Is there Life on Mars?
"We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time." - T.S. Eliot
Within the solar system, only the planet Earth is known to have indigenous life. But for centuries scientists have pondered whether other planets have life as well, all the while trying to understand the processes of biogenesis. Based upon the knowledge of life on Earth, biochemists had drawn the conclusion that Mars is by far the most plausible planet (besides Earth) that might be inhabited.
A number of biologists fully anticipated finding the molecular precursors of life-or, with long odds, perhaps some primitive organisms - on Mars. This conclusion led to an ambitious American program to search for life there. In 1976 two automated laboratories, the Viking landers, were sent to the red planet to perform this investigation. However, to the disappointment of biochemists, the Viking missions did not find terrestrial type of life at either of the two landing sites.
In 1984, biochemists around the world had a new hope with the discovery of a meteorite which appeared to have come from Mars. This meteorite was named ALH 84001. It was found in December 1984 in Antarctica by a U.S. meteorite-hunting expedition. When it was found, ALH 84001 weighed about 4 and 3/4 pounds (1.93 kilograms) ALH 84001 looked green inside, which really excited the expedition. But back at civilization, ALH 84001 looks much grayer inside than green. ALH 84001 is believed to have formed from molten lava, about 4.5 billion years ago, which is called its igneous age, possibly from an ancient Martian volcano.
It's debatable if ALH 84001 is from Mars, as people have never been to Mars and no rocks have ever been collected from its surface either. There are 11 other meteorites also, called the SNCs, that are believed to have come from Mars. The strongest evidence for their Martian origin is that they, including ALH 84001, contain traces of gases that is just like the Martian atmosphere. The composition of the Martian atmosphere is known because the Viking Lander spacecraft analyzed it, on Mars, in 1976.
From the Viking analyses, Mars' atmosphere is unique in the solar system, at least unique among the planets, atmospheres, and asteroids that have been sampled. In bulk elemental composition, Mars' atmosphere has unusual abundance of elements nitrogen (N), argon (Ar), krypton (Kr), and xenon (Xe) relative to each other. What's more interesting is that the Martian atmosphere is not like any other known source of gas. Also, the Viking landers found that Mars' atmosphere has unusual isotopic abundancies; it is rich in nitrogen with weight 14 (14N), and relatively rich in two isotopes that form during radioactive decay of other elements, 40Ar and 129Xe. It has also been showed by telescopic observation from Earth that the martian atmosphere is very rich in heavy hydrogen compared to the Earth. The presence of a "life-like" atmosphere on Mars therefore greatly enhances the chances of a possible existence of present and/or past life on the red planet.
However, some fundamental differences exist between the circulation patterns in the lower atmospheres of the Earth and that of Mars. Because Mars has no oceans, its entire surface responds very rapidly to change in incident sunlight. During the summer season in a given hemisphere, the hottest place on Mars is not at the equator, but displaced to the latitude where the Sun is directly overhead at noon. Most of the year through, however, the temperature at Mars reaches low extremes which is not feasible for life to exist. Mars' thin atmosphere also causes temperature just above the surface to drop significantly after the Sun sets. But when global dust storms occur on Mars, the highest dust-laden levels in the atmosphere does become substantially warmer. Thus, while the atmosphere of Mars is similar to that of the earth in some ways, the absence of essential elemental gases and the dilemma of extreme temperatures and pressures pose a serious question as to how life could, if it ever did, exist on the red planet.