WEST LAFAYETTE, Ind. - Scientists obtained the most detailed measurements of the moon's gravity and used that data to model for the first time the formation of a major impact basin and the rings that surround it.

The gravity measurements and modeling are detailed in two papers published Thursday (Oct. 28) in the journal Science and are part of NASA's GRAIL mission. Jay Melosh, a Purdue University Distinguished Professor of Earth, Atmospheric and Planetary Sciences, is a co-author of the papers and is co-investigator of the GRAIL mission, which launched two satellites to the moon in 2011. After the end of their successful mission, the satellites impacted the moon's surface in 2012. Other authors of the paper include two former Purdue doctoral students, Brandon Johnson and David Blair, and Andrew Freed, a Purdue professor of earth, atmospheric and planetary sciences.

The researchers focused on the Orientale basin, a crater that is 930 kilometers in diameter and at its lowest point is about 12 kilometers deep, measured from the tops of the mountains that surround it. It is called a basin rather than a crater because of its huge size and because it is encircled by tall mountainous rings.

The authors found that the gravity at the center of Orientale is larger by about 0.3 percent than the gravity on the surface outside the basin. While gravitational force can vary from place to place on Earth, those differences on the moon are 10 times greater than those found here.

"Those gravity variations have been known for some time, but not with the precision we have now," Melosh said. "If you were walking around on the surface of the moon, you probably wouldn't notice it. But if you had a pendulum clock, you'd probably see that it was running faster inside the basin."

That small difference in gravity shows that the center of the basin is underlain by denser rocks than elsewhere on the moon.

That gravity data helped confirm models that show how Orientale and its rings were formed some 3.85 billion years ago when an asteroid slammed into the surface of the moon.

They estimate that the asteroid was about 65 kilometers in diameter if it struck the moon vertically, although it would have been somewhat larger if it struck at an angle. At the time, the moon's interior was warmer than it is today, closer to modern-day Earth's temperatures. Those higher temperatures allowed rock from the mantle to move more freely, creating faults that are seen as rings around the Orientale basin.

"The warmer, deeper rocks in the moon were more fluid and moved inward after the impact, carrying the crustal rocks inward toward the center, and that's what made the faults," Melosh said.

Understanding the cratering of the moon 3.85 billion years ago can give scientists clues about the formation of other planets, including Earth.

"If the moon was getting smacked up by all these different impacts, the Earth absolutely had to as well," Melosh said. "The early Earth probably looked just like the moon. We're looking back in time and seeing what the Earth would have looked like had it not had plate tectonics and subsequent geologic activity that erased any major trace of those early impacts."

Some theories even include the idea that the bombardment of Earth by asteroids and other objects may have led to life on the planet, Melosh said.

Melosh will continue to model the formation of large moon craters and is working on a collaborative proposal to use robotic vehicles to bring moon samples back to Earth for study to learn more about the satellite's crater formation.

Writer: Brian Wallheimer: 765-532-0233, brian.wallheimer@gmail.com

Source: Jay Melosh, 765-494-3290, jmelosh@purdue.edu

ABSTRACT

Formation of the Orientale lunar multiring basin Brandon C. Johnson,1*† David M. Blair,2 Gareth S. Collins,3 H. Jay Melosh,4 Andrew M. Freed,4 G. Jeffrey Taylor,5 James W. Head,6 Mark A. Wieczorek,7 Jeffrey C. Andrews-Hanna,8 Francis Nimmo,9 James T. Keane,10 Katarina Miljković,1‡ Jason M. Soderblom,1 Maria T. Zuber1 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 2Massachusetts Institute of Technology Haystack Observatory, Route 40, Westford, MA 01886, USA. 3Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK. 4Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907, USA. 5Hawai’i Institute of Geophysics and Planetology, University of Hawai’i, Honolulu, HI 96822, USA. 6Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA. 7Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, Paris Cedex 13 75205, France. 8Southwest Research Institute, Boulder, CO 80302, USA. 9Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA 95064, USA. 10Department of Planetary Science, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA. *Corresponding author. Email: brandon_johnson@brown.edu Multiring basins, large impact craters characterized by multiple concentric topographic rings, dominate the stratigraphy, tectonics, and crustal structure of the moon. Using a hydrocode, we simulated the formation of the Orientale multiring basin, producing a subsurface structure consistent with high-resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft. The simulated impact produced a transient crater, ~390 kilometers in diameter, that was not preserved because of subsequent gravitational collapse. Our simulations indicate that the flow of warm weak material at depth was crucial to the formation of the basin's outer rings, which are large normal faults that formed at different times during the collapse stage. The key parameters controlling ring location and spacing are impactor diameter and lunar thermal gradients.

ABSTRACT