Life On Mars
For centuries people have speculated about the possibility of life on Mars due to the planet's proximity and similarity to Earth. Serious searches for evidence of life began in the 19th century, and they continue today via telescopic investigations and landed missions. While early work focused on phenomenology and bordered on fantasy, modern scientific inquiry has emphasized the search for water, chemicalbiosignatures in the soil and rocks at the planet's surface, and biomarker gases in the atmosphere.
Mars is of particular interest for the study of the origins of life because of its similarity to the early Earth. This is especially so since Mars has a cold climate and lacks plate tectonics or continental drift, so it has remained almost unchanged since the end of the Hesperianperiod. At least two thirds of Mars's surface is more than 3.5 billion years old, and Mars may thus hold the best record of the prebiotic conditions leading to abiogenesis, even if life does not or has never existed there. It remains an open question whether life currently exists on Mars or has existed there in the past, and fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries.
On January 24, 2014, NASA reported that current studies on the planet Mars by the Curiosity and Opportunity rovers will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic, and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have beenhabitable.The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.
Early speculation
Mars' polar ice caps were observed as early as the mid-17th century, and they were first proven to grow and shrink alternately, in the summer and winter of each hemisphere, by William Herschel in the latter part of the 18th century. By the mid-19th century, astronomers knew that Mars had certain other similarities to Earth, for example that the length of a day on Mars was almost the same as a day on Earth. They also knew that its axial tilt was similar to Earth's, which meant it experienced seasons just as Earth does — but of nearly double the length owing to its much longer year. These observations led to the increase in speculation that the darker albedo features were water, and brighter ones were land. It was therefore natural to suppose that Mars may be inhabited by some form of life.
In 1854, William Whewell, a fellow of Trinity College, Cambridge, who popularized the word scientist,theorized that Mars had seas, land and possibly life forms. Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent Martian canals — which were later found to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilization. This idea led British writer H. G. Wells to write The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet’s desiccation.
Spectroscopic analysis of Mars' atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen were present in theMartian atmosphere.[9] By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis
Past
Recent models have shown that, even with a dense CO2 atmosphere, early Mars was, in fact, colder than Earth. However, transiently warm conditions related to impacts or volcanism could have produced conditions favoring the formation of the late Noachian valley networks, even though the mid–late Noachian global conditions were probably icy. Local warming of the environment by volcanism and impacts would have been sporadic, but there should have been many events of water flowing at the surface of Mars. Both the mineralogical and the morphological evidence indicates a degradation of habitability from the mid Hesperian onward. The exact causes are not well understood but may be related to a combination of processes including loss of early atmosphere, or impact erosion, or both.
The loss of the Martian magnetic field strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability. When there was a magnetic field, the atmosphere would have been protected from erosion by solar wind, which would ensure the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars. The loss of the atmosphere was accompanied by decreasing temperatures. A part of the liquid water inventory sublimed and was transported to the poles, while the rest became trapped in a subsurface ice layer.
Observations on Earth and numerical modeling have shown that a crater-forming impact can result in the creation of a long lasting hydrothermal system when ice is present in the crust. For example, a 130 km large crater could sustain an active hydrothermal system for up to 2 million years, that is, long enough for microscopic life to emerge.
Soil and rock samples studied in 2013 by NASA's Curiosity rover's onboard instruments brought about additional information on several habitability factors. The rover team identified some of the key chemical ingredients for life in this soil, including sulfur, nitrogen, hydrogen, oxygen, phosphorus and possibly carbon, as well as clay minerals, suggesting a long-ago aqueous environment — perhaps a lake or an ancient streambed — that was neutral and not too salty. On December 9, 2013, NASA reported that, based on evidence from Curiosity studying Aeolis Palus, Gale Crater contained an ancient freshwater lake which could have been a hospitable environment for microbial life. The confirmation that liquid water once flowed on Mars, the existence of nutrients, and the previous discovery of a past magnetic field that protected the planet from cosmic and Solar radiation, together strongly suggest that Mars could have had the environmental factors to support life. However, the assessment of past habitability is not in itself evidence that Martian life has ever actually existed. If it did, it was probably microbial, existing communally in fluids or on sediments, either free-living or as biofilms, respectively.
Present
No definitive evidence for biosignatures or organics of Martian origin has been identified, and assessment will continue not only through the Martian seasons, but also back in time as the Curiosity rover studies what is recorded in the depositional history of the rocks in Gale Crater. While scientists have not identified the minimum number of parameters for determination of habitability potential, some teams have proposed hypotheses based on simulations
Source: en.wikipedia.org
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