Astronomy

We go about our daily lives sheltered under an invisible magnetic field generated deep inside Earth. It forms the magnetosphere, a region dominated by the magnetic field. Without that planetary protection shield, we’d experience harmful cosmic radiation and charged particles from the Sun.

Has Earth always had this deflector shield? Probably so, and the evidence is in old rocks. A team of researchers at University of Oxford and MIT found the earliest evidence for its existence in stones found along the coast of Greenland in a region called the Isua Supercrustal Belt.

Geologists have long known that iron particles in rocks “entrain” a print of the magnetic field that was in place when they formed. So, the research team uncovered rocks that formed some 3.7 billion years ago. It’s not an easy task, according to team lead Claire Nichols of the Department of Earth Sciences at Oxford. “Extracting reliable records from rocks this old is extremely challenging,” Nichols pointed out. “It was really exciting to see primary magnetic signals begin to emerge when we analyzed these samples in the lab. This is a really important step forward as we try and determine the role of the ancient magnetic field when life on Earth was first emerging.”

This 3.7-billion-year-old rock from Greenland. Entrained magnetic field fingerprints help scientists determine that our magnetosphere and magnetic field existed when this rock formed. Courtesy: Claire Nichols.

The team’s samples recorded a magnetic field strength of 15 microteslas at the time they formed. Today, Earth’s field strength is closer to 30 microteslas, so it’s obvious that our magnetic field and magnetosphere have been there for billions of years. It’s also clear that the field changes over time. The science team also found that early Earth’s magnetosphere was amazingly similar to the one it has today.

Tracking Earth’s Magnetosphere through Time

Our planet has a main dynamo at its heart. There are two cores—an inner one and an outer one. Motions in the core regions generate the magnetic field that defines our magnetosphere. Molten iron mixes and moves in the fluid outer core and the inner core solidifies. The two actions together create that dynamo. That’s what’s happening inside our planet today.

This cutaway of planet Earth shows the familiar exterior of air, water and land as well as the interior: from the mantle down to the outer and inner cores. Currents in hot, liquid iron-nickel in the outer core create our planet’s protective but fluctuating magnetic field and magnetosphere. Credit: Kelvinsong / Wikipedia

However, when Earth was first forming some 4.5 billion years ago, that solid inner core didn’t exist. Without the interaction we see today between the two parts of the core, it’s hard to know how any early magnetic field existed. That’s an open question among geologists and planetary scientists: how did it form and how was it sustained?

Another question relates to how much the planetary magnetic field has varied over time. Answering that one would help geologists understand just when the solid inner core formed. It would also show how much heat has escaped our planet from deep inside over time. Heat escape drives plate tectonics, which uses large “plates” of rock to shift things around on the surface over hundreds of millions of years.

What Do the Rocks Tell Us?

Rocks have a long and complex history. They form as a molten mixture that solidifies, or in the case of sandstones, are laid down in layers that then harden. In the case of molten rocks, they have magnetic field fingerprints entrained at the time of formation. In measuring those fingerprints, geologists account for any heating that could “reset” the magnetic signatures over time. The Greenland rocks are relatively pristine, meaning they haven’t been significantly heated since they formed. That means their magnetic fingerprints haven’t changed since formation.

Lava cooling after an eruption. This rock has an entrained magnetic field fingerprint from the time it formed. Credit: kalapanaculturaltours.com

Rocks also get weathered by wind, temperature changes and erosion, but the Isuan samples seem to be relatively pristine, according to Benjamin Weiss of MIT. “Northern Isua has the oldest known well-preserved rocks on Earth,” Weiss said. “Not only have they not been significantly heated since 3.7 billion years ago but they have also been scraped clean by the Greenland ice sheet.”

Rocks Through Time

The rocks the team studied date back to the Archean Eon—the second-oldest geologic eon in Earth’s history. That period began about 4 billion years ago, and during that time Earth was largely an ocean world with a limited amount of continental surface. Since then, Earth’s surface has changed a great deal, destroying or burying rocks from earlier times. So, finding rocks that date back that far in time is a big deal.

The Isuan rocks are relatively unchanged since they formed, and bear proof of a magnetic field existing less than a billion years after the planet formed. That same early magnetic field could have played a role in the development of our planet’s atmosphere, by assisting in removing xenon gas. Other old rocks may well tell scientists more about the birth of the magnetic field. There are rocks in Canada, Australia and South Africa that could give unique insight into the formation of the field and its role in making Earth habitable for life.

For More Information

Researchers Find Oldest Undisputed Evidence of Earth’s Magnetic Field
Possible Eoarchean Records of the Geomagnetic Field Preserved in the Isua Supracrustal Belt, Southern West Greenland

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When the first stars in the Universe formed, the only material available was primordial hydrogen and helium from the Big Bang. Astronomers call these original stars Population Three stars, and they were extremely massive, luminous, and hot stars. They’re gone now, and in fact, their existence is hypothetical.

But if they did exist, they should’ve left their fingerprints on nearby gas, and astrophysicists are looking for it.

The hunt for the Universe’s Population 3 (Pop III) stars is important in astrophysics. They were the first to form astronomical metals, elements heavier than hydrogen and helium. Only once these metals were available could rocky planets form. Their metals also fed into the next generation of stars, leading to the higher metallicity we observe in stars like our Sun.

Since Pop III stars were so massive and hot, they didn’t last long. None would have survived to this day. But the powerful JWST can expand the search for these crucial stars by looking back in time for their ancient light. That’s what the JWST-JADES (James Webb Space Telescope Advanced Deep Extragalactic Survey) is all about.

Researchers working with JADES data have found tantalizing evidence of Pop III stars in GN-z11, a high-redshift galaxy that’s one of the furthest galaxies from Earth ever observed. Their findings are in the paper “JWST-JADES. Possible Population III signatures at z=10.6 in the halo of GN-z11.” The lead author is Roberto Maiolino, a professor of Experimental Astrophysics at the Cavendish Laboratory (Department of Physics) and the Kavli Institute for Cosmology at the University of Cambridge. The research will be published in the journal Astronomy and Astrophysics.

“Finding the first generation of stars formed out of pristine gas in the early Universe, known as Population III (Pop III) stars, is one of the most important goals of modern astrophysics,” Maiolino and his colleagues write in their paper. “Recent models have suggested that Pop III stars may form in pockets of pristine gas in the halo of more evolved galaxies.”

GN-z11 is one such galaxy. At a redshift of z = 10.6034, the JWST sees the galaxy as it existed about 13.4 billion years ago, corresponding to about 400 million years after the Big Bang.

Pop III stars were massive and could be as much as 1000 times more massive than the Sun. These massive stars would’ve been exceptionally hot, which can provide a clue to their presence. Astrophysicists think all that heat could’ve doubly ionized nearby helium. So they search for the expected signature of that helium: prominent HeII nebular lines called the HeII?1640 emission line. To indicate the presence of Pop III stars, the HeII lines need to be unaccompanied by any metal lines.

The JWST observed the galaxy with its NIRSpec-IFU (Integrated Field Unit) and found a tentative detection of HeII?1640.

This figure from the research shows the detection of doubly-ionized Helium at 1.903 µm. in the galaxy GN-z11. Image Credit: Maiolino et al. 2024.

Detecting the doubly-ionized helium line was only the first step. Pop III stars aren’t the only objects that could’ve ionized the helium. To determine if the ancient stars were responsible, the researchers examined the galaxy and isolated several features.

Along with the HeII?1640, they also found Lyman-alpha emissions and CIII, or doubly-ionized carbon.

This figure from the research shows the detection of different emissions. The red star in the top images indicates the position of the continuum of GN-z11. The bottom row shows the lines mapped onto a JWST NIRCam image. The ‘fewer exposures’ on the top row indicates a lack of exposures in the upper portions of the panels due to a telescope-pointing error. Image Credit: Maiolino et al. 2024.

In the images above, the researchers note several features that are clues to the source of the helium ionization.

The HeII emissions show a plume extending to the west of GN-z11. It could be tracing gas photoionized by the galaxy’s active galactic nucleus (AGN.) Since CIII is so weak there, it could indicate very low metallicity gas photoionized by the AGN.

The image also shows a clump of HeII to the northeast of GN-z11. The researchers call this clump the “most intriguing” feature found. They analyzed the clump in the image below.

This figure from the research shows the spectra of the HeII clump. The observed emissions (blue) line up with the expected emissions from a galaxy at redshift z=10.600. Image Credit: Maiolino et al. 2024.

So what does this all add up to? Did the researchers find Pop III stars?

The spectral feature in the clump is strong evidence of photoionization by Pop III stars, according to the authors. “This wavelength corresponds to HeII?1640 at z=10.600, and it is fully consistent with the redshift of GN-z11,” they write. The same emission was detected over a larger area to the northeast, possibly with a second, fainter clump.

The authors say that the AGN could’ve photoionized the helium close to the galaxy’s center, but it can’t explain the HeII further away. Pop III stars are the best explanation, according to the authors.

Other evidence for Pop III stars comes from the emissions widths of the HeII lines. The high width suggests photoionization by metal poor Pop III stars rather than by Pop II stars with higher metallicity.

The extent of the ionization also indicates a certain mass for the Pop III stars, and the indicated mass agrees with simulations.

There’s another possibility: a direct collapse black hole (DCBH). “We also considered the alternative possibility of photoionization by a DCBH in the HeII clump,” the authors write. But the emission width should be lower in that scenario, although not by a lot. “Hence, this scenario remains another possible interpretation,” the authors write.

If future observations confirm the presence of Pop III stars in GN-z11, that’s a pretty big deal. But even if we have to wait for that confirmation, this research shows how powerful the JWST is again.

“These results have demonstrated the JWST’s capability to explore the primitive environment around galaxies in the early Universe, revealing fascinating properties,” the researchers conclude.

The post Astronomers Think They’ve Found Examples of the First Stars in the Universe appeared first on Universe Today.

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