[meteorite-list] Lunar Impacts

From: dorifry <dorifry_at_meteoritecentral.com>
Date: Mon, 23 Jan 2012 11:08:47 -0500
Message-ID: <FA8A2F940A194A07B34A725F5859D66C_at_DoriPC>

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
Received on Mon 23 Jan 2012 11:08:47 AM PST