Imagining and Planning Interstellar Exploration
Let’s break for a moment with interstellar issues to finish up a story I first covered at the beginning of the year. In 2022, members of the Interstellar Research Group led by Doug Loss began exploring the biological side of establishing a human presence on Mars. By ‘biological,’ what the team was looking at was how to create soil as opposed to regolith, soil with the microbial components needed to produce crops for human consumption on Mars. Alex Tolley wrote the idea up in MaRMIE: The Martian Regolith Microbiome Inoculation Experiment. Today’s post is the finalized document that has grown out of this effort, an attempt to foster further research by offering a framework for experiment. While the IRG lacks the means of executing these experiments itself, it offers this paper as a contribution to planetary studies to connect with those who can.
by Alex Tolley and Doug Loss*
* Contact: Doug Loss at firstname.lastname@example.org
The proposed designs for the settlement of Mars include various approaches for local food production. Food will most likely be based on traditional terrestrial crops to ensure that a variety of cuisines can be cooked for the well-being of the settlers. To farm on Mars, as well as provide an environment for plants and trees, will require establishing soils using the Martian regolith. The presence of (per)chlorates at levels toxic to plants and humans requires remediation of the regolith to remove the (per)chlorates. Prior work indicates that there is a knowledge gap in how to remediate the regolith to make it ready to support various crops for Martian agriculture. We propose a framework of experiments to help bridge the gap between the state of the regolith on the surface and the initial stages of soil creation.
With the renewed interest in settling Mars, there has been considerable attention on how to feed a base crew and its subsequent expansion into a larger settlement population. Unlike a human presence in low Earth orbit (LEO) and on the Moon where travel times are sufficiently short that food can be provided in regular shipments from Earth, the long 6 to 9-month, low-energy journeys to Mars that have 2-year gaps between flights, suggest that using local Martian resources to grow food would be a better option, both from economic and safety perspectives.
It should be noted that the flight times with current rocket transport technology are similar to that of the sailing ships traveling from England to the Botany Bay colony in Australia in the late 18th century. The resupply ship arrived 2 years later, with the colony starving from inadequate food supplies and an inability to successfully farm. Local food production on Mars would ensure that adequate, high-nutrition foods are available and avoid any supply problems from Earth.
The lower ambient light levels on Mars are sufficient for photosynthesis for a large range of plants from unicellular algae to many terrestrial crops . Additional light if needed can be supplied with mirrors or artificial lighting. The question then becomes what sort of plants should be cultivated? The simplest plants, such as cyanobacteria, have been proposed as they have short lifecycles and rapid growth, requiring small production areas and a few basic nutrients. However, anecdotal evidence from supplying astronauts to the ISS indicates that food quality is a very important factor in astronaut well-being [46,47,48]. Experiments with celebrity chef-developed meals have proven the popularity of meals that are similar to those on Earth and that are tasty, not just nutritious. For crew and settlers staying for long periods on Mars with minimum 2-year rotations, foods that can be prepared with different cuisines to be cooked by a base chef or personally would seem to be preferable. Cyanobacteria and algal species may grow quickly and be technically nutritious. However, algae is not a completely nutritious diet and only Spirulina  has been shown to be useful as a meal supplement, used for less than 1% of the diet, and therefore should be considered as feedstuffs for animals and as soil amendments.
Growing conventional food hydroponically  is often mooted as the means to grow conventional crops. It has the advantage of having a pedigree of experience in terrestrial farms as well as experimental success in space. Hydroponic food production can be carefully controlled which makes it attractive to those of technical expertise. However, hydroponics requires substantial inputs of nitrogen and phosphorus which are usually applied directly from external sources, and not all plants can be successfully grown hydroponically. In addition, an expansion for a growing settlement will require either transporting equipment from Earth or finding ways to manufacture at least the simple components locally on Mars. A more attractive approach has been to try growing conventional crops in the Martian regolith. Experiments using regolith simulants  have shown that given added nutrients and light, a number of common terrestrial leafy crops can be grown.
The advantage of using the Martian regolith as a medium to grow conventional crops is that it provides the needed anchorage and potentially water retention medium used by terrestrial plants. Martian agriculture would work like terrestrial agriculture which is done in a greenhouse. On Mars, the atmosphere and temperature would be controlled to maximize crop growth and it is feasible that some animal species might be transported to produce the high-protein foods. For example, fish eggs could be transported and herbivorous fish species such as Tilapia could feed on the algae and convert it for human consumption. However, it should be noted that soils are not simple, but include ecosystems with a large number of species including bacteria, fungi, and animals from annelid worms to insects..
Despite the research done to date, there are considerable gaps in our knowledge concerning how Martian agriculture should proceed. The Martian environment is very cold, and dry, with a thin atmosphere around 0.1% of Earth’s, composed mainly of carbon dioxide with a little nitrogen. While aqueous algal growth experiments have been done in conditions that approximate some of the Martian conditions, it is not known which conditions must be tightly
controlled for good growth of the algae. For complex plants that are to be grown in either regolith or hydroponically, what partial pressure of CO2 in the atmosphere and at what pressure is needed to ensure healthy growth? Crops grow in different soils on Earth, from near desert sandy soils for millet to rich dark loams and different acidities for different crops. We take for granted the quality of terrestrial soils, but on Mars, the regolith is considered sterile, with no organic carbon content to retain water and provide an environment for soil organisms.
Given that these conditions can be evaluated on Earth, the big gap in our knowledge is the issue of remediation of the toxic levels of (per)chlorates in the Martian regolith. All of the various experiments on growth conditions assume that none of these toxic compounds are present. Powdered terrestrial rocks and more carefully constructed Mars Regolith Simulants are free of (per)chlorates and therefore experiments on plant growth assume the (per)chlorates are removed. With (per)chlorate levels that are far higher than any found naturally on Earth, they are at levels found around sites that manufacture munitions where the compound is used as an oxidant. The US EPA has guidelines for the remediation of soils contaminated by (per)chlorates .
Soils can vary, with plants varying in requirements for water, nutrients, soil carbon, soil organisms, pH, climate, and weather conditions. Nutrients and organic carbon will need to be added, as well as soil organism inoculants to improve the regolith to become a soil capable of good crop production.
To get an agricultural food system working, which factors are critical? How best to detoxify the regolith? How best to amend its properties? Which crops are best suited and at which stages?
A low-mass approach is to employ bacteria that can metabolize (per)chlorates and grow locally. (Per)chlorate metabolizing organisms are proteobacteria of which there are more than 40 species known. Dechloromonas and Azospira genera appear to be ubiquitous on Earth. They have different pH tolerances and some can function in acidic conditions as low as pH 5  The Martian regolith has up to 1% of (per)chlorate  which is far higher than any uncontaminated place on Earth. (Per)chlorate reduction only occurs in anaerobic conditions . This suggests that regolith remediation may need to be kept isolated from the crop-growing areas. Experiments with Moorella sp show that these bacteria can grow on a variety of reduced carbon sources, optimally at neutral pH and warm temperatures (40-70C) . None of the experiments have tested the (per)chlorate-reducing metabolic rates and growth of the various potential bacterial inoculants under conditions between Mars and human habitation, such as lower atmospheric pressure, gas composition, and water requirements. As these bacteria need a carbon source, how would that source be provided by chemical means or by biological carbon fixation?
There is considerable interest in using cyanobacteria as carbon-fixing microorganisms. These hold promise to weather the regolith, release nitrogen and phosphorus for growth, create organic carbon to improve water retention, and allow a richer variety of solid organisms that may be needed for crop growth. These cyanobacteria have been tested in a variety of conditions to determine how they will fare under conditions closer to that on the surface of Mars. Resting states of cyanobacteria suitable for transport from Earth indicate that UV exposure is not tolerated, although survival in a vacuum is good . Cyanobacteria do require a lot of water to suspend the rock dust and particles, In CO2-dominant atmospheres, full terrestrial pressure reduces growth, partly because of the lowered pH of the aqueous media, while 100 mbar appeared more favorable. Temperatures need to be maintained between 15 and 30C. Most important is the finding that cyanobacteria cannot survive in (per)chlorate-contaminated conditions, requiring its removal before growth . Extensive testing of bacteria has shown that while a few can survive down to the 7 mbar of the Martian atmosphere, most require at least 25 mbar. Nitrogen-fixing bacteria can fix atmospheric nitrogen to as low as 1 mbar, but the 2.8% of nitrogen in the Martian atmosphere would require increasing the total local atmospheric pressure 50x.
From this prior work, it is clear that there is a difficulty in remediating the Martian regolith from its toxic state to a soil suited for crop growth. (Per)chlorate-reducing bacteria require reduced carbon sources with nitrogen and phosphorus for growth to detoxify the regolith. Ideally, this could be supplied by cyanobacteria that fix the CO2 in the atmosphere and can release the nitrogen and phosphorus from the regolith. The cyanobacteria can also provide the organic carbon in the soil to support crop growth. However, these cyanobacteria cannot tolerate the toxic (per)chlorates. Lastly, both the (per)chlorate-reducing bacteria and the cyanobacteria need to grow in aqueous conditions with the regolith particles separated to allow rapid microorganism growth. The regolith would then need to be drained and allowed to dry out before being suited to most crop growth, although rice might be able to grow in a “paddy field” of regolith that has settled. This suggests that there may need to be separate areas for removing the (per)chlorates, supplying needed nutrients for the (per)chlorate-reducing bacteria, by cyanobacteria growing in pre-treated regolith.
The following outline experiments are suggested to fill the gaps in treating the Martian regolith to make it suited for growing crops for the Martian settlement.
The exposed Martian regolith is both too cold and dry, as well as relatively airless, for bacteria to detoxify the (per)chlorates. Ideally, the detoxification would take place in optimal growth conditions for the bacteria. Given that maintaining atmospheric composition and pressure, as well as water and humidity conditions, incurs a mass penalty, it is important to determine what are the factors that can be reduced towards Mars’ conditions to reduce this cost. This will help decide whether the detoxification process must be carried out in a greenhouse suited to growing conventional crops, or whether simpler management of the regolith is sufficient. Other questions are also evident, such as the level of detoxification necessary before crops can be successfully grown in the treated regolith.
This suggests several experiments to test for these factors:
1. Composition of Bacterial inoculant
There are many known terrestrial (per)chlorate-metabolizing bacteria, e.g. Dechloromonas that can metabolize oxygenated chlorines. All are anaerobes and therefore may function with the existing composition of the Martian atmosphere. Questions to be considered are:
a. Should the inoculant be a single species or multiple?
b. Do other species need to be included to create a viable ecosystem, or are single-species populations both sufficient and effective?
2. Atmospheric pressure
Mars’s atmosphere is about 0.7% of that of Earth. While too low to support crop plants, how much pressure is needed for bacterial growth to be maintained? Unlike plants, the bacteria are aquatic, and therefore the needed atmospheric pressure need only be sufficient to prevent water from boiling off. In a sealed reactor, water vapor will provide the needed pressure to maintain the equilibrium. As the bacteria are anaerobes, the regolith would seem likely to be processed in separate areas from the crop plants, with the detoxified regolith then added to the agricultural area in the greenhouse to increase the cultivation area.
3. Atmospheric composition
Mars’ atmosphere is primarily CO2 with a little N2. This is not suitable for crop plants, but how much of a factor is this for the bacteria? Combined with atmospheric pressure, what composition is needed for the bacteria? For example, does the nitrogen partial pressure need to be increased to supply the needed nitrogen for bacterial growth, perhaps in combination with nitrogen-fixing bacteria in the inoculant, or just added as ammonia or nitrate? [c.f. Item 1 concerning species in the inoculant.]
Lastly, bacteria need wet conditions to grow and multiply. How wet does the regolith need to be for the bacteria? Do the bacteria survive and grow in an aqueous slurry, or would high humidity conditions be sufficient, saving water resources needed elsewhere?
To test these, experiments will need to be set up in conditions to test these various requirements, most probably in containers to maintain the conditions. It is assumed that surface UV and ionizing radiation do not need to be tested as simple shielding will be sufficient to mitigate these factors.
These experiments are primarily devoted to extending the existing work done on (per)chlorate removal by bacteria [2,10,13,22], extending prior work. If the regolith detoxification and preparation for traditional crops is to be the goal, the regolith will need additional preparation for crops, including nitrogen, phosphorus, and carbon supplements. Inoculants may be required to allow nitrogen-fixing bacteria to grow in association with the root nodules of crops like green beans. Prior experiments [30,34,40] with cyanobacteria have demonstrated the extraction of nitrogen from the regolith, suggesting this approach to fertilize the crop plants after the regolith has been cleared of the toxic chlorate and (per)chlorate.
A stretch goal might include gene splicing experiments to extend the capabilities of some microbial species. Can the (per)chlorate-reducing genes be added to cyanobacteria removing the need for the bacterial species? Conversely, can genes to extract the nitrogen and phosphorus from the regolith be inserted into the bacteria? Can the (per)chlorate genes be edited so that the oxygen is liberated safely in the organism, allowing the (per)chlorate to become an oxygen source for the Martian settlement? Suggestions as to possible ideas have been mooted [40, 49].
To start processing Martian regolith for food production on Mars, there is a substantial gap in our knowledge on getting this process underway in the volumes needed compared to the small-scale lab experiments. Firstly, the regolith must be detoxified to remove the (per)chlorates. While the lab experiments demonstrate that various species of bacteria can metabolize the (per)chlorates, there are two limitations. Firstly, the regolith needs to be in powdered form to expose the surfaces to the bacteria and be turned into an aqueous environment for the bacteria to survive. How wet the slurry needs to be is unknown and therefore the water requirements are also unknown. Secondly, the sterile regolith provides no useful food supplies for the bacteria to grow. How to supply the nutrients and from what source needs to be determined. Terrestrial starter kits may be inadequate for bulk regolith processing.
Cyanobacteria have been demonstrated in the lab to be able to fix the atmospheric CO2 and grow while extracting the needed nitrogen, phosphorus, and trace elements from the regolith, but only after the (per)chlorates have been removed.
Terrestrial crops are yet another step away as they need detoxified regolith, fertilizers, and organic carbon in the “soil” to grow successfully, suggesting that both the (per)chlorate-metabolizing bacteria and the cyanobacteria must preprocess the regolith.
While the bacterial cultures grow in aqueous conditions, terrestrial crops do not and are therefore subject to even more critical issues of the surrounding atmosphere: pressures, and composition.
Currently, it appears as if the regolith can be prepared by iteratively starting with (per)chlorate-metabolizing bacteria, followed by cyanobacteria to grow and produce the needed food for a larger amount of regolith to be detoxified so that large volumes of regolith can be prepared for conventional crops to be grown. Once the regolith has been prepared it is turned over to the agronomists to determine how best to provide the conditions and associated organisms to cultivate crops to feed the base crew or settlers. While hydroponics is favored for supplying small populations with food, more conventional agriculture using local resources including the regolith seems more likely to be the preferred approach once large settlements start to appear.
Atri, Dimitra, et al. “Estimating the Potential of Ionizing Radiation-induced Radiolysis for Microbial Metabolism in Terrestrial Planets With Rarefied Atmospheres.” arXiv (Cornell University), Cornell University, July 2022, https://doi.org/10.48550/arxiv.2207.14675.
Balk, Melike, et al. “(per)Chlorate Reduction by the Thermophilic Bacterium Moorella Perchloratireducens Sp. Nov., Isolated From Underground Gas Storage.” Applied and Environmental Microbiology, vol. 74, no. 2, American Society for Microbiology, Jan. 2008, pp. 403–09. https://doi.org/10.1128/aem.01743-07.
Bender, Kelly S., et al. “Identification, Characterization, and Classification of Genes Encoding Perchlorate Reductase.” Journal of Bacteriology, vol. 187, no. 15, American Society for Microbiology, Aug. 2005, pp. 5090–96. https://doi.org/10.1128/jb.187.15.5090-5096.2005.
Bennett, Jaemie. “The Experimentation of Growing Plants on Mars.” www.jhunewsletter.com, 2018, www.jhunewsletter.com/article/2018/10/the-experimentation-of-growing-plants-on-mars. Accessed 6 Aug. 2023.
Calderón, R., et al. “Perchlorate Levels in Soil and Waters From the Atacama Desert.” Archives of Environmental Contamination and Toxicology, vol. 66, no. 2, Springer Science+Business Media, Oct. 2013, pp. 155–61. https://doi.org/10.1007/s00244-013-9960-y.
Cannon, K. M., et al. “Mars Global Simulant MGS-1: A Rocknest-based Open Standard for Basaltic Martian Regolith Simulants.” Icarus, 1 Jan. 2019, doi.org/10.1016/j.icarus.2018.08.019.
Cartier, Kimberly. “Tests Indicate Which Edible Plants Could Thrive on Mars.” eos.org, 2018, eos.org/articles/tests-indicate-which-edible-plants-could-thrive-on-mars. Accessed 6 Aug. 2023.
Cartier, Kimberly M. S., and Kimberly M. S. Cartier. “Tests Indicate Which Edible Plants Could Thrive on Mars.” Eos, Jan. 2022, eos.org/articles/tests-indicate-which-edible-plants-could-thrive-on-mars.
—. “Tests Indicate Which Edible Plants Could Thrive on Mars.” Eos, Jan. 2022, eos.org/articles/tests-indicate-which-edible-plants-could-thrive-on-mars.
Coates, John D., and Laurie A. Achenbach. “Microbial Perchlorate Reduction: Rocket-fuelled Metabolism.” Nature Reviews Microbiology, vol. 2, no. 7, Nature Portfolio, July 2004, pp. 569–80. https://doi.org/10.1038/nrmicro926.
Daley, Jason. “Space Farmers Could Grow Crops in Lunar and Martian Soil, Study Suggests.” Smithsonian Magazine, 2019, www.smithsonianmag.com/smart-news/farmers-could-grow-crops-lunar-and-martian-soil-study-suggests-180973387/#:~:text=The%20radishes%2C%20cress%20and%20rye,it%20did%20not%20produce%20seeds. Accessed 7 Aug. 2023.
David, Leonard. “Toxic Mars: Astronauts Must Deal With Perchlorate on the Red Planet.” Space.com, 13 June 2013, www.space.com/21554-mars-toxic-perchlorate-chemicals.html.
Davila, Alfonso F., et al. “Perchlorate on Mars: A Chemical Hazard and a Resource for Humans.” International Journal of Astrobiology, vol. 12, no. 4, Cambridge UP, June 2013, pp. 321–25. https://doi.org/10.1017/s1473550413000189.
“The Experimentation of Growing Plants on Mars.” The Johns Hopkins News-Letter, 18 Oct. 2018, www.jhunewsletter.com/article/2018/10/the-experimentation-of-growing-plants-on-mars.
“—.” The Johns Hopkins News-Letter, 18 Oct. 2018, www.jhunewsletter.com/article/2018/10/the-experimentation-of-growing-plants-on-mars.
Fackrell, Laura, et al. “Growing Plants On Mars-Potential and Limitations OF Martian Regolith for In-Situ Resource Utilization.” www.hou.usra.edu, www.hou.usra.edu/meetings/ninthmars2019/eposter/6045.pdf. Accessed 6 Aug. 2023.
“Growing Crops in Mars Soil Simulant.” www.hmns.org, www.hmns.org/wp-content/uploads/2020/09/Growing-Crops-in-Mars-Soil-Simulant-Final-report.pdf. Accessed 6 Aug. 2023.
“Growing Green on the Red Planet – American Chemical Society.” American Chemical Society, www.acs.org/education/resources/highschool/chemmatters/past-issues/2016-2017/april-2017/growing-green-on-the-red-planet.html.
Growing Plants in Martian Soil | Chicago Botanic Garden. 13 Nov. 2017, www.chicagobotanic.org/blog/how_to/growing_plants_martian_soil.
Guinan, Edward, et al. “How to Grow Vegetables on Mars.” Scientific American, 2020, blogs.scientificamerican.com/observations/how-to-grow-vegetables-on-mars. Accessed 7 Aug. 2023.
Harris, Lynnette. “Farming Mars.” Farming Mars | USU, caas.usu.edu/cultivate/spring19/farming-mars. Accessed 7 Aug. 2023.
Harris, Rachel L., et al. “Transcriptional Response to Prolonged Perchlorate Exposure in the Methanogen Methanosarcina Barkeri and Implications for Martian Habitability.” Scientific Reports, vol. 11, no. 1, Nature Portfolio, June 2021, https://doi.org/10.1038/s41598-021-91882-0.
Hatzinger, Paul B. “Perchlorate Biodegradation for Water Treatment.” Environmental Science & Technology, vol. 39, no. 11, American Chemical Society, June 2005, pp. 239A-247A. https://doi.org/10.1021/es053280x.
He, Fang, et al. “Simultaneous Removal of Perchlorate and Nitrate Using Biodegradable Polymers Bioreactor Concept.” Journal of Geoscience and Environment Protection, vol. 02, no. 02, Scientific Research Publishing, Jan. 2014, pp. 42–47. https://doi.org/10.4236/gep.2014.22007.
ITRC Perchlorate Team. “Remediation Technologies for Perchlorate Contamination in Water and Soil.” ITRC, 2008, www.eosremediation.com/download/Perchlorate/ITRC%20PERC-2.pdf. Accessed 7 Aug. 2023.
“Jezero Delta Simulant (JEZ-1) – Perseverance Landing, Mars Space Dirt for Education and Research.” Exolith, exolithsimulants.com/products/jez-1-jezero-delta-simulant. Accessed 7 Aug. 2023.
Kasiviswanathan, Pooja, et al. “Farming on Mars: Treatment of Basaltic Regolith Soil and Briny Water Simulants Sustains Plant Growth.” PLOS ONE, vol. 17, no. 8, Public Library of Science, Aug. 2022, p. e0272209. https://doi.org/10.1371/journal.pone.0272209.
Kokkinidis, Ioannis. “Agriculture on Other Worlds.” Centauri Dreams, 2016, www.centauri-dreams.org/2016/03/11/agriculture-on-other-worlds. Accessed 7 Aug. 2023.
León, David San, and Juan Nogales. “Toward Merging Bottom–up and Top–down Model-based Designing of Synthetic Microbial Communities.” Current Opinion in Microbiology, vol. 69, Elsevier BV, Oct. 2022, p. 102169. https://doi.org/10.1016/j.mib.2022.102169.
Mapstone, Lydia J., et al. “Cyanobacteria and Microalgae in Supporting Human Habitation on Mars.” Biotechnology Advances, vol. 59, Elsevier BV, Oct. 2022, p. 107946. https://doi.org/10.1016/j.biotechadv.2022.107946.
Matthiä, Daniel, et al. “The Radiation Environment on the Surface of Mars – Summary of Model Calculations and Comparison to RAD Data.” Life Sciences in Space Research, vol. 14, Elsevier BV, Aug. 2017, pp. 18–28. https://doi.org/10.1016/j.lssr.2017.06.003.
NASA. “NASA’s Curiosity Rover Finds Biologically Useful Nitrogen on Mars.” NASA, 2015, www.nasa.gov/content/goddard/mars-nitrogen. Accessed 7 Aug. 2023.
Olsson-Francis, Karen, et al. “Survival of Akinetes (Resting-State Cells of Cyanobacteria) in Low Earth Orbit and Simulated Extraterrestrial Conditions.” Origins of Life and Evolution of Biospheres, vol. 39, no. 6, Springer Science+Business Media, Apr. 2009, pp. 565–79. https://doi.org/10.1007/s11084-009-9167-4.
Olsson-Francis, Karen, and Charles S. Cockell. “Use of Cyanobacteria for In-situ Resource Use in Space Applications.” Planetary and Space Science, vol. 58, no. 10, Elsevier BV, Aug. 2010, pp. 1279–85. https://doi.org/10.1016/j.pss.2010.05.005.
O’Neill, Mike. “Geologists Simulate Martian Soil Conditions to Figure Out How to Grow Plants on Mars.” SciTechDaily, 30 Oct. 2020, scitechdaily.com/geologists-simulate-martian-soil-conditions-to-figure-out-how-to-grow-plants-on-mars.
“Perchlorate Treatment Technology Update.” EPA, 2024, www.epa.gov/sites/default/files/2015-04/documents/perchlorate_542-r-05-015.pdf. Accessed 7 Aug. 2023.
Schuerger, Andrew C., et al. “Biotoxicity of Mars Soils: 1. Dry Deposition of Analog Soils on Microbial Colonies and Survival Under Martian Conditions.” Planetary and Space Science, vol. 72, no. 1, Elsevier BV, Nov. 2012, pp. 91–101. https://doi.org/10.1016/j.pss.2012.07.026.
Sia, Jin Sing. “ISRU Part IV: How to Grow Food on Mars.” Mars Society of Canada, 2020, www.marssociety.ca/2020/09/28/isru-part-iv-how-to-grow-food-on-mars/#:~:text=Farming%20on%20Mars,moist%20environments%2C%20such%20as%20greenhouses. Accessed 7 Aug. 2023.
Slank, Rachel A., et al. “Experimental Constraints on Deliquescence of Calcium Perchlorate Mixed With a Mars Regolith Analog.” The Planetary Science Journal, vol. 3, no. 7, July 2022, p. 154. https://doi.org/10.3847/psj/ac75c4.
Verseux, Cyprien, et al. “Sustainable Life Support on Mars – the Potential Roles of Cyanobacteria.” International Journal of Astrobiology, vol. 15, no. 1, Cambridge UP, Aug. 2015, pp. 65–92. https://doi.org/10.1017/s147355041500021x.
Wallis, Paul. “Op-Ed: Idiot Level Science — Can’t Grow Plants on Mars Despite Doing It in a Movie?” Digital Journal, Sept. 2021, www.digitaljournal.com/tech-science/idiot-level-science-cant-grow-plants-on-mars-despite-doing-it-in-a-movie/article.
Wamelink, G. W. W., et al. “Can Plants Grow on Mars and the Moon: A Growth Experiment on Mars and Moon Soil Simulants.” PLOS ONE, vol. 9, no. 8, Public Library of Science, Aug. 2014, p. e103138. https://doi.org/10.1371/journal.pone.0103138.
Wood, Charlie. “NASA Is Learning How to Farm on Mars and the Moon.” CNBC, 2021, www.cnbc.com/2021/06/20/space-agencies-are-learning-how-to-farm-on-mars-and-the-moon.html. Accessed 7 Aug. 2023.
Yamashita, Masamichi, et al. “Space Agriculture for Manned Space Exploration on Mars.” The Journal of Space Technology and Science, vol. 21, no. 2, Sept. 2005, pp. 1–10. https://doi.org/10.11230/jsts.21.2_1.
Yu, Lu, et al. “Uptake of Perchlorate in Terrestrial Plants.” Ecotoxicology and Environmental Safety, vol. 58, no. 1, Elsevier BV, May 2004, pp. 44–49. https://doi.org/10.1016/s0147-6513(03)00108-8.
Mars, Kelli. “Space Station 20th: Food on ISS.” NASA, Aug. 2020,www.nasa.gov/feature/space-station-20th-food-on-iss.
Dunbar, Brian. “NASA – Fresh Fruits and Vegetables in Space”. www.nasa.gov/audience/forstudents/9-12/features/F_Fruits_and_Vegetables_Space.html
Lupo, Lisa “Food in Space: Defying (Micro)Gravity to Feed our Astronauts“ NASA April 2015, www.qualityassurancemag.com/article/qa0415-food-in-space-nasa.
Cockell, Charles S. “Synthetic Geomicrobiology: Engineering Microbe–mineral Interactions for Space Exploration and Settlement.” International Journal of Astrobiology, vol. 10, no. 4, Cambridge UP, May 2011, pp.315–24. https://doi.org/10.1017/s1473550411000164.
We’re building some remarkably large telescopes these days. Witness the Giant Magellan Telescope now under construction in Chile’s Atacama desert. It’s to be 200 times more powerful than any research telescope currently in use, with 368 square meters of light collection area. It incorporates seven enormous 8.5 meter mirrors. That makes exoplanet work from the Earth’s surface a viable proposition, but look at the size of the light bucket we need to make it work. Three mirrors like that shown below are now in place, and the University of Arizona’s Mirror Lab is building number 6 now.
Image: University of Arizona Richard F. Caris Mirror Lab staff members Damon Jackson (left) and Conrad Vogel (right) in the foreground looking up at the back of primary mirror segment five, April 2019. Credit: Damien Jemison; Giant Magellan Telescope – GMTO Corporation. CC BY-NC-ND 4.0.
Imaging an exoplanet from the Earth’s surface is complicated by the Rayleigh Limit, which governs the resolution of our optical systems and their ability to separate two point sources. Stephen Fleming showed the equation in his talk on super-resolution imaging at the Interstellar Research Group’s recent meeting in Montreal. I use few equations on this site but I’ll show this one because it’s straightforward and short:
θ = 1.22 * (λ / D)
Here λ is the wavelength and D is the diameter of the mirror. What this says is that there is a minimum angular separation (θ) that allows two point sources to be clearly distinguishable, which in terms of astronomy means we can’t pull useful information out of the image when they are closer than this. I’ve pulled the image below out of Wikipedia (in the public domain, submitted by Spencer Bliven).
Image: Two Airy disks at various spacings: (top) twice the distance to the first minimum, (middle) exactly the distance to the first minimum (the Rayleigh criterion), and (bottom) half the distance. This image uses a nonlinear color scale (specifically, the fourth root) in order to better show the minima and maxima.
Here we have another useful term: An Airy disk is a diffraction pattern that is produced when light moves through the aperture of a telescope system. Light diffracts – it’s in the nature of the physics – and the Airy disk is the best focused spot of light that a perfect lens with a circular aperture can make. We’re looking at light interfering with itself, so in the image, we have a central bright spot with surrounding rings of light and dark. The diffraction pattern depends upon the wavelength being observed and the aperture itself. This diffraction can be described as a point spread function (PSF) for any optical system, and essentially governs how tightly that system can be focused.
Bigger apertures matter as we try to deal with these limitations, and the Giant Magellan Telescope will doubtless make many discoveries, as will all of the coming generation of Extremely Large Telescopes. But when we want to see ever smaller objects at astronomical distances, we run into a practical problem. Nothing in the physics prevents us from building a ground-based telescope that could see an Earth-class planet at Alpha Centauri, but if we want details, Fleming notes, we would need a mirror 1.8 kilometers in diameter to retrieve a 40 X 40 pixel image.
The point of Fleming’s talk, however, was that we can use quantum technologies to nudge into the Rayleigh limitations and extract information about amplitude and phase from the light we do collect. That, in turn, would allow us to distinguish between point sources that are closer than what the limit would imply. The operative term is super-resolution, a topic that is growing in importance in the literature of optics, though to this point not so much in the astronomical community. This may be about to change.
Counter-intuitively (at least insofar as my own intuitions run), a multi-aperture telescope does a better job with this than a large single-aperture. Instead of a 3-meter mirror you use three 1.7 meter mirrors that are spaced out over, perhaps, an acre. This hits at mirror economics as well, because the costs of these enormous mirrors goes up more than exponentially. The more you can break the monolithic mirror into an array of smaller mirrors, you can add to the data gain but also sharply reduce the expense.
In terms of the science, Fleming noted that the point spread function spreads out when multiple smaller mirrors are used, and objects become detectable that would not be with a monolithic single mirror instrument. The technique in play is called Binary Spatial Mode Demultiplexing. Here the idea is to extract quantum modes of light in the imaging system and process them separately. The central mode – aligned with the point spread function of the central star – is the on-axis light. The off-axis photons, sorted into a separate detector, are from what surrounds the star.
So in a way we’re nudging inside the Rayleigh Limit by processing the light, nulling out or dimming the star’s light while intensifying the signal of anything surrounding the star. I’m reminded, of course, of all the work that has gone into coronagraphs and starshades in the attempt to darken the star while revealing the planets around it. In fact, some of the earliest research that convinced me to write my Centauri Dreams book was the work of Webster Cash out at the University of Colorado on starshades for this purpose, with the goal of seeing continents and oceans on an exoplanet. I later learned as well of Sara Seager’s immense contributions to the concept.
Thus far the simulations that have been run at the University of Arizona by Fleming’s colleagues have shown far higher detection rates for an exoplanet around a star using multi-aperture telescopes. In fact, there is a 100x increase in sensitivity for multi-aperture methods. This early work indicates it should be possible to identify the presence of an exoplanet in a given system with this ground-based detection method.
Can we go further? The prospect of direct imaging using off-axis photons is conceivable if futuristic. If we could create an image like this one, we would be able to study this hypothetical world over time, watching the change of seasons and mining data on the land masses and oceans as the world rotates. The possibility of doing this from Earth’s surface is startling. No wonder super-resolution is a growing field of study, and one now being addressed within the astronomical community as well as elsewhere.
Take a look at the image below. It’s a jet coming off the quasar 3C273. I call your attention to the length of this jet, some 100,000 light years, which is roughly the distance across the Milky Way. Jeff Greason pointed out at the Montreal symposium of the Interstellar Research Group that images like this suggest it may be possible for humans to produce ‘pinched’ relativistic electron jets over the much smaller distances needed to propel a spacecraft out of the Solar System. This is an intriguing image if you’re interested in high-energy beams pushing payloads to nearby stars.
Greason is a self-described ‘serial entrepreneur,’ the holder of some 29 patents and chief technologist of Electric Sky, which is all about beaming energy to craft much closer to home. But he moonlights as chairman of the Tau Zero Foundation and is a well known figure in interstellar studies. Placing beaming into context is a useful exercise, as it suggests alternative ways to generate and use a beam. In all of these, we want to carry little or no fuel aboard the craft, drawing our propulsion from the home system.
Image: Composite false-color image of the quasar jet 3C273, with emission from radio waves to X-rays extending over more than 100,000 light years. The black hole itself is to the left of the image. Colors indicate the wavelength region where energetic particles give off most of their energy: yellow contours show the radio emission, with denser contours for brighter emission (data from VLA); blue is for X-rays (Chandra); green for optical light (Hubble); and red is for infrared emission (Spitzer). Credit: Y. Uchiyama, M. Urry, H.-J. Röser, R. Perley, S. Jester.
Laser beaming to a starship comes first to mind, going back as it does to the days of Robert Forward and György Marx, who explored options in the infancy of the technology. Later work on laser ad well as microwave beaming has included such luminaries as Geoffrey Landis, Gregory Matloff and James Benford, not to mention today’s intense laser effort via Breakthrough Starshot and the ongoing work at UC-Santa Barbara under Philip Lubin. A separate track has followed beamed options using elementary particles or, indeed, larger particles; the name Clifford Singer comes first to mind here, though Landis has done key work. A major problem: Beam power is inversely proportional to effective range. If we’re after faster, bigger ships, we need to find a way to extend the range of whatever kind of beam we’re sending.
We’ve lost some of the scientists who have dug deeply into these matters. Dana Andrews died last January, and Jordin Kare left us some six years ago (I will have more to say about Dr. Andrews in a future post). Kare developed ‘sailbeam,’ which was a string of micro-sails sent as fuel fodder to a larger starship. Pushing neutral particles to the long ranges we need faces problems of beam divergence, and charged particle beams are even more tricky, because like charges cause the beam to diverge.
Greason outlined another possibility at Montreal, one he described as ‘no more than half of an idea,’ but one he’s hoping to provoke colleagues to explore. This beaming option uses the ‘pinch’ phenomenon, in which charged beams in a low-density plasma can confine themselves over long distances. The mechanism: A beam carrying a current creates a circular axial magnetic field which in turn confines the beam. ‘Pinching’ is a means of self-confinement of the beam that has been studied since the 1930s. A pinch forming a jet explains why solar proton events can strike the Earth despite the 1 AU distance, and why galaxy-spanning jets like that in the image above can form.
Image: Jeff Greason, chief technologist and co-founder of Electric Sky.
We normally hear about a ‘pinch’ in the context of fusion research, but here we’re more interested in the beam’s persistence than its ability to compress and heat a plasma. The beam persists until it loses energy by collisions, which causes the current sustaining it to weaken and lose confinement. Although Greason said that ion beams may prove feasible, he noted that we’re getting into territory where we simply lack data to know what will work. Issues of charge neutralization and return currents from the beam come into play, as do long-range oscillations that can affect the beam. But the idea of applying a magnetic field to a stream of electrons along a specific axis to create the z-pinch is well established. If we can create an electron beam using this method, we can resurrect the idea of using charged particle beams to push our starship.
How to use power beamed in this fashion once it arrives at the target craft is a significant question. Greason spoke of the beam striking a plasma-filled waveguide which can ‘couple to backwards plasma wave modes,’ in effect launching plasma in the opposite direction as reaction mass. This keys to existing work on plasma accelerators (so-called “wakefield” accelerators), which use similar physics. How much of the beamed energy can be returned in this way remains up for investigation.
The consequences of mastering pinched beaming technologies would be immense. If we can increase the range of a beam from 0.1 AU to 1000 AU, we open up the possibility of sending much larger spacecraft, up to 105 larger, at the same power levels. We go from a gram-sized spacecraft as contemplated by Breakthrough Starshot’s laser methods to one of 10 kilograms. In doing this we have also changed the acceleration time from minutes to months. That increased payload size is particularly useful when it allows a braking system aboard for long-term study of the target.
This method demands a space-based platform – these ideas are inapplicable when applied to a ground installation and a beam through the atmosphere. Beaming from a location near the Sun offers obvious access to power and could be made possible through a near-Solar statite; i.e., an installation that ‘hovers’ over the Sun at Parker Solar Probe distances. Greason adds that to add maximum stability to the beam, the statite would have to transmit from a location between the Sun and the target star; i.e, the flow should be with the current of the solar wind as opposed to across the stream.
Image: Can we operate a statite at 0.05 AU from the Sun? This NASA visualization of the Parker Solar Probe highlights the kind of conditions the craft would be operating in.
The operative statite technology is thermionics, where electrons ‘boil’ out of a hot cathode and collect on a cold anode. Greason’s statite winds up with approximately 50 kilowatts per square meter of useful power; factoring in the thickness of the foils used in the installation, he calculates 150 kilowatts per square kilogram. A 1 gigawatt electron beam results. So operating at about 11 solar radii, we can produce the beam we need while also being forced to tackle the issues involved in maintaining a statite in position. One possibility is a plasma magnet sail to make use of the supersonic solar wind, a notion Greason has been exploring for years. See Alex Tolley’s The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar System and Beyond for more.
Greason’s tightly reasoned, no-nonsense approach makes him a hugely appealing speaker. He’s offering a concept that opens out into all kinds of research questions, and spurring interested parties to advance the construct. A symposium of like-minded scientists and engineers like that in Montreal provides the kind of venue to gin up that support. The implication of being able to reach 20 percent of lightspeed with a multi-kilogram spacecraft is driver enough. A craft like that could begin exploration of nearby stars in stellar orbit there, rather than blowing through the destination system within a matter of minutes. What smaller beam installations near Earth could do for interplanetary exploration is left to the imagination of the reader.
Writers have modeled the arrival of an extraterrestrial probe in our Solar System in a number of interesting science fiction texts, from Clarke’s Rendezvous with Rama (1973) to the enigmatic visitors of Ted Chiang’s “Story of Your Life,” which Hollywood translated into the film Arrival (2016). In between I might add the classic ‘saucer landing on the White House lawn’ trope of The Day the Earth Stood Still (1951), based on a Harry Bates short story. All these and many other stories raise the question: What if before we make a radio or optical SETI detection, an extraterrestrial scout actually shows up?
Graeme Smith (UC: Santa Cruz) goes to work on the idea in a recent paper in the International Journal of Astrobiology, where he focuses on the mechanism of interstellar dispersion. The model has obvious ramifications for ourselves. We are beings who have begun probing nearby space with vehicles like Pioneer and Voyager, and in our early stages of exploration we could conceivably be reached by an extraterrestrial civilization (ETC) before we can make such journeys ourselves. Smith is asking what form such contact would take. His paper cautiously tries to quantify how interstellar exploration likely proceeds based on velocity and distance in a steadily advancing technological culture.
This takes us back to the so-called ‘Wait Equation’ explored by Andrew Kennedy in 2006, where he dug into what he called ‘the incentive trap of progress.’ Kennedy made the natural assumption that as an interstellar program of exploration proceeded, it would continue to produce faster travel speeds, so that one probe might be overtaken by another (thus A. E. van Vogt’s ‘Far Centaurus’ scenario, where a starship crew comes out of hibernation at Alpha Centauri to find a thriving civilization of humans, all of whom came by much speedier means while the original exploration team slumbered enroute).
The question, then, becomes whether we should postpone an interstellar launch until a certain amount of further progress can be made. And exactly how long should we wait? But Smith looks at this from a different angle: What kind of probes would be first to arrive in a planetary system where a civilization like ours can receive them? Would they be the ‘lurkers’ Jim Benford has written about, left by beings who expected them to report home on what evolved in our Solar System? Or might they be more overt, making themselves known in some way, and advanced well beyond our understanding?
Image: Is this really what we might expect if an ETC arrived on Earth? From the 1951 version of The Day the Earth Stood Still. Frank Lloyd Wright is said to have been involved in the design of the craft for this movie, though some believe this to be no more than a Hollywood legend. It’s an interesting one if so. And about that spacecraft: Is it too low-tech to be realistic? Read on.
In Smith’s parlance, a civilization like ours is ‘passive,’ a specific usage meaning that it is able to probe its own system with spacecraft but does not yet have interstellar capabilities. He imagines two ETCs, one in this passive state and one capable of interstellar flight. Smith’s calculations then consider probes launched by an extraterrestrial civilization that are followed by increasingly advanced probes over time. You can see from this that the farther away the sending civilization is, the more likely that what will arrive at the passive ETC will be one of its more advanced probes, the earlier ones being still in transit.
If an active ETC is evolving rapidly in technology, or is exceedingly distant, then a vehicle of relatively advanced state may be more likely to first reach a passive collecting civilization. In this case, there could be a considerable mismatch in the technology level of the first-arrival probe and that of the passive ETC that it encounters. This would presumably have ramifications for what might eventuate if an artefact from an ETC were to arrive within the Solar System and enable first-contact with terrestrials. Hypothetical reverse engineering, for example, might be difficult given the technology gap.
Assuming the probe speed scales linearly with launch date, Smith uses as an example the Voyager probes and spins out increasingly fast generations of probes, noting how many such generations will be required to reach first the closest stars and then stars farther out, and calculating the time that separates the first encounter spacecraft with the initial, zero-generation probes. The situation accelerates if we assume probe speeds that scale exponentially with launch date. I send you to the paper for his equations, but the upshot is that this scenario heightens the likelihood that a first encounter probe will display a major disparity in technology from what it finds at the receiving end.
And depending on the distance of the sending civilization, the disparity between the ETC technology and our own could be such that we would have difficulty understanding, even comprehending, what we were looking at. Smith again (italics mine):
The key implication of this paper can be summarized as followed: if an actively space-faring ETC embarks on a program to send probes to interstellar destinations, and if the technology of this ETC advances with time, then the first probe to arrive at the destination of a less-advanced ETC is less likely to be one of the earliest probes launched, but one of more advanced capability. There may thus be a substantial disparity between the level of technology comprising the first-arrival probe and that developed by the receiving ETC, if it has no interstellar capability itself. The greater the initial separation of the two ETCs, or the greater the rate of probe development by the active ETC, the greater is the potential for a technological mismatch at first encounter.
Image: A language that can alter our perception of time, under study in the film Arrival, where the probes in question represent a technology that is baffling to Earth scientists.
The situation would change, of course, if the receiving civilization is also one possessing interstellar capabilities, in which case contact might not even occur on the home world or system of the receiving culture. As we are a passive civilization in Smith’s terms, we are likely to encounter a markedly advanced civilization if an artifact ever does show up in our system. David Kipping calls this result ‘contact inequality,’ and remember, “…increasing the distance Dmax of the first-contact horizon increases the likely generation number of a probe of first encounter, thereby enhancing a contact inequality with a passive ETC.”
Something entering our Solar System from another civilization should be highly sophisticated, well beyond our technological levels, and perhaps utterly opaque to our scrutiny. The scenario of, for example, the Strugatsky brothers’ Roadside Picnic seems more likely than that of The Day the Earth Stood Still. The 1972 novel depicts the ‘stalker’ Red Schuhart as he enters a ‘zone of visitation’ where an alien civilization has come to Earth and left behind bizarre and inexplicable traces. Now they’ve moved on. What is human culture to make of their detritus? From the novel:
He had never experienced anything like this before outside the Zone. And it had happened in the Zone only two or three times. It was as though he were in a different world. A million odors cascaded in on him at once—sharp, sweet, metallic, gentle, dangerous ones, as crude as cobblestones, as delicate and complex as watch mechanisms, as huge as a house and as tiny as a dust particle. The air became hard, it developed edges, surfaces, and corners, like space was filled with huge, stiff balloons, slippery pyramids, gigantic prickly crystals, and he had to push his way through it all, making his way in a dream through a junk store stuffed with ancient ugly furniture … It lasted a second. He opened his eyes, and everything was gone. It hadn’t been a different world—it was this world turning a new, unknown side to him. This side was revealed to him for a second and then disappeared, before he had time to figure it out.
Image: From the 1979 film Stalker, based loosely on the Strugatsky novel. No spacecraft, no aliens here, just the mystery of what they left and what it means.
It should hardly surprise us that an arriving interstellar probe would be well beyond our technology; otherwise, it couldn’t have gotten here. But if we factor in what Smith is saying, it appears that depending on how far away the sending ETC is, the technology gap between us and them becomes greater and greater. We’re talking about baffling and perplexing morphing into the all but unknowable. Indistinguishable from magic?
No grand arrivals, no opportunities for trade, no galactic encyclopedias. This is first contact as enigma, and if I had to put money on it, I suspect this is closer to what would happen if contact is achieved by a visitation to our planet. I return to Rendezvous with Rama, where odd geometric structures and a ‘cylindrical sea’ are found within the probe slingshotting around the Sun, and the vehicle departs as mysteriously as it came, leaving behind only one overwhelming fact: We are not alone.
The paper is Smith, “On the first probe to transit between two interstellar civilizations,”
International Journal of Astrobiology 22 (2023), 185-196 (abstract).
I sometimes imagine Claudio Maccone having a particularly vivid dream, a bright star surrounded by a ring of fire that all but grazes its surface. And from this ring an image begins to form behind him, kilometers wide, dwarfing him and carrying in its pixels the view of a world no one has ever seen. The dream is half visual, half diagrammatic, but it’s all about curving Einsteinian spacetime, so that light flows along the gravity well to be bent into a focus that extends into linear infinity.
My slightly poetic vision of what happens beyond 550 AU or so doesn’t do justice to the intrinsic beauty of the mathematics, which Maccone learned to unlock decades ago as he explored the concept of an ‘Einstein ring’ as fine-tuned by Von Eshleman at Stanford. When I met him (at one of Ed Belbruno’s astrodynamics conferences at Princeton in 2006), we and Greg Matloff and wife C talked about lensing at breakfast one morning. Even then he was afire with the concept. He’d been probing it since the late 1980s, and had submitted a mission proposal to the European Space Agency. He had written a short text that would later be expanded into the seminal Deep Space Flight and Communications (Springer, 2009).
Maccone said in his presentation at the Interstellar Research Group’s Montreal symposium that he was delighted to see the Sun’s gravitational focus moving into the hands of the next generation, citing the 2020 NASA grant to Slava Turyshev’s team at JPL, where a Solar Gravitational Lens mission is being worked out at the highest level of detail as an entrant into the sweepstakes known as the Heliophysics 2024 Decadal Survey. To see how far the concept has gone, have a look at, for example, Self-Assembly: Reshaping Mission Design, or A Mission Architecture for the Solar Gravity Lens, among numerous entries I’ve written on the JPL work.
Image: A meter-class telescope with a coronagraph to block solar light, placed in the strong interference region of the solar gravitational lens (SGL), is capable of imaging an exoplanet at a distance of up to 30 parsecs with a few 10 km-scale resolution on its surface. The picture shows results of a simulation of the effects of the SGL on an Earth-like exoplanet image. Left: original RGB color image with (1024×1024) pixels; center: image blurred by the SGL, sampled at an SNR of ~103 per color channel, or overall SNR of 3×103; right: the result of image deconvolution. Credit: Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II.
The astounding magnification we could achieve by using bent starlight was what drew me instantly to the concept when I first learned about it – how else to actually see not just pixels from an exoplanet around its star, but actual continents, weather patterns, oceans and, who knows, even vegetation on the surface? But at Montreal, after his praise for the JPL effort that could become our first attempt to exploit the gravitational lens if adopted by the Decadal survey, Maccone took a much more futuristic look at what humans might do with lensing, delving into the realm of communications. What about building a radio ‘bridge’?
The concept is even more audacious that reaching 650 AU with the payloads we’ll need to deconvolve imagery from another star. In fact, it’s downright science fictional. Suppose we achieve the technologies needed to send humans to Alpha Centauri. We have there in the form of Centauri A a G-class star much like the Sun (although we could also use the K-class star Centauri B). Both of these stars have their own distance from which gravitational lensing occurs, Align your spacecraft properly to look back towards the Earth from Centauri A and you can now connect to the ‘relay’ at the lensing distance from the Sun. You’ve drastically changed the communications picture by using lensing in both directions.
The consequences for contact and data transfer are enormous. Consider: If we want to talk to our crew now orbiting Centauri A and try to do so with one of the Deep Space Network’s 70-meter dish antennae using today’s standards for spacecraft communications, we’d have no usable signal to work with. Assume a transmitting power of 40 W and communications over the Ka band (32 GHz) at a rate of 32 kbps (these are the figures for the highest frequency used by the Cassini mission). The distances are too great; the power too weak. But if we factor in a receiver at the lensing point of Centauri A directly opposite to the Sun, we get the extraordinary gain shown in the diagram below.
This raises the eyebrows. Bit Error Rate expresses the quality of the signal, being the number of erroneous bits received divided by the total number of bits transmitted. Using a spacecraft at the solar gravitational lens distance from the Sun talking to one on the other side of Centauri A (alignment, of course, is critical here), we have a signal so strong that we have to go over 9 light years out before it begins to degrade. A radio bridge like this would allow communications with a colony at Alpha Centauri using power levels and infrastructure we have in place today.
Obviously, this is a multi-generational idea given travel times to and from Alpha Centauri. But it’s a step we may well need to take if we can solve all the problems involved in getting human crews to another star. Maccone told the audience at Montreal that in terms of channel capacity (as defined by Shannon information theory), the Sun used as a gravitational lens allows 190 gigabits per second in a radio bridge to Centauri A as opposed to the paltry 15.3 kilobits per second available without lensing.
Realizing that any star creates this possibility, Maccone has lately been working on the question of how a starfaring society of the future might use radio bridges to plot out expansion into nearby stars. He is in fact thinking about the best ‘trail of expansion’ humans might use to keep links being built and used between colonies at these stars. This turns out to be no easy task: The first goal must be to convert the list of nearby stars being studied (the number is arbitrary) into Cartesian coordinates centered on each star (their coordinates are currently given in terms of Right Ascension and Declination with respect to the Sun). Maccone calls this an exercise in spherical trigonometry, and it’s a thorny one.
A network of radio bridges between stars could evolve into a kind of ‘galactic internet,’ a term Maccone uses with an ironic smile as it plays to the journalist’s need to write dramatic copy. Be that as it may, the SETI component is intriguing, given that older civilizations may even now be exploiting gravitational lensing. It would be an interesting thing indeed if we were to discover a bridge relay somewhere at our Sun’s gravitational lensing distance, for its placement would allow us to calculate where the receiving civilization must be located. Using a gravitational lens for communications is, after all, extraordinarily directional. Might we one day discover at the lensing distance from the Sun an artifact that can open access to a networked conversation on the interstellar scale?
Human expansion to nearby stars would likely be a matter of millennia, but given the age of the galaxy, it would represent just a sliver of time. Whether humanity can survive for far shorter timeframes is an immediate question, but I think it’s refreshing indeed to look beyond the current work on reaching the solar gravitational lens to the implications that would follow from exploiting it. The radio bridge is great science fiction material – we might even call it the stuff of dreams – but solidly rooted in physics if we can find the tools to make it happen.
The exoplanet K2-18b has been all over the news lately, with provocative headlines suggesting a life detection because of the possible presence of dimethyl sulfide (DMS), a molecule produced by life on our own planet. Is this a ‘Hycean’ world, covered with oceans under a hydrogen-rich atmosphere? Almost nine times as massive as Earth, K2-18b is certainly noteworthy, but just how likely are these speculations? Centauri Dreams regular Dave Moore has some thoughts on the matter, and as he has done before in deeply researched articles here, he now zeroes in on the evidence and the limitations of the analysis. This is one exoplanet that turns out to be provocative in a number of ways, some of which will move the search for life forward.
by Dave Moore
124 light years away in the constellation of Leo lies an undistinguished M3V red dwarf, K2-18. Two planets are known to orbit this star: K2-18c, a 5.6 Earth mass planet orbiting 6 million miles out, and K2-18b, an 8.6 Earth mass planet orbiting 16 million miles out. The latter planet transits its primary, so from its mass and size (2.6 x Earth’s), we have its density (2.7 g/cm2), which class the planet as a sub-Neptune. The planet’s relatively large radius and its primary’s low luminosity make it a good target to get its atmospheric spectra, but what also makes this planet of special interest to astronomers is that its estimated irradiance of 1368 watts/m2 is almost the same as Earth’s (1380 watts/m2).
Determining an exosolar planet’s atmospheric constituents, even with the help of the James Webb telescope, is no easy matter. For a detectable infrared spectrum, molecules like H2O, CH4, CO2 and CO generally need to have a concentration above 100 ppm. The presence of O3 can function as a stand-in for O2, but molecules such as H2, N2, with no permanent dipole moment, are much harder to detect.
The Hubble telescope got a spectrum of K2-18b in 2019. Water vapor and H2 were detected, and it was assumed to have a deep H2/He/steam atmosphere above a high pressure ice layer over an iron/rocky core, much like Neptune. On September 11 of this year, the results of spectral studies by the James Webb telescope were announced: CH4 and CO2 were found as well as possible traces of DMS (Dimethyl sulfide). No signal of NH3 was found. Nor was there any sign of water vapor. The feature thought to be water vapor turned out to be a methane line of the same frequency.
Figure 1: Spectra of K2-18b obtained by the James Webb telescope
This announcement resulted in considerable excitement and speculation by the popular press. K2-18b was called a Hycean planet. It was speculated that it had an ocean, and the possible presence of DMS was taken as an indication of life because oceanic algae produce this chemical. But that was not what intrigued me. What caught my attention was the seemingly anomalous combination of CH4 and CO2in the planet’s atmosphere. How could a planet have CH4, a highly reduced form of carbon, in equilibrium with CO2, the oxidized form of carbon? A search turned up a paper from February 2021: “Coexistence of CH4, CO2, and H20 in exoplanet atmospheres,” by Woitke, Herbort, Helling, Stüeken, Dominik, Barth and Samra.
The authors’ purpose for this paper was to help with the detection of biosignatures. To quote:
The identification of spectral signatures of biological activity needs to proceed via two steps: first, identify combinations of molecules which cannot co-exist in chemical equilibrium (“non-equilibrium markers”). Second, find biological processes that cause such disequilibria, which cannot be explained by other physical non-equilibrium processes like photo-dissociation. […] The aim of this letter is to propose a robust criterion for step one…
The paper presents an exhaustive study for the lowest energy state (Gibbs free energy) composition of exoplanet atmospheres for all possible abundances of Hydrogen, Carbon, Oxygen, and Nitrogen in chemical equilibrium. To do that, they ran thermodynamic simulations of varying mixtures of the above atoms and looked at the resulting molecular ratios. At low temperatures (T ≤ 600K), they found that the only molecular species you get in any abundance are H2, H20, CH4, NH3, N2, CO2, O2. At higher temperature, the equilibrium shifts towards more H2, and CO begins to appear.
Some examples of their results:
If O > 0.5 x H + 2 x C ––> O2-rich atmosphere, no CH4
If H > 2 x O + 4 x C ––> H2-rich atmosphere, no CO2
If C > 0.25 x H + 0.5 x O ––> Graphite condensation, no H20
They also used the equations to tell what partial pressures of the elemental mixture will produce equal pressures of the various molecules:
If H = 2 x O then the CO2 level will equal CH4
If 12 C = 2 x O + 3 x H then the CO2level will equal H20
If 12 C = 6 x O + H then the H20 level will equal CH4
To summarize, I quote from their abstract:
We propose a classification of exoplanet atmospheres based on their H, C, O, and N element abundances below about 600 K. Chemical equilibrium models were run for all combinations of H, C, O, and N abundances, and three types of solutions were found, which are robust against variations of temperature, pressure, and nitrogen abundance.
Type A atmospheres[which] contain H20, CH4, NH3, and either H2 or N2, but only traces of CO2 and O2.
Type B atmospheres [which] contain O2, H20, CO2, and N2, but only traces of CH4, NH3, and H2.
Type C atmospheres [which] contain H20, CO2, CH4, and N2, but only traces of NH3, H2, and O2…
Type A atmospheres are found in the giant planets of our outer solar system. Type B atmospheres occur in our inner solar system. Earth, Venus and Mars fall under this classification, but we don’t see any planets with Type C atmospheres.
Below is a series of charts showing the results for each of the six main molecular species over a range of mixtures.
Figure 2: The vertical axis is the ratio of Hydrogen to Oxygen, starting at 100% Hydrogen at the bottom and running to 100% Oxygen at the top. The horizontal axis shows the proportion of Carbon in the total mixture (The ratio runs up to 35%.) Molecular concentrations are in chemical equilibrium as a function of Hydrogen, Carbon, and Oxygen element abundances, calculated for T = 400 K and p = 1 bar. The blank regions are concentrations of < 10−4.
The central grey triangle marks the region in which H20, CH4, and CO2 can coexist in chemical equilibrium. The thin grey lines bisecting the triangle indicate where two of the constituents are at an equal concentration. These lines are hard to discern unless you can magnify the original image. For H20 and CO2 at equal concentration, it’s the dashed line (the near vertical line running upwards from 0.2 on the horizontal scale.) For CO2 and CH4, it’s the horizontal line. And for H20 and CH4, it’s the dotted line swooping upwards toward the top right-hand corner.)
The color bars at the right-hand side of the charts are both a color representation of the concentration and show the proportion of Nitrogen tied up as N2, i.e. that which is not NH3. Not surprisingly, the more Hydrogen there is in the mix, the higher the proportion of NH3 there is.
Other Results from the Paper
In the area around the stoichiometric ratio for water you get maximum H20 production and supersaturation occurs. Clouds form and the water rains out. Therefore, you cannot get an atmosphere with very high concentrations of water vapor unless the temperature is over 650°K, the critical point of water. Precipitation results in the atmospheric composition moving out of the area that gives CO2/CH4 mixtures.
Atmospheres with high carbon concentrations and having Hydrogen and Oxygen near their stoichiometric ratio have most of the atmospheric constituents tied up as water, so at a certain point carbon forms neither CO2 nor CH4 but rains out as soot. This, however, only precludes mixtures in the very right hand side of the CO2/CH4 Triangle.
Full-equilibrium condensation models show that the outgassing from warm rock, such as mid-oceanic ridge basalt can naturally produce Type C atmospheres.
Thoughts and Speculations
i) While it is difficult to argue with the man who coined the term, I still think Madhusudhan’s description of K2-18b as Hycean is too broad. Watching Madhusudhan in a Youtube interview, he refers to his paper “Habitability and Biosignatures of Hycean Worlds,’ which suggests that ocean covered planets under a Hydrogen atmosphere can exists within a zone that reaches into a level of irradiance slightly greater than Earth’s; however, he doesn’t mention the work by Lous et al in their paper, “Potential long-term habitable conditions on planets with primordial H–He atmospheres,” that showed that inside irradiance levels equivalent to 2 au from our Sun or greater, the Hydrogen atmosphere required to maintain Earthlike temperatures and not cook it is so thin that it is lost quickly over geological timescales. (You can see this in more detail in my article Super Earths/Hycean Worlds.) I would therefore define a Hycean planet as a rocky world with a radius up to 1.8 x Earth’s outside the irradiance equivalent of 2 au from our sun. K2-18b, being both larger than this and less dense than a rocky world, would fall, in my mind, firmly into the category of sub-Neptune.
ii) Another way of thinking of Type A, Type B and Type C atmospheres is to denote them as Hydrogen dominated, Oxygen dominated and Carbon dominated. Carbon dominated atmospheres may have by far the bulk of their constituents being Hydrogen and Oxygen; but because the enthalpy of the Hydrogen-Oxygen reaction is so much greater than the other reactions, when Hydrogen and Oxygen are close to their stoichiometric ratio, they preferentially remove themselves from the mix leaving Carbon as the dominant constituent. There is no Nitrogen dominated atmosphere because for most of its range Nitrogen sticks to itself forming N2 and is inert.
iii) The lack of H20 spectral lines is puzzling. Madhusudhan in his interview suggests that the spectra was a shot of the high-dry stratosphere. To cross-check the plausibility of this, I looked up the physical data on DMS. Dimethyl Sulfide vaporizes at 37°C and freezes at -98°C, which is lower than CO2’s freezing point. It also has a much higher vapor pressure than water at below freezing temperatures, so this does not contradict the assumption.
iv) I’m surprised this paper is not more widely known as not only does it provide a powerful tool for the analysis of exosolar planets’ atmospheric spectra, but it can also point to other aspects of a planet.
After the Hubble results came out in 2017, papers were published to model the formation of K2-18b, and while a range of possibilities could match the planet’s characteristics, they all came from the assumption that the planet began via the formation of a rocky/iron core followed by the gas accretion of large amounts of H2, Helium, and H20. According to the coexistence paper though, you cannot have large amounts of H2 and get a CO2/CH4 mix with no NH3. So to arrive at this state, this planet must never have had much gas accretion in the first place, or lost large amounts of Hydrogen after it formed. This latter scenario would require the planet to gain a Hydrogen envelope while at less than full mass in a hot nebula and then at full mass, in a cooler environment, lose most of its Hydrogen.
It is much easier to explain the planet’s characteristics by assuming it formed outside the snowline, never gained much of a gas envelope in the first place and spiraled into its present position. If it was formed from icy bodies like Ganymede and Titan (density ~ 1.9 gm/cc), this would give a good match for its density (2.7 gm/cc) allowing for gravitational contraction. The snow line is also the zone where carbonaceous chondrites form, so this would give the planet a higher carbon content than a pure rocky/iron one.
v) Madhusudhan, again from his interview, seems to think that K2-18b is an ocean planet, but I’m dubious about this for two reasons:
The first is that from the work done on Hycean planets by Lous et al, any depth of atmosphere especially with the potent greenhouse mix of CO2 and CH4 is likely to result in a runaway-greenhouse steam atmosphere inside the classically defined habitable zone (inside 2 au. for our sun).
The planet’s CO2/CH4 mix also points against this. From the paper, if there is a slight excess of Hydrogen over the stoichiometric ratio for water, then condensing H20 out, as either water or high pressure ice, pushes the planet’s atmosphere towards a Type A Hydrogen excess with no CO2 and NH3 lines appearing.
All of this would point towards a planet with a rocky/iron core overlaid by high pressure ice, which would, at about the megabar level, transition to a gas atmosphere composed mainly of super-critical steam. This would make up a significant volume of the planet. At the top of this atmosphere, the water, now in the form of steam, would condense out as virago rain leaving a dry stratosphere consisting mainly of CO2, CH4, H2 and N2.
To test my assumption, I did a rough back of the envelope calculation using online calculators, and looked at the wet adiabatic lapse rate (the rate of increase in temperature when saturated air is compressed) per atm. pressure doubling starting from 1 bar at 20°C. This rate (1.5°C/1000 ft) is considerably less than the rate for dry gases (3°C/1000 ft).
It was all very ad hoc, but the first thing I noted was that for each pressure doubling, the boiling point of water goes up significantly–at 100 bar, water boils at 300°C–until its temperature approaches its critical point (374°C) where it levels off. So the lapse rate increase in temperature chases the boiling point of water as you go deeper and deeper into the atmosphere; however, from my calculations, it catches water’s boiling point at 270°C and 64 bar. The calculations are arbitrary—I was using Earth’s atmospheric composition and gravity–and small changes in the parameters can result in big changes in the crossover point; but what this does point to is that if the planet has an ocean, it could be a rather hot one under a dense atmosphere, and if the atmosphere has any great depth then the ocean is likely to be a supercritical fluid.
Also, for the atmosphere to be thin, the planet’s ratio of CO2, CH4 and H2 must be less than 1/10,000 that of H20, which is not something I regard as likely, given what we know about the outer solar system.
I’ll leave you with a phase diagram of water with (red line) the dry adiabat of Venus moved 25°C cooler to represent a dry Earth and the wet adiabat (blue line) the one I calculated out. It’s also a handy diagram to play with as it gives you an idea of how deep the ocean or critical fluid layer will be at a given temperature before it turns into a layer of high pressure ice.
vi) One final point, and this reinforces the purpose of the paper: that we need to thoroughly understand planetary chemistry to eliminate false bio-markers. DMS is widely touted as a biomarker, but if we look at the most thermodynamically stable forms of sulfur: In a Type A reducing atmosphere, it’s H2S; and in a wet, oxidizing, Type B atmosphere, it’s the Sulfate (SO42-) ion. Unfortunately, the authors of the paper did not extend their thermodynamic analysis to Sulfur, but if we look at DMS’s formula (CH3)2S, it looks an awful lot like a good candidate for the most thermodynamically stable form of Sulfur for a Type C atmosphere, not a biomarker.
N. Madhusudhan, S. Sarkar, S. Constantinou, M Holmberg, A. Piette, and J. Moses, Carbon-bearing Molecules in a Possible Hycean Atmosphere, Preprint, arXiv: 2309.05566v2, Oct 2023
P. Woitke, O. Herbort, Ch. Helling, E. Stüeken, M. Dominik, P. Barth and D. Samra, Coexistence of CH4, CO2, and H2O in exoplanet atmospheres, Astronomy & Astrophysics, Vol. 646, A43, Feb 2021
N. Madhusudhan, M. Nixon, L. Welbanks, A. Piette and R. Booth, The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b, The Astrophysical Journal Letters, 891:L7 (6pp), 2020 March 1
Super Earths/Hycean Worlds, Centauri Dreams 11 November, 2022
Youtube interview of Nikku Madhusudhan, Is K2-18b a Hycean Exoworld? on Colin Michael Godier’s Event Horizon