SAND-SIZED METEOROIDS ARE PEPPERING ASTEROID BENNU
Southwest Research Institute
A new study posits that the major particle ejections off the near-Earth asteroid Bennu may be the consequence of impacts by small, sand-sized particles called meteoroids onto its surface as the object nears the Sun. Launched in 2016, NASA's OSIRIS-REx spacecraft is currently orbiting Bennu with the aim of briefly touching on the surface and obtaining a sample from the asteroid in October 2020, and then returning to Earth. While in orbit, the spacecraft has been sending images of Bennu back to Earth. One of the most significant things noticed is that the asteroid is frequently ejecting materials into space. Tiny rocks are just flying off its surface, yet there is no evidence that they are propelled by sublimating ice, as one might expect from a comet. The biggest events launch rocks as large as a few centimetres. Even more curious is the fact that the observed major ejection events tend to occur in the late afternoon on Bennu. Many meteoroids originated on comets. As comets approach the Sun, pieces break off as a consequence of solar heating. Some comets even break apart, producing far more small particles than asteroid collisions in the asteroid belt. For this reason, comet fragments are thought to be the major source of meteoroids that fill the inner solar system. The study suggests that as Bennu draws closer to the Sun in its orbit, it experiences a higher number of meteoroid impacts. Moreover, sand-sized meteoroids are predicted to hit Bennu with the force of a shotgun blast about once every two weeks, with most striking in the head-on direction. Their impact location on Bennu corresponds to late afternoon and early evening.
HINTS OF LIFE ON VENUS?
RAS
Astronomers have announced the discovery of a rare molecule – phosphine – in the clouds of Venus. On Earth, this gas is only made industrially, or by microbes that thrive in oxygen-free environments. Astronomers have speculated for decades that high clouds on Venus could offer a home for microbes – floating free of the scorching surface, but still needing to tolerate very high acidity. The detection of phosphine molecules, which consist of hydrogen and phosphorus, could point to this extra-terrestrial ‘aerial’ life. The team first used the James Clerk Maxwell Telescope (JCMT) in Hawaii to detect the phosphine, and were then awarded time to follow up their discovery with 45 telescopes of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Both facilities observed Venus at a wavelength of about 1 millimetre, much longer than the human eye can see – only telescopes at high altitude can detect this wavelength effectively. Naturally cautious about the initial findings, the team was delighted to get three hours of time with the more sensitive ALMA observatory. Both observatories had seen faint absorption at the right wavelength to be phosphine gas, where the molecules are backlit by the warmer clouds below. Interpretation of the data showed that phosphine is present but scarce – only about twenty molecules in every billion. The astronomers then ran calculations to see if the phosphine could come from natural processes on Venus. They caution that some information is lacking – in fact, the only other study of phosphorus on Venus came from one lander experiment, carried by the Soviet Vega 2 mission in 1985. Massachusetts Institute of Technology led the work on assessing natural ways to make phosphine. Some ideas included sunlight, minerals blown upwards from the surface, volcanoes, or lightning, but none of these could make anywhere near enough of it. Natural sources were found to make at most one ten thousandth of the amount of phosphine that the telescopes saw. To create the observed quantity of phosphine on Venus, terrestrial organisms would only need to work at about 10% of their maximum productivity, according to calculations by Cambridge University. Any microbes on Venus will likely be very different to their Earth cousins though, to survive in hyper-acidic conditions.
Earth bacteria can absorb phosphate minerals, add hydrogen, and ultimately expel phosphine gas. It costs them energy to do this, so why they do it is not clear. The phosphine could be just a waste product, but other scientists have suggested purposes like warding off rival bacteria. Another MIT team-member was also thinking about searching for phosphine as a ‘biosignature’ gas of non-oxygen-using life on planets around other stars, because normal chemistry makes so little of it. The discovery raises many questions, such as how any organisms could survive. On Earth, some microbes can cope with up to about 5% of acid in their environment – but the clouds of Venus are almost entirely made of acid. Other possible biosignatures in the Solar System may exist, like methane on Mars and water venting from the icy moons Europa and Enceladus. On Venus, it has been suggested that dark streaks where ultraviolet light is absorbed could come from colonies of microbes. The Akatsuki spacecraft, launched by the Japanese space agency JAXA, is currently mapping these dark streaks to understand more about this “unknown ultraviolet absorber. The team believes their discovery is significant because they can rule out many alternative ways to make phosphine, but they acknowledge that confirming the presence of “life” needs a lot more work. Although the high clouds of Venus have temperatures up to a pleasant 30 degrees centigrade, they are incredibly acidic – around 90% sulphuric acid – posing major issues for microbes to survive there. The team are now eagerly awaiting more telescope time, for example to establish whether the phosphine is in a relatively temperate part of the clouds, and to look for other gases associated with life. New space missions could also travel to our neighbouring planet, and sample the clouds in situ to further search for signs of life.
JUPITER’S MOONS COULD BE WARMING EACH OTHER
University of Arizona
Jupiter's moons are hotter than they should be, for being so far from the Sun. In a process called tidal heating, gravitational tugs from Jupiter's moons and the planet itself stretch and squish the moons enough to warm them. As a result, some of the icy moons contain interiors warm enough to host oceans of liquid water, and in the case of the rocky moon Io, tidal heating melts rock into magma. Researchers previously believed that the gas giant Jupiter was responsible for most of the tidal
heating associated with the liquid interiors of the moons, but a new study found that moon-moon interactions may be more responsible for the heating than Jupiter alone. Understanding how the moons influence each other is important because it can shed light on the evolution of the moon system as a whole. Jupiter has nearly 80 moons, the four largest of which are Io, Europa, Ganymede and Callisto. Maintaining subsurface oceans against freezing over geological times requires a fine balance between internal heating and heat loss, and yet we have several pieces of evidence that Europa, Ganymede, Callisto and other moons should be ocean worlds. Io, the moon closest to Jupiter, shows widespread volcanic activity, another consequence of tidal heating, but at a higher intensity likely experienced by other terrestrial planets, like Earth, in their early history. Ultimately, scientists want to understand the source of all this heat, both for its influence on the evolution and habitability of the many worlds across the solar system and beyond. The trick to tidal heating is a phenomenon called tidal resonance. Resonance creates loads more heating. Basically, if you push any object or system and let go, it will wobble at its own natural frequency. If you keep on pushing the system at the right frequency, those oscillations get bigger and bigger, just like when you're pushing a swing. If you push the swing at the right time, it goes higher, but get the timing wrong and the swing's motion is dampened. Each moon's natural frequency depends on the depth of its ocean.
These tidal resonances were known before this work, but only known for tides due to Jupiter, which can only create this resonance effect if the ocean is really thin (less than 300 metres), which is unlikely. When tidal forces act on a global ocean, it creates a tidal wave on the surface that ends up propagating around the equator with a certain frequency, or period. According to the researchers' model, Jupiter's influence alone can't create tides with the right frequency to resonate with the moons
because the moons' oceans are thought to be too thick. It's only when the researchers added in the gravitational influence of the other moons that they started to see tidal forces approaching the natural frequencies of the moons. When the tides generated by other objects in Jupiter's moon system match each moon's own resonant frequency, the moon begins to experience more heating than that due to tides raised by Jupiter alone, and in the most extreme cases, this could result in the melting of ice or rock internally. For moons to experience tidal resonance, their oceans must be tens to hundreds of kilometres thick, which is in range of scientists' current estimates. However, there are some caveats to the researchers' findings. Their model assumes that tidal resonances never get too extreme. The team want to return to this variable in the model and see what happens when they lift that constraint.
PECULIAR PLANETARY SYSTEM AROUND ORION STARS
Carnegie Institution for Science
The discovery that our galaxy is teeming with exoplanets has also revealed the vast diversity of planetary systems out there and raised questions about the processes that shaped them. New work published by an international team could explain the architecture of multi-star systems in which planets are separated by wide gaps and do not orbit on the same plane as their host star's equatorial centre. In our Solar System, the eight planets and many other minor objects orbit in a flat plane around the Sun; but in some distant systems, planets orbit on an incline -- sometimes a very steep one. Understanding the origins of extremely oblique orbital angles such as these could help reveal details about the planetary formation process. Stars are born in nurseries of gas and dust called molecular clouds -- often forming in small groups of two or three. These young stars are surrounded by rotating disks of
leftover material, which accretes to form baby planets. The disk's structure will determine the distribution of the planets that form from it, but much about this process remains unknown. The team found the first direct evidence confirming the theoretical prediction that gravitational interactions between the members of multi-star systems can warp or break their disks, resulting in misaligned rings surrounding the stellar hosts. Over a period of 11 years, the researchers made observations of the GW Orionis triple-star system, located just over 1,300 light-years away in the Orion constellation. Their work was accomplished using the European Southern Observatory's Very Large Telescope and the Atacama Large Millimeter/submillimeter Array -- a radio telescope made up of 66 antennas. The images reveal an extreme case where the disk is not flat at all, but is warped and has a misaligned ring that has broken away from the disk. The findings were tested by simulations, which demonstrated that the observed disorder in the orbits of the three stars could have caused the disk to fracture into the distinct rings. Scientists predict that many planets on oblique, wide-separation orbits will be discovered in future planet imaging campaigns.
UNIQUE SUPERNOVA EXPLOSION
Florida State University
One-hundred million light years away from Earth, an unusual supernova is exploding. That exploding star -- is known as "supernova LSQ14fmg". This supernova's characteristics -- it gets brighter extremely slowly, and it is also one of the brightest explosions in its class -- are unlike any other. The exploding star is what is known as a Type Ia supernova, and more specifically, a member of the "super-Chandrasekhar" group. Stars go through a sort of life cycle, and these supernovae are the exploding
finale of some stars with low mass. They are so powerful that they shape the evolution of galaxies, and so bright that we can observe them from Earth even halfway across the observable Universe. The supernova LSQ14fmg exploded in a system with a central star losing a copious amount of mass through a stellar wind. When the mass loss abruptly stopped, it created a ring of material surrounding the star. Type Ia supernovae were crucial tools for discovering what's known as dark energy, which is the name given to the unknown energy that causes the current accelerated expansion of the universe. Despite their importance, astronomers knew little about the origins of these supernova explosions, other than that they are the thermonuclear explosions of white dwarf stars. But the research team knew that the light from a Type Ia supernova rises and falls over the course of weeks, powered by the radioactive decay of nickel produced in the explosion. A supernova of that type would get brighter as the nickel becomes more exposed, then fainter as the supernova cools and the nickel decays to cobalt and to iron.
After collecting data with telescopes in Chile and Spain, the research team saw that the supernova was hitting some material surrounding it, which caused more light to be released along with the light from the decaying nickel. They also saw evidence that carbon monoxide was being produced. Those observations led to their conclusion -- the supernova was exploding inside what had been an asymptotic giant branch (AGB) star on the way to becoming a planetary nebula. The explosion was
triggered by the merger of the core of the AGB star and another white dwarf star orbiting within it. The central star was losing a copious amount of mass through a stellar wind before the mass loss was turned off abruptly and created a ring of material surrounding the star. Soon after the supernova exploded, it impacted a ring of material often seen in planetary nebulae and produced the extra light and the slow brightening observed. This is the first strong observational proof that a Type Ia supernova can explode in a post-AGB or proto-planetary-nebula system and is an important step in understanding the origins of Type Ia supernovae. These supernovae can be particularly troublesome because they can mix into the sample of normal supernovae used to study dark energy. This research gives us a better understanding of the possible origins of Type Ia supernovae and will help to improve future dark energy research.
TELESCOPE SHOWS NO SIGNS OF ALIEN TECHNOLOGY
International Centre for Radio Astronomy Research
A radio telescope in outback Western Australia has completed the deepest and broadest search at low frequencies for alien technologies, scanning a patch of sky known to include at least 10 million stars. Astronomers used the Murchison Widefield Array (MWA) telescope to explore hundreds of times more broadly than any previous search for extraterrestrial life. The study observed the sky around the Vela constellation. But in this part of the Universe at least, it appears other civilisations are elusive, if they exist. The telescope was searching for powerful radio emissions at frequencies similar to FM radio frequencies, that could indicate the presence of an intelligent source. These possible emissions are known as 'technosignatures'. The MWA is a unique telescope, with an extraordinarily wide field-of-view that allows astronomers to observe millions of stars simultaneously. They observed the sky around the constellation of Vela for 17 hours, looking more than 100 times broader and deeper than ever before. And even though this was a really big study, the amount of space looked at was the equivalent of trying to find something in the Earth's oceans but only searching a volume of water equivalent to a large swimming pool. Since we can't really assume how possible alien civilisations might utilise technology, we need to search in many different ways. Using radio telescopes, we can explore an eight-dimensional search space. The MWA is a precursor for the instrument that comes next, the Square Kilometre Array (SKA), a 1.7 billion Euro observatory with telescopes in Western Australia and South Africa. Due to the increased sensitivity, the SKA low-frequency telescope to be built in Western Australia will be capable of detecting Earth-like radio signals from relatively nearby planetary systems. With the SKA, researchers will be able to survey billions of star systems, seeking technosignatures in an astronomical ocean of other worlds.
INGREDIENT MISSING FROM DARK MATTER THEORIES
ESA/Hubble Information Centre
Observations by the NASA/ESA Hubble Space Telescope and the European Southern Observatory's Very Large Telescope (VLT) in Chile have found that something may be missing from the theories of how dark matter behaves. This missing ingredient may explain why researchers have uncovered an unexpected discrepancy between observations of the dark matter concentrations in a sample of
massive galaxy clusters and theoretical computer simulations of how dark matter should be distributed in clusters. The new findings indicate that some small-scale concentrations of dark matter produce lensing effects that are 10 times stronger than expected. Dark matter is the invisible glue that keeps stars, dust, and gas together in a galaxy. This mysterious substance makes up the bulk of a galaxy's mass and forms the foundation of our Universe's large-scale structure. Because dark matter does not emit, absorb, or reflect light, its presence is only known through its gravitational pull on visible matter in space. Astronomers and physicists are still trying to pin down what it is. Galaxy clusters, the most massive and recently assembled structures in the Universe, are also the largest repositories of dark matter. Clusters are composed of individual member galaxies that are held together largely by the gravity of dark matter. The distribution of dark matter in clusters is mapped by measuring the bending of light -- the gravitational lensing effect -- that they produce. The gravity of dark matter concentrated in clusters magnifies and warps light from distant background objects. This effect produces distortions in the shapes of background galaxies which appear in images of the clusters. Gravitational lensing can often also produce multiple images of the same distant galaxy. The higher the concentration of dark matter in a cluster, the more dramatic its light-bending effect. The presence of smaller-scale clumps of dark matter associated with individual cluster galaxies enhances the level of distortions. In some sense, the galaxy cluster acts as a large-scale lens that has many smaller lenses embedded within it.
Hubble's crisp images were taken by the telescope's Wide Field Camera 3 and Advanced Camera for Surveys. Coupled with spectra from the European Southern Observatory's Very Large Telescope (VLT), the team produced an accurate, high-fidelity, dark-matter map. By measuring the lensing distortions astronomers could trace out the amount and distribution of dark matter. The three key galaxy clusters, MACS J1206.2-0847, MACS J0416.1-2403, and Abell S1063, were part of two Hubble surveys: The Frontier Fields and the Cluster Lensing And Supernova survey with Hubble (CLASH) programs. To the team's surprise, in addition to the dramatic arcs and elongated features of distant galaxies produced by each cluster's gravitational lensing, the Hubble images also revealed an unexpected number of smaller-scale arcs and distorted images nested near each cluster's core, where the most massive galaxies reside. The researchers believe the nested lenses are produced by the gravity of dense concentrations of matter inside the individual cluster galaxies. Follow-up spectroscopic observations measured the velocity of the stars orbiting inside several of the cluster galaxies to thereby pin down their masses. By combining Hubble imaging and VLT spectroscopy, the astronomers were able to identify dozens of multiply imaged, lensed, background galaxies. This allowed them to assemble a well-calibrated, high-resolution map of the mass distribution of dark matter in each cluster. The team compared the dark-matter maps with samples of simulated galaxy clusters with similar masses, located at roughly the same distances. The clusters in the computer model did not show any of the same level of dark-matter concentration on the smallest scales -- the scales associated with individual cluster galaxies.
Topic: Late September Astronomy Bulletin (Read 1395 times)