by Dr. David Kring
The Barringer Meteorite Crater is a research laboratory that provides scientists an opportunity to study processes that might affect rocks on other planetary surfaces. Research can thus reveal details about both the Barringer impact event and impact events elsewhere. A recent example of this broader application is a study completed by Megan E. Elwood Madden in 2005, with some assistance from David A. Kring and Robert J. Bodnar.
Testing a hypothesis
Impact sites like Barringer contain water in two forms. Water exists underground in pore space between mineral crystals in rock. This is called groundwater and is the substance that is tapped when drilling wells for household water. Smaller amounts of water are also trapped within mineral crystals. These small pockets of water are called fluid inclusions. They are very tiny, often less than 10 millionths of a meter long. They can also contain other volatile substances like carbon dioxide and methane.
Trapped fluid inclusions are important because they provide a historical record of water. They can be used, for example, to determine the composition and temperature of water that existed long ago in geologic time.
Although fluid inclusions are common in rocks on Earth, they are rare to non-existent in samples from Mars and other planets. These samples reach us in the form of meteorites. They were produced on other planetary surfaces and later launched towards Earth by impact cratering events.
Taken at face value, the lack of fluid inclusions in these meteoritic samples suggests that liquid water and other volatile substances did not exist on the other planets when and where the rocks formed. However, there is another possible interpretation. Perhaps there used to be fluid inclusions in the rocks, but those fluid inclusions were destroyed by the impact events that launched them towards Earth.
To test this hypothesis, Elwood Madden analyzed a suite of rocks from the Barringer Meteorite Crater and in rocks from surrounding geologic formations that were unaffected by the impact event. She found that the rocks unaffected by the impact event contained abundant fluid inclusions. In contrast, she found that fluid inclusions were destroyed in rocks that had been shock-metamorphosed by the impact event. This generated her first conclusion: the impact effectively dehydrated rocks at the impact site. She was also able to calibrate the destruction of fluid inclusions and determined that even relatively mild shock conditions destroy fluid inclusions. The meteorites launched from Mars have seen much higher shock pressures than those needed to destroy fluid inclusions. Consequently, this led to her second conclusion: the lack of fluid inclusions in samples from Mars does not necessarily mean liquid water and other volatile substances were absent when the rocks formed.
Hand Built for maximum Impact by The Cyrus Company. 1998.