Welcome back. Missed you. REH
----- Original Message ----- From: "Keith Hudson" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Monday, July 14, 2003 12:04 PM 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
