Moon Mystery: GRAIL Data Show How Gravitational Anomalies Developed
May 31, 2013

Scientists Solve Mystery Of Moon’s Irregular Gravitational Field

redOrbit Staff & Wire Reports - Your Universe Online

Researchers from Purdue University and MIT have solved the long-standing mystery of why the moon´s gravitational force is stronger in some areas than in others.

This irregular gravitational force has been observed ever since the first satellites were sent to the moon, when orbiting probes would pass over certain craters and impact basins, and periodically swerve off course before plunging toward the lunar surface and then pulling back up.

Scientists have suspected this lumpy gravitational field has to do with an excess distribution of mass below the lunar surface, and have even created a name for these regions' mass concentrations — “mascons.”

The Purdue University-led team of scientists, working as part of NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, confirmed this theory, finding that large, hidden concentrations of mass exist on the lunar surface that alter the gravity field.

"In 1968 these mass concentrations were an unwelcome discovery as scientists prepared for the Apollo landings, and they have remained a mystery ever since," said Jay Melosh, a member of the Gravity Recovery and Interior Laboratory, or GRAIL, science team who led the research.

These masses are responsible for either pulling a spacecraft in or pushing it off course.

"GRAIL has now mapped where they lay, and we have a much better understanding of how they developed. If we return to the moon, we can now navigate with great precision."

"A better understanding of these features also adds clues to the moon's origin and evolution and will be useful in studying other planets where mass concentrations also are known to exist including Mars and Mercury," said Melosh.

"We now know the ancient moon must have been much hotter than it is now and the crust thinner than we thought," said Melosh, a distinguished professor of earth, atmospheric and planetary sciences and physics. "For the first time we can figure out what size asteroids hit the moon by looking at the basins left behind and the gravity signature of the areas. We now have tools to figure out more about the heavy asteroid bombardment and what the ancient Earth may have faced."

The team confirmed the standing theory that the concentrations of mass were caused by massive asteroid impacts billions of years ago, and determined how these impacts changed the density of material on the moon's surface and, in turn, its gravity field.

"The explanation of mascons has eluded scientists for decades," said Maria Zuber, GRAIL principal investigator and professor at the Massachusetts Institute of Technology (MIT).

"Since their initial discovery they have also been observed on Mars and Mercury, and by understanding their formation on the moon we have greatly advanced knowledge of how major impacts modified planetary crusts."

The mass concentrations form a target pattern with a gravity surplus at the center surrounded by a ring of gravity deficit and an outer ring of gravity surplus. This pattern arises as a natural consequence of crater excavation, collapse and cooling following an impact, the researchers said.

The team determined that the increase in density and gravitational pull at the bulls-eye was caused by lunar material melted from the heat of the asteroid impact. The melting causes the material to become more concentrated, stronger and denser, and pulls in additional material from the surrounding areas, Melosh explained.

The large asteroid impacts also caused big holes into which the surrounding lunar material collapsed. As the cool, strong lunar crust slid into the holes it bent downward, forming a rigid, curved edge that held down the material beneath it and prevented it from fully rebounding to its original surface height. This causes a ring with less gravitational pull because the mass is held farther below the surface, the top of which is what most influences the gravitational signature, he said.

The outer ring of increased gravitational pull comes from the added mass of the material ejected by the initial impact that then piles on top of the lunar surface.

The researchers analyzed the Freundlich-Sharanov and Humorum mascon basins using expertise in specialized computer analysis methods called hydrocodes — computer programs originally created to analyze the flow of liquids — along with finite element codes to create computer simulations that could show the physical changes occurring from microseconds to millions of years.

Melosh is a pioneer in adapting computer hydrocodes to simulate how complex materials move when high-speed collisions occur, like that of a planetary collision. Hydrocodes can be used to study such phenomena on a time scale of microseconds to hours, but are not practical from time scales much longer than that, he said.

Team member Andrew Freed, associate professor of earth, atmospheric and planetary sciences at Purdue, is a leader in adapting finite element codes, like those used to study car crashes or to simulate changes in density of complex materials upon cooling.

The technique is also used to study the evolution of Earth and other planets on the time scale of hours to millions of years.

Using the GRAIL data set, which offers an unprecedented, detailed map of the distribution of masses in the moon, the team was able to assemble a picture of how the moon's crust and mantle behaved, and the development of the concentrations of mass in the aftermath of large asteroid impacts.

During their prime and extended missions, the two GRAIL spacecraft transmitted radio signals precisely defining the rate of change of distance between them. The distance between the crafts Ebb and Flow 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 scientists are now using this data to find detailed information about the moon's internal structure and composition.

The study was published online May 30 in the journal Science.