Word: planet
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Dates: during 1990-1999
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Although we live in an age where cars can talk, cuisinarts can blend and the World Wide Web can bring you news from around the planet, the accomplishments of an inhuman space probe still seem a little hard to believe in--too much sci-fi and too little fact...
...days earlier will enter Jupiter's atmosphere. "There won't be any dramatic pictures, just a data stream," notes Jaroff, "but this will help clear up a lot of speculation about the composition of the Jovian atmosphere. Scientists have been able to make a lot of inferences about the planet, but this would be the first time they've been able to sample the atmosphere. They expect to find a high water content, and some lightning, at the higher levels in the hour before the probe descends so far that Jupiter's extreme heat and pressure destroy...
...them back together. Mountains the size of the Himalayas shot skyward, hurling avalanches of rock, sand and mud down their flanks. The climate was in turmoil. Great ice ages came and went as the chemistry of the atmosphere and oceans endured some of the most spectacular shifts in the planet's history. And in one way or another, says Knoll, these dramatic upheavals helped midwife complex animal life by infusing the primordial oceans with oxygen...
...depleting processes, especially organic decay. Indeed, the vast populations of algae that smothered the Precambrian oceans generated tons of vegetative debris, and as bacteria decomposed this slimy detritus, they performed photosynthesis in reverse, consuming oxygen and releasing carbon dioxide, the greenhouse gas that traps heat and helps warm the planet...
...oxygen to rise, then, the planet's burden of decaying organic matter had to decline. And around 600 million years ago, that appears to be what happened. The change is reflected in the chemical composition of rocks like limestone, which incorporate two isotopes of carbon in proportion to their abundance in seawater - carbon 12, which is preferentially taken up by algae during photosynthesis, and carbon 13, its slightly heavier cousin. By sampling ancient limestones, Knoll and his colleagues have determined that the ratio of carbon 12 to carbon 13 remained stable for most of the Proterozoic Eon, a boggling expanse...