Astrobiology combines the study of the biological origin and evolution of life in addition to the study of extreme organisms (see extremophiles) in order to determine what the extreme limits of life are on Earth and possibly on extraterrestrial planets or satellites.
The goal of astrobiology is to determine if it is possible for life to survive on other planets or satellites in our solar system and beyond. Astrobiology includes research in the fields of biology, chemistry, biochemistry, geology, astronomy, planetary science and crosses over into other fields of science. Astrobiology is a fairly new science. NASA began the National Astrobiology Institute (NAI) in the year 1998. (Blumberg 2003)
Of particular interest to the field of astrobiology are the planet Mars, and the satellites of the outer solar system Europa, Enceladus, and Titan. These planetary bodies have what scientists believe are habitable environments with a range of temperature, pressure, and radiation that is tolerable to analogous life forms on Earth. These worlds also have the chemical and/or mineral resources available for organisms to metabolize, reproduce, and survive. Europa and Enceladus have an ice covered surface with a liquid ocean beneath. Both satellites have been photographed with water spewing out of the cracks in the crustal ice. NASA has reported that Enceladus has water and organic chemicals. (NASA 2008) These ingredients are essential to the study of astrobiology. Titan is the only known body in the solar system besides Earth to possess liquid that exists on the surface in rivers and lakes. However, the liquid on Titan is made up of methane and ethane. Astrobiologists know that liquid is essential to finding life on another world because liquid allows the chemical molecules to assemble into biological molecules. It is believed that assembly of such complex molecules can only happen with a liquid solvent. It has not been determined whether or not the liquid methane and ethane on the surface of Titan could be a solvent for complex biological molecules. On Earth this solvent is water. Recently, scientists have reported water may exist on the surface of Mars for brief periods of time. (LA Times 2013) If scientists prove liquid water is currently on the surface of Mars, it would be an ideal solvent for complex molecules to assemble into life.
Scientists know what is required for life to exist, but they have a hard time creating a true definition of what life is. Astrobiologists must determine what life is before they can unequivocally state its discovery on another world. This starts with an in depth study of the building blocks of life, organic matter in the universe, and extraterrestrial organic matter that is delivered to Earth from comets and meteorites as well as organic matter discovered on the aforementioned planetary bodies. A question that has concerned biologists for many years is, “when does chemistry become biology?” We must determine when molecules become life in order to determine if some of the groupings of compounds we may discover are biological or just interesting chemistry. Other issues involve the definition of life. As of yet, no true definition of life is mutually agreed upon. The more discoveries of extreme organisms scientists make, the more malleable the definition must become. This complicates the field of astrobiology.
Mars has been the focal point for astrobiologists since the inception of the science approximately 50 years ago. Since that time, NASA has sent a fleet of spacecraft to the Red Planet starting with the Mariner 4 mission in 1964. These spacecraft have all discovered different pieces of the puzzle for the field of astrobiology. The most recent success is the Mars Science Laboratory Curiosity Rover, which landed in August 2012. The most important piece of the puzzle so far has been the confirmation that liquid water existed on the surface of Mars and the evidence is mounting that it may still exist on the surface periodically. Mars has had liquid water throughout its history, some water was briny and some was fresh with a neutral pH. Briny water is water that has salts and mineral compounds within it. These compounds would be important for potential organisms to utilize as an energy gradient. Both types of water are important to the search for life on Mars. John Grotzinger is the Principal Investigator for the MSL Curiosity Rover Mission. He has stated, “We have found a habitable environment that is so benign and supportive of life that probably if this water was around you and you had been on the planet [Mars], you would have been able to drink it.” (NYT 2013) At the American Geophysical Union meeting held in San Francisco on December 9, 2013, scientists announced the Curiosity Rover discovered that Gale Crater once possessed a fresh water lake. Water is essential to life as we recognize it on Earth. Astrobiologists and NASA scientists have been using the “follow the water” slogan for several years. Each successive rover and lander sent to Mars has revealed pieces of the Martian water puzzle. It is now known the soil on Mars is made up of 2% water. Another clue for astrobiology discovered by the Curiosity Rover are the elements CHNOPS from her first drill sample. CHNOPS is the acronym used in the biological community that stands for the six essential ingredients to all life on Earth. These ingredients are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. As astrobiologists combine their data with the proof of water, habitability, and the essential ingredients for life, the evidence continues to mount for the potential of past or present life on Mars. (JPL 2013)
Mars is the most reasonable world to send astrobiologists to study at this time; however, with the discovery of extrasolar planets, we must hone our science to prepare for the future exploration of these worlds. Thousands of extrasolar planetary candidates have been detected and approximately 1,000 extrasolar planets have been confirmed. When applying this information to the Milky Way Galaxy, scientists extrapolate that there are 40 billion planets on the habitable zone of their parent star. (Petigura 2013) With all of the new data we are receiving and will gather in the future, the field of astrobiology is limitless.
Blumberg BS. 2003. “NASA Astrobiology Institute: early history and organization.” NCBI. Fall: 3(3):463-70. Accessed December 27, 2013. PMID:14678657 (Blumberg 2003)
NASA.gov. 2008. “Cassini Tastes Organic Material at Saturn’s Geyser Moon.” Accessed December 27, 2013. http://www.nasa.gov/mission_pages/cassini/media/cassini-20080326.html. (NASA 2008)
LA Times. 2013. “Does water still flow across Mars? Dark, mysterious tracks hold clues.” Accessed January 1, 2014. http://www.latimes.com/science/sciencenow/la-sci-sn-mars-water-salty-flows-equator-nasa-mro-agu-20131210,0,4096976.story#axzz2pCOrKOdb. (LA Times 2013)
New York Times. 2013. “Mars Could Have Supported Life Long Ago, NASA Says.” Accessed December 27, 2013. http://www.nytimes.com/2013/03/13/science/space/mars-could-have-supported-life-nasa-says.html?_r=2&. (NYT 2013)
JPL/NASA. 2013. “NASA Rover Finds Conditions Once Suited for Ancient Life on Mars.” Accessed January 1, 2014. http://www.jpl.nasa.gov/news/news.php?release=2013-092. (JPL 2013)
Petigura, Erik A., Howard, Andrew W., Marcy, Geoffrey W. “Prevalance of Earth-size planets orbiting Sun-like stars.” PNAS. 2013. 110(48) 19175-19176. Accessed December 27, 2013. doi:10.1073/iti4813110. (Petigura 2013)