General Astronomy/Life in the Solar System
Astrobiology is a relatively young approach to the study of life in the universe, with earth forming the basis of our knowledge and investigation. Astrobiology seeks to address many questions, including:
1. Are there other forms of life in the universe?
2. Are we alone in the universe?
Very importantly, astrobiology transcends many scientific field boundaries and creates an amalgam of interdisciplinary topics, ranging from biology, geology, astronomy, chemistry to planetary sciences, paleontology and physics. Astrobiology covers a huge breadth of knowledge; therefore, only a selected overview of the difficulties of extraterrestrial life and of the possibility and search for life beyond earth is given below. Many of the postulations are inconclusive.
Expected Restrictions & Prerequisites for Both Terrestrial and Extraterrestrial Life
- A source of energy such as from a nearby star is needed to drive endothermic, or energy-absorbing, reactions.
- Raw materials, predominantly carbon, build and form organic molecules. Other important elements for life on earth include hydrogen, nitrogen, oxygen, phosphorus, and sulfur (comprising the acronym SPONCH).
- Life requires some form of shielding from damaging radiation, implying the presence of an atmosphere.
- Also indispensable is the presence of a liquid or very dense gas as a solvent for biochemical reactions to occur quickly and stably. Liquid water, the life-sustaining universal solvent (at least on earth), entails certain atmospheric composition and temperatures.
- The characteristics of stars can also determine the capacity for life on surrounding, orbiting planetary bodies, namely through the location of the habitable zone. The habitable zone in a solar system is defined as the region around a star that is conducive to life, which usually implies regions with temperatures high enough to maintain liquid water. Earth falls under the habitable zone in our solar system. As the mass of a star increases, so does the distance of the habitable zone from the star. Also, stars hotter than the sun have habitable zones farther out while cooler stars have them closer in, but hotter stars produce excessively harmful ionizing UV radiation while cooler stars may “tidally lock” planets that are too close, meaning these planets will always show the same "face" to the star and hence have a very cold side and a very hot side. Stars both hotter and cooler than the sun therefore possess conditions hostile to life. The properties of a solar system’s star determine the likelihood of life’s existence on planets and/or moons. In turn, the planets and their moons' positioning relative to the habitable zone and to their star also influence their conduciveness to life. Locating habitable zones in the universe leads to discovering the possibility of life. However, the later-discussed planet Mars and moons in our solar system may shelter life even though they do not fall into the generally accepted habitable zone.
The difficulty of life on other celestial bodies is marked by several conditions. The atmospheric pressure and composition vary immensely and gravity is altered across different regions in space; for instance, Mars has only a tenth of earth’s atmosphere, its atmosphere is mainly constituted of carbon dioxide, and its gravity is only a third of earth’s. All the planets and moons currently being explored for life also face different solar and cosmic radiation systems than the earth since the bodies are farther out in the solar system. Though they are farther from the sun and so experience less solar radiation, none has an ozone layer to protect against life-damaging radiation although some do have an adequate atmosphere to offer some protection. Moreover, oxidants such as iron oxide made by UV radiation acting on mineral surfaces in the presence of atmospheric oxygen and extremely dry conditions are chemically hostile to many organic compounds. Space is overall a very inhospitable, bleak place for life. It includes a space vacuum, solar winds with protons, electrons, and alpha particles floating around, radiation, solar flares, cosmic radiation, and low temperatures, factors counter to the nourishing conditions on earth. 
Extremophiles and Their Role in Life Exploration
Though life seems only possible under specific conditions, extremophiles such as bacteria that live in high temperature, toxic, deep-sea hydrothermal volcanic vents on earth offer insight into life that could have developed on other planets that mostly share such extreme, inhospitable environments, which also can include freezing temperatures and minimal sun exposure. The range of conditions life here on earth has managed to flourish in reveals more outer space milieus as possible breeding grounds for life. A few species of halophilic or salt-loving bacteria have also been experimentally proven to survive exposure to the extreme space environment for about two weeks. This evidence inspires hope that life similar to earth’s extremophile microbes can exist in outer space on other planets or moons despite harsh conditions.
Other Solar System Candidates for Life
In 1976, a search for microbial life on the red planet was performed. Two Viking landers conducted three experiments on Martian soil to test for bacteria presence through: 1) detecting the fixation of carbon dioxide, 2) observing possible metabolic processes through radiorespirometry, and 3) measuring the production and uptake of carbon dioxide, nitrogen, methane, hydrogen, and oxygen. Some tests yielded positive results: one experiment demonstrated that carbon was indeed converted to some organic matter by Martian soil, but the materials were not necessarily living or biological and the matter was not confirmed as organic. The 1997 Pathfinder mission discovered magnesium, aluminum, iron, and phosphate in Martian rocks, which are all possible life-supporting materials. Methane was also discovered in Mars’ atmosphere. Methane is important because it is a building block molecule for compounds such as hydrogen cyanide, which are important for amino acids, which are vital for life. However, no conclusive organic matter has been found on Mars’ terrain.
Mars could have once had water but does not have any liquid water now because its air pressure is too low. Mars is also small, reducing its ability to retain a hold on its atmosphere and increasing the rate of heat loss, resulting in cold temperatures at -63 degrees Celsius, which further decreases the likelihood of surface liquid water significantly.
Some tentative evidence for the presence of water (and consequently maybe even life) possibly hidden under ice exists. Ancient craters that seem to have been eroded by water dot Mars’ landscape. The presence of craters suggests low to nil tectonic activity; however, some areas on Mars were smoothed over, therefore suggesting water activity. Dry, river-like features on Mars may have also been carved out by running water over long periods of time. The same water in the form of huge floods may have caused the broad channels observable on the dry planet.
Mars’ huge volcanoes such as Olympus Mons could contribute internal heat to sustain subsurface water. According to Bruce Jakosky, “there’s a possibility of hydrothermal vents [due to these volcanoes reminiscent of terrestrial extremophilic living conditions].”  Though carbon mostly as carbon dioxide, hydrogen, oxygen, and nitrogen were also found in the Martian atmosphere, increasing the possibility of life on the planet, still, no definite life forms have been uncovered.
The confirmed Martian Meteorite ALH84001 that hit and was recovered from Antarctica in 1984 could also shed light on the possibility of life on Mars. The meteorite consists of young, volcanic rocks with partially dissolved carbonate globules that indicate the infiltration of liquid water and with ancient fossilized bacteria-like objects about the size of viruses identified as nanobes. Nanobes have been hypothesized as the smallest life form and are filamental structures usually found in certain rocks and sediments. Magnetite and iron sulfide particles discovered in the meteorite could have been left behind by the bacteria, but compelling abiotic explanations do not allow them to coexist with partially dissolved carbonates. Furthermore, the apparent life forms may be contaminations from life on earth. All conclusions are tentative and heavily contested.
Europa is one of the four largest Jovian (Jupiter) moons found to be covered by a smooth layer of water-ice, suggesting a life-nurturing liquid ocean underneath global in extent. Though temperatures are a nippy -145 degrees Celsius, some possible factors may have kept the water from freezing. Tidal heating from friction due to Europa’s eccentric orbit around Jupiter has been proposed. The planet’s gravity pulls stronger on the near side than the far, creating tidal bulges that can crack the icy crust surface and heat the interior. Approximately 5–25 km thick ice sheets could also insulate the water beneath. Volcanoes deep down may harbor hydrothermal vents that provide an energy source to heat and maintain liquid water.
Europa has a very smooth surface and very few craters, which could not be the result of an atmosphere burning up or the weathering of the craters since Europa’s atmosphere is very thin, so the tentative explanation is ice covering an ocean that evens out the surface. Some parts of its surface show blocks of ice that are separated but seem to fit together like a puzzle; these icebergs could have been shifted by slushy or liquid water beneath. Ridges in Europa’s landscape suggest existent water seeping up the ice cracks, refreezing, and then forming higher and higher ridges. In 1996, the Galileo spacecraft detected a magnetic field on the planet’s surface, indicating that there must be some electrical conduction likely to derive from a salty ocean. The previously mentioned hydrothermal vents may be spewing and mixing energy, heat, and chemicals into Europa’s ocean, possibly fostering life. The aforementioned tides on Europa created by Jupiter’s pull on its surface could also mix life-supporting substances together in the ocean.
About the possibility of life on Europa, when Jupiter strongly irradiates Europa’s ice, which is shown by the Galileo spacecraft’s observations to contain carbon dioxide, the irradiated carbon dioxide can produce simple organic molecules such as formaldehyde that are steps towards life. As one of the fundamental preconditions for life is a source of energy, tidal heating and the decay of radioactive elements could provide such energy for underwater life near the volcanic vents. Life-supporting energy can also be supplied by high-energy particles from Jupiter’s magnetic field breaking apart molecules in the ice to produce hydrogen peroxide, oxygen molecules and hydrogen. Finally, in Europa’s water-immersed rocks, there is evidence of life-forming elements like those on other solar system candidates for life.
Hazy, orange Titan is Saturn’s largest moon and is the only moon in the solar system that possesses clouds and a thick, dense atmosphere that is about 1.5 times the pressure of earth’s atmosphere. It also contains many organic gases such as methane, ethane, nitrogen, and hydrocarbons, and possesses conditions that resemble those of early earth four billion years ago that allowed life to develop. Extremely tall mountains cap Titan’s landscape and have been recently found to contain methane as well. Dark spots on the moon’s surface may reveal lakes of liquid organic materials including methane and/or ethane.
The sun’s radiation reacts with atmospheric nitrogen and methane on the moon to produce the orange haze and a steady stream of organic substances such as black ethane that rains down from the sky, supplying more carbon-rich molecules that could generate life. The presence of methane gas in the air suggests that a localized reservoir of liquid methane and ethane slowly evaporates into gaseous form in the atmosphere. NASA Ames Research Center scientists believe that methane-producing microbes, methanogens, may flourish on Titan since the moon’s atmosphere and surrounding are so heavily saturated with the compound. However, liquid methane and ethane are much colder than liquid water by some 200 degrees Celsius and biochemical reactions would occur at painstakingly slow rates. These compounds are also far less capable as solvents compared to water and therefore much less life-inducing. Water is also missing from the picture due to freezing temperatures of -178 degrees Celsius. Long ago, Titan could have been warmer and had water and maybe even life. Water may still be under the presently icy and rocky surface, concealing signs of life.
Enceladus, another one of Saturn’s satellites, is currently the “search for life” priority hot-spot. Like Jupiter’s Europa, Enceladus has a very smooth, almost crater-free surface but which is caused by constant geological activities, which is characteristic of a relatively young planet or in this case, a moon associated with a planet. This inner, tiny moon of Saturn has a water vapor-based atmosphere and contains liquid water reservoirs approximately only a half kilometer beneath its icy south pole, implying a heat source originating from the south pole of the moon. Though general temperatures are well below freezing at -203 degrees Celsius, this south pole is actually a relative “hot spot” at -183 degrees Celsius. The south pole also has “tiger stripe” regions or cracks that are warmer spaces, signifying geothermal activity.
Water vapor and ice crystal plumes also from the moon's south pole were found to contain organic materials such as carbon dioxide, methane, and propane. Such plumes also suggest an organic soup in the oceanic reservoir. Like Earth, Enceladus could have underwater hydrothermal vents near its water reservoirs that produce energy to heat the water. However, how this small moon can generate so much heat is still puzzling. Perhaps an insulating shell containing water ice with gas particles could aid the retention of heat. The pressurized liquid water could also fuel the geysers that spew icy water matter into space that dissociates into hydrogen and oxygen.
These geysers may be erupting from superficial liquid-water pockets at above 0 degrees Celsius. Liquid water so near the surface rarely occurs on the moon, introducing more questions and speculations about life on the mysterious, distant moon. Currently, the NASA Cassini spacecraft has been taking high resolution photographs of Enceladus and confirming numerous theories interpreting radical findings on Enceladus. The spacecraft is scheduled to fly close enough for observations of the geysers in October 2008.
There may be many more planets and moons light-years away that are toeholds for life since we are turning up evidence for life on many planets and moons in our own neighborhood! Many of the diverse and often harsh planetary features discussed above that seem to suggest the existence of life broaden the conceptions and prospects of finding environments suitable for life throughout the universe, life not in the form of humanoid green-skinned Martians but of microbes perhaps distantly related to earth's extremophiles.
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