Nav: Home

Sandbox craters reveal secrets of planetary splash marks and lost meteorites

June 27, 2018

Look up above you. You might see blue sky, clouds, the Moon or stars. And while it might seem calm up there, the truth is it's nearly always raining. Every day, Earth is constantly bombarded by about 100 tons of falling objects from space, mostly simple dust or sand-sized particles that are destroyed as they hit the upper atmosphere.

But very rarely, a piece large enough to survive the intense heat of entry manages to fall all the way down to the Earth's surface, where its galactic journey ends with a bump.

Most extraterrestrial rocks that hit the Earth's surface - meteorites - are so small that they don't make a dent in the ground. Larger rocks, however, leave their mark in the form of bowl-shaped impact craters. One famous example is the 50,000-year-old Barringer Crater in Arizona, which is 1.2 kilometers across and 170 meters deep. But impact craters have been observed not just on Earth; scientists have also spied them on Mercury, Venus, and Mars, on our own Moon, and on the moons of Jupiter and Saturn.

One feature of craters has puzzled scientists for decades. A meteorite's force of impact turns the ground into powder and throws that powder high into the air in a cone-shaped trajectory. The flying powder settles around the crater to form a blanket. The question was: why are some blankets shaped like rays -- the long, radial streaks that fan out from the crater's center like spokes on a wheel?

In a new study published in Physical Review Letters (chosen as an Editors' Suggestion with an acommpanying Viewpoint article), scientists from the Okinawa Institute of Science and Technology Graduate University (OIST) have simulated these extraterrestrial impacts to shed light on how these mysterious crater rays form.

"You can't make a real crater with a real meteorite," said Associate Professor Pinaki Chakraborty, leader of OIST's Fluid Mechanics Unit, "but you can use an analog to simulate what goes on." An extensively studied simple experiment provides that analog: dropping a heavy metal ball onto a sand bed; the ball hurls out sand forming a crater surrounded by a blanket. "The problem is that these experiments don't produce crater rays," said Prof Chakraborty.

But there are some curious exceptions.

It wasn't until Dr. Tapan Sabuwala of the Continuum Physics Unit (Prof. Gustavo Gioia) was watching ball-drop experiments by high-school students on YouTube one day that he found the first clue about what might be causing the rays: "These experiments are popular in science classes. I noticed that some of their experiments were producing crater rays!"

So what was the unique feature of these experiments? In a word: messiness. Researchers generally even out the surface of the sand bed before dropping the ball, but the videos showed school students performing the experiment, who were skipping that step. Sure enough, when Dr. Sabuwala repeated the ball-drop experiment with an uneven surface, the mini-meteorites made crater rays. "That was the eureka moment".

It still wasn't clear why uneven landscapes caused crater rays to form. So the team conducted a second experiment in a flat sand bed imprinted with a regular pattern of hexagonal-shaped valleys. Upon impact, every one of the valleys touching the edge of the ball produced a ray. Christian Butcher, a technician in OIST's Fluid Mechanics Unit, repeated the experiment with different variables: "We changed the size of the ball, the distance between the valleys, the drop height of the ball, the grains in the bed, and so on," said Mr. Butcher. The only variables that affected the number of rays produced were the size of the ball and the distance between valleys.

For a closer look at the mechanism behind crater rays, the team turned to computer simulations. "The impacting ball creates shockwaves in the bed," says Prof Chakraborty. "The shockwaves focus the ejected sand grains from the valleys along radial streaks to form rays."

Having learned how the crater rays form, the scientists created a theoretical model to predict the number of rays. The model predictions matched well with the mini-meteorite experiments, allowing the scientists to predict what ray patterns would look like on the rough surfaces on real planets.

And there was another exciting twist to their model: it could also be used to learn about the meteorites that have vanished since making a crater. Based on how many rays a crater has, researchers can work out the diameter of the meteorite that created it.

"We can look at nearly any rayed crater with this model and learn about how it was made", said Prof Chakraborty.

From a simple ball dropped onto sand, the team has created an extraterrestrial lab like in the US TV show "CSI" able to investigate the violent history of the solar system.

Okinawa Institute of Science and Technology (OIST) Graduate University

Related Crater Articles:

Mars once had salt lakes similar to Earth
Mars once had salt lakes that are similar to those on Earth and has gone through wet and dry periods, according to an international team of scientists that includes a Texas A&M University College of Geosciences researcher.
Karla crater confirmed to be an impact structure
The Karla crater, one of the about 150 large impact structures on Earth, is situated near the border of the Republic of Tatarstan and Chuvash Republic, about 163 kilometers from Kazan Federal University.
ASU researchers study largest impact crater in the US, buried for 35 million years
About 35 million years ago, an asteroid hit the ocean off the East Coast of North America.
Mass anomaly detected under the moon's largest crater
A mysterious large mass of material has been discovered beneath the largest crater in our solar system -- the Moon's South Pole-Aitken basin -- and may contain metal from the asteroid that crashed into the Moon and formed the crater, according to a Baylor University study.
NASA finds possible second impact crater under Greenland ice
A NASA glaciologist has discovered a possible second impact crater buried under more than a mile of ice in northwest Greenland.
Chang'e 4 Rover comes into view
The Lunar Reconnaissance Orbiter got a closer look at Chang'e 4 on the lunar far side.
First look: Chang'e lunar landing site
On Jan. 30, NASA's Lunar Reconnaissance Orbiter caught views of the Chinese Chang'e 4 lander on the floor of the Moon's Von Kármán crater.
Curiosity's first attempt at gravimetry advances martian geology
By cleverly repurposing a device onboard Curiosity normally used to detect the rover's movements on Mars to measure slight variations in gravitational fields instead, researchers have refined the understanding of how Gale crater and the mountain at its center formed.
Sunset crater, San Francisco volcanic field
The San Francisco Volcanic Field is a 4,700 square kilometers (1,800 square miles) area in the southern boundary of the Colorado Plateau.
Overflowing crater lakes carved canyons across Mars
Today, most of the water on Mars is locked away in frozen ice caps.
More Crater News and Crater Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Rethinking Anger
Anger is universal and complex: it can be quiet, festering, justified, vengeful, and destructive. This hour, TED speakers explore the many sides of anger, why we need it, and who's allowed to feel it. Guests include psychologists Ryan Martin and Russell Kolts, writer Soraya Chemaly, former talk radio host Lisa Fritsch, and business professor Dan Moshavi.
Now Playing: Science for the People

#538 Nobels and Astrophysics
This week we start with this year's physics Nobel Prize awarded to Jim Peebles, Michel Mayor, and Didier Queloz and finish with a discussion of the Nobel Prizes as a way to award and highlight important science. Are they still relevant? When science breakthroughs are built on the backs of hundreds -- and sometimes thousands -- of people's hard work, how do you pick just three to highlight? Join host Rachelle Saunders and astrophysicist, author, and science communicator Ethan Siegel for their chat about astrophysics and Nobel Prizes.