The image is 1,000 km wide.
Credit: H. J. Melosh, Purdue University and the NASA GRAIL team
A mystery of the moon that imperiled astronauts and spacecraft on lunar missions has been solved by a Purdue University-led team of scientists as part of NASA's GRAIL mission.
Large concentrations of mass lurk on the lunar surface hidden like coral reefs beneath the ocean waves - an unseen and devastating hazard.
These concentrations change the gravity field and can either pull a spacecraft in or push it off course, sealing its fate to a crash on the face of the moon.
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| Jay Melosh |
"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, who is a distinguished professor of earth, atmospheric and planetary sciences and physics.
"We now know the ancient moon must have been much hotter than it is now and the crust thinner than we thought," he said.
"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.
A paper detailing the results will be published online by the journal Science on Thursday (May 30).
In addition to Melosh, Purdue team members include Andrew Freed, associate professor of earth, atmospheric and planetary sciences, and graduate students Brandon Johnson and David Blair.
Additional team members include Maria Zuber, GRAIL principal investigator and professor at the Massachusetts Institute of Technology; J. Andrews-Hanna of the Colorado School of Mines; S. Solomon of Columbia University; and the GRAIL Science Team.
"The explanation of mascons has eluded scientists for decades," Zuber said. "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 bulls-eye surrounded by a ring of gravity deficit and an outer ring of gravity surplus.
The team found that this pattern arises as a natural consequence of crater excavation, collapse and cooling following an impact.
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 said.
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.
More information: "The Origin of Lunar Mascon Basins," by H.J. Melosh et al. Science, 2013.
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