The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 408 2015 October 25
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ASTEROIDS ARE MOON'S MAIN WATER SUPPLY
Moscow Institute of Physics and Technology
Water found on the Moon is now thought to have been brought there by impacting asteroids and not by comets as was previously thought. Using computer simulation, scientists have discovered that one large asteroid can deliver more water to the lunar surface than the cumulative fall of comets over a thousand-million-year period. At the beginning of the space age, during the days of the Apollo programme, scientists believed the Moon to be completely dry. At the earliest stages in satellite evolution, the absence of an atmosphere and the influence of solar radiation were thought to be enough to evaporate all volatile substances into space. However, in the 1990s, scientists obtained data from the Lunar Prospector probe that were indicative of a larger fraction of hydrogen at the near-surface soil of some regions of the Moon, which one could interpret as a sign of the presence of water. In order to explain how water could be kept on the Moon's surface, scientists formulated a theory known as 'cold traps'. The axis of the Moon's rotation is nearly vertical, which is why in the polar regions there are craters whose floors are never exposed to sunlight. When comets consisting mostly of water ice fall, evaporated water can gravitate into those 'traps' and remain there indefinitely, as solar rays do not evaporate it.
In recent years, lunar missions (the Indian Chandrayan probe, the American LRO, data from the Cassini probe and Deep Impact) have brought scientists two new pieces of information. The first is that there are indeed considerable quantities of water and hydroxyl groups in the near-surface soil on the Moon. The LCROSS experiment, in which a probe was purposely crashed onto the Moon resulting in the release of a cloud of gas and dust that was later studied with a spectrometer, directly confirmed the existence of water and other volatile substances. The second piece of new information came when the Russian LEND apparatus on board LRO generated a map of water distribution on the Moon's surface. The scientists first considered whether comets could be the main water suppliers. The typical velocity of an ice comet ranges from 20 to 50 km/s; estimates suggested that such a high impact velocity causes from 95 to 99.9% of the water to evaporate into space beyond retrieve. There is a family of short-period comets whose velocity is much lower -- 8-10 km/s. Such comets could account for about 1.5% of lunar craters. Nevertheless, the simulation showed that when such short-period comets do impact, almost all the water evaporates and less than 1% of it remains at the impact point. Scientists concluded that only a very small amount of water that arrives with a comet stays on the Moon, so they decided to explore the possibility of an asteroid origin of lunar water. They noticed that asteroids consist of initially non-differentiated materials typical of the Solar System and contain a rather considerable proportion of water. In particular, carbonaceous chondrite, the most common type of asteroids and meteorites, can contain up to 10% water. However, water in chondrites is effectively protected: it is in a chemically bonded condition, and it is 'blocked' in a crystal lattice of minerals. Water starts to be lost only when it is heated to 300-1200 C depending on the type of mineral, so it could potentially remain in the crater together with the rest of the asteroid. Calculations indicate that between 2 and 4.5% of lunar craters could contain considerable supplies of water in the form of hydrated minerals. They are stable enough to contain water even in areas exposed to the Sun. That is important in relation to any project to establish a 'lunar base', because the polar cold traps are not convenient areas for such constructions: there is little solar energy, it is difficult to organize radio communication, and lastly, there are dramatically low temperatures. The possibility of obtaining lunar water in regions exposed to the Sun could make the issue of lunar exploration less difficult,
AIDA DOUBLE MISSION TO DIVERT DIDYMOS ASTEROID'S DIDYMOON
An ambitious joint US-European mission, called AIDA, is being planned to divert the orbit of a binary asteroid's small moon, as well as to give us new insights into the structure of asteroids. A pair of spacecraft, the ESA-led Asteroid Impact Mission (AIM) and NASA-led Double Asteroid Redirection Test (DART), will rendezvous with the asteroid Didymos nd its small natural satellite, known informally as Didymoon. Following a period of study of both asteroids and detailed mapping of Didymoon by AIM, DART will impact onto Didymoon and AIM will assess the mission's effectiveness in diverting the moon's orbit around Didymos. To protect the Earth from potentially hazardous impacts, we need to understand asteroids much better -- what they are made of, their structure, origins and how they respond to collisions. AIDA will be the first mission to study a binary asteroid system, as well as the first to test whether we can deflect an asteroid by an impact with a spacecraft. The European part of the mission, AIM, will study the structure of Didymoon and the orbit and rotation of the binary system, providing clues to its origin and evolution. Asteroids represent different stages in the road to planet formation, so they may be offering snapshots of the Solar System's history.
AIM is due for launch in 2020 October and to reach the binary system (65803) Didymos in 2022 May. Binary systems make up around 15% of the asteroid population. Egg-shaped Didymoon (about 160 metres in diameter) orbits the diamond-shaped Didymos asteroid (about 750 metres in diameter) every 12 hours at an altitude of 1.1 kilometres. Ground-based observations show that Didymos is probably a common chondrite, or stony asteroid, formed of dust from the primitive Solar System. At present, Didymoon's mass and density are unknown.
BLUE SKIES ON PLUTO
The first colour images of Pluto's atmosphere have been beamed back to Earth by the New Horizons spacecraft just after it sped by Pluto on July 14. The spacecraft's cameras were looking back at Pluto's night side as sunlight illuminated the fringe of blue around Pluto's circumference. A blue sky often results from scattering of sunlight by very small particles. On the Earth, those particles are nitrogen molecules. On Pluto they appear to be soot-like particles we call tholins. The term 'tholin' was coined to describe organic substances obtained in experiments on gas mixtures akin to the atmosphere of Saturn's moon Titan. On Pluto, tholins form high in the atmosphere where UV sunlight breaks apart nitrogen and methane molecules. The fragments re-combine to form complex macromolecules, which continue to combine and grow until they become tholins. Ironically, tholins themselves are not blue -- they merely scatter blue light. When tholins fall to the ground they show their true colours, grey or red. At least some of Pluto's patchy red colouring is thought to result from a gentle rain of such particles from the planet's atmosphere.
MASSIVE MAGNETIC STARS
A consortium known as 'Binarity and Magnetic Interactions in various classes of Stars' (BinaMIcS) has observed Epsilon Lupi, a third-magnitude B-type system whose duplicity was discovered more than 100 years ago. It is a pair of massive stars which have magnetic fields. Around 1/3 of stars in our Galaxy are thought to be in binary systems. They are interesting, because astronomers can in some cases determine their masses and relate that to their brightness -- which helps us understand how stars evolve.
Epsilon Lupi is the fourth-brightest star system in the southern constellation Lupus. The pair of stars is about 500 light-years away; both are B-type stars having between 7 and 8 times the mass of the Sun, and together the pair is around 6000 times as luminous as the Sun. Only comparatively recently it was discovered that the two giant stars have substantial magnetic fields. In cool stars, such as the Sun, magnetic fields are generated by 'dynamos' powered by strong convection in the outer layers of the star, where hot material rises, cools and falls back. But there is no convection in the envelopes of massive stars, so there is no support for a magnetic dynamo. Nevertheless, approximately 10% of massive stars have strong magnetic fields.
Two explanations have been proposed for their origin, both variants on the idea of a 'fossil' magnetic field, a field generated at some point in the star's past and then locked into the star's surface. The first hypothesis is that the magnetic field is generated while the star is being formed; a second is that the magnetic field originates in dynamos driven by the violent mixing of material when two already- formed stars in a close binary merge. The Epsilon Lupi discovery allows us to rule out the binary-merger scenario, but it does not change the basic finding of the BinaMIcS collaboration: fewer than 2% of massive stars in close binaries have magnetic fields, and we still do not know why that is. The research shows that the strengths of the magnetic fields are similar in the two stars, but their magnetic axes are anti-aligned, with the south magnetic pole of one star pointing in approximately the same direction as the north pole of the other. It may even be that the two stars share a single magnetic field, in any case it probably points to something significant about how the stars are interacting with one another. The stars are close enough that their magnetospheres are likely to be interacting during the whole of their orbit around each other. That means that their magnetic fields may act as a brake, slowing down the stars. As a result, in the long term, the two stars may be gradually spiralling in towards one another.
RIPPLES RACING THROUGH PLANET-FORMING DISC
Using images from the Very Large Telescope and the Hubble telescope, astronomers have discovered never-before-seen structures within a dusty disc surrounding a nearby star. The fast-moving wave-like features in the disc of the star AU Microscopii are unlike anything ever observed, or even predicted, before. The origin and nature of the features is unknown. AU Mic is a young, nearby star surrounded by a large disc of dust. Studies of such debris discs can provide valuable clues about how planets, which form from such discs, are created. Astronomers have been searching AU Mic's disc for any signs of clumpy or warped features, as such signs might give away the location of possible planets. In 2014 they used the more powerful high-contrast imaging capabilities of ESO's newly installed SPHERE instrument, mounted on the Very Large Telescope for their search -- and discovered something very unusual. Five wave-like arches at different distances from the star show up in the new images, reminiscent of ripples in water. After observing the features in the SPHERE data the team turned to earlier images of the disc taken by Hubble in 2010 and 2011 to see whether the features were also visible in those. They were not only able to identify the features on the earlier images, but they also discovered that they had changed in the four-year interval -- in fact they are moving very fast! They found that the arches are racing away from the star at speeds of up to about 10 km/s. The features further away from the star seem to be moving faster than those closer to it. At least three of the features are moving so fast that they could well be escaping from the gravitational attraction of the star. Such high speeds rule out the possibility that these are conventional disc features caused by objects such as planets disturbing material in the disc while orbiting the star. There must have been something else involved to speed up the ripples and make them move so quickly, implying that they are a sign of something truly unusual. The team does not know what caused the ripples, but it has considered and ruled out a series of phenomena as explanations, including the collision of two massive and rare asteroid-like objects releasing large quantities of dust, and spiral waves triggered by instabilities in the disc. One explanation for the strange structure links them to the star's flares. AU Mic is a star with high flaring activity -- it often lets off huge and sudden bursts of energy from on or near its surface. Such a flare could perhaps have triggered something on one of the planets -- if there are planets -- like a violent stripping of material which could now be propagating through the disc, propelled by the flare's force. The team plans to continue to observe the AU Mic system with SPHERE and other facilities, including ALMA, to try to understand what is happening.
RADIO TELESCOPES COULD OBSERVE STARS IN THE GALACTIC CENTRE
The centre of our Milky Way galaxy is a mysterious place. Not only is it thousands of light-years away, it is also cloaked in so much dust that most stars there are rendered invisible. Harvard researchers are proposing a new way to penetrate the fog and observe stars hiding there. They suggest looking for radio waves coming from supersonic stars. The long path from the centre of our Galaxy to the Earth is so choked with dust that in visible light there is about thirty magnitudes of extinction. Radio waves, however, can pass through the dust unimpeded. On their own, stars are not bright enough in the radio for us to detect them at such distances. However, if a star is travelling through gas faster than the speed of sound, material blowing off the star as a stellar wind can plough into the interstellar gases and create a shock wave. Through a process called synchrotron radiation, electrons accelerated by that shock wave produce radio emission that we can potentially detect. In a sense, it is the cosmic equivalent of a sonic boom from an aeroplane.
To create a shock wave, the star would have to be moving at a speed of thousands of km/s. That is possible in the Galactic Centre since the stars there are influenced by the strong gravity of a supermassive black hole. When an orbiting star reaches its closest approach to the black hole, it may well acquire the required speed. The researchers suggest looking for that effect from one already-known star called S2. That star, which is hot and bright enough to be seen in the infrared despite all the dust, and will make its closest approach to the Galactic Centre in late 2017 or early 2018. When it does, radio astronomers can look for emission from its shock wave. S2 will be a test: if it is seen in the radio, then potentially we can use the same method to find smaller and fainter stars that can not be seen in any other way.
BLACK HOLE IS 30 TIMES EXPECTED SIZE
The central supermassive black hole of a recently discovered galaxy has been found to be far larger than should be possible, according to current theories of galactic evolution. The galaxy, SAGE0536AGN, was initially discovered with the Spitzer space telescope in infrared light. Thought to be at least 9 billion years old, it contains an active galactic nucleus (AGN), an incredibly bright object resulting from the accretion of gas by a central supermassive black hole. The gas is accelerated to high velocities, and caused to emit light, by the black hole's immense gravitational field. The team has confirmed the presence of the black hole by measuring the speed of the gas moving around it. Using the Southern African Large Telescope, the scientists observed a hydrogen emission line whose broadening implies that the gas is moving at a speed that implies that the mass of the black hole in SAGE0536AGN is about 350 million times the mass of the Sun. But the mass of the galaxy itself, obtained through measurements of the movement of its stars, has been calculated to be 25 thousand million solar masses. That is seventy times that of the black hole, but the black hole is still thirty times more massive than expected for that size of galaxy. In ordinary galaxies the black hole would grow at the same rate as the galaxy, but in SAGE0536AGN the black hole has grown much faster, or the galaxy stopped growing prematurely. Because that galaxy was found by accident, there may be more such objects waiting to be discovered. Time will tell whether SAGE0536AGN really is an oddity, or simply the first in a previously unknown class of galaxies.
BARREN DWARF GALAXY FORMS BRILLIANT STAR CLUSTERS
National Radio Astronomy Observatory.
An international team of astronomers using the Atacama Large Millimetre/submillimetre Array (ALMA) has discovered an unexpected population of compact interstellar clouds hidden within the nearby dwarf irregular galaxy Wolf-Lundmark-Melotte, more commonly known as WLM. The clouds, which are within a heavy blanket of interstellar material, help to explain how dense star clusters are able to form in the tenuous environs of a galaxy thousands of times smaller and far more diffuse than our own Milky Way. For many reasons, dwarf irregular galaxies like WLM are poorly equipped to form star clusters. They are fluffy, with very low densities. They also lack the heavy elements that contribute to star formation. Such galaxies should form only dispersed stars rather than concentrated clusters, but that is clearly not the case. By studying that galaxy with ALMA, the astronomers were able to locate, for the first time, compact regions that appear able to emulate the nurturing environments found in larger galaxies. Those regions were discovered by pinpointing the almost imperceptible and highly localized millimetre-wavelength light emitted by carbon monoxide (CO) molecules, which are typically associated with star-forming interstellar clouds.
Earlier, an affiliated team of astronomers first detected CO in the WLM galaxy with the single-dish Atacama Pathfinder EXperiment (APEX) telescope. Those initial, low-resolution observations could not resolve where the molecules reside, but they did confirm that WLM contains the lowest abundance of CO ever detected in any galaxy. The lack of CO and other heavy elements should put a serious damper on star formation, the astronomers note. Molecules, and carbon monoxide in particular, play an important role in star formation. As gas clouds begin to collapse, temperatures and densities rise, pushing back against gravity. That is where molecules and dust particles come to the rescue by absorbing some of the heat through collisions and radiating it into space at infrared and sub-mm wavelengths. That cooling effect enables gravity to continue the collapse until a star forms. The problem previously was that in WLM and similar galaxies with very low abundances of heavy elements, astronomers simply did not see enough of that material to account for the new star clusters theyobserved. The reason that the CO was initially so difficult to see,the researchers discovered, is that unlike in normal galaxies, the WLM clouds are tiny compared to their overlying envelopes of molecular and atomic gas. To become effective star factories, the concentrated CO clouds need the enormous envelopes of transitional gas to bear down on them, giving the cores of CO a high enough density to allow them to form a normal cluster of stars. Those bundles of star-forming gas are under considerable pressure, even though the surrounding ocean of interstellar gas is much more shallow. By discovering that the carbon monoxide is confined to highly concentrated regions within a vast expanse of transitional gas, we could finally understand the mechanisms that led to the impressive stellar neighbourhoods we see in the galaxy today. Further studies with ALMA will also help determine the conditions that formed the globular clusters found in the halo of the Milky Way. It has been suggested that those much larger clusters may originally have formed in dwarf galaxies and later migrated to the halo after their host dwarf galaxies dispersed. WLM is a relatively isolated dwarf galaxy located approximately 3 million light-years away on the outer edges of the 'Local Group' -- the collection of galaxies that includes the Milky Way, the Magellanic Clouds, Andromeda, M33, and dozens of smaller galaxies.
MOST EARTH-LIKE WORLDS YET TO BE BORN
According to a new theoretical study, when our Solar System was born 4.6 billion years ago only 8% of the potentially habitable planets that will ever form in the Universe existed; the bulk of those planets had yet to be born. That conclusion is based on data collected by the Hubble and Kepler space telescopes. The main motivation for the study was understanding the Earth's place in the context of the rest of the Universe. Compared to all the planets that will ever form in the Universe, the Earth was actually formed quite early. Looking far away and far back in time, Hubble has given astronomers a 'family album' of galaxy observations that chronicle the Universe's star formation history as galaxies grew. The data show that the Universe was making stars at a fast rate 10 billion years ago, but the fraction of the Universe's hydrogen and helium gas that was involved was very low. Today, star birth is happening at a much slower rate than long ago, but there is so much leftover gas available that the Universe will keep forming stars and planets for a very longtime to come. There is enough material remaining to produce even more planets in the future, in the Milky Way and beyond.
Kepler's planet survey indicates that Earth-sized planets in a star's 'habitable zone', the distance at which water could pool on the surface, are ubiquitous in our galaxy. On the basis of that survey, scientists predict that there should be one billion Earth-sized worlds in the Milky Way galaxy at present, a good portion of them presumed to be rocky. That estimate skyrockets when extrapolated to the other 100 billion galaxies in the observable Universe. That leaves plenty of opportunity for untold more Earth-sized planets in habitable zones to arise in the future. The last star is not expected to burn out until 100 trillion years from now. That is plenty of time for literally anything to happen on the planet landscape. The researchers say that future Earths are more likely to appear inside giant galaxy clusters and also in dwarf galaxies, which have yet to use up all their gas for building stars and accompanying planetary systems. By contrast, our Milky Way galaxy has used up much more of the gas available for future star formation. A big advantage to our civilization from arising early in the evolution of the Universe is our being able to use powerful telescopes like Hubble to trace our lineage from the Big Bang through the early evolution of galaxies. The observational evidence for the Big Bang and cosmic evolution, encoded in light and other electromagnetic radiation, will be all-but erased away a trillion years from now by the expansion of space. Any far-future civilizations that might arise will be largely clueless as to how or if the Universe began and evolved.
LINK BETWEEN COMET AND ASTEROID SHOWERS & MASS EXTINCTIONS
For more than 30 years, scientists have argued about a controversial hypothesis relating to periodic mass extinctions and impact craters -- caused by comet and asteroid showers -- on the Earth. Now scientists offer new support linking the age of the craters with recurring mass extinctions of life, including the demise of the dinosaurs. They show a cyclical pattern over the studied period, with both impacts and extinction events taking place every 26 million years. That cycle has been linked to periodic passage of the Sun and planets through the dense mid-plane of our Galaxy. Scientists have theorized that gravitational perturbations of the distant Oort comet cloud that surrounds the Sun lead to periodic comet showers in the inner Solar System, where some comets strike the Earth. To test their hypothesis, the researchers performed time-series analyses of impacts and extinctions using newly available data offering more accurate age estimates. The correlation between the impacts and extinction events over the past 260 million years is striking and suggests a cause-and-effect relationship. In particular, they found that six mass extinctions of life during the studied period correlate with times of enhanced impact cratering. One of the craters considered in the study is the large (180 km diameter) Chicxulub impact structure in the Yucatan, which dates from about 65 million years ago, the time of a great mass extinction that included the dinosaurs. Moreover, they add, five out of the six largest impact craters of the last 260 million years on Earth correlate with mass-extinction events.
Bulletin compiled by Clive Down
(c) 2015 the Society for Popular Astronomy
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