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 back bibliography Life In The Solar System And Beyond --- --- Today scientists believe (based on discovered fossils) that life on Earth began between 3.9 and 3.5 billion years ago. The earliest versions of life we have found in fossils were deposits of the microscopic structure build of bacteria (stromatolites), and individual cells dating back to 3.5 billion years. **//Modern days stromatolites growing in Shark Bay, Australia//** ||=  **//Stromatolites on the ancient reef. Abigail, Allwood Australia 3.43 billion years ago//** ||
 * = [[image:Modern_days_stromatolites_growing_in_Shark_Bay,_Australia.PNG width="376" height="193"]]

Also there are 2 other important pieces of evidence found related to the origination of life on Earth:
 * 1) Life originated on Earth very quickly after Earth cooled down
 * 2) Life expanded very quickly all over Earth

 --- This means that despite the diversity of life forms on Earth and the ability of some life forms to exist in extreme environments, there are several prerequisites essential for life to get started:

>> Basically, this experiment was conducted to prove the theory that: **Raw materials** + **Energy** = **Life molecules.**
 * 1) **__Liquid water.__** Water represents the environment where chemical can be transported, dissolved as well as participate in chemical reactions.
 * 2) **__Access to the bioorganic elements__** that can build extended structures (elements such as H,O,N,C).
 * 3) **__Source of energy.__** There are several sources of energy that were available on Earth in earlier times such as lightning in atmosphere, sunlight, chemical energy or the heat of geothermal activities.
 * 4) **__Stability of the environment.__** The stability of geological activities, climate, and the absence of disruptive catastrophic events. Extreme changes in temperature, water availability, and radiation could be critical for life form survival.
 * 5) __**Source of organic molecules.**__ They are the building blocks for more complex molecules (such as proteins and amino acids) that represent living organisms. There are several possibilities for the origination of organic molecules:
 * __//Chemical reaction.//__ In 1953 Stanley Miller conducted an experiment which replicated the primeval conditions on Earth and had produced the chemicals that were essential for life to begin. He assembled a closed system into which he pumped a mixture of gases (methane, ammonia and hydrogen). There was a flask filled with boiling water that created a water vapour and the gases were circulated around the apparatus. A high voltage electrical discharge was put through the mixture. The product of the chemical reaction was cooled and then participated again in the reaction. The experiment ran for several weeks. The created substance was analysed, it had around 9 amino acids, 2% of the simplest, glycine and alanine, and traces of 7 others.
 * __//The product of geothermal activities//__ and chemical reactions at the bottom of the ocean where elements from the mantle interacted with water.
 * <span style="color: #21116a; font-family: Georgia, serif;">__//Organic molecules came from comets, asteroids and meteorites.//__ Some of the molecules can be brought with those objects in the atmosphere; some can be created due to hitting shock waves (initiating chemical reactions) when they passed through the atmosphere.

<span style="color: #21116a; font-family: Georgia, serif;">Based on these conditions there are 4 main ideas as to where life originated on Earth:
 * <span style="color: #21116a; font-family: Georgia, serif;">__//‘**Ball of ice theory’**//__ – life started under thick ice that was protecting the first life forms from Sun radiation
 * <span style="color: #21116a; font-family: Georgia, serif;">__//‘**Warm water theory’**//__ – life began in small ponds and lakes
 * <span style="color: #21116a; font-family: Georgia, serif;">__//‘**Hot saucepan theory’**//__ - life started in hot geysers and volcanoes
 * <span style="color: #21116a; font-family: Georgia, serif;">__//‘**Brought to Earth theory’**//__ – life was brought to Earth by comets and meteors that crashed against the planet.

<span style="color: #21116a; font-family: Georgia, serif;"> --- These are the conditions for life to start, as we know it on earth. These therefore would be the conditions that need to be considered when searching for life elsewhere. --- Based on our knowledge of the life requirements on Earth, the potential habitat for life elsewhere must have liquid water, carbon and other elements found in carbon compounds, an energy source, and the right amount of temperature and pressure that will allow the presence of liquid water (higher pressure allows water to be liquid in higher temperature, however temperature and pressure has to be low enough, so it will not break carbon chemical compounds). The zone around a star where that criterion is met is called the habitable zone.

The criteria of the habitable zone in a solar system based on the known requirements would be:
 * <span style="color: #21116a; font-family: Georgia, serif;">The planet has to be located within the habitable boundaries to be at the right distance from the star to allow for liquid water (temperature)
 * <span style="color: #21116a; font-family: Georgia, serif;">The planet has to be solid to allow for a liquid-solid interface, as this will enhance the exchange between molecules.
 * <span style="color: #21116a; font-family: Georgia, serif;">The planet should have an atmosphere (this would protect the planet from star radiation and would maintain [[image:russeldiagram.PNG width="306" height="278" align="right"]]temperature on the planet).
 * <span style="color: #21116a; font-family: Georgia, serif;">The planet has to have sufficient mass. This ensures that the atmosphere could provide enough pressure for water to be liquid, the planet would be geologically active, and that the atmosphere would not leak off to space…
 * <span style="color: #21116a; font-family: Georgia, serif;">The star has to be a __main sequence star__ //(phase in star life where the star radiates energy into space, temperatures and pressures in the core of the star are sufficient that it can ignite nuclear fusion, converting hydrogen atoms into helium)// See figure on the right ===>
 * <span style="color: #21116a; font-family: Georgia, serif;">Also, the existence of massive planets in the system will be additional gravitational shield, along with the Sun and protect the inner planets located within the habitable zone from asteroids, as they will be pulled towards the objects with greater gravitational pull.

<span style="color: #21116a; font-family: Georgia, serif;"> --- In the Solar system, Earth is positioned in the habitable zone. Venus is the closed planet in the inner boundary of the habitable zone. The temperature on Venus today is 460°C. It is too high to maintain life. If life there existed earlier when the Sun’s temperature was lower, then any traces of it would be lost due to extensive geological activity on the planet. Mars’ temperature lies within the boundary of habitable zone. However it is 1/10 of the mass of Earth, which results in too little of an atmosphere for life on the surface. This means that liquid water cannot stay on Mars’ surface and there is not enough atmosphere to protect life from radiation. The photos of the surface and geological evidence suggest that Mars could have had better life conditions and had liquid water on the surface long time ago. However, the test of the Viking’s probes in 1976 failed to find proof of the existence of life on the planet. In 1996, the oldest (13000 years) meteorite with microorganism fossils was discovered in Antarctica which chemical signature has been identified as Martian. Also, recent photographs from the Surveyor suggest that Mars had liquid water deep below its surface.

--- But the habitable zone may not be limited only to planets. Large planets may produce enough energy to heat the orbiting moons. Also, the newest researches show that life can thrive in extreme environments without light or at extremely low temperatures (Antarctica) or extremely high temperatures (volcanoes). This means that the habitable zone may extend beyond the conditions that are assumed favourable for life (on earth).

<span style="color: #ffffff; font-family: Georgia, serif;">--- <span style="color: #21116a; font-family: Georgia, serif;">When scientists look beyond a solar system for habitable planets they begin developing detection methods based on the characteristics of our solar system and theories of how our solar system was created. --- Few planets have been found in the HZ, but in April 2007, one of the most Earth-like planets ever found was discovered. It is called Gilese 581c, and is 12,000 miles in diameter, (much larger than Earth which has 8,000-mile diameter). It orbits a massive red star called Gilese 581, located in the Libra constellation, 20.5 light years from Earth. Because it lies in the HZ, scientists believe it has a developed atmosphere and is entirely covered by oceans. However, Gilese 581c does have some things working against it. Its gravity is about twice as strong as Earth's, and it receives significant doses of radiation from its star. --- Scientists think that most the probable form of life on planets would be a bacteria-like organism. These organisms are in abundance on Earth, and they are the first organisms that developed on Earth and they continue to exist for more than 3.5 billion years. They can be found in extreme environments and possibly on celestial objects, demonstrating the fact that intelligent life is not a requirement for survival and is considered a trait in the evolutionary process by scientists. --- But how likely are we to find intelligent life, or how likely is other intelligent life to exist, specifically in our own galaxy, the Milky Way? To estimate the number of civilizations in our galaxy that have life, an equation was developed by Frank Drake in the 1960’s. This equation describes interesting variables that are interconnected with our science and knowledge of the habitable zone and stars, and other factors. They give insight into the parameters of the existence of life, and by looking at the variables <span style="color: #241b5f; font-family: Georgia, serif;">one by one, we see the many different questions and sciences astrobiologists are exploring now.
 * <span style="color: #21116a; font-family: Georgia, serif;">[[image:hz3.PNG width="276" height="260"]] || <span style="color: #21116a; font-family: Georgia, serif;"> --- Titan, Saturn's largest moon can be considered a place where life can exist. It has a dense atmosphere composed primarily of nitrogen, with low percentages of argon and methane, and it also contains traces of organic compounds known as liquid hydrocarbons. Titan has hydrocarbon lakes, oceans, and sand dunes. Scientists think that the creation of organic molecules is possibly a consequence of lightning in the atmosphere. Using data from the Huygens, a space probe carried to Saturn's moon Titan as part of the Cassini-Huygens mission in 2005, Spanish scientists have proven that Titan has electrical storms too. ||~ <span style="color: #21116a; font-family: Georgia, serif;"> ||

Written mathematically, here is the Drake Equation: --- (from left to right)
 * Nc** = **Rs** x **Fp** x **N** x **F1** x **Fi** x **Fc** x **L**


 * <span style="color: #241b5f; font-family: Georgia, serif;">**Nc** || <span style="color: #241b5f; font-family: Georgia, serif;">The number of intelligent civilizations we would expect to communicate with us technologically ||


 * <span style="color: #241b5f; font-family: Georgia, serif;">** Rs ** || <span style="color: #241b5f; font-family: Georgia, serif;">The number of stars in the galaxy (the first step in estimating how many solar systems there are) ||


 * <span style="color: #241b5f; font-family: Georgia, serif;">**Fp** || <span style="color: #241b5f; font-family: Georgia, serif;">The fraction of stars that have planetary systems (few of the existing stars in our galaxy are suited to support life. Some burn out very quickly, and some are unfortunately located in parts of the galaxy with powerful streams of energy, making life on nearby - planets impossible) ||
 * <span style="color: #241b5f; font-family: Georgia, serif;">**N** || <span style="color: #241b5f; font-family: Georgia, serif;">The average number of habitable planets within an average solar system that could support life. This can interpreted as the fraction of the average number of planets within a solar system that lie in the habitable zone, around the average main sequence star (stars suited to support life - Fp) ||

<span style="color: #241b5f; font-family: Georgia, serif;">We believe that we are not alone. This is part of being an intelligent, social being.
 * <span style="color: #241b5f; font-family: Georgia, serif;">**F1** || <span style="color: #241b5f; font-family: Georgia, serif;">From the planets that are suitable to support life, what fraction of them does life actually arise on? ||
 * <span style="color: #241b5f; font-family: Georgia, serif;">**Fi** || <span style="color: #241b5f; font-family: Georgia, serif;">The fraction of planets with life, on which intelligent life arises. Every species on Earth, including past and extinct organisms, evolved and survived where the environment was just right, for the right amount of time, in the right places, in the right time periods… This variable is so complex and has so many factors, most of it we don’t know about yet. However, we have accumulated some knowledge, as when I described habitable zone, and primitive conditions that need to exist for life to develop and perhaps evolve into more complex organisms. ||
 * <span style="color: #241b5f; font-family: Georgia, serif;">**Fc** || <span style="color: #241b5f; font-family: Georgia, serif;">The fraction of intelligent species that are interested in communicating with other, far away civilizations (Us!). Which of the intelligent species will develop technology to communicate across the Galaxy? Which of them will have enough time, enough push, and other conditions needed for the evolution of technologically smart species? ||
 * <span style="color: #241b5f; font-family: Georgia, serif;">**L** || <span style="color: #241b5f; font-family: Georgia, serif;">The average lifetime of such a civilization. ||