Introduction
Illustrating enormous biodiversity, life on the Earth has been a prominent area of natural science research for centuries. The panspermia hypothesis is among the controversial theoretical propositions touching upon the possible origins of life on the planet and the existence of extraterrestrial life. Panspermia supposes that life in the universe can be distributed by means of microorganisms or life’s chemical precursors present in space that give rise to other living organisms when entering the right location. The ideas of life coming from space include the role of various moving bodies and objects, such as comets, planets, and even human-made vehicles traveling through space, in transporting life-initiating microorganisms. Panspermia-related propositions exist in several forms, each of which deserves a different amount of criticism from the scientific community. This report will review the evolution of the panspermia hypothesis and the mechanisms of life transfer that are not rejected as totally unrealistic.
Discussion
The Panspermia Theory’s Evolution
Panspermia has embarked on the path toward certification despite starting as a questionable idea. It began as Anaxagoras’s concept of “the seeds of life” permeating the universe (Kawaguchi 420). In the early nineteenth century, Svante Arrhenius continued the ongoing debate by hypothesizing that various microorganisms’ movement between planets is caused by radiation pressure from the Sun (Kawaguchi 420). In the 1970s, the age of science fiction, the hypothesis of life as a result of the deliberate transfer of seeds by extraterrestrial civilizations gained traction but never produced any viable evidence (Kawaguchi 420). The so-called soft or molecular panspermia emerged to argue that the pre-biotic building blocks of life form in space and are transferred to planets’ surfaces after entering the solar nebula and participating in condensation processes (Chukwudiegwu et al. 46). Another crucial proposed mechanism of life seeds’ transfer is the idea of lithopanspermia or microscopic spores traveling within meteorites, and it is currently considered as the most credible scenario (Kawaguchi 420). Every new proposition remains hypothetical and is imperfect in terms of verifiability because modeling the entire process of living organisms’ transfer in space poses a great methodological challenge.
Modern Understandings of Panspermia: Abiogenesis and Life Transfer as a Three-Stage Process
Soft Panspermia and the Detection of Sugars in Space
The soft panspermia hypothesis suggests that the building blocks of life originated in space, thus opposing the transition of living spores between various planets or constellations. The hypothesis, therefore, aligns with the theory of abiogenesis as the latter holds that life on the planet emerged from non-living substances (Chukwudiegwu et al. 46). Possible evidence for pseudo-panspermia comes from space exploration projects in which chemical substances that might serve as the materials to support the formation of life were found. Researchers have been studying meteoritic organic compounds since the 1960s, which resulted in the detection of four different biological sugars in meteorites, but the question of their extraterrestrial origin remained open (Furukawa et al. 24440). In their recent laboratory simulation experiment, Furukawa et al. conducted the analysis of sugars in C chondrites with reference to their stable isotope compositions and concluded the sugars’ ability to form in space (24440). As for the results’ implications for the theory of soft panspermia, extraterrestrial sugars could have contributed to the RNA’s formation, thus giving rise to life (Furukawa et al. 24440). Thus, the pseudo-panspermia hypothesis finds certain support in current research.
Lithopanspermia
The lithopanspermia hypothesis seeks to model interplanetary life transfer processes. The key proposition of the lithopanspermia theory is that microbial spores travel through space in meteorites, which creates optimal conditions for their survival (Kawaguchi 420). Despite the ongoing skepticism surrounding panspermia, this hypothesis gathers less critical reception since it considers the issue of ultraviolet radiation in space. Radiation in the interplanetary medium poses a crucial challenge to microbial spores’ life. However, the notion that traveling occurs within meteorites resolves this issue by explaining how microorganisms would travel long distances without losing the ability to reproduce (Kawaguchi 420). Another contributor to the theory’s relatively neutral acceptance is its reliance on the actually existing and observable phenomenon – the interplanetary exchange of meteorites (Kawaguchi 420). The meteors that surpass the Earth’s atmosphere and can reach the ground are often brittle. Their breakage could hypothetically enable dust and smaller rock pieces with microbial spores to enter the planet’s soil and give rise to new life (Kawaguchi 420). Therefore, lithopanspermia is among the relatively realistic mechanisms of life transfer in the universe.
Stage One of Panspermia and Microorganism Capture Experiments
The lithopanspermic hypothesis breaks the entire life transfer process into three separate stages, each of which is unique in terms of amenability to experimental validation. During stage one, microbes leave the donor planet, which would require them to survive rock ejection and remain alive despite severe temperature excursions and extreme shock pressures (Kawaguchi 420). Simulating this process with actual meteors is not possible, so the arguments for this stage’s possibility stem from microorganism capture experiments with air sampler devices and space aircraft. In 2008, Yang and colleagues extracted Deinococcus species from dust collected at the tropopause level (Kawaguchi 421). In other capture experiments at high altitudes, Bacillus species and non-spore-producing bacteria have been detected in air and dust (Kawaguchi 421). Therefore, the fact that cultivable microorganisms can leave the planet is used to argue for the first stage of the interplanetary life transition process as something possible.
Stage Two of Panspermia and Microbial/Extremophile Survival Research
The second stage of the hypothetical panspermia process centers on microbial spores’ space journey prior to reaching the recipient planet. During this stage, microorganisms are supposed to survive and maintain their ability to divide into daughter cells for a considerable amount of time before entering the other planet’s atmosphere. This process’s possibility has encouraged a heated discussion due to the existence of findings to show life’s survival in space and the barriers to it. On the one hand, since the 1960s, seven prominent exposure experiments with different microbial species have been conducted (Kawaguchi 423). In open space research, the best result for Bacillus subtilis spores without specific protections is three weeks of staying alive, but ultraviolet radiation still presents a major factor making their survival problematic (Kawaguchi 423). With protections that reduce exposure to radiation, such as covers or spores being assembled in multilayers rather than monolayers, Bacillus subtilis spores’ survival time can exceed five years (Kawaguchi 424). However, the need for special circumstances and stable physical shields against the Sun’s radiation could reduce these findings’ argumentative power in the panspermia debate.
Nevertheless, relatively recent microbial research in space suggests that the existence of extremophiles or organisms that survive in extreme environments lends credibility to the lithopanspermia hypothesis. In the Expose-E experiment, Xanthoria elegans, a lichenized fungus species capable of colonizing rocks, survived for fifteen months, and similar promising results were reported for Rhizocarpon geographicum (Kawaguchi 424). In their research within the frame of the Tanpopo space mission, Ott et al. exposed the cell pellets of Deinococcus radiodurans, an extremophilic radiation-resistant bacterium, to outer space environments (1). After thirty-six months in open space, Kawaguchi et al. reported that the bacteria were still alive (1). Interestingly, the degree of DNA damage and cell pellets’ protection from ultraviolet radiation led to the estimates that these bacteria would also survive up to eight years in space (Kawaguchi et al. 1). In rocks, as the authors conclude, the bacteria are anticipated to remain alive for several decades, which makes the lithopanspermia hypothesis more realistic (Kawaguchi et al. 9). Although extremophiles’ survival in space does not prove that life comes from space, it will facilitate panspermia modeling experiments in the future.
The evidence supporting the lithopanspermia hypothesis disproves the ideas of radiopanspermia effectively. The latter holds that the radiation pressure can transport microorganisms in space but does not consider the effects of ultraviolet radiation on spores’ viability (Chukwudiegwu et al. 46). Without at least minimal protection, such as rocks used as shields, microorganisms lose their viability and get deactivated relatively quickly (Chukwudiegwu et al. 46). Considering the amount of time needed to travel between different planets, the theory becomes extremely unrealistic. At the same time, lithopanspermia effectively addresses the drawbacks of the radiopanspermia model by introducing meteors as natural shields.
Stage Three of Panspermia
The third stage of the hypothesized life transfer process involves the landing of the microbial spores on the recipient planet. Compared to the preceding stages, the landing process has been researched relatively thoroughly within the frame of the lithopanspermia hypothesis. The key barrier to spores’ survival prior to landing is the heat entering the atmosphere. However, based on the example of the Alan Hills meteorite from Mars, about 0.3 cm of the meteorite’s surface melts during landing (Kawaguchi 425). For non-metallic meteorites, the temperature of the internal section of the rock is not expected to rise above 40°C during both landing and ejection, which would not pose risks to extremophiles’ survival (Kawaguchi 425). However, microbial survival and the ability to adapt to new environments in the post-landing stage are still under scientific investigation. Specifically, the European Space Agency’s team conducts experiments to expose terrestrial microorganisms to simulated Mars-like environments. Some preliminary results reported by Billi et al. suggest that Chroococcidiopsis cells would maintain their viability on Mars only for a few hours and need protection from radiation (158). Overall, there is much to be studied about the landing process.
Conclusion
In summary, despite remaining hypothetical, the original ideas of panspermia gave rise to life transfer models that can be partially tested or found support in current research. The three-stage lithopanspermia model holds that microorganisms survive ejection, remain alive in space, and land on the recipient planet safely due to using meteorites as shields. From experimental research, microorganisms can leave the planet and remain cultivable, and extremophilic organisms can survive in space for years. Based on the peculiarities of damage to meteors during falling, microorganisms would be able to survive inside of them. The ideas of soft panspermia also find certain support due to the detection of sugars originating in space. Nevertheless, these local arguments do not make panspermia accepted by the mainstream scientific community as the explanation of life’s origins.
Works Cited
Billi, Daniela, et al. “A Desert Cyanobacterium under Simulated Mars-Like Conditions in Low Earth Orbit: Implications for the Habitability of Mars.” Astrobiology, vol. 19, no. 2, 2019, pp. 158–169. Web.
Chukwudiegwu, Egbuim Timothy, et al. “Evolutionary Panspermia: Planets Micro-Life and Beyond.” Worldwide Journal of Multidisciplinary Research and Development, vol. 6, no. 11, 2020, pp. 45-52. Web.
Furukawa, Yoshihiro, et al. “Extraterrestrial Ribose and Other Sugars in Primitive Meteorites.” Proceedings of the National Academy of Sciences, vol. 116, no. 49, 2019, pp. 24440–24445. Web.
Kawaguchi, Yuko, et al. “DNA Damage and Survival Time Course of Deinococcal Cell Pellets during 3 Years of Exposure to Outer Space.” Frontiers in Microbiology, vol. 11, 2020, pp. 1-11. Web.
Kawaguchi, Yuko. “Panspermia Hypothesis: History of a Hypothesis and a Review of the Past, Present, and Future Planned Missions to Test This Hypothesis.” Astrobiology, edited by Akihiko Yamagishi et al., Springer Nature Singapore, 2019, pp. 419-428.
Ott, Emanuel, et al. “Molecular Repertoire of Deinococcus Radiodurans after 1 Year of Exposure Outside the International Space Station within the Tanpopo Mission.” Microbiome, vol. 8, no. 1, 2020, pp. 1-16. Web.