Origin of lunar MASCONS found in GRAIL data - JPL

The Moon's elusive, uneven gravity is clearly seen in this Free-Air Gravity map produced from data returned in 2012 by the twin GRAIL orbiters. Mare Imbrium, for example, at upper right presents a significant anomalous profile, concentrated near what may have been the original "transitory" crater boundary but equally reduced from the lunar average (blue) between that boundary and the outer reaches of Imbrium's present boundary [NASA/JPL/MIT].

Pasadena -- Investigators combing through the huge treasure trove of data returned to Earth by NASA's GRAIL (Gravity Recovery and Interior Laboratory) twin spacecraft Ebb and Flow in 2012 claim to have "uncovered the origin of massive invisible regions that make the moon's gravity uneven, a phenomenon affecting the stability and longevity of lunar-orbiting spacecraft," JPL announced Thursday.

"GRAIL data confirm that lunar mascons were generated when large asteroids or comets impacted the ancient moon, when its interior was much hotter than it is now," said Jay Melosh, a GRAIL co-investigator at Purdue University in West Lafayette, Ind., and lead author of the new research. "We believe the data from GRAIL show how the moon's light crust and dense mantle combined with the shock of a large impact to create the distinctive pattern of density anomalies that we recognize as mascons."

The origin of lunar mascons has been a mystery in planetary science since their discovery in 1968 by a team at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Researchers generally agree mascons resulted from ancient impacts billions of years ago. It was not clear until now how much of the unseen excess mass resulted from lava filling the crater or iron-rich mantle upwelling to the crust.

On a map of the moon's gravity field, a mascon appears in a target pattern. The bulls-eye has a gravity surplus. It is surrounded by a ring with a gravity deficit. A ring with a gravity surplus surrounds the bulls-eye and the inner ring. This pattern arises as a natural consequence of crater excavation, collapse and cooling following an impact. The increase in density and gravitational pull at a mascon's bulls-eye is caused by lunar material melted from the heat of a long-ago asteroid impact.

"Knowing about mascons means we finally are beginning to understand the geologic consequences of large impacts," Melosh said. "Our planet suffered similar impacts in its distant past, and understanding mascons may teach us more about the ancient Earth, perhaps about how plate tectonics got started and what created the first ore deposits."

"Mascons also have been identified in association with impact basins on Mars and Mercury," said GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology in Cambridge. "Understanding them on the moon tells us how the largest impacts modified early planetary crusts."

Launched as GRAIL A and GRAIL B in September 2011, the probes, renamed Ebb and Flow, operated in a nearly circular orbit near the poles of the moon at an altitude of about 34 miles (55 kilometers) until their mission ended in December 2012. The distance between the twin probes changed slightly as they flew over areas of greater and lesser gravity caused by visible features, such as mountains and craters, and by masses hidden beneath the lunar surface.

GRAIL twins begin science mission

Ebb (GRAIL-A) in continuously measuring the distance between itself and Flow (GRAIL-B), when measured against time and place will enable mapping the lunar interior with high precision [NASA/JPL/MIT].

NASA's Gravity Recovery And Interior Laboratory (GRAIL) spacecraft orbiting the moon officially have begun their science collection phase. During the next 84 days, scientists will obtain a high-resolution map of the lunar gravitational field to learn about the moon's internal structure and composition in unprecedented detail. The data also will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved.

"The initiation of science data collection is a time when the team lets out a collective sigh of relief because we are finally doing what we came to do," said Maria Zuber, principal investigator for the GRAIL mission at the Massachusetts Institute of Technology in Cambridge. "But it is also a time where we have to put the coffee pot on, roll up our sleeves and get to work."

The GRAIL mission's twin, washing-machine-sized spacecraft, named Ebb and Flow, entered lunar orbit on New Year's Eve and New Years Day. GRAIL's science phase began yesterday at 8:15 p.m. EST (5:15 p.m. PST). During this mission phase, the spacecraft will transmit radio signals precisely defining the distance between them. As they fly over areas of greater and lesser gravity caused by visible features such as mountains, craters and masses hidden beneath the lunar surface, the distance between the two spacecraft will change slightly. Science activities are expected to conclude on May 29, after GRAIL maps the gravity field of the moon three times.

"We are in a near-polar, near-circular orbit with an average altitude of about 34 miles (55 kilometers) right now," said David Lehman, GRAIL project manager from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "During the science phase, our spacecraft will orbit the moon as high as 31 miles (51 kilometers) and as low as 10 miles (16 kilometers). They will get as close to each other as 40 miles (65 kilometers) and as far apart as 140 miles (225 kilometers)."

Previously named GRAIL A and B, the names Ebb and Flow were the result of a nation-wide student contest to choose new names for the spacecraft. The winning entry was submitted by fourth graders from the Emily Dickinson Elementary School in Bozeman, Montana. Nearly 900 classrooms with more than 11,000 students from 45 states, Puerto Rico and the District of Columbia, participated in the contest.

JPL manages the GRAIL mission for NASA's Science Mission Directorate in Washington. The GRAIL mission is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space Systems in Denver built the spacecraft.

GRAIL-B joins New Years dance with GRAIL-A

GRAIL-B was still 6300 kilometers away and closing fast at 2038UT New Years Day and flight directors at JPL prepared for a burn that would begin a tandem flight with GRAIL-A. At the same moment GRAIL-A was reaching the 87.4 km apogee of a highly elliptical third orbit since its own insertion into lunar orbit 30 hours earlier, on New Years Eve. The high precision lunar gravity mapping mission, following five years of preparation, can be followed using NASA's web-based Eyes on the SOLAR SYSTEM GRAIL AT THE MOON module [NASA/JPL].

GRAIL-B performed a 39 minute lunar orbit insertion burn, Sunday afternoon, beginning at 2205 UT. Following the successful insertion of both GRAIL spacecraft mission principal investigator Dr. Maria Zuber of MIT is expected to announce the winners of a contest to formally name GRAIL A and B, having chosen from 20,000 entries submitter by elementary and middle school students.

Follow the story at SpaceflightNOW.

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