182 Encyclopedia of the Solar System
and hence are probably convective throughout. Barring ex-
otic chemical or fluid dynamical effects, then, one expects
that such oceans lack thermoclines. In many cases, these
oceans may be substantially thicker than Earth’s oceans; es-
timates suggest that Europa’s ocean thickness lies between
50 and 150 km.
The abundant life that occurs near deep-sea vents (“black
smokers”) in Earth’s oceans has led to suggestions that simi-
lar volcanic vents may help power life in Europa’s ocean. (In
contrast to Europa, any oceans in Callisto and Ganymede
would be underlain by high-pressure polymorphs of ice
rather than silicate rock, so such silicate–water interactions
would be weaker.) However, much of the biological rich-
ness of terrestrial deep-sea vents results from the fact that
Earth’s oceans are relatively oxygenated; when this oxidant-
rich water meets the reducing water discharged from black
smokers, sharp chemical gradients result, and the resulting
disequilibrium provides a rich energy source for life. Thus,
despite the lack of sunlight at Earth’s ocean floor, the biolog-
ical productivity of deep-sea vents results in large part from
the fact that the oceans are communicating with an oxygen-
rich atmosphere. If Europa’s ocean is more reducing than
Earth’s ocean, then the energy source available from chem-
ical disequilibrium may be smaller. Nevertheless, a range
of possible disequilibrium reactions exist that could provide
energy to drive a modest microbial biosphere on Europa.
5. Climate
Earth’s climate results from a wealth of interacting physi-
cal, chemical, and biological effects, and an understanding
of current and ancient climates has required a multidecadal
research effort by atmospheric physicists, atmospheric
chemists, oceanographers, glaciologists, astronomers, ge-
ologists, and biologists. The complexity of the climate
system and the interdisciplinary nature of the problem
have made progress difficult, and even today many as-
pects remain poorly understood. “Climate” can be defined
as the mean conditions of the atmosphere/ocean system—
temperature, pressure, winds/currents, cloudiness, atmo-
spheric humidity, oceanic salinity, and atmosphere/ocean
chemistry in three dimensions—when time-averaged over
intervals longer than that of typical weather patterns. It
also refers to the distribution of sea ice, glaciers, continen-
tal lakes and streams, coastlines, and the spatial distribution
of ecosystems that result.
5.1 Basic Processes—Greenhouse Effect
Earth as a whole radiates with an effective temperature of
255 K, and therefore its flux peaks in the thermal infrared
part of the spectrum. This effective temperature is 30 K
colder than the average temperature on the surface, and
quite chilly by human standards.
What ensures a warm surface is the wavelength-
dependent optical properties of the troposphere. In partic-
ular, infrared light does not pass through the troposphere
as readily as visible light. The Sun radiates with an effec-
tive temperature of 5800 K and therefore its peak flux is in
the visible part of the spectrum (or stated more correctly in
reverse, we have evolved such that the part of the spectrum
that is visible to us is centered on the peak flux from the Sun).
The atmosphere reflects about 31% of this sunlight directly
back to space, and the rest is absorbed or transmitted to the
ground. The sunlight that reaches the ground is absorbed
and then reradiated at infrared wavelengths. Water vapor
(H 2 O) and carbon dioxide (CO 2 ), the two primary green-
house gases, absorb some of this upward infrared radiation
and then emit it in both the upward and downward direc-
tions, leading to an increase in the surface temperature to
achieve balance. This is the greenhouse effect. Contrary to
popular claims, the elevation of surface temperature by the
greenhouse effect is not a situation where “the heat cannot
get out.” Instead, the heat must get out, and to do so in
the presence of the blanketing effect of greenhouse gases
requires an elevation of surface temperatures.
The greenhouse effect plays an enormous role in the
climate system. A planet without a greenhouse effect, but
otherwise identical to Earth, would have a global-mean sur-
face temperature 17◦C below freezing. The oceans would
be mostly or completely frozen, and it is doubtful whether
life would exist on Earth. We owe thanks to the green-
house effect for Earth’s temperate climate, liquid oceans,
and abundant life.
Water vapor accounts for between one third and two
thirds of the greenhouse effect on Earth (depending on
how the accounting is performed), with the balance re-
sulting from CO 2 , methane, and other trace gases. Steady
increases in carbon dioxide due to human activity seem
to be causing the well-documented increase in global sur-
face temperature over the past∼100 years. On Mars, the
primary atmospheric constituent is CO 2 , which together
with atmospheric dust causes a modest 5 K greenhouse ef-
fect. Venus has a much denser CO 2 atmosphere, which,
along with atmospheric sulfuric acid and sulfur dioxide,
absorbs essentially all the infrared radiation emitted by
the surface, causing an impressive 500 K rise in the sur-
face temperature. Interestingly, if all the carbon held in
Earth’s carbonate rocks were liberated into the atmosphere,
Earth’s greenhouse effect would approach that on Venus.
[SeeMars Atmosphere: History and Surface Inter-
action;Venus: Atmosphere.]5.2 Basic Processes—Feedbacks
The Earth’s climate evolves in response to volcanic erup-
tions, solar variability, oscillations in Earth’s orbit, and
changes in internal conditions such as the concentra-
tion of greenhouse gases. The Earth’s response to these