Mars is the planet nearest to Earth that we are most likely to explore and send manned missions to. It also is the planet that we have the best chance of finding life on. For both these things water is needed so exploration of Mars by space probes has concentrated on looking for water.
Humans have searched for evidence of water on Mars for many years. In 1895 American astronomer Percival Lowell thought that he had seen canals on the surface of Mars. These were disproved as tricks of the eye. Spacecraft in the 1960’s and 1970’s reported that Mars was very dry. However, spacecraft in the 1970’s to the 1990’s started to report evidence of water, either past or present. The atmosphere of Mars is very thin, only 1% of the Earth’s atmosphere, so it cannot hold water droplets for rain. But plenty of evidence was found of huge, catastrophic floods, such as channels on the surface, sedimentary layers and lake beds, even though these could have other explanations such as lava flows. Scientists have used their knowledge on how water shaped features appear on Earth to examine similar features on Mars and deduce the past or recent presence of water. Also geologists know which minerals are formed by water on Earth, so if these are found on Mars they indicate that water was once present. Also important in interpreting discoveries of water on Mars is the “triple point of water”. This is “the combination of pressure (6.1 millibars) and temperature (0.01 o
at which water can exist simultaneously as a liquid, solid or gas. http://science.nasa.gov/headlines/y2000/ast29jun_1m.htm The atmospheric pressure on Mars means that water is very close to the triple point of water. This means that it is very difficult for water to be liquid, rather than ice or water vapour. Liquid water cannot exist below 6.1 millibars, which is about the atmospheric pressure on Mars. Water as ice or water vapour is not useful for human exploration and the concern is that any liquid water would immediately evaporate. (taken from http://science.nasa.gov/headlines/y2000/ast29jun_1m.htm ) Introduction
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In 2002 NASA released its findings on water on Mars using the Mars Odyssey spacecraft, and was able to make the announcement that enough water to fill a large lake twice over was lying just below the surface on Mars, in the upper metre of soil. This was found by using an instrument called a gamma-ray spectrometer or GRS which detected hydrogen; a sign of water. The amount detected was 20% - 50% ice by mass in the lower layer which showed that the layer is possibly made up of ice and soil mixed. The hydrogen rich regions are found in very cold areas where the ice wouldn’t melt, in the north and south polar regions, which is another indication that the hydrogen is part of ice.
http://science.nasa.gov/headlines/y2002/28may_marsice.htm?list663144 The GRS works by detecting gamma rays and neutrons which fly out from the soil after cosmic rays hit the ground.
In 2002 the Mars Global Surveyor found evidence of water seeping from canyons and craters forming steep, dark gullies. Phillip Christensen of Arizona State University has proposed that these gullies are the result of trickling water from snow packs melting. If this is correct then this would be the first clue to there being fresh water flowing on the surface of the planet today. It seems that water ice could melt and flow beneath other ice and be sheltered from evaporating in the thin Martian atmosphere. This water would seep out and create these gullies. Apparently these gullies are very young as they haven’t been much eroded, and so point to water being recently present. Snow accumulates during cold periods and...
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