Universe Today

This image shows a highly porous crust on the lunar surface, a consequence of fractures generated by billions of years of impact cratering. Credit: NASA/JPL-Caltech/ IPGP

From looking at the Moon’s surface, we know it has taken a beating from asteroids and comets pummeling its surface. But new details from the GRAIL mission reveal the lunar interior just below the surface has been walloped as well, and is almost completely pulverized. This surprising finding, along with the discovery of deep fractures, suggests that in its first billion years, the Moon may have endured a history of massive impacts, more than previously thought. By inference, this means Earth and other terrestrial planets in the Solar System endured huge early impacts, too.

“It was known that planets were battered by impacts, but nobody had envisioned that the [Moon’s] crust was so beaten up,” said Maria Zuber, Principal Investigator for the GRAIL mission. “This is a really big surprise, and is going to cause a lot of people to think about what this means for planetary evolution.”

The new GRAIL data agrees with recent studies that suggest that the Late Heavy Bombardment may have lasted much longer than originally estimated and well into the time when early life was forming on Earth. Additionally, this “late-late” period of impacts — 3.8 billion to 2.5 billion years ago — was not for the faint of heart. Various blasts may have rivaled those that produced some of the largest craters on the Moon, and could have been larger than the dinosaur-killing impact that created the Chicxulub crater 65 million years ago.

From GRAIL’s measurements, Zuber and her team have now stitched together a high-resolution map of the Moon’s gravity (read more about it in our previous article.)

But the resulting map also reveals an interior gravitational field consistent with an incredibly fractured lunar crust. Compared to the surface, the map of the interior looks extraordinarily smooth. Except for the large impact basins, the Moon’s upper crust largely lacks dense rock structures and is instead likely made of porous, pulverized material.

This moon map shows the gravity gradients calculated by NASA’s GRAIL mission. Red and blue correspond to stronger gravity gradients. Image credit: NASA/JPL-Caltech/CSM

GRAIL’s lunar gravity map has also revealed numerous structures on the Moon’s surface that were unresolved by previous gravity maps of any planet, including volcanic landforms, impact basin rings, and many simple, bowl-shaped craters. From GRAIL’s measurements, scientists have determined that the Moon’s crust, ranging in thickness from 34 to 43 kilometers, is much thinner than planetary geologists had previously suspected. The crust beneath some major basins is nearly nonexistent, indicating that early impacts may have excavated the lunar mantle, providing a window into the interior.

“If you look at surface of the Moon and how heavily cratered it is,” said Zuber during a press briefing on Wednesday from the American Geophysical Union conference, “that tells us that all terrestrial planets looked that way, but Earth’s history is not preserved because of atmospheric and erosional processes on our planet. So, if we want to study those early periods, we need to go somewhere else, and the Moon is the perfect place for that.”

Zuber said that from finding an incredible fracturing of the Moon’s upper crust, we now know the crust of other planets likely have these same fractures as well. “We have reason to believe that the fractures on the terrestrial planets are deeper, and perhaps as in case of the Moon, even into the mantle. This effects planetary evolution, such as how planets lose heat,” she said.

Fractures also provide a pathway for fluids.

“Mars has been theorized to have an ancient ocean, and we wonder where it went,” said Zuber. “The ocean could well be underground, and we’ve seen evidence of water underground on Mars. If there were ever microbes on the surface of Mars, they could have gone very deep, so this finding opens up possibilities like that, and really opens a window to the early stages of our Solar System and just how violent a place it was.”

In addition to GRAIL’s discoveries, Zuber said another major accomplishment has been the performance of the spacecraft themselves. To achieve the mission’s science goals, the two probes, which can travel more than 200 kilometers apart, needed to be able to measure changes in the distance between them to within a few tenths of a micron per second. But GRAIL actually outperformed its measurement requirements by about a factor of five, resolving changes in spacecraft distance to several hundredths of a micron per second.

“On this mission, with two spacecraft, everything had to go perfectly twice,” Zuber says, adding proudly, “Imagine you’re a parent raising a twins, and your children sit down at the piano and play a duet perfectly. That’s how it feels.”

See an image gallery from the GRAIL mission here.

Sources: GRAIL press conference from AGU, MIT, JPL

This map shows the gravity field of the moon as measured by NASA’s GRAIL mission. Image credit: NASA/ARC/MIT

The first science results from NASA’s twin GRAIL lunar orbiters provide incredible detail of the Moon’s interior and the highest resolution gravity field map of any celestial body, including Earth.

The Gravity Recovery and Interior Laboratory (GRAIL) data shows ancient internal structures that were previously unknown, provides details that are up to five orders of magnitude better than previous studies of the Moon, and delivers unprecedented information about the Moon’s surface and gravity field.

The twin spacecraft, nicknamed Ebb and Flow, send radio signals to each other and any changes in distance between the two as they circle the Moon are measured, down to changes as small as 50 nanometers per second. “That’s 1/ 20,000th the velocity that a snail moves,” said Maria Zuber, GRAIL Principal Investigator, speaking at the American Geophysical Union conference today.

The new gravity maps reveals an abundance of features such as tectonic structures, volcanic landforms, basin rings, crater central peaks and numerous simple, bowl-shaped craters. Data also show the moon’s gravity field is unlike that of any terrestrial planet in our solar system.

The instruments on the GRAIL spacecraft can probe inside the planet. Incredible videos released today shows an abundance of detail that the team said they are only just beginning to study.

Subtracting away the gravity from surface features provides what is called a Bouguer gravity map. What remains is a view of mass anomalies inside the Moon due to either variations in crustal thickness or mantle density. In the video above, the prominent nearside circular highs (in red) indicate the well-known mass concentrations or ‘mascons,’ but many similar newfound far-side features are also visible.

“Ninety-eight percent of local gravity is associated with topography, while 2 percent are other gravitational features,” said Zuber. “You can see bull’s-eyes of the lunar mascons, but otherwise we see a smooth inner surface. The only way this could happen is if impacts to the early Moon shattered the inner surface.”

These maps of the moon show the “Bouguer” gravity anomalies as measured by NASA’s GRAIL mission. Image credit: NASA/JPL-Caltech/CSM

The Bougeur gravity map also revealed evidence for ancient volcanic activity under the surface of the Moon and strange linear gravitational anomalies.

“In the gradients of Bouguer gravity map, saw features we didn’t expect,” said Jeff Andrews-Hanna, GRAIL co-investigator. “We identified a large population of linear gravitational anomalies. We don’t see any expression of them on topography maps, so we infer that these are an ancient internal structures.”

A linear gravity anomaly intersecting the Crisium basin on the nearside of the moon has been revealed by NASA’s GRAIL mission. The GRAIL gravity gradient data are shown at left, with the location of the anomaly indicated. Red and blue correspond to stronger gravity gradients. Topography data over the same region from NASA’s Lunar Reconnaissance Orbiter’s Lunar Orbiter Laser Altimeter are shown at right; these data show no sign of the gravity anomaly. Credit: NASA/JPL-Caltech/CSM

For example, this image of Crisium Basin, which forms one of the ‘man on moon’s’ eyes, the gravity maps shows a linear feature crossing the basin while topography maps show no such correlating feature. “This tells us the gravity anomaly formed before the impacts,” Andrews-Hanna said.

These maps of the near and far side of the moon show the gravity gradients as measured by NASA’s GRAIL mission, highlighting a population of linear gravity anomalies. Image credit: NASA/JPL-Caltech/CSM

Additional data reveal that the Moon’s inner crust in almost completely pulverized. Read more about it in our second article on the new GRAIL results.

Other data reveal the crust of the Moon is thinner than previously thought.

“Using GRAIL gravity data, we found the average thickness of the crust is 32-34 kilometers which is about 10 km less than previous studies,” said Mark Wieczorek, GRAIL Co-Investigator. “We found the bulk abundance of aluminum on Moon is nearly the same as that of the Earth. This is consistent with a recent hypothesis that the Moon is derived of materials from the Earth when it was formed during a giant impact event.”

NASA’s GRAIL mission took this flyover video above the Mare Orientale basin of Earth’s moon. It was collected by the MoonKAM aboard GRAIL’s Ebb spacecraft on April 7 and 8, 2012. Movie credit: NASA/JPL-Caltech/Sally Ride Science

During its prime mission, the two GRAIL spacecraft orbited just 55 km above the Moon’s surface. This close distance is why GRAIL is producing the best gravity field data for any planet, including Earth.

“GRACE is still collecting great data about Earth’s gravity field, but because Earth has an atmosphere, GRACE has to orbit at 500 km,” said Zuber. “Nothing beats going low.”

Zuber said the GRAIL team learned from GRACE and were able to make “some judicious improvements.” She also suggested this technology should be used for every planetary body in the solar system, and threw out an enticing idea: “Imagine mapping currents beneath the surface of Europa,” she said.

GRAIL finishes the primary science mission in May and are currently working in an extended mission where the spacrafts’ altitude was lowered to just 23 km above the surface. “We are opening another window interms of geophysics, and so you’ll be hearing results from the new data sets soon,” said Sami Asmar, GRAIL team member.

At a AGU conference session later in the day, Zuber revealed that tomorrow, December 6, 2012, the team will lower the GRAIL spacecraft down to just 11 km above the lunar surface.

Artist concept of GRAIL mission, with two twin spacecraft in tandem orbits around the moon to measure its gravity field in unprecedented detail. Image credit: NASA/JPL

The extended mission will end soon, in mid-December, and soon after that, the two spacecraft will be crashed intentionally onto the lunar surface. The team said today that they are still formulating ideas for the impact scenario, and looking at the possibility of aiming the crashes so they are within the field-of-view of instruments on NASA’s Lunar Reconnaissance Orbiter.