For millennia, people looked at the sky and wondered about its mysteries. Observing celestial phenomena is a constant that unifies humanity throughout space and time. People followed the movements of the planets and the perpetual change of the constellations. They learned to read their changing patterns, and used them for navigation, agriculture, and warfare. The human hunger for exploration developed further, and man reached the surface of the moon. The search for life elsewhere sprung from a sudden cosmic loneliness that humanity experienced, and hence the question “are we alone?” From looking at the sky to probing other planets with robots, humanity is trying to decipher one of the biggest mysteries known to man. Taking into account the vastness of the universe, searching for life in the neighborhood would be the first step. But where in the solar system should explorers search for life, and what exactly to look for? To gain insight into where and how life could have happened, exploration begins with the only known example: life on Earth. Since Earth seems to be unique, life appears to be sparse. However, life on Earth is abundant and present in extreme environments, therefore it could have developed everywhere else in the universe. This paper provides an overview of the origins of life on Earth and its success in extreme environments as a possible suggestion that life could have developed elsewhere in the solar system, as well as a short research of plausible life on Mars, Europa, Titan and Enceladus.
Life on Earth started four billion years ago, as soon as the conditions on the planet became slightly favorable to ignite the spark of life. Favorable conditions allowed life to become firmly rooted in less than half a billion years. [1] To figure out how this happened and how did non-living matter turn into life, biologists imagined the process of evolution in reverse. Reversing evolution concluded that life on Earth started in simple forms, such as carbon-chained molecules capable of copying themselves. [2] These organisms did not have any bones or shell that could have been fossilized and preserved, and the evidence of their existence is scarce. However, ancient rocks dating over three billion years ago have revealed stromatolites, fossilized colonies of these single cell organisms. Life was thriving in oceans just a billion years after its beginnings. [3]
But what exactly is life? While there is no clear definition, one way of referring to life is by what living organisms do. Life is the process of extracting energy from the surrounding environment in order to maintain and reproduce an organism. [4] All life forms on Earth are carbon based and use water as a solvent. The cell is the basic unit of all living, the ambience in which the processes of life occur. Water is essential for the cell’s proper functioning. Liquid water acts as a solvent for various substances, enabling their transportation inside and between the cells, and it is an active participant in biochemical reactions. [5] Carbon is the best element for formic complex molecules. About all life’s biochemicals are compounds of carbon. Carbon in composition with other elements forms proteins, which have structural, transport, storage, and catalytic functions. The essence of protein synthesis is the RNA and DNA, complex organic compounds playing a role in the process by which organisms reproduce themselves. [6]
Life on Earth has also developed in extreme conditions. Hyperthermophiles live at high temperatures, between 80–121°C, in the high pressure environment of the ocean floor. Psychrophiles live in low temperatures of about -18°C in the deep oceans, glaciers, polar ice and snowfields. Halophiles live in salinities from 15 to 37.5 percent, alkalophiles live in the alkaline environment of soda lakes and chalky soils, and acidophiles are happy in environments with a pH close to that of vinegar. Some extremophiles can even live in level of radiation lethal to other species. [7] All these organisms are proof that life occurs wherever carbon and liquid water exist.
Having such a successful recipe for life on this planet, the search for life in the cosmic neighborhood starts with looking for the same ingredients: carbon compounds and liquid water. Potential habitats should be abundant in water and carbon, as well as other elements necessary in organic compounds, such as nitrogen. For the water to stay in liquid form, pressure should not be less than 610Pa. Temperature is also crucial: carbon based life can develop in environments with a temperature up to 160°C and down to several tens of degrees bellow 0°C. Habitats can exist either at the surface or below. [8] All these conditions create the so-called habitable zones.
The habitable zone is the range of distances from a star within which water occupies large territories and remains in liquid form on the surface of a planet. [9] It is however not enough for a planet to be in the habitable zone to develop life. Volatiles are needed to form an atmosphere. The planet has to have sufficient mass so that the atmosphere could provide pressure for water to stay liquid; it has to be geologically active; its atmosphere should not leak into space. [10] While Earth has been in the habitable zone throughout its history, Venus has lost its place, and Mars is at the outer boundary.
Water is an abundant substance in the solar system, whether in gaseous or solid form. However life requires liquid water. NASA's Mars Global Surveyor has revealed that some crater walls and hillsides on Mars seem to have formed recently, and liquid water below the surface could have burst out and created the landscape before evaporating. [11] Below surface liquid water is likely to be present also in the outer solar system. Ganymede and Callisto might have oceans trapped between two layers of ice, while Europa is believed to have oceans bellow its ice crust. [12] There seems to be liquid water in the solar system.
The key ingredients essential for life are nutrients and other elements needed for metabolism and reproduction, energy sources (sunlight, chemical reactions, internal heat), liquid water (or other liquid), catalysts such as enzyme proteins, and a stable environment. But life could exist beyond the classical habitable zone. Other habitats could be favorable for different life forms that would revolve around a different element than carbon. Some science fiction stories feature silicon-based life. Silicon belongs to the Group IV of the Periodic Table, together with carbon, and shares many characteristics: both have valence of four, bond to oxygen, form long chains (polymers), alternating with oxygen. However, carbon oxidizes into a gas (carbon dioxide), while silicon into a solid (silicon dioxide). Unlike carbon, silicon does not form many compounds that have handedness, and therefore it would be difficult for such a life form to achieve the functions that carbon-based enzymes perform. The chemistries of life do not seem possible in silicon. [13]
However, arsenic-based life forms have been discovered on Earth: the bacterium GFAJ-1 uses arsenic in its chemical building blocks. Marine algae were found that form organic molecules by incorporating arsenic. [14] Life obviously thrives in environments previously thought to be too harsh, and there is more than one chemical solution to the existence of life. Chlorine and sulfur, as well as nitrogen and phosphorus are also possible replacements for carbon. [15] Since carbon based life has proved to be successful, the search for life must start in the areas where such life forms may be possible. Mars is a candidate.
