Geology Reference
In-Depth Information
ambiguous.Sedimentsfrombothshallow-anddeep-waterenvironmentsarecommon.Life
began and thrived during that interval. How, then, could the early ocean have been liquid?
Certainly some of the heat deficiency caused by the much fainter Sun was compensated
for by a much hotter Earth. Following the crusting over of the primordial magma ocean,
there was still a lot of hot molten rock and volcanic activity to warm the surface. An ocean
on such a planet would have been continuously heated from below, as the black crust
slowly thickened and cooled.
The leading hypothesis to explain the faint Sun paradox points to an exaggerated green-
house warming effect caused by extremely high atmospheric concentrations of carbon di-
oxide—perhaps more than ten times the pressure of our present atmosphere (the same high
CO
2
concentrations that may have acidified the ocean and increased its salinity).
A second clever scenario posits that Earth, in its early black, then blue, phases, absorbed
a far higher percentage of the Sun's energy than the surface does today. Today the oceans
absorb more sunlight than the land—an effect possibly exaggerated long ago by the high
concentrations of iron in the earliest oceans. That increased solar absorption was coupled
withaprobabledearthoflight-scatteringclouds;today,plant-generatedparticlesandchem-
icals play a major role in nucleating clouds, but billions of years ago there were no plants
to trigger cloud formation.
Yet another hypothesis places a large amount of the potent greenhouse gas methane in
the early atmosphere. A curious consequence of a methane-rich atmosphere would have
been chemical reactions high in the atmosphere, where ultraviolet radiation would have
triggered the synthesis of a rich variety of organic molecules, including possible building
blocks of life. Such organic molecules might have caused a thick, smoglike haze, trans-
forming the blue Earth into a distinctly orange world, not unlike Saturn's big moon Tit-
an. And so while we don't yet know the exact combination of factors, we have more than
enough explanations as to how Earth kept well above the freezing point.
Whatwecansaywithconfidenceisthat,onceformed,thisglobaloceanshapedtheplan-
et's outermost layers—in sculpting the land, in the evolution of the increasingly diverse
mineral kingdom, and in the origin of the biosphere. Water still works its magic in every
facet of our lives, as the concentrator of mineral wealth, as the principal agent for surface
change, and as the medium for all life.
*
Though often attributed to Donald Rumsfeld's 2002 speeches, those quotes first appeared
severalyearsearlierintheprefacetomy1997bookwithMaxineSinger,
Why Aren't Black
Holes Black?
