Earth as a Planet: Atmosphere and Oceans 179
one third the mass of Earth’s oceans. On the other hand,
Earth’s oceans constitute only 0.02% of Earth’s total mass;
the mean oceanic thickness of 3.7 km pales in comparison
to Earth’s 6400 km radius, implying that the oceans span
only 0.06% of Earth’s width. The Earth is thus a relatively
dry planet, and the oceans truly are only skin deep.
It is possible that Earth’s solid mantle contains a mass of
dissolved water (stored as individual water molecules inside
and between the rock grains) equivalent to several oceans’
worth of water. Taken together, however, the total water in
Earth probably constitutes less than 1% of Earth’s mass.
In comparison, most icy satellites and comets in the outer
solar system contain∼40–60% H 2 O by mass, mostly in solid
form. This lack of water on Earth in comparison to outer
solar-system bodies reflects the relatively dry conditions in
the inner solar system when the terrestrial planets formed;
indeed, the plethora of water on Earth compared to Venus
and Mars has raised the question of whether even the paltry
amount of water on Earth must have been delivered from
an outer-solar-system source such as impact of comets onto
the forming Earth.
The modern oceans can be subdivided into the Pacific,
Atlantic, Indian, and Arctic Oceans, but these four oceans
are all connected, and this contiguous body of water is often
simply referred to as the global ocean.
4.1 Oceanic Structure
The top meter of ocean water absorbs more than half of
the sunlight entering the oceans; even in the sediment-
free open ocean, only 20% of the sunlight reaches a depth
of 10 m and only∼1% penetrates to a depth of 100 m
(depending on the angle of the Sun from vertical). Pho-
tosynthetic single-celled organisms, which are extremely
abundant near the surface, can thus only survive above
depths of∼100 m; this layer is called the photic zone. The
much thicker aphotic zone, which has too little light for pho-
tosynthetic production to exceed respiration, extends from
∼100 m to the bottom of the ocean. Despite the impos-
sibility of photosynthesis at these depths, the deep oceans
nevertheless exhibit a wide variety of life fueled in part by
dead organic matter that slowly sediments down from the
photic zone.
From a dynamical point of view, the ocean can be sub-
divided into several layers. Turbulence caused by wind and
waves homogenizes the top 20–200 m of the ocean (depend-
ing on weather conditions), leading to profiles of density,
temperature, salinity, and composition that vary little across
this layer, which is therefore called the mixed layer. Below
the mixed layer lies the thermocline, where the temperature
generally decreases with depth down to∼0.5–1 km. The
salinity also often varies with depth between∼100–1000 m,
a layer called the halocline. For example, regions of abun-
dant precipitation but lesser evaporation, such as the North
Pacific, have relatively fresh surface waters, so the salinity
increases with depth below the mixed layer in those re-
gions. The variation of temperature and salinity between
∼100–1000 m implies that density varies with depth across
this layer too; this is referred to as the pycnocline. Below
the thermocline, halocline, and pycnocline lies the deep
ocean, where temperatures are usually relatively constant
with depth at a chilly 0–4◦C.
The temperature at the ocean surface varies strongly
with latitude, with only secondary variations in longitude.
Surface temperatures reach 25–30◦C near the equator,
where abundant sunlight falls, but plummet to 0◦C near
the poles. In contrast, the deep oceans (>1 km) are gener-
ally more homogeneous and have temperatures between
0–4◦C all over the world. (When enjoying the bathtub-
temperature water and coral reefs during a summer va-
cation to a tropical island, it is sobering to think that if
one could only scuba dive deep enough, the temperature
would approach freezing.) This latitude-dependent upper-
ocean structure implies that the thermocline and pycnocline
depths decrease with latitude: They are about∼1 km near
the equator and reach zero near the poles.
Because warmer water is less dense than colder water,
the existence of a thermocline over most of the ocean im-
plies that the top∼1 km of the ocean is less dense than
the underlying deep ocean. The implication is that, except
for localized regions near the poles, the ocean is stable to
vertical convective overturning.4.2 Ocean Circulation
Ocean circulation differs in important ways from atmo-
spheric circulation, despite the fact that the two are gov-
erned by the same dynamical laws. First, the confinement
of oceans to discrete basins separated by continents pre-
vents the oceanic circulation from assuming the common
east–west flow patterns adopted by most atmospheres. (To-
pography can cause substantial north–south deflections in
an atmospheric flow, which may help explain why Earth’s at-
mospheric circulation involves more latitudinal excursions
than that of the topography-free giant planets; neverthe-
less, air’s ability to flow over topography means that atmo-
spheres, unlike oceans, are still fundamentally unbounded
in the east–west direction.) The only oceanic region un-
hindered in the east–west direction is the Southern Ocean
surrounding Antarctica, and, as might be expected, a strong
east–west current, which encircles Antarctica, has formed
in this region.
Second, the atmosphere is heated from below, but the
ocean is heated from above. Because air is relatively trans-
parent to sunlight, sunlight penetrates through the atmo-
sphere and is absorbed primarily at the surface, where it
heats the near-surface air at the bottom of the atmosphere.
In contrast, liquid water absorbs sunlight extremely well,
so that 99% of the sunlight is absorbed in the top 3%
of the ocean. This means, for example, that atmospheric