astronomy

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Saturn’s moon Titan has shorelines that appear to be shaped by waves

astronomy, Cassini, geology, planetary science, Saturn, Science, Titan / /

Surf the moon —

The liquid hydrocarbon waves would likely reach a height of a meter.

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Ligeia Mare, the second-largest body of liquid hydrocarbons on Titan.

Enlarge / Ligeia Mare, the second-largest body of liquid hydrocarbons on Titan.

During its T85 Titan flyby on July 24, 2012, the Cassini spacecraft registered an unexpectedly bright reflection on the surface of the lake Kivu Lacus. Its Visual and Infrared Mapping Spectrometer (VIMS) data was interpreted as a roughness on the methane-ethane lake, which could have been a sign of mudflats, surfacing bubbles, or waves.

“Our landscape evolution models show that the shorelines on Titan are most consistent with Earth lakes that have been eroded by waves,” says Rose Palermo, a coastal geomorphologist at St. Petersburg Coastal and Marine Science Center, who led the study investigating signatures of wave erosion on Titan. The evidence of waves is still inconclusive, but future crewed missions to Titan should probably pack some surfboards just in case.

Troubled seas

While waves have been considered the most plausible explanation for reflections visible in Cassini’s VIMS imagery for quite some time, other studies aimed to confirm their presence found no wave activity at all. “Other observations show that the liquid surfaces have been very still in the past, very flat,” Palermo says. “A possible explanation for this is at the time we were observing Titan, the winds were pretty low, so there weren’t many waves at that time. To confirm waves, we would need to have better resolution data,” she adds.

The problem is that this higher-resolution data isn’t coming our way anytime soon. Dragonfly, the next mission to Titan, isn’t supposed to arrive until 2034, even if everything goes as planned.

To get a better idea about possible waves on Titan a bit sooner, Palermo’s team went for inferring their presence from indirect cues. The researchers assumed shorelines on Titan could have been shaped by one of three candidate scenarios. They first assumed there was no erosion at all; the second modeled uniform erosion caused by the dissolution of the bedrock by the ethane-methane liquid; and the third assumed erosion by wave activity. “We took a random topography with rivers, filled up the basin-flooding river valleys all around the lake. Then, we then used landscape evolution computer model to erode the coast to 50 percent of its original size,” Palermo explains.

Sizing the waves

Palermo’s simulations showed that wave erosion resulted in coastline shapes closely matching those actually observed on Titan.

The team validated its model using data from closer to home. “We compared using the same statistical analysis to lakes on Earth, where we know what the erosion processes are. With certainty greater than 77.5 percent, we were able to predict those known processes with our modeling,” Palermo says.

But even the study that claimed there were waves visible in the Cassini’s VIMS imagery concluded they were roughly 2 centimeters high at best. So even if there are waves on Titan, the question is how high and strong are they?

According to Palermo, wave-generation mechanisms on Titan should work just like they do on Earth, with some notable differences. “There is a difference in viscosity between water on Earth and methane-ethane liquid on Titan compared to the atmosphere,” says Palermo. The gravity is also a lot weaker, standing at only one-seventh of the gravity on Earth. “The gravity, along with the differences in material properties, contributes to the waves being taller and steeper than those on Earth for the same wind speed,” says Palermo.

But even with those boosts to size and strength, could waves on Titan actually be any good for surfing?

Surf’s up

“There are definitely a lot of open questions our work leads to. What is the direction of the dominant waves? Knowing that can tell us about the winds and, therefore, about the climate on Titan. How large do the waves get? In the future, maybe we could tell that with modeling how much erosion occurs in one part of the lake versus another in estimated timescales. There is a lot more we could learn,” Palermo says. As far as surfing is concerned, she said that, assuming a minimum height for a surfable wave of around 15 centimeters, surfing on Titan should most likely be doable.

The key limit on the size and strength of any waves on Titan is that most of its seas are roughly the size of the Great Lakes in the US. The largest of them, the Kraken Mare, is roughly as large as the Caspian Sea on Earth. There is no such thing as a global ocean on Titan, and this means the fetch, the distance over which the wind can blow and grow the waves, is limited to tens of kilometers instead of over 1,500 kilometers on Earth. “Still, some models show that the waves on Titan be as high as one meter. I’d say that’s a surfable wave,” Palermo concluded.

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Astronomers think they’ve figured out how and when Jupiter’s Red Spot formed

astronomy, geophysics, Giovanni Cassini, great red spot, Jupiter, Science, science history / /

a long-lived vortex —

Astronomers concluded it is not the same and that Cassini’s spot disappeared in 1708.

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Enhanced image of Jupiter’s Great Red Spot, as seen from a Juno flyby in 2018. The Red Spot we see today is likely not the same one famously observed by Cassini in the 1600s.

Enlarge / Enhanced Juno image of Jupiter’s Great Red Spot in 2018. It is likely not the same one observed by Cassini in the 1600s.

The planet Jupiter is particularly known for its so-called Great Red Spot, a swirling vortex in the gas giant’s atmosphere that has been around since at least 1831. But how it formed and how old it is remain matters of debate. Astronomers in the 1600s, including Giovanni Cassini, also reported a similar spot in their observations of Jupiter that they dubbed the “Permanent Spot.” This prompted scientists to question whether the spot Cassini observed is the same one we see today. We now have an answer to that question: The spots are not the same, according to a new paper published in the journal Geophysical Research Letters.

“From the measurements of sizes and movements, we deduced that it is highly unlikely that the current Great Red Spot was the ‘Permanent Spot’ observed by Cassini,” said co-author Agustín Sánchez-Lavega of the University of the Basque Country in Bilbao, Spain. “The ‘Permanent Spot’ probably disappeared sometime between the mid-18th and 19th centuries, in which case we can now say that the longevity of the Red Spot exceeds 190 years.”

The planet Jupiter was known to Babylonian astronomers in the 7th and 8th centuries BCE, as well as to ancient Chinese astronomers; the latter’s observations would eventually give birth to the Chinese zodiac in the 4th century BCE, with its 12-year cycle based on the gas giant’s orbit around the Sun. In 1610, aided by the emergence of telescopes, Galileo Galilei famously observed Jupiter’s four largest moons, thereby bolstering the Copernican heliocentric model of the solar system.

(a) 1711 painting of Jupiter by Donato Creti showing the reddish Permanent Spot. (b) November 2, 1880, drawing of Jupiter by E.L. Trouvelot. (c) November 28, 1881, drawing by T.G. Elger.

Enlarge / (a) 1711 painting of Jupiter by Donato Creti showing the reddish Permanent Spot. (b) November 2, 1880, drawing of Jupiter by E.L. Trouvelot. (c) November 28, 1881, drawing by T.G. Elger.

Public domain

It’s possible that Robert Hooke may have observed the “Permanent Spot” as early as 1664, with Cassini following suit a year later and multiple more sightings through 1708. Then it disappeared from the astronomical record. A pharmacist named Heinrich Schwabe made the earliest known drawing of the Red Spot in 1831, and by 1878 it was once again quite prominent in observations of Jupiter, fading again in 1883 and at the onset of the 20th century.

Perhaps the spot is not the same…

But was this the same Permanent Spot that Cassini had observed? Sánchez-Lavega and his co-authors set out to answer this question, combing through historical sources—including Cassini’s notes and drawings from the 17th century—and more recent astronomical observations and quantifying the results. They conducted a year-by-year measurement of the sizes, ellipticity, area, and motions of both the Permanent Spot and the Great Red Spot from the earliest recorded observations into the 21st century.

The team also performed multiple numerical computer simulations testing different models for vortex behavior in Jupiter’s atmosphere that are the likely cause of the Great Red Spot. It’s essentially a massive, persistent anticyclonic storm. In one of the models the authors tested, the spot forms in the wake of a massive superstorm. Alternatively, several smaller vortices created by wind shear may have merged, or there could have been an instability in the planet’s wind currents that resulted in an elongated atmospheric cell shaped like the spot.

Sánchez-Lavega et al. concluded that the current Red Spot is probably not the same as that observed by Cassini and others in the 17th century. They argue that the Permanent Spot had faded by the start of the 18th century, and a new spot formed in the 19th century—the one we observe today, making it more than 190 years old.

Comparison between the Permanent Spot and the current Great Red Spot. (a) December 1690. (b) January 1691. (c) January 19, 1672. (d) August 10, 2023.

Enlarge / Comparison between the Permanent Spot and the current Great Red Spot. (a) December 1690. (b) January 1691. (c) January 19, 1672. (d) August 10, 2023.

Public domain/Eric Sussenbach

But maybe it is?

Others remain unconvinced of that conclusion, such as astronomer Scott Bolton of the Southwest Research Institute in Texas. “What I think we may be seeing is not so much that the storm went away and then a new one came in almost the same place,” he told New Scientist. “It would be a very big coincidence to have it occur at the same exact latitude, or even a similar latitude. It could be that what we’re really watching is the evolution of the storm.”

The numerical simulations ruled out the merging vortices model for the spot’s formation; it is much more likely that it’s due to wind currents producing an elongated atmospheric shell. Furthermore, in 1879, the Red Spot measured about 24,200 miles (39,000 kilometers) at its longest axis and is now about 8,700 miles (14,000 kilometers). So, the spot has been shrinking over the ensuing decades and becoming more rounded. The Juno mission’s most recent observations also revealed the spot is thin and shallow.

The question of why the Great Red Spot is shrinking remains a matter of debate. The team plans further simulations aiming to reproduce the shrinking dynamics and predict whether the spot will stabilize at a certain size and remain stable or eventually disappear like Cassini’s Permanent Spot presumably did.

Geophysical Research Letters, 2024. DOI: 10.1029/2024GL108993  (About DOIs).

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Supermassive black hole roars to life as astronomers watch in real time

active galactic nuclei, astronomy, astrophysics, black holes, Physics, Science, Very Large Telescope / /

Sleeping Beauty —

A similar awakening may one day occur with the Milky Way’s supermassive black hole

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Artist’s animation of the black hole at the center of SDSS1335+0728 awakening in real time—a first for astronomers.

In December 2019, astronomers were surprised to observe a long-quiet galaxy, 300 million light-years away, suddenly come alive, emitting ultraviolet, optical, and infrared light into space. Far from quieting down again, by February of this year, the galaxy had begun emitting X-ray light; it is becoming more active. Astronomers think it is most likely an active galactic nucleus (AGN), which gets its energy from supermassive black holes at the galaxy’s center and/or from the black hole’s spin. That’s the conclusion of a new paper accepted for publication in the journal Astronomy and Astrophysics, although the authors acknowledge the possibility that it might also be some kind of rare tidal disruption event (TDE).

The brightening of SDSS1335_0728 in the constellation Virgo, after decades of quietude, was first detected by the Zwicky Transient Facility telescope. Its supermassive black hole is estimated to be about 1 million solar masses. To get a better understanding of what might be going on, the authors combed through archival data and combined that with data from new observations from various instruments, including the X-shooter, part of the Very Large Telescope (VLT) in Chile’s Atacama Desert.

There are many reasons why a normally quiet galaxy might suddenly brighten, including supernovae or a TDE, in which part of the shredded star’s original mass is ejected violently outward. This, in turn, can form an accretion disk around the black hole that emits powerful X-rays and visible light. But these events don’t last nearly five years—usually not more than a few hundred days.

So the authors concluded that the galaxy has awakened and now has an AGN. First discovered by Carl Seyfert in 1943, the glow is the result of the cold dust and gas surrounding the black hole, which can form orbiting accretion disks. Gravitational forces compress the matter in the disk and heat it to millions of degrees Kelvin, producing radiation across the electromagnetic spectrum.

Alternatively, the activity might be due to an especially long and faint TDE—the longest and faintest yet detected, if so. Or it could be an entirely new phenomenon altogether. So SDSS1335+0728 is a galaxy to watch. Astronomers are already preparing for follow-up observations with the VLT’s Multi Unit Spectroscopic Explorer (MUSE) and Extremely Large Telescope, among others, and perhaps even the Vera Rubin Observatory slated to come online next summer. Its Large Synoptic Survey Telescope (LSST) will be capable of imaging the entire southern sky continuously, potentially capturing even more galaxy awakenings.

“Regardless of the nature of the variations, [this galaxy] provides valuable information on how black holes grow and evolve,” said co-author Paula Sánchez Sáez, an astronomer at the European Southern Observatory in Germany. “We expect that instruments like [these] will be key in understanding [why the galaxy is brightening].”

There is also a supermassive black hole at the center of our Milky Way galaxy (Sgr A*), but there is not yet enough material that has accreted for astronomers to pick up any emitted radiation, even in the infrared. So, its galactic nucleus is deemed inactive. It may have been active in the past, and it’s possible that it will reawaken again in a few million (or even billion) years when the Milky Way merges with the Andromeda Galaxy and their respective supermassive black holes combine. Only much time will tell.

Astronomy and Astrophysics, 2024. DOI: 10.1051/0004-6361/202347957  (About DOIs).

Listing image by ESO/M. Kornmesser

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Polarized light yields fresh insight into mysterious fast radio bursts

astronomy, astrophysics, CHIME, fast radio bursts, Physics, Science / /

CHIME-ing in —

Scientists looked at how polarization changed direction to learn more about origins

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Artist’s rendition of how the angle of polarized light from an FRB changes as it journeys through space.

Enlarge / Artist’s rendition of how the angle of polarized light from a fast radio burst changes as it journeys through space.

CHIME/Dunlap Institute

Astronomers have been puzzling over the origins of mysterious fast radio bursts (FRBs) since the first one was spotted in 2007. Researchers now have their first look at non-repeating FRBs, i.e., those that have only produced a single burst of light to date. The authors of a new paper published in The Astrophysical Journal looked specifically at the properties of polarized light emitting from these FRBs, yielding further insight into the origins of the phenomenon. The analysis supports the hypothesis that there are different origins for repeating and non-repeating FRBs.

“This is a new way to analyze the data we have on FRBs. Instead of just looking at how bright something is, we’re also looking at the angle of the light’s vibrating electromagnetic waves,” said co-author Ayush Pandhi, a graduate student at the University of Toronto’s Dunlap Institute for Astronomy and Astrophysics. “It gives you additional information about how and where that light is produced and what it has passed through on its journey to us over many millions of light years.”

As we’ve reported previously, FRBs involve a sudden blast of radio-frequency radiation that lasts just a few microseconds. Astronomers have over a thousand of them to date; some come from sources that repeatedly emit FRBs, while others seem to burst once and go silent. You can produce this sort of sudden surge of energy by destroying something. But the existence of repeating sources suggests that at least some of them are produced by an object that survives the event. That has led to a focus on compact objects, like neutron stars and black holes—especially a class of neutron stars called magnetars—as likely sources.

There have also been many detected FRBs that don’t seem to repeat at all, suggesting that the conditions that produced them may destroy their source. That’s consistent with a blitzar—a bizarre astronomical event caused by the sudden collapse of an overly massive neutron star. The event is driven by an earlier merger of two neutron stars; this creates an unstable intermediate neutron star, which is kept from collapsing immediately by its rapid spin.

In a blitzar, the strong magnetic fields of the neutron star slow down its spin, causing it to collapse into a black hole several hours after the merger. That collapse suddenly deletes the dynamo powering the magnetic fields, releasing their energy in the form of a fast radio burst.

So the events we’ve been lumping together as FRBs could actually be the product of two different events. The repeating events occur in the environment around a magnetar. The one-shot events are triggered by the death of a highly magnetized neutron star within a few hours of its formation. Astronomers announced the detection of a possible blitzar potentially associated with an FRB last year.

Only about 3 percent of FRBs are of the repeating variety. Per Pandhi, this is the first analysis of the other 97 percent of non-repeating FRBs, using data from Canada’s CHIME instrument (Canadian Hydrogen Intensity Mapping Experiment). CHIME was built for other observations but is sensitive to many of the wavelengths that make up an FRB. Unlike most radio telescopes, which focus on small points in the sky, CHIME scans a huge area, allowing it to pick out FRBs even though they almost never happen in the same place twice.

Pandhi et al. decided to investigate how the direction of the light polarization from 128 non-repeating FRBs changes to learn more about the environments in which they originated. The team found that the polarized light from non-repeating FRBs changes both with time and with different colors of light. They concluded that this particular sample of non-repeating FRBs is either a separate population or more evolved versions of these kinds of FRBs that are part of a population that originated in less extreme environments with lower burst rates. That’s in keeping with the notion that non-repeating FRBs are quite different from their rarer repeating FRBs.

The Astrophysical Journal, 2024. DOI: 10.3847/1538-4357/ad40aa  (About DOIs).

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Radio telescope finds another mystery long-repeat source

astronomy, astrophysics, neutron stars, pulsars, Science, weird stuff / /

File under W for WTF —

Unlike earlier object, the new source’s pulses of radio waves are erratic.

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Image of a purple, glowing sphere with straight purple-white lines emerging from opposite sides, all against a black background.

Enlarge / A slowly rotating neutron star is still our best guess as to the source of the mystery signals.

Roughly a year ago, astronomers announced that they had observed an object that shouldn’t exist. Like a pulsar, it emitted regularly timed bursts of radio emissions. But unlike a pulsar, those bursts were separated by over 20 minutes. If the 22 minute gap between bursts represents the rotation period of the object, then it is rotating too slowly to produce radio emissions by any known mechanism.

Now, some of the same team (along with new collaborators) are back with the discovery of something that, if anything, is acting even more oddly. The new source of radio bursts, ASKAP J193505.1+214841.0, takes nearly an hour between bursts. And it appears to have three different settings, sometimes producing weaker bursts and sometimes skipping them entirely. While the researchers suspect that, like pulsars, this is also powered by a neutron star, it’s not even clear that it’s the same class of object as their earlier discovery.

How pulsars pulse

Contrary to the section heading, pulsars don’t actually pulse. Neutron stars can create the illusion by having magnetic poles that aren’t lined up with their rotational pole. The magnetic poles are a source of constant radio emissions but, as the neutron star rotates, the emissions from the magnetic pole sweep across space in a manner similar to the light from a rotating lighthouse. If Earth happens to be caught up in that sweep, then the neutron star will appear to blink on and off as it rotates.

The star’s rotation is also needed for the generation of radio emissions themselves. If the neutron star rotates too slowly, then its magnetic field won’t be strong enough to produce radio emissions. So, it’s thought that if a pulsar’s rotation slows down enough (causing its pulses to be separated by too much time), it will simply shut down, and we’ll stop observing any radio emissions from the object.

We don’t have a clear idea of how long the time between pulses can get before a pulsar will shut down. But we do know that it’s going to be far less than 22 minutes.

Which is why the 2023 discovery was so strange. The object, GPM J1839–10, not only took a long time between pulses, but archival images showed that it had been pulsing on and off since at least 35 years ago.

To figure out what is going on, we really have two options. One is more and better observations of the source we know about. The second is to find other examples of similar behavior. There’s a chance we now have a second object like this, although there are enough differences that it’s not entirely clear.

An enigmatic find

The object, ASKAPJ193505.1+214841.0, was discovered by accident when the Australian Square Kilometre Array Pathfinder telescope was used to perform observations in the area due to detections of a gamma ray burst. It picked up a bright radio burst in the same field of view, but unrelated to the gamma ray burst. Further radio bursts showed up in later observations, as did a few far weaker bursts. A search of the telescope’s archives also spotted a weaker burst from the same location.

Checking the timing of the radio bursts, the team found that they could be explained by an object that emitted bursts every 54 hours, with bursts lasting from 10 seconds to just under a minute. Checking additional observations, however, showed that there were often instances where a 54 minute period would not end with a radio burst, suggesting the source sometimes skipped radio emissions entirely.

Odder still, the photons in the strong and weak bursts appeared to have different polarizations. These differences arise from the magnetic fields present where the bursts originate, suggesting that the two types of bursts differ not only in total energy, but also that the object that’s making them has a different magnetic field.

So, the researchers suggest that the object has three modes: strong pulses, faint pulses, and an off mode, although they can’t rule out the off mode producing weak radio signals that are below the detection capabilities of the telescopes we’re using. Over about eight months of sporadic observations, there’s no apparent pattern to the bursts.

What is this thing?

Checks at other wavelengths indicate there’s a magnetar and a supernova remnant in the vicinity of the mystery object, but not at the same location. There’s also a nearby brown dwarf at that point in the sky, but they strongly suspect that’s just a chance overlap. So, none of that tells us more about what produces these erratic bursts.

As with the earlier find, there seem to be two possible explanations for the ASKAP source. One is a neutron star that’s still managing to emit radiofrequency radiation from its poles despite rotating extremely slowly. The second is a white dwarf that has a reasonable rotation period but an unreasonably strong magnetic field.

To get at this issue, the researchers estimate the strength of the magnetic field needed to produce the larger bursts and come up with a value that’s significantly higher than any previously observed to originate on a white dwarf. So they strongly argue for the source being a neutron star. Whether that argues for the earlier source being a neutron star will depend on whether you feel that the two objects represent a single phenomenon despite their somewhat different behaviors.

In any case, we now have two of these mystery slow-repeat objects to explain. It’s possible that we’ll be able to learn more about this newer one if we can get some information as to what’s involved in its mode switching. But then we’ll have to figure out if what we learn applies to the one we discovered earlier.

Nature Astronomy, 2024. DOI: 10.1038/s41550-024-02277-w  (About DOIs).

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Nova explosion visible to the naked eye expected any day now

astronomy, astrophysics, nova, Science, white dwarf / /
Image of a blue sphere, surrounded by blue filaments, and enclosed in a partial sphere of pink specks.s

Enlarge / Aftermath of a nova at the star GK Persei.

When you look at the northern sky, you can follow the arm of the Big Dipper as it arcs around toward the bright star called Arcturus. Roughly in the middle of that arc, you’ll find the Northern Crown constellation, which looks a bit like a smiley face. Sometime between now and September, if you look to the left-hand side of the Northern Crown, what will look like a new star will shine for five days or so.

This star system is called T. Coronae Borealis, also known as the Blaze Star, and most of the time, it is way too dim to be visible to the naked eye. But once roughly every 80 years, a violent thermonuclear explosion makes it over 10,000 times brighter. The last time it happened was in 1946, so now it’s our turn to see it.

Neighborhood litterbug

“The T. Coronae Borealis is a binary system. It is actually two stars,” said Gerard Van Belle, the director of science at Lowell Observatory in Flagstaff, Arizona. One of these stars is a white dwarf, an old star that has already been through its fusion-powered lifecycle. “It’s gone from being a main sequence star to being a giant star. And in the case of giant stars, what happens is their outer parts eventually get kind of pushed into outer space. What’s left behind is a leftover core of the star—that’s called a white dwarf,” Van Belle explained.

The white dwarf stage is normally a super peaceful retirement period for stars. The nuclear fusion reaction no longer takes place, which makes white dwarfs very dim. They are still pretty hot, though, and they’re super dense, with a mass comparable to our Sun squeezed into a volume resembling the Earth.

But the retirement of the white dwarf in T. Coronae Borealis is hardly peaceful, as it has a neighbor prone to littering. “Its companion star is in the red giant phase, where it is puffed up. Its outer parts are getting sloughed off and pushed into space. The material that is coming off the red giant is now falling onto the white dwarf,” Van Belle said.

Ticking time bomb

And it doesn’t take much littering to make the white dwarf explode. “The material from the red giant will accumulate on the white dwarf’s surface until it forms a layer that’s actually not that thick. Just a few meters—the depth of a deep swimming pool,” Van Belle explained. Most of the material coming off the red giant is hydrogen. And since the red dwarf is still hot, there will eventually be a spark that triggers a runaway nuclear fusion reaction. “That is what causes the explosion,” Van Belle said.

The explosion is a nova, which means it doesn’t kill either the white dwarf or the red giant as a supernova would. “Only about 5 percent of the hydrogen layer fuses into heavier elements like helium, and the rest just gets ejected into space. Then the process starts all over again because the explosion isn’t large enough to disrupt the red giant, the donor of all this hydrogen, so it just keeps doing its thing,” Van Belle told Ars. This is why we can predict this event with such precision.

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How the perils of space have affected asteroid Ryugu

asteroid, astronomy, magneticsm, planetary science, Ryugu, Science / /

Magnets: how do they stop working? —

Ryugu’s parent body appears to have had a fair amount of water present, too.

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Grey image of a complicated surface composed of many small rocks bound together by dust.

Enlarge / The surface of Ryugu. Image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST

An asteroid that has been wandering through space for billions of years is going to have been bombarded by everything from rocks to radiation. Billions of years traveling through interplanetary space increase the odds of colliding with something in the vast emptiness, and at least one of those impacts had enough force to leave the asteroid Ryugu forever changed.

When the Japanese Space Agency’s Hayabusa2 spacecraft touched down on Ryugu, it collected samples from the surface that revealed that particles of magnetite (which is usually magnetic) in the asteroid’s regolith are devoid of magnetism. A team of researchers from Hokkaido University and several other institutions in Japan are now offering an explanation for how this material lost most of its magnetic properties. Their analysis showed that it was caused by at least one high-velocity micrometeoroid collision that broke the magnetite’s chemical structure down so that it was no longer magnetic.

“We surmised that pseudo-magnetite was created [as] the result of space weathering by micrometeoroid impact,” the researchers, led by Hokkaido University professor Yuki Kimura, said in a study recently published in Nature Communications.

What remains…

Ryugu is a relatively small object with no atmosphere, which makes it more susceptible to space weathering—alteration by micrometeoroids and the solar wind. Understanding space weathering can actually help us understand the evolution of asteroids and the Solar System. The problem is that most of our information about asteroids comes from meteorites that fall to Earth, and the majority of those meteorites are chunks of rock from the inside of an asteroid, so they were not exposed to the brutal environment of interplanetary space. They can also be altered as they plummet through the atmosphere or by physical processes on the surface. The longer it takes to find a meteorite, the more information can potentially be lost.

Once part of a much larger body, Ryugu is a C-type, or carbonaceous, asteroid, meaning it is made of mostly clay and silicate rocks. These minerals normally need water to form, but their presence is explained by Ryugu’s history. It is thought that the asteroid itself was born from debris after its parent body was smashed to pieces in a collision. The parent body was also covered in water ice, which explains the magnetite, carbonates, and silicates found on Ryugu—these need water to form.

Magnetite is a ferromagnetic (iron-containing and magnetic) mineral. It is found in all C-type asteroids and can be used to determine their remanent, or remaining, magnetization. The remanent magnetization of an asteroid can reveal how intense the magnetic field was at the time and place of the magnetite’s formation.

Kimura and his team were able to measure remanent magnetization in two magnetite fragments (known as framboids because of their particular shape) from the Ryugu sample. It is proof of a magnetic field in the nebula our Solar System formed in, and shows the strength of that magnetic field at the time that the magnetite formed.

However, three other magnetite fragments analyzed were not magnetized at all. This is where space weathering comes in.

…and what was lost

Using electron holography, which is done with a transmission electron microscope that sends high-energy electron waves through a specimen, the researchers found that the three framboids in question did not have magnetic chemical structures. This made them drastically different from magnetite.

Further analysis with scanning transmission electron microscopy showed that the magnetite particles were mostly made of iron oxides, but there was less oxygen in those particles that had lost their magnetism, indicating that the material had experienced a chemical reduction, where electrons were donated to the system. This loss of oxygen (and oxidized iron) explained the loss of magnetism, which depends on the organization of the electrons in the magnetite. This is why Kimura refers to it as “pseudo-magnetite.”

But what triggered the reduction that demagnetized the magnetite in the first place? Kimura and his team found that there were more than a hundred metallic iron particles in the part of the specimen that the demagnetized framboids had come from. If a micrometeorite of a certain size had hit that region of Ryugu then it would have produced approximately that many particles of iron from the magnetite framboids. The researchers think this mystery object was rather small, or it would have had to have been moving incredibly fast.

“With increasing impact velocity, the estimated projectile size decreases,” they said in the same study.

Pseudo-magnetite might sound like an imposter, but it will actually help upcoming investigations that seek to find out more about what the early Solar System was like. Its presence indicates the former presence of water on an asteroid, as well as space weathering, such as micrometeoroid bombardment, that affected the asteroid’s composition. How much magnetism was lost also affects the overall remanence of the asteroid. Remanence is important in determining an object’s magnetism and the intensity of the magnetic field around it when it formed. What we know of the Solar System’s early magnetic field has been reconstructed from remanence records, many of which come from magnetite.

Some magnetic properties of those particles might have been lost eons ago, but so much more could be gained in the future from what remains.

Nature Communications, 2024.  DOI: 10.1038/s41467-024-47798-0

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Monster galactic outflow powered by exploding stars

astronomy, astrophysics, galaxy evolution, Science / /

A big burp —

Star death and birth both contribute to driving material out of a galaxy.

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Image of a galaxy showing lots of complicated filaments of gas.

Enlarge / All galaxies have large amounts of gas that influence their star-formation rates.

Galaxies pass gas—in the case of galaxy NGC 4383, so much so that its gas outflow is 20,000 light-years across and more massive than 50 million Suns.

Yet even an outflow of this immensity was difficult to detect until now. Observing what these outflows are made of and how they are structured demands high-resolution instruments that can only see gas from galaxies that are relatively close, so information on them has been limited. Which is unfortunate, since gaseous outflows ejected from galaxies can tell us more about their star formation cycles.

The MAUVE (MUSE and ALMA Unveiling the Virgo Environment) program is now changing things. MAUVE’s mission is to understand how the outflows of galaxies in the Virgo cluster affect star formation. NGC 4383 stood out to astronomer Adam Watts, of the University of Australia and the International Centre for Radio Astronomy Research (ICRAR), and his team because its outflow is so enormous.

The elements it releases into space can reveal the galaxy’s potential to form (or stop forming) stars. “Understanding the physics of stellar feedback-driven outflows… is essential to completing our picture of galaxy evolution,” the researchers said in a study recently published in Monthly Notices of the Royal Astronomical Society.

Star potential

Stellar feedback, which is all the radiation, particle winds, and other materials that stars blast into the interstellar medium, is what forms outflows as huge as that in NGC 4383. Much of this material comes from either bursts of star formation or the insides of massive stars when they die and go supernova. It includes heavier elements that escape into space with the outflow and float there for an indefinite amount of time, sometimes ending up in other galaxies.

Star formation in a galaxy depends on several processes. There has to be the right balance of gas accretion (growth from added gas), consumption (the burning of hydrogen and helium by stars), and ejection (when interstellar gas is blown out of the galaxy) between the intergalactic medium and circumgalactic medium, the gas surrounding galaxies. Some of the gas and other materials, such as iron and other heavy elements, that form stars can be recycled from supernova explosions.

The supply of gas is key because large amounts of gas eventually collapse in on themselves because of their immense gravity, eventually forming stars. A deficit of gas can squelch the formation of potential stars.

Watts and his team think that one source of the stellar feedback pushing star-forming gas out of NGC 4383 is multiple supernovae that occurred relatively close together. Supernovae can form gargantuan bubbles of scorching gas that eventually break out of a galactic disk vertically, extending from the top and bottom of the galaxy.

Hot gas continues into cooler regions of the interstellar medium, with its gravity pulling in more gas on the way out of the galaxy and increasing the total mass of the outflow (known as mass loading). The loss of so much gas decreases the chances of star formation even further.

Lost in space

Outflows can be observed at many different wavelengths. Emissions of X-rays from elements such as hydrogen and compounds such as carbon monoxide can be detected. It is also possible to observe outflows using UV, optical, and infrared. Some of the region’s emissions had already been observed with other telescopes, which was combined with MAUVE imaging of the Virgo Cluster and NGC 4383 at different wavelengths.

The problem with observing outflows accurately is that the scattered materials are notoriously difficult to spatially resolve, which means figuring out the distance of the entire outflow based on pixels. MAUVE, NGC 4383, and the Virgo Cluster were observed at a spatial resolution of about 261 light-years, so each pixel represented a square in space that measured 261 light-years on every side. Clumps of ionized gas that showed up in these pixels told the research team there was a bipolar outflow leaving the galaxy from the top and bottom.

So, does NGC 4383 have reduced star formation because of its massive outflow of star stuff? It turns out that stars are actually forming at the galaxy’s edge. While no stars form in the stream escaping the galaxy, there are still areas where there is enough accreted gas to give birth to them.

These starbursts, or areas of rapid star formation, are also providing stellar feedback—it’s not just supernovae. “There is an extension of blue knots that are much brighter in the near-UV and are clear evidence of star formation occurring outside the main body of the galaxy,” the researchers said in the same study.

Something that remains unclear about NGC 4383 is whether the gas outflow was set off by stellar feedback alone or whether a gravitational interaction with another galaxy intensified existing outflows. There is possibly evidence for this on the eastern side, where a disturbance in the gas suggests that a nearby dwarf galaxy might have interacted with it. For now, the research team is confident that the outflow is primarily driven by starbursts and supernovae.

There is still more that the researchers want to find out about NGC 4383 and its outflow. As telescopes become more advanced and spatial resolution improves, maybe something else will be revealed inside those clouds of gas.

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Glow of an exoplanet may be from starlight reflecting off liquid iron

astronomy, atmospheres, exoplanets, Science / /

For all the glory —

A phenomenon called a “glory” may be happening on a hellishly hot giant planet.

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Image of a planet on a dark background, with an iridescent circle on the right side of the planet.

Enlarge / Artist impression of a glory on exoplanet WASP-76b.

Do rainbows exist on distant worlds? Many phenomena that happen on Earth—such as rain, hurricanes, and auroras—also occur on other planets in our Solar System if the conditions are right. Now we have evidence from outside our Solar System that one particularly strange exoplanet might even be displaying something close to a rainbow.

Appearing in the sky as a halo of colors, a phenomenon called a “glory” occurs when light hits clouds made up of a homogeneous substance in the form of spherical droplets. It might be the explanation for a mystery regarding observations of exoplanet WASP-76B. This planet, a scorching gas giant that experiences molten iron rain, has also been observed to have more light on its eastern terminator (a line used to separate the day side from the night side) than its western terminator. Why was there more light on one side of the planet?

After observing it with the CHEOPS space telescope, then combining that with previous observations from Hubble, Spitzer, and TESS, a team of researchers from ESA and the University of Bern in Switzerland now think that the most likely reason for the extra light is a glory.

Seeing the light

Over three years, CHEOPS made 23 observations of WASP-76B in both visible and infrared light. These included phase curves, transits, and secondary eclipses. Phase curves are continuous observations that track a planet’s complete revolution and show changes in its phase or the part of its illuminated side that is facing the telescope. The telescope may see more or less of that side as the planet orbits its star. Phase curves can determine the change in the total brightness of the planet and star as the planet orbits.

Secondary eclipses happen when a planet passes behind its host star and is eclipsed by it. The light seen during such an eclipse can later be compared with the total light both before and after the occultation to give us a sense of the light that’s reflected off the planet. Hot Jupiters like WASP-76B are commonly observed through secondary eclipses.

Phase-curve observations can continue while the planet is eclipsing its star. While it was observing the phase curve of WASP-76B, CHEOPS saw a pre-eclipse excess of light on its night side. This had also been seen in TESS phase-curve and secondary-eclipse observations that had been made earlier.

End of the rainbow?

An advantage of WASP-76b is that it is an ultra-hot Jupiter, so at least its day side does not have the clouds and hazes that often obscure the atmospheres of cooler hot Jupiters. This makes atmospheric emissions much easier to detect. That we had already observed an asymmetry in iron content between the day-side and night-side terminators, discovered in a previous study, made the planet especially intriguing. There was not much gaseous iron in the upper atmosphere of the day-side limb compared to that of the night-side limb. This is probably because it rains iron on the day side of WASP-76b, which then condenses into clouds of iron on the night side.

Observations from Hubble suggested that thermal inversion—when the air near the surface of a planet begins cooling—was occurring on the night side. Cooling on that side would cause iron that had previously condensed into clouds, rained down onto the day side, and then evaporated from the intense heat to condense again. Drops of liquid iron can then form clouds.

These clouds are critical since light from the host star, reflecting off these drops in those clouds, can create the effect of a glory.

“Explaining the observation with the glory effect would require spherical droplets of highly reflective, spherically shaped aerosols and clouds on the planet’s eastern hemisphere,” the researchers said in a paper recently published in Astronomy & Astrophysics.

Glories have been seen off Earth before. They are also known to form in the clouds of Venus. Just like WASP-76b, more pre-eclipse light was observed on Venus, so while a glory is all but definite for the exoplanet, future observations with a more powerful telescope could help determine how similar the phenomenon on WASP-76 is to that on Venus. If they match, this will be the first glory ever observed on an exoplanet.

If future research figures out a definite way to tell whether this is really a glory, these phenomena could tell us more about the atmospheric makeup of exoplanets, depending on the kinds of elements or molecules light is reflecting off of. They might even give away the presence of water, which could mean habitability. While the hypothesized glory on WASP-76b has not been definitively demonstrated, it is anything but a rainbow in the dark.

Astronomy & Astrophysics, 2024. DOI: 10.1051/0004-6361/202348270

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We may have spotted the first magnetar flare outside our galaxy

astronomy, astrophysics, gamma rays, giant flare, magnetar, neutron star, Science / /

Magnetars: how do they work? —

Not all gamma-ray bursts come from supernovae.

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Image of a whitish smear running diagonally across the frame, with a complex, branching bit of red material in the foreground.

Enlarge / M82, the site of what’s likely to be a giant flare from a magnetar.

NASA, ESA and the Hubble Heritage Team

Gamma rays are a broad category of high-energy photons, including everything with more energy than an X-ray. While they are often created by processes like radioactive decay, few astronomical events produce them in sufficient quantities that they can be detected when the radiation originates in another galaxy.

That said, the list is larger than one, which means detecting gamma rays doesn’t mean we know what event produced them. At lower energies, they can be produced in the areas around black holes and by neutron stars. Supernovae can also produce a sudden burst of gamma rays, as can the merger of compact objects like neutron stars.

And then there are magnetars. These are neutron stars that, at least temporarily, have extreme magnetic fields—over 1012 times stronger than the Sun’s magnetic field. Magnetars can experience flares and even giant flares where they send out copious amounts of energy, including gamma rays. These can be difficult to distinguish from gamma-ray bursts generated by the merger of compact objects, so the only confirmed magnetar giant bursts have happened in our own galaxy or its satellites. Until now, apparently.

What was that?

The burst in question was spotted by the ESA’s Integral gamma-ray observatory, among others, in November 2023. GRB 231115A was short, lasting only about 50 milliseconds at some wavelengths. While longer gamma-ray bursts can be produced by the formation of black holes during supernovae, this short burst is similar to those expected to be seen when neutron stars merge.

The directional data from Integral placed GRB 231115A right on top of a nearby galaxy, M82, which is also known as the Cigar Galaxy. M82 is what is called a starburst galaxy, which means that it’s forming stars at a rapid clip, with the burst likely to have been triggered by interactions with its neighbors. Overall, the galaxy is forming stars at a rate more than 10 times that of the Milky Way. That means lots of supernovae, but it also means a large population of young neutron stars, some of which will form magnetars.

That doesn’t rule out the possibility that M82 happened to be sitting in front of a gamma-ray burst from a distant event. However, the researchers use two different methods to show that this is pretty improbable, which leaves something happening inside the galaxy as being the most likely source of the gamma rays.

It could still be a gamma-ray burst happening within M82, except the estimated total energy of the burst is much lower than we’d expect from those events. A supernova should also be detected at other wavelengths, but there was no sign of one (and they typically produce longer bursts anyway). An alternative source, the fusion of two compact objects such as neutron stars, would have been detectable using our gravitational wave observatories, but no signal was apparent at this time. These events also frequently leave behind X-ray sources, but no new sources are visible in M82.

So, it looks like a magnetar giant flare, and the potential explanations for a brief burst of gamma radiation don’t really work for GRB 231115A.

Looking for more

The exact mechanism by which magnetars produce gamma rays isn’t entirely worked out. It is thought to involve the rearrangement of the crust of the neutron star, forced by the intense forces generated by the staggeringly intense magnetic field. Giant flares are thought to require magnetic field strengths of at least 1015 gauss; Earth’s magnetic field is less than one gauss.

Assuming that the event sent radiation off in all directions rather than directing it toward Earth, the researchers estimate that the total energy released was 1045 ergs, which translates to roughly 1022 megatons of TNT. So, while it’s less energetic than neutron star mergers, it’s still an impressively energetic event.

To understand them better, however, we probably need more than the three instances in our immediate neighborhood that are obviously associated with magnetars. So, being able to consistently identify when these events happen in more distant galaxies would be a big win for astronomers. The results could help us develop a template for distinguishing when we’re looking at a giant flare instead of alternative sources of gamma rays.

The researchers also note that this is the second candidate giant flare associated with M82 and, as mentioned above, starburst galaxies would be expected to be relatively rich in magnetars. Focusing searches on it and similar galaxies might be what we need to boost the frequency of our observations.

Nature, 2024. DOI: 10.1038/s41586-024-07285-4  (About DOIs).

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Io: New image of a lake of fire, signs of permanent volcanism

astronomy, Io, juno, Jupiter, planetary science, Science, volcanism / /
Io: New image of a lake of fire, signs of permanent volcanism

Ever since the Voyager mission sent home images of Jupiter’s moon Io spewing material into space, we’ve gradually built up a clearer picture of Io’s volcanic activity. It slowly became clear that Io, which is a bit smaller than Mercury, is the most volcanically active body in the Solar System, with all that activity driven by the gravitational strain caused by Jupiter and its three other giant moons. There is so much volcanism that its surface has been completely remodeled, with no signs of impact craters.

A few more details about its violence came to light this week, with new images being released of the moon’s features, including an island in a lake of lava, taken by the Juno orbiter. At the same time, imaging done using an Earth-based telescope has provided some indications that this volcanism has been reshaping Io from almost the moment it formed.

Fiery, glassy lakes

The Juno orbiter’s mission is primarily focused on studying Jupiter, including the dynamics of its storms and its internal composition. But many of its orbital passes have taken it right past Io, and this week, the Jet Propulsion Laboratory released some of the best images from these flybys. They include a shot of Loki Patera, a lake of lava that has an island within it. Also featured: the impossibly sheer slopes of Io’s Steeple Mountain.

Looking more closely at the lake, the Juno team found that some of the areas within it were incredibly smooth, raising the possibility that obsidian glass had formed on the surface where it had cooled enough to solidify. Given the level of volcanism on Io, this may be more widespread than the Loki Patera.

Volcanic ash would also create a relatively smooth surface, and is likely to be even more common, but it would have significantly different reflective properties.

How long has this been going on?

But we don’t have to send hardware to Jupiter to learn something about Io. A US-based team got time on the Atacama Large Millimeter Array (ALMA) and used it to record emissions from atoms in Io’s sparse atmosphere. By combining the imaging power of lots of smaller telescopes scattered across a plateau, ALMA is able to spot regional differences in the presence of specific elements in Io’s atmosphere, as well as identify different isotopes of those elements.

What can isotopes tell us? Any atoms that reach Io’s upper atmosphere are at risk of being lost to space. And, because of their relative atomic weights, lighter isotopes have a higher probability of being lost. So, it’s possible to compare the present ratio of elements in the atmosphere with the expected ratio, and we can make inferences about the history of loss of lighter isotopes. And, since the material is put into the atmosphere by volcanoes in the first place, that tells us something about the history of volcanism.

The research team focused on two particular elements: sulfur and chlorine. Sulfur has two common non-radioactive isotopes, 32S and 34S, and chlorine, its neighbor on the periodic table, has 35Cl and 37Cl. There are differences in the ratio of these isotopes throughout the bodies of the Solar System, but those differences are generally small. And, because we think we know what sort of material contributed to the formation of Io, we can focus on the ratios found in bodies that have a similar origin.

Chlorine enters the atmosphere from volcanoes primarily in the form of sodium and potassium salts. These have a very short half-life before they’re split up by exposure to light and radiation. The ALMA data indicated both these chemicals were present in localized regions, likely corresponding to active volcanic plumes. The data from the chlorine isotopes were a bit noisy, so were largely used as a sanity check for the ones obtained from sulfur isotopes.

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Second-biggest black hole in the Milky Way found

astronomy, astrophysics, black holes, Gaia, Science, stars / /
A dark background with a bright point at the end of a curved path, and a small red circle.

Enlarge / The star’s orbit, shown here in light, is influenced by the far more massive black hole, indicated by the red orbit.

As far as black holes go, there are two categories: supermassive ones that live at the center of the galaxies (and we’re unsure about how they got there) and stellar mass ones that formed through the supernovae that end the lives of massive stars.

Prior to the advent of gravitational wave detectors, the heaviest stellar-mass black hole we knew about was only a bit more than a dozen times the mass of the Sun. And this makes sense, given that the violence of the supernova explosions that form these black holes ensures that only a fraction of the dying star’s mass gets transferred into its dark offspring. But then the gravitational wave data started flowing in, and we discovered there were lots of heavier black holes, with masses dozens of times that of the Sun. But we could only find them when they smacked into another black hole.

Now, thanks to the Gaia mission, we have observational evidence of the largest black hole in the Milky Way outside of the supermassive one, with a mass 33 times that of the Sun. And, in galactic terms, it’s right next door at about 2,000 light-years distant, meaning it will be relatively easy to learn more.

Mapping the stars

Although stellar-mass black holes are several times the mass of the Sun, they aren’t really all that heavy in the grand scheme of things. The sorts of stars that tend to leave black holes behind also tend to lead violent existences, spewing a lot of themselves into space before dying. And the supernova that forms the black hole obviously expels a lot of the star’s mass, rather than feeding it into the black hole. It had been thought that these processes set limits on how big a stellar mass black hole could be when it forms.

The discovery of larger black holes through gravitational wave detectors suggested that this wasn’t true. While there are ways for black holes to get bigger after they form—excessive feeding, mergers—it wasn’t clear that these events occurred often enough to explain the frequency of heavy black holes that we were seeing. And detecting them via gravitational waves doesn’t tell us anything about the history of how they got that large.

Which is why the discovery of Gaia BH3 (which is what the research team is using to avoid having to retype Gaia DR3 4318465066420528000 all the time) is so intriguing. The black hole is sitting calmly in a binary system, not doing anything in particular. But we know it’s there due to its gravitational influence.

Gaia is an ESA mission to map the location and movement of many of the Milky Way’s brighter stars by imaging them multiple times from different perspectives. It also gathers basic data on the stars’ light, allowing us to estimate things like age and composition. And, in addition to their movement across the galaxy, Gaia can measure their movement relative to Earth, a method that is useful for the detection of orbital interactions, such as the presence of companion stars or exoplanets.

The Gaia team was busy preparing for the fourth release of the data from the spacecraft and were running validation tests on the software used to detect binary star systems when they stumbled across Gaia BH3. While normally they’d publish its discovery at the same time as the data release, they consider the new object too important to wait: “We took the exceptional step of the publication of this paper based on preliminary data ahead of the official DR4 due to the unique nature of the discovery, which we believe should not be kept from the scientific community until the next release.”

Finding the invisible

Every star in our galaxy is in motion relative to every other. They orbit the center of our galaxy and may have a history that has imparted additional momentum—gravitational interactions with neighbors, having been part of a smaller galaxy that was consumed by the Milky Way, and so on. But that motion only changes on very long time scales. By contrast, any star in an orbit experiences regular changes in its motion in addition to its overall travel through the galaxy. As part of processing its data, the Gaia team attempts to identify both overall motion and any indications that a star is orbiting as part of a binary system.

The star that is orbiting Gaia BH3 is similar in mass to the Sun but shows the sort of periodic wobbles that indicate it’s in a mutual orbit with a companion. The companion itself, however, was completely invisible, which means it is almost certainly a black hole (the Gaia data had already been used to identify black holes this way). And, based on the mass and orbital motion of the visible star, it’s possible to estimate the mass of the invisible companion.

The estimate ended up being 32 solar masses, which is significantly larger than anything else identified in the Gaia dataset. So, the Gaia team wanted to confirm this wasn’t a software issue and used Earth-based telescopes to observe the same system. Three different observatories confirmed it was there, and the resulting mass estimates were slightly larger than those derived from the Gaia data alone: just under 33 solar masses.

Assuming it’s a single object and not two black holes orbiting each other closely, that makes it the largest non-supermassive black hole known in the Milky Way. And it places it in the mass range that had been difficult to explain via formations in supernovae.

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