INTRODUCTION
Mars serves as a primary focus for astrobiology research not solely due to its similarities with our planet, but also because of other technical reasons. Billions of years ago, Mars hosted liquid water and boasted an environment having significant similarities to Earth. Surviving on Mars poses daunting challenges due to its scarcity of liquid water, extremely low temperatures, a thin atmosphere rich in carbon dioxide, gravity only a third of Earth's, excessive surface radiation, and absence of organic nutrients. Ensuring secure habitation on Mars for future missions necessitates reducing reliance on Earth's resources. This goal requires enhancing astronauts' mental and physical well-being, implementing effective safety protocols, and optimizing energy and resource utilization.
Craterhab, a concept introduced by Mareekh Dynamics, illustrates a colossal composite-fabric dome of high-strength cables constructed over small craters on Mars to a achieve micro-terraformed environment on an otherwise uninhabitable world. Martian atmosphere mimics a vacuum, featuring significantly lower pressure compared to Earth's. To sustain life on Mars, the Craterhab must withstand an internal pressure of 0.5 to 1 bar. Maintaining a pressure level of approximately 0.5 to 0.6 inside the dome will reduce strain while sustaining an optimal balance. This pressure setting is conducive not only to attain long-term habitability of humans in a shirt-sleeve environment inside Craterhabs without the confines of a space-suit, but also to the successful growth of plants in the Martian greenhouse (Hussain, 2023).
To ensure the optimal survival of humans on Mars, a comprehensive variety of nutrients must be available to the crew residents throughout their stay. Moreover, crops in the greenhouse need to meet nutritional standards and consistently yield sufficient food.
Figure 1: A.I. Illustration of Martian Greenhouse by Mareekh Dynamics
2. CHALLENGES AND GROWTH CONDITIONS REQUIRED
The crop cultivation in the Craterhab, or similar large-scale habitation environments will be done either by through modification of Martian regolith to turn it into soil or through a hydroponics system, in which plants are grown in nutrient-rich water (Siranush Babakhanova, 2019). This vigilantly designed system will provide the best conditions for plant growth, keeping factors like light, temperature, nutrients, and plant spacing under high consideration. Plant species selection will be crucial, as unique conditions of the farming Craterhab will be taken into account. The diameter will be around 500m, and translucent silicon-composite segments will allow 5-10% sunlight to reach the growing crops. An internal atmospheric pressure of 0.5 bar will be maintained and water extracted from the extensive Martian permafrost. As for the temperature, careful consideration will allow 10 to 20 degrees Celsius to be sustained inside the greenhouse.
Furthermore, around 50 to 60 % humidity is required to get rid of the dryness of the soil. However, that is only in case if plants are being grown in modified and nutrient-rich soil and not in a hydroponic system. The soil on the red planet lacks living microorganisms that help plants to growth and help in the fertility of the soil, hence, farmers on Mars will also need to augment the soil or hydroponic system with biologically rich materials (Cartier, 2018). There is still no sign of reactive Nitrogen (NO3, NO4) on Mars, although there is some evidence that it might be present. Nitrogen in reactive form is required heavily by the plants to grow well. Here on Earth, mineralization of organic compounds is the main source of reactive Nitrogen which does not occur on Mars (G. W. Wieger Wamelink 1. ,., 2014).
Given the progress in biotechnology, innovative research continues to unfold along with the introduction of GMOs (genetically modified organisms), hence now it seems quite feasible and much more likely to create an independent ecological life system on Mars (Tolentino, 2023).
Figure 2: Martian Craterhab Greenhouse Specifications
3. PLANT SELECTION
On Earth, plants have evolved to a much greater extent in order to sustain their growth and thrive under harsh conditions, such as less amount of sunlight, low-pressure environments, high altitudes, or even low-oxygen conditions. The advanced mechanisms of plants include adaptations in their morphology, physiology, and metabolism. According to the researchers, certain plants that are capable of flourishing under unfavourable environments show growth during the developmental stages, some even exhibit significantly improved results (Antonio Quesada Ramos, 2019). Therefore, providing the controlled CraterHabs greenhouse environment to these plants on Mars, with a more refined modified Martian soil, increases the likelihood of their successful growth.
Aerenchyma tissues are present in some plants, they can be defined as specialized plant tissue or air pockets that play a vital role in plant survival under different conditions by facilitating the gas exchange process in plants. Hence, those plants that respond to stressful conditions through the development of aerenchyma tissue might be well suited for the Martian greenhouse (Heng Zhang, 2020).
Plants thriving in low pressure environments
In response to low pressure, plants tend to alter their metabolic processes to survive. Antioxidant production is enhanced for the protection against oxidative stress caused by low oxygen levels. Plants can also shift their growth patterns in response to encounters with low-pressure environments. Reduction in shoot elongation and increase in root growth might be observed in order to enhance nutrient uptake and anchorage.
During a research experiment done on the Wheat plant, researchers observed that it was able to grow in pressures as low as 0.1 bar. The growth showed to be similar or a bit higher at 0.2 bar as compared to normal. Throughout the vegetative stage (plant growth between germination and flowering), the growth of wheat in low pressure was possible. As for the leaf appearance, it was similar in both low-pressure and high-pressure condition. In another study, wheat was grown under 50kPa low pressure, as a result the average rate of photosynthesis and transpiration of wheat enhanced by 9.23% and 11.54% respectively, compared to ambient pressure. On decrease of oxygen partial pressure, the rates of photosynthesis and transpiration continued to increase. The yield increased at 10.5 and 5.0 kPa oxygen partial pressure (Shuangsheng Guo, 2008).
Table 1: The contents of nutrients of wheat seed under 50kPa and 10.5 oxygen partial pressure and 5.0 oxygen partial pressure
The aforementioned Aerenchyma might be significant for the plants growing inside the Martian greenhouse, as they play a crucial role in efficient gas exchange like O2 and CO2. Moreover, these air pockets would also facilitate diffusion, ensuring that viable gas remains available for respiration and photosynthesis. A study showed carrot (Daucus carota) roots cultivated without aeration also had increased aerenchyma. This indicated the adaptation of the plant to cope with hypoxia, which might be a response to improve oxygen diffusion and maintain root function in lower oxygen conditions (Feng Que, 2018). All the metabolic adjustments Antioxidant production and growth pattern alterations will be facilitated by the maintenance of an internal bar pressure of 0.5 inside the Martian Craterhabs.
Plants tolerating cold temperatures
Another major challenge for agriculture on Mars is the frigid temperature, not only the extreme heat but cold can also be lethal, threatening to rupture plant cells while disrupting the vital processes. Nature has proposed multiple ways for plants to combat cold stress. These strategies offer potential ways for adapting them to the environment of Mars.
According to a study, the role of a certain transcription factor in plants, DREB1 (dehydration-responsive element binding protein1) acts as a command centre and triggers the expression of cold-resistant genes when temperature drops, leading to the production of proteins that protect molecules that stabilize membranes and lower the freezing point of cellular fluids. DREB1 activity can be boosted and genes can be manipulated to increase their expression, which will provide a broader spectrum of protection against cold. Aerenchyma tissues can also aid in regulating temperature, reducing oxidative stress from radiation, and also help in the enhancement of nutrient uptake from soil with some potential deficiencies (Aimen Nasir I. S., 2023).
Table 2: Adaptation of Transgenic Plants to Chilling Stress
Engineering cold-tolerant plants
Two cold-sensing pathways in plants were revealed. One is the Day/Night Pathway, which adapts to both sudden and gradual temperature changes and operates continuously to protect against harsh conditions. Next is the Daylight Pathway which is triggered by both rapid and gradual temperature alterations. This pathway stays active during the day. Further analysis and research on these pathways are important for Mars agriculture plans, it will be possible to engineer better cold-tolerant plants that will be feasible for icy Mars' climate (Kidokoro, 2017).
Table 3: Plants that are likely to grow on Mars due to their ability to thrive under harsh conditions
4. PLANT ADAPTATION TO MARS-LIKE CONDITIONS
The adaptation of plants to endure Mars stressors is by activation of stress-responsive genes, leading to adjusting osmotic levels and ion homeostasis regulations. To withstand extreme conditions, plants develop hydrophobic barriers i.e., waxes and cuticles to shield against water loss and sustain water balance in low humidity. Furthermore, cryoprotective glycoproteins and antifreeze proteins are also produced by some plants to protect against chilling and frost stress. To tolerate high temperatures and thermal stress, Heat-shock proteins (HSPs) are present in plants.
Sequestering heavy metals in vacuoles and producing compounds like phytochelatins and metallothioneins for detoxification, can help plants to cope with heavy metal stress. Antioxidant systems are also activated by plants to relieve oxidative damage caused by abiotic stresses. (Aimen Nasir, 2023). The stress tolerance of plants can be enhanced by genetic engineering techniques. For example, CRISPR/Cas9 can be used to improve the abiotic stress tolerance of plants. It enables them to endure the harsh conditions on Mars. Other than that, Aluminium is one of the common elements found in Martian soil. A trace amount of this element is beneficial for lowering the soil pH, however, excessive Aluminium content in soil results in plant toxicity. Aluminium-resistant genes are expressed by some plants i.e., wheat possessing the gene TaALMT1, barley (HvAACT1), and tobacco expressing HvAACT1 (P R Ryan, 2011).
Figure 3: Modified plants using CRISPR/Cas9
5. STRATEGIES FOR PERCHLORATE REDUCTION
Martian regolith contains an enriched amount of Perchlorate (ClO4−) which is toxic for plants. Therefore, the presence of perchlorate on Mars is a huge obstacle in the way of developing a successful agriculture system on the planet (Christopher Oze, 2021). There are some microorganisms that have the ability to degrade perchlorate via enzymatic reactions. Perchlorate degradation activity can be enhanced in bacteria through engineering. Microbial species such as, dissimilatory perchlorate-reducing bacteria (DPRB) are known to obtain energy under anaerobic conditions by reducing perchlorate. They belong to the Proteobacteria phylum, Dechloromonas sp. and Azospira sp (Mamie Nozawa-Inoue, 2005).
Bioremediation employs using microorganisms that play a crucial role in breaking down or transforming perchlorate into less harmful and toxic substances. This can be executed through the process of bioaugmentation, where specific perchlorate-degrading bacteria are introduced into polluted areas. Bio-stimulation can also be done here to solve the problem which involves optimizing the conditions and providing necessary nutrients to enhance the activity of perchlorate-degrading microbes. Phytoremediation is an eco-friendly approach that utilizes the ability of plants to uptake and metabolize pollutants, including perchlorate. There are some plants that have the ability to accumulate and degrade perchlorate in their tissues. With the approaches and techniques of genetic engineering, specific genes can be introduced in perchlorate-degrading microbes to further improve their efficiency.
Finally, another solution of perchlorate reduction can be through chemical and physical methods. By chemical methods, the use of reducing agents such as ferrous iron, zero-valent iron, or hydrogen gas to convert perchlorate into less harmful chloride ions is achieved. The physical methods involve ion exchange, electrochemical reduction, and membrane filtration, which can efficiently remove perchlorate from H2O sources (Christopher Oze, 2021).
6. GENOME STABILIZATION STRATEGY IN PLANTS AGAINST RADIATION
The investigation and study of plants that have adapted to radioactive outcomes of the Chernobyl disaster yield valuable and significant insights into how plants will alter and adapt to radiation exposure on Mars.
Homologous recombination (HR) is a DNA repair process that brings changes in the genome arrangement under harsh radiation stress, and plants in Chernobyl show reduced levels of HR. This demonstrates that there will be a shift towards non-HR mechanisms, which have the tendency to cause errors. However, there is an increase in recombination levels over time as shown by the Chernobyl plants. This indicates the potential adaptation of the plants to radiation exposure. Repair mechanisms evolve and there is a selection for radiation-resistant genotypes, although the changes in gene expression do not alter the DNA sequence still these changes are inheritable and provide a quick response to stress caused by unfavourable environmental conditions. Radiation-resistant crops can only be engineered if scientists’ study how Chernobyl plants adapted to radiation (Igor Kovalchuk, 2004).
Figure 4: Mutated cucumber plants near Chernobyl, a result of the 1986 nuclear power station explosion, serve as a unique study subject for understanding plant adaptation to Mars. (Image source: PUBLIC HEALTH ENGLAND / SCIENCE PHOTO LIBRARY)
7. MICROORGANISMS FOR MARS GREENHOUSE
Martian regolith contains heavy metals and metalloids, which will cause hindrance and pose possible challenges for plants to thrive under Mars soil. Not only that but there is also no known sign of reactive Nitrogen present on Mars. As mentioned above, plants require a reactive form of nitrogen (NO3, NH4) to grow as it is one of the essential minerals for them. On Earth, it usually comes from the breakdown or mineralization of organic matter, which is absent on Mars and the Moon as well. However, studies have shown that the reactive form of Nitrogen is part of solar wind and some material in our solar system, this might be the source of reactive Nitrogen on Mars. However, there is no such evidence to prove the presence of this important mineral on Mars. Apart from this, another expected source of reactive Nitrogen is the effect of lightning or volcanic activity (G. W. Wieger Wamelink J. Y., 2014).
A proposed solution to solve this problem and to improve the Martian soil fertility for plants can be the addition of microorganisms that have the ability to cause nitrogen fixation or perchlorate reduction in order to make the soil habitable for plants and crops. Extensive research has been done to figure out which microbial species are best suited to grow under the environment of Mars. According to research, the genus Pseudomonas is an intriguing candidate for further analysis. The study demonstrates the species of this genus are capable of metabolizing toxic compounds from soils and these microbes are considered denitrifying bacteria which causes Nitrogen fixation. One of its species Pseudomonas putida possesses metabolism which degrades organic compounds. This Gram-negative soil bacterium also has the ability to tolerate heavy metals and metalloids. Due to its environmental pollutant degrading activity, Pseudomonas putida is also used in bioremediation.
Figure 5: Significance of Microorganisms in Greenhouse on Mars
Another species of Pseudomonas known as Pseudomonas stutzeri is also a denitrifying bacterium and it can be easily found in the environment. This species almost has the same properties as Pseudomonas putida including pollutant degradation and interaction with toxic metals. The research has also shown another exciting possibility, both of these species P. putida and P. stutzeri were tolerant to environmental conditions similar to those of Mars.
The idea of utilizing nitrogen-fixing bacteria or denitrifying bacteria in Martian soil is an optimistic approach for ISRU (In-Situ Resource Utilization). These microorganisms can convert the atmospheric Nitrogen into reactive form of nitrogen which can be accessible to the plants. This may reduce the potential need for importing fertilizers from Earth.
8. WATER THROUGH MARTIAN PERMAFROST
In 2008, NASA’s Phoenix spacecraft reached the Arctic plains of northern Mars and revealed a landscape there that was quite familiar to the polar regions on Earth. The images showed a pattern of interlocking polygon shapes that usually form in permafrost seasonally, hence, portraying evidence of a large quantity of frozen water. The permafrost on Mars is the permanently frozen surface that consists of ice and soil (Richard B. Hoover, 2005).
Figure 6: Mars image courtesy NASA/JPL-Caltech/University of Arizona. Earth photograph courtesy Olafur Ingolfsson.
So, the polar regions of Mars, the northern and southern hemispheres are believed to possess permafrost. This is of interest to the scientists because it shows the possibility of water on Mars. Therefore, due to this there is a prospect to transform Mars from a barren land into a world where life exists and thrives at its best or perhaps a place where plants can also grow. The southern hemisphere of Mars is characterized by its extensively cratered surface, moreover, it is also known to have the oldest and best-preserved ice-rich permafrost on Martian surface (H.D. Smith, 2005). Subsequently, there will be a need for automated robotic machines with drills to dig up the surface and melt the ice. The heat inside Mars and the effective sun rays can also be used providing an excessive amount of life-giving water for plants and humans inside the Craterhabs.
Figure 7: Frost deposits in Louth Crater appears to remain through the year, as found in Mars Reconnaissance Orbiter HiRISE photos of the region. Credit: NASA/JPL/University of Arizona
9. Conclusion
A self-sufficient and productive agricultural system on Mars is essentially required for the establishment of a sustainable human settlement. This article explored the challenges and potential advantages associated with plant and crop cultivation on Mars within the confines of Craterhab. A suitable plant species must be considered that can thrive under the unique environment of Mars. Exploiting biological organisms like microbes for nitrogen fixation, soil remediation, and perchlorate reduction can enhance soil fertility as well. In future, the advancement of robotic technologies and automation tools can further support the idea of Martian agriculture. These systems can be very useful in reducing labour and can perform tasks such as robot-assisted planting, harvest management, and environmental control. The chance of human error is also reduced along with other risks associated with the environment with limited resources.
The idea of micro-terraforming on Mars within the Craterhabs represents a more practical, viable, and sustainable strategy. Through utilizing these controlled biospheres, much more feasible living conditions can be provided for a significant human population without disturbing the natural state of Mars.
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