Thursday, September 16, 2010

LRO's LOLA reveals distinct populations in bombardment record and Diviner finds "no pristine lunar mantle," even within SPA

Reduced laser altimetry data from the LOLA instrument on-board the Lunar Reconnaissance Orbiter is presented in this topographic map of Mare Orientale, straddling the western limb and marking the border between the Moon's near and far side hemispheres. New studies using these data show the relatively late, dramatic Orientale "basin-forming impact" may have marked a more definitive change in the history of Earth-Moon bombardment than previously understood [NASA/GSFC/LOLA/Brown University].

LRO project management announced Thursday an investigation using data from the Lunar Reconnaissance Orbiter (LRO) laser altimeter (LOLA) have created the first-ever comprehensive catalog of large craters on the moon. One immediate result is the discovery of distinct periods and populations in the Moon's bombardment record.

Data from the LRO Diviner instrument used in two studies has uncovered a richer complexity to in the anorthosite-rich lunar highlands and, more surprisingly, no evidence of materials composed of the pristine lunar mantle Diviner was partly designed to detect.

The history of the Moon is also the history of Earth.

In a new study, Dr. James Head of Brown University describes results obtained from a detailed global topographic map of the moon created LOLA data.

"Our new LRO LOLA dataset shows the older highland impactor population can be clearly distinguished from a younger population in giant impact basins, inundated with solidified lava flows," Head writes. "The highlands have a greater density of large craters compared to smaller ones, implying that the earlier population of impactors had a proportionally greater number of large fragments than the population that characterizes the more recent lunar history."

The Moon, Mars, and Mercury all bear scars of ancient bombardment, impact craters hundreds or even thousands of kilometers across. Earth must have been subjected to this same assault as well.

Large impacts that occurred long after the advent of life on Earth appear to have resulted in Great Extinctions. The partially buried crater at Chicxulub, in the Yucatan, is from a 65 million years old impact widely believed to have led or contributed to the end of Age of Dinosaurs (and many other lifeforms, as well).

Scientists trying to reconstruct the bombardment history on Earth face difficulties because impact craters are relatively swiftly eroded by wind and water, or destroyed by plate tectonics. A rich record, however, is well-preserved on the Moon. The only source of significant erosion comes from other impacts, small and steady or large and less frequent.

"The moon is a Rosetta Stone for understanding the bombardment history of Earth," said Head. "Like Egypt's Rosetta Stone, the lunar record can be used to translate the hieroglyphics of a poorly preserved impact record on Earth."

Head and his team used the LOLA instrument on-board LRO to build a map highlighting lunar craters with unprecedented clarity.

LOLA sends laser pulses to the lunar surface, measures the interval needed for these pulses to reflect back to the spacecraft and then, with a very precise knowledge of the LRO's orbit, convert these data into increasingly more detailed topographic maps of the Moon, said Head.

Objects hitting the moon can be categorized into distinct populations. Each population has its own characteristics. Head also used LOLA maps to determine the times when these populations changed.

"Using the crater counts from within the basalt-inundated impact basins, the familiar "seas" of the Moon's near side, for example, and examining populations superposed upon older craters, we can date these transitions. The LRO LOLA impact crater database shows a transition occurred about the time of the Orientale impact basin forming event, about 3.8 billion years ago.

"The implication is this change in populations occurred around the same time as the large impact basins stopped forming, and this raises questions of whether or not these factors are related. The answers has implications for the earliest history of the inner solar system, including Earth," said Head.

Map showing locations (in purple) of anorthositic crust exhibiting compositional anomalies. The iron and magnesium-rich maria appear red while calcium-rich highlands appear blue green. The five anomalous silicic features are labeled. Full size figure 11, HERE. (Read the Diviner news release HERE) [Science].

In two other studies, researchers describe how data from the Diviner Lunar Radiometer Experiment instrument (Diviner) on LRO are showing that the geologic processes that forged the lunar surface were complex, also. Data revealed previously unseen compositional differences in the crustal highlands, and these have confirmed a presence of an anomalously silica-rich material in five distinct regions.

Every mineral, and therefore every rock, absorbs and emits energy with a unique spectral signature that can be measured to reveal its identity and formation mechanisms. For the first time LRO's Diviner instrument is providing scientists with global, high-resolution infrared maps of the moon, enabling the definitive identification of silicate minerals in the Moon's crust.

"Diviner is literally viewing the moon in a whole new light," said Benjamin Greenhagen of NASA’s Jet Propulsion Laboratory, and lead author of one of the Diviner papers.

Lunar geology can be roughly broken down into two categories – the anorthositic highlands, rich in calcium and aluminum, and basaltic maria, abundant in iron and magnesium. Both of these crustal rock types are deemed by geologists as 'primitive,' i.e., the direct result of crystallization from lunar mantle material, a partially molten layer beneath the crust.

Diviner observations have confirmed most lunar terrains have spectral signatures consistent with compositions that fall into these two broad categories, but also reveal the lunar highlands are far less homogeneous than previously believed.

In a wide range of terrains, Diviner reveals a presence of fine lunar surface material with compositions more sodium rich than typical anorthosite crust. The widespread nature of these "fines" hint there may have been variations in the chemistry and cooling rate of the "magma ocean" which is now thought to have formed the earliest lunar crust, or these could be the result of a secondary processing of the earliest lunar crust.

Most impressively, in several locations around the moon Diviner detects a presence of highly silicic minerals, like quartz, potassium-rich and sodium-rich feldspar - minerals only associated with highly evolved lithologies, rocks that have undergone extensive molten processing.

Detection of silicic minerals at certain locations is significant because these occur in areas previously shown to exhibit unusually high abundances of the element thorium, yet another proxy for highly evolved lithologies.

"The silicic features we've found on the moon are fundamentally different from the more typical basaltic mare and anorthositic highlands," said Timothy Glotch, assistant professor of geosciences at Stony Brook University in New York, and lead author of a second Diviner Science paper. "The fact that we see this composition in multiple geologic settings suggests that there may have been multiple processes producing these rocks."

Read "New types of rock found on Moon by researchers at Stony Brook," HERE.

No evidence for pristine lunar mantle material

Using data from the Diviner Lunar Radiometer, an instrument uniquely capable of identifying common lunar silicate minerals, scientists at Stony Brook University in New York and NASA’s Jet Propulsion Laboratory have found previously unseen compositional differences in the crustal highlands of the Moon, and have confirmed the presence of anomalously silica-rich material in five distinct regions. Diviner data superimposed on a Lunar Orbiter IV mosaic of Aristarchus crater. Red and orange colors indicate silicic compositions [NASA/GSFC/UCLA/Stony Brook].

One thing not apparent in the data is evidence for pristine lunar mantle material, which previous studies have suggested may be exposed at some places on the lunar surface. Such material, rich in iron and magnesium, would be readily detected by Diviner.

Even in the South Pole Aitken basin (SPA), the largest, oldest, and deepest impact crater yet to be identified on the moon, deep enough to have penetrated through the crust and into the mantle, presented no evidence of pristine mantle material.

It's reported likely if the impact that formed SPA or Apollo basins did excavate any mantle material, it has since mixed with crustal material from later impacts, inside and outside the 2100 km-wide SPA impact.

"The new Diviner data will help in selecting the appropriate landing sites for potential future robotic missions to return samples from SPA. We want to use these samples to date the SPA-forming impact and potentially study the lunar mantle, so it's important to use Diviner data to identify areas with minimal mixing," says Greenhagen.

1 comment:

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