Kurt Vonnegut said it all in Sirens of Titan. Or maybe we are part of a galactic (or inter-galactic) double blind study.
arthur -----Original Message----- From: Keith Hudson [mailto:[EMAIL PROTECTED] Sent: Monday, July 14, 2003 12:04 PM To: [EMAIL PROTECTED] Subject: [Futurework] Life from Mars? Some FWers might be interested in an article fom this week's New Scientist: <<<< BORN LUCKY Against all the odds, life establoished itself remarkably quickly on earth. Does this tgell us anything about where it began? Yes, says theoretical physicist Paul Davies, and the answer is out of this world. Paul Davies It's a shame that there are precious few hard facts when it comes to the origin of life. We have a rough idea when it began on Earth, and some interesting theories about where, but the how part has everybody stumped. Nobody knows how a mixture of lifeless chemicals spontaneously organised themselves into the first living cell. It may have been a straightforward sequence of unexceptional chemical processes, or a bizarre accident. The chief bugbear is that we have just the one sample of life to study -- Earth life. And the mere existence of life on Earth tells us nothing at all about how likely or unlikely it is, or whether it has happened elsewhere. The great hope for astrobiologists is that another sample of life will be discovered somewhere beyond Earth, giving us a crucial comparison. But while we wait, there is one key fact about life on Earth that does provide us with an important data point, a fact that could tell us something about where it all began. This is the often-discussed fact that life started up with almost suspicious haste. Following the birth of the solar system 4.55 billion years ago, the planets were pounded mercilessly by huge asteroids and comets. It is believed that around this time a horrendous cataclysm occurred when a rock the size of Mars slammed into Earth, creating the moon. This would have melted the planet and left the surface roasting hot for tens of millions of years. Even smaller impacts would have caused mayhem by swathing the globe in incandescent rock vapour, creating a searing heat blanket that would have boiled the oceans dry and sterilised the rock deep into the crust. The record of impact craters on the moon suggests that this hellish barrage began to abate only about 3.8 billion years ago. From the record of the rocks, most palaeobiologists are convinced that microbial life was flourishing on our planet as long as 3.5 billion years ago. Fossils from the Pilbara region of Western Australia imply that colonies of fairly sophisticated bacteria were already hard at work. The Pilbara fossils remain the oldest clear traces of life on Earth, but presumably there would have been an extended phase of evolution preceding this. Studies of carbon isotopes in Greenland rocks dating as far back as 3.85 billion years have hinted at still earlier traces of life, though the evidence for this is hotly contested. While the fossil hunters continue to follow the tenuous thread of life on Earth further and further back in time, it is already clear that life established itself almost as soon as the cosmic bombardment tailed off. Can this simple fact tell us anything about the origin of life? Many scientists believe it can. The late Carl Sagan cited the rapidity of life's appearance as evidence that it must be easy to make, and might therefore be expected to arise on many other planets in the galaxy. The argument seems intuitively correct, but how much can be inferred from a single sample? Statistically speaking, the thorniest problem is that we haven't a clue what the probability is of life forming from scratch on a planet like ours. So we can't use this single example to infer whether life was inevitable or a sheer fluke. We can, however, use it to turn hand-waving arguments into probabilities. Last year Charles Lineweaver and Tamara Davis of the University of New South Wales applied a statistical analysis to the problem (Astrobiology, vol 2, p293, 2002). Their argument is best explained by analogy with winning a lottery -- with life being the jackpot. In most lotteries punters have a good idea of what their chances of winning might be. But suppose a gambler enters some weekly lottery with no idea at all of the odds. If the gambler won the prize after, say, three weeks, what would he infer? He might have just got lucky and won a million-to-one prize on the third attempt. But most people would infer that the odds were very much lower -- closer to 1in 3 than 1 in1million. If we assume that planet Earth could have spawned life at any time between, say, 3.9 billion years ago and today, then it gives us pause for thought that the great lottery called Life was won in just a few hundred million years -- maybe less. Using a refined version of the gambler's argument, Lineweaver and Davis conclude that there is at least a 13 per cent probability that life will form at some stage on a planet where the requisite conditions for life to arise have existed for at least one billion years. Of course, the authors are quick to point out this does not guarantee the galaxy will be teeming with life, because we still don't know how "Earth-like" another planet might need to be for it to lie in the same reference class as Earth for such statistical reasoning. But their work at least quantifies what many scientists have long claimed in a somewhat vague way. Something else needs to be taken into account. The factors that determine how long life takes to get going on an Earth-like planet will depend on unknown details of physics and chemistry, for example how long it will be before certain rare but crucial molecules form by chance in a primordial soup. On the other hand, the factors that determine how long a planet remains congenial for life will be determined by completely different aspects of science, such as the life expectancy of stars, which depends in turn on gravitation and nuclear physics. Because there is no physical connection between these two timescales, it would be an unlikely coincidence if they happened to be about the same. Therefore, it seems reasonable to believe that life will either form much faster or much slower than typical stars take to burn up most of their fuel (between 5 and 10 billion years). Most biologists are conservative on the matter and opt for the latter: the molecules of life are hard to form by chance, and on average would take very much longer to arise than the window of opportunity of several billion years during which an Earth-like planet would be able to sustain life. It happened on Earth by a fluke, they say. But why did it happen so quickly? After all, life could have taken another billion years to get going, and there would still be time for scientists to evolve and squabble over the subject before the sun turns into a red giant. One response is to shrug this detail aside. If the chances of life starting on Earth at some stage are, say, a mere trillion to one, then the chances of it starting as soon as it did might be ten trillion to one. So what? It is practically a miracle anyway, and one might as well be hanged for a sheep as for a lamb. But this would be too hasty. It is still possible to draw an interesting conclusion from Lineweaver and Davis's analysis, even if life is a fluke and unlikely to emerge more than once in the observable universe. In common with most researchers, Lineweaver and Davis assume that life arose on Earth from scratch. But there is a contending explanation for why life got going so fast: perhaps it did not originate here at all. Maybe it came from somewhere else in the universe. If the early asteroid-battered Earth were subjected to a continual influx of extraterrestrial organisms, it would be no surprise if life colonised our planet pretty much as soon as conditions became favourable. The theory that life came to Earth from space is far from new. It was much discussed in the i9th century, and became the subject of a book published in 1906 by the Swedish chemist Svante Arrhenius, who called the idea "panspermia" meaning "seeds everywhere". Arrhenius had in mind swarms of bacteria wafting naked across the galaxy, propelled by the pressure of starlight. Nowadays panspermia is not much favoured by astrobiologists, with the notable exception of Chandra Wickramasinghe at the University of Wales. The main problem with the theory is the high levels of radiation in space, which would prove lethal to all known microorganisms in fairly short order. But another scenario circumvents the radiation hazard. It was first mooted in rather vague terms in 1871 by the physicist Lord Kelvin, and is now receiving increasing support among scientists. It goes like this. The same bombardment that threatened early life also blasted vast quantities of rock into space from the surfaces of numerous planets. Some of this debris would have eventually hit the Earth. If another planet in the solar system had already spawned life, then microbes cocooned inside these ejected rocks, shielded from the radiation, could have made the journey unscathed. If life did arrive here inside a meteorite, where did it come from? The prime candidate is Mars. A couple of dozen Martian rocks have been identified on Earth and many more must be lying around undetected. It has been estimated that an average of one Martian rock a month falls to Earth. And Mars fits the bill when it comes to the cradle of life, too. Although a freeze-dried desert today, the Red Planet was once warm and wet, possessing a thick atmosphere, liquid water and active volcanism. Being a smaller planet, it cooled quicker than Earth and could have been ready for life at least half a billion years earlier. The first life forms may have cowered deep in the crust, which would have afforded a measure of protection against the bombardment. This subsurface zone on Mars may have cooled enough to host life as long ago as 4.5 billion years, while Earth's crust still sizzled. The implications for the statistical argument are striking. If Mars had a "window of opportunity" several times longer than Earth's in the period prior to life's first known appearance, the chances of life starting there rather than here are correspondingly higher. But it goes much deeper that that. The size of the "Mars advantage" hinges not just on the longer window of opportunity that the Red Planet enjoyed. It also depends on the number of difficult steps involved in generating life. If a single very improbable event -- such as the spontaneous assembly of a lucky combination of key molecules -- sparked life, then it is about five times as likely that life started on Mars and came here in a meteorite as the orthodox terrestrial alternative. But if more than one very unlikely step is involved in getting life started, things get seriously interesting. If life required three unlikely molecular combinations -- say, one to make long strands of nucleic acid, another to make certain proteins, and another to make the right sort of cellular structure -- that would make a Martian origin 125 times as likely; a 99 per cent probability that it happened on Mars and not on Earth. More steps than that and the odds favouring Mars become overwhelming. There is of course another possibility: that life came from outside the solar system altogether. After all, the Earth is only about a third as old as the universe -- there may have been Earth-like planets in the galaxy billions of years before the solar system formed. In the run-up to the period when life is known definitely to have existed here on Earth, life would have had much longer to arise on these planets than it would on Mars. But against this advantage of a longer window of opportunity must be set the much slimmer chance that such extra-solar life could traverse the many light years of interstellar space needed to bring it to the solar system. Jay Melosh of the University of Arizona and his colleagues have studied the fate of rocky ejecta both within and between star systems, and they doubt that a single interstellar rock has ever hit Earth throughout its entire history. Unless the origin of life required several exceedingly unlikely steps -- thus amplifying the relative advantage of a longer window of opportunity -- Mars remains the best bet for being the cradle of life. What supporting evidence can we find? Perhaps we can invoke the emergence of human beings -- the product of a long sequence of events, starting with biogenesis and extending through many evolutionary steps tc intelligence. The economist and philosopher Richard Hanson from George Mason University in Virginia, elaborating on an argument first used 20 years ago by the physicist Brandon Carter of the Meudon Observatory in Paris, has shown that, if a sequence of steps -- some relatively easy, some hard -- are needed to attain a certain goal in a fixed length of time, then the expected time left over when the last step has been successfully completed is roughly the same as the time taken by the hardest step in the total sequence. If it is true that the first step towards our emergence -- the origin of life -- is the hardest, taking on average much longer than the age of the Earth, then Hanson's argument suggests its flukey occurrence took roughly as long as the remaining time left. Astronomers reckon the sun will become too hot for life on Earth in about another billion years, so that suggests life took about a billion years to form -- a figure implausibly long for the Earth, but much closer to the few hundred million years during which Mars would have been a suitable incubator before life appeared here. None of this tells us anything about how life began, only that if the process was hard -- as many biologists say it was -- it probably came from Mars, and if it was easy it should have formed spontaneously on both Earth and Mars at some stage. And of course statistical reasoning is no substitute for hard facts. The clincher will come if a future mission to Mars provides unambiguous evidence for terrestrial-type life there before it existed here. Other hard facts closer to home would also help: a Martian provenance of life would be greatly strengthened if Earth's known window of opportunity were to become sharply narrowed, for example as a result of a clear sign of life at work here 3.9 billion years ago coupled with confirmation of the hostility of the early bombardment. But in the absence of any more reliable data about life on Earth, Mars or beyond, statistical reasoning is about all we have to go on. And, for the moment, the odds make it a racing certainty that we are all descended from Martians. New Scientist 12 July 2003 Paul Davies is a theoretical physicist in the Australian Centre forAstrobiology at Macquarie University, Sydney. His 1999 book, The Fifth Miracle, has been reissued by Penguin as The Origin of Life. >>>> Keith Hudson, 6 Upper Camden Place, Bath, England _______________________________________________ Futurework mailing list [EMAIL PROTECTED] http://scribe.uwaterloo.ca/mailman/listinfo/futurework _______________________________________________ Futurework mailing list [EMAIL PROTECTED] http://scribe.uwaterloo.ca/mailman/listinfo/futurework
