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ebb and flow

NASA’s Ebb and Flow gravity mapping twin satellites have ended their mission to the Moon.

Ebb and Flow were commanded to slam into a 2 km-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. Sally 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.

Together known as Grail (Gravity Recovery and Internal Laboratory), the pair hit the flank of the lunar-nearside mountain about 3 km 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.

NASA’s Ebb and Flow gravity mapping twin satellites have ended their mission to the Moon

NASA’s Ebb and Flow gravity mapping twin satellites have ended their mission to the Moon

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.

The Grail mission has produced the highest resolution, highest quality global gravity maps for any planetary body in the Solar System, including Earth.

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 analyzed but already scientists are getting some tantalizing 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 pulverized 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.

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 recognize 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. Maria 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.”

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Cryosat, the European radar spacecraft launched in 2010 to monitor changes in the thickness and shape of polar ice, is now watching the ebb and flow of Arctic sea ice with high precision.

Scientists have spent the past two years getting to grips with its data.

And on Tuesday, they reported that Cryosat was now delivering an unprecedented view of the seasonal growth and retreat of sea ice spanning the entire Arctic basin.

The researchers also released a map showing the difference in height across the Greenland ice sheet.

“The message is that Cryosat is working extremely well. Its data are very reliable and the measurements we have match reality,” said Prof. Volker Liebig, the director of Earth Observation at the European Space Agency (ESA).

“We now have a very powerful tool to monitor the changes taking place at the poles,” he said.

The ESA director delivered an update on the mission at London’s Royal Society. The information was also being released here at the European Geosciences Union (EGU) meeting in Vienna, Austria.

Several satellites have already detailed the recent and rapid erosion of summer sea ice extent as the Arctic has warmed.

But Crysosat’s innovation has been to provide a means to get at a figure for ice volume – a far more significant number in terms of understanding the long-term viability of the ice.

To do this, the satellite carries one of the highest resolution synthetic aperture radars ever put in orbit.

The instrument sends down pulses of microwave energy which bounce off both the top of the ice and the water in the cracks, or leads, which separate the floes.

Cryosat was launched in 2010 to monitor changes in the thickness and shape of polar ice

Cryosat was launched in 2010 to monitor changes in the thickness and shape of polar ice

By measuring the difference in height between these two surfaces, scientists can, using a relatively simple calculation, work out the overall volume of the marine cover.

The Cryosat team, led from University College London, has spent the period since launch working through the satellite’s measurements, validating and calibrating them against a number of independent observations.

These include data from plane-borne instruments, from direct on-the-ice assessments, and even from scientific sea-floor moorings that profile the ice floes as they pass overhead.

“We can now say with good confidence that Cryosat’s maps of ice thickness are correct to within 10-20 cm,” said Dr. Seymour Laxon, from UCL’s Centre for Polar Observation and Modelling (CPOM).

Tuesday’s release shows a complete seasonal cycle, from October 2010, when the Arctic Ocean was beginning to freeze up following the summer melt, right through to March 2011, when the sea ice was approaching peak thickness. Cryosat found the volume (area multiplied by thickness) of sea ice in the central Arctic in March 2011 to have been 14,500 cubic kilometres.

This figure is very similar to that suggested by PIOMAS (Panarctic Ice Ocean Modeling and Assimilation System), an influential computer model that has been used to estimate Arctic sea ice volume, and which has been the basis for several predictions about when summer sea ice in the north might disappear completely.

In addition to the announcement on sea ice, the Cryosat team also published a digital elevation model (DEM) of Greenland.

The big island, too, has experienced some rapid changes of late and is losing tens of billions of tons of its ice cover to the ocean annually.

The DEM is a map of varying height, and the visualization on this page incorporates a year’s worth of data.

For Cryosat, it is another illustration of its capability. Radar satellites have traditionally struggled to discern the detail in the steep slopes and ridges that mark the edges of ice sheets, but the ESA spacecraft can recover far more information thanks to a special interferometric observing mode that uses two antennas.

“This is really the first demonstration of the interferometer in action,” said Prof. Andrew Shepherd from Leeds University.

“The DEM contains about 7.5 million data points, and we’re pretty confident this will be the best elevation model for Greenland, by some margin. Our next step is to compare it to previous data to see how Greenland has changed.”

Cryosat’s principal investigator, Prof. Duncan Wingham – formerly of UCL but now chief executive of the UK’s National Environment Research Council – summed up: “We have years of data to come, but I think it’s quite clear that we will provide synoptic, accurate, Arctic-wide thickness; and that we will be able to determine the accuracy of the predictions of when the Arctic will be ice-free in Summer.

“And I think it’s also clear we can now sustain coverage of [ice sheets on Antarctica and Greenland] right down to the coast.”

The Cryosat update was timed to coincide with this week’s 50th anniversary of UK activity in orbit.

April 1962 was the month Britain became a space-faring nation with the launch of its first satellite, Ariel-1.