Geoscience Reference
In-Depth Information
monia, methane, sulfuric acid, or hydrogen fluoride (HF); at the right
temperature and pressure, they share some (but not all) of the proper-
ties of water. Aside from numerous science fiction books and movies,
there is also an active scientific literature on this fascinating topic. Dis-
cussions of life in these alternative solutions are, however, highly specu-
lative; one might even say imaginative. Therefore, I'll take the easy road,
and as far as we know for certain, water is the perfect and only solvent
for life.
To summarize, we have highlighted three basic ingredients for life.
These are energy, the chemical components that make up cells, and
water. We will see that the availability of each of these is linked by spe-
cial properties of planet Earth.
Let's start with water. It's no secret that Earth is a watery planet. From
NASA's spell-binding images of our “blue planet” from space, to the
“Rime of the Ancient Mariner” by Samuel Taylor Coleridge, we are re-
minded of the boundless expanse of the global oceans. We will not
concern ourselves at length with why Earth has so much water—likely a
combination of early degassing from its interior as well as delivery from
comets—but rather with why the water we have is, well, wet. The an-
swer of course is that most of the planet is of the right temperature,
lying between the boiling and freezing points of water. But why? Here,
at least in part, we are lucky. We can think of it like this. Earth sits at
a certain distance from the Sun as dictated by its orbit. The Sun has a
certain brightness as dictated by its size and chemical composition.
The amount of the Sun's warmth intercepted by Earth depends on a
combination of these two factors. However, as all planets of our solar
system are warmed by the same Sun, let's consider distance from the
Sun as the key variable. It's easy to imagine that if Earth was closer to
the Sun it would receive more warmth, and less warmth if further away.
As it turns out, Earth resides at a distance from the Sun where the
warmth is sufficient to allow liquid water to persist. If closer to the Sun
like Venus, the temperature becomes too hot, and liquid water is boiled
away into the atmosphere in a so-called runaway greenhouse. Some of
this water may even be completely lost through chemical processes in
the stratosphere. If further from the Sun, like Mars, the surface would
become too cold and therefore frozen. The zone defining the optimal
 
Search WWH ::




Custom Search