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Plutonium in Earth Rocks Signals Long-ago Cosmic Collision

A small lump of rock pulled up from the Pacific Ocean seafloor in 1976 is giving scientists new clues about an ancient cosmic event. More than a hundred million years ago, two neutron stars collided. The resulting energetic kilonova sent a rain of long-lived elements, such as isotopes of plutonium, through space. Eventually, this stellar “debris” settled onto Earth. Some sank to the bottom of the ocean and got incorporated into a chunk of ferromanganese rock. Hidden inside were a few hundred atoms of plutonium radioisotopes. They provide the strongest clues about what created them in the merger and how long ago it happened.

The plutonium is in the form of Pu-244, which has a half-life of 81.3 million years. That helped a team of scientists from the Helmholtz-Zentrum Dresden-Rossendorf institution in Germany and researchers at Australia’s Nuclear Science and Technology Organisation (ANSTO), put the epoch of the explosion a near 100 million years ago. They also found that the sample lacked another element related to the collision: curium 247. It has a half-life of 16 million years.

“The absence of the curium radioisotope Cm-247, which was also produced in the explosion, tells us it happened a very long time ago,” said ANSTO’s Dr. Michael Hotchkis. “But not more than about 1 billion years ago; otherwise the Pu-244 would also be undetectable.”

Research team member Dominic Koll holds a sample of the rocky crust recovered from the Pacific Ocean. Courtesy ANSTO.

Drilling Cores Reveal Elements from a Kilonova

To get to the hidden PU-244 and figure out the age of the neutron star merger debris, the science team drilled out three cores in the rock. Then they began a careful chemical analysis. The cores were dated using the beryllium isotope Be-10, which has a half-life of 1.5 million years. They also found traces of the iron isotope Fe-60 in one core. Earth’s crust grows so slowly that each core, measuring up to 3 cm, spanned more than ten million years.

The remaining crust was imaged with computed x-ray tomography and encased in resin. This allowed the scientists to cut thin layers that each corresponded to ~1 million years of growth. Then, each sample was divided up and processed to extract the plutonium. During this analysis, the team also found traces of material from known supernova events that occurred 2 and 7 million years ago. They also found some curium, but not the specific isotope that would have been created in the neutron star collision, according to Hotchkis. “The only possible explanation is that the cosmic explosion responsible for the plutonium happened so long ago that the curium has already decayed away to practically nothing,” he said.

Making Elements

We all know that elements such as helium, carbon, nitrogen, oxygen — all the way up to iron — are made inside stars, a process called stellar nucleosynthesis. The Sun, for example, is fusing hydrogen in its core to make helium. In a few billion years, it will start to fuse helium to make carbon, and then continues on to make carbon and oxygen. When the Sun begins the transition to become a white dwarf, it will release all the elements to space. In stars much more massive than the Sun, the process is more complex, but basically, it continues up to the creation of iron. Since it takes more energy to make iron and anything heavier, the process stops, the core collapses and the star explodes all its elements to space. Elements such as gold, platinum, uranium, nickel, and zinc get created in such events.

About half the heaviest elements are made in colossal events such as neutron star collisions that result in kilonova events. That process is called the “r-process” and includes such elements as thorium and uranium, and transuranics, such as plutonium and curium. Theories of r-process nucleosynthesis suggest that both Cm-247 and Pu-244 are produced simultaneously, in roughly equal proportions in such an event. Since the curium decays more rapidly than the plutonium, that puts a lower bound on the age of the neutron star merger, while the Pu-244 helps define the upper bound.

*The periodic table of the elements with the origin of each element highlighted. Elements heavier than iron are created in supernovae, while some are created only in neutron star mergers. Courtesy Cmglee. CC BY-SA 3.0*

Exploring the R-process Dust on Earth and Beyond

The detailed study of these isotopes, plus others found in the ocean-bottom rock sample, show the debris from cosmic events can arrive at Earth in pulses. Some are linked to nearby supernova explosions. However, the tiny sample of Pu-244 existed throughout all layers of the rock slices. That means the plutonium very likely came from the neutron-star merger/kilonova. It has been showing up at Earth as a continuous flux throughout the 100 million years since the event.

The research team is looking for other samples to bolster the neutron-star merger discovery using radioisotope samples. There should be more pieces of ancient crust on Earth that contain the products of the r-process that occurred. The dust from that long-ago event could well have settled onto the Moon and other worlds. The Apollo rocks could be fair game for study, and future missions could provide another way to access dust from the ancient past.

Space-based missions such as the Chandra X-ray Observatory, James Webb Space Telescope, and others have seen neutron star mergers in various wavelengths. So, scientists knew they took place. However, this “chemical analysis” of debris from such events is a big step forward in dating the events and observing the results of r-process nucleosynthesis.

An artist’s view of a neutron star merger, accompanied by two views taken by the Chandra X-ray Observatory. This type of event results in extremely high-energy conditions conducive to the creation of some of the heavier elements such as plutonium.

For More Information

Stardust, the Sea, and an Ancient Cosmic Collision

The Timing of the Last R-process Event Near Earth from Interstellar ⁶⁰Fe, ²⁴⁴Pu and ²⁴⁷Cm Deposition on Earth

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Gang members targeted victims at secluded L.A. lookout points, D.A. says. They face long prison sentences

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Four gang members were sentenced to life in prison without the possibility of parole on Thursday for their roles in two armed robberies that resulted in the death of three people at lookout points in the Angeles National Forest and Rancho Palos Verdes in 2023.

A fifth defendant, also a gang member, was sentenced to a minimum of 30 years to life in jail.

The defendants in the case were identified as Marco Antonio Hernandez, 21, Luis Ventura, 27, Abraham Alvarenga Cortez, 24, Rossel Jose Hernandez-Ponce, 24, and Wendy Sarai Cerritos, 23, according to the Los Angeles County district attorney’s office.

Hernandez was found guilty of three counts of first-degree murder, two counts each of second-degree robbery and attempted second-degree robbery, and one count of conspiracy to commit robbery, prosecutors said.

Ventura and Cortez were found guilty of one count of first-degree murder, two counts of second degree robbery and one count of conspiracy to commit a robbery. Ventura was the only defendant not facing life in prison without the possibility of parole.

Separately, Hernandez-Ponce and Cerritos were also convicted on two counts of first-degree murder, two counts of second-degree robbery and one count of conspiracy to commit a robbery, authorities said.

The convictions and jail sentences stemmed from an incident on July 22, 2023 at 3:30 a.m. when Hernandez, Ventura and Cortez drove to the Angeles Crest National Forest where they found Jessie Enrique Munoz, 32, and a friend sitting in a vehicle. The three men held the two at gunpoint and robbed them. During the encounter Munoz “refused to turn over his car keys and put his vehicle in reverse” and was shot and killed by Cortez, according to prosecutors. The passenger in the vehicle was unharmed, court records show.

In another robbery two days later at 2 a.m., in the parking lot outside Terreanea Resort in Rancho Palos Verdes, Cerritos, Hernandez-Ponce and Hernandez approached two people who were sitting in their vehicle, identified as Jorge Ramos, 36, and Taylorraven Whittaker, 26, and killed them after the victims refused to “hand over their valuables,” authorities said.

The defendants had been in custody since their arrests in late July and August 2023.

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Education Department Lowers Student Loan Interest Rates for Two Years

The department, citing high default rates, is reducing interest rates on federal student loans by up to 1 percentage point for two years.

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What Would Happen if the Sun Stopped? Part 3: The Photon Traffic Jam

(This is Part 3 of a series on what would happen if the Sun stopped. Read Part 1 and Part 2 first.)

Imagine you’re standing in the middle of a crowded room. Not just any crowded room. A packed one. Shoulder to shoulder. So crowded you can’t take more than a single step in any direction before bumping into somebody. And every time you bump into someone, you get spun around to face a brand new random direction. You can’t see the walls. You can’t see the doors. All you can do is push, bump, spin. Push, bump, spin.

You can feel your blood pressure climbing already. You want out. Now. How long does it take you?

The answer depends on how big the room is, naturally, but it also depends on something subtler. You aren’t walking out of the room. You’re random walking out of the room. Every step lands in a completely random direction. Half the time you’re blundering deeper into the crowd without realizing it. Sometimes you go in circles. Sometimes you make a little progress and then immediately undo it.

This is not an efficient way to travel.

There’s some math describing how long this takes, and the math is frustrating, especially if you happen to be in a hurry. It says that to cover a given distance by random walk, you can’t just take the number of steps a straight walk would need. You have to take the square of that number. If the door is 4 steps away on a normal walk, it’s 16 steps away on a random walk. If it’s 10 steps in an empty room, it’s 100 in a packed one.

Every photon born in the core of the Sun is in exactly this predicament. Worse, actually. The Sun’s interior isn’t a gas, it’s a plasma, every atom stripped down to bare nuclei and free electrons drifting everywhere. And photons absolutely love to interact with free electrons. A photon born in the core travels about one centimeter before slamming into an electron, scattering off in a completely random direction, traveling another centimeter, slamming into another electron, scattering again. And again. And again.

One. Centimeter. The Sun’s radius is 70 billion of them. That’s the straight-line, empty-room, normal-walk distance. For a photon actually stuck inside the Sun, it’s 70 billion squared steps.

If you tried to count them off at one per second, it would take you longer than the current age of the universe. Several times over.

Each step takes only a fraction of a nanosecond, which is good. But there are a staggering number of them, which is bad. Run the arithmetic, and a photon born in the core of the Sun takes around 100,000 years to claw its way out to the surface.

A hundred thousand years.

If photons could simply stream straight out, the trip would take about two seconds. Instead, bouncing around like the unluckiest pinball in history, the journey takes 100,000 years. The random walk inflates the travel time by a factor of roughly a trillion.

The photon striking your face right now was born around the time anatomically modern humans were first spreading beyond Africa. Neanderthals were still around. Agriculture hadn’t been invented. Spoken language as we’d recognize it didn’t yet exist. Every civilization, every religion, every memory in all of human history is younger than the trip that photon just finished.

Sunlight is REALLY old.

And by the way, it isn’t even the same photon that started the trip. Photons in the solar interior don’t merely ricochet around like billiard balls. They are constantly being swallowed by electrons and then re-emitted, in new random directions and at slightly different energies. So the gamma ray born in the core, carrying around a million electronvolts, gets ground down step by patient step into longer, softer, lower-energy light. By the time it escapes the surface it’s visible light, about one electronvolt, peaking conveniently in the very wavelengths our eyes evolved to catch. The energy survived the journey. The original photon, not so much.

Most of that century-long crawl happens in what we call the radiative zone, the inner 70 percent of the Sun by radius, where the plasma is dense and hot and the photons are trapped in their pinball nightmare. Above the radiative zone sits the convective zone, where the plasma finally turns cool and opaque enough that radiation can’t carry the energy along fast enough anymore. So the Sun gives up on radiation and starts to BOIL. Bulk motion takes over: hot blobs of plasma physically rise to the surface, dump their heat, and sink back down. Once energy reaches the convective zone, it pops out to the surface in just a few months.

The upshot is that anything happening in the core of the Sun stays invisible from the surface for about 100,000 years. The light you see from the Sun today is reporting on conditions in the core during the last ice age. If the fusion rate at the heart of the Sun had been quietly drifting for the past 50,000 years, we would have no idea. As far as light is concerned, the Sun’s surface is a 100,000-year delayed broadcast.

The Sun is gigantic. The Sun is crowded. Changes deep inside it take an enormous amount of time to propagate outward. We already knew that fusion is so inefficient that the Sun is basically coasting on stored heat, and that the Kelvin-Helmholtz mechanism could keep the lights on for tens of millions of years all on its own.

Now layer on top of that the fact that the surface itself is broadcasting from a hundred millennia in the past.

You can see where this is going.

In Part 4, we finally pull the trigger, switch off fusion, and trace exactly how, and how slowly, the Sun would actually die.