Once upon a time there were two adjacent planets orbiting a run-of the-mill star in one of the arms of an unremarkable spiral galaxy. Both were warm, both were wet, both had substantial atmospheres, both had vulcanism, both had oceans, seas and rivers, and both were in or on the edge of their star's habitable zone. Life, we are certain began on one, but on the other - well we're not too sure. The planets in question are of course the Earth and Mars.
Everyone is fascinated by Mars. From an earlier less-informed age, science fiction by Ray Bradbury and Edgar Rice Burroughs, or the imagined canals of astronomer Percival Lowell has fired our imagination, and has ensured that the Red Planet now has a special place in both our hearts and popular folklore. The real Mars is even more fascinating however, and the planet's formation and history can be the subject of some fascinating speculation. Mars is still one of the few places in the solar system that humans can think realistically about exploring on foot.
Did life arise on Mars in its early past like it did on Earth? Even more speculative, did life arise on one planet, only to be transported by ejecta to the other after an asteroid impact? Many scientists think that life, well microbial life at any rate, protected from cosmic rays and a fiery entry into the Earth's atmosphere inside a space rock can traverse the vast distances between planets. One of the meteorites discovered on the snows of the Allen Hills of Antarctica showcased by NASA in 1996, and confirmed as Martian by isotopic analysis, contains tantalising crystal structures that may be either chemical in origin or fossilised bacteria (albeit very small bacteria!). Meteorite ALH84001 may surprise us yet.
Will future geologists as they explore Mars discover fossils in the sedimentary rocks that are so indicative of the planet's wet and warmer past? Did creatures swim in the seas and rivers of Mars - were they washed up on the now high and dry fossilised Martian beaches that we've identified with our Mars orbiters? Did they take the ultimate white-knuckle ride over waterfalls to dwarf Niagara in the Vallis Marineris, a gargantuan canyon the width of North America? As the late NASA astronomer and planetary scientist Carl Sagan (1994) speculated, "Now that would be a world to explore - unfortunately we are four billion years too late!"
Whether such speculation turns out to be confirmed, things started to go wrong about 3.8 billion years ago, about the time life got started on Earth. Mars is about half the size of the Earth so its interior began to radiate heat to space much more quickly and its core began to solidify. Without a molten iron core acting as a dynamo, any magnetic field surrounding the planet started to dissipate exposing the atmosphere and surface to the Sun's charged particles. Any tentative carbon cycle would grind to a halt too -- despite having the largest volcano in the solar system (Olympus Mons), vulcanism would cease, and with it any possibility of recycling the planet's carboniferous rocks.
In addition, with its gravity and hence escape velocity only 40% that of the Earth, and with no protective ozone layer, ultra violet radiation would pummel the Martian atmosphere disassociating water and carbon dioxide molecules into their constituent atoms with hydrogen and oxygen drifting off into space. With steadily decreasing atmospheric pressure, the Martian greenhouse effect would be thrown into reverse. Temperatures would plummet, the planet's remaining water would freeze either in permafrost or subterranean glaciers, and life, if it had existed would be forced to retreat into the last protected under- the-surface niches and habitats.
Is it still there, hiding in the caves of Mars or in the subsurface soils, clays and rocks, away from the desiccated, radiation-fried environment above? Is this the cause of the methane out gassing detected by NASA - or does this possible bio-signature have chemical or volcanic origins?
We know from a plethora of studies in some of the most inhospitable places on Earth such as Antarctica, deep in the oceans, in sulphurous volcanic springs, even in nuclear reactors and in solar radiation-saturated NASA hardware brought back by astronauts from the surface of the Moon that extremophiles are tenacious in the extreme! Once life has a foothold, extinguishing it is phenomenally difficult.
However, NASA/JPL's's two Viking spacecraft that touched down in mid-1976 gave inconclusive results in their analysis of the Martian soil. Gases were exchanged when a nutrient soup was added to the soil, but no organic molecules were found on the Martian surface. However, the Vikings were designed to detect only a small subset of possible life - that found on the Earth. There's no guarantee that extraterrestrial bugs will adhere to terrestrial rules.
NASA/JPL's Mars Science Laboratory is slated for launch in 2011, and with a battery of on-board tests and equipment may start to provide some more substantial tantalising evidence of the signatures of life. Previous unmanned spacecraft have participated in NASA's "follow the water" initiative --both the Spirit and Opportunity Mars Exploration Rovers have found abundant evidence of sulphate rocks formed in water and stratified sedimentary rocks exposed on the Martian surface. The Mars Phoenix lander found copious amounts of water ice underneath its landing site, and evidence of perchlorate-saturated water condensed on its legs.
Mars is still a fascinating, enigmatic and lovely world with wonders aplenty to keep our robot emissaries, and eventually astronauts busy for decades and centuries to come. Its river channels, waterfalls, lakes and seas may now be desiccated, and its warmest days may be barely above the freezing point of water, but finding life on the Red Planet has been a dream of humanity for centuries. And sometimes dreams come true.
Sagan, C., Pale Blue Dot: A Vision of the Human Future in Space, Random House, (November 1994)
Andy Fleming is the author of the Andromeda Child blog about astronomy and much, much more: http://andromedachild.blogspot.com/.
Andromeda Child features an eclectic mix of interesting articles, news, views and reviews on a diverse range of subjects including astronomy, cosmology, science in general, railways and transport, music, radio, television, movies, politics, psychology, philosophy, history, education and sport (especially cricket!) This list is not exhaustive, and of course I'm open to suggestions for topics for posts!
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