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Jonathan Amos By Jonathan Amos Science correspondent, BBC News
The US space agency's (Nasa) Ebb and Flow gravity mapping satellites have ended
their mission to the Moon.
The duo were commanded to slam into a 2km-high mountain in the far lunar north.
The deliberate ditching avoids the possibility of an uncontrolled descent on to
locations of historic importance, such as the Apollo landing sites.
Nasa's deep-space radio-tracking system confirmed the loss of signal from the
satellites just before 22:30 GMT.
Afterwards, it was announced the impact site would be named for Sally Ride, the
first female American astronaut who died earlier this year. Ms Ride's
educational programme had run the outreach cameras on the spacecraft.
The satellite twins returned some remarkable data during their operational
mission, which got under way in March. Their maps of the subtle variations in
gravity across the Moon's surface are expected to transform many areas of
planetary science.
"Ebb and Flow have removed a veil from the Moon and removing this veil will
enable discoveries about the way the Moon formed and evolved for many years to
come," said principal investigator Prof Maria Zuber from the Massachusetts
Institute of Technology, US.
Nasa photo dated June 1983 shows Sally Ride on the Challenger The Sally Ride
Science organisation ran the "Moonkams" on the Grail satellites
Together known as Grail (Gravity Recovery and Interior Laboratory), the pair
hit the flank of the lunar-nearside mountain about 3km and 30 seconds apart.
The peak - located at 75 degrees North latitude close to a crater named
Goldschmidt - was in darkness at the time.
Being only the size of washer-driers, and having completely depleted their fuel
tanks, the pair were not expected to produce any sort of impact flash visible
to Earth observers.
That said, another of Nasa's missions at the Moon, its Lunar Reconnaissance
Orbiter (LRO), was looking out for the crashes.
If it was lucky, LRO's ultraviolet imager might have seen some volatile
materials being driven off the surface by the heat from the impacts. The
orbiter will also image the site in a couple of weeks to see if it can discern
any new craters.
Deep views
The Grail mission has produced the highest resolution, highest quality global
gravity maps for any planetary body in the Solar System, including Earth.
Mountain The mountain (red) represents the partially buried rim of an old
impact basin
The gravity differences the satellites have measured are the result of an
uneven distribution of mass across the Moon.
Obvious examples at the surface include big mountain ranges or deep impact
basins, but even inside the lunar body the rock is arranged in an irregular
fashion, with some regions being denser than others.
Much of the twins' data has yet to be analysed but already scientists are
getting some tantalising new insights into the Moon's structure and history.
"One of the major results that we've found is that the lunar crust is much
thinner than we had believed before, and that a couple of the large impact
basins probably excavated the Moon's mantle, which is very useful in terms of
trying to understand the composition of the Moon as well as the Earth, because
we actually think that the Earth's mantle has a similar composition to the
Moon's mantle," said principal investigator Prof Maria Zuber from the
Massachusetts Institute of Technology, US.
The gravity data also shows the lunar body's top-most layers to be far more
fractured than anyone had previously suspected. These pulverised and porous
materials that coat the surface bear witness to the brutal battering the Moon
received in the first few hundred million years of its existence.
In addition, Ebb and Flow found evidence for great lava-filled fissures just
under all this impact debris.
Grail gravity map Scientists have really only just begun to delve into the
Grail gravity data
These dykes, some hundreds of km long, appear to reach deep into the Moon, and
may hint at an early expansion phase in its history when the hot body expanded
outwards, before eventually cooling and contracting.
Grail data will be critical in tying down ideas for how the Moon came into
existence. The dominant theory calls for a giant impact billions of years ago
between the Earth and a Mars-sized object which threw material into space that
ultimately coalesced into the familiar body we recognise in the sky today.
Some scientists have argued that Earth may once even have had two moons which
later merged - although the Grail data could have sunk this idea.
"We have looked for evidence of the second moon and we have not seen any of the
suggested characteristics of the internal structure of the Moon that would be
consistent with the idea of a second companion," said Prof Zuber.
"That in itself does not rule out that idea at this point. We and others can
look at this in more detail, but nothing jumps out in that regard."
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos