Reducing the Search Space with the SETI Ellipsoid

by Paul Gilster | Jul 11, 2023 | Astrobiology and SETI | 25 comments

SETI’s task challenges the imagination in every conceivable way, as Don Wilkins points out in the essay below. A retired aerospace engineer with thirty-five years experience in designing, developing, testing, manufacturing and deploying avionics, Don is based in St. Louis, where he is an adjunct instructor of electronics at Washington University. He holds twelve patents and is involved with the university’s efforts at increasing participation in science, technology, engineering, and math. The SETI methodology he explores today offers one way to narrow the observational arena to targets more likely to produce a result. Can spectacular astronomical phenomena serve as a potential marker that could lead us to a technosignature?

by Don Wilkins

Finite SETI search facilities searching a vast search volume must set priorities for exploration. Dr. Jill Tarter, Chair Emeritus for SETI Research, describes the search space as a “nine-dimensional haystack” composed of three spatial, one temporal (when the signal is active), two polarization, central frequency, sensitivity, and modulation dimensions. Methods to reduce the search space and prioritize targets are urgently needed.

One method for limiting the search volume is the SETI Ellipsoid, Figure 1, which is reproduced from a recent paper in The Astronomical Journal by lead author James R. A. Davenport (University of Washington: Seattle) and colleagues. [1]

Image: This is Figure 1 from the paper. Caption: Schematic diagram of the SETI Ellipsoid framework. A civilization (black dot) could synchronize a technosignature beacon with a noteworthy source event (green dot). The arrival time of these coordinated signals is defined by the time-evolving ellipsoid, whose foci are Earth and the source event. Stars outside the Ellipsoid (blue dot) may have transmitted signals in coordination with their observation of the source event, but those signals have not reached Earth yet. For stars far inside the Ellipsoid (pink dot), we have missed the opportunity to receive such coordinated signals. Credit: Davenport et al.

In this approach, an advanced civilization (black dot) synchronizes a technosignature beacon with a significant astronomical event (green dot). The astronomical event, in the example, is SN 1987A, a type II supernova in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. The explosion occurred approximately 51.4 kiloparsecs (168,000 light-years) from the Sun.

Arrival time of the coordinated signals is defined by a time-evolving ellipsoid, with foci at Earth (or an observation station within the Solar System) and the source event. The synchronized signals arrive from an advanced civilization based on the distance to the Solar System or other system with a technological system (d1), and the distance from the advanced civilization to the astronomical event (d2). Signals from civilizations (blue dot) outside the Ellipsoid coordinated with the source event have not reached the Solar System. Stars inside the Ellipsoid (pink dot) but on line between the advanced civilization and the Solar System will not receive the signals intended for the Solar System. However, the advanced civilization could beam new signals to the pink star and form a new Ellipsoid.

The source event acts as a “Schelling Point” to facilitate communication between observers who have not coordinated the time or place of message exchanges. A Schelling point is a game theory concept which proposes links can be formed between two would-be communicators simply by using common references, in this case a supernova, to coordinate the time and place of communication. In addition to supernovae, source events include gamma-ray bursts, binary neutron star mergers, and galactic novae.

In conjunction with the natural event which attracts the attention of other civilizations, the advanced civilization broadcasts a technosignature signal unambiguously advertising its existence. The technosignature might, as an example, mimic a pulsar’s output: modulation, frequency, bandwidth, periods, and duty cycle.

The limiting factor in using the SETI Elliposoid to search for targets is the unavailability of precise distance measurements to nearby stars. The Gaia project remedies that problem. The mission’s two telescopes provide parallaxes, with precision 100 times better than its predecessors, for over 1.5 billion sources. Distance uncertainties are less than 10% for stars within several kiloparsecs of Earth. This precision directly translates into lower uncertainties on the timing for signal coordination along the SETI Ellipsoid.

“I think the technique is very straightforward. It’s dealing with triangles and ellipses, things that are like high-school geometry, which is sort of my speed,” James Davenport , University of Washington astronomer and lead author in the referenced papers, joked with GeekWire. “I like simple shapes and things I can calculate easily.” [2]

An advanced civilization identifies a prominent astronomical event, as an example, a supernova. It then determines which stars could harbor civilizations which could also observe the supernova and the advanced civilization’s star. An unambiguous beacon is transmitted to stars within the Ellipsoid. The volume devoted to beacon propagation is significantly reduced, which reduces power and cost, when compared to an omnidirectional beacon.

At the receiving end, the listeners would determine which stars could see the supernova and which would have time to send a signal to the listeners. The listening astronomers would benefit by limiting their search volume to stars which meet both criteria.

For example, astronomers on Earth only observed SN 1987A in 1987, thirty six years ago. If the advanced civilization beamed a signal at the Solar System a century ago, our astronomers would not have the necessary clue, the observation of SN 1987A, to select the advanced civilization’s star as the focus of a search. Assuming both civilizations are using SN 1987A as a coordination beacon, human astronomers should listen to targets within a hemisphere defined by a radius of thirty-six light-years.

The following is written with apologies to Albert Einstein. The advanced civilization could observe the motion of stars and predict when a star will come within the geometry defined by the Ellipsoid. In the case of the Earth and SN1987A, the advanced civilization could have begun transmissions thirty-six years ago.

The recently discovered SN 2023ixf in the spiral galaxy M101 could serve as one of the foci of an Ellipsoid. 108 stars within 0.1 light-year of the SN 2023ixf – Earth SETI Ellipsoid. [3]

Researchers propose to use the Allen Telescope Array (ATA), designed specifically for radio technosignature searches, to search this Ellipsoidal. The authors point out the utility of the approach and caution about its inherent anthropocentric biases:

“…there are numerous other conspicuous astronomical phenomena that have been suggested for use in developing the SETI Ellipsoid, including gamma-ray bursts (Corbet 1999), binary neutron star mergers (Seto 2019), and historical supernovae (Seto 2021). We cannot know what timescales or astrophysical processes would seem “conspicuous” to an extraterrestrial agent with likely a much longer baseline for scientific and technological discovery (e.g., Kipping et al. 2020; Balbi & Ćirković 2021). Therefore we acknowledge the potential for anthropogenic bias inherent in this choice, and instead focus on which phenomena may be well suited to our current observing capabilities.”

1. James R. A. Davenport , Bárbara Cabrales, Sofia Sheikh , Steve Croft , Andrew P. V. Siemion, Daniel Giles, and Ann Marie Cody, Searching the SETI Ellipsoid with Gaia, The Astronomical Journal, 164:117 (6pp), September 2022, https://doi.org/10.3847/1538-3881/ac82ea

2. Alan Boyle, How ‘Big Data’ could help SETI researchers intensify the search for alien civilizations, 22 June 2022, https://www.geekwire.com/2022/how-big-data-could-help-seti-researchers-intensify-the-search-for-alien-civilizations/

3. James R. A. Davenport, Sofia Z. Sheikh, Wael Farah, Andy Nilipour, B´arbara Cabrales, Steve Croft, Alexander W. Pollak, and Andrew P. V. Siemion, Real-Time Technosignature Strategies with SN2023ixf, Draft version June 7, 2023.

Abundant Phosphorus in Enceladus Ocean Increases Habitability: But is Enceladus Inhabited?

by Paul Gilster | Jun 20, 2023 | Astrobiology and SETI | 15 comments

Finding the right conditions for life off the Earth justifiably drives many a researcher’s work, but nailing down just what might make the environment beneath an icy moon’s surface benign isn’t easy. The recent wave of speculation about Enceladus revolves around the discovery of phosphorus, a key ingredient for the kind of life we are familiar with. But Alex Tolley speculates in the essay below that we really don’t have a handle on what this discovery means. There’s a long way between ‘habitable’ and ‘inhabited,’ and many data points remain to be analyzed, most of which we have yet to collect. Can we gain the knowledge we need from a future Enceladus plume mission?

by Alex Tolley 

There has been abundant speculation about the possibility of life in the subsurface oceans of icy moons. Europa’s oceans with possible hydrothermal vents mimicking Earth’s abyssal oceans and the probable site of the origin of life, caught our attention now that Mars has no extant surface life. Arthur C Clarke had long suggested Europa as an inhabited moon in his novel 2010: Odyssey Two. (1982). While Europa’s hot vents are still speculative based on interpretations of the surface features of its icy crust, Saturn’s moon, Enceladus, showed visible aqueous plumes at the southern pole. These plumes ejected material that contributes to the E-Ring around Saturn as shown below.

While most searches for evidence for life focus on organic material, it has been noted that of the necessary elements for terrestrial life, Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur, and Phosphorus (CHONSP), phosphorus is the least abundant cosmically. Phosphorus is a key component in terrestrial life, from energy management (ATP-ADP cycle) and information molecules DNA, and RNA, with their phosphorylated sugar backbones.

If phosphorus is absent, terrestrial biology cannot exist. Phosphorus is often the limiting factor for biomass on Earth, In freshwater environments phosphorus is the limiting nutrient [1]. Typically, algae require about 10x as much nitrogen as phosphorus. If the amount of available nitrogen is increased, the algae cannot use that extra nitrogen as the amount of available phosphorus now determines how large the algal population can grow. The biomass-to-phosphorus ratio is around 100:1. When phosphorus is the limiting nutrient, then the available phosphorus will limit the biomass of the local plants and therefore animals, regardless of the availability of other nutrients like nitrogen, and other factors such as the amount of sunlight, or water. Agriculture fertilizer runoff can cause algal blooms in aqueous environments and may result in dead zones as oxygen is depleted by respiration as phytoplankton blooms die or are consumed by bacteria.

While nitrogen can be fixed by bacteria from the atmosphere, phosphorus is derived from phosphate rocks, and rich sources of phosphorus for agriculture were historically gleaned from bird guano.

A recent paper in Nature about the detection of phosphorus in the grains from the E-ring by the Cassini probe’s Cosmic Dust Analyzer (CDA) suggested that phosphorus is very abundant. As these grains are probably sourced from Enceladus’ plumes, this implies that this moon’s subsurface ocean has high levels of dissolved phosphorus.

The authors of the paper have modeled, and experimentally confirmed the model, and make the claim that Enceladus’ ocean is very rich in phosphorus:

…phosphorus concentrations at least 100-fold higher in the moon’s plume-forming ocean waters than in Earth’s oceans.

around 100-fold greater than terrestrial phosphorus abundance levels. They show that the CDA spectrum [figure 1) is consistent with a solution of disodium phosphate (Na2HPO4) and trisodium phosphate (Na3PO4) (figure 2) The source of these salts on Enceladus is likely from the hot vents chemically releasing the material from the carbonaceous chondritic rocky core and the relatively alkaline ocean. Contrary to intuition, the greater CO2 concentrations in cold water with the hydroxyapatite-calcite and whitlockite-calcite buffer system maintain an alkaline solution that allows for the high phosphate abundance in the plume material that produces the grains in Saturn’s E-ring.

Figure 1. CDA cation spectrum co-added from nine baseline-corrected individual ice grain spectra. The mass lines signifying a high-salinity Type 3 spectrum are Na + (23 u) and (NaOH)Na + (63 u) with secondary Na-rich signatures of (H₂O)Na + (41 u) and Na 2+ (46 u). Sodium phosphates are represented by phosphate-bearing Na-cluster cations, with (Na₃ PO₄)Na + (187 u) possessing the highest amplitude in each spectrum followed by (Na₂HPO₄ )Na + (165 u) and (NaPO₃)Na + (125 u). The first two unlabelled peaks at the beginning of the spectrum are H + and C +, stemming from target contamination 3 (source nature paper). a.u., arbitrary units.

Figure 2. Spectrum from the LILBID analogue experiment reproducing the features in the CDA spectrum. An aqueous solution of 0.420 M Na₂HPO₄ and 0.038 M Na₃PO₄ was used. All major characteristics of the CDA spectrum of phosphate-rich grains (Fig. 1) are reproduced at the higher mass resolution of the laboratory mass spectrometer (roughly 700 m/?m). Note: this solution is not equivalent to the inferred ocean concentration. To derive the latter quantity, the concentration determined in these P-rich grains must be averaged over the entire dataset of salt-rich ice grains. (source Nature paper).

Fig. 3: Comparison of observed and calculated concentrations of ΣPO_43– in fluids affected by water–rock reactions within Enceladus. a, Relation between ΣPO_43– and ΣCO₂ at a temperature of 0.1 °C for the hydroxyapatite-calcite buffer system (solid lines) and the whitlockite-calcite buffer system (dashed lines). Constraints on ΣCO₂ obtained in previous studies are indicated by the blue shaded area. The area highlighted in pink represents the range of ΣPO_43– constrained in this study from CDA data. b, Dependence of ΣPO_43– on temperature for the hydroxyapatite-calcite buffer and different values of pH and ΣCO₂. A similar diagram for the whitlockite-calcite buffer can be found in Extended Data Fig. 11.

The simple conclusion to draw from this is that phosphorus is very abundant in the Enceladan ocean and that any extant life could be very abundant too.

While the presence of phosphorus ensures that the necessary conditions of elements for habitability are present on Enceladus, it raises the question: “Does this imply Enceladus is also inhabited?”

On Earth, phosphorus is often, the limiting factor for local biomass. On Enceladus, if phosphorus was the limiting factor, then one would not expect it to be detected as inorganic phosphate, but rather in an organic form, bound with biomolecules.

But suppose Enceladus is inhabited, what might account for this finding?

1. Phosphorus is not limiting on Enceladus. Perhaps another element is limiting allowing phosphates to remain inorganic. In Earth’s oceans, where iron (Fe) can be the limiting factor, adding soluble Fe to ocean water can increase algal blooms for enhanced food production and possible CO2 sequestration. On Enceladus, the limiting factor might be another macro or micronutrient. [This may be an energy limitation as Enceladus does not have the high solar energy flux on Earth.]

2. Enceladan life may not use phosphorus. Some years ago Wolfe-Simon claimed that bacteria in Mono Lake used arsenic (As) as a phosphorus substitute. [2] This would have been a major discovery in the search for “shadow life” on Earth. However, it proved to be an experimental error. Arsenic is not a good substitute for phosphorus, especially for life already evolved using such a critical element, and as is well-known, arsenic is a poison for complex life.

3. The authors’ modeling assumptions are incorrect. Phosphorus exists in the Enceladan ocean, but it is mostly in organic form. The plume material is non-biological and is ejected before mixing in the ocean and being taken up by life. The authors may also have wildly overestimated the true abundance of phosphorus in the ocean.

Of these explanations allowing for Enceladus to be inhabited, all seem to be a stretch that life may be in the ocean despite the high inorganic phosphorus abundance. Enceladan biomass may be constrained by the energy derived from the moon’s geochemistry. On Earth, sunlight is the main source of energy maintaining the rich biosphere. In the abyssal darkness, life is very sparse, although it can huddle around the deep ocean’s hot vents.

However, if life is not extant, then the abundance of inorganic phosphorus salts is simply the result of chemical equilibria based on the composition of Enceladus rocky core and abundant frozen CO2 where it formed beyond the CO2 snow line.

While the popular press often conflate habitability with inhabited, the authors are careful to make no such claim, simply arguing that the presence of phosphorus completes the set of major elements required for life:

Regardless of these theoretical considerations, with the finding of phosphates the ocean of Enceladus is now known to satisfy what is generally considered to be the strictest requirement of habitability.

With this detection, it would seem Enceladus should be the highest priority candidate for a search for life in the outer solar system. Its plumes would likely contain evidence of life in the subsurface ocean and avoid the difficult task of drilling through many kilometers of ice crust to reach it. A mission to Enceladus with a suite of life-detecting instruments would be the best way to try to resolve whether life is extant on Enceladus.

The paper is Postberg, F., Sekine, Y., Klenner, F. et al. Detection of phosphates originating from Enceladus’s ocean. Nature 618, 489–493 (2023). https://doi.org/10.1038/s41586-023-05987-9

References

1. Smil, V (2000) Phosphorus in the Environment: Natural Flows and Human Interferences. Annual Review of Energy and the Environment Volume 25, 2000 Smil, pp 53-88

2. Wolfe-Simon F, et al (2010) “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus. SCIENCE 2 Dec 2010, Vol 332, Issue 6034 pp. 1163-1166
DOI:10.1126/science.1197258

SETI: Asking the Right Questions

by Paul Gilster | May 25, 2023 | Astrobiology and SETI | 13 comments

Did Carl Sagan play a role in the famous Arecibo message transmitted toward the Hercules Cluster in 1974? I’ve always assumed so, given Sagan’s connection with Frank Drake, who was then at Cornell University, where Sagan spent most of his career. But opinion seems to vary. Artist/scientist Joe Davis, who now has affiliations with both MIT’s Laboratory of Molecular Structure and Harvard Medical School, noted in an email this morning that Sagan’s widow, Ann Druyan, supports the connection, but according to Davis, Drake himself denied Sagan’s role in the composition or transmission of the message.

I mention all this because of Tuesday’s post on the simulated SETI signal being sent via ESO’s Mars ExoMars Trace Gas Orbiter, as a kind of work of art in its own right as well as a test case in building public involvement in the decoding of an unusual message. The idea of doing that irresistibly recalled Joe Davis because in 1988 Davis performed his own act of scientific art involving SETI, one that involved the Arecibo message and raised the question of whether any recipients would recognize it, much less decode it.

Image: A color-coded version of the Arecibo message highlighting its separate parts. The binary transmission itself carried no color information. Credit: Arne Nordmann / Wikimedia Commons. CC BY-SA 3.0.

The project, called “A Message in Many Bottles,” was set up at MIT’s Hayden Library in 1988. Davis used 1679 ‘Boston round’ 16 ounce glass bottles arrayed in a set of partitioned racks that were displayed in stacks. This is remarkably clever stuff: Each of 18 aisles in the library contained racks of bottles mounted, as Davis told me, 23 across. Empty bottles served as 0s in this digital message, while bottles filled with water represented the 1s. The whole thing reproduced the 1974 Arecibo message.

Now remember, this is MIT. You would think that if there is any place where a population of scientists, academics and students might puzzle out an enigmatic artifact like this, it would be here. Davis puts it this way in his email:

Hayden Library is MIT’s science library and contains all of the information needed to decode the message, all information the message refers to, and supposedly, better-than-average terrestrial intelligence. To the best of my knowledge, nobody decoded it. Instead, there were arguments…about whether or not the racks of bottles constituted works of art.

Image: An evidently baffled student contemplates the “Message in Many Bottles.” Credit: Joe Davis.

In 1997, a year after Sagan’s death, Davis reinstalled the display at MIT’s then new biology facility (Building 68), dedicating the work to the memory of Sagan. A short article on the matter in Nature (27 March 1997) noted the project as an homage to Sagan that accurately reproduced the Arecibo signal, going on to note:

Philip Sharp, chairman of MIT’s biology department, describes the exhibit as a “fitting tribute” to Sagan’s work. “It brings the abstraction of a radar message into an accessible, physical form,” says Sharp. He says he sees “numerous benefits” in having an artist who approaches issues from an unorthodox perspective working alongside more formally trained scientists.

Labeled as a tribute to Sagan and explained so that viewers could decode the message, “A Message in Many Bottles” served as an effective exhibit inhabiting the muzzy borderland where science meets art and creative minds translate research into shapes and forms that interrogate the meaning of our experiments. For that matter, was the Arecibo message itself not a kind of art, given that with a target 25,000 light years away, there was no conceivable way to see it as an actual communication?

Back in 2009 Joe Davis wrote “RuBisCo Stars” and the Riddle of Life for Centauri Dreams, presenting his own work at Arecibo, which wound up, on the 35th anniversary of the Arecibo message, in a new message based on molecular biology that was sent to three nearby stars. How he did this using, remarkably, an analog audio file on his iPhone interfacing with Arecibo’s technology is explained in the second part of his 2009 post, “RuBisCo Stars”: Part II. These two posts are, as everything involving Joe Davis’ work continues to be, invigorating and startlingly thought-provoking.

Image: At Arecibo, Joe Davis ponders transmission options as he holds the possible answer. Credit: Ashley Clark.

In fact, Davis notes in part II, in the midst of explaining to Arecibo’s then interim director Michael Nolan what his project is about, that “projects concerned with the search for extraterrestrial intelligence are really more about a search for ourselves; that they make us look much more intensely at ourselves than we look away into space and that nobody seems to see that part of it.” Nor could the myriad well-trained minds who encountered the Arecibo message in “A Message in Many Bottles” decode its meaning.

Science is so often about asking the right question. What are we staring at right now that we are not seeing? Are we asking the right questions about SETI?

First Contact: A Global Simulation

by Paul Gilster | May 23, 2023 | Astrobiology and SETI | 9 comments

Now and again scientists think of interesting ways to use our space missions in contexts for which they were not designed. I’m thinking, for example, of the ‘pale blue dot’ image snapped by Voyager 1 in 1990, an iconic view that forcibly speaks to the immensity of the universe and the smallness of the place we inhabit. Voyager’s cameras, we might recall, were added only after a debate among mission designers, some of whom argued that the mission could proceed without any cameras aboard.

Fortunately, the camera advocates won, with results we’re all familiar with. Now we have a project out of The SETI Institute that would use a European Space Agency mission in a novel way, one that also challenges our thinking about our place in the cosmos. Daniela de Paulis, who serves as artist in residence at the institute, is working across numerous disciplines with researchers involved in SETI and astronautics to create A Sign in Space, the creation of an ‘extraterrestrial’ message. This is not a message beamed to another star, but a message beamed back at us.

The plan is this: On May 24, 2023, tomorrow as I write this on the US east coast, ESA’s ExoMars Trace Gas Orbiter, in orbit around Mars, will transmit an encoded message to Earth that will act as a simulation of a message from another civilization. The message will be detected by the Allen Telescope Array (ATA) in California, the Green Bank Telescope (GBT) in West Virginia and the Medicina Radio Astronomical Observatory in Italy. The content of the message is known only to de Paulis and her team, and the public will be in on the attempt to decode and interpret it. The message will be sent at 1900 UTC on May 24 and discussed in a live stream event beginning at 1815 UTC online.

The signal should reach Earth some 16 minutes after transmission, hence the timing of the live stream event. This should be an enjoyable online gathering. According to The SETI Institute, the live stream, hosted by Franck Marchis and the Green Bank Observatory’s Victoria Catlett, will feature key team members – scientists, engineers, artists and more – and will include control rooms from the ATA, the GBT, and Medicina.

Daniela de Paulis points to the purpose of the project:

“Throughout history, humanity has searched for meaning in powerful and transformative phenomena. Receiving a message from an extraterrestrial civilization would be a profoundly transformational experience for all humankind. A Sign in Space offers the unprecedented opportunity to tangibly rehearse and prepare for this scenario through global collaboration, fostering an open-ended search for meaning across all cultures and disciplines.”

The data are to be stored in collaboration with Breakthrough Listen’s Open Data Archive and the storage network Filecoin, the idea being to make the signal available to anyone who wants to have a crack at decoding it. A Sign in Space offers a Discord server for discussion of the project, while findings may be submitted through a dedicated form on the project’s website. For a number of weeks after the signal transmission, the A Sign in Space team will host Zoom discussions on the issues involved in reception of an extraterrestrial signal, with the events listed here.

Assembly theory (AT) – A New Approach to Detecting Extraterrestrial Life Unrecognizable by Present Technologies

by Paul Gilster | May 16, 2023 | Astrobiology and SETI | 20 comments

With landers on places like Enceladus conceivable in the not distant future, how we might recognize extraterrestrial life if and when we run into it is no small matter. But maybe we can draw conclusions by addressing the complexity of an object, calculating what it would take to produce it. Don Wilkins considers this approach in today’s essay as he lays out the background of Assembly Theory. A retired aerospace engineer with thirty-five years experience in designing, developing, testing, manufacturing and deploying avionics, Don tells me he has been an avid supporter of space flight and exploration all the way back to the days of Project Mercury. Based in St. Louis, where he is an adjunct instructor of electronics at Washington University, Don holds twelve patents and is involved with the university’s efforts at increasing participation in science, technology, engineering, and math. Have a look at how we might deploy AT methods not only in our system but around other stars.

by Don Wilkins

A continuing concern within the astrobiology community is the possibility alien life is detected, then misclassified as built from non-organic processes. Likely harbors for extraterrestrial life — if such life exists — might be so alien, employing chemistries radically different from those used by terrestrial life, as to be unrecognizable by present technologies. No definitive signature unambiguously distinguishes life from inorganic processes. [1]

Two contentious results from the search for life on Mars are examples of this uncertainty. Lack of knowledge of the environments producing the results prevented elimination of abiotic origins for the molecules under evaluation. The Viking Lander’s metabolic experiments provide debatable results as the properties of Martian soil were unknown. An exciting announcement of life detection in the ALH 84001 meteorite is challenged as the ambiguous criteria to make the decision are not quantitative.

Terrestrial living systems employ processes such as photosynthesis, whose outputs are potential biosignatures. While these signals are relatively simple to identify on Earth, the unknown context of these signals in alien environments makes distinguishing between organic and inorganic origins difficult if not impossible.

The central problem arises in an apparent disconnect between physics and biology. In accounting for life, traditional physics provides the laws of nature, and assumes specific outcomes are the result of specific initial conditions. Life, in the standard interpretation, is encoded in the initial period immediately after the Big Bang. Life is, in other words, an emergent property of the Universe.

Assembly theory (AT) offers a possible solution to the ambiguity. AT posits a numerical value, based on the complexity of a molecule, that can be assigned to a chemical, the Assembly Index (AI), Figure 1. This parameter measures the histories of an object, essentially the complexity of the processes which formed the molecule. Assembly pathways are sequences of joining operations, from basic building blocks to final product. In these sequences, sub-units generated within the sequence combine with other basic or compound sub-units later in the sequence, to recursively generate larger structures. [2]

The theory purports to objectively measure an object’s complexity by considering how it was made. The assembly index (AI) is produced by calculating the minimum number of steps needed to make the object from its ingredients. The results showed, for relatively small molecules (mass?<?~250 Daltons), AI is approximately proportional to molecular weight. The relationship with molecular weight is not valid for large molecules greater than 250 Daltons. Note: One Dalton or atomic mass unit is a equal to one twelfth of the mass of a free carbon-12 atom at rest.

Figure 1. A Comparison of Assembly Indices for Biological and Abiotic Molecules.

Analyzing a molecule begins with basic building blocks, a shared set of objects, Figure 2. AI measures the smallest number of joining operations required to create the object. The assembly process is a random walk on weighted trees where the number of outgoing edges (leaves) grows as a function of the depth of the tree. A probability estimate an object forms by chance requires the production of several million trees and calculating the likelihood of the most likely path through the “forest”. Probability is related to the number of joining operations required or the path length traversed to produce the molecule. As an example, the probability of Taxol forming ranges between 1:10³⁵ to 1:10⁶⁰ with a path length of 30. In Figure 2, alpha biasing controls how quickly the number of joining operations grows with the depth of the tree.

Figure 2. Calculating Complexity

AT does not require extremely fine-tuned initial conditions demanded in the physics-based origins of life. Information to build specific objects accumulates over time. A highly improbable fine-tuned Big Bang is no longer needed. AT takes advantage of concepts borrowed from graph (networks of interlinked nodes) theory. [3] According to Sara Walker of Arizona State University and a lead AT researcher, information “is in the path, not the initial conditions.”

To explain why some objects appear but others do not, AT posits four distinct classifications, Figure 3. All possible basic building block variations are allowed in the Assembly Universe. Physics, temperature or catalysis are examples, constraining the combinations, eliminating constructs which are not physical in the Assembly Possible. Only objects that can be assembled comprise the Assembly Contingent level. Observable objects are grouped in the Assembly Observed.

Figure 3. The four “universes” of Assembly Theory

Chiara Marletto, a theoretical physicist at the University of Oxford, with David Deutsch, a physicist also at Oxford, are developing a theory resembling AT, the constructor theory (CT). Mimicking the thermodynamics Carnot cycle, CT uses machines or constructors operating in a cyclic fashion, starting at a original state, processing through a pattern until the process returns to the original state to explain a non-probabilistic Universe.

A team headed by Lee Cronin of the University of Glasgow and Sara Walker proposes AT as a tool to distinguish between molecules produced by terrestrial or extraterrestrial life and those built by abiotic processes. [4] AT analysis is susceptible to false negatives but current work produces no false positives. After completing a series of demonstrations, the researchers believe an AT life detection experiment deployable to extraterrestrial locations is possible.

Researchers believe AI estimates can be made using mass or infrared spectrometry. [5-6] While mass spectrometry requires physical access to samples, Cronin and colleagues showed a combination of AT and infrared spectrometry sensors similar to those on the James Webb Space Telescope could analyze the chemical environment of an exoplanet, possibly detecting alien life.

References

[1] Philip Ball, A New Idea for How to Assemble Life, Quanta, 4 May 2023,
https://www.quantamagazine.org/a-new-theory-for-the-assembly-of-life-in-the-universe-20230504?mc_cid=088ea6be73&mc_eid=34716a7dd8

[2] Abhishek Sharma, Dániel Czégel, Michael Lachmann, Christopher P. Kempes, Sara I. Walker, Leroy Cronin, “Assembly Theory Explains and Quantifies the Emergence of Selection and Evolution,”
https://arxiv.org/abs/2206.02279

[3] Stuart M. Marshall, Douglas G. Moore, Alastair R. G. Murray, Sara I. Walker, and Leroy Cronin, Formalising the Pathways to Life Using Assembly Spaces, Entropy 2022, 24(7), 884, 27 June 2022, https://doi.org/10.3390/e24070884

[4] Yu Liu, Cole Mathis, Micha? Dariusz Bajczyk, Stuart M. Marshall, Liam Wilbraham, Leroy Cronin, “Ring and mapping chemical space with molecular assembly trees,” Science Advances, Vol. 7, No. 39
https://www.science.org/doi/10.1126/sciadv.abj2465

[5] Stuart M. Marshall, Cole Mathis, Emma Carrick, Graham Keenan, Geoffrey J. T. Cooper, Heather Graham, Matthew Craven, Piotr S. Gromski, Douglas G. Moore, Sara I. Walker, Leroy Cronin, “Identifying molecules as biosignatures with assembly theory and mass spectrometry,” Nature Communications volume 12, article number: 3033 (2021)
https://www.nature.com/articles/s41467-021-23258-x

[6] Michael Jirasek, Abhishek Sharma, Jessica R. Bame, Nicola Bell1, Stuart M. Marshall,Cole Mathis, Alasdair Macleod, Geoffrey J. T. Cooper!, Marcel Swart, Rosa Mollfulleda, Leroy Cronin, “Multimodal Techniques for Detecting Alien Life using Assembly Theory and Spectroscopy,” https://arxiv.org/ftp/arxiv/papers/2302/2302.13753.pdf