Scientists predict greater
longevity for planets with life
Roughly a billion years from now, the ever-increasing radiation
from the sun will have heated Earth into inhabitability; the carbon
dioxide in the atmosphere that serves as food for plant life will
disappear, pulled out by the weathering of rocks; the oceans will
evaporate; and all living things will disappear.
Or maybe not quite so soon, say researchers from the California
Institute of Technology (Caltech), who have come up with a
mechanism that doubles the future lifespan of the biosphere—while
also increasing the chance that advanced life will be found
elsewhere in the universe.
A paper describing their hypothesis was published June 1 in the
early online edition of the Proceedings of the National Academy of
Sciences (PNAS).
Earth maintains its surface temperatures through the greenhouse
effect. Although the planet's greenhouse gases—chiefly water vapor,
carbon dioxide, and methane—have become the villain in global
warming scenarios, they're crucial for a habitable world, because
they act as an insulating blanket in the atmosphere that absorbs
and radiates thermal radiation, keeping the surface comfortably
warm.
As the sun has matured over the past 4.5 billion years, it has
become both brighter and hotter, increasing the amount of solar
radiation received by Earth, along with surface temperatures. Earth
has coped by reducing the amount of carbon dioxide in the
atmosphere, thus reducing the warming effect. (Despite current
concerns about rising carbon dioxide levels triggering detrimental
climate change, the pressure of carbon dioxide in the atmosphere
has dropped some 2,000-fold over the past 3.5 billion years;
modern, man-made increases in atmospheric carbon dioxide offset a
fraction of this overall decrease.)
The problem, says Joseph L. Kirschvink, the Nico and Marilyn Van
Wingen Professor of Geobiology at Caltech and a coauthor of the
PNAS paper, is that "we're nearing the point where there's not
enough carbon dioxide left to regulate temperatures following the
same procedures."
Kirschvink and his collaborators Yuk L. Yung, a Caltech professor
of planetary science, and graduate students King-Fai Li and Kaveh
Pahlevan, say that the solution is to reduce substantially the
total pressure of the atmosphere itself, by removing massive
amounts of molecular nitrogen, the largely nonreactive gas that
makes up about 78 percent of the atmosphere. This would regulate
the surface temperatures and allow carbon dioxide to remain in the
atmosphere, to support life, and could tack an additional 1.3
billion years onto Earth's expected lifespan.
In the "blanket" analogy for greenhouse gases, carbon dioxide would
be represented by the cotton fibers making up the blanket. "The
cotton weave may have holes, which allow heat to leak out,"
explains Li, the lead author of the paper.
"The size of the holes is controlled by pressure," Yung says.
"Squeeze the blanket," by increasing the atmospheric pressure, "and
the holes become smaller, so less heat can escape. With less
pressure, the holes become larger, and more heat can escape," he
says, helping the planet to shed the extra heat generated by a more
luminous sun.
Strikingly, no external influence would be necessary to take
nitrogen out of the air, the scientists say. Instead, the biosphere
itself would accomplish this, because nitrogen is incorporated into
the cells of organisms as they grow, and is buried with them when
they die.
In fact, "This reduction of nitrogen is something that may already
be happening," says Pahlevan, and that has occurred over the course
of Earth's history. This suggests that Earth's atmospheric pressure
may be lower now than it was earlier in the planet's history.
Proof of this hypothesis may come from other research groups that
are examining the gas bubbles formed in ancient lavas to determine
past atmospheric pressure: the maximum size of a forming bubble is
constrained by the amount of atmospheric pressure, with higher
pressures producing smaller bubbles, and vice versa.
If true, the mechanism also would potentially occur on any
extrasolar planet with an atmosphere and a biosphere.
"Hopefully, in the future we will not only detect Earth-like
planets around other stars but learn something about their
atmospheres and the ambient pressures," Pahlevan says. "And if it
turns out that older planets tend to have thinner atmospheres, it
would be an indication that this process has some
universality."
Adds Yung: "We can't wait for the experiment to occur on Earth. It
would take too long. But if we study exoplanets, maybe we will see
it. Maybe the experiment has already been done."
Increasing the lifespan of our biosphere—from roughly 1 billion to
2.3 billion years—has intriguing implications for the search for
life elsewhere in the universe. The length of the existence of
advanced life is a variable in the Drake equation, astronomer Frank
Drake's famous formula for estimating the number of intelligent
extraterrestrial civilizations in the galaxy. Doubling the duration
of Earth's biosphere effectively doubles the odds that intelligent
life will be found elsewhere in the galaxy.
"It didn't take very long to produce life on the planet, but it
takes a very long time to develop advanced life," says Yung. On
Earth, this process took four billion years. "Adding an additional
billion years gives us more time to develop, and more time to
encounter advanced civilizations, whose own existence might be
prolonged by this mechanism. It gives us a chance to meet."
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California Institute of Technology
Thanks to California Institute of
Technology for this article.