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Jupiter may have had a head-on collision with a massive protoplanet

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K. Suda & Y. Akimoto/Mabuchi Design Office

Planet-forming disks start out as a mix of dust and gas, but the gas doesn’t stick around for long. As the star at their center ignites, the radiation it emits starts driving off the gas, eventually leaving a disk with nothing but dust behind. That creates a narrow window for the formation of gas giants, which have to grow big enough to start sweeping in gas before the star drives it all off.

Our current models suggest that the best way to do this is to start with a large solid body, roughly 10 times the mass of Earth. That’s big enough to draw in gas quickly and start a runaway process by which the ever-increasing mass pulls in more material from farther away in the disk. This would suggest that, buried deep below the clouds and layers of metallic hydrogen on Jupiter, there’s a solid core that would dwarf the Earth if it were ever stripped of all the material above it.

Among other things, the Juno mission was intended to test this idea by studying the gravitational field of the giant planet. But the data it has been sending back suggests something strange is going on inside Jupiter, with more heavy material outside the immediate core area than we’d expect. Now, an international team of researchers is providing a possible explanation: Jupiter’s core was shattered by a head-on collision with a massive protoplanet.

What lies beneath

We obviously can’t directly image what’s going on inside Jupiter. Instead, we have to figure out what’s there based on inferences made from the planet’s gravitational field. And Juno was the first probe specifically designed to improve our understanding of that gravitational field. While further data is still coming in, a preliminary analysis suggests that one explanation for what we’re seeing is that the planet has a core that the new paper describes as “dilute.” Instead of the heavier, solid material being concentrated at the core, some of the heavier elements appear to be spread widely across the planet’s interior, reaching up to about halfway to the planet’s surface.

How that happened is not at all clear, given that we think the only way for a planet like Jupiter to happen is to start with a solid core. It’s possible that further Juno data will indicate that a diffuse core is unlikely. Alternatively, our models of planet formation could be wrong. But the researchers start with the premise that everything is correct and that there’s something unexpected going on in Jupiter’s interior.

One option is that the metallic hydrogen layer of Jupiter has gradually eroded the core, but we don’t know whether metallic hydrogen is capable of that or how heavier elements would mix in it. Instead, the authors consider the possibility that Jupiter’s core was disrupted by a collision, much like the one that formed the Earth-Moon system—although completely unlike it in scale.

Collisions could be driven by Jupiter’s formation itself. A 10-Earth-mass core is only about 5% of Jupiter’s final mass, and the runaway process that surrounded it with gas would have enhanced its gravitational pull by a factor of 30 in less than a million years. Any other bodies nearby could be drawn in to a collision. And since Jupiter’s core is thought to have formed via a series of collisions among smaller bodies, there’s a reasonable chance there was something nearby that could undergo a collision.

To test this idea, the researchers ran a large number of simulations of the early Solar System, varying the precise configuration of Jupiter and any nearby orbital bodies. They found that in many of these simulations, the growth of Jupiter caused anything nearby to cross orbits, frequently resulting in collisions. Because of the immense pull of Jupiter, most of the collisions ended up being head-on, sending the protoplanet directly to the core of Jupiter.

Core shattering

They then turned to a different set of simulations, looking into what happened to the core of Jupiter as a result. The exact details depend on the size of what hits Jupiter and the size of the giant planet at the time of the impact. The simulation they ran in detail involves Jupiter being run into by an eight-Earth-mass core surrounded by two Earth masses of gas. Smaller objects, including Earth-sized protoplanets, would disintegrate in the atmosphere before reaching the core.

Despite the staggering scale of this collision, it only adds a small amount to the total energy delivered to Jupiter during its formation. But it does change the energetics of the core itself, which begins to oscillate. And convection starts bringing the products of these oscillations higher up into the planet’s envelope. Within a matter of a few days, Jupiter settles into a state where its core is diffuse and extends nearly half-way to the surface of the planet.

Snapshots of the collision simulation.
Enlarge / Snapshots of the collision simulation.
Shang-Fei Liu/Sun Yat-sen University

Of course, this event occurred over four billion years ago, and it would have to have remained stable for the intervening time to be detected by Juno. The researchers found that this was possible if the internal temperature of Jupiter stabilized at 30,000 Kelvin. Any hotter and convection becomes high enough to eliminate the gradient between the core and its surroundings, which stabilizes the presence of heavier material above the core. Any cooler and convection isn’t strong enough, and heavier material settles back into the core.

Because most planets are thought to have been built by multiple collisions among protoplanets and smaller bodies, the authors think it’s worth exploring whether diffuse cores could be a common feature of gas giants. There have been a number of giant exoplanets that appear to have high metal content in their atmospheres, which might be the product of similar events.

There’s no obvious way to test these things at the moment, and there’s still a chance that further data from Juno will suggest alternate explanations. But if the idea holds up, planetary scientists will undoubtedly start considering the implications of these collisions and might come up with some overt indication of the marks they leave on gas giants.

Nature, 2019. DOI: 10.1038/s41586-019-1470-2  (About DOIs).

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The weird, repeating signals from deep space just tripled – CNET

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We’re picking up more signals from deep space.


Danielle Futselaar

Scientists suddenly have a whole lot more data on one of the strangest and most recent mysteries in the cosmos, so-called fast radio bursts (FRBs). First discovered in 2007, these fleeting blasts of radio waves originate thousands, millions or even billions of light years from Earth. 

FRBs have influenced the design of new radio telescopes like the Canadian Hydrogen Intensity Mapping Experiment (CHIME). And now a team of Canadian and American researchers using CHIME has reported a major new set of FRB detections that could fine-tune our understanding of where these enigmatic signals come from and what produces them. 

The group says it’s discovered eight new FRBs that repeat.


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“Repeating FRBs are highly valuable from an observational perspective since their repeating nature make them better candidates for localizing their host galaxies and multi-wavelength follow-up observations that can help determine if FRBs emit at wavelengths other than radio,” said Ryan McKinven, one of the researchers who is based at the University of Toronto and co-author of a paper about the FRBs.

Those follow-up observations could provide details about the origins of the strange bursts, he added. A larger sample size of repeating FRBs to study could also help scientists answer one of the obvious questions about non-repeating FRBs: Could they actually be repeating FRBs that just haven’t been recorded as repeating yet?

While dozens of FRBs have been detected and cataloged over the past 12 years, few of those deep space signals had been known to repeat themselves. Two have been documented so far in published, peer-reviewed journals. Two others — one via a Russian radio telescope, the other via Australia — have been reported but not yet reviewed. 

So with this batch of bursts, the number of reported repeaters has tripled — from four to 12. 

The team laid out its findings in a draft paper that’s been submitted to the Astrophysical Journal and was posted this month on the Arxiv pre-print site

Discovering different types of FRBs at an unexpected rate, we will soon open new windows into understanding the cosmological origin of these high-energy astrophysical phenomena,” said co-author Masoud Rafiei-Ravandi of the Perimeter Institute for Theoretical Physics. 

In addition to the sheer number of repeating FRBs discovered in one haul, one of the newfound repeaters appears to be much closer to Earth than the handful of fast radio bursts that have been traced back to a source galaxy. So far, traceable FRBs seem to come from sources on the other side of the universe — we’re talking billions of light years away.

However, in the new paper, the authors suggest that one of the repeating FRBs could actually originate near the edge of our own Milky Way galaxy but caution that more study is needed to better localize the signal. 

“Knowing that we are observing every patch of sky visible to CHIME once every day, it was only a matter of time before we detected a very nearby source,” co-author Pragya Chawla of McGill University said.

Studying relatively nearby FRBs will hopefully allow scientists to get a better idea of just what the heck is throwing off these signals, which could be anything from far-fetched notions like alien starships to the less fantastic but literally more powerful sources, like neutron stars.

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Climate Crisis: Hurricanes Are Making Some Spiders More Aggressive – Inverse

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In the most uncomfortable piece of climate crisis-related news yet, a team of scientists believe that the increasing tropical cyclones may be changing the temperament of a “super abundant” spider. As the storms continue to increase in tandem with the planet’s temperature, some of our eight-legged friends are starting to get more aggressive, report scientists in a paper published Monday in Nature Ecology and Evolution.

Specifically, these findings refer to a species of group-dwelling spiders called Anelosimus studiosus. They’re “hardly majestic,” lead study author Jonathan Pruitt, Ph.D., an associate professor at the University of California Santa Barbara, tells Inverse, but they happen to have very intricate social lives. Along the Gulf Coast, the spiders form multi-female groups that hunt in packs, dwell in group webs, and sometimes rear each other’s children. But the climate crisis may be shifting these old spider traditions toward a less interconnected lifestyle.

In the paper, Pruitt and his team show that hurricanes are actually changing the social and behavior dynamics of these spider colonies. The aggressive spiders in the colonies are well-equipped to handle the chaos, but the less aggressive ones are not. That inequality, he explains, may reshape life in the colony.

“There’s a behavioral tipping point when very very aggressive colonies stop working together, start killing each other, and the group wisely disbands,” he says. “Combine hurricane increases with global warming and I think you could get something like that.”

Anelosimus studiosus, a social cobweb spider, can live in groups, but if the group becomes too aggressive it can disband. 

The species as a whole will fare just fine, he says, in case you’re worried about losing even more animal and insect species to climate change. But the spiders are a good example of the way incomprehensibly large events — say, increases in large storms — can cause minute but significant changes too, like the behavior of a five-millimeter-long spider.

How Storms Change Spider Behavior

Anelosimus studiosus have two “behavioral” phenotypes (traits) that seem to be heritable, suggesting that they each have a genetic underpinning.

Some individuals are naturally more aggressive, which means that they swiftly attack in large numbers, kill their mates, are more wasteful their their prey, and are prone to fight among themselves; they also happen to be better at foraging when resources are scarce. The other individuals tend to be more docile, so they’re better at coexisting. To survive in a colony, you need a balance of both.

But Pruitt’s work suggests that tropical cyclones are selecting for the aggressive spiders. He observed 240 colonies before and after Hurricane Florence, Hurricane Michael, and tropical storm Alberto in the fall of 2018, finding that, while roughly 75 percent of each colony survived the storm, the colonies with more aggressive foraging responses produced more egg cases than the colonies with less aggressive tendencies.

Over time, this process shifts the nature of the colony toward the more aggressive types.

What Aggressive Behavior Means For the Species

While this shift likely won’t impact the species’ chances of survival, it does edge in on a “behavioral” tipping point. A colony of overly aggressive spiders, honed by the hostile summer cyclones of the Southern USA, is unlikely to cohabitate, says Pruitt. So if this trend continues, these spiders, which traditionally live in tight communities, may each decide to go it alone.

“I think the species as a whole will fare fine. But, if tropical cyclones start striking some regions all of the time (e.g., annually), then we might see this species revert back to is ancestral solitary state, where females no longer work together and they go it alone,” he explains.

Already, we know that extreme environmental disturbances (like the once predicted with increasing global temperature), will profoundly affect which species will live and die. But Pruitt’s work also shows a more nuanced approach to how climate change will impact species, as Eric Ameca, an ecologist who studies biodiversity and response to extreme climate events, adds in a commentary accompanying the new paper. While some species do have adaptive responses (so they’ll probably make it out okay), Ameca writes, they may look or behave a lot differently due to these extreme weather events.

Pruitt for one, sees the changing behavior of his spiders as a puzzle to be solved — and maybe applied across species in the future.

“It also means that the future of life, how it operates, and who prevails in the face of changing environments is going to be a very difficult puzzle to solve. Thankfully, humans like puzzles,” he says.

Abstract:

Extreme events, such as tropical cyclones, are destructive and influential forces. However, observing and recording the ecological effects of these statistically improbable, yet pro- found ‘black swan’ weather events is logistically difficult. By anticipating the trajectory of tropical cyclones, and sampling populations before and after they make landfall, we show that these extreme events select for more aggressive colony phe- notypes in the group-living spider Anelosimus studiosus. This selection is great enough to drive regional variation in colony phenotypes, despite the fact that tropical cyclone strikes are irregular, occurring only every few years, even in particularly prone regions. These data provide compelling evidence for tropical cyclone-induced selection driving the evolution of an important functional trait and show that black swan events contribute to within-species diversity and local adaptation.

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Hurricanes, spiders and Waffle Houses: How a McMaster evolutionary biologist spent his summer – TheSpec.com

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The findings might seem relevant only to spider colonies, but in a broader sense, studying the evolutionary tendencies of species following extreme weather events could have wide-ranging applications, Pruitt suggested, given climate models predicting more violent storms in the future.

“The fact is, we know surprisingly little about what kind of species do better after these storms; how it affects diversity, how storms can cause the evolution of certain collective traits, or individual traits … It hints that tropical storms could drive the evolution of aggressiveness in other species.”

Anelosimus studiosus spiders are not harmful to humans. Pruitt, an engaging and often humorous speaker — and a Canada 150 Research Chair — said that the spiders are just five millimetres long and, while terrifying to, say, a fly, “I could put some on your Red Lobster salad, and they would drown in the dressing before you would ever know they were there.”


He chased results in the field following hurricanes Florence, Michael and Alberto.

In the process, Pruitt said he observed more than 1,000 spiders in 240 colonies, listened to four fantasy novels in his car while logging 33,000 km on the road and ate frequently at U.S. road-trip staple Waffle House, which never seems to close.

He recorded spider behaviour by putting a little piece of paper in the webbed colony, and then used a mechanical toothbrush with a metal thread attached to vibrate the paper and coax spiders out of their home, as though food, like a moth, was waiting.

Docile spiders “take their sweet time” coming out, and aggressive ones emerge quickly. He noted his findings on the spot.

He laughs imagining the sight he must have been for locals who spotted him poking around a large silken web, wearing his beat-up Pokemon T-shirt (a childhood enthusiasm) and holding a toothbrush “that looks like a narwhal.”

Next up for the insect/storm chaser — McMaster colleagues call him a “swarm chaser’ — is Northern Australia for three months, starting in January, to examine cyclonic storm impact on spiders and a wide range of other insects.

“I’m narrowly missing what could be a cold winter here,” said Pruitt, who grew up in Florida.

“I chase summer.”

jwells@thespec.com

905-526-3515 | @jonjwells

jwells@thespec.com

905-526-3515 | @jonjwells

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