Colorado School of Mines Professor Helps Solve A Lunar Mystery

Earth's moon as observed in visible light (left), topography (center, where red is high and blue is low), and the GRAIL gravity gradients (right). The Procellarum region is a broad region of low topography covered in dark mare basalt. The gravity gradients reveal a giant rectangular pattern of structures surrounding the region. Image Credit: NASA/GSFC/JPL/Colorado School of Mines/MIT

Earth’s moon as observed in visible light (left), topography (center, where red is high and blue is low), and the GRAIL gravity gradients (right). The Procellarum region is a broad region of low topography covered in dark mare basalt. The gravity gradients reveal a giant rectangular pattern of structures surrounding the region.
Image Credit: NASA/GSFC/JPL/Colorado School of Mines/MIT

Golden, Colorado. October 1, 2014 – Colorado School of Mines Geophysics Associate Professor Jeff Andrews-Hanna is the lead author of a study that uses data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) to solve a lunar mystery that is almost as old as the moon itself.

Andrews-Hanna is part of a team that examined the subsurface structure of the Procellarum region, also known as the Ocean of Storms. GRAIL scientists believe the Ocean of Storm’s rocky outline is the result of ancient rift valleys, and not an asteroid impact as some previous theories suggested. The lava-flooded rift valleys are unlike anything found anywhere else on the Moon, and may at one time have resembled the rift zones on the Earth, Mars and Venus.

“The nearside of the moon has been studied for centuries, and yet continues to offer up surprises for scientists with the right tools,” said Maria Zuber, principal investigator of NASA’s GRAIL mission, from the Massachusetts Institute of Technology, Cambridge. “We interpret the gravity anomalies discovered by GRAIL as part of the lunar magma plumbing system — the conduits that fed lava to the surface during ancient volcanic eruptions.”

GRAIL gravity data is now allowing scientists to look beneath the lunar surface at structures that are hidden from view, using the subtle gravitational pulls on the orbiting spacecraft.

“This dataset has provided us with the highest resolution gravity map of any object in the solar system, including the Earth,” said Zuber.

The rifts are buried beneath dark volcanic plains on the nearside of the moon and have been detected only in the gravity data provided by GRAIL. Using the gradients in the gravity data, the researchers can now clearly and completely see structures that were only hinted at by previous surface observations. This newly discovered rectangular pattern has an area of approximately 2.5 million square miles (6.5 million square kilometers) and covers 17 percent of the surface of the Moon.

The full Moon as seen from the Earth, with the Procellarum border structures superimposed in red. Image Credit: Kopernik Observatory/NASA/Colorado School of Mines/MIT/JPL/Goddard Space Flight Center

The full Moon as seen from the Earth, with the Procellarum border structures superimposed in red. Image Credit: Kopernik Observatory/NASA/Colorado School of Mines/MIT/JPL/Goddard Space Flight Center

The study also noted a surprising similarity between the rectangular pattern of structures on the moon, and those surrounding the south polar region of Saturn’s icy moon Enceladus. Both patterns appear to be related to volcanic and tectonic processes operating on their respective worlds.

“This rectangular structure covers a larger fraction of the surface area of the Moon than do North America, Europe and Asia combined on the Earth,” Andrews-Hanna said. “This goes to show that there are still big discoveries waiting for us on all of the planets.”

The rectangular pattern, with its angular corners and straight sides, is at odds with the notion that Procellarum might be an ancient impact basin, as that hypothesis would predict a circular basin rim. Instead, the new work suggests that internally driven processes dominated the evolution of this region.

In contrast, previous work by Andrews-Hanna and colleagues in 2008 used gravity data from Mars to reveal an enormous elliptical structure in the northern hemisphere of that planet, supporting the idea that the northern lowlands of Mars were formed by a giant impact that excavated the ‘Borealis Basin.’

“In two separate studies, we have used gravity data to support the existence of the largest impact basin in the solar system on Mars, and to refute the proposed second largest basin in the solar system on the Moon,” said Andrews-Hanna. “Our gravity data is opening up a new chapter of lunar history, during which the Moon was a more dynamic place than suggested by the cratered landscape that is visible to the naked eye.”

More work is needed to understand the cause of this newfound pattern of gravity anomalies and the implications for the history of the Moon.

GRAIL A and B, later renamed Ebb and Flow, were launched to the Moon in September 2011. The twin spacecraft flew in a nearly circular orbit near the poles of the moon at an altitude of about 34 miles (55 kilometers). 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.

The mission ended on December 17, 2012, when the probes were intentionally sent into the moon’s surface. NASA later named the impact site in honor of late astronaut Sally K. Ride, who was America’s first woman in space and a member of the GRAIL mission team.

The GRAIL mission was managed by JPL, a division of the California Institute of Technology in Pasadena, California, for NASA’s Science Mission Directorate in Washington. The mission was part of the Discovery Program managed at NASA’s Marshall Space Flight Center in Huntsville, Alabama. GRAIL was built by Lockheed Martin Space Systems in Denver.

The findings are published online in the journal Nature.