A new 'hit-and-run' Giant Impact scenario

Figure 1a: Five snapshots from the 30° impact angle and 1.30vesc impact velocity case (cC06) showing cuts through the impact plane. Colour coded is the type and origin of the material. Dark and light blue indicate target and impactor iron; Red and orange show corresponding silicate material. The far right shows the situation at the time of impact. At 0.52h, it can be seen how the impactor ploughs deep through the targets mantle and pushes considerable amount of target material into orbit. A spiral arm of material forms and gravitationally collapses into fragments. The outer portions of the arm mainly consist of impactor silicates and escapes due to having retained a velocity well above escape velocity. The silicate fragments further inward are stronger decelerated and enter eccentric orbits around the target. The impactor's iron core also looses much of its angular momentum to the outer parts of the spiral arm and re-impacts the proto-Earth. -  Figure 1b: The origin of the disk material highlighted, half a collisional timescale ( (Rimp + Rtar) / vimp ) after impact. In the grazing reference case (cA08), the majority of the proto-lunar disk originates from a spill-over of the impactor. In the head-on cases (cC01, fB06, iA10), much more material comes from the target mantle, being pushed out into orbit by the impactor core. Colours are identical to figure 1. Turquoise on the right shows water ice for the icy impactor case iA10.

Reufer, Meier, Benz & Wieler
Universität Bern
Eidgenössische Technische Hochschule Zürich
Lund University, Sölvegatan
 
The formation of the Moon from the debris of a slow and grazing giant impact of a Mars-sized impactor on the proto-Earth (Cameron & Ward 1976, Canup & Asphaug 2001) is widely accepted today. We present an alternative scenario with a hit-and-run collision (Asphaug 2010) with a fractionally increased impact velocity and a steeper impact angle.

Hydrodynamical simulations have identified a slow, grazing impact in being able to reproduce the Moon's iron deficiency and the angular momentum of the Earth-Moon-system. But in this canonical scenario, the Moon forms predominantly from impactor material, thus contradicting the Moon's close geochemical similarity to Earth. Furthermore, due to the slow impact velocity, only limited heat input is provided for the aftermath of the collision. Post-impact mechanisms (Pahlevan & Stevenson 2007) required to match the impact scenario with the compositional observations, depend on the thermal conditions in the post-impact debris disk. We show that a new class of hit and-run collisions with higher impact velocities and a steeper impact angles is also capable of forming a post-impact debris disk from which the Earth's Moon can later form, but leads to a much hotter post-impact debris disk. Furthermore, the ratio of target body material in the debris disk is considerably larger, compared to the canonical scenario. This new class of impacts was previously rejected due to the limited resolutions 26 of early simulations (Benz 1989).

View the (pdf) Icaris manuscript, at arXiv 1207.5224

Figure 2: Comparing post-impact temperatures of the proto-Earth between the grazing reference simulation left (cA08) and the head-on case on the right (cC06). Color coded is temperature in K in logged scale. The initial average temperature before the impact inside the target mantle is ~2000K.

"While the Moon has an iron core like Earth, it does not have the same fraction of iron - and computer models supporting the Theia impact idea show just the same thing. 

"However, the ratio of the Earth's and the Moon's oxygen isotopes is nearly identical, and not all scientists agree on how that may have come about.

"Confounding the issue further, scientists reporting in Nature Geoscience in March said that a fresh analysis of lunar samples taken by the Apollo missions showed that the Moon and the Earth shared an uncannily similar isotope ratio of the metal titanium."

Moon formation: Was it a 'hit and run' accident?
BBC News, Science & Environment, July 27, 2012

Shackleton harbors ice after all

Spoke too soon! When JAXA released this Kaguya Terrain Camera image, showing the deep interior of Shackleton crater for the first time in 2008, scientists claimed it disappointingly showed no indication of ice, though no one yet can say how a slurry of lunar volatiles might appear. Now, however, researchers analyzing laser altimetry returned by the LOLA instrument on-board the Lunar Reconnaissance Orbiter (LRO) cite strong evidence of ice content in the permanently shadowed interior.  The Moon's south pole is serendipitously situated on Shackleton's rim, directly under all of LRO's nearly twenty thousand polar orbits since 2009, affording extraordinary study [JAXA/SELENE]..

Jennifer Chu

MIT News

If humans are ever to inhabit the moon, the lunar poles may well be the location of choice: Because of the small tilt of the lunar spin axis, the poles contain regions of near-permanent sunlight, needed for power, and regions of near-permanent darkness containing ice — both of which would be essential resources for any lunar colony.

The area around the moon’s Shackleton crater could be a prime site. Scientists have long thought that the crater — whose interior is a permanently sunless abyss — may contain reservoirs of frozen water. But inconsistent observations over the decades have cast doubt on whether ice might indeed exist in the shadowy depths of the crater, which sits at the moon’s south pole.

Now scientists from MIT, Brown University, NASA’s Goddard Space Flight Center and other institutions have mapped Shackleton crater with unprecedented detail, finding possible evidence for small amounts of ice on the crater’s floor. Using (the LOLA) laser altimeter on the Lunar Reconnaissance Orbiter (LRO) spacecraft, the team essentially illuminated the crater’s interior with laser light, measuring its albedo, or natural reflectance. The scientists found that the crater’s floor is in fact brighter than that of other nearby craters — an observation consistent with the presence of ice, which the team calculates may make up 22 percent of the material within a micron-thick layer on the crater’s floor.
 

http://youtu.be/j2NAZODSdwM 

The group published its findings today in the journal Nature.

In addition to the possible evidence of ice, the group’s map of Shackleton reveals a “remarkably preserved” crater that has remained relatively unscathed since its formation more than three billion years ago. The crater’s floor is itself pocked with several smaller craters, which may have formed as part of the collision that created Shackleton.

The crater, named after the Antarctic explorer Ernest Shackleton, is more than 12 miles wide and two miles deep — about as deep as Earth’s oceans. Maria Zuber, the team’s lead investigator and the E.A. Griswold Professor of Geophysics in MIT’s Department of Earth, Atmospheric and Planetary Sciences, describes the crater’s interior as “extremely rugged … It would not be easy to crawl around in there.”

Mapping the dark. Slipping past the Moon's south pole on the brightly lit rim of Shackleton crater, the dark of the permanently shadowed interior of the crater quickly overtakes a very steep crater wall, like the terrestrial oceans. LRO has skipped through thousands of polar orbits eventually carrying the vehicle over every area on the Moon's surface and over Shackleton, high at the top of everyone's list of priority targets, during every orbit,   LROC Narrow Angle Camera (NAC) M142464150L, LRO orbit 6128, October 23, 2010, 89.21° angle of incidence, 0.87 meters resolution from 41.91 kilometers [NASA/GSFC/Arizona State University].

The group was able to map the crater’s elevations and brightness in extreme detail, thanks in part to the LRO’s path: The spacecraft orbits the moon from pole to pole as the moon rotates underneath. With each orbit, the LRO’s laser altimeter maps a different slice of the moon, with each slice containing measurements of both poles. The upshot is that any terrain at the poles — Shackleton crater in particular — is densely recorded. Zuber and her colleagues took advantage of the spacecraft’s orbit to obtain more than 5 million measurements of the polar crater from more than 5,000 orbital tracks.

“We decided we would study the living daylights out of this crater,” Zuber says. “From the incredible density of observations we were able to make an extremely detailed topographic map.”

The team used the (LOLA) to map the crater’s elevations based on the time it took for laser light to bounce back from the moon’s surface: The longer it took, the lower the terrain’s elevation. Through these measurements, the group mapped the crater’s floor and the slope of its walls.

A quaking theory.The researchers also used the laser altimeter to measure the crater’s brightness, sending out pulses of infrared light at a specific wavelength. The crater’s surface absorbed some light based on its own natural albedo, reflecting the rest back to the spacecraft. The researchers calculated the difference, and mapped the relative brightness throughout the crater’s floor and walls.

While the crater’s floor was relatively bright, Zuber and her colleagues observed that its walls were even brighter. The finding was at first puzzling: Scientists had thought that if ice were anywhere in a crater, it would be on the floor, where very little sunlight penetrates. The upper walls of Shackleton crater, in comparison, are occasionally illuminated, which could evaporate any ice that accumulates.

How to explain the bright walls? The team studied the measurements, and came up with a theory: Every once in a while, the moon experiences seismic shaking brought on by collisions, or gravitational tides from Earth. Such “moonquakes” may have caused Shackleton’s walls to slough off older, darker soil, revealing newer, brighter soil underneath.

Until very recently luna incongnita, the permanently shadowed 10.3 km-wide interior of Shackleton, shouldering the Moon's south pole (blue arrow), today seems much like hundreds of other lunar craters of similar age and dimension. Its ink-black interior has steadily been brightly unveiled in a steady build-up of laser data points collected over the course of three years in polar orbit by the LOLA instrument on LRO. As it is on Earth, however, in Real Estate, "location is everything" [NASA/GSFC/LOLA].

Zuber says there may be multiple explanations for the observed brightness throughout the crater: For example, newer material may be exposed along its walls, while ice may be mixed in with its floor. Her team’s ultra-high-resolution map, she says, provides strong evidence for both.

Ben Bussey, staff scientist at Johns Hopkins University’s Applied Physics Laboratory, says the group’s evidence for ice in Shackleton crater may help determine the course for future lunar missions.

“Ice in the polar regions has been sort of an enigmatic thing for some time … I think this is another piece of evidence for the possibility of ice,” Bussey says. “To truly answer the question, we’ll have to send a lunar lander, and these results will help us select where to send a lander.”

Zuber adds that the group’s topographic map will help researchers understand crater formation and study other uncharted areas of the moon.

“I will never get over the thrill when I see a new terrain for the first time,” Zuber says. “It’s that sort of motivation that causes people to explore to begin with. Of course, we’re not risking our lives like the early explorers did, but there is a great personal investment in all of this for a lot of people.”

The research was supported by the Lunar Reconnaissance Orbiter Mission under the auspices of NASA’s Exploration Systems Mission Directorate and Science Mission Directorate.

Japan's scientists may have leaped to conclusions when they over-confidently announced there was no ice inside Shackleton (upper left), after releasing the first image of the crater's interior a few years ago, but their iconic high-definition image of an orbital Earthrise from November 2007 still takes the breath away [JAXA/NHK/SELENE].

Further evidence of recent lunar geologic activity

Close-up of the "Virtanen graben" field, near the 18.29°N, 180.79°E, on the central meridian of the lunar far side. From LROC Narrow Angle Camera observation M136355592RE (LRO orbit 5228, August 13, 2010; resolution 0.66 meters from 59.85 km). This LROC NAC frame, along with M136362376, were used by Mark Robinson and colleagues at Arizona State University to create a Digital Terrain Model of the Virtanen graben in November 2010. That DTM can be explored HERE.

Images and elevation models from NASA's Lunar Reconnaissance Orbiter (LRO) appear to show the Moon's crust is being stretched, forming miniature valleys in a few small places on the lunar surface. A team of investigators will present their findings at the upcoming Lunar and Planetary Science Conference as evidence that this geologic activity occurred less than 50 million years ago, a very recent time in relation to the Moon's estimated age of roughly 4.575 billion years.

Researchers analyzing high-resolution images obtained by the Lunar Reconnaissance Orbiter Camera (LROC) have shown many small, narrow trenches typically much longer than they are wide, indicating the lunar crust is being pulled apart at these locations. These linear valleys, known as graben, form when the moon's crust stretches, breaks and drops down along two bounding faults. A handful of these graben systems have already been identified across the lunar surface and are cited as evidence the Moon may be shrinking.

"We think the moon is in a general state of global contraction because of cooling of a still hot interior," said Thomas Watters of the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum, lead author of a paper on this research appearing in the March issue of the journal Nature Geoscience.

"The graben tell us forces acting to shrink the moon were overcome in places by forces acting to pull it apart. This means the contractional forces shrinking the moon cannot be large, or the small graben might never form."

Full width (about 5 km wide) of LROC NAC DTM"Virtanen
graben 1;" the small rectangle is the field of view seen in the
image above [NASA/GSFC/Arizona State University].

The weak contraction suggests that the moon, unlike terrestrial planets, did not completely melt in the very early stages of its evolution. Rather, observations support an alternative view that only the moon's exterior initially melted forming an ocean of molten rock.

In August 2010, the team used LROC images to identify physical signs of contraction on the lunar surface, in the form of lobe-shaped cliffs known as lobate scarps.

The scarps are evidence the moon shrank globally in the geologically recent past and might still be shrinking today. The team saw these scarps widely distributed across the moon and concluded it was shrinking as the interior slowly cooled.

Based on the size of the scarps, it is estimated that the distance between the moon's center and its surface shrank by approximately 300 feet. The graben were an unexpected discovery and the images provide contradictory evidence that the regions of the lunar crust are also being pulled apart.

"This pulling apart tells us the moon is still active," said Richard Vondrak, LRO Project Scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "LRO gives us a detailed look at that process."

As the LRO mission progresses and coverage increases, scientists will have a better picture of how common these young graben are and what other types of tectonic features are nearby. The graben systems the team finds may help scientists refine the state of stress in the lunar crust.

"It was a big surprise when I spotted graben in the far side highlands," said co-author Mark Robinson of the School of Earth and Space Exploration at Arizona State University, principal investigator of LROC. "I immediately targeted the area for high-resolution stereo images so we could create a three-dimensional view of the graben.  It's exciting when you discover something totally unexpected and only about half the lunar surface has been imaged in high resolution.  There is much more of the moon to be explored."

DERIVATION OF ABSOLUTE MODEL AGES FOR LUNAR LOBATE SCARPS
van der Bogert, Hiesinger, Banks, Watters and Robinson, LPSC #1847

Lobate Scarp or Fluidized Ejecta (November 10, 2011)

LROC: Lunar Landslides! (October 15, 2011)

LROC: Tectonics at the edge of Procellarum (October 13, 2011)

Scarps in Schrödinger (September 28, 2011)

LROC: lobate scarp in Xenophanes (September 14, 2011)

Wrinkled Planet (May 3, 2011)

Too brief an expedition to a lobate scarp (August 24, 2010)

Moon geologically active, cooling and shrinking (August 19, 2010)

Updated map of lunar graben, lobate scarps and further more recent topographic features broadly hinting Earth's Moon is not "dead," as once assumed, but geologically active [NASA/GSFC/DLR/Smithsonian CEPS/Arizona State University].

Sample redated, study reports a "younger Moon"

Photograph of 60025 sample used in the Borg study, "Note large proportion of pyroxene (green)" [LPSC 2011, #1127].

Redating a lunar sample after a weak acid bath has led workers to speculate the Moon may be 200 million years younger then generally thought. The report on a study appearing in Nature, by David Shiga at New Scientist, was previously presented to the 42nd Lunar and Plantary Science Conference in March 2011.

Lars Borg and colleagues at Lawrence Livermore based their conclusions following redating lunar sample FAN 60025, collected by Young & Duke during the Apollo 16 expedition to the lunar highlands north of the Descartes Formation in December 1972.

"But Clive Neal of the University of Notre Dame," Shiga wrote, "says some of the plagioclase - including this sample - might simply have melted again after the moon formed. Different minerals solidify at different temperatures, so if a heavy mineral solidified before a lighter one beneath it, it would sink, pushing magma upwards. This could melt the plagioclase and reset its age. "I remain to be convinced that the moon is as young as suggested by this paper," he says.

The report in New Scientist.
Citation appearing online by Nature

The Age of lunar Ferroan Anorthosite 60025 with implications for the interpretation of lunar chronology and the Magma Ocean Model

42nd Lunar and Planetary Science Conference, #1127

Did Earth start out with two moons?

Did Earth, for some protracted period in its earliest history, have "two moons?" Too be precise, for the largely held view of our Moon's origin to be fact, the Moon must once have had many, many of those planetesimals, through fairly rapid accretion, becoming one. A suggestion just published in the science journal Nature posits the origin of our present Moon's farside highlands, and many of its aspects reflecting a lack of uniformity, may have their origins in a primeval collision between two progenitor moons. This possibility may also explain the relative size of our Moon and Earth, among the rocky terrestrial planets. And while were setting our imaginations on a new, unrestrained course, who knows? One of those two moons might have had. at least for a time, a substantial atmosphere [NASA/JAXA].

Forming the lunar farside highlands by accretion of a companion moon

Jutzi & Asphaug
Nature 476, 69–72 (04 August 2011)

The most striking geological feature of the Moon is the terrain and elevation dichotomy¹ between the hemispheres: the nearside is low and flat, dominated by volcanic maria, whereas the farside is mountainous and deeply cratered. Associated with this geological dichotomy is a compositional and thermal variation^{2, 3}, with the nearside Procellarum KREEP (potassium/rare-earth element/phosphorus) Terrane and environs interpreted as having thin, compositionally evolved crust in comparison with the massive feldspathic highlands. The lunar dichotomy may have been caused by internal effects (for example spatial variations in tidal heating⁴, asymmetric convective processes⁵ or asymmetric crystallization of the magma ocean⁶) or external effects (such as the event that formed the South Pole/Aitken basin¹ or asymmetric cratering⁷). Here we consider its origin as a late carapace added by the accretion of a companion moon. Companion moons are a common outcome of simulations⁸ of Moon formation from a protolunar disk resulting from a giant impact, and although most coplanar configurations are unstable⁹, a ~1,200-km-diameter moon located at one of the Trojan points could be dynamically stable for tens of millions of years after the giant impact¹⁰. Most of the Moon’s magma ocean would solidify on this timescale^{11, 12}, whereas the companion moon would evolve more quickly into a crust and a solid mantle derived from similar disk material, and would presumably have little or no core. Its likely fate would be to collide with the Moon at ~2–3 km s⁻¹, well below the speed of sound in silicates. According to our simulations, a large moon/Moon size ratio (~0.3) and a subsonic impact velocity lead to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness consistent with the dimensions of the farside highlands^{1, 13} and in agreement with the degree-two crustal thickness profile⁴. The collision furthermore displaces the KREEP-rich layer to the opposite hemisphere, explaining the observed concentration^{2, 3}.

LRO's Diviner continues to map lunar terrain

Thermal data gathered during repeated orbital passes using LRO's Diviner shows the range of materials of similar composition in and around 109 million year-old Tycho, putting the familiar crater in a new light, including the distinctive twin ray trails [NASA/GSFC/UCLA/UW].

Eric Hand
Nature: The Great Beyond

Scientists are using temperature measurements to map the rockiest parts of the Moon – and the results could help NASA choose better landing sites for missions.

Infrared radiation readings taken by the Diviner Lunar Radiometer Experiment, an instrument on NASA’s Lunar Reconnaissance Orbiter (LRO) mission, have enabled researchers to see the moon’s temperature variations in detail. Not surprisingly, the surface heats up during the day and cools down at night. But rocks tend to retain their heat longer than the regolith, or lunar soil, and so they stay warm throughout the night.

Mapping these hot spots has provided a quick and quantitative way to assess rock abundance over vast areas of the Moon, says planetary scientist Josh Bandfield of the University of Washington in Seattle, who presented his results on Thursday at the Third Annual NASA Lunar Science Forum, held at NASA Ames Research Center at Moffett Field, California.

Because rocks get worn away over time, older craters tend to be less rocky. But a young crater like Tycho “just lights up” on the rock abundance map, says Bandfield (see image).

The Diviner team has also mapped spots that are cold enough to retain water ice. Around the Moon’s south pole, the surfaces of crater floors fulfill this criteria – but there are even larger surrounding areas where water ice would be stable below the surface, said planetary scientist David Paige of the University of California, Los Angeles at the forum. These regions might warm up during the hottest part of the year, but the subsurface would stay cool enough to preserve water ice for billions of years, he said.