(September 30, 2015) Water reserves found on the moon are the result of asteroids
acting as “delivery vehicles” and not of falling comets as was previously
thought. Using computer simulation, scientists from MIPT and the RAS Geosphere
Dynamics Institute have discovered that a large asteroid can deliver more water
to the lunar surface than the cumulative fall of comets over a billion year
period. Their research is discussed in an article recently published in the
journal Planetary and Space Science.
At the beginning of the space age, during the days of the
Apollo program, scientists believed the moon to be completely dry. At these
earliest stages in satellite evolution, the absence of an atmosphere and the
influence of solar radiation were thought enough to evaporate all volatile
substances into space. However, in the1990s, scientists obtained data from the
Lunar Prospector probe that shook their confidence: the neutron current from
the satellite surface was indicative of a larger fraction of hydrogen at the
near-surface soil of some regions of the moon, which one could interpret as a
sign of the presence of water.
In order to explain how water could be kept on the moon’s
surface, scientists formulated a theory known as “cold traps.” The axis of the
moon’s rotation is nearly vertical, which is why in the polar regions there are
craters with floors that are never exposed to sunlight. When comets consisting
mostly of water ice fall, evaporated water can gravitate into those “traps” and
remain there indefinitely, as solar rays do not evaporate it.
In recent years, lunar missions (the Indian Chandrayan
probe, the American LRO, data from the Cassini probe and Deep Impact) have
brought scientists two pieces new information. The first is that there are
indeed considerate quantities of water and hydroxyl groups in the near-surface
soil on the moon. The LCROSS experiment, in which a probe purposely crashed
onto the moon resulting in the release of a cloud of gas and dust that was
later studied with the use of a spectrometer, directly confirmed the existence
of water and other volatile substances. The second piece of new information
came when the Russian LEND apparatus mounted on board LRO generated a map of
water distribution on the moon’s surface.
Temperature of the
surface around the southern pole of the moon according to LRO data © NASA.
But this second piece has only partly proven their theory:
the map of “cold traps” did not correspond to the map of water deposits. The
scientists had to refine the theory, and the idea of “lunar congelation” was
proposed. It allowed accepting that “survival” of water ice in the regions
exposed to sunlight is possible under a soil blanket. It was also suggested
that a substantial part of “water” seen by the probes is implanted solar wind:
hydrogen atoms from solar wind react with oxygen atoms and form an unstable
“dew” of water molecules and hydroxyl groups. Scientists left the possibility
open that water could exist in a bound state, i.e. in hydrated minerals.