Geology Reference
In-Depth Information
this hottest of planets also the most desiccated. Venus, the next planet out, may initially
have had an Earth-like share of water, but today it appears to be almost totally lacking in
near-surface H
2
O. Its thick, superheated carbon dioxide atmosphere speaks of a runaway
greenhouse effect and ancient loss of whatever near-surface water might have been there
when it formed.
Mars, with its white polar ice caps that expand and recede in concert with the 687-day
cycleofMartianseasons,isanaltogetherdifferentstory.Astronomershavelongspeculated
that the red planet might be a wet, living world. In the 1870s, during a particularly close
orbital approach of Mars to Earth, Italian astronomer Giovanni Schiaparelli documented
dark, linear features that he interpreted as natural, possibly water-bearing channels, or
can-
ali
in Italian. When the English translation of his original descriptions erroneously called
them canals, implying high-tech engineered structures, the idea of an intelligent, extinct
Martian race took on a life of its own. Most notable of these Mars life enthusiasts was
Harvard-trained astronomer Percival Lowell, who became obsessed by Schiaparelli's dis-
coveries in the 1890s. He used his family's wealth to construct a private observatory in
Flagstaff, Arizona, and there he devoted himself to the study of Mars. Employing a state-
of-the-art twenty-four-inch telescope under the clear Arizona skies, he imagined that he
could resolve a vast network of canals stretching from the presumably ice-covered poles to
thedesiccatedEquator.In his immensely popular topics,
Mars
(1895),
Mars and Its Canals
(1905), and
Mars as the Abode of Life
(1908), Lowell describes the last desperate techno-
logical masterpiece of a vanished, water-starved race.
Lowell's colorful imaginings fueled a wave of science fiction stories and novels (in-
cluding H. G. Wells's 1898 classic,
The War of the Worlds
) but did little to convince the
scientific community that Mars is wet, much less living. In spite of more than a century
of subsequent studies employing larger and larger telescopes, supplemented by a flurry of
sophisticated Mars flyby missions (commencing with Mariner 4 in 1965), orbiters (Marin-
er 9 was the first in 1971), and landers (starting with Viking in 1976), definitive evidence
for Martian water repositories and their significant extent proved elusive. The presence of
watericeinthenorthernpolarregionswasfinally,unambiguouslydocumented bytheVik-
ing missions with spectral measurements in the late 1970s, but only since 2000, with an
arsenal of instruments on the latest generation of satellites, coupled with scraping tools on
the Phoenix lander and the Spirit and Opportunity rovers, has the true vast extent of water
and the nature of its repositories on Mars been revealed.
TodaymostofMars'swateroccursassubsurfacepermafrostandperhapsasgroundwater
in deeper warm zones—potentially huge repositories that remain hidden from the dry out-
ermost layer. Hints of the extent of this subsurface water were provided in 2002 by the
Mars Odyssey spacecraft, which carried a sophisticated neutron spectrometer. When cos-
mic rays bombard Mars's surface, they can dislodge neutrons from hydrogen-rich (that is,
