592 PART 5^ |^ LIFE
liquid as the medium in which the processes of life can occur.
Water is a cosmically abundant substance with properties such as
high heat capacity that set it apart from other common molecules
that are liquid at the temperatures of planetary surfaces. Maybe
there are other liquids that can support the processes of life on
other planets, but water, aside from being common in the uni-
verse, has characteristics that cause Earth scientists to regard it as
special, and not just because they are made of water themselves.
Many worlds in the solar system can be eliminated imme-
diately as hosts for water-based life because liquid water is not
possible there. Th e moon and Mercury are airless, and water
would boil away into space immediately. Venus has traces of
water vapor in its atmosphere, but it is too hot for liquid water
to survive on the surface. Th e Jovian planets have deep atmo-
spheres, and at certain altitudes it is likely that water condenses
into liquid droplets. However, it seems unlikely that life could
have originated there. Th e Jovian planets do not have solid
surfaces (see Chapters 23 and 24), so isolated water droplets
cannot mingle to mimic the rich primordial oceans of Earth,
where organic molecules grew and interacted. Additionally,
powerful downdraft currents in the atmospheres of the giant
planets would quickly carry any reproducing molecules that
did form there into inhospitably hot lower regions.
As you learned in Chapter 23, at least one of the Jovian
satellites could potentially support life. Jupiter’s moon Europa
appears to have a liquid-water ocean below its icy crust, and
minerals dissolved in the water could provide a source of raw
material for chemical evolution. Europa’s ocean is kept warm
and liquid now by tidal heating. Th ere also may be liquid waterfl oor or in hot rock deep underground. Others suggest that the
heavy bombardment during the end of the solar system’s forma-
tion (see Chapter 19) would have repeatedly boiled much of
Earth’s oceans away. If life had already begun by then, the only
organisms that survived this phase of the planet’s history to become
our ancestors by natural selection would have been heat resistant.
Life may have required special circumstances to start, but
once it started, biological evolution allowed life to spread across
Earth and adapt to a wide range of conditions. Eventually all
niches—even extreme environments—became occupied. Th is
means that, if life begins on a planet, even if most of the
environment of the planet later becomes inhospitable as seems
to have happened on Mars (see Chapter 22), some life could
continue to survive.
It is diffi cult to pin down a range of environments and be
sure that life cannot exist outside those conditions, so long as
there is even occasionally some liquid water present. Scientists
searching for life on other worlds must keep in mind Earth’s
extremophiles and the harsh conditions in which they thrive.
Life in Our Solar System
Could there be carbon-based life elsewhere in our solar system?
Liquid water seems to be a requirement of carbon-based life,
necessary both as the medium for vital chemical reactions and to
transport nutrients and wastes. It is not surprising that life devel-
oped in Earth’s oceans and stayed there for billions of years before
it was able to colonize the land.
Scientists are in general agreement that any world harboring
living things must have signifi cant quantities of some type of
■ Figure 26-8
Every life form on Earth has evolved to survive in some ecological niche. (a) Wekiu bugs live with the astronomers at an altitude of 13,800 feet atop the
Hawaiian volcano Mauna Kea. The bugs inhabit spaces between the icy cinders and eat insects carried up by ocean breezes. (Kris Koenig/Coast Learning Systems)
(b) Grand Prismatic Spring in Yellowstone National Park is about 90 meters (300 feet) across. Colonies of thermophilic (heat-loving) single-celled organisms
thrive around the edge of the pool at temperatures up to 72°C (162°F) and produce the yellow, orange, and red pigments. The water in the center of the pool
is too hot even for thermophiles. (S. Leach/U. Colorado Denver)
a b