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.
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. 
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. 
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.  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.  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.
 Philip Ball, A New Idea for How to Assemble Life, Quanta, 4 May 2023,
 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,”
 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
 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
 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)
 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
Here’s an image that brings out the philosopher in me, or maybe the poet. It’s Voyager 1, as detected by a new processing system called COSMIC, now deployed at the Very Large Array west of Socorro, New Mexico. Conceived as a way of collecting data in the search for technosignatures, COSMIC (Commensal Open-Source Multimode Interferometer Cluster) taps data from the ongoing VLASS (Very Large Array Sky Survey) project and shunts them into a receiver designed to spot narrow channels, on the order of one hertz wide, to spot possible components of a technosignature.
Technosignatures fire the imagination as we contemplate advanced civilizations going about their business and the possibility of eavesdropping upon them. But for me, the image below conjures up thoughts of human persistence and a gutsy engagement with the biggest issues we face. Why are we here, and where exactly are we in the galaxy? In the cosmos? Spacecraft like the Voyagers were part of the effort to explore the Solar System, but they now push into realms not intended by their designers. And here we have the detection of doughty Voyager 1, still working the mission, somehow still sending us priceless information.
Image: The detection of the Voyager I spacecraft using the COSMIC instrument on the VLA. Launched in 1977, the Voyager I spacecraft is now the most distant piece of human technology ever sent into space, currently around 14.8 billion miles from Earth. Voyager’s faint radio transmitter is difficult to detect even with the largest telescopes, and represents an ideal human “technosignature” for testing the performance of SETI instruments. The detection of Voyager’s downlink gives the COSMIC team high confidence that the system can detect similar artificial transmitters potentially arising from distant extraterrestrial civilizations. Credit: SETI Institute.
Voyager 1 is thus a dry run for a technosignature detection, and COSMIC is said to offer a sensitivity a thousand times more comprehensive than any previous SETI search. The detection is unmistakable, combining and verifying the operation of the individual antennas that comprise the array to show the carrier signal and sideband transmissions from the spacecraft. The most distant of all human-made objects, Voyager 1 is now 24 billion kilometers from home. For one participant in COSMIC, the spacecraft demonstrates what can be done by combing through the incoming datastream of VLASS. Thus Jack Hickish (Real Time Radio Systems Ltd):
“The detection of Voyager 1 is an exciting demonstration of the capabilities of the COSMIC system. It is the culmination of an enormous amount of work from an international team of scientists and engineers. The COSMIC system is a fantastic example of using modern general-purpose compute hardware to augment the capabilities of an existing telescope and serves as a testbed for technosignatures research on upcoming radio telescopes such as NRAO’s Next Generation VLA.”
COSMIC is the result of collaboration between The SETI Institute (working with the National Radio Astronomy Observatory) and the Breakthrough Listen Initiative. The key here is efficiency – the technosignature search draws on data already being taken for other reasons, and given the challenge of obtaining large amounts of telescope time, an offshoot method of tracking pulsed and transient signals simply makes use of existing resources, with approximately ten million star systems within its scope.
Technosignatures are fascinating, but I come back to Voyager. We’ve gotten used to the scope of its achievement, but what fires the imagination is the details. It wasn’t all that long ago, for example, that controllers decided to switch to the use of the spacecrafts’ backup thrusters. The reason: The primary thrusters, having gone through almost 350,000 thruster cycles, were pushing their limits. When the backup thrusters were fired in 2017, the spacecraft had been on their way for forty years. The “trajectory correction maneuver,” or TCM, thrusters built by Aerojet Rocketdyne (also used on Cassini, among others), dormant since Voyager 1’s swing by Saturn in 1980, worked flawlessly.
In his book The Interstellar Age (Dutton, 2015), Jim Bell came up with an interesting future possibility for the Voyagers before we lose them forever. Bell worked as an intern on the Voyager science support team at JPL starting in 1980, and he would like to see some of the results of the mission stored up for a potentially wider audience. Right now there is nothing aboard each spacecraft that tells their stories. Bell quotes Jon Lomberg, who worked on the Voyager Golden Records and has advanced the idea of a digital message to be uploaded to New Horizons:
‘One thing I wish could have been on the Voyager record… is that I wish we could have had something of ‘here’s what Voyager was and here’s what Voyager found,’ because it’s one of the best things human beings have ever done. If they ever find Voyager they won’t know about its mission. They won’t know what it did, and that’s sad.’
And Bell goes on to say:
…let’s try to upload the Earth-Moon portrait; the historic first close-up photos of Io’s volcanoes and Europa and Ganymede’s cracked icy shells; the smoggy haze of Titan; the enormous cliffs of Miranda; the strange cantaloupe and geyser terrain of Triton; the swirling storms of Jupiter, Saturn and Neptune; the elegant, intricate ring systems of all four giant planets; the family portrait of our solar system. Let’s arm our Voyagers with electronic postcards so they can properly tell their tales, should any intelligence ever find them.
Could images be uploaded to the Voyager tape recorders at some point before communication is lost? It’s an intriguing thought about a symbolic act, but whether possible or not, it reminds us of the distances the Voyagers have thus far traveled and the presence of something built here on Earth that will keep going, blind and battered but more or less intact, for eons.
The old trope about signals from Earth reaching other civilizations receives an interesting twist when you ponder just what those signals might be. In his novel Contact, Carl Sagan has researchers led by Ellie Arroway discover an encrypted TV signal showing images from the Berlin Olympics in 1936. Thus returned, the signal announces contact (in a rather uncomfortable way). More comfortable is the old reference to aliens watching “I Love Lucy” episodes in their expanding cone of flight that began in 1951. How such signals could be detected is another matter.
I’m reminded of a good friend whose passion for classical music has caused him to amass a collection of recordings that rival the holdings of a major archive. John likes to compare different versions of various pieces of music. How did Beecham handle Delius’ “A Walk in the Paradise Garden” as opposed to Leonard Slatkin? Collectors find fascination in such things. And one day John called me with a question. He was collecting the great radio broadcasts that Toscanini had made with the NBC Symphony Orchestra beginning in 1937. His question: Are they still out there somewhere?
Image: A screenshot of Arturo Toscanini from the World War II era film ‘Hymn of the Nations,’ December, 1943. Credit: US Office of War Information.
John’s collection involved broadcasts that had been preserved in recordings, of course, but he wanted to know if somewhere many light years away another civilization could be listening to these weekly broadcasts, which lasted (on Earth) until 1954. We mused on such things as the power levels of such signal leakage (not to mention the effect of the ionosphere on AM radio wavelengths!), and the fact that radio transmissions lose power with the square of distance, so that those cherished Toscanini broadcasts are now hopelessly scattered. At least John has the Earthly versions, having finally found the last missing broadcast, making a complete set for his collection.
Toscanini was a genius, and these recordings are priceless (John gave me the complete first year on a set of CDs – they’re received a lot of play at my house). But let’s play around with this a bit more, because a new paper from Reilly Derrick (UCLA) and Howard Isaacson (UC-Berkeley) tweaks my attention. The authors note that when it comes to the leakage of signals into space, a 5 MW UHF television picture has effective radiated power of 5 x 106 W and an effective isotropic radiated power (EIRP) of approximately 8 x 106 W. ERP tells us the strength of an actual signal in a specific direction, while EIRP describes an isotropic ideal antenna.
It’s interesting to see that a much more powerful signal than our TV broadcasts comes from the Deep Space Network as it communicates with our spacecraft. Derrick and Isaacson say that DSN transmissions at 20 kW power have an EIRP of 1010 W, making such signals (103 times higher than leakage, thus more likely to be detected. With this in mind, the authors come up with a new way to identify SETI targets; viz., find stars that are within the background of sky positions occupied by our spacecraft at such times as the transmissions to them from the DSN were active.
Ingenious. Remember that identifying interesting SETI targets has led us to study such things as the ‘Earth Transit Zone,’ which would identify stars so aligned as to be able to see transits of Earth across the Sun. In a similar way, we can study stars whose ecliptic planes align with our line of sight to intercept possible communications in those systems. It turns out that most of our outbound radio traffic to spacecraft occurs near the ecliptic, and the assumption would be that other civilizations might do the same.
Image: With the Pluto/Charon flyby, we have performed reconnaissance on every planet and dwarf planet in our solar system, with the help of the Deep Space Network. The DSN comprises three facilities separated by about 120 degrees around the Earth: Goldstone, California; near Madrid, Spain; and near Canberra, Australia. Above is the 70-meter Deep Space Station 14 (DSS-14), the largest Deep Space Network antenna at the Goldstone Deep Space Communications Complex near Barstow, California. Credit: NASA/JPL-Caltech.
So what Derrick and Isaacson are doing is an extension of this earlier work (see SETI: Knowing Where to Look and Seeing Earth as a Transiting World for some of the archival material I’ve written on these studies). The authors want to know where our DSN signals went after they reached our spacecraft, and that means building an ephemeris for our deep space missions that are leaving or have left the Solar System: Voyagers 1 and 2, Pioneer 10, Pioneer 11, and New Horizons. They then consult the positions of over 300,000 stars within 100 parsecs as drawn from the Gaia Catalogue of Nearby Stars, checking to ensure that the stars they identify will not leave the radius of the search in the time it takes for the cone of transmission to reach them.
The Voyagers were launched in 1977, with both of them now outside the heliosphere. As to the others, Pioneer 10 (launched in 1972) crossed the orbit of Neptune in 1983, while Pioneer 11 (launched in 1973) crossed Neptune’s orbit in 1990. New Horizons, launched in 2006, crossed Neptune’s orbit in 2014. All these craft received or are receiving DSN transmissions, though the Pioneers have long since gone silent. Their ephemerides thus end on the final day of communication, while the other missions are ongoing.
The universe is indeed prolific – the Gaia Catalogue includes 331,312 stars within 100 parsecs, and as the authors note, it is complete for stars brighter than M8 and contains 92 percent of the M9 dwarfs in this range. We learn that Pioneer 11 – I should say the signals sent to Pioneer 11 and thus beyond it – encounters the largest group of stars at 411, while New Horizons has the least, 112. The figures on Voyager 1, 2 and Pioneer 10 are 289, 325, and 241 stars, respectively. Transmissions to Voyager 2, Pioneer 10 and Pioneer 11 have already encountered at least one star, while Voyager 1 and New Horizons signals will encounter stars in the near future. From the paper:
Transmissions to Voyager have already encountered an M-dwarf, GJ 1154, and a brown dwarf, Gaia EDR3 6306068659857135232. Transmissions to Pioneer 10 have encountered a white dwarf, GJ 1276. Transmissions to Pioneer 11 have encountered a M-dwarf, GJ 359. We have shown that the radio transmissions using the DSN are stronger than typical leakage and are useful for identifying good technosignature targets. Just as the future trajectories of the Voyager and Pioneer spacecraft have been calculated and their future interactions with distant stars cataloged by Bailer-Jones & Farnocchia (2019), we now also consider the paths of DSN communications with those spacecraft to the stars beyond them.
The paper provides a table showing stars encountered by transmissions to the spacecraft sorted by the year we could expect a return transmission if a civilization noted them, along with data on the time spent by the star within the transmission beam. DSN transmissions are several orders of magnitude smaller in EIRP than the Arecibo planetary radar (1012 W), but it’s also true that the positions of the spacecraft, and hence background stars during transmissions, are better documented than background stars that would have encountered the Arecibo signals.
So what we have here is a small catalog of stars whose systems are in the background of DSN transmissions, and the dates when each star will encounter such signals. The goal is to offer a list of higher value targets for scarce SETI time and resources, especially concentrating on those stars nearest the Sun where civilizations may have noticed us. I don’t hold out high hopes for our receiving a signal from any of these stars, but find the process fascinating. Derrick and Isaacson offer a new way of considering our position in the galaxy in relation to the stars that surround us.
The paper is Derrick & Isaacson, “The Breakthrough Listen Search for Intelligent Life: Nearby Stars’ Close Encounters with the Brightest Earth Transmissions,” available as a preprint. Thanks to my friend Antonio Tavani for the pointer to this work. The Bailer-Jones & Farnocchia paper mentioned above is likewise interesting. It’s “Future Stellar Flybys of the Voyager and Pioneer Spacecraft,” Research Notes of the AAS Vol. 3, No. 4 (April 2019) 59 (full text).
The enigmatic ‘Oumuamua continues to stir controversy. Last week we looked at a new paper from Jennifer Bergner (UC-Berkeley) and Darryl Seligman (Cornell University), discussing a mechanism for the interstellar object’s unusual non-gravitational acceleration. The researchers explored the possibility that ice impacted by high-energy particles like cosmic rays would dissociate water in a comet to create molecular hydrogen within the ice. Was the warming of this hydrogen, all but undetectable according to the authors, the cause of outgassing and the anomalous acceleration?
Image: This very deep combined image shows the interstellar object ‘Oumuamua at the center of the image. It is surrounded by the trails of faint stars that are smeared as the telescopes tracked the moving comet. Credit: ESO/K. Meech et al.
Answering the question in a paper just submitted to the arXiv site is Harvard’s Avi Loeb, working with Thiem Hoang (Korea University of Science and Technology), who home in on Bergner and Seligman’s finding that the surface temperature of ‘Oumuamua can exceed 140 K at perihelion, enough to produce this evaporation. Loeb and Hoang argue that this calculation ignores the effect of evaporative cooling of the molecular hydrogen. The authors proceed to take such cooling into account and find that the surface temperature of H2 water ice is lower than that calculated by Bergner and Seligman by a factor of 9. This is turn reduces the projected outgassing.
From the paper:
…we found that the evaporative cooling is much more efficient than radiative cooling at temperatures above 20 K (see Figure 1, left panel). By taking into account the evaporative cooling by H2 evaporation, our results (see Figure 1, right panel) show that the surface temperatures of H2-water ice are lower by a factor of 9 than the temperature obtained by Bergner & Seligman (2023) (see their figure 3). Therefore, the thermal speed of outgassing H2 is decreased by a factor of 3.
Image: This is Figure 1 from the paper. Caption: Left panel: comparison of heating and cooling rates when the object is located at 1.4 times the Earth separation from the sun. Evaporative cooling by H2 is dominant over radiative cooling. The intersection of heating and total cooling determines the equilibrium surface temperature. Right panel: surface temperature at different distances, calculated for the case with (solid lines) and without (dashed-dotted line) evaporative cooling. Different ratio of H2 to water is assumed. Evaporative cooling by H2 decreases significantly the surface temperature compared to the case without evaporative cooling (dashed-dotted line).
And this is a problem for the molecular hydrogen evaporation scenario. The result of this decrease in outgassing is that ‘Oumuamua would have had to have been what the authors call an ‘oxygen iceberg’ to produce enough molecular hydrogen to drive the observed acceleration, a highly unlikely scenario for the following reason:
Given this constraint, the requirement for a surface layer that is made of pure molecular hydrogen will not survive the journey through interstellar space as a result of heating by starlight (Hoang & Loeb 2020). Moreover, the lower surface temperature also influences the thermal annealing of water ice, a key process that is appealed to by Bergner & Seligman (2023) to release H2.
The paper is Hoang & Loeb, “Implications of evaporative cooling by H2 for 1I/‘Oumuamua” (full text).
Alex Tolley follows up his analysis of agriculture on Mars with a closer look at the Interstellar Research Group’s MaRMIE project – the Martian Regolith Microbiome Inoculation Experiment. Growing out of discussions on methods beyond hydroponics to make the Red Planet fertile, the project is developing an experimental framework, as described below, to test our assumptions about Martian regolith here on Earth. A path forward through simulation and experiment could help us narrow the options for what may be possible for future colonists. Fertile regolith, achieved through perchlorate removal, would open up possibilities far beyond what is achievable through hydroponics.
by Alex Tolley
Successful settlement of distant locations requires living off the land, which requires resourcing food. Failure can lead to disaster, as experienced by some of the early American colonies. While near Earth space settlements could be supplied with packaged food, this would be too costly for an expanding Mars base over the long term. Food and air must be supplied from local sources, a point that has been emphasized by the Mars Society’s president, Robert Zubrin (Zubrin, 2011).
In the mid-20th century, it was assumed that agriculture on Mars would be like that on Earth, with crops growing in the Martian soil, but under clear domes to maintain air pressure, and light for photosynthesis. As a result, the focus for settlement was on the shiny technologies of transport and the design of Martian bases and cities.
Image: The Martian Base: Painting for The Exploration of Space by Arthur C Clarke. Credit: Leslie Carr.
This rosy picture of farming on Mars was disturbed after the Apollo missions when it became apparent that plants did not grow well in lunar regolith samples. The Phoenix lander’s discovery of perchlorates on the surface of Mars meant that the Martian regolith would be toxic to plant growth without remediation. Perchlorates are found on Earth, for example in the Atacama desert, but in far lower concentrations than the 0.5-1.0% concentrations found on the surface of Mars. Perchlorates are used in industry, and the US EPA regulates perchlorate contamination because of its toxicity.
Because of the adverse nature of regolith on plant growth, the focus shifted to soilless agriculture using hydroponics or aquaponics, but as we saw in the previous post, there are limitations on the use of hydroponics. Plants with extensive root systems needed for support, especially trees, can’t be grown this way, eliminating the availability of tree fruits and nuts. Most of our grains cannot be grown using current hydroponic methods either. It really would be useful if the regolith could be altered to make it suitable for traditional agriculture, perhaps more like the farms in arid areas, such as the Middle East.
In 2022, after participating in a panel discussion on establishing a sustainable human presence on Mars at a science fiction convention (LibertyCon) in Tennessee, members of the Interstellar Research Group (IRG) including Doug Loss, Joe Meany, and Jeff Greason, considered how some experiments could be done to test how best the regolith might be treated to remove the perchlorates with bioremediation using bacteria, and convert the sterile regolith into soil suitable for agriculture. Some species of bacteria metabolize chlorates and perchlorates for energy, and therefore could be used to remediate the regolith. Relatively small, low mass cultures could be brought from Earth and exponentially cultured to meet the requirements for the volumes of regolith to be treated.
Bioremediation of perchlorate contaminated soils is established practice (Hatzinger 2005), suggesting that if it could be adapted to Martian conditions, this may be a viable solution to remove the perchlorates and solve the toxicity issue.
This use of bacteria, a low mass approach to remediate the regolith was the inspiration for core IRG members to propose a project, the Martian Regolith Microbiome Inoculation Experiment.(MaRMIE).
Mars is almost certainly too dry and cold to just irrigate the regolith on the exposed Martian surface with an inoculant of perchlorate metabolizing organisms. Knowledge about the required conditions for successful large scale regolith bioremediation, especially of temperature and pressure, was required, as well as the issue of UV and ionizing radiation.
Simulating the Martian Surface
The initial idea was to run experiments in a sealed chamber that mimicked the Martian surface environment to determine whether a terrestrial type soil might be created in which agriculture could be practiced. This Mars simulation chamber would contain a Martian regolith simulant (MRS) with added perchlorate, and inoculated with suitable bacteria. If the bacteria could break down the perchlorate, it would indicate that this approach could, in principle, be used to remediate the regolith from the surrounding area, which would then be used inside a greenhouse to grow the food crops. By doing so, the mass, complexity, and likely equipment failures of a hydroponic system could be avoided, and a more traditional agricultural approach could be practiced. This was a far more scalable solution than a technical one, allowing food production anywhere it would be needed, and was in much closer alignment with ISRU.
Early thinking was to design the experiment and have an outside PI with expertise and funding to refine the design and run the experiments. The IRG would publish a review article, and at some point participate in writing a paper on the results.
At this point, the initial group decided to invite others who might be interested in providing input and expertise to investigate the biological remediation of regolith. Of particular importance was the need to design experiments that could be done at suitable facilities. IRG hopes the guidelines that develop out of this work may be of use to anyone pursuing research into agriculture for future use on Mars, and offers them to any organization that chooses to draw on them.
Prior work had identified various bacteria that had the genes that encoded the enzymes to reduce the [per]chlorate and extract energy from it (Balk 2008, Bender 2005, Coates 2004). As the genes coding for the various enzymes for perchlorate metabolism were known it has been suggested that by just liberating the oxygen from the perchlorate the regolith could be a useful source of life support and rocket fuel oxidant (Davil 2013), therefore offering another avenue of ISRU using an engineered bacterium.
Will bioremediation need to be taken inside the base, and if so, can or should it be done as close to Martian conditions as possible, or should it be done as close to the living or working conditions, and plant growing conditions in the agricultural greenhouse? Would it be better to grow the bacteria in a bioreactor rather than in situ, or even extract the enzymes to treat the regolith, thus controlling the bacteria growth and both reducing the perchlorates and liberating the oxygen as a useful side product?
These questions can only be answered with experiments testing the various bacterial inoculants under varying conditions from terrestrial to Martian, as well as applying economic and other analyses to determine the more effective way to use bioremediation on the regolith as an initial step to making it a proactive soil for farming.
While bioremediation is one approach to removing perchlorates, the fact that they are readily water-soluble suggests that if free water is available, the regolith could be simply washed to flush out the perchlorates. This would require more plant to wash the regolith and then remove the salts to recycle the water. This method would work more effectively on regolith than soil and would not require the controlled conditions of bacterial growth and the time to build the culture.
The Phoenix lander detected the perchlorates as they deliquesced on exposure. Experiments have shown that the perchlorates will deliquesce under Martian conditions (Slank 2022).
Removal of the toxic perchlorates is just the start of the process to make the regolith fertile. There have been a number of experiments with regolith simulant to grow a variety of plants and crops under terrestrial conditions of temperature and pressure, the sort of conditions that might be expected in a Mars greenhouse that has humans managing the farm.
By far the best results have been achieved by increasing the illumination to terrestrial levels and adding carbon-rich soils to the regolith, which now will also include the many soil organisms that improve the soils. A partial solution that has also worked is to grow cover crops like alfalfa grass or reuse the waste from prior crops to be added into the regolith to improve its water retention and nutrient supply. (Kasiviswanathan 2022).
So far none of these crop growing experiments have been attempted at pressures and temperatures that differ from optimal terrestrial conditions. There is considerable space to repeat these experiments under different conditions, especially if it proves important to build structurally lighter greenhouses, or even use artificial illumination in below-ground farms, much like container farming today. While oxygen can be extracted from Martian air, water and rocks, nitrogen is less readily available, as is phosphorus. These macronutrients and the other micronutrients will have to be found and extracted to support plant growth whatever farming method is used.
As a result of all these questions, the MaRMIE project has expanded in scope beyond bioremediation, to include crop growth experiments under non-terrestrial conditions.
An Experimental Framework
The project has generated an outline of the experiments that might be done, starting with bioremediation, and extending out into the more general issue of agriculture under conditions that differ from terrestrial ones. Even this is the tip of the iceberg as gene engineered organisms might well be better adapted to conditions on Mars, reducing containment costs, nutrients, and allowing faster scale-up to support an expanding settlement.
The experimental framework encompassing the ideas to date has 4 phases:
1. Remediating perchlorates in the regolith, and any problematic chemicals produced as a result of the remediation. This requires acquiring Martian regolith simulants (MRS) and the addition of perchlorates, testing a number of bacterial and microfungal agents to remediate the MRS under terrestrial conditions, and then in stages of pressure and temperature modified towards Martian conditions.
2. Developing a microbiome tailored to Martian conditions with which to inoculate the regolith. The microbiome should lessen or remove tendencies toward cementation of the regolith as well as gradually convert it into actual soil, if possible. “Actual soil” implies the provision of required nutrients for plant growth. This includes testing microbiomes to add to the MRS along with testing pioneer plant species to condition the regolith to become more like soil.
3. Testing plant growth in microbiome-inoculated regolith under Martian lighting levels and atmospheric conditions, gradually increasing the atmospheric pressure until plant growth is acceptable.
4. Continuing plant growth testing per #3, but gradually lowering ambient temperatures toward Martian levels until plant growth diminishes unacceptably.
5. Developing agricultural structures to provide appropriate conditions, with inoculated regolith, lighting levels, atmospheric pressure, and temperature levels previously determined, and with shielding from ionizing radiation.
As for output, the initial idea to publish some sort of review paper on the known issues and prior work, indicating the direction of experimental work needed, is still in process.
As noted at the outset, the IRG cannot execute these experiments and offers this work as a contribution to the field of planetary studies. IRG hopes that this framework will be seen and used as a structure for designing experiments and building on the results of previous experiments, by any researchers interested in the ultimate goal of viable large-scale agriculture on Mars.
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Kasiviswanathan P, Swanner ED, Halverson LJ, Vijayapalani P (2022) Farming on Mars: Treatment of basaltic regolith soil and briny water simulants sustains plant growth. PLoS ONE. 17(8): e0272209.