Tag Archives: impact crater

Several city- and state-sized asteroids impacted young Earth. Probably.

Early Earth might have caught some significantly larger asteroids than we assumed, a new paper reports. These could have ranged from a city to a small province in size, the authors explain. Still, in the end, these impacts could have helped to shape Earth into what it is today.

Rendering of Mars’ Victoria Crater. Image credits Alexander Antropov.

Take someone living today back to the days when our Earth was young, and they probably wouldn’t recognize it. Even after its crust cooled and solidified, the blue planet wasn’t particularly blue, but rather, a bit barren. It was also pockmarked by asteroid impacts and, occasionally, impacted by asteroids.

Most traces of this past have slowly been ground away by tectonics, erosion, and weathering. So we don’t have much in the way of direct evidence (i.e. craters) to study. Still, researchers are pretty confident that the Earth was hit by a significant number of large asteroids, rocks over 10 km (6.2 mi) in diameter, in the past, and that this helped shape its chemical properties, eventually culminating in the appearance of life. But new research presented at the 2021 Goldschmidt Geochemistry Conference proposes that these large asteroids were much larger than we believed.

Big Rock

While our planet slowly grinds away traces of asteroid impacts unlike, say, the Moon or Mars, we can still find evidence of them happening in the shape of spherules. These are round, glassy beads that are produced by super-heated material ejected during an asteroid impact. As they’re propelled away from the impact through the air, they cool to form a spherical shape and eventually land back on the surface. Over geological time, they become encased by rocks. The greater the impact, the more of these particles it would produce, and the wider they would spread around the crater.

A large enough impact could even spread spherules across the world.

The team developed a statistical model to analyze our records of spherule layers so far. Their model suggests that the number of known impacts in the past “severely underestimates” the real number of impacts. According to the results, there were likely 10 times more impacts between 3.5 and 2.5 billion years ago than we assumed. That’s equivalent to one Chicxulub-sized impact (the one that wiped out the dinosaurs) once every 15 million years.

The authors add that although we have very little information regarding their number and magnitude, these impacts had a profound effect on how the Earth’s surface and atmosphere evolved throughout the ages. For example, they explain that atmospheric levels of oxygen likely varied significantly during these impacts. They could help account for the dips we see in oxygen levels throughout history, before they stabilized around 2.5 billion years ago, for example.

Given how important oxygen eventually became for the evolution of both the Earth and the life upon it, a better understanding of ancient impacts could help us better understand how we came to be here.

Naturally, the impacts also caused widespread disruption and destruction, but we don’t really have enough evidence to estimate their true effect; very few rocks survive from that period. This lack of direct data is what prompted the team to develop a statistical model to study these impacts in the first place.

The oldest known meteorite in the UK struck about 1.2 billion years ago

The meteorite crater was discovered near Ullapool, Scotland.

Britain might be a green place that’s full of life today, but 1.2 billion years ago, it was a completely different place — the whole world was. Plants hadn’t migrated onto land yet, and all macroscopic life was still in the sea. Judging by its dry land, Earth was a barren planet. At that time, Scotland was around the equator, making it an arid, Mars-like landscape.

When the meteorite struck, the change in landscape wouldn’t have been observed by anyone, although the impact was probably felt by any microorganisms unfortunate enough to lie too close to the impact site. For geologists, however, the site of the impact was quite lucky: it was quickly covered by sediment favoring its preservation.

At about 1 km across, it wasn’t a huge meteorite, but it came down at 40,000mph, striking the Earth with a force 940 million times greater than that of the Hiroshima bomb. Although it created a 40 km-wide crater, the fact that it was preserved was quite lucky. Lead author Dr. Ken Amor explains:

“The material excavated during a giant meteorite impact is rarely preserved on Earth, because it is rapidly eroded, so this is a really exciting discovery. It was purely by chance this one landed in an ancient rift valley where fresh sediment quickly covered the debris to preserve it”.

“It would have been quite a spectacle when this large meteorite struck a barren landscape, spreading dust and rock debris over a wide area,” he adds.

World map with the largest discovered meteorite craters.

The site was discovered near Ullapool, in northwest Scotland and it’s the oldest meteorite impact ever recorded in the UK. The largest ever meteorite was discovered in Free State, South Africa, and it’s called the Vredefort crater. Vredefort crater has an estimated radius of 118 miles (190 kilometers), making it the world’s largest known impact structure. This crater was declared a UNESCO World Heritage Site in 2005.

Meteorite impacts were a bit more common back then due to all the leftover debris from the formation of the solar system still floating around. Even now, it’s unclear how common and likely this sort of impact was at the time. A commonly used estimate states that collisions with an object of about 1 km in size occur once every 100,000-1,000,000 years. However, these estimates can vary quite a bit — one of the reasons we don’t have better estimates is because these impact sites aren’t often preserved. Meanwhile, smaller impacts (where the meteorite is only a few meters across) are much more common, occurring about once every 25 years on average. In December 2018, a meteorite exploded in the atmosphere above the Kamchatka Peninsula in Russia and we barely even noticed.

Journal Reference: ‘The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography, and geochemistry’ is available to view in Journal of the Geological Society here: https://doi.org/10.1144/jgs2018-093

A new impact crater has been found under more than a mile of ice in northwest Greenland. Credit: NASA Goddard.

NASA finds a second huge impact crater beneath Greenland’s ice sheet

A new impact crater has been found under more than a mile of ice in northwest Greenland. Credit: NASA Goddard.

A new impact crater has been found under more than a mile of ice in northwest Greenland. Credit: NASA Goddard.

Scientists didn’t think they would be able to find evidence of ancient impact craters in places such as Greenland or Antarctica, which should have been cleared away by erosion by underlying ice. But, in November, researchers found a huge 30-kilometer-wide crater (19 miles) beneath Greenland’s Hiawatha Glacier. It was the first meteorite impact found beneath an ice sheet, a breakthrough moment in geoscience that combined the latest imaging technologies. Now, researchers report finding an even larger crater beneath Greenland’s thick ice. With a width of 36.5 km (22 miles), if confirmed, the new crater would be the 22nd largest impact crater found on Earth.

Beneath the ice

The new site was identified just 183 km (114 miles) from Hiawatha, but, judging from current evidence, the two impacts weren’t likely made during the same time. Both are fairly recent though. Scientists estimate that Hiawatha is no more than three million years old, while the new impact site in northwest Greenland was likely formed by an asteroid impact within the past 2.6 million years. Although it doesn’t have a formal name yet, researchers are considering naming it the Paterson crater, in honor of the late glaciologist Stan Paterson, who helped reconstruct climate data for the past 100,000 years using ice cores from Greenland.

“We’ve surveyed the Earth in many different ways, from land, air, and space—it’s exciting that discoveries like these are still possible,” said Joe MacGregor, a glaciologist with NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who made contributions to the discovery of both craters.

MacGregor was inspired by last year’s discovery to scour topographic maps for more signs of other craters. He eventually noticed a circular pattern in an ice surface map made using data from NASA’s Terra and Aqua satellites. His suspicions were confirmed by raw radar data, including those collected by NASA’s Operation IceBridge, which were used to study the topography of the bedrock beneath the ice.

The images that MacGregor and colleagues assembled show all the hallmarks of an impact crater, including a flat, bowl-shaped depression in the bedrock, surrounded by an elevated rim and centrally located peaks. Measurements also revealed a negative gravity anomaly over the area, which is also characteristic of impact craters.

“The only other circular structure that might approach this size would be a collapsed volcanic caldera,” MacGregor said in a statement. “But the areas of known volcanic activity in Greenland are several hundred miles away. Also, a volcano should have a clear positive magnetic anomaly, and we don’t see that at all.”

Given the proximity of the two craters, it’s plausible that a double asteroid system impacted the area. In order to investigate this possibility, the researchers studied the erosion rates of the two craters. The findings suggest that the new crater is far more eroded than the Hiawatha crater, and consequently older. Previously, two pairs of geographically close craters in Ukraine and Canada were also found to be unrelated.

“The existence of a third pair of unrelated craters is modestly surprising but we don’t consider it unlikely,” MacGregor said. “On the whole, the evidence we’ve assembled indicates that this new structure is very likely an impact crater, but presently it looks unlikely to be a twin with Hiawatha.”

The findings appeared in the journal Geophysical Research Letters.

145 million year old body of seawater found under Chesapeake Bay

  • Chesapeake Bay is one of the few oceanic impact craters on Earth
  • When the huge impact took place ~35 million years ago, it sealed the ancient oceanic water
  • The water has remained virtually unchanged since then

 

A new study published in Nature provides chemical, isotopic and physical evidence that groundwater found at about 1.5 km deep under the Chesapeake Bay is actually a 145 million year old remnant of the Cretaceous North Atlantic Sea.

Image credit: Sanford WE et al.

Image credit: Sanford WE et al.

Metaphorically speaking, the aquifer described is just like a very ancient fly trapped in amber – preserving the exact conditions of the time it was sealed; the water is two times more salty than modern saltwater, providing valuable information about Cretaceous salinity. The entire setting was created with the “help” of a massive comet or meteorite that struck the area, its impact basically creating Chesapeake Bay.

“Previous evidence for temperature and salinity levels of geologic-era oceans around the globe has been estimated indirectly from various types of evidence in deep sediment cores. In contrast, our study identifies ancient seawater that remains in place in its geologic setting, enabling us to provide a direct estimate of its age and salinity,” said lead author Dr Ward Sanford of U.S. Geological Survey.

Chesapeake Bay is one of only a few impact craters that have been identified and described in oceanic waters. It’s estimated that the impact took place some 35 million years ago, ejecting enormous quantities of debris and creating humongous tsunamis that probably reached as far as the Blue Ridge Mountains, over 150 km away. This study not only highlights an underground structure that can provide valuable information about the marine environment from the Cretaceous, but it also helped geologists better understand the Chesapeake Bay itself.

“This study gives us confidence that we are working directly with seawater that dates far back in Earth’s history,” said Jerad Bales, acting U.S. Geological Survey’s Associate Director for Water. “The study also has heightened our understanding of the geologic context of the Chesapeake Bay region as it relates to improving our understanding of hydrology in the region.”

So how exactly was the aquifer preserved? Well generally speaking, an aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, or silt), trapped between impermeable rocks. You can think of is basically as a wet sponge in a horizontal bottle – the sponge is the permeable rocks holding water, the bottle is the layers of impermeable rock. When the big impact took place, it created such an impermeable layer, trapping the ancient water beneath it.

Researchers had a hunch that they might find something interesting there by drilling boreholes, but they had no idea just how interesting things would get.

Scientific Reference: Evidence for high salinity of Early Cretaceous sea water from the Chesapeake Bay crater. Ward E. Sanford, Michael W. Doughten, Tyler B. Coplen, Andrew G. Hunt & Thomas D. Bullen. Nature 503, 252–256  doi:10.1038/nature12714

Seas of molten and solidified rock on the Moon can be mistaken for pristine rocks

A new analysis of data from NASA’s Lunar Orbiter Laser Altimeter (LOLA) shows that molten rock created by lunar impacts has been around for much longer than previously believed.

moon melting

During its earliest days, the Moon was covered in an ocean of molten rock, pretty much like every planet out there. As that lunar magma ocean cooled over millions of years, a process called igneous differentiation took over. Basically, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process (or cooling, emplacement or eruption).

However, according to Brown University researchers, this wasn’t the only time the lunar surface was melted on a massive scale. The Moon, unlike the Earth, has no atmosphere to protect it from meteorite impacts; these impacts are much more frequent and much more violent there. Such is the case with the impact event that formed the Orientale basin on the Moon’s western edge and far side: it produced a “sea” of lava some 350 km (220 miles) across, and almost 10 km (6 miles) deep. Similar processes happened at various times in the Moon’s history in at least 30 other large impact basins.

Microphogoraphs of lunar samples, as seen through cross polarized light.

Microphogoraphs of lunar samples, as seen through cross polarized light.

Vaughan and his colleagues show that as these melt seas cooled, they igneously differentiated in pretty much the same way it did during the Moon’s initial cooling phase, so rocks found there could actually be mistaken for pristine, original ones.

“This work adds the concept of impact melt magma seas to the lexicon of lunar rock-forming processes,” said planetary geologist James W. Head III, the Scherck Distinguished Professor of Geological Sciences and the senior researcher involved in the study. “It emphasizes that one must consider the detailed point of origin of the rocks in order to interpret them correctly.”

Bad thing is, these rocks include the ones brought back by the Apollo project and Russia’s Luna missions. It’s quite possible, the researchers say, that impact melt material is present in lunar samples thought to be representative of the early formation of the lunar crust, which if true, could raise some big question on previous interpretation. If lunar samples do include melt material, it would help to explain some puzzling findings from lunar samples.

The thing is, for differentiation to take place, it would have to remain liquid for several thousands of years, which is very likely, when you’re dealing with “seas” as big as the ones described here. The next question was what that differentiation might look like, and how can we determine the composition of the impact melt sea.

“This is a mechanism by which the Moon was later modified to add petrologic complexity,” Vaughan said. “It helps make sense of mineralogical data that doesn’t always fit in this lunar magma ocean idea.”