(Box) 3. Could it have happened twice?
Earth may not have been the only beneficiary of cosmic deliveries of organic materials, Mars may have been equally blessed. That Mars was bombarded with comets and meteorites is not in doubt; the scars, in the form of impact craters, are clearly evident. But exactly what those foreign bodies may have brought down with them has a bearing on the arguments raging over the most famous meteorite to have landed on Earth - ALH84001, the Martian meteorite that may or may not contain the remnants of alien life.
The battle over this small chunk of rock has been fought for more than two years (Chem. Br., September 1996, p20 and July 1997, p17), with much of the original evidence now looking far from convincing; for instance,supposed fossils of primitive microorganisms could result from crystallisation of shock-melted rock. Scott Sandford, co-leader of Nasa's astrochemistry laboratory and one of the expedition members that found ALH84001 in Antarctica, says that even he isn't convinced by the evidence.
The discovery of PAHs in the meteorite was also claimed as proof of the existence of life on Mars, with Nasa researchers arguing that these were the remnants of Martian biological activity. This conclusion has been heavily contested, with Luann Becker from Scripps Institute of Oceanography, California, US, suggesting that most of the PAHs in ALH84001 are due to terrestrial contamination by ice melt water. Becker also argued that if a small percentage of the PAHs are of Martian origin then they were simply brought to Mars by meteorites or interplanetary dust.
However, new research led by Simon Clemett of Stanford University, California, US, and reported in Faraday Discussions (July 1998, 109, 417), found no evidence of terrestrial contamination and concluded that all the PAHs in ALH84001 were probably of extraterrestrial origin. While these findings don't exactly support the notion of a biological basis for the PAHs, they do give credence to the idea that complex organic molecules fell on Mars. Whether, at some point, they developed into life is still entirely open to question.