Topic: Mid April Astronomy Bulletin

[Clive]

CLEAREST LOOK AT MARTIAN CORE

NASA

A pair of quakes in 2021 sent seismic waves deep into the Red Planet’s core, giving scientists the best data yet on its size and composition. While NASA retired its InSight Mars lander in December, the trove of data from its seismometer will be pored over for decades to come. By looking at seismic waves the instrument detected from a pair of temblors in 2021, scientists have been able to deduce that Mars’ liquid iron core is smaller and denser than previously thought. The findings, which mark the first direct observations ever made of another planet’s core, were detailed in a paper published April 24 in the Proceedings of the National Academies of Sciences. Occurring on Aug. 25 and Sept. 18, 2021, the two temblors were the first identified by the InSight team to have originated on the opposite side of the planet from the lander – so-called farside quakes. The distance proved crucial: The farther a quake happens from InSight, the deeper into the planet its seismic waves can travel before being detected.

The two quakes occurred after the mission had been operating on the Red Planet for well over a full Martian year (about two Earth years), meaning the Marsquake Service – the scientists who initially scrutinize seismographs – had already honed their skills. It also helped that a meteoroid impact caused one of the two quakes; impacts provide a precise location and more accurate data for a seismologist to work with. (Because Mars has no tectonic plates, most marsquakes are caused by faults, or rock fractures, that form in the planet’s crust due to heat and stress.) The quakes’ size was also a factor in the detections. One of the challenges in detecting these particular quakes was that they’re in a “shadow zone” – a part of the planet from which seismic waves tend to be refracted away from InSight, making it hard for a quake’s echo to reach the lander unless it is very large. Detecting seismic waves that cross through a shadow zone is exceptionally difficult; it’s all the more impressive that the InSight team did so using just the one seismometer they had on Mars. (In contrast, many seismometers are distributed on Earth.) A previous paper that offered a first glimpse of the planet’s core relied on seismic waves that reflected off its outer boundary, providing less precise data. Detecting seismic waves that actually travelled through the core allows scientists to refine their models of what the core looks like. Based on the findings documented in the new paper, about a fifth of the core is composed of elements such as sulphur, oxygen, carbon, and hydrogen.

CURIOSITY ROVER GETS MAJOR SOFTWARE UPGRADE

NASA

Years in the making, a major software update that has been installed on NASA’s Curiosity rover will enable the Mars robot to drive faster and reduce wear and tear on its wheels. Those are just two of 180 changes implemented during the update, which required the team to put Curiosity’s science and imaging operations on hold between April 3 and April 7. “

Planning for this update goes back to 2016, when Curiosity last received a software overhaul. Some changes this time around are as small as making corrections to the messages the rover sends back to mission controllers on Earth. Others simplify computer code that has been altered by multiple patches since Curiosity touched down in 2012. The biggest changes will help keep Curiosity rolling more efficiently for years to come. The rover can now do more of what the team calls “thinking while driving” – something NASA’s newest Mars rover, Perseverance, can perform in a more advanced way to navigate around rocks and sand traps. When Perseverance drives, it constantly snaps pictures of the terrain ahead, processing them with a dedicated computer so it can autonomously navigate during one continuous drive. Curiosity doesn’t have a dedicated computer for this purpose. Instead, it drives in segments, halting to process imagery of the terrain after each segment. That means it needs to start and stop repeatedly over the course of a long drive. The new software will help the venerable rover process images faster, allowing it to spend more time on the move.

The team also wants to maintain the health of Curiosity’s aluminium wheels, which began showing signs of broken treads in 2013. When engineers realized that sharp rocks were chipping away at the treads, they came up with an algorithm to improve traction and reduce wheel wear by adjusting the rover’s speed depending on the rocks it’s rolling over. The new software goes further by introducing two new mobility commands that reduce the amount of steering Curiosity needs to do while driving in an arc toward a specific waypoint. With less steering required, the team can reach the drive target quicker and decrease the wear that inherently comes with steering. Overall, the new software will streamline the task of Curiosity’s human drivers, who have to write complex plans containing hundreds of commands. The software update will also enable them to upload software patches more easily than in past. And it will help engineers plan the motions of Curiosity’s robotic arm more efficiently and point its “head” atop the mast more accurately.

ASTEROID’S TAIL IS NOT MADE OF DUST

NASA/Goddard Space Flight Center

We have known for a while that asteroid 3200 Phaethon acts like a comet. It brightens and forms a tail when it's near the Sun, and it is the source of the annual Geminid meteor shower, even though comets are responsible for most meteor showers. Scientists had blamed Phaethon's comet-like behaviour on dust escaping from the asteroid as it's scorched by the Sun. However, a new study using two NASA solar observatories reveals that Phaethon's tail is not dusty at all but is actually made of sodium gas. Asteroids, which are mostly rocky, do not usually form tails when they approach the Sun. Comets, however, are a mix of ice and rock, and typically do form tails as the Sun vaporizes their ice, blasting material off their surfaces and leaving a trail along their orbits. When Earth passes through a debris trail, those cometary bits burn up in our atmosphere and produce a swarm of shooting stars -- a meteor shower. After astronomers discovered Phaethon in 1983, they realized that the asteroid's orbit matched that of the Geminid meteors. This pointed to Phaethon as the source of the annual meteor shower, even though Phaethon was an asteroid and not a comet. In 2009, NASA's Solar Terrestrial Relations Observatory (STEREO) spotted a short tail extending from Phaethon as the asteroid reached its closest point to the Sun (or "perihelion") along its 524-day orbit. Regular telescopes hadn't seen the tail before because it only forms when Phaethon is too close to the Sun to observe, except with solar observatories. STEREO also saw Phaethon's tail develop on later solar approaches in 2012 and 2016. The tail's appearance supported the idea that dust was escaping the asteroid's surface when heated by the Sun. However, in 2018, another solar mission imaged part of the Geminid debris trail and found a surprise. Observations from NASA's Parker Solar Probe showed that the trail contained far more material than Phaethon could possibly shed during its close approaches to the Sun.

Scientists wondered whether something else, other than dust, was behind Phaethon's comet-like behaviour. Comets often glow brilliantly by sodium emission when very near the Sun, so we suspected sodium could likewise serve a key role in Phaethon's brightening. An earlier study, based on models and lab tests, suggested that the Sun's intense heat during Phaethon's close solar approaches could indeed vaporize sodium within the asteroid and drive comet-like activity.Hoping to find out what the tail is really made of, Zhang looked for it again during Phaethon's latest perihelion in 2022. He used the Solar and Heliospheric Observatory (SOHO) spacecraft -- a joint mission between NASA and the European Space Agency (ESA) -- which has color filters that can detect sodium and dust. Zhang's team also searched archival images from STEREO and SOHO, finding the tail during 18 of Phaethon's close solar approaches between 1997 and 2022. In SOHO's observations, the asteroid's tail appeared bright in the filter that detects sodium, but it did not appear in the filter that detects dust. In addition, the shape of the tail and the way it brightened as Phaethon passed the Sun matched exactly what scientists would expect if it were made of sodium, but not if it were made of dust. This evidence indicates that Phaethon's tail is made of sodium, not dust. Still, one important question remains: If Phaethon doesn't shed much dust, how does the asteroid supply the material for the Geminid meteor shower we see each December? The team suspects that some sort of disruptive event a few thousand years ago -- perhaps a piece of the asteroid breaking apart under the stresses of Phaethon's rotation -- caused Phaethon to eject the billion tons of material estimated to make up the Geminid debris stream. But what that event was remains a mystery.

WEBB TAKES IMAGE OF PLANET URANUS

NASA/Goddard Space Flight Center

Following in the footsteps of the Neptune image released in 2022, NASA's James Webb Space Telescope has taken a stunning image of the solar system's other ice giant, the planet Uranus. The new image features dramatic rings as well as bright features in the planet's atmosphere. The Webb data demonstrates the observatory's unprecedented sensitivity for the faintest dusty rings, which have only ever been imaged by two other facilities: the Voyager 2 spacecraft as it flew past the planet in 1986, and the Keck Observatory with advanced adaptive optics. The seventh planet from the Sun, Uranus is unique: It rotates on its side, at roughly a 90-degree angle from the plane of its orbit. This causes extreme seasons since the planet's poles experience many years of constant sunlight followed by an equal number of years of complete darkness. (Uranus takes 84 years to orbit the Sun.) Currently, it is late spring for the northern pole, which is visible here; Uranus' northern summer will be in 2028. In contrast, when Voyager 2 visited Uranus it was summer at the south pole. The south pole is now on the 'dark side' of the planet, out of view and facing the darkness of space. When Voyager 2 looked at Uranus, its camera showed an almost featureless blue-green ball in visible wavelengths. With the infrared wavelengths and extra sensitivity of Webb we see more detail, showing how dynamic the atmosphere of Uranus really is. On the right side of the planet there's an area of brightening at the pole facing the Sun, known as a polar cap. This polar cap is unique to Uranus -- it seems to appear when the pole enters direct sunlight in the summer and vanish in the fall; these Webb data will help scientists understand the currently mysterious mechanism. Webb revealed a surprising aspect of the polar cap: a subtle enhanced brightening at the centre of the cap. The sensitivity and longer wavelengths of Webb's NIRCam may be why we can see this enhanced Uranus polar feature when it has not been seen as clearly with other powerful telescopes like the Hubble Space Telescope and Keck Observatory.

At the edge of the polar cap lies a bright cloud as well as a few fainter extended features just beyond the cap's edge, and a second very bright cloud is seen at the planet's left limb. Such clouds are typical for Uranus in infrared wavelengths, and likely are connected to storm activity.This planet is characterized as an ice giant due to the chemical make-up of its interior. Most of its mass is thought to be a hot, dense fluid of "icy" materials -- water, methane, and ammonia -- above a small rocky core. Uranus has 13 known rings and 11 of them are visible in the Webb image. Some of these rings are so bright with Webb that when they are close together, they appear to merge into a larger ring. Nine are classed as the main rings of the planet, and two are the fainter dusty rings (such as the diffuse zeta ring closest to the planet) that weren't discovered until the 1986 flyby by Voyager 2. Scientists expect that future Webb images of Uranus will reveal the two faint outer rings that were discovered with Hubble during the 2007 ring-plane crossing. Webb also captured many of Uranus' 27 known moons. In 2022, the National Academies of Sciences, Engineering, and Medicine identified Uranus science as a priority in its 2023-2033 Planetary Science and Astrobiology decadal survey. Additional studies of Uranus are happening now, and more are planned in Webb's first year of science operations.

TWINKLING STARS FUEL INTERSTELLAR DUST

University of Tokyo

Of the many different kinds of stars, asymptotic giant branch (AGB) stars, usually slightly larger and older than our own Sun, are known producers of interstellar dust. Dusty AGBs are particularly prominent producers of dust, and the light they shine happens to vary widely. For the first time, a long-period survey has found the variable intensity of dusty AGBs coincides with variations in the amount of dust these stars produce. As this dust can lead to the creation of planets, its study can shed light on our own origins. Llong before the JWST took to the skies, two other IR space telescopes, AKARI and WISE, have been surveying the cosmos, both of which have ended their initial missions, but produced so much valuable data that astronomers are still finding new discoveries with it. The latest finding from that could have implications for the study of the origins of life itself. Interstellar dust is not the same stuff that accumulates on your floor when you forget to vacuum for a few days; it's a name given to heavy elements that disperse from stars and lead to the formation of solid objects including planets. Although it's long been known that AGBs, and especially so-called dusty AGBs, are the main producers of dust, it's not known what the main drivers of dust production are and where we should be looking to find this out.

Thanks to long-period IR observations, astronomers have found that the light from dusty AGBs varies with periods longer than several hundred days. They also found that the spherical shells of dust produced by and then ejected by these stars have concentrations of dust that vary in step with the stars' changes in luminosity. Of the 169 dusty AGBs surveyed, no matter their variability period, the concentrations of dust around them would coincide. So, we're certain these are connected. Finding a connection between the concentration of dust and the variability of stars' brightness is just the first step in this investigation however. Now the team wishes to explore the possible physical mechanisms behind the production of dust. For this, they intend to monitor various AGB stars for many years continuously. The University of Tokyo is nearing completion of a large ground-based telescope project, the University of Tokyo Atacama Observatory, in Chile, which will be dedicated to making infrared observations.

DO EARTH-LIKE EXOPLANETS HAVE MAGNETIC FIELDS?

National Science Foundation

Earth's magnetic field does more than keep everyone's compass needles pointed in the same direction. It also helps preserve Earth's sliver of life-sustaining atmosphere by deflecting high energy particles and plasma regularly blasted out of the sun. Researchers have now identified a prospective Earth-sized planet in another solar system as a prime candidate for also having a magnetic field -- YZ Ceti b, a rocky planet orbiting a star about 12 light-years away from Earth. Researchers observed a repeating radio signal emanating from the star YZ Ceti using the Karl G. Jansky Very Large Array, a radio telescope operated by the U.S. National Science Foundation's National Radio Astronomy Observatory. The search for potentially habitable or life-bearing worlds in other solar systems depends in part on being able to determine if rocky, Earth-like exoplanets actually have magnetic fields and this research shows not only that this particular rocky exoplanet likely has a magnetic field but provides a promising method to find more. A planet's magnetic field can prevent that planet's atmosphere from being worn away over time by particles spewed from its star. The researchers theorize that the stellar radio waves they detected are generated by the interactions between the magnetic field of the exoplanet and the star it orbits. However, for such radio waves to be detectable over long distances, they must be very strong. While magnetic fields have previously been detected on massive Jupiter-size exoplanets, doing so for a comparatively tiny Earth-sized exoplanet requires a different technique. Because magnetic fields are invisible, it's challenging to determine if a distant planet actually has one. What astronomers are doing is looking for a way to see them, looking for planets that are really close to their stars and are a similar size to Earth. These planets are way too close to their stars to be somewhere you could live, but because they are so close the planet is passing through plasma coming from the star. If the planet has a magnetic field and it ploughs through enough plasma it will cause the star to emit bright radio waves.

The small red dwarf star YZ Ceti and its known exoplanet, YZ Ceti b, provided an ideal pair because the exoplanet is so close to the star that it completes a full orbit in only two days. (By comparison, the shortest planetary orbit in our solar system is Mercury's at 88 days.) As plasma from YZ Ceti careens off the planet's magnetic "plough," it then interacts with the magnetic field of the star itself, which generates radio waves strong enough to be observed on Earth. The strength of those radio waves can then be measured, allowing researchers to determine how strong the magnetic field of the planet might be. This is telling us new information about the environment around stars. The Sun's high energy particles and sometimes huge bursts of plasma create solar weather closer to home, around Earth. Those ejections from the Sun can disrupt global telecommunications and short-circuit electronics in satellites and even on Earth's surface. The interaction between solar weather and Earth's magnetic field and atmosphere also creates the phenomenon of the aurora borealis, or northern lights. The interactions between YZ Ceti b and its star also produce an aurora, but with a significant difference: The aurora is on the star itself.

NEW DETAILS SEEN IN CASSIOPEIA  A

NASA/Goddard Space Flight Center

The explosion of a star is a dramatic event, but the remains the star leaves behind can be even more dramatic. A new mid-infrared image from NASA's James Webb Space Telescope provides one stunning example. It shows the supernova remnant Cassiopeia A (Cas A), created by a stellar explosion seen from Earth 340 years ago. Cas A is the youngest known remnant from an exploding, massive star in our galaxy, which makes it a unique opportunity to learn more about how such supernovae occur. Cassiopeia A is a prototypical supernova remnant that has been widely studied by a number of ground-based and space-based observatories, including NASA's Chandra X-ray Observatory. The multi-wavelength observations can be combined to provide scientists with a more comprehensive understanding of the remnant. The striking colours of the new Cas A image, in which infrared light is translated into visible-light wavelengths, hold a wealth of scientific information the team is just beginning to tease out. On the bubble's exterior, particularly at the top and left, lie curtains of material appearing orange and red due to emission from warm dust. This marks where ejected material from the exploded star is ramming into surrounding circumstellar gas and dust. Interior to this outer shell lie mottled filaments of bright pink studded with clumps and knots. This represents material from the star itself, which is shining due to a mix of various heavy elements, such as oxygen, argon, and neon, as well as dust emission. The stellar material can also be seen as fainter wisps near the cavity's interior. Perhaps most prominently, a loop represented in green extends across the right side of the central cavity.

Among the science questions that Cas A may help answer is: Where does cosmic dust come from? Observations have found that even very young galaxies in the early Universe are suffused with massive quantities of dust. It's difficult to explain the origins of this dust without invoking supernovae, which spew large quantities of heavy elements (the building blocks of dust) across space. However, existing observations of supernovae have been unable to conclusively explain the amount of dust we see in those early galaxies. By studying Cas A with Webb, astronomers hope to gain a better understanding of its dust content, which can help inform our understanding of where the building blocks of planets and ourselves are created. Supernovae like the one that formed Cas A are crucial for life as we know it. They spread elements like the calcium we find in our bones and the iron in our blood across interstellar space, seeding new generations of stars and planets. The Cas A remnant spans about 10 light-years and is located 11,000 light-years away in the constellation Cassiopeia.

ULTRA-LUMINOUS X-RAY SOURCES

NASA /JPL

Exotic cosmic objects known as ultra-luminous X-ray sources produce about 10 million times more energy than the Sun. They’re so radiant, in fact, that they appear to surpass a physical boundary called the Eddington limit, which puts a cap on how bright an object can be based on its mass. Ultra-luminous X-ray sources (ULXs, for short) regularly exceed this limit by 100 to 500 times, leaving scientists puzzled. In a recent study published in The Astrophysical Journal, researchers report a first-of-its-kind measurement of a ULX taken with NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR). The finding confirms that these light emitters are indeed as bright as they seem and that they break the Eddington limit. A hypothesis suggests this limit-breaking brightness is due to the ULX’s strong magnetic fields. But scientists can test this idea only through observations: Up to billions of times more powerful than the strongest magnets ever made on Earth, ULX magnetic fields can’t be reproduced in a lab. Particles of light, called photons, exert a small push on objects they encounter. If a cosmic object like a ULX emits enough light per square foot, the outward push of photons can overwhelm the inward pull of the object’s gravity. When this happens, an object has reached the Eddington limit, and the light from the object will theoretically push away any gas or other material falling toward it.

TINY GALAXY GENERATES HIGH RATE OF STARS

University of Minnesota

Using first-of-their-kind observations from the James Webb Space Telescope, a team of astronomers looked more than 13 billion years into the past to discover a unique, minuscule galaxy that generated new stars at an extremely high rate for its size. The galaxy is one of the smallest ever discovered at this distance—around 500 million years after the Big Bang—and could help astronomers learn more about galaxies that were present shortly after the Universe came into existence. The researchers were one of the first teams to study a distant galaxy using the James Webb Space Telescope, and its findings will be among the first ever published. This galaxy is far beyond the reach of all telescopes except the James Webb, and these first-of-their-kind observations of the distant galaxy are spectacular. The James Webb telescope can observe a wide enough field to image an entire galaxy cluster at once. The researchers were able to find and study this new, tiny galaxy because of a phenomenon called gravitational lensing—where mass, such as that in a galaxy or galaxy cluster, bends and magnifies light. A galaxy cluster lens caused this small background galaxy to appear 20 times brighter than it would if the cluster were not magnifying its light. The researchers then used spectroscopy to measure how far away the galaxy was, in addition to some of its physical and chemical properties. Studying galaxies that were present when the Universe was this much younger can help scientists get closer to answering a huge question in astronomy regarding how the Universe became reionized. The galaxies that existed when the Universe was in its infancy are very different from what we see in the nearby Universe now and this discovery can help us learn more about the characteristics of those first galaxies, how they differ from nearby galaxies, and how the earlier galaxies formed. The James Webb telescope can collect about 10 times as much light as the Hubble Space Telescope and is much more sensitive at redder, longer wavelengths in the infrared spectrum. This allows scientists to access an entirely new window of data. We're seeing things that previous telescopes would have ever been able to capture.

60-YEAR MYSTERY OF QUASARS SOLVED

University of Sheffield

Scientists have unlocked one of the biggest mysteries of quasars -- the brightest, most powerful objects in the Universe -- by discovering that they are ignited by galaxies colliding. First discovered 60 years ago, quasars can shine as brightly as a trillion stars packed into a volume the size of our Solar System. In the decades since they were first observed, it has remained a mystery what could trigger such powerful activity. New work led by scientists at the Universities of Sheffield and Hertfordshire has now revealed that it is a consequence of galaxies crashing together. The collisions were discovered when researchers, using deep imaging observations from the Isaac Newton Telescope in La Palma, observed the presence of distorted structures in the outer regions of the galaxies that are home to quasars. Most galaxies have supermassive black holes at their centres. They also contain substantial amounts of gas -- but most of the time this gas is orbiting at large distances from the galaxy centres, out of reach of the black holes. Collisions between galaxies drive the gas towards the black hole at the galaxy centre; just before the gas is consumed by the black hole, it releases extraordinary amounts of energy in the form of radiation, resulting in the characteristic quasar brilliance.

The ignition of a quasar can have dramatic consequences for entire galaxies -- it can drive the rest of the gas out of the galaxy, which prevents it from forming new stars for billions of years into the future. This is the first time that a sample of quasars of this size has been imaged with this level of sensitivity. By comparing observations of 48 quasars and their host galaxies with images of over 100 non-quasar galaxies, researchers concluded that galaxies hosting quasars are approximately three times as likely to be interacting or colliding with other galaxies. The study has provided a significant step forward in our understanding of how these powerful objects are triggered and fuelled. Quasars are one of the most extreme phenomena in the Universe, and what we see is likely to represent the future of our own Milky Way galaxy when it collides with the Andromeda galaxy in about five billion years. Quasars are important to astrophysicists because, due to their brightness, they stand out at large distances and therefore act as beacons to the earliest epochs in the history of the Universe. One of the main scientific motivations for NASA's James Webb Space Telescope was to study the earliest galaxies in the Universe, and Webb is capable of detecting light from even the most distant quasars, emitted nearly 13 billion years ago. Quasars play a key role in our understanding of the history of the Universe, and possibly also the future of the Milky Way."