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days of us； and that is only if a telescope happened to be trained on it；which is unlikely because even now the number of people searching for such objects ismodest。 the arresting analogy that is always made is that the number of people in the worldwho are actively searching for asteroids is fewer than the staff of a typical mcdonald鈥檚restaurant。 （it is actually somewhat higher now。 but not much。）while gene shoemaker was trying to get people galvanized about the potential dangers ofthe inner solar system； another development鈥攚holly unrelated on the face of it鈥攚as quietlyunfolding in italy with the work of a young geologist from the lamont doherty laboratory atcolumbia university。 in the early 1970s； walter alvarez was doing fieldwork in a elydefile known as the bottaccione gorge； near the umbrian hill town of gubbio； when he grewcurious about a thin band of reddish clay that divided two ancient layers of limestone鈥攐nefrom the cretaceous period； the other from the tertiary。 this is a point known to geology asthe kt boundary；1and it marks the time； sixty…five million years ago； when the dinosaurs androughly half the world鈥檚 other species of animals abruptly vanish from the fossil record。
alvarez wondered what it was about a thin lamina of clay； barely a quarter of an inch thick；that could account for such a dramatic moment in earth鈥檚 history。
at the time the conventional wisdom about the dinosaur extinction was the same as it hadbeen in charles lyell鈥檚 day a century earlier鈥攏amely that the dinosaurs had died out overmillions of years。 but the thinness of the clay layer clearly suggested that in umbria； if1it is kt rather than ct because c had already been appropriated for cambrian。 depending on which sourceyou credit； the k es either from the greek kreta or german kreide。 both conveniently mean 鈥渃halk；鈥潯hichis also what cretaceous means。
nowhere else； something rather more abrupt had happened。 unfortunately in the 1970s notests existed for determining how long such a deposit might have taken to accumulate。
in the normal course of things； alvarez almost certainly would have had to leave theproblem at that； but luckily he had an impeccable connection to someone outside hisdiscipline who could help鈥攈is father； luis。 luis alvarez was an eminent nuclear physicist；he had won the nobel prize for physics the previous decade。 he had always been mildlyscornful of his son鈥檚 attachment to rocks； but this problem intrigued him。 it occurred to himthat the answer might lie in dust from space。
every year the earth accumulates some thirty thousand metric tons of 鈥渃osmicspherules鈥濃�攕pace dust in plainer language鈥攚hich would be quite a lot if you swept it intoone pile； but is infinitesimal when spread across the globe。 scattered through this thin dustingare exotic elements not normally much found on earth。 among these is the element iridium；which is a thousand times more abundant in space than in the earth鈥檚 crust （because； it isthought； most of the iridium on earth sank to the core when the planet was young）。
alvarez knew that a colleague of his at the lawrence berkeley laboratory in california；frank asaro； had developed a technique for measuring very precisely the chemicalposition of clays using a process called neutron activation analysis。 this involvedbombarding samples with neutrons in a small nuclear reactor and carefully counting thegamma rays that were emitted； it was extremely finicky work。 previously asaro had used thetechnique to analyze pieces of pottery； but alvarez reasoned that if they measured the amountof one of the exotic elements in his son鈥檚 soil samples and pared that with its annual rateof deposition； they would know how long it had taken the samples to form。 on an octoberafternoon in 1977； luis and walter alvarez dropped in on asaro and asked him if he wouldrun the necessary tests for them。
it was really quite a presumptuous request。 they were asking asaro to devote months tomaking the most painstaking measurements of geological samples merely to confirm whatseemed entirely self…evident to begin with鈥攖hat the thin layer of clay had been formed asquickly as its thinness suggested。 certainly no one expected his survey to yield any dramaticbreakthroughs。
鈥渨ell； they were very charming； very persuasive；鈥潯saro recalled in an interview in 2002。
鈥渁nd it seemed an interesting challenge； so i agreed to try。 unfortunately； i had a lot of otherwork on； so it was eight months before i could get to it。鈥潯e consulted his notes from theperiod。 鈥渙n june 21； 1978； at 1：45 p。m。； we put a sample in the detector。 it ran for 224minutes and we could see we were getting interesting results； so we stopped it and had alook。鈥�
the results were so unexpected； in fact； that the three scientists at first thought they had tobe wrong。 the amount of iridium in the alvarez sample was more than three hundred timesnormal levels鈥攆ar beyond anything they might have predicted。 over the following monthsasaro and his colleague helen michel worked up to thirty hours at a stretch （鈥渙nce youstarted you couldn鈥檛 stop；鈥潯saro explained） analyzing samples； always with the same results。
tests on other samples鈥攆rom denmark； spain； france； new zealand； antarctica鈥攕howedthat the iridium deposit was worldwide and greatly elevated everywhere； sometimes by asmuch as five hundred times normal levels。 clearly something big and abrupt； and probablycataclysmic； had produced this arresting spike。
after much thought； the alvarezes concluded that the most plausible explanation鈥攑lausible to them； at any rate鈥攚as that the earth had been struck by an asteroid or et。
the idea that the earth might be subjected to devastating impacts from time to time was notquite as new as it is now sometimes presented。 as far back as 1942； a northwesternuniversity astrophysicist named ralph b。 baldwin had suggested such a possibility in anarticle in popular astronomy magazine。 （he published the article there because no academicpublisher was prepared to run it。） and at least two well…known scientists； the astronomerernst ？pik and the chemist and nobel laureate harold urey； had also voiced support for t