Forum Replies Created
-
New fossils may capture the minutes after the dinosaur-killing asteroid impact
A North Dakota site appears to hold fish, other organisms swiftly buried in the strike’s wakehttps://www.sciencenews.org/article/new-fossils-north-dakota-dinosaur-killing-asteroid-impact
About 66 million years ago, a giant asteroid smashed into Earth off the coast of what’s now Mexico. Less than an hour later, the impact sent water in a riverbed 3,000 kilometers away sloshing violently back and forth, swiftly burying freshwater fish, plants and other organisms in the heavy sediment, a new study finds. Evidence of those surges, as well as tiny traces of the impact itself, appear to be preserved in a meter-thick layer of rock in southwestern North Dakota.
The site, found in the Hell Creek Formation and dubbed Tanis, represents a unique snapshot of what happened on land in the immediate aftermath of the impact, says paleontologist Robert DePalma of the University of Kansas in Lawrence.
“It’s a critical moment in time,” DePalma says. “We have a high-resolution image of the first couple of hours after the impact. That level of detail is not really known elsewhere.”
Despite the potential uniqueness of the find, the publication of the PNAS paper was overshadowed by a profile of DePalma that had been published online on March 29 in the New Yorker, just a few days before the study’s planned release. The profile included tantalizing hints of fossilized dinosaurs and pterosaurs and even rarely preserved feathers that the researchers say they have found at the Tanis site. If so, the site could hold the answer to a much larger question: Was it really the asteroid strike that killed all nonbird dinosaurs, or were they already dying out? But the PNAS paper doesn’t discuss those fossils, and paleontologists have expressed skepticism and frustration over how to evaluate the claims.
Moment in time
The PNAS paper does provide evidence that the fossil site opens a window on a key time in Earth’s history. At Tanis, a river once drained eastward from a vast inland sea. Sandy deposits reveal where the meandering river carved a deep channel into the rock. Above that channel lies an unusual rock sequence that DePalma and his colleagues call the “event deposit.” That 1.3-meter-thick layer of rock has two distinct sublayers. The bottom layer has large pebbles at its base and finer-grained sediment toward the top, ending in fine silt. Overlying that is another layer that starts with large sand grains and then gets finer toward the surface. This pattern, and the direction of water flow preserved by the grains, point to some kind of massive inundation, DePalma says.
The deposit also contains tiny glass spheres, remnants of vaporized rock cast into the atmosphere from the impact that then rained back down potentially thousands of kilometers away. Fossils are also abundant in the deposit, particularly bits of logs and groups of fish skeletons. The fish, the researchers suggest, may have died en masse after becoming rapidly buried by mud displaced during the inundation. Some of the fish’s bellies contain the tiny spherules, possibly snagged from the water just before death.
Above the event deposit is a thin, 1- to 2-centimeter-thick layer of volcanic ash-turned-clay that also outcrops in other parts of the central United States. This layer contains impact spherules and dates to the KPg, helping connect the Tanis site to the extinction event.
Because a vast shallow sea covered much of the ancient central United States at one time, the team first suspected that the back-and-forth sloshing indicated that a giant tsunami had swept northward from the Gulf of Mexico in the wake of the impact. But it’s not clear how large the sea was or if it still existed at the time of the impact. And later calculations by DePalma’s team suggested that such a tsunami would have taken at least 18 hours to travel from the Gulf to Tanis.
However, the spherules found embedded in the event deposit indicate that the powerful wave action must have occurred nearly instantly after the impact, DePalma says. Even a swift tsunami wouldn’t have been that fast. Instead, the team suggests, strong seismic waves might have shaken up a local body of water, such as river or lake, producing the deposits.
Plausible past
The team convincingly argues that the whole sequence of events took only a few hours, says Paul Olsen, a paleontologist and geologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y. Fish consumed the spherules that rained into the water, became entombed in sediment displaced during the violent sloshing of the water, and were then covered by a second layer of sediment bearing iridium, an element enriched in asteroids. “I think they’ve got that nailed. It’s hard to imagine how that would occur any other way,” Olsen says.
Jessica Whiteside, a geochemist at the University of Southampton in England, agrees that the sedimentary evidence supports the idea that the impact produced violent sloshing at Tanis. And the possibility that a massive earthquake provoked those waves is plausible, she adds. As the authors say in the study, the 2011 magnitude 9.2 Tohoku earthquake in Japan is known to have triggered a 1.5-meter-high waves in a Norwegian fjord some 8,000 kilometers away.
“But it’s not the only plausible sequence of events that could have happened,” Whiteside says. And there may be no way to know for sure if the scenario is the right one, or what the exact timing would have been of the seismic waves’ arrival, because there so many unknowns about the lay of the land 66 million years ago.
“That said, I find it really exciting work,” Whiteside says.
Antoine Bercovici, a paleobotanist and sedimentologist at the Smithsonian Institution in Washington, D.C., has worked in North Dakota for years, studying the KPg boundary within the Hell Creek Formation. He agrees that the sedimentary evidence described in the study is compelling. “It’s extraordinary that [the sediments of] this oxbow lake were able to record in a detailed way the moment of the impact,” he says. “It would be hard to find another place like that.”
The fish fossils, he adds “are pretty amazing,” and their preservation is exceptional. Yet, he says, he’s a bit skeptical that the haphazard orientation of the fossils definitively represents a snapshot of mass death right as the waves impacted the animals, spherules still held in their mouths. “It’s a bit dramatic and hard to verify,” he says.
Dino drama
But the drama of a mass fish grave is minimal compared with the reaction to some of the other fossils DePalma says that he has found at Tanis, as described in the New Yorker profile: a mammal burrow, dinosaur feathers, a ceratopsian hip bone with a skin impression (the ceratopsian fossil is briefly mentioned, though not officially described or pictured in detail, in the supplemental material accompanying the PNAS paper).
A general dearth of fossils dating to just before the impact has led some scientists to speculate that perhaps the animals were already vanishing before the asteroid slammed into Earth. So the discovery of dinosaur fossils at Tanis, perhaps representing animals that drowned and were buried in sediment, could help prove the impact was the true culprit after all.
That wouldn’t, in itself, be all that surprising. Most paleontologists already think that the strike killed the dinosaurs. Still, such extraordinary claims for what Tanis holds require extraordinary evidence, says Thomas Holtz, a paleontologist at the University of Maryland in College Park. “I think a lot of the reaction to this stems from the particulars of how the announcement came out,” Holtz says, referring to the online publication of the New Yorker article a few days ahead of the PNAS study. “Perhaps [those claims] are indeed accurate, but if so [the researchers] should either have been included in the initial paper or they should not be reporting it so far in advance if they aren’t ready to back up their claims with data.”
DePalma and colleagues’ data on those tantalizing dinosaur fossils remain unpublished, for now. The PNAS paper “is not a paper about dinosaurs,” DePalma says, adding that he didn’t want the information about the dinosaur fossils to appear in the New Yorker, either. “Of course we’d seek to keep that under wraps; we want to publish it later,” he says.
Whether or not Tanis contains dinosaur fossils, the site is still interesting, Bercovici says. “Tanis is the first unequivocal site of this kind,” in that it represents the first minutes of impact, he says. Furthermore, he adds, although dinosaurs tend to take center stage in the popular imagination, “the KPg extinction is not a nonavian dinosaur story. It involved the demise of entire and complex ecosystems.”
And as interesting as Tanis is, he adds, scientists studying the KPg extinction have many lingering questions — who died, who survived, was the extinction gradual or abrupt, was the recovery fast or slow — that extend far beyond that one snapshot in time. After all, the majority of the dying happened later, over the ensuing decades.
“The immediate effect of the impact certainly killed many plants and animals,” he adds. But what really placed particular pressures on different ecosystems, and ultimately led to mass extinctions, were the longer-term effects on the climate: dramatic cooling and warming, the acid rain, the emission of soot and the darkening of the sky, he says.
The Event Horizon Telescope may soon release first-ever black hole image
A photo of the Milky Way’s central black hole, Sagittarius A*, would be the first of its kind.No, you can’t actually take a picture of a black hole. But astronomers have promised to do the next best thing: To image the seething chaos just outside the black hole, known as its event horizon. To capture this region, just on the cusp of the black hole itself, astronomers have had to link telescopes from across the globe and focus them on the closest, most massive black holes known: Sagittarius A* (pronounced “A-star”), which resides at the center of our own Milky Way galaxy, as well as the even larger supermassive black hole that sits at the center of nearby galaxy M87.
The result, known as the Event Horizon Telescope (EHT) had its big observing run in April of 2017. Researchers warned that it would take time to piece together the data. And the team has repeatedly dropped hints that the results could be ready soon, only for the project to continue on. But based on their upcoming press event, set for April 10, it seems that time may have come, and that viewers are about to see the first-ever picture of a black hole’s event horizon.
Teamwork
EHT is actually a team of telescopes working together in a process known as interferometry. This lets the connected telescopes behave as if they had one enormous collecting area. Of course, there are gaps between the individual observatories, and each telescope is unique and behaves in slightly different ways – as well as experiencing different weather, and having a different view of the black hole, though this last is actually the feature that makes the combined imaging so accurate. But figuring out how to stitch all that data together is why researchers have taken so long to turn the 2017 data into a presentable image.But the cooperation pays off. Individually, the telescopes are world-class. And together, they deliver enough observing power that a person standing in New York City could use the EHT to read the writing on a quarter in Los Angeles, something none of them could do individually.
It’s not clear which of the black holes targeted by EHT may be ready to show off to the public. It’s also not for certain that they’ve actually accomplished the feat yet. But after such a wait, the pictures should be stunning. The National Science Foundation, which helps fund EHT, will be hosting the press conference. Due to the collaboration being spread across the globe, other press conferences will happen simultaneously in Brussels, Santiago, Shanghai, Taipei and Tokyo, highlighting the cooperation and vast resources it takes to make a project this large succeed.
==
6 Supermassive Questions On The Eve Of The Event Horizon Telescope’s Big Announcement
In science, there’s no moment more exciting than when you get to confront a longstanding theoretical prediction with the first observational or experimental results. Earlier this decade, the Large Hadron Collider revealed the existence of the Higgs boson, the last undiscovered fundamental particle in the Standard Model. A few years ago, the LIGO collaboration directly detected gravitational waves, confirming a longstanding prediction of Einstein’s General Relativity.
And in just a few days, on April 10, 2019, the Event Horizon Telescope will make a much-anticipated announcement where they’re expected to release the first-ever image of a black hole’s event horizon. At the start of the 2010s, such an observation would have been technologically impossible. Yet not only are we about to see what a black hole actually looks like, but we’re about to test some fundamental properties of space, time, and gravity as well.
If you want to image any object in the Universe, you have to meet the following two challenges:
You must gather enough light to see your target, and reveal its details against the background noise of both your instruments and the other objects in the vicinity of your object of interest.
You need sufficient resolution (or resolving power) to reveal the structure of the object you’re looking at, otherwise all of your data will be confined to one mere pixel.So if you want to image a black hole’s event horizon, you need to both gather enough light that the radiation around the black hole stands out against the rest of the environment, and also to probe angular scales that are narrower than the diameter of the event horizon itself.
Two of the possible models that can successfully fit the Event Horizon Telescope data thus far, as of earlier in 2018. Both show an off-center, asymmetric event horizon that’s enlarged versus the Schwarzschild radius, consistent with the predictions of Einstein’s General Relativity. A full image has not yet been released to the general public, but is expected in just a few days in 2019. R.-S. LU ET AL, APJ 859, 1
The only way we have of doing both of those is with an enormous, ultra-sensitive array of radio telescopes that observe the largest black holes, in terms of angular size, that are visible from Earth. The more massive your black hole is, the bigger the diameter of its event horizon will be, but it will appear smaller dependent on its distance. That means the largest black hole will be Sagittarius A*, the supermassive one at the center of the Milky Way, while the second largest will be the ultramassive one at the center of the galaxy M87, some 60 million light-years away.
While single-dish radio telescopes might be able to detect the emissions from either one — i.e., they have sufficient light-gathering power — they cannot resolve the event horizon. But an array of telescopes, all observing the target together, can get us there.
Black holes ought to be surrounded by matter that’s in the slow process of being devoured. This material will be strewn about the black hole’s outsides, spinning around, heating up, and emitting radiation as it falls in. That radiation should come in the radio part of the spectrum, and be observable to a sensitive-enough telescope array.
The Event Horizon Telescope (EHT) is exactly the radio array we need — with the most stunning advances coming from the inclusion of ALMA in South America — to not only gather the radio information, but to get that excessive resolution. The EHT consists of scores of individual dishes with enough combined light-gathering power to reveal the radiation surrounding the black hole, with the distances between the dishes providing the resolution necessary to image the event horizons in question themselves.
We’ve used this technique before, of long-baseline interferometry, to reveal details that would be invisible with even an enormous single-dish telescope. So long as the features you’re attempting to observe are bright enough and show up in the telescopes you’re using to take the observations simultaneously, you can achieve imaging resolutions that correspond to the distance between the telescopes, rather than of the diameter of the individual telescopes themselves.
Most spectacularly, telescope arrays have been used thus far to image erupting volcanoes on the surface of Jupiter’s moon Io, even at the moment that Io falls into the shadow of another of Jupiter’s moons.
The EHT uses this exact same concept to probe the radiation coming from around the black holes with the largest angular diameters as seen from Earth. Here are the six things we’re poised to learn when the first-ever images are released.
The black hole at the center of our Milky Way, simulated here, is the largest one seen from Earth’s perspective. The Event Horizon Telescope should, on April 10, 2019, come out with their first image of what this central black hole’s event horizon looks like, while the one at the center of M87, the second largest, might be visible as well with this technology. The white circle represents the Schwarzschild radius of the black hole, while the dark region should be devoid of emission due to the instability of orbits around it. UTE KRAUS, PHYSICS EDUCATION GROUP KRAUS, UNIVERSITÄT HILDESHEIM; BACKGROUND: AXEL MELLINGER
1.) Do black holes have the correct sizes that General Relativity predicts? According to Einstein’s theory, based on the measured gravitational mass of the black hole of the Milky Way’s center, the event horizon itself should be 11 micro-arc-seconds (μas) in diameter, but there should be no emissions coming from within 37 μas, owing to the fact that within that angular diameter, matter should quickly spiral in towards the singularity. With a resolution of 15 μas, the EHT should be able to see a horizon, and measure whether the size matches our predictions or not. It will be a fabulous test of General Relativity.
The orientation of the accretion disk as either face-on (left two panels) or edge-on (right two panels) can vastly alter how the black hole appears to us. ‘TOWARD THE EVENT HORIZON—THE SUPERMASSIVE BLACK HOLE IN THE GALACTIC CENTER’, CLASS. QUANTUM GRAV., FALCKE & MARKOFF (2013)
2.) Are the accretion disks aligned with the black hole, the host galaxy, or randomly? We’ve never observed an accretion disk before, and in fact the only real indication we have about the orientation of the matter surrounding black holes comes from the cases where either:
there’s an emitted jet that we can detect from the black hole,
or there’s extended emission coming from the surrounding region.But neither one of those observations are a substitute for a direct measurement. The EHT, when these first images come out, should be able to tell us whether the accretion disk is edge-on, face-on, or at any other orientation.
Some of the possible profile signals of the black hole’s event horizon as simulations of the Event Horizon Telescope indicate. HIGH-ANGULAR-RESOLUTION AND HIGH-SENSITIVITY SCIENCE ENABLED BY BEAMFORMED ALMA, V. FISH ET AL., ARXIV:1309.3519
3.) Is a black hole’s event horizon circular, as predicted, or does it take on a different shape? Although all physically realistic black holes are expected to spin to some degree, the event horizon’s shape is predicted to be indistinguishable from that of a perfect sphere.
But other shapes are possible. Some objects bulge along their equators when they rotate, creating a shape known as an oblate spheriod, such as planet Earth. Others creep up along their rotational axes, resulting in a football-like shape known as a prolate spheroid. If General Relativity is correct, a sphere is what we anticipate, but there’s no substitute for making the critical observations ourselves. When the images come out on April 10, we should have our answers.
Five different simulations in general relativity, using a magnetohydrodynamic model of the black hole’s accretion disk, and how the radio signal will look as a result. Note the clear signature of the event horizon in all the expected results, but also how they may appear differently in detail depending on turbulence, magnetic field strength, etc. GRMHD SIMULATIONS OF VISIBILITY AMPLITUDE VARIABILITY FOR EVENT HORIZON TELESCOPE IMAGES OF SGR A*, L. MEDEIROS ET AL., ARXIV:1601.06799
4.) Why do black holes flare? When a black hole is in a non-flaring state, there are specific signatures that we anticipate will show up around the event horizon. But then, when a black hole flares, there are different features that the radiation surrounding it will exhibit.
But what will those emissions look like? Will there be turbulent features that show up in the disk at all times? Will there be “hot spots,” as predicted, that are most visible in the flaring state? If we get lucky and see either of these signatures, we might be well on our way to learning why black holes flare, just by observing the extended radio emissions surrounding them. We should also learn, based on these observations, additional information about the strength of the magnetic fields surrounding these black holes.
5.) Are the X-ray estimates of a black hole’s mass biased towards lower values? There are, at present, two ways to infer the mass of a black hole: from measuring its gravitational effects on stars (and other objects) that orbit it, and from the (X-ray) emissions of the gas that orbits it. We can easily make the gas-based measurements for most black holes, including the one at the center of the Milky Way, which gives us a mass of approximately 2.5-2.7 million solar masses.
But the gravitational measurement is far more direct, despite being a greater observational challenge. Still, we’ve done it in our own galaxy, and have inferred a mass of approximately 4 million solar masses: about 50% higher than the X-ray observation indicates. We fully expect that this will be the size of the event horizon we measure. If the measurements of M87 show a higher value than the X-ray emission indicates, we could learn that the X-ray estimates are systematically low, showing us there’s new astrophysics (but not new fundamental physics) at play.
A large slew of stars have been detected near the supermassive black hole at the Milky Way’s core. In addition to these stars and the gas and dust we find, we anticipate there to be upwards of 10,000 black holes within just a few light years of Sagittarius A*, but detecting them had proved elusive until earlier in 2018. Resolving the central black hole is a task that only the Event Horizon Telescope can rise to, and may yet detect its motion over time. S. SAKAI / A. GHEZ / W.M. KECK OBSERVATORY / UCLA GALACTIC CENTER GROUP
6.) Can we see the black hole “jittering” over time, as predicted? This one may not come out right away, especially if all we get from these initial observations is a single image of one or two black holes. But one of the science goals of the EHT is to observe how black holes evolve with time, meaning that they plan on taking multiple images at different times and reconstructing a movie of these black holes.
Because of the presence of stars and other masses, the apparent position of the black hole will change significantly over time, as it gets gravitationally pushed around. Although it will likely take years to observe a black hole move by an appreciable amount, we have data that was taken over the course of a long time. At the centers of galaxies, EHT-imaged black holes may begin to exhibit signs of this jitter: the cosmic equivalent of Brownian motion.
The critical observations for creating the first image of a black hole, assuming the EHT publishes one of the black hole at the Milky Way’s center, were taken way back in 2017: two full years ago. It’s taken this long to analyze, clean up, cut, adjust, and synthesize the full suite of data, which equates to some 27 petabytes for the critical observation. (Although only about 15% of that data is relevant and usable for constructing an image.)
At 9 AM Eastern Time (6 AM Pacific Time) on April 10, the EHT collaboration will hold a press conference where they’re expected to release the first image of an event horizon, and it’s possible that many — or possibly even all — of these questions will be answered. Whatever the results, this is a monumental step forward for physics and astrophysics, and ushers in a new era of science: direct tests and images of a black hole’s event horizon itself!
