Geology Reference
In-Depth Information
ward and try to make the star bigger. The Sun, in its present stately hydrogen-burning
phase, has achieved a stable diameter of about 870,000 miles—the size that has persisted
for 4.5 billion years and will continue to persist for about 5 billion more.
The Sun is large enough that when the hydrogen-burning phase is finally over, a new,
frantically energetic helium-burning phase will begin. Helium, the by-product of hydrogen
fusion reactions, can fuse to other helium atoms to make the element carbon, but this new
solar strategy will have catastrophic consequences for the inner planets. Because of the
more energetic helium reactions, the Sun will swell larger and larger, like a crazy super-
heated balloon, into a pulsating red giant star. It will swell past the orbit of little Mercury,
engulfing the tiny planet. It will swell past the orbit of our neighbor Venus, swallowing
that sister world as well. It will swell to more than one hundred times its present diamet-
er—even past the orbit of Earth.
The exact details of Earth's endgame are murky. According to some bleak scenarios, the
red giant Sun will simply overwhelm Earth, which will vaporize in the solar atmosphere
and be no more. Other models have the Sun shedding more than a third of its present mass
in unimaginable solar winds (which would ceaselessly blast Earth's dead surface). As the
Sun becomes less massive, Earth's orbit will move outward—perhaps just enough to avoid
being engulfed. But if we are not engorged by the expanding Sun, all that would remain of
ouroncebeautifulblueworldwouldbeanutterlybarrenorbitingcinder.Sparsesubsurface
microbialecosystemsmaypersevereforanotherbillionyears,butneveragainwilltheland
be lush and verdant.
Desert World: Two Billion Years from Now
Eversoslowly,eveninitspresentcalmhydrogen-burningstate,theSunisgettinghotter.In
the beginning, 4.5 billion years ago, the Sun shone with 70 percent of its present light. The
Great Oxidation Event 2.4 billion years ago found a Sun shining with perhaps 85 percent
of today's intensity. And a billion years from now, the Sun will be brighter still.
Foratime,perhapsformanyhundredsofmillionsofyears,Earth'sfeedbacksmaymod-
erate the change. More heat means more evaporation, which produces more clouds, which
reflects more sunlight back into space. More heat means faster weathering of rocks, which
consumesmorecarbondioxide,whichlowerstheamountofgreenhousegases.Andsoneg-
ative feedback loops may keep Earth habitable for a long time.
But there will come a tipping point. Smaller Mars reached that critical time billions of
yearsago,asalmost allitssurfacewaterwaslost.Abillion yearsfromnow,Earth'soceans
will have begun to evaporate at an alarming rate, and the atmosphere will have become a
perpetualsauna.Noicecapsorglaciers willremain,aseventhepoleswillbecometropical
zones. For a span of many millions of years, life may thrive in such a hothouse environ-
