Our Earth is constantly battered by tiny pieces of space rock.
Most of them burn up in the atmosphere, and these are called meteors.
But when these chunks of rock reach the ground, we call them meteorites.
Thousands of meteorites hit the surface of the planet each year, often falling harmlessly into the ocean.
Understanding more about how meteorites affect our planet is not only important for Earth science, but for modeling the formation and evolution of planets across our solar system.
A team of researchers led by Sally June Tracy, staff scientist at the Carnegie Institution for Science, recently shed light on how meteorite strikes may affect quartz, one of the most abundant materials in the Earth’s crust.
Tracy and her colleagues —Professor Thomas Duffy and Senior Scientist Stefan Turneaure — used the incredibly bright X-ray beams of the Advanced Photon Source (APS) to not only figure out more about how this planet and others may be affected by meteorites, but to begin to settle a decades-old dispute about the way quartz transforms under pressure.
What the team found is a new crystal structure of quartz, one that only lasts for about 100 nanoseconds, or 100 billionths of a second after impact. (Light, which moves at 186,282 miles per second, can travel about a foot in a nanosecond.)
This pivotal discovery was not predicted by any theoretical models, but will influence them going forward, adding to our understanding of one of our planet’s most common materials.
The research was published in Science Advances.
“The high-pressure structure of quartz is something that has been studied extensively,” Tracy said.
“We know a lot about what happens with quartz when you compress it in a static way using a press or compression cell. The question we wanted to answer is, what happens when you compress it under a faster timescale?”
That’s a question that has divided geoscientists since the 1960s, when a new form of quartz, called stishovite, was discovered at meteorite impact sites.
Scientists determined that this stishovite was caused by high pressure, the meteorite impact forcing the quartz to re-form itself into a structure with denser atomic packing.
Stishovite is an extremely hard form of silicon dioxide that is rare on Earth’s surface, but which scientists think may reside in the planet’s lower mantle, the inner layer that spans from 400 to 1,800 miles below the surface.
But the details of the structural transformation quartz undergoes when subjected to a high-velocity impact is the subject of some debate.
The research team’s experiments were designed to shed light on this question by analyzing the very moment of impact and determining the temporal evolution of the atomic-level change as it occurs.
“We wanted to send a shockwave through material and probe it as the wave travels through it,” she said. “The duration of the entire experiment is a couple hundred nanoseconds, so we’re probing very quickly. Capturing a picture of the material in this state is challenging.”
To accomplish it, the team turned to the Dynamic Compression Sector (DCS) at the APS. Here scientists have access to a unique combination of experimental capabilities to capture the moment of impact on a material sample, using the ultra-bright X-rays of the APS to take snapshots of a material’s structure at extremely short timescales.
It’s that combination of capabilities that makes DCS a “wonderful playground for knowledge,” in the words of Yogendra Gupta, professor at Washington State University and director of the Institute for Shock Physics.
DCS is managed and operated by Washington State, and Gupta said its ability to both create shock impacts in materials and take vivid X-ray images of the effects sets it apart.
“DCS allows us to look at the atomic level using a variety of dynamic compression platforms,” he said. “This has not been possible before.”
In the case of Tracy’s experiment, a hydrogen gas gun was used to fire a projectile roughly half an inch in diameter at a sample of quartz.
Gupta said the velocity of these gas gun projectiles can hit 18,000 miles an hour, which is roughly the speed a space shuttle must travel to achieve orbit around the Earth. The X-ray beam was then used to probe the changes the quartz went through in the nanoseconds during and after impact.
“We can adjust the time of when the picture is taken to collect a series of snapshots,” Tracy said.
“We carried out on the order of 20 experiments, which we can use to reconstruct a physical picture. We can translate our data into information about how the bonding and structure of the material changes under pressure.”
The newly discovered phase, according to Duffy, turned out to fall between two predicted states of quartz. It is neither the dense stishovite found at meteorite sites nor the amorphous, glassy state that some models suggested.
“Finding an intermediate phase is not unexpected,” Duffy said. “Some of the atoms migrate, others cannot. It makes sense. But we didn’t predict it.”
It was the availability of the capabilities at DCS, Tracy said, that made her experiment possible.
“Having these instruments available at user facilities opens them up to more people, and broadens the type of science questions we can ask,” she said. “The combination of tools is unique to the APS, you can’t do this experiment elsewhere.”
The APS is currently undergoing a massive upgrade that will increase the brilliance of its X-ray beams by up to 500 times. According to both Tracy and Gupta, the upgrade will make dynamic compression experiments like this one even more effective. The brighter beams will allow for sharper, much more detailed X-ray images with stronger contrast for fine features.
Even before that, though, this new result, Tracy said, will help inform the way we think about the formation of our own planet, and of others.
“We are refining our understanding of how these types of natural impact processes dictate how planetary systems evolved,” she said.
“And we are revisiting some of the old assumptions we made with our limited understanding, and posing new data that challenges our picture of what is going on.”
Written by Andre Salles