Although generating tremendous public enthusiasm, Lowell's ideas were rejected by most astronomers. The dark areas, which could be seen to change seasonally, were now thought to be tracts of vegetation. The presence of oceans was no longer accepted, so the paradigm changed to an image of Mars as a. These observations, coupled with the fact that Mars has a 2. Early telescopic observers correctly assumed that the white polar caps and clouds were indications of water's presence. Mars Express orbiter has also provided essential data in this quest. Mars Odyssey, Mars Exploration Rovers (MERs), Mars Reconnaissance Orbiter (MRO), and Mars Phoenix lander have been instrumental in answering key questions about water's abundance and distribution on Mars. The damaging effects of ionizing radiation on cellular structure is another one of the prime limiting factors on the survival of life on the surface. In addition, Mars lacks a thick atmosphere, ozone layer, and magnetic field, allowing solar and cosmic radiation to strike the surface unimpeded. 8 billion years ago, Mars may have had a denser atmosphere and higher surface temperatures. Almost all water on Mars today exists as ice, though it also exists in small quantities as vapor in the atmosphere. An artist's impression of what ancient Mars may have looked like, based on geological data.
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