Word: cometted
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...first has to do with the period's cataclysmic close. In lots of people's minds, the mystery of what killed the dinosaurs and other species - paving the way for the rise of mammals - was solved a couple of decades ago: a giant asteroid or comet slamming into the Earth, resulting in a dust cloud that shrouded the sun, cooled the planet dramatically and killed off plants and animals wholesale. It's a compelling story, but plenty of scientists never completely bought it. The dinos died pretty quickly, they admit, but not quite abruptly enough to be explained this...
...other recent challenge to conventional Cretaceous wisdom comes from a paper in the journal Science, published Thursday. It's pretty certain from many lines of evidence that the world was much hotter then (which is why a post-comet cold snap would have been pretty tough on the dinosaurs). During a period called the Turonian, about 90 million years ago, things got especially toasty: In some places, during what's often called the "super-greenhouse" years, the ocean's surface temperature approached 100 degrees F, and alligators thrived in the Arctic...
...does smack into Mars, every telescope on Earth will be pointed in that direction - just as they were in 1994 when Comet Shoemaker-Levy 9 crashed into Jupiter. In that case, the comet broke up while it was still in orbit, so astronomers watched nearly two dozen individual impacts. But Jupiter is made mostly of thick clouds, so there was no lasting scar, and because it lies so far from Earth, the event wasn't quite as spectacular as this one promises to be. Asteroid 2007 WD5 should release some 3 megatons of energy if it slams into solid ground...
...impact calls to mind a loosely related incident that occurred almost exactly 100 years ago, when something exploded above the Tunguska region of Siberia, flattening trees in a 25-mile radius, their trunks pointing outward from the epicenter of the blast. Scientists are pretty sure it was a comet or asteroid - about the same size as 2007 WD5, as it happens - that disintegrated from its own shock wave as it plowed through the atmosphere. (UFO enthusiasts have long been convinced it was a flying saucer that somehow made it across trillions of miles of interstellar space safely, only to blow...
Holmes is nowhere near the sun, so what's causing its outburst? One theory suggests that repeated melting and refreezing of the surface has created a hard crust on the snowball-like comet; heating by the sun has warmed the inside, raising the pressure until the crust cracked and released the pent up vapors with explosive force. The show, say astronomers could go on for several weeks...