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'So long, Ebb and Flow': NASA crashes probes into moon

By CNN Staff
updated 9:21 PM EST, Mon December 17, 2012
An artist's depiction of the twin probes Ebb and Flow that comprise NASA's Gravity Recovery and Interior Laboratory mission.
An artist's depiction of the twin probes Ebb and Flow that comprise NASA's Gravity Recovery and Interior Laboratory mission.
STORY HIGHLIGHTS
  • GRAIL probes Ebb and Flow crash into a mountain near the moon's north pole
  • The site is named for the late American astronaut Sally Ride, who died in July
  • The probes have been diligently mapping the moon and collecting evidence this year

(CNN) -- A pair of robotic twins that have been diligently mapping the moon this year went out with a bang Monday.

As scheduled, the Gravity Recovery and Interior Laboratory probes Ebb and Flow crashed into a mountain on the moon, ending a fruitful mission to study the surface and composition of the celestial body.

"The two probes were sent purposely into the moon because they no longer had enough altitude or fuel to continue science operations," NASA said.

The agency named the site where the spacecraft crashed for Sally Ride, the first American woman astronaut in space. She died in July.

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Ebb and Flow crashed on a mountain near the moon's north pole at 2:28 p.m. PT and 2:29 p.m. PT at a speed of 3,760 mph, NASA said. They were about the size of a washer and dyer.

"We will miss our lunar twins, but the scientists tell me it will take years to analyze all the great data they got, and that is why we came to the moon in the first place," said GRAIL project manager David Lehman of NASA's Jet Propulsion Laboratory. "So long, Ebb and Flow, and we thank you."

Thanks to GRAIL, scientists now have the highest-resolution gravity field map of any celestial body, NASA has said. That means the probes have been making a high-quality map of the gravitational field of the moon, giving scientists unprecedented insight into what's below the surface and how the moon may have formed.

Results from this mission have delivered several other important findings, principal investigator Maria Zuber of the Massachusetts Institute of Technology told reporters last week.

These include:

-- Evidence that the crust of the moon is much thinner than scientists believed.

-- Some of the large impact basins dug into the moon's mantle, which is useful because scientists want to understand the moon's composition. It is believed that the composition of the Earth's mantle is similar to that of the moon.

-- The moon's crust is much more fractured than scientists thought.

-- It appears the crusts of planets, including Mars, have been bombarded in a similar way. If there was water on Mars, as scientists believe there may have once been, where would it have gone? GRAIL, looking at the moon, provides a clue:

"These fractures provide a pathway deep inside the planet, and it's very easy to envision now how a possible ocean at the surface could have found its way deep into the crust of a planet," Zuber said.

Another major finding: large, lava-filled cracks (dykes), which are hundreds of miles long and exist about 6 miles below the surface. There's no record of them at the surface of the moon.

"These dykes actually provide evidence for early expansion of the moon shortly after it formed," Zuber said. "This had been predicted by models, but no evidence had been found."

CNN's Elizabeth Landau contributed to this report.

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