Geology Reference
In-Depth Information
water-bearing) deposits. The spectrometer was designed to detect such neutrons as they
spray out from a wide swath of the Martian surface, from the Equator to high latitudes.
However, these intriguing results raised as many questions as they answered, for the exact
form of water—liquid, ice, or mineral bound—could not be determined.
In2007NASA'sMarsReconnaissanceOrbiter,usingground-penetratingradar,provided
a much higher-resolution picture of this buried water. These pioneering measurements de-
tected glacier-size accumulations of ice in the mid-southern latitudes. More recently, the
European Space Agency's Mars Express Orbiter employed a similar radar system to detect
deep ice across a wider swath of the planet. Some areas near the south pole reveal ice-rich
zonesmorethanfifteenhundredfeetdeep.Indeed,Marsmayholdaquantityofsubsurface
frozenwaterequivalenttoaglobe-spanningoceanhundredsoffeetdeep.SoEarth'soceans
may have once had a Martian cousin.
Water can also be revealed by the presence of distinctive rocks and minerals. NASA's
Phoenixlanderanditspluckyrovers,SpiritandOpportunity,foundabundantcomplement-
ary evidence in the form of minerals formed through water-rock interactions. Water-rich
clay minerals turn out to be a common phenomenon in the near-surface environment of
Mars, and they may represent much of the hydrogen-rich material that the neutron experi-
ments observed several years earlier. Evaporite minerals characteristic of dried-up lakes or
oceans are also common, as is opal—a poorly crystallized variety of quartz that typically
forms in wet sediments on the ocean floor.
As planetary scientists examine the red planet with new eyes, they also see more and
more evidence that water once flowed freely across the sculpted Martian surface. High-
resolution photos reveal ancient river valleys and gullies strewn with boulders, teardrop-
shapedislands,slumps,andbraidedstreamchannels.Theselandformscutthroughlayersof
sedimentthatappeartohavebeenlaiddowninshallowlakesorseas.Indeed,thebeachlike
terraces that girdle the northern hemisphere of Mars imply that northern-latitude oceans
may once have covered more than a third of the Martian surface. If so, then cooler Mars
may have been a blue, life-giving planet many millions of years before Earth.
And then there's the Moon—a key to understanding the history of water on its larger
companion, Earth. The Moon is bone-dry by conventional wisdom (actually drier than
bone, which retains a significant water component even when baked in the desert sun).
Multiple lines of evidence point to this aridity: Earth-based telescopes reveal no character-
istic infrared absorption; Moon rocks from all six Apollo landing sites held no detectable
tracesofwater(atleastby1970analyticalstandards);andthefindingofunrustedironmet-
al after four billion years on the lunar surface would seem to preclude even a trace of cor-
rosive water.
It's a funny thing about conventional wisdom, though. Eventually someone will chal-
lengewhateveryoneelseknowstobetrue,andonceinawhilesomethingreallyinteresting
