English
中文Question 5
If extraterrestrial life did exist, the most likely place scientists would point to would be Mars. Comparatively, even though Mars with a core radius of 1700 Km compared to 3485 Km,1is smaller than Earth, it is the closest planet to Earth in terms of size and proximity to the sun. Also in common, are its surface topology of volcanoes, valleys, deserts and polar ice caps, and its rotational tilt and the existence of seasonal cycles. However, the present environment on Mars is unfriendly and the planet, unlike Earth, has not evolved to support life.
Today, the Martian environment is one that is extreme. With dry and cold conditions, what life would survive on a planet where the surface rarely exceeds above freezing point and the average temperature is -63ºC?1 Its atmosphere consists of 95.3%carbon dioxide, 2.7% nitrogen, 1.6% argon, 0.13% oxygen and 0.08% carbon monoxide.1 Loss of its magnetosphere leaves the Martian atmosphere unprotected from solar winds which erodes it surface.2 Consequently, this thin atmosphere does not trap any heat radiated from the sun to warm the planet, and its lack of ozone offers little protection from cosmic radiation. With little atmosphere to separate between the surface and space, there is a lack of atmospheric pressure, which sophisticated life form need.1 As a result, water cannot exist in a liquid state. The surface gravity on Mars is 3.7 m s-2 compared to 9.8 m s-2 on Earth, and the solar irradiance is 589 Wm-2 compared to 1,367 W m-2, respectively.Any previously abundant surface water has disappeared to form water ice below the surfaces of the northern and southern polar ice caps,leaving the terrain dry.3 Water ice as much as 1.8 Km wide and 3.5 Km deep has been detected below the surface of the northern and southern ice caps, respectively, by pulse radio waves from the Mars Advanced Radar for Subsurface and Ionosphere Sounding.3 It has been predicted that if these polar icecaps were to melt they would cover the whole surface of the planet in a layer of water up to 10 meters deep.3
Evidently, today the conditions on Earth are ideal for life. Interestingly, the current environmental conditions on Mars may not be too dissimilar to those on Earth 4 billion years ago and it is possible that conditions on early Mars were also favourable for the existence of life. Young Earth was undoubtedly inhabitable, but life did evolve from a very harsh environment of very cold temperatures, little oxygen or liquid water. Following the bigbang, the input of heat energy produced from radiation and impacting meteorites melted the Earth. Upon cooling, an atmosphere consisting of water vapour, carbon monoxide, carbon dioxide,nitrogen and methane formed. Water vapour condensed and torrential rains filled up basins in the surface, forming oceans. It was in this setting that life began. It is thought that life evolved from basic molecules e.g. methane, ammonium, water, hydrogen sulphide,carbon dioxide and monoxide, phosphate. Conventionally, the natural non-biological beginning of life on Earth has been described by the formation of bimolecules from small organic molecules followed by the self-organisation of these molecules into a sophisticated life-form. Energy from lightning, volcanic activity and ultraviolet radiation enabled this transformation. Photosynthetic cells would have introduced oxygen into the atmosphere, forming the ozone layer, which then shielded the Earth from ultraviolet radiation.The formation of polymers of proteins, built-up from amino acids developed the ability to reproduce and pass on genetic information from generation to generation. Natural selection influenced survival best suited to the environment and gave rise to the variety of life on Earth.
The similarities between early Earth and Mars, suggest that Mars could also have developed through the process of chemical evolutionin the same way, but something must have happened to prevent this from occurring. It has been predicted that water was abundant in oceans, riverbeds and lakes,2 signifying the planet’s potential to support life. It has also been predicted that the planet, like Earth today, was surrounded by a thicker atmosphere,keeping the environment warm. If the temperature had been warm enough to sustain liquid water, it may have been warm enough for chemical reactions to have occurred, favourable for life. However,the significant decrease in temperature due to the loss of the protective atmosphere would have bought evolution to a halt. In addition, irradiation from ultraviolet light would have destroyed any organic material.
If chemical evolution had occurred, it would have had more chance lower under the surface of the planet, where temperatures would have been higher and protection from ultra violet radiation would have been offered. Volcanic activity, which was widespread in the past, would have caused the recycling of elements between the surface and core of the planet. However, NASA’s Viking mission reported no findings of traces of organic matter beneath the surface and suggested that, if present, it would be oxidised into carbon dioxide by the highly oxidising minerals on the surface.2 This may explain the high percentage of carbon dioxide in the atmosphere. Volcanoes are now dormant and the absence of any tectonic activity removes the opportunity for the terrestrial recycling of carbon dioxide, which is paramount in the Earth’s atmosphere. While these conditions prevail on Mars, it is unlikely that chemical evolution, as hypothesized today would be occurring
The environmental conditions on Mars are extreme and would be hostile to the life we know on Earth. The planet has evolved to lack in the elements, which are essential for life on Earth. Although there are life forms on Earth, which can survive a harsh environment, if life did emerge on Mars, it would certainly have been much challenged and may be different from that we already know.
