HOUSTON -- Scientists using images taken by NASA's Lunar Reconnaissance Orbiter have found scars on the moon's surface left by the planned impact of two gravity-mapping satellites in December.

This before-and-after Lunar Reconnaissance Orbiter Camera image shows the impact site of GRAIL A (Ebb spacecraft) before and after the spacecraft's descent to the lunar surface. Credit: NASA/GSFC/ASU

The two impact craters, each less than 20 feet across, were carved by the crash of NASA's twin GRAIL spacecraft Dec. 17.

"Finding the impact craters was like finding a needle in a haystack," said Mark Robinson, lead scientist for the Lunar Reconnaissance Orbiter Camera at Arizona State University.

Speaking at the 44th Lunar and Planetary Science Conference near Houston, Robinson described gazing at an image of the predicted impact site looking for any sign of GRAIL's crash location. After hours of looking, he finally spotted the craters out of the corner of his eye.

LRO's camera gathered more detailed imagery in late February, making several passes over the GRAIL impact site and taking pictures to create a stereo three-dimensional view of the region, Robinson said.

The GRAIL satellites, each about the size of a washing machine, crashed into a ridge near the moon's north pole, striking the surface at a low angle and tossing rock and soil along their direction of travel.

"I was a little skeptical that we would even find these impact craters because the spacecraft were so small," Robinson said.

The $471 million Gravity Recovery and Interior Laboratory, or GRAIL, mission mapped the moon's gravity field, shedding light on the lunar interior and revealing the moon's crust is thinner than predicted.

GRAIL also found buried dikes underneath the lunar surface. The features - likely composed of ancient solidified lava - indicate the moon's size grew soon after it formed.

NASA operated the GRAIL satellites, nicknamed Ebb and Flow, until they ran low on fuel in late 2012, then engineers devised a scheme to empty their propellant tanks and deliberately crash the spacecraft into the moon at a velocity of about 3,800 mph.

The Lunar Reconnaissance Orbiter was orbiting 100 miles above the moon near the crash site at the time of the Dec. 17 impacts, which occurred in quick succession about one-and-a-half miles apart.

LRO's ultraviolet imaging spectrograph, officially named the Lyman Alpha Mapping Project, observed a stream of debris launched into space by the dual impacts, finding high concentrations of mercury and atomic hydrogen.

"We were especially excited to see this detection of atomic hydrogen coming out of the impact sites," said Kurt Retherford, principal investigator for the LAMP instrument from Southwest Research Institute.

LRO also detected elevated mercury and hydrogen levels in the ejecta streaming from the lunar surface during the impact of NASA's LCROSS mission in 2009, in which engineers guided an empty Centaur rocket stage into a permanently-shadowed crater near the moon's south pole.

Scientists believe eternally-dark regions like the LCROSS impact site could harbor frozen water ice.

Data from the GRAIL impact site, which sees sunlight, will add to researchers' understanding of the lunar water cycle, Retherford said.