Geology Reference
In-Depth Information
will be found. In 1994 a single flyby of the Clementine mission produced radar measure-
ments that were consistent with water ice, though many planetary scientists were uncon-
vinced. Four years later the Lunar Prospector employed neutron spectroscopy to detect a
significant concentration of hydrogen atoms, and hence possibly water ice or water-con-
taining minerals, near the poles. Still, many experts pointed to implanted hydrogen ions
from the Sun's solar wind as a more likely source of the signal. Then in October 2009
NASA smashed the upper stage of an Atlas rocket into one of these craters (the Cabeus
crater, near the southern lunar pole) and scrutinized the plume of impact debris for signs
of H
2
O. Sure enough, the flurry of dust incorporated a small but significant amount of the
life-giving stuff—enough to renew interest in lunar water and its possible origins. Three
back-to-back articles in
Science
that same October established that evidence for water on
the Moon is now unambiguous.
Enter Erik Hauri and his colleagues at the Carnegie Institution. Using an ion micro-
probe—a highly sensitive instrument that hadn't been available to the first generation of
scientists who studied the Apollo samples—Hauri's team has revisited the colorful glass
beads of the sort I studied in my first geology job, picking out Moon specks, back in 1976.
Other scientists had examined the glass beads for signs of water decades earlier, but their
detection capacities were no match for the ion microprobe's ability to resolve measure-
ments at the scale of a millionth of an inch. Hauri and his coworkers polished a variety of
glass beads so that their round cross sections were revealed in the ion probe. The beads'
outer rims proved to be very dry, with only a few parts per million water, but the cores
of the largest beads have as much as a hundred parts per million. Over billions of years,
most of the glass beads' original water has evaporated to space, more from the outsides
than from the cores. However, based on the significant amount of remaining water deep
inside the beads, Hauri and his colleagues calculate that the original water content of the
Moon's magma may have been as high as 750 parts per million—a lot of water, compar-
able to many volcanic rocks on Earth, and more than enough to drive surface volcanism
that would have dispersed magma in explosive eruptions billions of years ago.
If that much water powered volcanoes in the Moon's past, then a great deal of water
must still be locked somewhere in the Moon's frozen interior. And since the Moon formed
primarily by Theia's wholesale excavation of Earth's primordial mantle, our planet's deep
interior likely holds prodigious amounts of unseen water as well.
The Visible Water Cycle
However much water we end up finding on Mars or the Moon (and it now appears there's
a lot), Earth remains our Solar System's one and only water world. The story of Earth's
water—how much there is, what form it assumes, where it resides, and how it moves—is
