METEORITES THAT FORMED EARTH FROM OUTER SOLAR SYSTEM
Tokyo Institute of Technology
Our Solar System is believed to have formed from a cloud of gas and dust, the so-called solar nebula, which began to condense on itself gravitationally ~ 4.6 billion years ago. As this cloud contracted, it began to spin and shaped itself into a disk revolving about the highest gravity mass at its centre, which would become our Sun. Our solar system inherited all of its chemical composition from an earlier star or stars which exploded as supernovae. Our Sun scavenged a general sample of this material as it formed, but the residual material in the disk began to migrate based on its propensity to freeze at a given temperature. As the Sun grew dense enough to initiate nuclear fusion reactions and become a star, it scavenged a general sample of this material as it formed, but the residuals in the disk formed solid materials to form planetary bodies based on its propensity to freeze at a given temperature. As the Sun irradiated the surrounding disk, it created a heat gradient in the early solar system. For this reason, the inner planets, Mercury, Venus, Earth and Mars, are mostly rock (mostly composed of heavier elements, such as iron, magnesium and silicon), while the outer planets are largely composed of lighter elements, especially hydrogen, helium, carbon, nitrogen and oxygen. Earth is believed to have formed partly from carbonaceous meteorites, which are thought to come from outer main-belt asteroids. Telescopic observations of outer main-belt asteroids reveal a common 3.1 mm reflectance feature that suggests their outer layers host either water ices or ammoniated clays, or both, which are only stable at very low temperatures. Interestingly, though several lines of evidence suggest carbonaceous meteorites are derived from such asteroids, the meteorites recovered on Earth generally lack this feature. The asteroid belt thus poses many questions for astronomers and planetary scientists.
A new study led by researchers at Tokyo Institute of Technology suggests these asteroidal materials may have formed very far out in the early Solar System then been transported into the inner Solar System by chaotic mixing processes. In this study, a combination of asteroid observations using the Japanese AKARI space telescope and theoretical modelling of chemical reactions in asteroids suggests that the surface minerals present on outer main-belt asteroids, especially ammonia (NH3)-bearing clays, form from starting materials containing NH3 and CO2 ice that are stable only at very low temperature, and under water-rich conditions. Based on these results, this new study proposes that outer main-belt asteroids formed in distant orbits and differentiated to form different minerals in water-rich mantles and rock-dominated cores. To understand the source of the discrepancies in the measured spectra of carbonaceous meteorites and asteroids, using computer simulations, the team modelled the chemical evolution of several plausible primitive mixtures designed to simulate primitive asteroidal materials. They then used these computer models to produce simulated reflectance spectra for comparison to the telescopically obtained ones. Their models indicated that in order to match the asteroid spectra, the starting material had to contain a significant amount of water and ammonia, a relatively low abundance of CO2, and react at temperatures below 70K, suggesting the asteroids formed much further out than their present locations in the early solar system. In contrast, the lack of the 3.1 mm feature in meteorites can be attributed to reaction possibly deeper inside asteroids where temperatures reached higher values thus, recovered meteorites may sample deeper portions of asteroids. If true, this study suggests that Earth's formation and unique properties result from peculiar aspects of the Solar System's formation. There will be several opportunities to test this model, for example, this study provides predictions for what the analysis of Hayabusa 2 returned samples will find. This distant origin of asteroids, if correct, predicts that there will be ammoniated salts and minerals in Hayabusa 2's returned samples. A further check on this model will be provided by the analyses of returned materials from NASA's OSIRIS-Rex mission.
RIP COMET LEONARD
Gizmodo
Leonard will not be making its next 80,000-year-long journey around the Sun, as the mile-wide comet appears to have disintegrated in late February. Astronomer Gregory Leonard from the Catalina Sky Survey was the first human to spot the comet, which he did on January 3, 2021, a year before its perihelion on January 2, 2022. This closest approach to the Sun proved to be Leonard’s undoing, as all signs point to the demise of this dirty snowball—the brightest of the previous year. It is now moving away from the Sun and has not only faded but is now missing its two most important parts: its nucleus (core) and its coma (the nebulous envelope surrounding the nucleus, which appears when a comet passes near the Sun. At perihelion, Leonard, also known as Comet C/2021 A1, came to within 90 million kilometres of the Sun. By comparison, Earth during perihelion comes to within 147 million km of the star. The Sun’s warmth brought Leonard to life, causing it to release gas and dust, but the comet’s sojourn through the inner solar system turned out to be its undoing.
Scientists were able to study Leonard using the Solar Terrestrial Relations Observatory-A (STEREO-A), operated by NASA, and Solar Orbiter, a joint project of NASA and ESA. Comets tend to be like snowflakes, as no two are alike in terms of their behaviour, and they’re virtually impossible to predict. When Gregory Leonard first observed the comet, it was still way out near the orbit of Jupiter. Speculation emerged that it might become visible to the unaided eye as it got closer, but this turned out not to be the case, as the comet could only be seen with telescopes and binoculars. And like any comet cruising for a solar flyby, Leonard was at risk of disintegrating, a possibility that appears to have transpired. The 1-mile-wide comet, which had just barely started its 40,0000-year return journey to the outer solar system, seems to have fizzled out. Astronomers began to notice the signs of the comet’s doom around the time of its perihelion. These included fluctuations in its brightness every three to five days and wonky structural changes in its tail—a potential signal that chunks were falling off the nucleus, according to EarthSky. On February 23, Leonard, which could only be seen in the southern hemisphere, had become a fading streak in space, without a glowing head. The most likely scenario is that the nucleus disintegrated or evaporated, or a combination of the two. Regardless, Leonard will go down in history as the brightest comet of 2021. It’s sad to lose a fellow traveller, but comets do appear in the sky with decent regularity. There’s bound to be another one soon, and the next one may even be visible to the naked eye.
NASA PREDICTED ASTEROID IMPACT
NASA
Asteroid 2022 EB5 was too small to pose a hazard to Earth, but its discovery marks the fifth time that any asteroid has been observed before impacting into the atmosphere. A small asteroid hit Earth’s atmosphere over the Norwegian Sea before disintegrating on March 11, 2022. But this event wasn’t a complete surprise: Astronomers knew it was on a collision course, predicting exactly where and when the impact would happen. Two hours before the asteroid made impact, Piszkéstető Observatory in northern Hungary first reported observations of the small object to the Minor Planet Center – the internationally recognized clearinghouse for the position measurements of small celestial bodies. The object was posted on the Minor Planet Center’s Near-Earth Object Confirmation Page to flag it for additional observations that would confirm it as a previously unknown asteroid. NASA’s “Scout” impact hazard assessment system then took these early measurements to calculate the trajectory of 2022 EB5. As soon as Scout determined that 2022 EB5 was going to hit Earth’s atmosphere, the system alerted the Center for Near Earth Object Studies (CNEOS) and NASA’s Planetary Defense Coordination Office, and flagged the object on the Scout webpage to notify the near-Earth object observing community. Maintained by CNEOS at NASA’s Jet Propulsion Laboratory in Southern California, Scout automatically searches the Minor Planet Center’s database for possible new short-term impactors. CNEOS calculates every known near-Earth asteroid orbit to improve impact hazard assessments in support of the Planetary Defense Coordination Office.
Scout had only 14 observations over 40 minutes from one observatory to work with when it first identified the object as an impactor. Astronomers were able to determine the possible impact locations, which initially extended from western Greenland to off the coast of Norway. Fully interactive, Eyes on Asteroids uses science data to help visualize asteroid and comet orbits around the Sun.
ASTEROID RYUGU MAY BE EXTINCT COMET REMNANT
Nagoya City University
Asteroids hold many clues about the formation and evolution of planets and their satellites. Understanding their history can, therefore, reveal much about our solar system. While observations made from a distance using electromagnetic waves and telescopes are useful, analysing samples retrieved from asteroids can yield much more detail about their characteristics and how they may have formed. An endeavour in this direction was the Hayabusa mission, which, in 2010, returned to Earth after 7 years with samples from the asteroid Itokawa. The successor to this mission, called Hayabusa2, was completed near the end of 2020, bringing back material from Asteroid 162173 "Ryugu," along with a collection of images and data gathered remotely from close proximity. While the material samples are still being analysed, the information obtained remotely has revealed three important features about Ryugu. Firstly, Ryugu is a rubble-pile asteroid composed of small pieces of rock and solid material clumped together by gravity rather than a single, monolithic boulder. Secondly, Ryugu is shaped like a spinning top, likely caused by deformation induced by quick rotation. Third, Ryugu has a remarkably high organic matter content. Of these, the third feature raises a question regarding the origin of this asteroid. The current scientific consensus is that Ryugu originated from the debris left by the collision of two larger asteroids. However, this cannot be true if the asteroid is high in organic content (which will confirmed once the analyses of the returned samples are complete). What could, then, be the true origin of Ryugu?
In a recent effort to answer this question, astronomers proposed an alternative explanation backed up by a relatively simple physical model. They suggest that Ryugu, as well as similar rubble-pile asteroids, could, in fact, be remnants of extinct comets. Comets are small bodies that form on the outer, colder regions of the solar system. They are mainly composed of water ice, with some rocky components (debris) mixed in. If a comet enters the inner solar system -- the space delimited by the asteroid belt "before" Jupiter -- heat from the solar radiation causes the ice to sublimate and escape, leaving behind rocky debris that compacts due to gravity and forms a rubble-pile asteroid. This process fits all the observed features of Ryugu. Ice sublimation causes the nucleus of the comet to lose mass and shrink, which increases its speed of rotation. As a result of this spin-up, the cometary nucleus may acquire the rotational speed required for the formation of a spinning-top shape. Additionally, the icy components of comets are thought to contain organic matter generated in the interstellar medium. These organic materials would be deposited on the rocky debris left behind as the ice sublimates. To test their hypothesis, the research team conducted numerical simulations using a simple physical model to calculate the time it would take for the ice to sublimate and the increase in rotational speed of the resulting asteroid due to it. The results of their analysis suggested that Ryugu has likely spent a few tens of thousands of years as an active comet before moving into the inner asteroid belt, where the high temperatures vaporized its ice and turned it into a rubble-pile asteroid. Overall, this study indicates that spinning top-shaped, rubble-pile objects with high organic content, such as Ryugu and Bennu (the target of the OSIRIS-Rex mission) are comet-asteroid transition objects (CATs). "CATs are small objects that were once active comets but have become extinct and apparently indistinguishable from asteroids. ains Dr. Miura. Due to their similarities with both comets and asteroids, CATs could provide new insights into our solar system. Hopefully, detailed compositional analyses of the samples from both Ryugu and Bennu will shed more light on these issues.
COMET 67P’S ABUNDANT OXYGEN AN ILLUSION
Johns Hopkins University Applied Physics Laboratory
When the European Space Agency's Rosetta spacecraft discovered abundant molecular oxygen bursting from comet 67P/Churyumov-Gerasimenko (67P) in 2015, it puzzled scientists. They had never seen a comet emit oxygen, let alone in such abundance. But most alarming were the deeper implications: that researchers had to account for so much oxygen, which meant reconsidering everything they thought they already knew about the chemistry of the early solar system and how it formed. A new analysis however, shows Rosetta's discovery may not be as strange as scientists first imagined. Instead, it suggests the comet has two internal reservoirs that make it seem like there's more oxygen than is actually there. In reality, the comet doesn't have this high oxygen abundance, at least not as far as its formation goes, but it has accumulated oxygen that gets trapped in the upper layers of the comet, which then gets released all at once. While common on Earth, molecular oxygen (two oxygen atoms doubly linked to each other) is markedly uncommon throughout the Universe. It quickly binds to other atoms and molecules, especially the universally abundant atoms hydrogen and carbon, so oxygen appears only in small amounts in just a few molecular clouds. That fact led many researchers to conclude any oxygen in the protosolar nebula that formed our solar system likely had been similarly scooped up. When Rosetta found oxygen pouring out of comet 67P, however, everything turned on its head. Nobody had seen oxygen in a comet before, and as the fourth most abundant molecule in the comet's bright coma (after water, carbon dioxide and carbon monoxide), it needed some explanation. The oxygen seemed to come off the comet with water, causing many researchers to suspect the oxygen was either primordial -- meaning it got tied up with water at the birth of the solar system and amassed in the comet when it later formed -- or formed from water after the comet had formed.
As the comet's dumbbell shape gradually rotates, each "bell" (or hemisphere) faces the Sun at various points, meaning the comet has seasons so the oxygen-water connection might not be present all the time. On short time frames, volatiles could potentially turn on and off as they thaw and refreeze with the seasons. Taking advantage of these seasons, the team examined the molecular data on short- and long-time periods just before the comet's southern hemisphere entered summer and then again just as its summer ended. As reported in their study, the team found that as the southern hemisphere turned away and was sufficiently far from the Sun, the link between oxygen and water disappeared. The amount of water coming off the comet dropped precipitously, so instead the oxygen seemed strongly linked to carbon dioxide and carbon monoxide, which the comet was still emitting. If oxygen were primordial and tied to water in its formation, there shouldn't be any time that oxygen strongly correlates with carbon monoxide and carbon dioxide but not water. The team instead proposed the comet's oxygen doesn't come from water but from two reservoirs: one made of oxygen, carbon monoxide and carbon dioxide deep inside the comet's rocky nucleus, and a shallower pocket closer to the surface where oxygen chemically combines with water ice molecules. The idea goes like this: A deep reservoir of oxygen, carbon monoxide and carbon dioxide ice is constantly emitting gases because oxygen, carbon dioxide and carbon monoxide all vaporize at very low temperatures. As oxygen traverses from the comet's interior toward the surface, however, some chemically inserts into water ice (a major constituent of the comet's nucleus) to form a second, shallower oxygen reservoir. But water ice vaporizes at a much higher temperature than oxygen, so until the Sun sufficiently heats the surface and vaporizes the water ice, the oxygen is stuck. The consequence is that oxygen can accumulate in this shallow reservoir for long periods until the comet surface is finally warmed enough for water ice to vaporize, releasing a plume far richer in oxygen than was actually present in the comet.
NEARBY STAR MIGHT EXPLAIN SUN’S 70 YEAR SUNSPOT LULL.
Penn State
The number of sunspots on our Sun typically ebbs and flows in a predictable 11-year cycle, but one unusual 70-year period when sunspots were incredibly rare has mystified scientists for three hundred years. Now a nearby Sun-like star seems to have paused its own cycles and entered a similar period of rare starspots. Continuing to observe this star could help explain what happened to our own Sun during this "Maunder Minimum" as well as lend insight into the Sun's stellar magnetic activity, which can interfere with satellites and global communications and possibly even affect climate on Earth. The star -- and a catalogue of 5 decades of starspot activity of 58 other Sun-like stars -- is described in a new paper. Starspots appear as a dark spot on a star's surface due to temporary lower temperatures in the area resulting from the star's dynamo -- the process that creates its magnetic field. Astronomers have been documenting changes in starspot frequency on our Sun since they were first observed by Galileo and other astronomers in the 1600s, so there is a good record of its 11-year cycle. The exception is the Maunder Minimum, which lasted from the mid 1600s to early 1700s and has perplexed astronomers ever since. Astronomers e don't really know what caused the Maunder Minimum, and have been looking to other Sun-like stars to see if they can offer some insight. They have identified a star that has entered a state similar to the Maunder Minimum.
They plan to continue to observe this star during, and hopefully as it comes out of, this minimum, which could be extremely informative about the Sun's activity three hundred years ago. The research team pulled data from multiple sources to stitch together 50 to 60 years of starspot data for 59 stars. This included data from the Mount Wilson Observatory HK Project -- which was designed to study stellar surface activity and ran from 1966 to 1996 -- and from planet searches at Keck Observatory which include this kind of data as part of their ongoing search for exoplanets from 1996 to 2020. The researchers compiled a database of stars that appeared in both sources and that had other readily available information that might help explain starspot activity. The team also made considerable efforts to standardize measurements from the different telescopes to be able to compare them directly and otherwise clean up the data. The team identified or confirmed that 29 of these stars have starspot cycles by observing at least two full periods of cycles, which often last more than a decade. Some stars did not appear to have cycles at all, which could be because they are rotating too slowly to have a dynamo and are magnetically 'dead' or because they are near the end of their lives. Several of the stars require further study to confirm whether they have a cycle. According to the researchers, the star -- called HD 166620 -- was estimated to have a cycle of about 17 years but has now entered a period of low activity and has shown no signs of starspots since 2003.
NASA CONFIRMS 5,000 EXOPLANETS
NASA
Not so long ago, we lived in a Universe with only a small number of known planets, all of them orbiting our Sun. But a new raft of discoveries marks a scientific high point: More than 5,000 planets are now confirmed to exist beyond our solar system. The planetary odometer turned on March 21, with the latest batch of 65 exoplanets – planets outside our immediate solar family – added to the NASA Exoplanet Archive. The archive records exoplanet discoveries that appear in peer-reviewed, scientific papers, and that have been confirmed using multiple detection methods or by analytical techniques. The 5,000-plus planets found so far include small, rocky worlds like Earth, gas giants many times larger than Jupiter, and “hot Jupiters” in scorchingly close orbits around their stars. There are “super-Earths,” which are possible rocky worlds bigger than our own, and “mini-Neptunes,” smaller versions of our system’s Neptune. Add to the mix planets orbiting two stars at once and planets stubbornly orbiting the collapsed remnants of dead stars. Our galaxy likely holds hundreds of billions of such planets. The steady drumbeat of discovery began in 1992 with strange new worlds orbiting an even stranger star. It was a type of neutron star known as a pulsar, a rapidly spinning stellar corpse that pulses with millisecond bursts of searing radiation. Measuring slight changes in the timing of the pulses allowed scientists to reveal planets in orbit around the pulsar.
The first planet detected around a Sun-like star, in 1995, turned out to be a hot Jupiter: a gas giant about half the mass of our own Jupiter in an extremely close, four-day orbit around its star. A year on this planet, in other words, lasts only four days. More such planets appeared in the data from ground-based telescopes once astronomers learned to recognize them – first dozens, then hundreds. They were found using the “wobble” method: tracking slight back-and-forth motions of a star, caused by gravitational tugs from orbiting planets. But still, nothing looked likely to be habitable. Finding small, rocky worlds more like our own required the next big leap in exoplanet-hunting technology: the “transit” method. Astronomer William Borucki came up with the idea of attaching extremely sensitive light detectors to a telescope, then launching it into space. The telescope would stare for years at a field of more than 170,000 stars, searching for tiny dips in starlight when a planet crossed a star’s face. That idea was realized in the Kepler Space Telescope whose launch in 2009 opened a new window on the Universe. The more than 5,000 exoplanets confirmed in our galaxy so far include a variety of types – some that are similar to planets in our solar system, others vastly different. Among these are a mysterious variety known as “super-Earths” because they are larger than our world and possibly rocky.
RADIO TELESCOPE REVEALS HUGE SPACE OBJECT
RAS
A South African radio telescope has captured the clearest image yet of a massive and mysterious space object. Sixteen times larger than our Milky Way galaxy, or about a million light years across. The so-called “odd radio circle” has fascinated astronomers since it was first discovered in 2019. South Africa’s Inter-University Institute for Data Intensive Astronomy compiled the image of the gigantic glowing rings with the help of the MeerKAT radio telescope, which consists of 64 dish antennas. Only five odd radio circles have been found since first being discovered with a similar Australian radio telescope in 2019. Odd radio circles, or ORCs for short, are not visible with optical, infrared or x-ray telescopes. While there is no explanation for what causes odd radio circles, the new data apparently shows spherical rings centred around a galaxy where only a roundish blob was previously visible. That added detail has helped researchers narrows it down to three leading theories: they could be the aftermath of a huge explosion at the centre of a galaxy, powerful jets of energy shooting out of a galaxy’s centre, or the shock wave from the formation of stars. Astronomers know odd radio circles are rings of faint radio emissions surrounding a galaxy with a highly active black hole at its centre, but they don’t yet know what causes them, or why they are so rare.
Topic: Late March Astronomy Bulletin (Read 1691 times)