212 Encyclopedia of the Solar System
8. Earth as a Rosetta Stone
The Earth is unique among its partners in our solar system in
that it has had liquid water oceans for most of its history, has
a highly mobile crust, and a dynamically convecting interior.
This combination means that the surface is and has been
constantly driven by the movement of the interior, such
that the oldest terrestrial subaerial landscapes are at most
∼10% of the age of the planet, and the oldest submarine
landscapes are only a little more than 10% of that. Thus, the
Earth not only has one of the most globally dynamic surfaces
in the solar system, but its interior is also one of the most dy-
namic. Only the tidally wracked and volcanically incessant
surface of Io, Jupiter’s innermost satellite, may be younger
and more active. Driven by internal forces, the periodic
conglomeration and separation of continental landmasses,
causing opening and closing of oceans, and construction
and destruction of mountain ranges profoundly impact the
global climate. The environmental stresses caused by such
reshuffling of the surface may themselves have influenced
the progress of evolution on the planet—evolution that was
possibly reset every 100 Myr or so by devastating asteroidal
impacts. In the final analysis, the Earth is the only plane-
tary body with which the human species has had intimate
experience—for millenia. Thus, beyond being our home,
the Earth is for us a crucial yardstick—a Rosetta stone—by
which we will measure and interpret the processes, inter-
nal structure, and overall histories of other planets in this
solar system and, someday, of other planets around other
stars. [Also SeeEarth as aPlanet:Atmosphere and
Oceans.]
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