[meteorite-list] GRAIL Moon Mission Shares Insights into Giant Impacts

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=6662

NASA Moon Mission Shares Insights into Giant Impacts
Jet Propulsion Laboratory
October 27, 2016

Fast Facts:

* Orientale basin is a giant, ringed impact crater on Earth's moon.
* Until now, how impact craters with rings form had not been well 
understood.
* Scientists have reconstructed Orientale's formation using data 
from NASA's GRAIL mission.

New results from NASA's Gravity Recovery and Interior Laboratory (GRAIL) 
mission are providing insights into the huge impacts that dominated the 
early history of Earth's moon and other solid worlds, like Earth, Mars, 
and the satellites of the outer solar system.

In two papers, published this week in the journal Science, researchers 
examine the origins of the moon's giant Orientale impact basin. The research 
helps clarify how the formation of Orientale, approximately 3.8 billion 
years ago, affected the moon's geology.

Located along the moon's southwestern limb -- the left-hand edge as seen 
from Earth -- Orientale is the largest and best-preserved example of what's 
known as a "multi-ring basin." Impact craters larger than about 180 miles 
(300 kilometers) in diameter are referred to as basins. With increasing 
size, craters tend to have increasingly complex structures, often with 
multiple concentric, raised rings. Orientale is about 580 miles (930 kilometers) 
wide and has three distinct rings, which form a bullseye-like pattern.

Multi-ring basins are observed on many of the rocky and icy worlds in 
our solar system, but until now scientists had not been able to agree 
on how their rings form. What they needed was more information about the 
crater's structure beneath the surface, which is precisely the sort of 
information contained in gravity science data collected during the GRAIL 
mission.

The powerful impacts that created basins like Orientale played an important 
role in the early geologic history of our moon. They were extremely disruptive, 
world-altering events that caused substantial fracturing, melting and 
shaking of the young moon's crust. They also blasted out material that 
fell back to the surface, coating older features that were already there; 
scientists use this layering of ejected material to help determine the 
age of lunar features as they work to unravel the moon's complex history.

The Importance of Orientale

Because scientists realized that Orientale could be quite useful in understanding 
giant impacts, they gave special importance to observing its structure 
near the end of the GRAIL mission. The orbit of the mission's two probes 
was lowered so they passed less than 1.2 miles (2 kilometers) above the 
crater's mountainous rings.

"No other planetary exploration mission has made gravity science observations 
this close to the moon. You could have waved to the twin spacecraft as 
they flew overhead if you stood at the ring's edge," said Sami Asmar, 
GRAIL project scientist at NASA's Jet Propulsion Laboratory, Pasadena, 
California.

Of particular interest to researchers has been the size of the initial 
crater that formed during the Orientale impact. With smaller impacts, 
the initial crater is left behind, and many characteristics of the event 
can be inferred from the crater's size. Various past studies have suggested 
each of Orientale's three rings might be the remnant of the initial crater.

In the first of the two new studies, scientists teased out the size of 
the transient crater from GRAIL's gravity field data. Their analysis shows 
that the initial crater was somewhere between the size of the basin's 
two innermost rings.

"We've been able to show that none of the rings in Orientale basin represent 
the initial, transient crater," said GRAIL Principal Investigator Maria 
Zuber of the Massachusetts Institute of Technology in Cambridge, lead 
author of the first paper. "Instead, it appears that, in large impacts 
like the one that formed Orientale, the surface violently rebounds, obliterating 
signs of the initial impact."

The analysis also shows that the impact excavated at least 816,000 cubic 
miles (3.4 million cubic kilometers) of material -- 153 times the combined 
volume of the Great Lakes.

"Orientale has been an enigma since the first gravity observations of 
the moon, decades ago," said Greg Neumann, a co-author of the paper at 
NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We are now 
able to resolve the individual crustal components of the bullseye gravity 
signature and correlate them with computer simulations of the formation 
of Orientale."

Reproducing the Rings

The second study describes how scientists successfully simulated the formation 
of Orientale to reproduce the crater's structure as observed by GRAIL. 
These simulations show, for the first time, how the rings of Orientale 
formed, which is likely similar for multi-ring basins in general.

"Because our models show how the subsurface structure is formed, matching 
what GRAIL has observed, we're confident we've gained understanding of 
the formation of the basin close to 4 billion years ago," said Brandon 
Johnson of Brown University, Providence, Rhode Island, lead author of 
the second paper.

The results also shed light on another moon mystery: Giant impacts like 
Orientale should have dredged up deep material from the moon's mantle, 
but instead, the composition of the crater's surface is the same as that 
of the lunar crust. So, scientists have wondered, where did the mantle 
material go?

The simulation shows that the deep, initial crater quickly collapses, 
causing material around the outside to flow inward, and covering up the 
exposed mantle rock.

The new GRAIL insights about Orientale suggest that other ringed basins, 
invisible in images, could be discovered by their gravity signature. This 
may include ringed basins hidden beneath lunar maria -- the large, dark 
areas of solidified lava that include the Sea of Tranquility and the Sea 
of Serenity.

"The data set we obtained with GRAIL is incredibly rich," said Zuber. 
"There are many hidden wonders on the moon that we'll be uncovering for 
years to come."

The twin GRAIL probes were launched in 2011. The mission concluded in 
2012.

The GRAIL mission was managed by NASA's Jet Propulsion Laboratory 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.

For more information about GRAIL, visit:

http://www.nasa.gov/grail

News Media Contact
DC Agle
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-9011
agle at jpl.nasa.gov

Written by
Preston Dyches/JPL

2016-281