Geoscience Reference
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Centauri. Occasionally perturbed by a passing star, the Oort Cloud
may be the source of the comets that race towards the inner regions
of the solar system.
So, in our solar system there really is water everywhere, and a diver-
sity of oceans, some larger than those of Earth. Let us count them up.
Including Earth, there are currently seven (eight if we count Titan's
two-storey ocean system, with hydrocarbons above and water below).
Most are subsurface, separated from the exterior by an icy crust: only
Earth and Titan possess surface seas, of water and hydrocarbons
respectively. They vary in temperature and chemistry and physical
structure, and most remain profoundly mysterious in many aspects,
being sensed only indirectly by the modern magic of spacecraft-borne
instrumentation. At least two planets, Venus and Mars, have lost their
oceans—not counting any short-lived hydrospheres attached to the
planetesimals that collided and aggregated to form the existing plan-
ets. More might acquire or reacquire them when the Sun eventually
inflates into a red giant (Mars briefly, as we have seen, and perhaps
with more relevance for future biology, distant Titan). At least three
of the non-Earth oceans (Europa, Titan, and Callisto) and one of the
dead ones (Mars) have been linked with the possibility of alien life.
That is just the tally for one normal planetary system circling around
a small-to-medium-sized, perfectly banal star.
Planets and Kuiper Belts have already been detected around other
nearby stars. There must, then, be oceans on worlds so distant from
us that even their existence has only just been detected by the most
sophisticated of modern techniques. But even this far out in space, we
can detect the faintest signals of water, and can use that to begin to
reconstruct oceans in distant star systems.
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