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New doctoral thesis questions dating of known lunar crater

The moon. Photo.
The bright side of the moon. The enormous Tycho crater is visible in white at the bottom edge of the image. Photo: PHOTO: NASA/JPL/USGS.

Over one hundred million years ago, the impact of an asteroid on the moon formed the giant crater Tycho, which is visible from Earth. The date of this event is established as being 109 million years ago, but a new doctoral thesis from Lund University in Sweden has now shown that the crater is probably much older.

For several hundred years, humans have been fascinated by the Tycho crater, which is 85 kilometres wide and 4.8 kilometres deep. Shaped like a fried egg, the crater has long tantalised the world’s astronomers from the highlands of the southern part of the moon’s visible face. Our knowledge of its rocky massifs and terraced slopes is largely based on analyses of gravel collected by Apollo 17 astronaut Eugene Cernan on the most recent moonwalk in 1972. Researchers agree that the Tycho crater was formed by a gigantic asteroid impact but the currently accepted dating of the crater to 109 million years ago, based on isotope analyses of material gathered several thousand kilometres from Tycho, is not set in stone. Other dating processes have shown that the crater could be either more ancient or more recent. Now, in her new doctoral thesis, Ellinor Martin has launched a theory suggesting the Tycho crater could be more than 117 million years old.

Woman mixing material in a container in a laboratory. Photo.
Ellinor Martin is looking for titanium-rich chrome-spinel grains in the Astrogeobiology laboratory in Lund. Photo: Kennet Ruona.

“I have analysed a ton of sediment that was deposited on the floor of the Pacific Ocean during the period in which Tycho is thought to have been formed. The age of the samples ranges from 103 to 117 million years. However, I did not find any extra-terrestrial mineral grains originating from Tycho, which should have been the case considering the size of the asteroid impact”, says Ellinor Martin.

Together with her colleagues at the Astrogeobiology laboratory, she dissolved the sediment in strong acids to isolate extra-terrestrial material in the form of titanium-rich chrome spinels. These microscopic grains are extremely resistant to weathering and function as a kind of time capsule. A few space grains were discovered, but only one could have originated from the moon.

“However, the potential moon grain does not have the high titanium content one could expect and probably has nothing to do with the Tycho crater; it most probably represents the common background flow of dust from the moon”, says Ellinor Martin.

The age of the Tycho crater is an important reference point in studies calculating the frequency of impacts on the moon throughout its existence. More precise dating would improve the models currently used for analysis and would also facilitate the dating of surfaces on other celestial bodies in the solar system.

“Our study shows that Tycho is probably more than 117 million years old. Previous analyses of sediments of the noble gas Helium-3, among other things, do not show any indication that the crater could have been formed more recently than 100 million years ago. So now, we must quite simply start looking for traces of Tycho in even older sediments”, says Ellinor Martin.

The study is published in Ellinor Martin’s doctoral thesis: The micrometeorite flux to Earth through the Phanerozoic Eon: Reconstructed using sediment-dispersed extraterrestrial spinels.

The doctoral thesis in the Lund University Research Portal

The article has also been accepted for publication in a special volume of the Geological Society of America Special Papers honouring palaeontologist Walter Alvarez on his 80th birthday.