A massive crater hides under Greenland’s ice | Science News for Students

A massive crater hides under Greenland’s ice

The asteroid collision that caused it may have triggered a period of cooling called the Younger Dryas
Jan 2, 2019 — 6:45 am EST
An artists illustration of incoming asteroids breaking up in Earth's atmosphere, as seen from Earth orbit

An iron-rich asteroid breaks up in Earth’s atmosphere in this artist’s conception. It suggests the event that long ago created a large crater in northwest Greenland.

Natural History Museum of Denmark, Cryospheric Sciences Lab/GSFC/NASA

Something big lurks beneath Greenland’s ice. Using ice-penetrating radar, scientists have discovered a crater larger than the city of Paris. This 31-kilometer- (19.3-mile-) wide pit in northwest Greenland lies buried under as much as 930 meters (3,000 feet) of ice.

It was formed by a meteorite that slammed into Earth. Scientists now calculate that this space rock must have been about 1.5 kilometers (0.9 mile) across. One so big would have caused notable environmental damage across the entire Northern Hemisphere, a new analysis finds. Glaciologist Kurt Kjær led the team that found this crater. He works at the University of Copenhagen in Denmark.

a map showing where the crater was discovered and a radar image showing the crater
A crater has just been discovered at the edge of Greenland’s Hiawatha Glacier (upper image). Airborne radar data (lower image) show a round depression (green circle) under nearly a kilometer of ice. Deformed quartz minerals and other signatures of an ancient impact were found just outside the edge of the ice (black dot).
K.H. Kjær et al/Science Advances 2018

No one yet knows how old the crater is. But glacial debris and computer studies on rates of ice flow suggest the asteroid impact might have happened between 2.6 million and 11,700 years ago. That was during what is known as the Pleistocene Epoch (PLY-stuh-seen EP-ok).

The newfound crater could breathe new life into a controversial idea. It argues that an impact some 13,000 years ago triggered a mysterious 1,000-year cold snap. Scientists refer to that surprise cold period as the Younger Dryas.

Members of the research team first spotted hints of the crater in 2015. They were curious about a big rounded shape at the edge of Hiawatha Glacier in northwest Greenland. It showed up in a radar scan of the region. NASA’s Operation IceBridge was flying an aircraft over the area. It was part of a program to map the thickness of ice at Earth’s poles. The rounded shape looked like it might mark the edge of a crater, Kjær says.

His team decided to take a more detailed look. They hired a plane from Germany’s Alfred Wegener Institute. Its radar sent pulses of energy toward the ice at a large number of frequencies. Using data collected between 1997 and 2014 by several NASA programs, as well as lots of data collected in 2016 with this “ultra-wideband radar,” the team mapped its target.

The object is almost certainly an impact crater, Kjær says: “It became clear that our idea had been right from the beginning.” It is the first crater found in Greenland and one of the 25 or so largest ever spotted on Earth. And, Kjær adds, from its elevated rim to its bowl-shaped depression, this crater has held its shape beautifully.

His team reported its new find November 14 in Science Advances.

Hidden in plain sight

“It’s so conspicuous in the satellite imagery now,” says team member John Paden. He’s an electrical engineer at the University of Kansas in Lawrence. “There’s not another good explanation,” he says.

On the ground, the scientists hunted for chemical and geological clues of an asteroid impact. They couldn’t sample the crater. It remains buried under deep ice. But just beyond the edge of the ice, meltwater from the base of the glacier has, over the years, washed rocky sediment out of the glacier. The scientists sampled this so-called “outwash.”

a microscopic image of a grain of quartz showing how the crystal structure is deformed
This grain of quartz shows evidence that its crystal structure is deformed. That deformation is a signature of a massive impact event.
Natural History Museum of Denmark, Cryospheric Sciences Lab/GSFC/NASA

It contained several telltale signs of an impact. These included grains of quartz, a crystalline mineral, that had been “shocked.” This means it had deformed portions and glassy bits that might be flash-melted rock. The sample also contained amounts of certain elements that were higher than what’s normally found in Earth’s crust. These included nickel, cobalt, platinum and gold.

That grouping of elements points to an asteroid impact, the researchers say. And it suggests that the space rock would have been a relatively rare iron meteorite.

Figuring out when it slammed into Earth is trickier.

The ice-penetrating radar revealed that the crater’s bowl contains several different layers of ice. There appears to be continuous sequence of layers. They would have come from the gradual deposits of snow and ice through the most recent 11,700 years of Earth’s history. That period is known as the Holocene (HO-loh-seen).

Below that “well-behaved” layer is an older, debris-rich layer. It is one that has been seen elsewhere in ice cores from Greenland. That debris-strewn layer is thought to represent a cold period known as the Younger Dryas. It spanned from about 12,800 to 11,700 years ago. Beneath that is another large layer that is jumbled and rough.

“You see folding and strong disturbances,” says study coauthor Joseph MacGregor. He’s a glaciologist with Operation IceBridge. “And below that, we see yet deeper, complex basal ice.” Radar images showed the bottommost ice inside the crater was not flat but instead had curious peaks. MacGregor says these could represent material from the ground that got trapped in the ice.

Trying to date the cratering event

“What you have is a snapshot of an ice sheet that looked fairly normal during the Holocene, but was quite disturbed before that,” Kjær says. He says that clearly suggests the impact is at least 11,700 years old. The rim of the crater appears to cut through an ancient channel for a river that must have flowed across the land before Greenland became covered with ice roughly 2.6 million years ago.

That time span — essentially, the entire Pleistocene Epoch — is a large range. The team is now working to narrow the range in its likely age. To do that, they’ll look at more sediment. They’ll also run computer models of the rate of ice flow. They may even study ice cores collected from within the crater. The age range for the impact that caused the crater includes the onset of the Younger Dryas.

Planetary scientist Clark Chapman works at the Southwest Research Institute in Boulder, Colo. “There are plenty of roughly circular landforms on Earth of many different sizes,” he notes. Most of them are not impact craters. Still, he says, the paper presents several clues that strongly support the object being a crater.

But Chapman thinks it’s “quite unlikely” that the crater formed within the last couple of million years. Such strikes are rare, he notes. Asteroids barreling into Earth are far more likely to land somewhere in an ocean, he points out. And, he adds, “It would be at least a hundred times less likely that it could have happened so recently as to have affected the Younger Dryas.”

Whenever the crater formed, it is “a straight-up exciting discovery,” MacGregor says. “And we’re just happy not to have to keep it a secret anymore.”

A huge, newly discovered crater, long buried underneath Greenland’s ice sheet, may be the remnants of an asteroid that struck Earth more than 11,000 years ago.
Science News/YouTube

Power Words

(more about Power Words)

asteroid     A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

basal     Of or relating to the base or bottom layer of something.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

coauthor     One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.

computer model     A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

core     Something — usually round-shaped — in the center of an object. (in geology) Earth’s innermost layer. Or, a long, tube-like sample drilled down into ice, soil or rock. Cores allow scientists to examine layers of sediment, dissolved chemicals, rock and fossils to see how the environment at one location changed through hundreds to thousands of years or more.

crater     A large, bowl-shaped cavity in the ground or on the surface of a planet or the moon. They are typically caused by an explosion or the impact of a meteorite or other celestial body. Such an impact is sometimes referred to as a cratering event.

crust     (in geology) Earth’s outermost surface, usually made from dense, solid rock.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

debris     Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.

depression     A low spot, such as in a field or the surface of a rock.

electrical engineer     An engineer who designs, builds or analyzes electrical equipment.

element     A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

epoch     (in geology) A span of time in the geologic past that was shorter than a period (which is itself, part of some era ) and marked when some dramatic changes occurred.

geological     Adjective to describe things related to Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists.

glacier     A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.

Greenland     The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically part of North America (sitting just east of Northern Canada), Greenland has been linked more politically to Europe. Indeed, Vikings arrived in Greenland around the 10th century, and for a time the island was a colony of Denmark. In June 2009, Greenland became an independent nation. Ice covers roughly 80 percent of Greenland. Indeed, the Greenland ice sheet is the world’s largest. If its frozen water were to melt, it could raise sea levels around the world by 6 meters (about 20 feet). Although this is the 12th biggest nation (based on surface area), Greenland averages the fewest people per square kilometer of its surface area.

Holocene     The current period in geologic time. Meaning “entirely recent,” the Holocene began at the end of the last ice age, 11,700 years ago, and continues to the present time.

ice sheet     A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.

iron     A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.

meltwater     The water that comes from melting ice. The quantities can be large and show up quickly when it comes from melting glaciers, ice sheets and snow-capped mountains.

meteorite     A lump of rock or metal from space that passes through Earth’s atmosphere and collides with the ground.

mineral     Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

nickel     Number 28 on the periodic table of elements, this hard, silvery element resists oxidation and corrosion. That makes it a good coating for many other elements or for use in multi-metal alloys.

platinum     A naturally occurring silver-white metallic element that remains stable (does not corrode) in air. It is used in jewelry, electronics, chemical processing and some dental crowns.

poles     (in Earth science and astronomy) The cold regions of the planet that exist farthest from the equator; the upper and lower ends of the virtual axis around which a celestial object rotates. 

quartz     A type of mineral made from silicon dioxide. The most common mineral on Earth, it can occur in any rock type: igneous, metamorphic or sedimentary.

radar     A system for calculating the position, distance or other important characteristic of a distant object. It works by sending out periodic radio waves that bounce off of the object and then measuring how long it takes that bounced signal to return. Radar can detect moving objects, like airplanes. It also can be used to map the shape of land — even land covered by ice.

range     The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.

satellite     A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.

sediment     Material (such as stones and sand) deposited by water, wind or glaciers.

simulation     (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.

Younger Dryas     A period of abrupt climate change that occurred about 14,500 years ago. A glacial period had ended and the world was warming when a sudden cooling off took place. This near-glacial period ended about 11,500 years ago. It’s named for a cold-adapted flower (Dryas octopetala) that flourished throughout this period in Europe.

Citation

Journal: K.H. Kjær et al. A large impact crater beneath Hiawatha Glacier in northwest Greenland. Science Advances. November 14, 2018. doi: 10.1126/sciadv.eaar8173.