[meteorite-list] Lunar Impacts

[dorifry]

The study of lunar zircons helps establish the dates of impact craters.

http://www.sciencealert.com.au/news/20122301-23036-2.html


Meteorites definitely struck Moon Curtin University
      Tuesday, 24 January 2012

      The presence of zircon in rocks collected during the Apollo missions 
provides unequivocal evidence that meteorites have collided with our Moon.
      Image: NASA/JPL
      Research led by Curtin University geologists has uncovered a wealth of 
new evidence in the mineral zircon from lunar rock samples recovered during 
NASA's Apollo missions, revealing indisputable proof of meteorite collisions 
on the Moon.

      Headed by microstructural geology experts Dr Nick Timms and Professor 
Steven Reddy of the Western Australian School of Mines (WASM), the study 
documents the discovery of impact-related shock features in lunar zircon, 
giving scientists a new conceptual framework to explain the history and 
timing of meteorite impact events in our solar system.

      Dr Timms said the discovery was made while looking more closely at 
lunar zircon mineral grains, with the use of microscopy facilities at 
Curtin, and finding the presence of preserved microscopic details, known as 
planar deformation features (PDFs), as well as micro-twins (impact 
indicators), which are only ever produced by large-scale meteorite impacts.

      "This research is the first to report the presence of PDFs and 
micro-twins in lunar zircon, which provide unequivocal evidence of the 
immense pressures that occur during an impact event," Dr Timms said.

      "This research also provides a new explanation of how these features 
form. As shock waves pass through a rock, fractions of a second after a 
meteorite impact, these features form like microscopic crumple zones which 
are caused by directional differences in zircon's elasticity."

      Dr Timms said the research, which characterises the impact shock 
features, would provide a new framework for scientists to interpret 
impact-related data.

      "The new conceptual framework allows lunar scientists to recognise 
whether complex zircon grains can be explained by a single impact event, or 
require more than one impact event," he said.

      "Furthermore, our new approach allows us to recognise impact-related 
features in zircon in lunar and terrestrial rocks that would otherwise be 
overlooked or difficult to find.

      "This helps us to overcome one of the major problems with studying the 
impact history of the Earth, as direct evidence of impacts, such as craters, 
become eroded and destroyed through processes of plate tectonics, so much so 
that none are preserved from the earliest periods of the Earth's history."

      Dr Timms said the research was a step closer to the major scientific 
goal of establishing the absolute timing of meteorite impact events on the 
Moon, and consequently, the inner solar system.

      "The current paradigm for the early impact history of our solar system 
stems from studies of lunar rocks and involves a period of intense impact 
events around 3.9 billion years ago, known as the 'Late Heavy Bombardment'," 
he said.

      "Recent dating of grains of the mineral zircon in lunar samples by the 
research group at Curtin shows a range of ages that challenges this view and 
we anticipate the new framework will help us to test if this bombardment is 
recorded in similar age zircon grains on Earth."

      This research was the result of a collaborative effort between the 
Curtin research group in Applied Geology, Dr Nick Timms, Professor Steven 
Reddy, Associate Professor Alexander Nemchin, Dr Marion Grange and Professor 
Bob Pidgeon, as well as Dr Rob Hart from the Materials Characterisation 
Group in Curtin Applied Physics and Dr Dave Healy at the University of 
Aberdeen, UK.

      The Curtin research group in Applied Geology is a pioneer in its field 
and is currently leading the world in the application of quantitative 
microstructural techniques to zircon research. In 2006, they also made the 
discovery that zircon could deform in the Earth's crust and that the 
structures formed in this deformation could help modify the geochemistry of 
zircon.

      The group's most recent paper, Resolution of impact-related 
microstructures in lunar zircon: A shock deformation mechanism map, is 
published in the internationally esteemed journal, Meteoritics and Planetary 
Science.



Phil Whitmer
Joshua Tree Earth & Space Museum