This week spacefans, we are taking a close look at the Moon, which despite its proximity to us, still has much to teach us space scientists. We also have a very nice Vegas video by our one and only Space Pope!
MOON MISSION HELPS SOLVE IMPACT MYSTERIES
New results from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission are providing valuable clues into the frequent, massive meteor and cometary impacts that dominated the young 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 left-hand edge as seen from Earth — Orientale is the large and well preserved example of what’s known as a “multi-ring basin.” Impact craters larger than about 300 kilometers in diameter are referred to as basins. With increasing size, craters tend to have increasingly complex structures, often with multiple concentric rings. Orientale is about 930 kilometers wide and has three distinct rings, which form a bullseye-like pattern.
Credit: sky&telescope.com
Multi-ring basins are observed on many of the rocky and icy worlds in our solar system, but until now scientists have only been able to speculate as to their formation. 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.
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 craggy 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 calculated 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 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.”
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 pulled 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.”
GRAIL MISSION LOOKS FOR THE ORIGINS OF THE OCEAN OF STORMS
Staying with the Moon and NASA’s GRAIL mission to study our closest neighbour’s mysteries, the Mission is also helping scientists figure out the story behind a region of the Moon we see every time its visible. This area dominates the part of the Moon we can see from earth.
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
Early theories suggested the uneven outline of a region of the moon’s surface known as Oceanus Procellarum, or the Ocean of Storms, was caused by an asteroid impact. If this theory had been correct, the basin it formed would be the largest asteroid impact basin on the moon. However, mission scientists studying GRAIL data believe they have found evidence the craggy outline of this rectangular region — roughly 2,600 kilometers across — is actually the result of the formation of ancient rift valleys.
“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.”
The rifts are concealed beneath dark volcanic plains on the nearside of the moon and have been detected only in the gravity data provided by GRAIL. The lava-flooded rift valleys are unlike anything found anywhere else on the moon and may at one time have resembled rift zones on Earth, Mars and Venus. Another theory arising from recent data analysis suggests this region formed as a result of churning deep in the interior of the moon that led to a high concentration of heat-producing radioactive elements in the crust and mantle of this region. Scientists studied the gradients in gravity data from GRAIL, which revealed a rectangular shape in resulting gravitational anomalies.
“The rectangular pattern of gravity anomalies was completely unexpected,” said Jeff Andrews-Hanna, a GRAIL co-investigator at the Colorado School of Mines in Golden, Colorado, and lead author of the paper. “Using the gradients in the gravity data to reveal the rectangular pattern of anomalies, we can now clearly and completely see structures that were only hinted at by surface observations.”
This rectangular pattern, with its angular corners and straight sides, goes against the theory that Procellarum is an ancient impact basin, since such an impact would create a circular basin. Instead, the new research suggests processes beneath the moon’s surface dominated the evolution of this region. Over time, the region would cool and contract, pulling away from its surroundings and creating fractures similar to the cracks that form in mud as it dries out, but on a much larger scale.
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.
“Our gravity data are 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,” said Andrews-Hanna. “More work is needed to understand the cause of this newfound pattern of gravity anomalies, and the implications for the history of the moon.”
The spacecraft used for the GRAIL mission were impacted on the Moon’s surface back in 2012, but the dat theya found are still coming up with surprises.
SPACEPOPE TOURS THE SOLAR SYSTEM
As the last but far from least presenter at Eve Vegas, my friend Charles White from JPL/NASA AKA our beloved Space Pope talked about his views on The Solar system with a peppering of Eve related Easter eggs, enjoy!
Credit: CCP
