On planet Earth, most organisms draw their life-fuel from the Sun. Our plant life uses photosynthesis to convert the Sun’s rays into food, and our animal life forms consume these plants either directly or indirectly. The fact that the Sun is absolutely essential to life on Earth has led many scientists to conclude that a central star similar to our Sun and a planetary location similar to our Earth is necessary for life on any planet. However, examples of life forms on Earth living in extreme environments suggest that this may not be the case.
There are areas on Earth that do not receive any sunlight, requiring life to adapt and develop alternative sources of energy. In deep ocean environments where sunlight cannot penetrate, organisms subsist on the energy from deep sea vents attached to the ocean floor. The process of energy conversion they use is called chemosynthesis instead of photosynthesis, because they do not use sunlight but these deep sea chemicals as food. Similarly, in underground environments, such as deep mines, life must find a way to survive without using sunlight.
Some subsurface life forms use geochemical or geothermal processes to survive, similar to the deep ocean organisms. Others survive using radiolysis, a process where radioactive substances provide energy radiation instead of sunlight. Radioactive materials that can be found deep underground give off particles which are used to create food by these life forms. One example is a type of bacteria found 2 miles deep in a South African mine which consumes hydrogen formed by particles emitted by radioactive Uranium, Thorium, and Potassium. These organisms are rare compared to those that use the Sun’s energy, but they can be found in many places on Earth.
One reason these organisms are not as prevalent is because the radiation they consume is often dangerous to Earth life. The Sun’s radiation is relatively weak, but the abundant supply of sunlight make it a good source for metabolic processes. Ionizing radiation, on the other hand, is much stronger. This radiation can ionize, or strike an atom or molecule with enough force to knock off at least one of its electrons. This type of radiation can interact with DNA and cause irreparable damage, such as health problems or death. Despite these risks, exposure to ionizing radiation can enable some organisms to develop the ability to survive these conditions. Not only do some species develop resistance to the harmful effects, but they can also evolve to flourish under this radiation.
Melanins, naturally produced materials found in many species on Earth, can help species adapt to extreme radiation. One property of melanins is their ability to absorb radiation, acting as either a shield from harm or as a transducer, converting the radiation to energy to fuel life. Humans have melanins that allow them to absorb some of the Sun’s harmful radiation, which can result in sunburn. Specialized melanins can help organisms living in environments of extreme radiation from succumbing to harmful side-effects, and can allow the useful conversion of this radiation to energy. For example, several fungal species found in Arctic and Antarctic regions and the Evolution Canyon in Israel contain more melanin than other species and respond to ionizing radiation with enhanced growth.
Besides the ability of some species to use it directly, another way that ionizing radiation can contribute to life is through its interactions with ices. Experiments have shown that when ionizing radiation interacts with ice, particles are produced which can support life. This has dramatic implications for life on distant, icy bodies in space such as Jupiter’s moon, the ice-encased Europa. Given our knowledge of extreme life forms on Earth and ionizing radiation interactions with ice, it seems possible for life to form and survive on Europa.
This paper links one type of radiation to supporting radiolysis-based life for the first time. Galactic cosmic rays (GCRs) are extremely high energy particles originating outside of our solar system. This paper proposes three ways in which galactic cosmic rays may provide the source of life-giving radiation on planets.
One way for GCRs to permit life is through cosmic ray-induced radiolysis. Cosmic rays strike Earth’s atmosphere and explode in a shower of unstable particles, which quickly decay into smaller packets of energy. These smaller energy particles are some of the same particles that are produced by radioactive materials and consumed in the process of radiolysis underground on Earth. Life on other planets could potentially consume these secondary particles from cosmic rays in the exact same way that Earth life consumes them through radiolysis.
If a planet does not have an atmosphere to break up the impact of cosmic rays, the rays will be able to directly strike the planets surface and propagate underground. However, as the rays pass through the surface of the planet, they lose a huge portion of their harmful energy. As the rays progress deeper and become less harmful, this energy can also be used directly by specialized life forms to produce food.
A second way for cosmic rays to permit life is through organic synthesis. When cosmic rays penetrate into certain types of ice, they initiate diverse chemical reactions. Lab experiments have shown that several types of ice mixtures produce organic materials when exposed to cosmic rays. This is a significant result in the development of extraterrestrial life.
A third way for cosmic rays to support life is through the direct capture of ionizing radiation. Life can form melanins to protect itself from the harmful effects of this high-energy radiation. In the presence of this radiation, it is also possible that melanins will evolve the ability to transduce, or convert the energy for biological use.
While we know that it is feasible, and perhaps ideal, for life to form following the conditions present on Earth, we must also examine the possibility of other circumstances allowing life to flourish. Even on Earth, there are organisms that have formed in the most unlikely of circumstances; away from water, oxygen, sunlight. If it’s possible for life to form in the most seemingly inhospitable environments on Earth, it becomes much more realistic to consider the various ways for life to form in unlikely circumstances elsewhere.
It is not necessary to have a radiation source similar to our Sun; ubiquitous cosmic rays may be enough to form and sustain life anywhere in the Universe.
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