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NASA Astronomy Picture of the Day
April 7, 2025
NGC 4414: A Flocculent Spiral Galaxy
How much mass do flocculent spirals hide? The featured image of flocculent spiral galaxy NGC 4414 was taken with the Hubble Space Telescope to help answer this question. Flocculent spirals galaxies without well-defined spiral arms are a quite common form of galaxy, and NGC 4414 is one of the closest. Stars and gas near the visible edge of spiral galaxies orbit the center so fast that the gravity from a large amount of unseen dark matter must be present to hold them together. Understanding the matter and dark matter distribution of NGC 4414 helps humanity calibrate the rest of the galaxy and, by deduction, flocculent spirals in general. Further, calibrating the distance to NGC 4414 helps humanity calibrate the cosmological distance scale of the entire visible universe.
https://apod.nasa.gov/apod/astropix.html
NASA astronaut captures stunning aurora images from space station
Updated 13:15, 07 Apr 2025
An astronaut has captured stunning images of green aurora displays over Earth.
NASA astronaut Don Pettit captured the night sky spectacle, known as the Northern Lights in the northern hemisphere, from the International Space Station (ISS) on Saturday (April 5).
Auroras are a spectacular light display that is typically seen in high-latitude regions near the Arctic and Antarctic, known as the aurora borealis and aurora australis, respectively.
Auroras are caused by large explosions of activity from the Sun, which throw out huge amounts of charged particles – called solar wind – into space.
When these particles are captured by the Earth's magnetic field they can collide with gas molecules in the atmosphere in an event called a geomagnetic storm.
Light is emitted from these collisions at various wavelengths, forming spectacular colourful displays in the sky.
Pettit described the sight from 260 miles above sea level as "green vaporous turbulence", seen while ISS orbit was "passing between Australia and Antarctica."
The Aurora australis, also known as the Southern Lights, is the counterpart of the aurora borealis – or Northern Lights – and occurs in the southern hemisphere, primarily around the Antarctic Circle.
Pettit, a member of the Expedition 72 crew aboard the ISS, is known for his astrophotography.
The mission, which ends in Spring 2025, involves astronauts conducting science experiments and maintaining the space station.
This is not the first time Pettit has captured stunning aurora images from space. The aurora display of October 2024 was a widespread show of the Northern Lights that reached large parts of the UK.
During the severe magnetic storm, the aurora borealis display was captured by Pettit from the ISS.
"Stunning was the word," Pettit wrote in a post on X on October 11, 2024. "The sun goes burp and the atmosphere turns red.
Spectacular not only from Earth but from orbit as well." Pettit admitted the night sky event had caught him off guard.
He added: "It looked like @Space_Station had been shrunk to some miniature dimension and inserted into a neon sign. We were not flying above the aurora; we were flying in the aurora. And it was blood red.
"Caught off guard, we hastily set up our cameras, four of them, all snapping shutters as fast as they could, creating a syncopated rhythm that accented Nature's artistic display presented before us."
https://www.manchestereveningnews.co.uk/news/uk-news/nasa-astronaut-captures-stunning-aurora-31369095
https://x.com/astro_Pettit/status/1908297775478890956
https://x.com/astro_Pettit/status/1908615642053255548
https://qalerts.app/?n=2222
North America is 'dripping' down into Earth's mantle, scientists discover
April 6, 2025
An ancient slab of Earth's crust buried deep beneath the Midwest is sucking huge swatches of present-day's North American crust down into the mantle, researchers say.
The slab's pull has created giant "drips" that hang from the underside of the continent down to about 400 miles (640 kilometers) deep inside the mantle, according to a new study.
These drips are located beneath an area spanning from Michigan to Nebraska and Alabama, but their presence appears to be impacting the entire continent.
The dripping area looks like a large funnel, with rocks from across North America being pulled toward it horizontally before getting sucked down.
As a result, large parts of North America are losing material from the underside of their crust, the researchers said.
"A very broad range is experiencing some thinning," study lead author Junlin Hua, a geoscientist who conducted the research during a postdoctoral fellowship at The University of Texas (UT) at Austin, said in a statement.
"Luckily, we also got the new idea about what drives this thinning," said Hua, now a professor at the University of Science and Technology of China.
The researchers found that the drips result from the downward dragging force of a chunk of oceanic crust that broke off from an ancient tectonic plate called the Farallon plate.
The Farallon plate and the North American plate once formed a subduction zone along the continent's west coast, with the former sliding beneath the latter and recycling its material into the mantle.
The Farallon plate splintered due to the advance of the Pacific plate roughly 20 million years ago, and remnant slabs subducted beneath the North American plate slowly drifted off.
One of these slabs currently straddles the boundary between the mantle transition zone and the lower mantle roughly 410 miles (660 km) beneath the Midwest.
Dubbed the "Farallon slab" and first imaged in the 1990s, this piece of oceanic crust is responsible for a process known as "cratonic thinning," according to the new study, which was published March 28 in the journal Nature Geoscience.
Cratonic thinning refers to the wearing away of cratons, which are regions of Earth's continental crust and upper mantle that have mostly remained intact for billions of years.
Despite their stability, cratons can undergo changes, but this has never been observed in action due to the huge geologic time scales involved, according to the study.
Now, for the first time, researchers have documented cratonic thinning as it occurs.
The discovery was possible thanks to a wider project led by Hua to map what lies beneath North America using a high-resolution seismic imaging technique called "full-waveform inversion."
This technique uses different types of seismic waves to extract all the available information about physical parameters underground.
"This sort of thing is important if we want to understand how a planet has evolved over a long time," study co-author Thorsten Becker, a distinguished chair in geophysics at UT Austin, said in the statement.
"Because of the use of this full-waveform method, we have a better representation of that important zone between the deep mantle and the shallower lithosphere [crust and upper mantle]."
To test their results, the researchers simulated the impact of the Farallon slab on the craton above using a computer model.
A dripping area formed when the slab was present, but it disappeared when the slab was absent, confirming that — theoretically, at least — a sunken slab can drag rocks across a large area down into Earth's interior.
Dripping beneath the Midwest won't lead to changes at the surface anytime soon, the researchers said, adding that it may even stop as the Farallon slab sinks deeper into the lower mantle and its influence over the craton wanes.
The findings could help researchers piece together the enormous puzzle of how Earth came to look the way it does today. "It helps us understand how do you make continents, how do you break them, and how do you recycle them," Becker said.
https://www.space.com/the-universe/earth/north-america-is-dripping-down-into-earths-mantle-scientists-discover
https://www.nature.com/articles/s41561-025-01671-x