THE FATE OF THE SUN AND THE EARTH!
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Life on the Earth should thrive
for hundreds of millions more
years. But wait till the oceans
start boiling. . . . . . .
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I ’VE ALWAYS BEEN FASCINATED BY H. G. WELLS’S VIVID description, in his 1895 classic THE TIME MACHINE, of a journey into Earth’s future. Wells describes the English country-side descending first into a blur of years, then ages: “I saw trees growing and changing like puffs of vapour....... I saw huge buildings rise up faint and fair, and pass like dreams. The whole surface of the earth seemed changed ------melting and flowing under my eyes.”
Our view of Earth’s distant future has progressed enormously since Wells’s day. And while time machines remain science fiction, our understanding of the Sun’s evolution makes it possible to sketch out Earth’s very distant future in a way that’s both detailed and scientifically sound.
If we could watch a time-lapse movie of Earth over the next 200 million years or so, we would see the continents merge to form a future version of Pangaea, which in turn will rift into a new configuration of continental plates. Earth’s interior heat is supplied primarily by the radioactive decay of uranium-238, which has a half-life of 4.5 billion years. So Earth’s innards are now producing heat at about half the original rate. As the interior cools, Earth’s processes of geological renewal will gradually shut down, and the planet will enter the senescence of old age. Volcanism will sputter out, and the continents will gradually settle into a final configuration. Over the next billion years, ocean levels will decline slightly as water is absorbed into the steadily cooling crust.
The cooling of Earth’s interior will not, however, be accompanied by cooling on the surface . Inexorably, since its birth, the Sun has been slowly brightening. Our very warm, life-giving star is on a road to become the scalding bane of Earth’s existence.
By: Gregory P. Laughlin
AN UNEASY BALANCE
Like most stars, the Sun exists in a stable, settled balance ---— between gravity pulling inward, and pressure, maintained by heat from nuclear fusion, pushing outward . This balance is self-regulating. Any slight contraction of the Sun thus increases its central density and temperature and speeds up the nuclear reactions, whose extra heat make the Sun expand back out. A slight expansion cools the interior a bit, slowing the nuclear energy production and causing the Sun to thus re-contract. So the Sun remains in equilibrium.
But as the years run into the hundreds of millions, the balance of the equilibrium slowly shifts. The amount of hydrogen in the Sun’s core declines as hydrogen fuses to form helium. The helium is inert, so the core must shrink and grow hotter to maintain fusion. In the 4.34 billion years since its birth, the Sun has burned a little less than half of its initial hydrogen and has grown a substantial helium-rich core. As its gas gauge continues creeping lower, the Sun will respond with a long series of tortuous events, none of which spells cheer for Earth.
As a main-sequence star like the Sun burns its hydrogen and grows denser at the core, the central temperature climbs high enough that hydrogen fuses all the faster. Since its formation, the Sun has already grown 30% brighter. During the next 1.2 billion years its luminosity will increase another 10% as it swells slightly and its surface grows about 15o0C (2700F) hotter.
A very interesting question is how Earth’s complex systems will respond. So far, Earth has done a reasonably good job of maintaining a stable temperature in the face of the Sun’s steadily growing luminosity. There have, however, been dramatic swings in the distant past, including at least two ancient “snowball Earth” phases before life emerged on land, in which the planet seems to have frozen over most completely from poles to equator. There have also been much warmer epochs than now, when temperate forests covered the poles.
Over the next century and more, we’ll be carrying out a dramatic large-scale experiment as we continue to release a huge pulse of carbon dioxide (C02) into the atmosphere. The resulting shock to Earth’s climate will provide a valuable improvement in our understanding of how the planet will respond, over a vastly longer term, to the increased energy output of the Sun.
Taking the really long view, a much hotter Earth might actually be a boon for life, judging by that fact that the biosphere gets ever richer as we look from the arctic to the tropics . Life thrives in a hothouse. Perhaps — given millions of years for evolution to do its slow adaptive work — this trend will produce an ever richer biosphere right up to the boiling point of water.
But eventually, that fateful temperature will arrive. Cloud-free climate models indicate that Earth will reach a runaway moist greenhouse in the next L2 billion years. This will be far more devastating than anything we can do by releasing C02, as water vapor itself is a powerful greenhouse gas when there’s enough of it. In the runaway moist greenhouse, very hot temperatures lead to greatly increased evaporation from the oceans, more water vapor in the atmosphere, thus even hotter temperatures, and so on until the oceans boil dry In a little over a billion years, we expect Earth to become a baked, barren, powder-dry desert. It’s hard to imagine how multicellular life could survive.
What will happen to all that water vapor above the bone-dry land? Currently, water vapor is almost completely confined to the troposphere, the atmosphere’s low layer where weather occurs. Very little exists in the stratosphere and even less above the ozone layer, which blocks most solar ultraviolet light. Therefore a very little of Earth’s water issubject to photolysis, in which ultraviolet sunlight destroys an H20 molecule by breaking off a hydrogen atom. So our water stays intact.
At the top of Earth’s atmosphere the temperature is high enough that any free hydrogen is gradually lost to space. So if water gets above the ozone layer, it’s on its way to being lost for good. After the moist-greenhouse upheaval, Earth then gradually loses all its water, and the planet makes a new transition into an even more hellish dry greenhouse, like that currently reigning on Venus.
Indeed, Venus may once have been much more clement and Earth-like. Measurements of its atmospheric deuterium-to-hydrogen ratio indicate that Venus has lost a lot of water during its history, arguably even an ocean’s worth.Although Venus has always received more solar energy than Earth does now, it too received 30% less during the Sun’s early era. If we can figure out exactly when Venus went bad, we may be able to make a more accurate long-term climate forecast for Earth.
Models by James Kasting (Penn State University) suggest that the dry greenhouse will befall Earth in approximately 3 billion years — a couple billion after the last liquid water has boiled away At that time the temperature will soar anew to a lead-melting 4000C (75 00F). And yet the Sun will be only 40% brighter than now, with several billion years of hydrogen-burning time to go.
POSTPONING THE INEVITABLE
In 2000 I joined with Fred Adams (University of Michigan) and Don Korycansky (University of California, Santa Cruz) to devise a very feasible plan to stabilize Earth’s current pleasant temperature against the Sun’s increasing heat for several billion years to come. The key is the vast amount of time available. We showed that, with only a tiny expenditure of energy it would be possible over thousands or millions of years to maneuver a larger asteroid or Kuiper Belt object into making repeated close passes by Earth and Jupiter, in such a way as to transfer some of Jupiter’s huge store of orbital energy to Earth. One such pass every 10,000 years would be enough to move Earth’s orbit outward at just the right rate to compensate for the Sun’s gradual brightening.
Although the technology to start such a project is nearly at hand, humanity’s own stability and long-term planning will have to improve by many orders of magnitude first. It’s not just that a small miscalculation could cause an Earth-sterilizing collision. The current state of intelligent life was brought home to us after we published our work. The media reported, completely incorrectly, that this scheme could stave off short-term, human-induced global warming. Such reports prompted newspaper editorials, laudatory talk-show blather, and an overwhelming avalanche of angry calls and letters. Ten million years will be plenty of time to wait before getting started. The real issues concern whether civilization will last for a thousandth that long.
But no matter what our distant descendents may do, eventually the Sun’s evolution will speed up. Highly refined computer models, as well as detailed observations of older Sun-like stars, establish that hydrogen burning will shift to a shell outside the Sun’s core and happen faster and faster. Energy production will increase dramatically, causing the Sun’s outer layers to balloon in size.
Current models tell us that 6.36 billion years from now the Sun will be 2.2 times as luminous as it is today, and Mars will receive the same heat Earth does now. This thaw, however, will likely come too late for Mars — its gravity is too low to hold a significant warm atmosphere, making it a nonstarter for late habitability.
During the following 730 million years the Sun will grow to 2.7 times its present luminosity and 2.3 times its present diameter. At that point the solar system is desolate indeed.
Venus and Earth are sulfurous twins; Mars is a torrid desert. The Jovian satellites are still frozen, but their melt-down is imminent. Over the next 590 million years the Sun will ascend the red-giant branch of stellar evolution, and the consequences will be more drastic than anything yet.
Today you can make a scale model of Earth and Sun by holding a grain of sand in one hand and a dime in the other as far apart as you can stretch. As the Sun grows into a red giant, the dime must be replaced first by a quarter, then a lemon, then a grapefruit. At that point Jupiter’s moons Europa, Ganymede, and Callisto will develop thick water-vapor atmospheres and enter moist-greenhouse eras; then their water will be photolyzed and lost to space.
Saturn’s largest moon, Titan, twice as far out, will warm nicely. Chris McKay (NASA/Ames Research Center) and his collaborators find a period of several hundred million years when Ii uid-ammonia oceans will be stable on Titan. Complex chemical reactions will proceed among the abundant organic molecules there, perhaps allowing a new formation of life — but not for very long.
And Earth? The question now is whether the barren planet will survive at all.
As our scale-model Sun swells from grapefruit to basketball and then beachball size, it will at least engulf Mercury. But as its luminosity increases to several hundred times the current value, and as its surface gravity lessens as it balloons, the Sun will begin to blow off a strong wind that will ultimately carry away close to a quarter of the solar mass. The Sun will therefore lose a quarter of its gravitational pull, so the orbits of the planets will expand outward.
As the Sun nears the apex of its red-giant phase it will swell to more than 200 times its current size, easily enough to engulf the present orbit of Venus and almost to the present orbit of Earth. But by then Venus will probably have receded to where Earth is today, and Earth will lie near the current orbit of Mars. So at first glance it looks as if Earth will barely escape.
Recently, however, Kacper R. Rybicki (Institute of Geophysics, Polish Academy of Sciences) and Carlos Denis (University of Liege, Belgium) made a detailed study of the effect the red-giant Sun will have on the solar system. They caine to an important realization: tides will play a decisive role in determining the fate of the Earth. As the Sun swells, Earth’s gravity will raise a very slight bulge on the solar su face. Friction causes the bulge to lag slightly as it attempts to track Earth’s position in orbit. This lag m turn creates a persistent gravitational drag on our planet, causing it to slowly spiral inward. The situation is sin-iilar to how the Moon’s tidal effect on Earth is causing the Moon to creep outward — except that it will happen in reverse, because the Sun will be rotating slower than we circle it, rather than faster.
Given all the uncertainties, it’s not yet clear whether Earth will escape just being engulfed by the red-giant Sun.
The Sun’s first red-giant phase ends abruptly when the core temperature reaches 100 million degrees Celsius and helium begins to fuse into carbon, providing a fresh new energy source. The Sun responds to this newfound wealth by (rather paradoxically) shrinking drastically and cutting its overall luminosity by a factor of nearly 100. For a few hundred million years the Sun bums helium at a steady rate, and the solar system takes a break from the long string of disasters. Earth is still intact (the smart money puts the chance at about 75%), it will likely have a rocky surface of fused silicates bare to the vacuum of space, and the daily noon temperature will be a toasty 600 degrees C. Any evidence that a biosphere once existed on our planet will have long since been melted and re-crystallized into oblivion.
The near-exhaustion of helium will leave the Sun with a white-dwarf-like core made of carbon and oxygen. The Sun’s outer layers, meanwhile, re-expand and cool, and the Sun becomes a red giant for a second time. Astronomers refer to this second red-giant episode as the “asymptotic branch” stage, for its position on the Hertzsprung-Russell diagram of stellar evolution. Once again the Sun becomes a serious threat to Earth’s physical survival. During the second red-giant phase it experiences several epochs of enormous energy output — “helium flashes” — that lead to massive, roughly 10,000-year long pulsations in size. It’s distinctly possible that Earth could be briefly engulfed during these pulses without having time to spiral all the way in to total destruction.
Then, roughly 100 million years after the second red-giant phase begins, the Sun will throw off its massive outer layers completely to form a planetary nebula, perhaps , leaving behind a brilliantly hot but very tiny white dwarf.
And along the way comes yet another threat.
We have seen that mass loss from the Sun will be essential to Earth’s physical survival. But if the Sun loses much more mass than expected, giant Jupiter and Saturn could find themselves interacting gravitationally in a serious way They would goad each other into sweeping crazily on long, elliptical loops that would in turn disrupt the rest of the solar system. In the ensuing chaos, objects would end up colliding, being flung away to interstellar space, and falling onto the Sun. The likely wreckage of such disrupted solar systems has been found at two and possibly as many as 40 white-dwarf~ stars, including the one whose recently lost mass forms the familiar Helix Nebula.
The Sun’s transformation into a white dwarf brings us to the end of the Sun’s role in the story. The surviving planets, including perhaps the cinders of Earth and Mars, will orbit the white dwarf quietly and stably for hundreds of billions of years as they and the Sun’s little remnant cool ever closer to absolute zero — awaiting a bizarre sequence of subsequent cosmic disasters.
As the white-dwarf Sun slowly fades to black, the splendor of the night will also diminish. The universe is currently entering an era of accelerating expansion, which, within several hundred billion years, will carry all galaxies except those in the gravitational hold of our Local Group beyond our causal horizon. The Milky Way itself will have long since merged with the Andromeda Galaxy to form a large elliptical galaxy . Within roughly 100 trillion years, we can expect a random white-dwarf remnant to pass close enough to tear Earth from its orbit and send it loose into the now -dark galaxy, where it will roam alone for perhaps another thousand quadrillion years.
Peering even further, if Earth can avoid a collision with the galaxy’s central black hole, chance encounters with passing stellar remnants will eventually fling it out of the galaxy completely If Earth is liberated in this manner, continued cosmic expansion will carry the Milky Way beyond the horizon out of sight or any causal contact, and our Earth will enter vast stretches of time in an inky black void, utterly alone. *
GREGORY P. LAUGHLIN hunts and studies extrasolar planets at the University of California, Santa Cruz. He and FRED ADAMS authored The Five Ages of the Universe: Inside the Physics of
Eternity. In the August 1998 Sky & Telescope they sketched out the next 10 to 120 power years.
Sky & Telescope Magazine
June 2007. (Pgs. 32-36)
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