Planet Earth and Earth systems - Physical Processes in Earth and Environmental Sciences - page 4

The resulting polar molecule has
the ability to (1) dissolve ionic
solids and (2) hydrate surface ions

Ice

H

+ve net charge
in this region

Water

Sinking and mixing
cool dense
freshwater

H

Vortices
indicative
of shear
and instability

O

-ve net charge
in this region

+ve net charge
in this region

Fig. 1.4 In the water molecule, the oxygen atom is strongly
electronegative, attracting a higher electron density than the two
bonding hydrogen atoms. Charge field is outlined by grayscale line.

Fig. 1.3 Water circulation in the oceans is aided by density contrasts
due to temperature and salinity variations, illustrated here by the
downflow of dense water from melting ice.

lithospheric plates moving above a flowing and convecting
upper mantle. Major consequences of plate tectonics are
volcanic eruptions and mountain building by the growth of
folds and faults caused by deformation: both phenomena
are associated with earthquakes.

1.2.1 Water, plant life, and plate tectonics

Fig. 1.5 Water molecules aggregate in clusgers of four (tetrahedral
form) due to hydrogen bonding: a pair (dimer) is shown bonded
opposite. Each net

The thermal history and temperature equilibrium of Earth
has allowed copious water to remain at and close to the
planetary surface. The other rocky planets closer to the
Sun may have lost their water by solar boiling early on.
The Martian landscape still bears telltale signs of extensive
past surface water transfer (channels, subaqueous ripple
structures). Today water is stored as permafrost at
the poles and as seasonal water vapor in weathered surface
layers (regolith).
Water has a distinctive polar molecular structure
(Figs 1.4 and 1.5). It is important for the following reasons:
1 Water vapor is the most important greenhouse gas , absorbing
infrared radiation in several absorption bands from reflected
incoming short wavelength solar radiation. Water vapor in
clouds thus plays an important role in the process of climate
regulation because of its feedback role in reflecting and absorbing
incoming and outgoing radiation.
2 Its very high thermal heat capacity causes ocean currents
to flow and, in conjunction with the atmosphere, enables
heat to be transferred tremendous distances meridionally.

ve
charged oxygen from an adjacent molecule. The tetrahedral clusters
cause high surface tension and capillary pressure. The increased
frequency of hydrogen bonding below 4

ve charged hydrogen atom attracts a net

C causes the anomalous

expansion of water.

3 The coexistence of solid, liquid, and gaseous phases at
the Earth's surface enables rapid heat transfers to be made
as the phases are forced to change from one to another in
response to motions of the atmosphere and oceans, for
example, the latent heat released as water vapor forms
liquid rain droplets.
4 It provides life's medium via cellular development and
photosynthesis.
5 In its liquid state, together with oxygen and carbon
dioxide, it helps cause continental rock weathering
and ocean crust alteration: it is thus responsible for global
elemental redistribution called geochemical cycling .
6 Streams and waves of water physically transport weath-
ered material far and wide over the planetary surface.