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December 22, 2019 at 1:34 pm #9982Anonymous
THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 505 2019 December 22
Here is the latest round-up of news from the Society for Popular Astronomy. The SPA is arguably Britain's liveliest astronomical society, with members all over the world. We accept subscription payments online at our secure site and can take credit and debit cards. You can join or renew via a secure server or just see how much we have to offer by visiting
FIRST IDENTIFIED COMET TO VISIT FROM ANOTHER STAR
NASA/Goddard Space Flight Center
When astronomers see something in the Universe that at first glance seems like one-of-a-kind, it's bound to stir up a lot of excitement and attention. Enter comet 2I/Borisov. This mysterious visitor from the depths of space is the first identified comet to arrive here from another star. We don't know from where or when the comet started heading toward our Sun, but it won't hang around for long. The Sun's gravity is slightly deflecting its trajectory, but can't capture it because of the shape of its orbit and high velocity of about 100,000 miles per hour. Telescopes around the world have been watching the fleeting visitor. The Hubble Space Telescope has provided the sharpest views as the comet skirts by our Sun. Hubble revealed that the heart of the comet, a loose agglomeration of ices and dust particles, is probably no more than about 3,200 feet across. Though comet Borisov is the first of its kind, no doubt there are many other comet vagabonds out there, plying the space between stars. Astronomers will eagerly be on the lookout for the next mysterious visitor from far beyond. Crimean amateur astronomer Gennady Borisov discovered the comet on 2019 August 30, and reported the position measurements to the International Astronomical Union's Minor Planet Center in Cambridge, Massachusetts. The Center for Near-Earth Object Studies
at NASA's Jet Propulsion Laboratory in Pasadena, California, working with the Minor Planet Center, computed an orbit for the comet, which shows that it came from elsewhere in our Milky Way galaxy, point of origin unknown. Nevertheless, observations by numerous telescopes show that the comet's chemical composition is similar to that of the comets found inside our solar system, providing evidence that comets also form around other stars. By the middle of 2020 the comet will have already zoomed past Jupiter's distance of 500 million miles on its way back into the frozen abyss of interstellar space.
NASA's TREASURE MAP FOR WATER ICE ON MARS
NASA has big plans for returning astronauts to the Moon in 2024, a stepping stone on the path to sending humans to Mars. But where should the first people on the Red Planet land? A new paper published in Geophysical Research Letters will help by providing a map of water ice believed to be as little as 2.5 centimetres below the surface. Water ice will be a key consideration for any potential landing site. With little room to spare aboard a spacecraft, any human missions to Mars will have to harvest what's already available for drinking water and making rocket fuel. NASA calls this concept “in situ resource utilization”. and it's an important factor in selecting human landing sites on Mars. Satellites orbiting Mars are
essential in helping scientists determine the best places for building the first Martian research station. The authors of the new paper make use of data from two of those spacecraft, NASA's Mars Reconnaissance Orbiter (MRO) and Mars Odyssey orbiter, to locate water ice that could potentially be within reach of astronauts on the Red Planet.
Liquid water can't last in the thin air of Mars; with so little air pressure, it evaporates from a solid to a gas when exposed to the atmosphere. Martian water ice is locked away underground throughout the planet's mid-latitudes. The regions near the poles have been studied by NASA's Phoenix lander, which scraped up ice, and MRO, which has taken many images from space of meteor impacts that have excavated this ice. To find ice that astronauts could easily dig up, the study's authors relied on two heat-sensitive instruments: MRO's Mars Climate Sounder and the Thermal Emission Imaging System (THEMIS) camera on Mars Odyssey. Why use heat-
sensitive instruments when looking for ice? Buried water ice changes the temperature of the Martian surface. The study's authors cross-referenced temperatures suggestive of ice with other data, such as reservoirs of ice detected by radar or seen after meteor impacts. Data from Odyssey's Gamma Ray Spectrometer, which is tailor-made for mapping water ice deposits, were also useful. As expected, all those data suggest a trove of water ice throughout the Martian poles and mid-latitudes. But the map reveals particularly shallow deposits that future mission planners may want to study further.
While there are lots of places on Mars scientists would like to visit, few would make practical landing sites for astronauts. Most scientists have homed in on the northern and southern mid-latitudes, which have more plentiful sunlight and warmer temperatures than the poles. But there's a heavy preference for landing in the northern hemisphere, which is generally lower in elevation and provides more atmosphere to slow a landing spacecraft. A large portion of a region called Arcadia Planitia is the most tempting target in the northern hemisphere. The map shows lots of blue and purple in this region, representing water ice less than 30 centimetres below the surface; warm colours are over 60 centimetres deep. Sprawling black zones on the map represent areas where a landing spacecraft would sink into fine dust.
'TIGER STRIPES' OF ENCELADUS
University of California – Davis
Saturn's tiny, frozen moon Enceladus is a strange place. Just 300 miles across, the moon is thought to have an outer shell of ice covering a global ocean 20 miles deep, encasing a rocky core. Slashed across Enceladus' south pole are four straight, parallel fissures or “tiger stripes” from which water erupts. The fissures aren't quite like anything else in the Solar
System. Saturn's gravity exerts tidal forces on Enceladus, which cause heating and cooling of the tiny world. Those forces are strongest at the poles. As liquid water solidifies into ice under the outer ice shell, it expands in volume, putting pressure on the ice until it cracks. Enceladus'
surface temperature is about minus 200 degrees Celsius, so if a crack formed in the ice, you would expect it to freeze shut pretty quickly. Yet the south polar fissures remain open, and in fact reach all the way to the liquid ocean below. That's because liquid water within the fissure is sloshed around by tidal forces produced by Saturn's gravity, releasing energy as heat. That stops the crack from freezing shut. The release of pressure from the fissures stops new cracks from forming elsewhere on the moon, such as at the north pole. But at the same time, water vented from the crack falls back as ice, building up the edges of the fissure and weighing it down a bit. That causes the ice sheet to flex, the researchers calculate, just enough to set off a parallel crack about 20 miles away.
OSIRIS-REx EXPLAINS BENNU MYSTERY PARTICLES
Shortly after NASA's OSIRIS-REx spacecraft arrived at asteroid Bennu, an unexpected discovery by the mission's science team revealed that the asteroid could be active, or consistently discharging particles into space. The ongoing examination of Bennu — and its sample that will eventually be returned to Earth — could potentially shed light on why this intriguing phenomenon is occurring. The OSIRIS-REx team first observed a particle-
ejection event in images captured by the spacecraft's navigation cameras taken on Jan. 6, just a week after the spacecraft entered its first orbit around Bennu. At first glance, the particles appeared to be stars behind the asteroid, but on closer examination, the team realized that the asteroid was ejecting material from its surface. After concluding that these particles did not compromise the spacecraft's safety, the mission began dedicated observations in order fully to document the activity. The team observed the three largest particle-ejection events on Jan. 6 and 19 and Feb. 11, and concluded that the events originated from different locations on Bennu's surface. The first event originated in the southern hemisphere, and the second and third events occurred near the equator. All three events took place in the late afternoon on Bennu.
The team found that, after ejection from the asteroid's surface, the particles either briefly orbited Bennu and fell back to its surface or escaped from Bennu into space. The observed particles travelled up to 3 metres per second, and measured smaller than 10 centimetres in size. Approximately 200 particles were observed during the largest event, which took place on Jan. 6. The team investigated a wide variety of possible mechanisms that may have caused the ejection events and narrowed the list to three candidates: meteoroid impacts, thermal stress fracturing and released water vapour. Meteoroid impacts are common in the deep space neighbourhood of Bennu, and it is possible that these small fragments of space rock could be hitting Bennu where OSIRIS-REx is not observing it, shaking loose particles with the momentum of their impact. The team also determined that thermal fracturing is another reasonable explanation. Bennu's surface temperatures vary drastically over its 4.3-hour rotation period. Although it is extremely cold during the night hours, the asteroid's surface warms significantly in the mid-afternoon, which is when the three major events occurred. As a result of this temperature change, rocks may begin to crack and break down, and eventually particles could be ejected from the surface. This cycle is known as thermal stress fracturing.
Water release may also explain the asteroid's activity. When Bennu's water-locked clays are heated, the water could begin to release and create pressure. It is possible that as pressure builds in cracks and pores in boulders where absorbed water is released, the surface could become agitated, causing particles to erupt. But nature does not always allow for simple explanations. It could be that more than one of these possible mechanisms at play, for example, thermal fracturing could be chopping the surface material into small pieces, making it far easier for meteoroid impacts to launch pebbles into space. If thermal fracturing, meteoroid
impacts or both are in fact the causes of these ejection events, then this phenomenon is probably happening on all small asteroids, as they all experience these mechanisms. However, if water release is the cause of these ejection events, then that phenomenon would be specific to asteroids that contain water-bearing minerals, like Bennu.
Bennu's activity presents larger opportunities once a sample is collected and returned to Earth for study. Many of the ejected particles are small enough to be collected by the spacecraft's sampling mechanism, meaning that the returned sample may possibly contain some material that was ejected and returned to Bennu's surface. Determining that a particular particle had been ejected and returned to Bennu might be a scientific feat similar to finding a needle in a haystack. The material returned to Earth from Bennu, however, will almost certainly increase our understanding of asteroids and the ways they are both different and similar, even as the particle-ejection phenomenon continues to be a mystery whose clues we'll also return home with in the form of data and further material for study. Sample collection is scheduled for summer 2020, and the sample will be delivered to Earth in September 2023.
FIRST WHITE DWARF PLANET FOUND
Researchers using the Very Large Telescope have, for the first time, found evidence of a giant planet associated with a white-dwarf star. The planet orbits the hot white dwarf, the remnant of a Sun-like star, at close range, causing its atmosphere to be stripped away and form a disc of gas around the star. This unique system hints at what our own Solar System might look like in the distant future. The team had inspected around 7000 white dwarfs
observed by the Sloan Digital Sky Survey and found one to be unlike any other. By analysing subtle variations in the light from the star, they found traces of chemical elements in amounts that scientists had never before observed at a white dwarf. To get a better idea of the properties of this unusual star, named WDJ0914+1914, the team analysed it with the X-shooter instrument in the Chilean Atacama Desert. These follow-up observations confirmed the presence of hydrogen, oxygen and sulphur associated with the white dwarf. By studying the fine details in the spectra taken by ESO's X-shooter, the team discovered that these elements
were in a disc of gas swirling into the white dwarf, and not coming from the star itself. The detected amounts of hydrogen, oxygen and sulphur are similar to those found in the deep atmospheric layers of icy, giant planets like Neptune and Uranus. If such a planet were orbiting close to a hot white dwarf, the extreme ultraviolet radiation from the star would strip away its outer layers and some of the stripped gas would swirl into a disc, itself accreting onto the white dwarf. That is what scientists think they are seeing around WDJ0914+1914: the first evaporating planet orbiting a white dwarf.
Combining observational data with theoretical models, the team of astronomers from the UK, Chile and Germany were able to paint a clearer image of that unique system. The white dwarf is small and, at 28 000 degrees Celsius (five times the Sun's temperature), extremely hot. By contrast, the planet is icy and large — at least twice as large as the star. Since it orbits the
hot white dwarf at close range, making its way around it in just 10 days, the high-energy photons from the star are gradually blowing away the planet's atmosphere. Most of the gas escapes, but some is pulled into a disc swirling into the star at a rate of 3000 tonnes per second. It is that disc that makes the otherwise hidden Neptune-like planet visible. Stars like our Sun burn hydrogen in their cores for most of their lives. Once they run out of that fuel, they puff up into red giants, becoming hundreds of times larger and engulfing nearby planets. In the case of the Solar System, this will include Mercury, Venus, and even Earth, which will all be consumed by the red-giant Sun in about 5 billion years. Eventually, Sun-like stars lose their outer layers, leaving behind only a burnt-out core, a white dwarf. Such stellar remnants can still host planets, and many such star systems are thought to exist in our galaxy. However, until now, scientists had never found evidence of a surviving giant planet around a white dwarf. The
detection of an exoplanet in orbit around WDJ09 14+1914, located about 1500 light years away in the constellation Cancer, may be the first of many orbiting such stars. According to the researchers, the exoplanet now found with the help of ESO's X-shooter orbits the white dwarf at a distance of only 10 million kilometres, or 15 times the solar radius, which would have been deep inside the red giant. The unusual position of the planet implies that at some point after the host star became a white dwarf, the planet moved closer to it. The astronomers believe that the new orbit could be the result of gravitational interactions with other planets in the system,
meaning that more than one planet may have survived its host star's violent transition.
ANCIENT STARBURST FOUND IN MILKY WAY
Astronomers using the Very Large Telescope (VLT) have observed the central part of the Milky Way and uncovered new details about the history of star birth in our galaxy. Thanks to the new observations, astronomers have found evidence for a dramatic event in the life of the Milky Way: a burst of star formation so intense that it resulted in over a hundred thousand supernova explosions. In the study, the team found that about 80% of the stars in the Milky Way central region formed in the earliest years of our galaxy, between eight and 13.5 billion years ago. This initial period of star formation was followed by about six billion years during which very few stars were born. This was brought to an end by an intense burst of star formation around one billion years ago when, over a period of less than 100 million years, stars with a combined mass possibly as high as a few tens of million Suns formed in this central region. The conditions in the studied region during this burst of activity must have resembled those in “starburst” galaxies, which form stars at rates of more than 100 solar masses per year. At present, the whole Milky Way is forming stars at a rate of about one or
two solar masses per year. This burst of activity, which must have resulted in the explosion of more than a hundred thousand supernovae, was probably one of the most energetic events in the whole history of the Milky Way. During a starburst, many massive stars are created; since they have shorter lifespans than lower-mass stars, they reach the end of their lives much
faster, dying in violent supernova explosions.
Bulletin compiled by Clive Down
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