火星是否適合人類居住?

不適合

事實上，對於移居其他月球或火星的可行性，霍金的回答也顯得相當無奈。他這樣說：「……但月球和火星都很細小，而且缺乏或完全沒有大氣層，除非離開太陽系，否則找不到像地球一樣美好的地方。」人類生於地球，我們的身體只為適應地球生態而生，離開了天然的生境地，去到任何一個星球，當然都不及地球美好。

火星的體積不到地球的六分之一，火星大氣比地球大氣稀薄得多，主要成分是二氧化碳，佔95％，其餘3%是氮，氧氣和水蒸氣含量極微，它的平均溫度只有－30℃左右，約等於地球南極冬季的平均溫度，至於火星究竟有沒有水？有，不過全都結成了冰。火星這樣「溫和」氣候，是否吸引到你？
就算他日發現到有一個和地球一模一樣的星球，能夠適合人類居住，但我們仍然要無止境不顧一切地追求經濟增長和物質生活，繼續採礦、砍伐森林、燃燒化石燃料、製造各式各樣的化學品，歷史只會重演。地球是我們的家園，文明社會把這個家弄得污煙瘴氣，然後棄之，再到別處佔據別人的家園。這種想法充份反映文明社會的思維模式，以發展和進步之名，首先破毀本土的森林和天然環境，然後到別國開發當地的森林和土地。現在，地球很快再無地可開發、或再不適合生物或人類居住，就開始打其他的星球主意。我們又是否真的有權去侵佔其他星球？

The current understanding of planetary habitability—the ability of a world to develop and sustain life—favors planets that have liquid water on their surface. This most often requires that the orbit of a planet lie within the habitable zone, which for the Sun currently extends from just beyond Venus to about the semi-major axis of Mars. During perihelion Mars dips inside this region, but the planet's thin (low-pressure) atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water, however, demonstrates the planet's potential for habitability. Recent evidence has suggested that any water on the Martian surface would have been too salty and acidic to support terran life.
The lack of a magnetosphere and extremely thin atmosphere of Mars are a greater challenge: the planet has little heat transfer across its surface, poor insulation against bombardment and the solar wind, and insufficient atmospheric pressure to retain water in a liquid form (water instead sublimates to a gaseous state). Mars is also nearly, or perhaps totally, geologically dead; the end of volcanic activity has stopped the recycling of chemicals and minerals between the surface and interior of the planet.
Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there is still unclear. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites, and had some apparently positive results, including a temporary increase of CO2 production on exposure to water and nutrients. However this sign of life was later disputed by many scientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have found life.

2009-09-13 22:33:25 補充：
A re-analysis of the now 30-year-old Viking data, in light of modern knowledge of extremophile forms of life, has suggested that the Viking tests were also not sophisticated enough to detect these forms of life.

2009-09-13 22:33:51 補充：
The tests may even have killed a (hypothetical) life form. Tests conducted by the Phoenix Mars Lander have shown that the soil has a very alkaline pH and it contains magnesium, sodium, potassium and chloride.

2009-09-13 22:34:19 補充：
The soil nutrients may be able to support life, but life would still have to be shielded from the intense ultraviolet light.

2009-09-13 22:34:43 補充：
At the Johnson space center lab organic compounds have been found in the meteorite ALH84001, which is supposed to have come from Mars.

2009-09-13 22:34:53 補充：
They concluded that these were deposited by primitive life forms extant on Mars before the meteorite was blasted into space by a meteor strike and sent on a 15 million-year voyage to Earth.

2009-09-13 22:35:16 補充：
Also, small quantities of methane and formaldehyde recently detected by Mars orbiters are both claimed to be hints for life, as these chemical compounds would quickly break down in the Martian atmosphere.

2009-09-13 22:35:21 補充：
It is possible that these compounds may be replenished by volcanic or geological means such as serpentinization.

不太適合
因為火星需然有大氣層、水
但它的大氣層大多是二氧化碳,也十分薄,它的溫差也很大,一天可以尤50℃變成-400℃,

適合
因為有水
而水係由氧和氫組成
所以適合人類居住