About 50,000 years ago, a nickel-iron meteorite some 50 meters across plowed into the Pleistocene-era grasslands of what is now Northern Arizona. It was traveling fast—about 13 kilometers per second. In just a few seconds, an impact dug out a crater just over a kilometer wide and spread rocks from the site for miles around.
For years, scientists have worked to understand all the forces at work in such an impact event like the one that carved out Meteor Crater. Clearly, impacts have huge effects. The aftermath of the collision affects the landscape and leaves behind a scene of destruction. Yet, as often as Earth has been hit, obvious craters like the one in Arizona are relatively rare. That’s because erosion, weathering, and plate tectonics erase them over geologic time. Unless you know exactly where to look, you might not be able to find obvious evidence that something smacked into our planet.
The Clearwater East & West impact craters in Quebec, Canada (image credit: Google Earth). These forms are still visible, even though they are filled with water. Other craters on Earth, such as the Chixculub site in Mexico, are harder to identify.
So, how to understand the forces at work in an impact? According to Professor Falko Langenhorst from the University of Jena, scientists need to study the indirect effects of impacts. These include precision examination of shocked minerals and impact glass—often referred to as lamellar structures. When something hits the ground with tremendous force, it affects materials down to the crystalline level of minerals. These lamellar structures are best studied using electron microscope techniques.
Finding Evidence of Shocked Grains
“For more than 60 years, these lamellar structures have served as an indicator of an asteroid impact, but no one knew until now how this structure was formed in the first place,” Liermann said, discussing a set of techniques that allowed them to study these shocked grains. “We have now solved this decades-old mystery.”
Langenhorst’s team came up with a way to simulate the incredible forces of an asteroid impact in the lab. The idea was to put quartz crystals (similar to the rocks shocked by the Meteor Crater event) under extremely high pressure inside a laboratory instrument. They used something called a “dynamic diamond anvil cell” (dDAC). It allows the science team to control pressures inside and change them very quickly. This simulates the rapidly changing pressures and temperatures at work during an actual impact event.
The impact simulated at the Jena lab creates tiny glass lamellae in quartz crystald. These structures are only tens of nanometers wide, so they had to be studied using an electron microscope. Courtesy: Falko Langenhorst, Christoph Otzen (University of Jena).
With this device, the scientists compressed single, tiny quartz crystals, putting them under tremendous pressure. At the same time, they shone an intense X-ray light through the crystals. This allowed them to witness changes to the crystal structure. “The trick is to let the simulated asteroid impact proceed slowly enough to be able to follow it with the X-ray light, but not too slowly, so that the effects typical of an asteroid impact can still occur,” Liermann said.
Looking at an Impact Second-by-Second
Experiments on a scale of seconds proved to be the right duration. This roughly simulates just how quickly an impactor can affect the landscape it’s encountering. Essentially, it transforms a quiet grassland into a rapidly expanding upheaval, melting rock and turning the surface into a hole in an extremely short period of time. The experiment in Jena focused on the split-second actions of the impact.
“We observed that at a pressure of about 180,000 atmospheres, the quartz structure suddenly transformed into a more tightly packed transition structure, which we call rosiaite-like,” reported team member Christoph Otzen, who is writing his doctoral thesis on these studies. (Technically, rosiaite is an oxidic mineral and the namesake for the crystal structure that is known from various materials. It does not consist of silica, but is a lead antimonate (a compound of lead, antimony, and oxygen).)
“In this crystal structure, the quartz shrinks by a third of its volume. The characteristic lamellae form exactly where the quartz changes into this so-called metastable phase, which no one has been able to identify in quartz before us,” said Otzen.
Looking Beyond Impacts
Understanding asteroid impacts on our planet (and others), gives a lot of insight into the interaction between these space rocks and planetary surfaces. After all, impact events shaped our worlds—starting from the first collisions of planetesimals in the early solar system. Earth has been hit many times and is not yet free from the dangers of impacts. So, it’s important to understand the intricate forces at work when something from space smacks into our planet.
However, the Jena team’s study has implications beyond the study of cratering events, according to Langenhorst. “What we observed could be a model study for the formation of glass in completely different materials such as ice,” Langenhorst pointed out. “It might be the generic path that a crystal structure transforms into a metastable phase in an intermediate step during rapid compression, which then transforms into the disordered glass structure. We plan to investigate this further because it could be of great importance for materials research.”
For More Information
Asteroid Impact in Slow Motion
Evidence for a rosiaite-structured high-pressure silica phase and its relation to lamellar amorphization in quartz
