicon-based life, will we ever find aliens?

Example of the principle of analyzing assembly steps (image source: original paper)

The researchers assigned MA to 2.5 million molecules in a chemical database. Phosphine, seen by some scientists as a biological sign of Venus, consists of one phosphorus atom and three hydrogen atoms linked by a symmetrical phosphorus-hydrogen single bond and has an MA of only 1. In contrast, the tryptophan molecule, which has a more complex structure consisting of 11 carbon atoms, 12 hydrogen atoms, two nitrogen atoms and two oxygen atoms, has an MA of 12.

If it's silicon-based life, will we ever find aliens?

Schematic representation of the molecular structure of tryptophan (image credit: NEUROtiker/Wikipedia)

To verify the validity of MA, the researchers tested it with real molecules. Because molecules with high MA have more chemical bonds and relatively lower symmetry, the researchers predicted that they would generate more peaks in mass spectrometry (each peak representing a different ion in the mixture), while the opposite was true for molecules with low MA. The experimental results were consistent with their prediction - there was a linear relationship between the number of peaks and MA, with a correlation of 0.89.

If it's silicon-based life, will we ever find aliens?

On the left are the three molecules used as examples, and on the right are their corresponding mass spectra. It can be seen that the more complex molecules have higher MA and show more peaks on the mass spectra. (Image source: Original paper)

The "threshold" of life

After establishing a link between theory and practice, the researchers further tested their core hypothesis that molecules with high MA can be produced almost exclusively by organisms. They examined the mass spectra of a variety of mixture samples, including E. coli, plant alkaloids, coal, granite and even beer, and estimated their MA values based on linear relationships. The researchers found that only samples with living organisms had MAs higher than 15.

Lee Cronin, a chemist at the University of Glasgow and leader of the study, said that when a molecule has an MA greater than 15, its probability of arising in abiotic processes under Earth-like conditions is extremely low (less than 1 in 6 x 1023). Therefore, molecules with MA values greater than or equal to 15 can be produced almost exclusively by life. That is, we can discover life through mixtures with MA greater than a certain threshold.

So, is the MA value of 15 an absolute criterion for determining life and non-life? No, it is not. First, many molecules with low MA values may also be biological traces, such as the simple structure of the oxygen molecule that organisms release into the Earth's atmosphere through photosynthesis. Second, Cronin points out that while on Earth, whether the MA is greater than 15 appears to be the threshold condition for the presence or absence of life, this threshold may be different in planetary environments that are very different from Earth.

To test their theory, study co-author Heather Graham, an astrobiologist at NASA's Goddard Space Flight Center, sent a set of blind samples to Cronin. One was a fossilized creature from millions of years ago, and the other was a sample of the Murchison meteorite. The Murchison meteorite, a fiery meteorite that fell to Earth in 1969, is rich in organic, carbon-containing compounds but not biogenic.

If it's silicon-based life, will we ever find aliens?

Murchison meteorite specimen at the American Museum of Natural History (Photo credit: Basilicofresco/Wikipedia)

This is a test of the assembly theory, and the results are exciting, showing that a complex sample composition alone does not mean that life was involved, and that "complex molecules" that reflect the complexity of chemical organization are the key element of life. It is the "complex molecules" that reflect the complexity of chemical organization that are the key elements of life.

Putting it into practice

Previous NASA interplanetary missions have collected some mass spectrometry data on other planets. Greene and NASA scientists were curious if assembly theory could be used to look for signs of life in them.

Greene first considered the Cassini vehicle, which collected water vapor samples from Enceladus. Unfortunately, Cassini's mass spectrometer can only detect molecules smaller than 100 atomic mass units (amu), but assembly theory only applies to molecules larger than 150 amu. And although NASA's Curiosity and Perseverance Mars rovers carry mass spectrometers that can detect molecules above 150 amu, they lack the ability to study a single molecule enough to analyze the MA value.

All future space exploration missions should be equipped with mass spectrometers that can measure larger molecules and perform more accurate analysis, Green said. Dragonfly, which will fly to Saturn's moon Titan in 2034, is expected to achieve that goal. It will probe Titan's atmosphere and surface to find the components of life. Although Dragonfly's mass spectrometer does not have the full capabilities of a laboratory mass spectrometer, it has the ability to detect complex molecules.

If it's silicon-based life, will we ever find aliens?

Artistic imagery of the Dragonfly mission at Titan (Photo credit: NASA/Wikipedia)

There are still places in our solar system where life may exist waiting to be explored by humans, and large telescopes are searching for potentially habitable planets for us in the vastness of the universe. Assembly theory offers a new perspective on the universe at the molecular scale, guiding our search for extraterrestrial life by focusing on the complexity that is unique to life. Right now, there are uncountable complex molecules synthesizing, flowing, and working within our bodies that make us so different in the universe.

Written by Natalie Elliot