Using chemical, solar, or nuclear rockets, 6-9 months are required for a one-way voyage to Mars. Fourth from the Sun and never closer to the Earth than 56 million kilometers, the Red Planet has baited human astronomers and fiction authors for centuries and tantalized space explorers for decades.
Early telescopic observers learned that Mars has polar caps, like the Earth. Some detected seasonal changes and occasional clouds. At the limits of resolution, a number of influential nineteenth-century astronomers reported a network of fine lines on the planet's surface—the legendary canals.
By the early twentieth century, these observations and science fiction of authors, including H.G. Wells and Edgar Rice Burroughs, firmly entrenched the idea of a Martian civilization in the human psyche. On this small and arid world, a fictitious ancient civilization had constructed canals to divert water from the polar regions to the equatorial cities. But as the atmosphere was escaping and the water reserves were emptying, Martian interplanetary imperialists began to turn their attention to the third planet from the Sun, our fertile Earth. Fortunately, it is now evident that the canals were illusory and that indigenous higher life forms do not exist on Mars.
Unlike the Moon with its month-long "day" and the NEOs with their widely variable rotation rate, the Martian "Sol" is only slightly longer (40 minutes) than the terrestrial day. But that's where the similarity between the two worlds ends.
Mars has an exceedingly thin atmosphere, with a surface pressure less than 1% that of the Earth and a surface gravity about 38% that of Earth. Although the polar caps do indeed have some water ice, their major constituent is dry ice—frozen carbon dioxide. This is not surprising since CO2 is the predominant gas in the planet's thin atmosphere and frigid temperatures near the Martian poles are low enough to cause it to freeze.
Impact craters abound on Mars, as do enormous ancient volcanoes. One of these giants, Olympus Mons, dwarfs Mount Everest in both girth and height. Another major feature is Vallis Marineris, an equatorial canyon system far greater in size than North America's Grand Canyon.
Most tantalizing of all Martian surface features are the unique landforms produced by ancient rivers and seas. At some time in the distant past, Mars shimmered with abundant surface water, perhaps resembling a somewhat smaller version of the Earth.
Recent evidence from Martian rovers and orbiters indicates that some portions of Mars have been water-covered comparatively recently. Water may indeed be common in subsurface layers of the planet's regolith. One trace constituent of the Martian atmosphere is methane, which is a byproduct of terrestrial biological processes. This data, in combination with the ambiguous bacteria-like structures found in the mid-1990s in a meteorite from Mars, has led many exobiologists to the conclusion that Martian life was abundant in the past and may still exist in subsurface locations or caves today.
Future robotic and human expeditions to the Red Planet are planned, many ofwhich will search for life on Mars or return samples directly to the Earth. Hopefully, we will know before too many decades have elapsed whether Gaia (Mother Earth) has a near neighbor in the solar system.
The search for Martian biology is a double-edged sword for would-be colonists of this planet. Early human expeditions to the Red Planet will almost certainly be driven by astrobiology. But if Martian life is actually discovered with a tenuous foothold in caves or below the planet's surface, a furious ethical debate is sure to ensue. Some will argue that Mars should be declared a preserve for indigenous Martians, no matter how primitive and that the human technological ability to cross the void is not a divine right to impose our biology on a distant biosphere. Others are sure to respond with the Darwinian argument that terrestrial life forms, having demonstrated the ability to survive interplanetary travel, are more fit to survive than the residue, stay-at-home Martians. Also, as happens on the Earth, where life that adapted to extreme environments such as the polar caps, deep-ocean floor, or upper atmosphere, can coexist successfully with our form ofabundant surface life, perhaps Martian and terrestrial life forms can successfully share the Red Planet.
If people do ultimately settle on Mars, the first settlements may be shielded from cosmic radiation by Martian soil offering shelter not provided by the planet's atmosphere. The domed cities of science fiction may be constructed later as terraforming efforts begin the thickening of the atmosphere. After hundreds or thousands of years, perhaps with the aid of greenhouse gases imported from the Earth, the Martian atmosphere might be thick enough and the surface warm enough to support free-standing bodies of water, forests, parks, and outdoor agriculture. Although Mars may never be a twin of the Earth, it may ultimately become a second planetary biosphere in Sol's system.
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