the U N I V E R S E T O D A Y
Space Exploration News From Around the Internet, Updated Every Weekday.
October 3, 2000 - Issue #316
Pennsylvania State University
University Park, Pennsylvania
February 20, 2000
"The sun, like all main sequence stars, is getting brighter with time and that affects the Earth's climate," says Dr. James F. Kasting, professor of meteorology and geosciences. "Eventually temperatures will become high enough so that the oceans evaporate."
At 140 degrees Fahrenheit, water becomes a major constituent of the atmosphere. Much of this water migrates to the stratosphere where it is lost to the vacuum. Eventually, the oceans will evaporate into space.
"Astronomers always knew that the oceans would evaporate, but they typically thought it would occur only when the sun left the main sequence," Kasting told attendees today (Feb. 20) at the annual meeting of the American Association for the Advancement of Science. "That will be in 5 billion years."
Stars leave the main sequence when they stop burning hydrogen. The sun, a yellow, G-2 star, will then become a red giant encompassing the orbit of Mercury. Mercury will disappear and Venus will lose its atmosphere and become a burnt out planet. The Earth will suffer the same fate, even though it is outside the red giant's immediate reach.
"However, the oceans may evaporate much earlier," says Kasting, a faculty member with the College of Earth and Mineral Sciences. "My calculations are somewhat pessimistic and present a worst case scenario that does not include the effects of clouds, but they say a billion years."
This model was developed with Ken Caldeira, now at Lawrence Livermore Laboratory.
Things may go bad long before the Earth is a waterless desert. As the climate becomes warmer, the cycle of silicate rock weathering speeds up. This cycle removes carbon dioxide from the atmosphere and sequesters it in the oceans as calcium carbonate.
"The silicate weathering cycle stabilizes the Earth's climate for a time," says Kasting. "Eventually, atmospheric carbon dioxide levels will become so low that it will not be able to do so, but before then, there will not be sufficient carbon dioxide to sustain most plants."
Plants use carbon dioxide in photosynthesis to convert the sun's energy to sugars and other carbohydrates. Two main kinds of photosynthesis exist, C3 and C4. In a half billion years, the models predict that carbon dioxide will be at the compensation point for C3 plants which make up 95 percent of all plants. Below the compensation point, carbon dioxide is not concentrated enough for these plants to photosynthesize. C3 plants include trees and most crops.
C4 plants, which include corn, sugar cane and other tropical grasses, can still photosynthesize because they have an internal mechanism to concentrate carbon dioxide, but these plants cannot sustain the biosphere as we know it today.
"If carbon dioxide levels in the atmosphere continue to increase over the next few centuries, they could remain high for a very long time," says Kasting. "Then, after fossil fuels run out, it would take a million years or so for levels to return to present."
But even if there is a pulse of high carbon dioxide in the near future, by a half billion years, levels will be too low for productive plant life.
"Obviously, a billion, even a half billion years, are a long way off in the future," says Kasting. "However, these models can help us refine our understanding of the time that a planet remains in an orbit where life can exist."
Only a narrow spherical shell of space exists at a distance from a star that is neither too cold nor too warm for life. As a sun matures and brightens, that spherical shell moves outward. A planet must remain in the livable shell for long enough for life to evolve, even while that band moves outward. If planets lose their water supply, a mandatory requirement for life, earlier than previously thought, then that creates a shorter window for livable planets.
"If we calculated correctly, Earth has been habitable for 4.5 billion years and only has a half billion years left," says Kasting.
The University of Michigan
Ann Arbor, Michigan 48109-1399
February 15, 2000
Adams presented his vision of the solar system's fate in a presentation at the annual meeting of the American Association for the Advancement of Science held here Feb. 17-22.
During the next 7 billion years, our aging sun will gradually exhaust its fuel supply and collapse into a white dwarf, says Adams, an associate professor of physics at the University of Michigan. The sun will mushroom in size before it collapses and shine so brightly that it will incinerate the Earth and the inner planets in the solar system. But as soon as 3.5 billion years from now -- long before the planets go up in smoke -- all life in the Earth's fragile biosphere already will have perished from the heat.
There is one escape clause in this fiery scenario, according to Adams. We may be rescued by a close encounter between our solar system and a passing star. Adams and Gregory Laughlin, a scientist at NASA's Ames Research Laboratory, used a computer and statistical processing calculations to model possible interactions between nearby binary stars and the orbits of the Earth, sun and outer planets, especially Jupiter.
"Jupiter is vulnerable to gravitational interactions with a passing star," Adams explains. "Because of its large mass, even a modest disruption of Jupiter's orbit could have a catastrophic effect on Earth. The chances of such an encounter either hurling the Earth out into space or plunging it into the sun during the next 3.5 billion years are about one in 100,000."
Should the Earth be thrown out of the solar system into deep space, its oceans would not freeze solid for about 1 million years, according to Adams. "Life could continue to thrive near hydrothermal vents on the ocean floor, which are warmed by radioactive heat from deep within the Earth," he says. In fact, Adams maintains that the most likely place to find extra-terrestrial life may be in liquid oceans beneath thick ice sheets on planets or moons of giant planets, such as Jupiter's moon, Europa.
In the far, far distant future -- long after our solar system has met its ultimate fate -- the galaxy will move into what Adams calls the Degenerate Era. "The only stellar objects remaining will be white dwarfs, brown dwarfs, neutron stars and black holes," he says. During this era, galaxies will begin to relax dynamically with some remnants of stars moving out to the edge of the galaxy and others falling to the center. Invisible dark matter gradually will be captured and converted into energy to keep the few remaining white dwarf stars shining weakly for a little while longer.
"Eventually the supply of dark matter particles will be exhausted," Adams says. "Then the mass of white dwarfs and neutron stars will begin to dissipate through a process called proton decay. A white dwarf fueled by proton decay would generate approximately 400 watts or enough to run a few light bulbs."
Even black holes won't last forever. Fed by material falling to the galaxy's center, black holes will grow larger for a long time. But even their enormous mass must eventually dissipate into thermal radiation, photons and other decay products.
Once the black holes have radiated away, Adams says all that remains will be a diffuse sea of electrons, positrons, neutrinos and radiation suspended in nearly complete and total blackness.
This U-M research study was supported by the National Science Foundation, NASA and the U-M Department of Physics.
Iowa State University
Lee Anne Willson, an Iowa State University professor of physics and astronomy and George Bowen, professor emeritus of physics and astronomy, say new computer models of the later stages of the Sun's life show that it will expand and engulf our planet before it shrinks back in size and fades away as a white dwarf.
"Earth will get scorched as part of the process the Sun will go through as it transforms from being a red giant into a white dwarf," said Willson, who will present the research at the annual meeting of the American Association for the Advancement of Science, Feb. 17-22, in Washington, D.C.
Most astronomers agree that as the Sun evolves over the next 5 thousand million years, it will become about twice as bright as it is today. However, when the center of the Sun runs out of fuel and the Sun becomes a red giant, it will grow much brighter and larger. At question is the maximum size the Sun will reach and whether Earth will be able to get out of the way before the Sun gets too big.
Willson and Bowen studied the mass loss of red giant stars and used that information to predict the fate of the Sun and its effects on the planets. Their computer models are able to predict in great detail the conditions that Earth and the other planets will encounter as the Sun ages. Willson and Bowen's research predicts a fiery end to our planet.
"We know stars like the Sun lose about 40 percent of their mass during their advanced red giant evolution," Willson said. "However, it has not been clear whether that mass comes off very gradually, over millions of years, or relatively abruptly, over the last tens or hundreds of thousands of years. And that turns out to make all the difference in the world."
From detailed computer models of the rate of mass loss of red giants, also known as Mira variables, Bowen and Willson have found that the final mass loss process ramps up steeply only near the very end of the red giant stage, and therefore is over in a much shorter period of time, not giving Earth much of a chance to get out of the way.
"The Sun will expand and include Earth's orbit (a distance nominally of 93 million miles) before the Sun loses enough mass to let Earth move away," Willson said. "Most likely, the Sun will expand to hide the Earth for a few centuries and then retreat to reveal it, still mostly intact, at least once before the end."
Like in real estate, the planets will find that location is everything.
"We are confident that Jupiter, Saturn, Uranus and Neptune (the large, gaseous outer planets) will survive after the Sun fades to a dim remnant," she adds. "We are not sure about Mars. Mercury and Venus? They're toast long before Earth gets into trouble."