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A Case for Life on Mars
A multitude of arguments supporting the possible existence of life on Mars have surfaced after the discovery and examination of the ALH84001 meteorite. The polycyclic aromatic hydrocarbons (PAHs) found within, plus detailed examination of the ratios of certain metabolites, all have various interpretations supporting or opposing their organic origin.
In a recent NASA workshop, the results of careful sectioning, imaging, identification of mineral constituents, measurement of isotope ratios and analysis of the organic matter in the meteorite were compared. In the process of filtering out the evidence supporting microbes, new avenues of investigation are being conceived, leading to the current flourishing of exobiological methodology.
The geological record of Mars suggests that its environment was similar to the Earth's prior to about 3.5 billion years (Gyr) ago, when terrestrial life was first emerging. In particular, there is abundant evidence for liquid water, in the form of rivers, lakes and possibly even larger bodies of water, on the Martian surface at that time.
CO2 clouds can regulate the Martian climate by re-scattering infrared (IR) radiation, effectively warming the planet to habitable temperatures. As liquid water is thought to be essential for biology, the environment on early Mars may well have been favourable for the development of life.
Although some remain sceptical, the knowledge to date validates a concise effort to search for evidence of either past life or presently existing life. Recent research has revealed that there is active biota on Earth living in environments once thought impossible.
Underwater vents, where the pressure approaches 100 atm and the temperature is 80-120 oC, provide everything necessary for organisms to live and flourish. Even more astonishing is the fact that a species of mite called tardigrades, less than half a millimetre long has been found to be able to survive boiling, freezing and exposure to a vacuum.
The results show that these microscopic animals, can withstand pressures of up to 6000 atmospheres by entering a state of suspended animation, which can be maintained for more than a century.
To accomplish this they reduce their body weight by 50% or more, accompanied by an almost total loss of water using a sugar called trehalose to stabilise their cell membranes. Such evidence points towards a re-evaluation of our current beliefs on how essential liquid water is to the development and preservation of life.
Examination of terrestrial biota reveals that life-forms have invaded an enormous variety of 'non-optimum' niches, for example cold polar regions and desert belts, where the essential ingredients for life are rare.
The simple conclusion therefore is that one of life's most conspicuous properties is its aggressive versatility, which arises from its fundamental ability to create new experimental organisms during evolution. Life on modern Mars would admittedly be very challenging, the largest obstacle being the lack of ample liquid water which would hinder metabolism, mobility and reproduction.
All other obstacles though, like UV, ionising radiation and lack of organic compounds are all routinely accommodated through numerous strategies adopted by terrestrial organisms. Similar strategies may have been developed by organisms on Mars and could have manipulated the natural resources to the same effect.
The notion that photosynthetic organisms would not be able to survive on the surface due to UV radiation should be reconsidered after considering that the natural environment on Mars could be used to attenuate UV radiation with only minor attenuation of visible light.
A number of natural absorbers are readily available on the planet, such as nitrate, nitrite and sulphate salts. Fe-containing silica glass can be used as a cuticle protecting cell contents. Furthermore, the ferric iron (Fe203) in the environment may have protected phototrophs in ancient microbial mats in shallow aquatic environments.
Recent in-situ analysis has demonstrated regolith chemical constituents that allow the above cases to manifest themselves on Mars. Volcanic sources in conjunction with gases produced by the action of UV photons on the atmosphere, would allow chemolithoautotrophic life forms to exploit solar energy without the need for direct capture of photons.
The combination of an appropriate energy source with an effective UV shielding mechanism thus demonstrates how an organism can take advantage of a multitude of niches existing in its environment.
In addition, there is growing evidence that cross-contamination between Earth and Mars may have occurred, leading to the possible conclusion of a single origin of life in the solar system, the panspermia theory. This hypothesis alone forms a realistic scenario supporting the case for life on Mars.
Certainly new evidence is demonstrating to us that life has a profound ability to adapt to extreme and variable conditions and it is entirely plausible that if life once existed on Mars, it may have found a niche and indeed still exists.
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Microorganisms Grow At Low Pressures: And Maybe On Mars
Fayetteville - Aug 26, 2002
Using a unique device known as the Andromeda Chamber to simulate conditions found on Mars, University of Arkansas researchers discovered that certain microorganisms called methanogens could grow at low pressures.