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Earth-Deadly-Future.Pdf

Earth-Deadly-Future.Pdf

The evolving system

“BLACK SMOKERS” are bastions of at hydrothermal vents in today’s oceans. They get their names from the soot-like look of ’s the mineral-rich material they eject. NOAA

he first things to go will be A brightening will boil the Earth’s glaciers and polar ice caps. Warming and bake the continents a will turn billion from now. But that’s ice to , leading to a deadly slow but steady rise in levels. But it Tdoesn’t stop there. Eventually, tempera- nothing compared with what tures will rise high enough for seawater to boil away, leaving Earth bereft of this we can expect further down vital substance. With that, life on our world will need to relocate underground the road. ⁄⁄⁄ BYcr Ri ha d Talcott or emigrate from our home . This apocalyptic scenario is more than an inconvenient truth — it’s our inevitable destiny. And it has nothing future to do with changes humans may work on our fragile environment. The agent for this transformation is far beyond our control. The culprit: our current life-sustaining source of and , the Sun. Ask most people familiar with when to expect this coming apocalypse, and you’ll hear answers of around 5 billion years — once the Sun swells into a giant. But the end is nearer than that. The Sun is currently growing brighter, and has been since the it was born. Life on the A BILLION YEARS FROM NOW, the Sun’s When the Sun was a baby, it was rather increasing will have boiled off miserly by today’s standards. It emitted most of Earth’s water. In this view, water roughly 30-percent less energy then exists only in deep ocean trenches, where than it does now. The Sun officially thermophilic bacteria cling to life. Lyttene Cook became a when it started fusing

© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com www.astronomy.com 29 gases the will contain. Most on the move scientists expect the level of atmospheric Today dioxide to drop in the distant future. This will come about as photosynthetic Sun Earth organisms extract from the 0.38 AU 0.72 AU 1.00 AU 1.52 AU atmosphere and weathering incorporates some of it into silicate rocks, which then Sun and planetary shown to scale; planet sizes not to scale are subducted into the mantle. As the oceans start to evaporate, the Sun’s high-energy will break the water molecules into their 6.5 billion years from now constituents, and . The lightweight hydrogen gas will escape Sun as Earth’s gravitational hold and bleed into 0.88 solar- Venus Earth Mars space. It might take another billion years 0.93 AU 1.17 AU 1.85 AU for ocean water to disappear completely, but by then, any remaining life will have had to make other plans. One viable option might be Mars. As Earth becomes too warm for most life to 6.7 billion years from now survive, the Red Planet should be getting balmy. If humans can make it till then, Mars would offer some attractive real estate. Sun as asymptotic giant 0.66 solar-mass Earth Mars 1.61 AU 2.46 AU Into the deep future To this distant point, the Sun and Earth have taken nearly opposite paths. Even a billion or two years from now, the Sun will As the SUN ages, it will lose some of its mass. This trend will accelerate when it becomes a red giant, and grow even greater when it look basically the same on the outside as it ICY could prove to be a watery haven in the distant future, when increasing swells into an asymptotic-giant-branch star. This mass loss will cause the orbits of the planets to migrate outward. Astronomy: Roen Kelly does now — a little bigger and brighter, but solar radiation will render the inner planets uninhabitable. NASA/JPL still recognizable. The Sun’s internal struc- hydrogen into in its core. These And that’s the rub. The nuclear reactions weren’t much, if any, colder. That’s good ture, however, will have changed markedly. Location, location, location nuclear reactions release energy according in the Sun’s core essentially convert four news as far as life is concerned. The first Its center will be largely helium, although When the South Pole feels more like the Amazon jungle a few billion years from now, any 2 to Einstein’s famous equation: E=mc . This hydrogen atoms into one helium atom. Gas single-cell organisms arose some 3.5 billion lots of hydrogen will exist in the core. The life on Earth will be looking for a way out. The Sun’s increasing luminosity will render Earth energy source defines any star’s main pressure, however, depends in part on the years ago, and they presumably required hydrogen continues to fuse into helium and uninhabitable, and worried eyes will look skyward. sequence life — where it spends the vast number of particles in the gas. The ongoing liquid water. But the Sun wasn’t hot enough add to that element’s growing abundance. In a reversal of -fiction proportions, the first stop may well be Mars. Unlike H. G. majority of its days. fusion reduces the number of particles, so by itself to melt terrestrial ice until roughly For Earth, on the other hand, the sur- Wells’ classic novel, in which dying Martians looked longingly toward a more hospitable We tend to think of a main sequence the pressure drops. To maintain hydrostatic 2 billion years ago. face would hardly be recognizable. Our Earth, earthlings may decide to head for cooler martian climes. Mars has a distinct advan- star like the Sun as constant, but it’s not. equilibrium, the Sun must compensate. The We can thank our lucky for the “” will be more of a muted tage: Not only will it likely serve as humans’ first permanent outpost in the , It maintains what call hydro- core shrinks, raising both the greenhouse effect. The presence of water brown, and blistering temperatures will but it also holds the promise of being clement for an extended period. static equilibrium — the outward pressure and . That, in turn, increases the rate vapor and carbon dioxide in the atmos­ make it uninhabitable. But the deep interior But even Mars will grow too hot once the Sun becomes a red giant. Then, the only exerted by the core’s hot gas balances the of nuclear reactions, and the Sun generates phere warms our planet well above what it won’t see much effect. Although it will have ­reasonable outposts will be on the of the gas-giant planets. Several of them — including ’s , Europa, and , and ’s , , and inward crush of . If the Sun’s central even more energy. would otherwise be. Even today, Earth is cooled modestly as the total mass of radio- — already come with huge complements of ice. Raise the Sun’s temperature significantly, temperature were to drop slightly, for These changes operate slowly. Although some 60° Fahrenheit (33° Celsius) warmer active elements decreases, a 21st-century and all may afford ocean-front property at some future point. example, the gas pressure would also fall. a hundred million years may sound like a than it would be without greenhouse geologist would still recognize it. But the reality of the Sun’s demise is that by the Jupiter or Saturn become viable Gravity then would force our star to con- long time, for the Sun, it’s a blip on the warming. In the distant past, when Earth’s But as time continues to march on, abodes, any surviving civilization should seek other solar systems. After several billion tract and heat up, restoring its equilibrium. screen, representing 1 percent of its interior was hotter and volcanic eruptions changes in the Sun and the rest of the solar years of calling home, a few million extra years won’t seem like much. It will be time The Sun started life as a uniform mix life span. And in a hundred million years, likely belched significantly more green- system will become more pronounced. The to become citizens of the . — R. T. of approximately 73-percent hydrogen, the Sun’s luminosity rises less than 1 per- house gases into the atmosphere, the effect real changes start roughly 5 billion years 25-percent helium, and 2-percent heavier cent. The energy increase prompts the Sun would have been greater. from now, when the Sun exhausts the ­elements, by mass. The outer parts of the to expand at a comparably lethargic pace. The push to higher solar hydrogen fuel in its core and prepares to of water) there, but not extreme enough to jumps, the overlying layers will expand and Sun still maintain that balance. But in the Its diameter grows at about the same rate as continues. Roughly 1 to 2 billion years leave the main sequence. As the Sun takes ignite helium. Meanwhile, hydrogen in the cool. The star will be on its way to becom- core, where rules, helium human fingernails: 1 to 2 inches per . from now, Earth’s surface temperature its first tentative steps into old age, it will outer core will continue to burn. ing a red giant. levels continuously rise. Since the Sun’s will approach the point of no return, shine some 70-percent brighter than it does With no source of energy at the center, birth, about 5 percent of its total mass has Crystal-ball gazing when water will start evaporating and now. That won’t last long, however. the core will contract and heat up. Like Monster star been converted into helium. If the Sun is warmer now than it was in the ­herald an end to above-ground life. The Sun’s inner core then will contain adding gasoline to a fire, the increased heat It will take the Sun between 1 and 1.5 billion past, what were conditions like on Earth a Several unknowns affect the timing. only helium. It’ll be hot (some 50 million will cause the hydrogen-burning shell to years to evolve from the close of its main Richard Talcott is a senior editor of Astronomy. few billion years ago? Surprisingly, they Most important: The fraction of greenhouse ) and dense (10,000 the density kick into overdrive. As the Sun’s luminosity sequence life to a full-fledged red giant. By

30 astronomy ⁄⁄⁄ july 07 www.astronomy.com 31 then, its surface temperature will have dropped to around 3,500 K, just over half of what it was on the main sequence. The cool surface will mean the star radiates most of its energy at longer wavelengths, in the red part of the spectrum. Still, the Sun will put out 1,000 times more energy than today. To release this much energy from a cooler surface requires the Sun to swell dra- matically. As a red giant, it will appear 100 times bigger than today, taking it beyond Mercury’s and swallowing the inner- most planet. If any people were to visit Earth on a spaceship from the more tem- perate outer solar system, they would see the Sun as a bloated red spanning some 50° of the . If our planet still rotated once every 24 hours, it would take the Sun more than 3 hours to rise and set. In reality, Earth’s will have slowed significantly by then, lengthening and further. In the red giant’s distended outer layers, gravity will be so weak that the solar will blow a million times stronger than it WHEN THE SUN DIES, it will puff off its outer layers in a final blaze of glory. The resulting

does today. During the course of its red- planetary , like NGC 2440 seen here, will last about 50,000 years. NASA/ESA/K. Noll (STScI) giant phase, the Sun will lose approximately 10 percent of its total mass. This gradual mass loss will reduce the it’s déjà vu all over again. Carbon ash will The Sun’s internal instability during this Sun’s overall gravitational pull, so it no build up in the center, surrounded by a asymptotic-giant-branch stage will cause ­longer will hold the planets as tightly. The helium-burning shell which, in turn, will our star to pulsate with a period measured planets will spiral outward a bit — except be surrounded by a hydrogen-burning in hundreds of days. It will be a vari- for Mercury, of course, which already will shell. Once more, the core will contract, able star, named after the prototype star in have succumbed to the Sun’s appetite. heating the interior and spiking the nuclear- the Cetus. As hydrogen continues burning in a rates. The star swells again; but In just a few tens of thousands of years, shell, it’ll dump more helium “ash” onto the this time, it’ll grow even bigger and more the Sun will puff off its outer layers. The inner core. Eventually, the temperature at luminous than on the first go-round. It is Sun’s core, made of carbon and oxygen, the center will rise to 100 million K — hot now an “asymptotic-giant-branch star.” will be left behind as a -. enough to ignite helium. The Sun will tap At the height of this phase, the Sun will The star then will contain more than half into this second energy source with a ven- be 500 times its current diameter and swell the Sun’s current mass compressed into a geance, fusing helium into carbon and beyond the current orbit of Mars. Its outer sphere the size of Earth — a density equiva- some oxygen in its core while still fusing layers will claim their second victim as they lent to crushing a car to the size of a grape. hydrogen to helium in a surrounding shell. swallow Venus. But the Sun also loses mass The will have an initial tem- Ironically, the initiation of helium fusion at a greater rate this time around, turning perature of 100,000 K, so it’ll emit lots of will lower the Sun’s luminosity as it causes the into a full-blown hurricane. ultraviolet . This high-energy radiation the core to expand and cool. The star as The Sun’s mass will drop to two-thirds of will energize the expanding shell that was a whole will shrink, and its surface will what it is now, and Earth’s orbit will grow previously the Sun’s outer layers, causing it warm. It will stay in this stable configura- by approximately 60 percent. to glow. This will light up tion for approximately 100 million years. Current computer models can’t tell for about 50,000 years before the shell dis- Two bright stars visible from Earth — whether Earth will survive the onslaught sipates into the . Mean- and — are at this stage or not — it looks to be a close call. Mars while, the remnant white dwarf will slowly of evolution now. should make it easily, although its days of but steadily cool off, eventually extinguish- relative tranquility will be long over. The ing the light that nurtured billions of years Supersize me best place to be could be on one of the of life in the solar system. A RED-GIANT SUN looms over a dead and waterless planet Earth some 6 billion years As with all nuclear reactions, a small tem- moons of the outer planets. They may enjoy perature increase causes a big jump in the a brief period of springlike . And To watch a simulation of the Sun’s in the future. Lynette Cook for Astronomy ONLINE reaction rate. That’s why the Sun will burn with large stores of ice currently on some EXTRA evolution in the distant future, visit www.astronomy.com/toc. through its helium fuel so rapidly. Then, of them, precious water could be plentiful.

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