It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Tuesday, April 16, 2024
SPACE
Environmental concerns raised by rocket flights over San Diego County
by Phil Diehl, The San Diego Union-Tribune
Credit: Unsplash/CC0 Public Domain
Plans by SpaceX and other companies to boost the number of rocket launches sometimes seen streaking across San Diego County's skies have prompted the California Coastal Commission to question the environmental effects.
Residents near Vandenberg Space Force Base, on the state's Central Coast, say the launches shake their homes and rattle their nerves. People don't know when to expect them because the lift-off time varies and can be delayed by weather conditions.
"I find it difficult to believe that there are no impacts on (wildlife) species due to SpaceX launches," said Carpinteria resident Rebecca Stebbins in an April 5 letter to the Coastal Commission.
"I, along with thousands of other residents of the South Coast, am significantly impacted with each launch, including being woken up from a deep sleep on occasion, while my dogs are terrified, my house shakes, and the sonic booms are felt physically, with a deep shock."
Conservationists say the noise disturbs native wildlife such as red-legged frogs, the western snowy plover, seals and sea lions, and it interferes with commercial and recreational fishing. Nearby public beaches and fishing grounds are often closed during the launches.
"The launches are extremely loud and destructive," said Mandy Sackett in San Diego, senior California policy coordinator for the Surfrider Foundation.
"Sound impacts are grossly underestimated," Sackett said, and she urged the Coastal Commission to "pump the brakes" on the increase.
Another downside are the latex weather balloons released before every flight to check atmospheric conditions. The balloons carry batteries and electronics that reach the stratosphere and then burst from the pressure before falling back to earth or into the ocean, where the equipment sinks with little chance of being recovered.
As many as 30 balloons were released before each launch until recently, a Vandenberg official said. A launch now needs as few as 10, and the number is decreasing as technology improves.
Space companies pay mitigation fees of $10 for each pound of unrecoverable debris they create, and the money goes into a fund for the collection of lost fishing gear such as monofilament line and nets. But commissioners, at their meeting Wednesday in Long Beach, said that amount may be insufficient.
"A battery is hazardous waste," said Commissioner Kristina Kunkel. "It's not comparable to fishing gear."
Air quality may be the first concern of anyone who has seen the rocket's long trail of vapor, yet the reported emissions are well below applicable state and federal standards. The fuel is rocket-grade kerosene combined with liquid oxygen. When it burns, it produces a negligible amount of soot and nitrogen oxide in the exhaust.
The U.S. Space Force and SpaceX, owned by electric-car magnate Elon Musk, have asked the Coastal Commission to approve an increase to as many as 36 launches a year at Vandenberg. The SpaceX launches averaged six annually over the past five years, although they have been increasing steadily, reaching a total of 19 in 2022 and 28 in 2023.
The company has been ramping up launches as it builds a network of nearly 42,000 Starlink satellites to provide worldwide direct-to-cell internet service. Each Falcon 9 rocket carries up to 22 satellites.
SpaceX also uses bases in Texas and Florida, and as of March had launched more than 5,500 satellites. The company has a roster of other launch customers, including NASA and the Pentagon.
The Coastal Commission reached no decision on the request Wednesday. Instead, the commissioners voted to postpone the matter so staffers can look further into the cumulative effects of the launches and return with more information in a month or longer.
Other private companies and federal agencies also launch rockets at Vandenberg. Last year, there were 37 launches in all, said Space Force Col. Bryan Titus, operations vice commander at the base.
"We're asking for 36 right now (for SpaceX alone), and we do plan to ask for more later," Titus said at Wednesday's commission meeting. The base has the capacity to do as many as 110 launches a year, which could increase with plans to build an additional launch platform.
The launches are allowed based on the Coastal Commission's previous determination that the environmental effects of the events are relatively insignificant. Also, there are questions about whether the state agency can regulate actions by the federal government that Titus said are vital to national security.
"All launches support the Defense Department and our allies," Titus said.
About 25% of all SpaceX rockets include a Defense Department payload, he said. The United States also benefits from the company's Starlink system of satellites
"Starlink has been absolutely critical in the situation in Ukraine," he said, referring to the U.S. support of the country in its war with Russia.
Landings of the rocket's reusable first stage also will increase under the SpaceX plan, another concern for Central Coast residents.
While the launch of the rocket creates a thunderous roar, it does not create a sonic boom, Titus said. Only the return of the first stage, less than 10 minutes after liftoff, creates a sonic boom that can be heard from 80 miles away or farther, depending on atmospheric conditions.
The rocket stage can return to Vandenberg, or, if that's too far, it can land on a floating platform at sea. SpaceX also is asking the Coastal Commission to allow an expansion of the ocean landing zone to cover an area beginning at least 31 miles from the coast and extending out as far as several hundred miles, anywhere between the latitudes of Los Angeles and the middle of Baja California.
At sea, landings occur on a barge-like drone ship that is towed to the general area. Once there, it can remotely adjust its position.
Some of the commissioners questioned the need for so many launches, especially when most of the profits go to private companies such as SpaceX and Firefly Aerospace, a Texas-based aerospace company.
Some of the commissioners said they would prefer to see statistics for all the launches, including those by NASA, the Defense Department and private companies. They also noted that no one representing SpaceX was present at the meeting.
"I am concerned about the piecemealing of this," said Commissioner Ann Notthoff. "We can't really assess what this exponential growth is. We have to get a handle on that."
2024 The San Diego Union-Tribune. Distributed by Tribune Content Agency, LLC.
Glittering threads of stars around the Milky Way may hold answers to one of our biggest questions about the universe: what is dark matter? With images taken through six different color filters mounted to the largest camera ever built for astronomy and astrophysics, Vera C. Rubin Observatory's upcoming Legacy Survey of Space and Time will reveal never-before-seen stellar streams around the Milky Way—and the telltale effects of their interactions with dark matter.
As mesmerizing as rivers that glitter in the sunlight, stellar streams trace sparkling arcs through and around our home galaxy—the Milky Way. Stellar streams are composed of stars that were originally bound in globular clusters or dwarf galaxies but have been disrupted by gravitational interactions with our galaxy and drawn into long, trailing lines.
These slender trails of stars often show signs of disturbance, and scientists suspect that, in many cases, dark matter is the culprit. Vera C. Rubin Observatory will soon provide a wealth of data to illuminate stellar streams, dark matter, and their complex interactions.
Dark matter makes up 27% of the universe, but it can't be observed directly, and scientists currently don't know exactly what it is. To learn more, they use a variety of indirect methods to investigate its nature. Some methods, like weak gravitational lensing, map the distribution of dark matter on large scales across the universe. Observing stellar streams allows scientists to probe a different aspect of dark matter because they showcase the fingerprint of dark matter's gravitational effects at small scales.
Vera C. Rubin Observatory, located in Chile, will use an 8.4-meter telescope equipped with the largest digital camera in the world to conduct a 10-year survey of the entire southern hemisphere sky beginning in late 2025. The resulting data, with images taken through six different color filters, will make it easier than ever for scientists to isolate stellar streams among and beyond the Milky Way and examine them for signs of dark matter disruption.
"I'm really excited about using stellar streams to learn about dark matter," said Nora Shipp, a postdoctoral fellow at Carnegie Mellon University and co-convener of the Dark Matter Working Group in the Rubin Observatory/LSST Dark Energy Science Collaboration. "With Rubin Observatory, we'll be able to use stellar streams to figure out how dark matter is distributed in our galaxy from the largest scales down to very small scales."
Rubin Observatory will begin science operations in late 2025. Rubin Observatory is a Program of NSF NOIRLab, which, along with SLAC National Accelerator Laboratory, will jointly operate Rubin.
Evidence suggests that a spherical halo of dark matter surrounds the Milky Way, made up of smaller dark matter clumps. These clumps interact with other structures, disrupting their gravitational dynamics and changing their observed appearance. In the case of stellar streams, the results of dark matter interactions appear as kinks or gaps in the starry trails.
Rubin Observatory's incredibly detailed images will make it possible for scientists to identify and examine very subtle irregularities in stellar streams and thus infer the properties of the low-mass dark matter clumps that caused them—even narrowing down what types of particles these clumps are made of.
"By observing stellar streams, we'll be able to take indirect measurements of the Milky Way's dark matter clumps down to masses lower than ever before, giving us really good constraints on the particle properties of dark matter," said Shipp.
Stellar streams in the outer regions of the Milky Way are especially good candidates for observing the effects of dark matter because they're less likely to have been affected by interactions with other parts of the Milky Way, which can confuse the picture. Rubin Observatory will be able to detect stellar streams at a distance of about five times farther than we can see now, allowing scientists to discover and observe an entirely new population of stellar streams in the Milky Way's outer regions.
Stellar streams are challenging to distinguish from the many other stars of the Milky Way. To isolate stellar streams, scientists search for stars with specific properties that indicate they likely belonged together as globular clusters or dwarf galaxies. They then analyze the motion or other properties of these stars to identify those connected as a stream.
"Stellar streams are like strings of pearls, whose stars trace the path of the system's orbit and have a shared history," said Jaclyn Jensen, a Ph.D. candidate at the University of Victoria who plans to use Rubin/LSST data for her research on the progenitors of stellar streams and their role in the formation of the Milky Way.
"Using properties of these stars, we can determine information about their origins and what kind of interactions the stream may have experienced. If we find a pearl necklace with a few scattered pearls nearby, we can deduce that something may have come along and broken the string."
Rubin Observatory's 3200-megapixel LSST Camera is equipped with six color filters—including, notably, for stellar stream scientists like Shipp and Jensen, an ultraviolet filter. Rubin's ultraviolet filter will provide critical information on the blue-ultraviolet end of the light spectrum that will enable scientists to distinguish the subtle differences and untangle the stars in a stream from look-alike stars in the Milky Way.
Overall, Rubin will provide scientists with thousands of deep images taken through all six filters, giving them a clearer view of stellar streams than ever before.
The avalanche of data that Rubin will provide will also inspire new tools and methods for isolating stellar streams. As Shipp notes, "Right now it's a labor-intensive process to pick out potential streams by eye—Rubin's large volume of data presents an exciting opportunity to think of new, more automated ways to identify streams."
A new study shows how measuring the Sun’s gravitational field could search for additional physics. Credit: NASA/ESA
It's an exciting time for the fields of astronomy, astrophysics, and cosmology. Thanks to cutting-edge observatories, instruments, and new techniques, scientists are getting closer to experimentally verifying theories that remain largely untested. These theories address some of the most pressing questions scientists have about the universe and the physical laws governing it—like the nature of gravity, dark matter, and dark energy.
For decades, scientists have postulated that either there is additional physics at work or that our predominant cosmological model needs to be revised.
While the investigation into the existence and nature of dark matter and dark energy is ongoing, there are also attempts to resolve these mysteries with the possible existence of new physics. In a paper, a team of NASA researchers proposed how spacecraft could search for evidence of additional physical within our solar systems. This search, they argue, would be assisted by the spacecraft flying in a tetrahedral formation and using interferometers. Such a mission could help resolve a cosmological mystery that has eluded scientists for over half a century.
The proposal is the work of Slava G. Turyshev, an adjunct professor of physics and astronomy at the university of California Los Angeles (UCLA) and research scientist with NASA's Jet Propulsion Laboratory. He was joined by Sheng-wey Chiow, an experimental physicist at NASA JPL, and Nan Yu, an adjunct professor at the university of South Carolina and a senior research scientist at NASA JPL.
Their research paper has appeared online and has been accepted for publication in Physical Review D.
Turyshev's experience includes being a Gravity Recovery And Interior Laboratory (GRAIL) mission science team member. In previous work, Turyshev and his colleagues have investigated how a mission to the sun's solar gravitational lens (SGL) could revolutionize astronomy. In a previous study, he and SETI astronomer Claudio Maccone also considered how advanced civilizations could use SGLs to transmit power from one solar system to the next.
To summarize, gravitational lensing is a phenomenon where gravitational fields alter the curvature of spacetime in their vicinity. This effect was originally predicted by Einstein in 1916 and was used by Arthur Eddington in 1919 to confirm his general relativity (GR). However, between the 1960s and 1990s, observations of the rotational curves of galaxies and the expansion of the universe gave rise to new theories regarding the nature of gravity over larger cosmic scales. On the one hand, scientists postulated the existence of dark matter and dark energy to reconcile their observations with GR.
On the other hand, scientists have advanced alternate theories of gravity (such as modified Newtonian dynamics (MOND), modified gravity (MOG), etc.). Meanwhile, others have suggested there may be additional physics in the cosmos that we are not yet aware of.
As Turyshev told Universe Today via email, "We are eager to explore questions surrounding the mysteries of dark energy and dark matter. Despite their discovery in the last century, their underlying causes remain elusive. Should these 'anomalies' stem from new physics—phenomena yet to be observed in ground-based laboratories or particle accelerators—it's possible that this novel force could manifest on a solar system scale."
For their latest study, Turyshev and his colleagues investigated how a series of spacecraft flying in a tetrahedral formation could investigate the sun's gravitational field. These investigations, said Turyshev, would search for deviations from the predictions of general relativity at the solar system scale, something that has not been possible to date.
"These deviations are hypothesized to manifest as nonzero elements in the gravity gradient tensor (GGT), fundamentally akin to a solution of the Poisson equation. Due to their minuscule nature, detecting these deviations demands precision far surpassing current capabilities—by at least five orders of magnitude. At such a heightened level of accuracy, numerous well-known effects will introduce significant noise. The strategy involves conducting differential measurements to negate the impact of known forces, thereby revealing the subtle, yet nonzero, contributions to the GGT."
The mission, said Turyshev, would employ local measurement techniques that rely on a series of interferometers. This includes interferometric laser ranging, a technique demonstrated by the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, a spacecraft pair that relies on laser range finding to track Earth's oceans, glaciers, rivers, and surface water. The same technique will also be used to investigate gravitational waves by the proposed space-based Laser Interferometry Space Antenna (LISA).
The spacecraft will also be equipped with atom interferometers, which use the wave character of atoms to measure the difference in phase between atomic matter waves along different paths. This technique will allow the spacecraft to detect the presence of non-gravitational noise (thruster activity, solar radiation pressure, thermal recoil forces, etc.) and negate them to the necessary degree. Meanwhile, flying in a tetrahedral formation will optimize the spacecraft's ability to compare measurements.
"Laser ranging will offer us highly accurate data on the distances and relative velocities between spacecraft," said Turyshev. "Furthermore, its exceptional precision will allow us to measure the rotation of a tetrahedron formation relative to an inertial reference frame (via Sagnac observables), a task unachievable by any other means. Consequently, this will establish a tetrahedral formation leveraging a suite of local measurements."
Ultimately, this mission will test GR on the smallest of scales, which has been sorely lacking to date. While scientists continue to probe the effect of gravitational fields on spacetime, these have been largely confined to using galaxies and galaxy clusters as lenses. Other instances include observations of compact objects (like white dwarf stars) and supermassive black holes (SMBH) like Sagittarius A*—which resides at the center of the Milky Way.
"We aim to enhance the precision of testing GR and alternative gravitational theories by more than five orders of magnitude. Beyond this primary objective, our mission has additional scientific goals, which we will detail in our subsequent paper. These include testing GR and other gravitational theories, detecting gravitational waves in the micro-Hertz range—a spectrum not reachable by existing or envisioned instruments— and exploring aspects of the solar system, such as the hypothetical Planet 9, among other endeavors."
Astronomers have found the most massive stellar black hole in our galaxy, thanks to the wobbling motion it induces on a companion star. This artist's impression shows the orbits of both the star and the black hole, dubbed Gaia BH3, around their common center of mass. This wobbling was measured over several years with the European Space Agency's Gaia mission. Additional data from other telescopes, including ESO's Very Large Telescope in Chile, confirmed that the mass of this black hole is 33 times that of our sun. The chemical composition of the companion star suggests that the black hole was formed after the collapse of a massive star with very few heavy elements, or metals, as predicted by theory. Credit: ESO/L. Calçada
Astronomers have identified the most massive stellar black hole yet discovered in the Milky Way galaxy. This black hole was spotted in data from the European Space Agency's Gaia mission because it imposes an odd 'wobbling' motion on the companion star orbiting it. Data from the European Southern Observatory's Very Large Telescope (ESO's VLT) and other ground-based observatories were used to verify the mass of the black hole, putting it at an impressive 33 times that of the sun.
Stellar black holes are formed from the collapse of massive stars and the ones previously identified in the Milky Way are on average about 10 times as massive as the sun. Even the next most massive stellar black hole known in our galaxy, Cygnus X-1, only reaches 21 solar masses, making this new 33-solar-mass observation exceptional.
Remarkably, this black hole is also extremely close to us—at a mere 2,000 light-years away in the constellation Aquila, it is the second-closest known black hole to Earth. Dubbed Gaia BH3 or BH3 for short, it was found while the team was reviewing Gaia observations in preparation for an upcoming data release.
"No one was expecting to find a high-mass black hole lurking nearby, undetected so far," says Gaia collaboration member Pasquale Panuzzo, an astronomer at the Observatoire de Paris, part of France's National Center for Scientific Research (CNRS). "This is the kind of discovery you make once in your research life."
To confirm their discovery, the Gaia collaboration used data from ground-based observatories, including from the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on ESO's VLT, located in Chile's Atacama Desert. These observations revealed key properties of the companion star, which, together with Gaia data, allowed astronomers to precisely measure the mass of BH3.
Astronomers have found similarly massive black holes outside our galaxy (using a different detection method), and have theorized that they may form from the collapse of stars with very few elements heavier than hydrogen and helium in their chemical composition. These so-called metal-poor stars are thought to lose less mass over their lifetimes and hence have more material left over to produce high-mass black holes after their death. But evidence directly linking metal-poor stars to high-mass black holes has been lacking until now.
Stars in pairs tend to have similar compositions, meaning that BH3's companion holds important clues about the star that collapsed to form this exceptional black hole. UVES data showed that the companion was a very metal-poor star, indicating that the star that collapsed to form BH3 was also metal-poor—just as predicted.
The research, led by Panuzzo and titled "Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry" is published in Astronomy & Astrophysics.
"We took the exceptional step of publishing this paper based on preliminary data ahead of the forthcoming Gaia release because of the unique nature of the discovery," says co-author Elisabetta Caffau, also a Gaia collaboration member from the CNRS Observatoire de Paris. Making the data available early will let other astronomers start studying this black hole right now, without waiting for the full data release, planned for late 2025 at the earliest.
Further observations of this system could reveal more about its history and about the black hole itself. The GRAVITY instrument on ESO's VLT Interferometer, for example, could help astronomers find out whether this black hole is pulling in matter from its surroundings and better understand this exciting object.
More information: Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry. Astronomy & Astrophysics (aanda.org/10.1051/0004-6361/202449763).
Astronomers detect radio halo in a massive galaxy cluster
by Tomasz Nowakowski , Phys.org
Archival Chandra image of ACT-CL J0329. Credit: Sikhosana et al., 2024.
An international team of astronomers has performed radio observations of a massive galaxy cluster known as ACT-CL J0329.2-2330, which resulted in the detection of a new radio halo in this cluster. The finding was reported in a research paper published April 5 on the pre-print server arXiv.
Radio halos are enormous regions of diffuse radio emission, usually found at the centers of massive galaxy clusters, showcasing a regular morphology, which tends to trace the X-ray emitting intracluster medium (ICM). However, diffuse emissions generally have very low surface brightness, particularly at GHz frequencies, which makes them hard to detect. Their brightness increases at lower frequencies, unveiling the presence of these regions.
Now, a group of astronomers led by Sinenhlanhla Precious Sikhosana of the University of KwaZulu-Natal in Durban, South Africa, has found a new radio halo in ACT-CL J0329.2-2330 (or ACT-CL J0329 for short)—a galaxy cluster with a mass of about 970 trillion solar masses, at a redshift of 1.23. The discovery is a result of L-band and UHF-band observations of this cluster with the MeerKAT radio telescope as part of the MeerKAT Massive Distant Cluster Survey (MMDCS).
"In this letter, we have presented MeerKAT L and UHF-band observations of ACT-CL J0329.2-2330, a galaxy cluster at z=1.23. The low-resolution images reveal a radio halo in the cluster. (...) The MeerKAT observations were carried out at L-band with a total on-target time of 3.5 hours, using a dump rate of 8 seconds and 4,096 channels," the researchers wrote.
By analyzing MeerKAT images, Sikhosana's team identified extended emission at the center of ACT-CL J0329, with a largest linear size of 3.59 million light years at 1.28 GHz. MeerKAT images also show that the radio halo in ACT-CL J0329 has a smooth, regular morphology that traces the thermal bremsstrahlung emission of the intracluster medium (ICM).
Based on these results, the astronomers classified this emission as a radio halo, which means that it is the highest redshift halo so far detected.
The study found that the newly discovered radio halo has a flux density of 3.44 and 6.11 mJy at L and UHF-band, respectively. The integrated spectral index of the halo was calculated to be 1.3, while its radio power was estimated to be of 4.4 YW/Hz.
These results suggest that the halo in ACT-CL J0329 is as luminous as the halos found in nearby massive galaxy clusters, which seems to confirm that there is rapid magnetic field amplification in galaxy clusters at high redshifts.
In concluding remarks, the authors of the paper underlined that the spectral index map of ACT-CL J0329 showcases distinguishable fluctuations as steeper spectral index values are concentrated in the eastern region. This may indicate that the turbulent energy is not homogeneously dissipated in the halo volume.
More information: S. P. Sikhosana et al, The MeerKAT Massive Distant Clusters Survey: A Radio Halo in a Massive Galaxy Cluster at z = 1.23, arXiv (2024). DOI: 10.48550/arxiv.2404.03944
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker
The mystery of how Pluto got a giant heart-shaped feature on its surface has finally been solved by an international team of astrophysicists led by the University of Bern and members of the National Center of Competence in Research (NCCR) PlanetS. The team is the first to successfully reproduce the unusual shape with numerical simulations, attributing it to a giant and slow oblique-angle impact.
Ever since the cameras of NASA's New Horizons mission discovered a large heart-shaped structure on the surface of the dwarf planet Pluto in 2015, this "heart" has puzzled scientists because of its unique shape, geological composition, and elevation. A team of scientists from the University of Bern, including several members of the NCCR PlanetS, and the University of Arizona in Tucson have used numerical simulations to investigate the origins of Sputnik Planitia, the western teardrop-shaped part of Plutos heart surface feature.
According to their research, Pluto's early history was marked by a cataclysmic event that formed Sputnik Planitia: a collision with a planetary body about 700 km in diameter, roughly twice the size of Switzerland from east to west. The team's findings, which were recently published in Nature Astronomy, also suggest that the inner structure of Pluto is different from what was previously assumed, indicating that there is no subsurface ocean.
A divided heart
The heart, also known as the Tombaugh Regio, captured the public's attention immediately upon its discovery. But it also immediately caught the interest of scientists because it is covered in a high-albedo material that reflects more light than its surroundings, creating its whiter color.
However, the heart is not composed of a single element. Sputnik Planitia (the western part) covers an area of 1,200 by 2,000 kilometers, which is equivalent to a quarter of Europe or the United States. What is striking, however, is that this region is three to four kilometers lower in elevation than most of Pluto's surface.
"The bright appearance of Sputnik Planitia is due to it being predominantly filled with white nitrogen ice that moves and convects to constantly smooth out the surface. This nitrogen most likely accumulated quickly after the impact due to the lower altitude," explains Dr. Harry Ballantyne from the University of Bern, lead author of the study.
The eastern part of the heart is also covered by a similar but much thinner layer of nitrogen ice, the origin of which is still unclear to scientists, but is probably related to Sputnik Planitia.
Artistic representation of the huge and slow impact on Pluto that led to the heart-shaped structure on its surface. Credit: University of Bern, Illustration: Thibaut Roger
An oblique impact
"The elongated shape of Sputnik Planitia strongly suggests that the impact was not a direct head-on collision but rather an oblique one," points out Dr. Martin Jutzi of the University of Bern, who initiated the study.
So the team, like several others around the world, used their Smoothed Particle Hydrodynamics (SPH) simulation software to digitally recreate such impacts, varying both the composition of Pluto and its impactor, as well as the velocity and angle of the impactor. These simulations confirmed the scientists' suspicions about the oblique angle of impact and determined the composition of the impactor.
"Pluto's core is so cold that the rocks remained very hard and did not melt despite the heat of the impact, and thanks to the angle of impact and the low velocity, the core of the impactor did not sink into Pluto's core, but remained intact as a splat on it," explains Ballantyne.
"Somewhere beneath Sputnik is the remnant core of another massive body, that Pluto never quite digested," adds co-author Erik Asphaug from the University of Arizona. This core strength and relatively low velocity were key to the success of these simulations: lower strength would result in a very symmetrical leftover surface feature that does not look like the teardrop shape observed by New Horizons.
"We are used to thinking of planetary collisions as incredibly intense events where you can ignore the details except for things like energy, momentum and density. But in the distant solar system, velocities are so much slower, and solid ice is strong, so you have to be much more precise in your calculations. That's where the fun starts," says Asphaug.
The two teams have a long record of collaborations together, exploring since 2011 already the idea of planetary "splats" to explain, for instance, features on the far side of the moon. After our moon and Pluto, the University of Bern team plans to explore similar scenarios for other outer solar system bodies such as the Pluto-like dwarf planet Haumea.
No subsurface ocean on Pluto
The current study sheds new light on Pluto's internal structure as well. In fact, a giant impact like the one simulated is much more likely to have occurred very early in Pluto's history. However, this poses a problem: a giant depression like Sputnik Planitia is expected to slowly move toward the pole of the dwarf planet over time due to the laws of physics, since it has a mass deficit. Yet it is paradoxically near the equator.
The previous theorized explanation was that Pluto, like several other planetary bodies in the outer solar system, has a subsurface liquid water ocean. According to this previous explanation, Pluto's icy crust would be thinner in the Sputnik Planitia region, causing the ocean to bulge there, and since liquid water is denser than ice, you would end up with a mass surplus that induces migration toward the equator.
However, the new study offers an alternative perspective. "In our simulations, all of Pluto's primordial mantle is excavated by the impact, and as the impactor's core material splats onto Pluto's core, it creates a local mass excess that can explain the migration toward the equator without a subsurface ocean, or at most a very thin one," explains Martin Jutzi.
Dr. Adeene Denton from the University of Arizona, also co-author of the study, is currently conducting a new research project to estimate the speed of this migration. "This novel and inventive origin for Pluto's heart-shaped feature may lead to a better understanding of Pluto's origin," she concludes.
More information: Harry A. Ballantyne et al, Sputnik Planitia as an impactor remnant indicative of an ancient rocky mascon in an oceanless Pluto, Nature Astronomy (2024). DOI: 10.1038/s41550-024-02248-1
An illustration of Mimas’s ice shell evolution, in which the changes in ice shell thickness (y-axis) lag behind the eccentricity decay (inverted x-axis). Time increases to the right of the plot, while eccentricity decreases. Points A, B, and C have associated interior structure cartoons, where the equilibrium ice shell thickness (light blue dashed line, “EQ thickness”) moves to thicker values as the eccentricity drops. Hence, the ice shell will first thin, then achieve temporary thermal equilibrium, and finally thicken. Credit: Alyssa Rhoden
Saturn's moon Mimas could have grown a huge underground ocean as its orbital eccentricity decreased to its present value and caused its icy shell to melt and thin.
"In our previous work, we found that for Mimas to be an ocean world today, it must have had a much thicker icy shell in the past. But because Mimas's eccentricity would have been even higher in the past, the pathway to get from thick ice to thinner ice was less clear," said Planetary Science Institute Senior Scientist Matthew E. Walker. "In this work we showed that there is a pathway for the ice shell to be thinning currently even as the eccentricity is dropping due to tidal heating, but the ocean must be very young, geologically speaking."
Walker is co-author of "The evolution of a young ocean within Mimas", which appears in Earth and Planetary Science Letters. Alyssa Rose Rhoden of the Southwest Research Institute is lead author.
"Eccentricity is what drives the tidal heating. Right now it is very high as compared with other active ocean moons, like neighboring Enceladus. We think that tidal heating is the heat source responsible for currently thinning the shell," Walker said. "Tidal heating is not free energy though, so as it melts the shell, it pulls energy out of the orbit, which drops that eccentricity until it eventually circularizes it and shuts the whole thing down."
The onset of melting had to occur when Mimas's eccentricity was two to three times the present value. A thinning ice shell over the past 10 million years of Mimas's evolution is consistent with its geology.
"Generally when we think of ocean worlds we don't see a lot of craters because the environment is resurfaced and ends up erasing them, like Europa or the south pole of Enceladus. The shape, central peak, and undisrupted interior of Herschel crater require that the shell must have been thicker in the past, when Herschel formed. In order to get the crater morphology that we observe, the shell must have been at least 55 kilometers when it got hit," Walker said.
"Craters can provide clues as to the presence of an ocean and the thickness of the ice shell through their morphology—such as the ratio between the crater diameter and its depth and the existence of a central peak."
Mimas has a radius of just under 200 kilometers. The thickness of the outer hydrosphere, made up of ice and liquid, is roughly estimated to be around 70 kilometers. The current estimates of ice shell thickness are 20 to 30 kilometers, based on the precession (the rotational motion of the axis of a spinning body), or a narrower range of 24 to 31 kilometers from the libration (a slight wobble in the rotation rate of the moon that makes it appear to nod back and forth) measurements, leaving an ocean that is about 40 to 45 kilometers deep before it hits the rock.
"We may be seeing Mimas at a particularly interesting time. In order to match the current eccentricity and the thickness constraints based on the libration info, we think this whole thing must have started off no more than about 25 million years ago. In other words, we think Mimas was completely frozen until 10 to 25 million years ago, at which point its ice shell began melting. What changed to kick off that epoch of melting is still under investigation," Walker said.
More information: Alyssa Rose Rhoden et al, The evolution of a young ocean within Mimas, Earth and Planetary Science Letters (2024). DOI: 10.1016/j.epsl.2024.118689
Representative μCT slices from chips of the Winchcombe meteorite and contoured orientation data on the long and short shape axes of chondrules plotted on a lower hemisphere stereographic projections and n denotes the number of chondrules measured for each plot. Credit: Meteoritics & Planetary Science (2024). DOI: 10.1111/maps.14164
Intensive new nano-analysis of the Winchcombe meteorite has revealed how it was affected by water and repeatedly smashed apart and reassembled on the journey it took through space before landing in an English sheep field in 2021.
Researchers from dozens of institutions in the UK, Europe, Australia, and the U.S. collaborated on the research. Together, they subjected mineral grains in fragments of the Winchcombe meteorite to a diverse range of cutting-edge analytical techniques.
Their work, which was conducted on a scale more typically reserved for investigating samples returned to Earth by multibillion-dollar space missions, has given them unparalleled insight into the history of the Winchcombe meteorite in the process.
Their analysis has helped them roll back the clock to the meteorite's earliest days as an ice-bearing dry rock, then trace its transformation through the melting of the ice into a ball of mud which was broken apart and rebuilt over and over again.
The Winchcombe meteorite is an unusually well-preserved example of a group of space rocks called CM carbonaceous chondrites, which were formed during the earliest periods of the solar system. They carry minerals altered by the presence of water on their parent asteroid.
Analysis of those minerals within the Winchcombe meteorite will help scientists unravel the answers to questions around the processes which formed our solar system, including the possible origins of the Earth's wate
Unlike most meteorites, which can lie undiscovered for months or years after entering the Earth's atmosphere, the Winchcombe meteorite was recovered within hours of hitting the ground. Members of the public, citizen scientists and the amateur meteorite enthusiast community recognized that rocks had hit the ground and helped scientists to identify the location of samples, aiding their recovery.
The speed of its recovery helped prevent it from being further altered by exposure to the Earth's atmosphere, offering scientists a rare opportunity to learn more about CM chondrites by scrutinizing it down to the atomic level.
In a paper published in the journal Meteoritics and Planetary Science, researchers describe how they explored the complex breccia of the Winchombe meteorite.
A breccia is rock formed from chunks of other rocks cemented together in a structure called a cataclastic matrix. The team's analysis carried out using sophisticated techniques including transmission electron microscopy, electron backscatter diffraction, time of flight secondary ion mass spectrometry and atom probe tomography, showed that the Winchcombe breccia contains eight distinct types of CM chondrite rocks.
The team found that each type of rock has been altered to different degrees by the presence of water, not just between the types of rocks but also, surprisingly, within them. The team found many examples of unaltered mineral grains next to completely altered ones, even down to the nano-scale. For comparison, a human hair is around 75,000 nanometers thick.
The team suggests that the likely explanation for the jumbled nature of the different types of rocks and their extreme variation in aqueous alteration is that the Winchcombe asteroid was repeatedly smashed into pieces by impacts with other asteroids before being pulled back together.
Another significant finding of the analysis is the unexpectedly high proportion of carbonate minerals like aragonite, calcite, and dolomite, along with minerals that have subsequently replaced carbonates, in the samples the team analyzed.
This suggests that the Winchcombe meteorite was more carbon-rich than previously thought and likely accumulated abundant frozen CO2 before it melted to form the carbonate minerals the team observed. The team's analysis could help explain the large carbonate veins that have been observed on the surface of the Asteroid Bennu by NASA's OSIRIS-REx mission.
The study was led by Dr. Luke Daly of the University of Glasgow, who is also the lead author of the paper. Dr. Daly also led the search party which recovered the largest fragment of the Winchcombe meteorite after it was spotted as a fireball streaking across the skies over Gloucestershire on February 28th, 2021
Dr. Daly said, "We were fascinated to uncover just how fragmented the breccia was within the Winchcombe sample we analyzed. If you imagine the Winchcombe meteorite as a jigsaw, what we saw in the analysis was as if each of the jigsaw pieces themselves had also been cut into smaller pieces, and then jumbled in a bag filled with fragments of seven other jigsaws.
"However, what we've uncovered in trying to unjumble the jigsaws through our analyses is new insight into the very fine detail of how the rock was altered by water in space. It also gives us a clearer idea of how it must have been battered by impacts and reformed again and again over the course of its lifetime since it swirled together out of the solar nebula, billions of years ago."
Dr. Leon Hicks from the University of Leicester and co-author of the study said, "This level of analysis of the Winchcombe meteorite is virtually unprecedented for materials that weren't directly returned to Earth from space missions, like moon rocks from the Apollo program or samples from the Ryugu asteroid collected by the Hayabusa 2 probe."
Paper co-author Dr. Martin Suttle from the Open University said, "The speed which the fragments of Winchcombe were recovered left us with some pristine samples for analysis, from the centimeter scale all the way down to individual atoms within the rocks. Each grain is a tiny time capsule that, taken together, helps us build a remarkably clear view into the formation, re-formation, and alteration that occurred over the course of millions of years."
Dr. Diane Johnson from Cranfield University, a co-author of the paper, added, "Research like this helps us understand the earliest part the formation of our solar system in a way that just isn't possible without detailed analysis of materials that were right there in space as it happened. The Winchcombe meteorite is a remarkable piece of space history and I'm pleased to have been part of the team that has helped tell this new story."
More information: Luke Daly et al, Brecciation at the grain scale within the lithologies of the Winchcombe Mighei‐like carbonaceous chondrite, Meteoritics & Planetary Science (2024). DOI: 10.1111/maps.14164
Credit: Results in Physics (2024). DOI: 10.1016/j.rinp.2024.107344
The idea that the universe is expanding dates from almost a century ago. It was first put forward by Belgian cosmologist Georges Lemaître (1894–1966) in 1927 and confirmed observationally by American astronomer Edwin Hubble (1889-1953) two years later. Hubble observed that the redshift in the electromagnetic spectrum of the light received from celestial objects was directly proportional to their distance from Earth, which meant that bodies farther away from Earth were moving away faster and the universe must be expanding
A surprising new ingredient was added to the model in 1998 when observations of very distant supernovae by the Supernova Cosmology Project and the High-Z Supernova Search Team showed that the universe is accelerating as it expands, rather than being slowed down by gravitational forces, as had been supposed. This discovery led to the concept of dark energy, which is thought to account for more than 68% of all the energy in the currently observable universe, while dark matter and ordinary matter account for about 27% and 5% respectively.
"Measurements of redshift suggest that the accelerating expansion is adiabatic [without heat transfer] and anisotropic [varying in magnitude when measured in different directions]," said Mariano de Souza, a professor in the Department of Physics at São Paulo State University (UNESP) in Rio Claro, Brazil. "Fundamental concepts in thermodynamics allow us to infer that adiabatic expansion is always accompanied by cooling due to the barocaloric effect [pressure-induced thermal change], which is quantified by the Grüneisen ratio [Γ, gamma]."
In 1908, German physicist Eduard August Grüneisen (1877–1949) proposed a mathematical expression for Γeff, the effective Grüneisen parameter, an important quantity in geophysics that often occurs in equations describing the thermoelastic behavior of material. It combines three physical properties: expansion coefficient, specific heat, and isothermal compressibility.
Almost a century later, in 2003, Lijun Zhu and collaborators demonstrated that a specific part of the Grüneisen parameter called the Grüneisen ratio, defined as the ratio of thermal expansion to specific heat, increases significantly in the vicinity of a quantum critical point owing to the accumulation of entropy. In 2010, Souza and two German collaborators showed that the same thing happens near a finite-temperature critical point.
Now Souza and fellow researchers at UNESP have used the Grüneisen parameter to describe intricate aspects of the expansion of the universe in an article published in the journal Results in Physics, presenting part of the Ph.D. research of first author Lucas Squillante, currently a postdoctoral fellow under Souza's supervision.
"The dynamics associated with the expansion of the universe are generally modeled as a perfect fluid whose equation of state is ω = p/ρ, where ω [omega] is the equation of state parameter, p is pressure, and ρ [rho] is energy density. Although ω is widely used, its physical meaning hadn't yet been appropriately discussed. It was treated as merely a constant for each era of the universe. One of the important results of our research is the identification of ω with the effective Grüneisen parameter by means of the Mie-Grüneisen equation of state," Souza said.
The Mie–Grüneisen equation of state relates to pressure, volume and temperature, and is often used to determine the pressure in a shock-compressed solid.
The authors show, using the Grüneisen parameter, that continuous cooling of the universe is associated with a barocaloric effect that relates pressure and temperature and occurs owing to adiabatic expansion of the universe. On this basis, they propose that the Grüneisen parameter is time-dependent in the dark energy-dominated era (the current universe era).
One of the interesting aspects of this research is its use of thermodynamics and solid-state physics concepts such as stress and strain to describe the anisotropic expansion of the universe. "We show that the Grüneisen parameter is naturally embodied in the energy–momentum stress tensor in Einstein's famous field equations, opening up a novel way to investigate anisotropic effects associated with the expansion of the universe. These don't rule out the possibility of a Big Rip," Souza said.
The Big Rip hypothesis, first put forward in 2003 in an article published in Physical Review Letters, posits that if the quantity of dark energy is sufficient to accelerate the expansion of the universe beyond a critical velocity, this could tear the "fabric" of space-time and rip apart the universe.
"Also in the perspective of the Grüneisen parameter, we conjecture that the shift from a decelerating expansion regime [in the radiation and matter-dominated eras] to an accelerating expansion regime [in the dark energy-dominated era] resembles a thermodynamic phase transition. This is because Γeff changes sign when the expansion changes from decelerating to accelerating. The sign change resembles the typical signature of phase transitions in condensed matter physics," Souza said.
Dark energy is often associated with the cosmological constant Λ [lambda], originally introduced by Einstein in 1917 as a repulsive force required to keep the universe in static equilibrium. Einstein later rejected the concept, according to some accounts. It was rehabilitated when the expansion of the universe was found to be accelerating instead of decelerating. The hegemonic model, known as Λ-CMD (Lambda-Cold Dark Matter), gives the cosmological constant a fixed value. That is, it assumes that the density of dark energy remains constant as the universe expands. However, other models assume that the density of dark energy, and hence Λ, vary over time.
"Assigning a fixed value to lambda means also assigning a fixed value to omega, but recognition of ω as the effective Grüneisen parameter enables us to infer time dependency for ω as the universe expands in the dark energy-dominated era. This directly entails time dependency for Λ, or the universal gravitation constant," Souza said.
The study could lead to important developments insofar as it affords a glimpse of a novel interpretation of the expansion of the universe in terms of thermodynamics and condensed matter physics.
Besides Souza and Squillante, the other co-authors of the article are Antonio Seridonio (UNESP Ilha Solteira), Roberto Lagos-Monaco (UNESP Rio Claro), Gabriel Gomes (Institute of Astronomy, Geophysics and Atmospheric Sciences, University of São Paulo, IAG-USP), Guilherme Nogueira (UNESP Rio Claro), and Ph.D. candidate Isys Mello, supervised by Souza.
More information: Lucas Squillante et al, Exploring the expansion of the universe using the Grüneisen parameter, Results in Physics (2024). DOI: 10.1016/j.rinp.2024.107344
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