Water detected in rock samples from the Moon's lunar highlands
indicates the celestial body may have had a watery landscape from
its inception, calling into question long-held theories about its
The question of just how watery the Moon is has long been a
contentious one. The reigning theory explaining the satellite's
creation is that a Mars-sized piece of rock slammed into the Earth
in the early stages of its formation about four billion years
ago, breaking off a chunk that would become the Moon as the hot
liquid magma cooled. The force and heat of the impact would have
vapourised any potential liquid -- hence the formation theory is
known as the anhydrous Moon hypothesis -- so trace water detected
in samples brought back from Nasa's Apollo missions was
explained away as being brought there by collisions with frozen meteorites or comets.
However, over the years, the quantities of water detected in
Moon samples has not quite added up. In 2008, bubbles of water
(more specifically hydroxyl, a relation of the water molecule) were
detected in lunar magma formed three billion years ago and
collected in the 1970s. Using an ion microprobe, the water content
was calculated as being 50 parts per million (ppm). Follow up
investigations in 2009 by the Chandrayaan-1 probe discovered that lunar dust was coated in
water -- about a litre for every cubic metre. This
data backed up earlier finds from Nasa's Deep Impact probe and
In 2011, the volcanic rock
samples were revisited (specifically, its melt inclusions) and
it was discovered
that the water quantities were between 615 and 1410ppm, around the
same as Earth's 500 to 1000ppm (in the upper mantle). Together,
these finds led planetary scientists to wonder whether sections of
the Moon have in fact always been naturally (and relatively) rich
The latest find, carried out by a team of geologists, not only
calls into question how water arrived on the Moon, but how the
satellite was created in the first place, since the mineral samples
from the lunar highland's upper crust are thought to be a record of
the most ancient parts of the Moon.
"Because these are some of the oldest rocks from the Moon, the
water is inferred to have been in the Moon when it formed," said a
member of the team, Youxue Zhang of the University of Michigan.
"That is somewhat difficult to explain with the current popular
Moon-formation model, in which the Moon formed by collecting the
hot ejecta as the result of a super-giant impact of a Martian-size
body with the proto-Earth."
Using infrared spectroscopy, Zhang and his team analysed the
samples, which were mainly plagioclase minerals that made their way
to the Moon's surface after being crystallised during the magma
phase. Though the amounts of hydroxyl found did not compare to that
detected in other surface rocks, it still amounted to about 6ppm in
the primary crust and 2.7mm in the highland upper crust, levels the
paper calls "significant".
"The presence of water could imply a more prolonged
solidification of the lunar magma ocean than the once-popular
anhydrous Moon scenario suggests," said Hejiu Hui of the University
of Notre Dame, lead author on the paper.
The find suggests that even in its molten form there was water
present on the Moon -- predicted to be around 320 ppm -- and that
water persisted despite the extreme heat and through the
solidification process. When the ocean of magma crystallised, the
team estimates, water content could have been as much as 1.4
Aside from calling into question the history of the Moon and its
formation, it's another good indicator for astronauts hoping to one
day use the Moon as a landing post to refuel and get supplies. The
Kennedy Space Centre and the Canadian Space Agency plan to land a rover on the Moon called Resolve that's
specifically designed to track down traces of water so we'll know
exactly where's best to setup shop.
The study was published in the journal Nature Geoscience.