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Debate: Life on Mars

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Has there ever been organic life on Mars?

Background and context

A meteorite, found in Antarctica in 1984 and known as ALH 84001, gives the main fodder to scientists wishing to establish a history of organic life on the planet Mars. It is believed to be around 4.6 billion years old and to have been covered with micro-organisms soon afterwards (soon being between 0.5 and 1 billion years). Around 16 million years ago, scientists believe, a comet or asteroid struck Mars causing fragments such as ALH 84001 to be thrown into space; this one fell into the Earth’s atmosphere some 13,000 years ago. ALH 84001 is a 1.9kg igneous plutonic rock, consisting of c. 95% orthopyroxene grains, c. 2% chromite, c. 1% maskelynite, c. 1% carbonate and other trace elements.

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Argument #2

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Yes

‘Polycyclic aromatic hydrocarbons’ (PAHs) are benzene rings which occur as the result of the decay of micro-organisms on Earth. PAHs were discovered on ALH 84001, and therefore it is likely that micro-organisms decayed there too. Great care was taken to ensure that terrestrial PAHs did not contaminate the meteor while it was being examined.

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No

These PAHs could have been inserted deep into the meteor while it lay in Antarctica. As it is black, it would warm up in the sun; melting snow may have seeped into its fissures, concentrating PAHs on the inside, and those left on the outside would be destroyed by ultraviolet light giving the impression that it brought its own samples internally.

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Argument #1

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Yes

Although ALH 84001 was originally thought to be a diogenite (from Vesta), a 1994 study highlighted many petrologic features more commonly identified with orthopyroxenite, a type of martian. Its oxygen isotope composition is also indistinguishable from the martian Nakhlites.

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No

The very age of ALH 84001 makes it unlikely to be a martian rock. The other examples of the same clan (SNCs) of martians are no older than 1.3 billion years, and some as young as 180 million years. The similarity of its petrologic features with other martians is circumstancial rather than compelling.

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Argument #3

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Yes

Inside fissures in ALH 84001 are carbonate deposits or ‘globules’. Magnetite and iron-sulphide particles inside the globules are chemically, structurally and morphologically similar to magnetosome particles produced by bacteria on Earth. Some studies show that these carbonates were formed at ‘cool’ temperatures, below 212 degrees - which allows for the birth of bacteria.

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No

Several studies on the globules conclude that they were formed at temperatures in excess of 1,200 degrees - far too hot for life.

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Argument #4

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Yes

Inside the meteor’s globules are what appear to be fossil remnants of bacterial organisms, measuring between 20 and 200 nanometers. These could be the caused by dissolution of the carbonates, themselves precipitated by marine bacteria.

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No

These fossil forms could also be the remains of nannobacteria from Earth, which entered the meteor after it landed. Similar fossils were found in Viterbo, Italy, and deposited by hot springs.

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Argument #5

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Yes

In 1976 the Viking robots from NASA landed on Mars, scooped up soil, heated it to 500 degrees C for 30 seconds and analysed the vapour. Their conclusion was that it was a cold, arid desert, with no signs of carbon-based molecules.

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No

In the ultra-violet light of Mars, carbon-based molecules would have been dismantled or transmuted into something else - the most stable of which would be carboxylic acid which takes longer than 30 seconds to turn to vapour. A new test in 2003, by the British lander Beagle II, may detect this substance.

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