WHAT MAKES SATURN’S ATMOSPHERE SO HOT?
University of Arizona
The upper layers in the atmospheres of gas giants -- Saturn, Jupiter, Uranus and Neptune -- are hot, just like Earth's. But unlike Earth, the Sun is too far from these outer planets to account for the high temperatures. Their heat source has been one of the great mysteries of planetary science. New analysis of data from the Cassini spacecraft finds a viable explanation for what's keeping the upper layers of Saturn, and possibly the other gas giants, so hot: auroras at the planet's north and south
poles. Electric currents, triggered by interactions between solar winds and charged particles from Saturn's moons, spark the auroras and heat the upper atmosphere. (As with Earth's northern lights, studying auroras tells scientists what's going on in the planet's atmosphere.) The work is the most complete mapping yet of both temperature and density of a gas giant's upper atmosphere -- a region that has been poorly understood. By building a complete picture of how heat circulates in the atmosphere, scientists are better able to understand how auroral electric currents heat the upper layers of Saturn's atmosphere and drive winds. The global wind system can distribute this energy, which is initially deposited near the poles toward the equatorial regions, heating them to twice the temperatures expected from the Sun's heating alone. Cassini observed Saturn for more than 13 years before exhausting its fuel supply. The mission plunged it into the planet's atmosphere in September 2017, in part to protect its moon Enceladus, which Cassini discovered might hold conditions suitable for life. But before its plunge, Cassini performed 22 ultra-close orbits of Saturn, a final tour called the Grand Finale.
It was during the Grand Finale that the key data was collected for the new temperature map of Saturn's atmosphere. For six weeks, Cassini targeted several bright stars in the constellations of Orion and Canis Major as they passed behind Saturn. As the spacecraft observed the stars rise and set behind the giant planet, scientists analysed how the starlight changed as it passed through the atmosphere. Measuring how dense the atmosphere is gave scientists the information they needed to find the temperatures. Density decreases with altitude, and the rate of decrease depends on temperature. They found that temperatures peak near the auroras, indicating that auroral electric currents heat the upper atmosphere. Density and temperature measurements together helped scientists figure out wind speeds. Understanding Saturn's upper atmosphere, where planet meets space, is key to understanding space weather and its impact on other planets in our solar system and exoplanets around other stars.
‘OUMUAMA - NEW FORMATION THEORY FOR INTERSTELLAR OBJECT
University of California - Santa Cruz
Since its discovery in 2017, an air of mystery has surrounded the first known interstellar object to visit our solar system, an elongated, cigar-shaped body named 'Oumuamua (Hawaiian for "a messenger from afar arriving first"). How was it formed, and where did it come from? A new study offers a first comprehensive answer to these questions. Astronomers used computer simulations to show how objects like 'Oumuamua can form under the influence of tidal forces like those felt by Earth's oceans. Their formation theory explains all of 'Oumuamua's unusual characteristics. They showed that 'Oumuamua-like interstellar objects can be produced through extensive tidal fragmentation during close encounters of their parent bodies with their host stars, and then ejected into interstellar space. Discovered on October 19, 2017, by the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1) in Hawaii, 'Oumuamua is absolutely nothing like anything else in our solar system. Its dry surface, unusually elongated shape, and puzzling motion even drove some scientists to wonder if it was an alien probe. It is really a mysterious object, but some signs, like its colours and the absence of
radio emission, point to 'Oumuamua being a natural object. Astronomers had expected that the first interstellar object they detected would be an icy body like a comet. Icy objects like those populating the Oort cloud, a reservoir of comets in the outermost reaches of our solar system, evolve at very large distances from their host stars, are rich in volatiles, and are often tossed out of their host systems by gravitational interactions. They are also highly visible due to the sublimation of volatile compounds, which creates a comet's coma (or "tail") when it is warmed by the Sun. 'Oumuamua's dry appearance, however, is similar to rocky bodies like the solar system's asteroids, indicating a different ejection scenario.
Other researchers have calculated that there must be an extremely large population of interstellar objects like 'Oumuamua. The discovery of 'Oumuamua implies that the population of rocky interstellar objects is much larger than we previously thought. On average, each planetary system should eject in total about a hundred trillion objects like 'Oumuamua. When a smaller body passes very close to a much bigger one, tidal forces of the larger body can tear the smaller one apart, as happened to comet Shoemaker-Levy 9 when it came close to Jupiter. The tidal disruption processes can eject some debris into interstellar space, which has been suggested as a possible origin for 'Oumuamua. But whether such a process could explain 'Oumuamua's puzzling characteristics remained highly uncertain. The team found that if the object comes close enough to the star, the star can tear it into extremely elongated fragments that are then ejected into the interstellar space. The elongated shape is more compelling when the variation of material strength during the stellar encounter was considered. The ratio of long axis to short axis can be even larger than ten to one. The researchers' thermal modelling showed that the surface of fragments resulting from the disruption of the initial body would melt at a very short distance from the star and re-condense at greater distances, thereby forming a cohesive crust that would ensure the structural stability of the elongated shape. Heat diffusion during the stellar tidal disruption process also consumes large amounts of volatiles, which not only explains 'Oumuamua's surface colours and the absence of visible coma, but also elucidates the inferred dryness of the interstellar population. Nevertheless, some high-sublimation-temperature volatiles buried under the surface, like water ice, can remain in a condensed form. Observations of 'Oumuamua showed no cometary activity, and only water ice is a possible outgassing source to account for its non-gravitational motion. If 'Oumuamua was produced and ejected by the scenario, plenty of residual water ice could be activated during its passage through this solar system. The resulting outgassing would cause accelerations that match 'Oumuamua's comet-like trajectory. The tidal fragmentation scenario not only provides a way to form one single 'Oumuamua, but also accounts for the vast population of asteroid-like interstellar objects. The researchers' calculations demonstrate the efficiency of tidal forces in producing this kind of object. Possible progenitors, including long-period comets, debris disks, and even super-Earths, could be transformed into 'Oumuamua-size pieces during stellar encounters. This work supports estimates of a large population of 'Oumuamua-like interstellar objects. Since these objects may pass through the domains of habitable zones, the possibility that they could transport matter capable of generating life (called panspermia) cannot be ruled out. These interstellar objects could provide critical clues about how planetary systems form and evolve.
BROWN DWARF WIND SPEED MEASURED
NASA/Jet Propulsion Laboratory
For the first time, scientists have directly measured wind speed on a brown dwarf, an object larger than Jupiter but not quite massive enough to become a star. To achieve the finding, they used a new method that could also be applied to learn about the atmospheres of gas-dominated planets outside our solar system. The work combines observations by a group of radio telescopes with data from the recently retired infrared observatory, the Spitzer Space Telescope. Officially named 2MASS J10475385+2124234, the target of the new study was a brown dwarf located 32 light-years from Earth. The researchers detected winds moving around the planet at 2,293 kph. For comparison, Neptune's atmosphere features the fastest winds in the solar system, which whip through at more than 2,000 kph. Measuring wind speed on Earth means clocking the motion of our gaseous atmosphere relative to the planet's solid surface. But brown dwarfs are composed almost entirely of gas, so "wind" refers to something slightly different. The upper layers of a brown dwarf are where portions of the gas can move independently. At a certain depth, the pressure becomes so intense that the gas behaves like a single, solid ball that is considered the object's interior. As the interior rotates, it pulls the upper layers -- the atmosphere -along so that the two are almost in synch. In their study, the researchers measured the slight difference in speed of the brown dwarf's atmosphere relative to its interior. With an atmospheric temperature of over 600 degrees Celsius, this particular brown dwarf radiates a substantial amount of
infrared light. Coupled with its close proximity to Earth, this characteristic made it possible for Spitzer to detect features in the brown dwarf's atmosphere as they rotate in and out of view. The team used those features to clock the atmospheric rotation speed. To determine the speed of the interior, they focused on the brown dwarf's magnetic field. A relatively recent discovery found that the interiors of brown dwarfs generate strong magnetic fields. As the brown dwarf rotates, the magnetic field accelerates charged particles that in turn produce radio waves, which the researchers detected with the radio telescopes in the Karl G. Jansky Very Large Array in New Mexico.
The new study is the first to demonstrate this comparative method for measuring wind speed on a brown dwarf. To gauge its accuracy, the group tested the technique using infrared and radio observations of Jupiter, which is also composed mostly of gas and has a physical structure similar to a small brown dwarf. The team compared the rotation rates of Jupiter's atmosphere and interior using data that was similar to what they were able to collect for the much more distant brown dwarf. They then confirmed their calculation for Jupiter's wind speed using more detailed data collected by probes that have studied Jupiter up close, thus demonstrating that their approach for the brown dwarf worked. Scientists have previously used Spitzer to infer the presence of winds on exoplanets and brown dwarfs based on variations in the brightness of their atmospheres in infrared light. And data from the High Accuracy Radial velocity Planet Searcher (HARPS) -- an instrument on the European Southern Observatory's La Silla telescope in Chile -- has been used to make a direct measurement of wind speeds on a distant planet. But the new paper represents the first time scientists have directly compared the atmospheric speed with the speed of a brown dwarf's interior. The method employed could be applied to other brown dwarfs or to large planets if the conditions are right, according to the
authors. The Spitzer Space Telescope was decomissioned on Jan. 30, 2020, after more than 16 years in space.
SUPERNOVA OUTSHINES ALL OTHERS
University of Birmingham
A supernova at least twice as bright and energetic, and likely much more massive than any yet recorded has been identified by an international team of astronomers, led by the University of Birmingham. The team believe the supernova, dubbed SN2016aps, could be an example of an extremely rare 'pulsational pair-instability' supernova, possibly formed from two massive stars that merged before the explosion. Such an event so far only exists in theory and has never been confirmed
through astronomical observations. In a typical supernova, the radiation is less than 1 per cent of the total energy. But in SN2016aps, the radiation was five times the explosion energy of a normal-sized supernova. This is the most light ever seen emitted by a supernova. In order to become this bright, the explosion must have been much more energetic than usual. By examining the light spectrum, the team were able to show that the explosion was powered by a collision between the supernova and a massive shell of gas, shed by the star in the years before it exploded. While many supernovae are discovered, most are in massive galaxies. This one immediately stood out for further observations because it seemed to be in the middle of nowhere. Astronomers weren't able to see the galaxy where this star was born until after the supernova light had faded. The team observed the explosion for two years, until it faded to 1 per cent of its peak brightness. Using these measurements, they calculated the mass of the supernova was between 50 to 100 times greater than our Sun (solar masses). Typically supernovae have masses of between 8 and 15 solar masses.
Stars with extremely large mass undergo violent pulsations before they die, shaking off a giant gas shell. This can be powered by a process called the pair instability, which has been a topic of speculation for physicists for the last 50 years. If the supernova gets the timing right, it can catch up to this shell and release a huge amount of energy in the collision. We think this is one of the most compelling candidates for this process yet observed, and probably the most massive. SN2016aps also contained another puzzle. The gas detected was mostly hydrogen -- but such a massive star would usually have lost all of its hydrogen via stellar winds long before it started pulsating. One explanation is that two slightly less massive stars of around, say 60 solar masses, had merged before the explosion. The lower mass stars hold onto their hydrogen for longer, while their combined mass is high enough to trigger the pair instability. Now that we know such energetic explosions occur in nature, NASA's new James Webb Space Telescope will be able to see similar events so far away that we can look back in time to the deaths of the very first stars in the Universe.
LINK BETWEEN DARK MATTER HALOS AND GALAXY FORMATION
DOE/SLAC National Accelerator Laboratory
Just as the Sun has planets and the planets have moons, our galaxy has satellite galaxies, and some of those might have smaller satellite galaxies of their own. The Large Magellanic Cloud (LMC), a relatively large satellite galaxy visible from the Southern Hemisphere, is thought to have brought at least six of its own satellite galaxies with it when it first approached the Milky Way, based on recent measurements from the European Space Agency's Gaia mission. Astrophysicists believe that dark matter is responsible for much of that structure, and now researchers at the Department of Energy's SLAC National Accelerator Laboratory and the Dark Energy Survey have drawn on observations of faint galaxies around the Milky Way to place tighter constraints on the connection between the size and structure of galaxies and the dark matter halos that surround them. At the same time, they have found more evidence for the existence of LMC satellite galaxies and made a new prediction: If the scientists' models are correct, the Milky Way should have an additional 150 or more very faint satellite galaxies awaiting discovery by next-generation projects such as the Vera C. Rubin Observatory's Legacy Survey of Space and Time. The new study is part of a larger effort to understand how dark matter works on scales smaller than our galaxy. Astronomers have long known the Milky Way has satellite galaxies, including the Large Magellanic Cloud, which can be seen by the naked eye from the Southern Hemisphere, but the number was thought to be around just a dozen or so until around the year 2000. Since then, the number of observed satellite galaxies has risen dramatically. Thanks to the Sloan Digital Sky Survey and more recent discoveries by projects including the Dark Energy Survey (DES), the number of known satellite galaxies has climbed to about 60. Such discoveries are always exciting, but what's perhaps most exciting is what the data could tell us about the cosmos. The DES team are using data from a comprehensive search over most of the sky to ask different questions, including how much dark matter it takes to form a galaxy, how many satellite galaxies we should expect to find around the Milky Way and whether galaxies can bring their own satellites into orbit around our own -- a key prediction of the most popular model of dark matter.
The possibility of detecting a hierarchy of satellite galaxies first arose some years back when DES detected more satellite galaxies in the vicinity of the Large Magellanic Cloud than they would have expected if those satellites were randomly distributed throughout the sky. Those observations are particularly interesting in light of the Gaia measurements, which indicated that six of these satellite galaxies fell into the Milky Way with the LMC. To study the LMC's satellites more thoroughly, the team analysed computer simulations of millions of possible universes. Those simulations model the formation of dark matter structure that permeates the Milky Way, including details such as smaller dark matter clumps within the Milky Way that are expected to host satellite galaxies. To connect dark matter to galaxy formation, the researchers used a flexible model that allows them to account for uncertainties in the current understanding of galaxy formation, including the relationship between galaxies' brightness and the mass of dark matter clumps within which they form. Astronomers produced the crucial final step: a model of which satellite galaxies are most likely to be seen by current surveys, given where they are in the sky as well as their brightness, size and distance. Those components in hand, the team ran their model with a wide range of parameters and searched for simulations in which LMC-like objects fell into the gravitational pull of a Milky Way-like galaxy. By comparing those cases with galactic observations, they could infer a range of astrophysical parameters, including how many satellite galaxies should have tagged along with the LMC. The results were consistent with Gaia observations: Six satellite galaxies should currently be detected in the vicinity of the LMC, moving with roughly the right velocities and in roughly the same places as astronomers had previously observed. The simulations also suggested that the LMC first approached the Milky Way about 2.2 billion years ago, consistent with high-precision measurements of the motion of the LMC from the Hubble Space Telescope. In addition to the LMC findings, the team also put limits on the connection between dark matter halos and galaxy structure. For example, in simulations that most closely matched the history of the Milky Way and the LMC, the smallest galaxies astronomers could currently observe should have stars with a combined mass of around a hundred suns, and about a million times as much dark matter. According to an extrapolation of the model, the faintest galaxies that could ever be observed could form in halos up to a hundred times less massive than that. And there could be more discoveries to come: If the simulations are correct, there are around 100 more satellite galaxies -- more than double the number already discovered -- hovering around the Milky Way. The discovery of those galaxies would help confirm the researchers' model of the links between dark matter and galaxy formation, he said, and likely place tighter constraints on the nature of dark matter itself.
ANDROMEDA AND MILKY WAY ALREADY TOUCHING
Physics-Astronomy.com
The Milky Way and Andromeda galaxy won’t collide for next 4 billion years. But a recent discovery of a massive halo of hot gas close to Andromeda Galaxy may mean that our galaxies are already touching. A group of scientists using the Hubble Space Telescope have detected an enormous halo of hot, ionized gas about 2 million light years in diameter around the galaxy. The Andromeda Galaxy and Milky Way are the largest members of a group of some 54 galaxies, called the Local Group. Andromeda, with almost a trillion stars — twice as many as the Milky Way — shines 25% brighter and can be seen with the naked eye from outlying and rural skies. If the recently discovered halo spreads at least a million light years in our direction, our two galaxies are much closer to touching than previously thought. Halos are the “gaseous atmospheres of galaxies”. Regardless of its huge size, Andromeda’s nimbus is almost invisible. To observe and study the halo, astronomers sought out quasars, distant star-like objects that emit incredible amounts of energy as matter is sucked into the supermassive black holes. The brightest quasar, 3C273 in Virgo, can be easily observed with a 6-inch telescope. Quasars’ bright, pinpoint nature make them perfect probes. As the light from the quasars travels toward Hubble, the halo’s gas will absorb some of that light and make the quasar appear a little darker in just a very small wavelength range. By measuring the dip in brightness, we can tell how much halo gas from M31 there is between us and that quasar.
Astronomers have studied halos around 44 other galaxies but they never discovered one as massive as Andromeda where so many quasars are accessible to clearly outline its extent. The previous 44 were all very distant galaxies, with only a lone quasar or data point to regulate halo size and structure. The halo is projected to comprise half the mass of the stars in the Andromeda galaxy itself, in the form of a hot, rambling gas. Simulations propose that it was formed at the same time as the rest of the Andromeda galaxy. Even though mostly composed of ionized hydrogen — bare protons and electrons — Andromeda’s aura is rich in heavier elements, possibly supplied by supernovae that explode inside the visible galaxy and aggressively blow iron, silicon, oxygen and other similar elements far into space. Previous researches have shown that over Andromeda’s lifetime, almost half of all the heavy elements assembled by its stars have been ejected far outside the galaxy’s 200,000-light-year-diameter stellar disk.
POWERFUL JET EMERGING FROM COLLIDING GALAXIES
Clemson University
A team of researchers has reported the first definitive detection of a relativistic jet emerging from two colliding galaxies -- in essence, the first photographic proof that merging galaxies can produce jets of charged particles that travel at nearly the speed of light. Furthermore, scientists had previously discovered that these jets could be found in elliptical-shaped galaxies, which can be formed in the merging of two spiral galaxies. Now, they have an image showing the formation of a jet from two younger, spiral-shaped galaxies. The fact that the jet is so young enabled the researchers to clearly see its host. Typically, a jet emits light that is so powerful we can't see the galaxy behind it. Jets are the most powerful astrophysical phenomena in the universe. They can emit more energy into the universe in one second than our Sun will produce in its entire lifetime. That energy is in the form of radiation, such as intense radio waves, X-rays, and gamma-rays. Jets were thought to be born from older, elliptical-shaped galaxies with an active galactic nucleus (AGN), which is a super-massive black hole that resides at its centre. As a point of reference, scientists believe all galaxies have centrally located super-massive black holes, but not all of them are AGNs. For example, our Milky Way's massive black hole is dormant. Scientists theorize that the AGNs grow larger by gravitationally drawing in gas and dust through a process called accretion. But not all of this matter gets accreted into the black hole. Some of the particles become accelerated and are spewed outward in narrow beams in the form of jets.
It's hard to dislodge gas from the galaxy and have it reach its centre. You need something to shake the galaxy a little bit to make the gas get there. The merging or colliding of galaxies is the easiest way to move the gas, and if enough gas moves, then the super-massive black hole will become extremely bright and could potentially develop a jet. The team believes that the image captured the two galaxies, a Seyfert 1 galaxy known as TXS 2116-077 and another galaxy of similar mass, as they were colliding for the second time because of the amount of gas seen in the image. Eventually, all the gas will be expelled into space, and without gas, a galaxy cannot form stars anymore. Without gas, the black hole will switch off and the galaxy will lay dormant. Billions of years from now, our own Milky Way will merge with the nearby Andromeda galaxy. Scientists have carried out detailed numerical simulations and predicted that this event may ultimately lead to the formation of one giant elliptical galaxy. Depending on the physical conditions, it may host a relativistic jet, but that's in the distant future. The team captured the image using one of the largest land-based telescopes in the world, the Subaru 8.2-meter optical infrared telescope located on a mountain summit in Hawaii. They performed subsequent observations with the Gran Telescopio Canarias and William Herschel Telescope on the island of La Palma off the coast of Spain, as well as with NASA's Chandra X-Ray Observatory space telescope.
MOST ENERGETIC FLOW FROM DISTANT QUASAR DETECTED
Association of Universities for Research in Astronomy (AURA)
Researchers using the Gemini North telescope on Hawai'i's Maunakea have detected the most energetic wind from any quasar ever measured. This outflow, which is travelling at nearly 13% of the speed of light, carries enough energy to dramatically impact star formation across an entire galaxy. The quasar is known as SDSS J135246.37+423923.5. As well as measuring its outflow, the team was also able to infer the mass of the supermassive black hole powering the quasar. This monstrous object is 8.6 billion times as massive as the Sun -about 2000 times the mass of the black hole in the centre of our Milky Way and 50% more massive than the well-known black hole in the galaxy Messier 87. Despite its mass and energetic outflow, the discovery of this powerhouse languished in a quasar survey for 15 years before the combination of Gemini data and the team's innovative computer modelling method allowed it to be studied in detail. Quasars -- also known as quasi-stellar objects -- are a type of extraordinarily luminous astrophysical object residing in the centres of massive galaxies. Consisting of a supermassive black hole surrounded by a glowing disk of gas, quasars can outshine all the stars in their host galaxy and can drive winds powerful enough to influence entire galaxies. SDSS J135246.37+423923.5 is a particularly windy quasar, whose outflow is so thick that it's difficult to detect the signature of the quasar itself at visible wavelengths. Despite the obstruction, the team was able to get a clear view of the quasar using the Gemini Near-Infrared Spectrograph (GNIRS) on Gemini North to observe at infrared wavelengths. Using a combination of high-quality spectra from Gemini and a pioneering computer modelling approach, the astronomers uncovered the nature of the outflow from the object -- which proved, remarkably, to be more energetic than any quasar outflow previously measured.
MERCURY MISSION DEPARTS EARTH
BBC Science
BepiColombo, the joint European-Japanese mission to Mercury, has swung past the Earth - a key milestone in its seven-year journey to reach the "iron planet." The gravitational flyby enabled the two-in-one space probe to bend a path towards the inner Solar System and bleed off some speed. The mission needs to make sure it isn't travelling too fast when it arrives at Mercury in 2025 or it won't be able to go into orbit around the diminutive world. As well as this flyby of Earth, Bepi must perform two similar manoeuvres at Venus and six at Mercury itself to get itself into position. The only alternative would have been to give the spacecraft a colossal volume of fuel to use in a braking engine. An impractical solution. Bepi came within 13,000km of the Earth's surface.
