868 Encyclopedia of the Solar System
FIGURE 11 Habitable zone (large
rectangle) in terms of planetary
temperatures and planetary mass.
The objects of the Solar System
(open squares) are shown as well as
the newly discovered extrasolar
planets (filled circles).atmosphere that kept Mars warm has dissipated, presum-
ably into carbonate rocks located on the floor of ancient
lake and ocean basins on Mars. Mars lacks plate tectonics
because it is too small, 10 times smaller than the Earth,
to maintain the active heat flows that drive tectonic activ-
ity. The low gravity of Mars and the absence of a magnetic
field also contributed to the loss of its atmosphere. Hence,
planetary size and its effect on geological activity also play
a role in determining the surface temperature and thereby
the presence of liquid water and life. Figure 11 shows the
habitable zone for a planet in terms of its surface tempera-
ture and mass. The planets of the Solar Systems and some
of the extrasolar planets discovered as of 2006 are shown.
9. Conclusion
Life is a planetary phenomenon. We see its profound influ-
ences on the surface of one planet—the Earth. Its origin,
history, present reach, and global scale interactions remain
a mystery primarily because we have only one datum. Many
questions about life await the discovery of another life form
with which to compare. Mars in its early history is probably
the best prospective target in the search for extraterrestrial
lifeforms, although Europa and Enceladus are also promis-
ing candidates because of the likely presence of liquid water
beneath a surface ice shell and the possibility of associated
hydrothermal vent activity. In any case, it is likely that our
true understanding of life is to be found in the exploration
of other worlds—both those with and without life forms.
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