Is This “Zombie” Plant the Secret Key to Finally Terraforming Mars?

The Study

The search for organisms capable of withstanding the harsh environment of space has long fascinated biologists and astronomers. While microscopic animals like tardigrades often capture the public imagination for their durability, the resilience of plant life is equally vital for the future of human space exploration. A recent study led by researchers at Hokkaido University has provided compelling evidence that the spores of a common moss, Physcomitrium patens, can survive prolonged exposure to the vacuum and radiation of low Earth orbit.

This research focuses on the fundamental question of how terrestrial life can adapt to extraterrestrial conditions. As humanity looks toward establishing long-term habitats on the Moon or Mars , the ability to cultivate plants for oxygen and food becomes a necessity. Understanding the limits of plant survival in space helps scientists select the best candidates for these future agricultural systems. The findings indicate that moss spores, protected by their natural casings, possess a robust resistance to extreme cold, radiation, and vacuum, potentially serving as pioneer species for terraforming efforts.

The study employed a combination of rigorous ground-based simulations and a nine-month exposure experiment aboard the International Space Station. By isolating different stages of the moss life cycle, the team identified the sporophyte stage as the most durable. This discovery offers new insights into the evolutionary history of land plants and their capacity to endure environmental stress, suggesting that the mechanisms allowing early plants to colonize Earth’s landmasses also equip them for survival in the cosmos.

The Model Organism Physcomitrium patens

The subject of this investigation is Physcomitrium patens, a moss species frequently used as a model organism in plant biology. It is recognized for its structural simplicity, ease of genetic manipulation, and fully sequenced genome. In the wild, this moss grows on muddy soil, but its evolutionary lineage traces back to the first plants that transitioned from aquatic to terrestrial environments approximately 500 million years ago. This transition required the development of mechanisms to survive desiccation, fluctuating temperatures, and ultraviolet radiation, traits that are theoretically useful for space survival.

The life cycle of P. patens includes several distinct tissues, each with different biological functions and stress tolerances. The researchers examined three specific types: protonemata, brood cells, and sporophytes. Protonemata are the juvenile, filamentous cells that grow from germinating spores. They represent the vegetative growth phase and are generally delicate. Brood cells are specialized, stress-induced structures that act as vegetative diaspores, allowing the plant to clone itself and survive difficult conditions.

The third tissue type, the sporophyte, is the diploid reproductive structure. It contains spores encased within a protective layer called the sporangium. In the natural life cycle, the sporangium shields the developing spores from the environment until they are ready for release. The research team hypothesized that this natural armor might provide superior protection against the rigors of space compared to the exposed cells of the protonemata or brood cells.

Simulating Space Conditions on Earth

Before sending samples to orbit, the team conducted extensive ground-based experiments to determine the baseline tolerance of the different moss tissues. These tests simulated specific aspects of the space environment, including high doses of ultraviolet C (UVC) radiation, deep freezing, extreme heat, and vacuum conditions. The goal was to identify which tissue type offered the highest probability of survival.

The first stressor tested was UVC radiation. While the Earth’s atmosphere filters out most solar UVC, it is abundant in space and highly damaging to biological DNA. The protonemata showed high sensitivity, dying completely when exposed to a dose of 10 kilojoules per square meter. Brood cells fared better, with some surviving moderate doses, but they succumbed entirely at higher levels. In sharp contrast, the spores encased in sporophytes exhibited remarkable resilience. They retained a germination rate of 27 percent even after exposure to 12 megajoules per square meter, a dose roughly 1,000 times higher than what the brood cells could withstand.

Freezing tolerance was another primary concern. Space environments and planetary surfaces like Mars experience extreme cold. The researchers exposed the samples to negative 80 degrees Celsius for up to 30 days. The protonemata died within four days. Brood cells showed declining survival rates, with none surviving the full 30 days. The spores remained robust, maintaining an 80 percent germination rate at the end of the month-long freeze. Further tests at negative 196 degrees Celsius showed that while survival dropped, some spores could still germinate, highlighting their capacity for cryopreservation.

Heat tolerance tests yielded similar results. When exposed to 55 degrees Celsius, protonemata and brood cells died within four days. The spores maintained a 36 percent germination rate even after 30 days of continuous heat. This ability to withstand elevated temperatures is significant for storage and transport logistics in space missions, where thermal control might fluctuate.

The final ground test involved exposing the sporophytes to a vacuum and vacuum ultraviolet (VUV) radiation. VUV has a shorter wavelength than UVC and poses a significant threat in space. The spores showed a 99 percent germination rate after 29 days in a vacuum. Furthermore, they withstood VUV doses exceeding what would be accumulated during a year in low Earth orbit. These comprehensive ground tests confirmed that the sporophyte stage was the only viable candidate for the actual spaceflight experiment.

The Tanpopo Mission

Following the successful ground validation, the researchers prepared for the space exposure phase. This experiment was part of the Tanpopo mission, an astrobiology initiative conducted on the Japanese Experiment Module, known as KIBO, on the International Space Station. The objective was to expose biological samples to the raw environment of space and assess their survival upon return.

The samples were housed in specially designed aluminum exposure units. These units featured magnesium fluoride windows, which are transparent to ultraviolet light, allowing the full spectrum of solar radiation to reach the samples. Inside the units, the moss sporophytes were secured in small wells using a specialized adhesive derived from cyanobacteria. This natural glue ensured the samples remained in place during the intense vibrations of launch and the microgravity environment of orbit without introducing toxic chemicals that might skew the results.

To differentiate the effects of UV radiation from other space factors like vacuum and temperature fluctuations, the team prepared four distinct experimental groups. The “Ground Dark” group remained in the laboratory on Earth as a control. The “Space Dark” group was flown to the ISS but shielded from all light. The “Space Non-UV” group was exposed to space sunlight but covered with a filter that blocked UV radiation. Finally, the “Space UV” group was exposed to the full space environment, including direct solar UV radiation.

The exposure unit launched aboard a Cygnus spacecraft in early 2022. Once docked, the unit was placed on the Exposed Facility outside the KIBO module. There, the moss samples remained for 283 days, orbiting the Earth approximately every 90 minutes. During this period, they experienced rapid cycles of heating and cooling as the station moved between sunlight and shadow, along with constant exposure to cosmic rays and the hard vacuum of space.

Survival Rates in Orbit

After nine months in orbit, the samples were returned to Earth via a SpaceX Dragon capsule. The researchers immediately extracted the spores from the recovered sporophytes and sowed them on nutrient agar to observe germination. The results were strikingly positive. Spores from all four experimental groups successfully germinated and grew into healthy protonemata.

The “Ground Dark” control group exhibited a 97 percent germination rate, providing a baseline for maximum viability. The “Space Dark” group showed a 95 percent germination rate. This high survival rate indicates that the combination of microgravity, vacuum, and temperature fluctuations alone had a negligible impact on the spores’ ability to revive. It suggests that the physical environment of space, absent radiation, is not inherently lethal to these dormant plant structures.

The “Space Non-UV” group, which received visible light but was shielded from UV, also achieved a 97 percent germination rate. This finding reinforces the conclusion that vacuum and general cosmic radiation background are not the primary limiting factors for moss survival over this timeframe. The visible spectrum of sunlight in space did not cause significant harm to the reproductive viability of the samples.

The “Space UV” group, which faced the harshest conditions, showed a germination rate of approximately 86 percent. While this represents a statistically significant decrease compared to the controls, it is an unexpectedly high survival rate. Despite the bombardment of high-energy photons that can shred DNA and destroy cellular machinery, the vast majority of the spores remained viable. The reduction in germination confirms that UV radiation is the primary stressor in space, but the high survival rate proves that P. patenspossesses robust defense mechanisms.

The Protective Role of the Sporangium

The exceptional survival of the spores is attributed largely to the sporangium, the multicellular structure that encases them. This casing acts as a physical barrier, shielding the spores from direct exposure to vacuum and attenuating the intensity of incoming radiation. The researchers hypothesize that the sporangium functions similarly to a seed coat in vascular plants, providing a layer of “maternal” tissue that sacrifices itself to protect the next generation.

The sporangium likely employs both physical and chemical defenses. Physically, the cell layers absorb a portion of the UV radiation before it can penetrate to the spores inside. Chemically, the tissues may contain antioxidant compounds like flavonoids and carotenoids. These molecules are known to absorb UV light and neutralize reactive oxygen species, which are harmful byproducts of radiation exposure. The study suggests that this dual protection strategy is a key factor in the moss’s extreme tolerance.

This finding has evolutionary implications. Bryophytes, the group of plants that includes mosses, were among the first to colonize land. The development of the sporangium was likely a pivotal adaptation that allowed these early plants to survive the harsh, unfiltered sunlight and desiccation of the terrestrial environment. The fact that this same structure provides protection against the even harsher environment of space suggests that the evolutionary toolkit for land survival overlaps significantly with the requirements for space survival.

Pigment Degradation and Hyperspectral Analysis

While the spores survived, the exposure to space was not without consequence. The researchers used hyperspectral imaging to analyze the pigment composition of the returned sporophytes. This non-invasive technique allowed them to measure the levels of chlorophyll and carotenoids without destroying the valuable samples.

The analysis revealed that chlorophyll a, the primary pigment responsible for photosynthesis, was significantly degraded in the space-exposed samples. The levels of chlorophyll a dropped by approximately 20 percent in both the “Space UV” and “Space Non-UV” groups compared to the dark controls. This indicates that the degradation was caused by exposure to intense visible and infrared light, rather than UV radiation specifically.

Chlorophyll a is known to be sensitive to photodegradation. In the intense light environment of orbit, where there is no atmosphere to scatter solar radiation, the pigments absorb more energy than they can process, leading to the generation of reactive oxygen species that break down the chlorophyll molecules. Interestingly, levels of chlorophyll b and carotenoids remained relatively stable.

The degradation of chlorophyll in the outer sporangium layers does not necessarily affect the viability of the spores inside, as the spores are dormant and do not actively photosynthesize until germination. However, the loss of pigment in the protective casing serves as a biological dosimeter, recording the intensity of the light exposure. It also highlights an important consideration for active plants in space: prolonged exposure to unfiltered solar radiation can bleach photosynthetic pigments, potentially hindering growth if not managed.

Comparative Resilience in Astrobiology

To place the resilience of P. patens in context, it is useful to compare it with other extremophiles known to science. The bacterium Deinococcus radiodurans is famous for its ability to withstand massive doses of radiation. However, in terms of UVC tolerance, the moss spores demonstrated a lethal dose threshold significantly higher than that of D. radiodurans.

Specifically, the study calculated that the dose required to kill 90 percent of the moss spores was approximately 24.5 megajoules per square meter. This is orders of magnitude higher than the tolerance of most bacteria and fungi. It also exceeds the tolerance of many vascular plant seeds, such as those of the sunflower. Some seeds, like those of Arabidopsis thaliana (thale cress) and tobacco, have shown higher tolerance in other studies, but the moss’s performance is nonetheless top-tier among biological organisms.

Microscopic animals like tardigrades, while resilient to vacuum and desiccation, generally have lower UV tolerance than the dormant spores of P. patens. The difference likely stems from the presence of specialized UV-screening pigments in plants that animals lack. The study suggests that for long-duration exposure on the exterior of spacecraft or on planetary surfaces, plant spores and seeds may be more durable carriers of life than even the toughest microscopic animals.

Implications for Terraforming and Life Support

The demonstrated resilience of P. patens has significant implications for the concept of terraforming and the development of bioregenerative life support systems (BLSS). Terraforming involves modifying the atmosphere and surface of a planet to make it habitable for Earth life. Mosses are considered ideal pioneer species for this process because they can grow on rocky substrates, help create soil, and produce oxygen.

The ability of moss spores to survive transport through space with minimal protection reduces the logistical burden of moving biological material to places like Mars. If spores can withstand the journey and the initial harsh conditions, they could be seeded to begin the long process of soil formation. Bryophytes are efficient at carbon fixation and can thrive in low-light conditions, making them suitable for the dimmer sunlight found on Mars or in shaded craters on the Moon.

In the context of life support systems within spacecraft or habitats, mosses offer an alternative to algae and higher crops. They require less complex infrastructure than vascular plants and are easier to manage than liquid algae cultures. The study confirms that moss spores can be stored in vacuum or freezing conditions for extended periods and then revived, providing a reliable biological backup for oxygen generation and waste recycling systems.

Limitations and Future Inquiries

Despite the success of the experiment, the researchers acknowledge several limitations. The study focused on a single species, P. patens, and the results may not apply to all bryophytes. Additionally, the exposure duration was limited to nine months. While the germination rates were high, extrapolating survival out to years or decades requires caution. A linear regression model suggested the spores could theoretically survive for 15 years in space, but biological decay often follows non-linear patterns.

Another limitation was the inability to precisely quantify the light intensity that caused the chlorophyll degradation. While the degradation was measured, the exact flux of photons was not recorded for every moment of the exposure. Future experiments will need to include more detailed environmental logging to correlate specific stress events with biological damage.

The mechanism behind the sporangium’s protection also remains partially understood. While physical shielding is a major factor, the specific genetic and chemical pathways that confer such high resistance need further exploration. Comparing the gene expression of spores before and after spaceflight could reveal which repair mechanisms are activated during germination.

Evolutionary Perspectives

The study sheds light on the evolutionary history of life on Earth. The transition of plants from water to land was one of the most significant events in biological history. It required the evolution of a suite of protective traits to deal with UV radiation, desiccation, and temperature swings. The fact that a modern moss, which retains many primitive characteristics of those early land plants, is so well-suited to the space environment suggests that the hurdles of conquering land were, in some ways, a rehearsal for conquering space.

This “pre-adaptation” implies that the genetic toolkit for space survival already exists within the plant kingdom. It is not necessary to engineer entirely new synthetic organisms to find life capable of enduring space; nature has already done much of the hard work. By studying these primitive plants, scientists can unlock the secrets of stress tolerance that have been refined over nearly half a billion years of evolution.

Summary

The research conducted by the team at Hokkaido University marks a significant step forward in the field of astrobiology. By demonstrating that the spores of Physcomitrium patens can survive nine months of direct exposure to the space environment with an 86 percent survival rate, the study establishes moss as a prime candidate for future space missions. The findings highlight the protective power of the sporangium and the innate resilience of bryophytes. As humanity prepares to venture further into the solar system, these humble plants may well be the companions that help turn barren worlds into green, breathable habitats. The durability of life, forged in the transition from ocean to land, appears strong enough to bridge the gap between worlds.