The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 410 2015 November 22
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MARS' MOON PHOBOS SLOWLY FALLING APART
Orbiting only 6,000 kilometres above the surface of Mars, Phobos is closer to its planet than any other moon in the Solar System. Mars' gravity is drawing in Phobos, the larger of its two moons, by about 2 metres every hundred years. Scientists expect the moon to be pulled apart in 30 to 50 million years. Grooves on the surface of Phobos were long thought to be fractures caused by the impact that formed Stickney, a crater nearly half the diameter of Phobos itself. That collision was so powerful, it came close to shattering Phobos. However, scientists eventually determined that the grooves do not radiate outward from the crater itself but from a focal point nearby. More recently, researchers have proposed that the grooves may instead be produced by many smaller impacts of material ejected from Mars. But new modelling supports the view that the grooves are more like 'stretch marks' that occur when Phobos gets deformed by tidal forces exerted on Phobos by Mars. The Earth and the Moon pull on each other in the same way, producing tides in the oceans and making both the planet and Moon slightly egg-shaped rather than perfectly round. The same explanation was proposed for the grooves decades ago, after the Viking spacecraft sent back images of Phobos. At the time, however, Phobos was thought to be more-or-less solid all the way through. When the tidal forces were calculated, it seemed that the stresses were not great enough to fracture a solid moon of that size.
The recent thinking, however, is that the interior of Phobos could be a rubble pile, barely holding together, surrounded by a layer of powdery regolith about 100 metres thick. An interior like that could distort easily because it has very little strength and forces the outer layer to readjust. The researchers think that the outer layer of Phobos behaves elastically and builds stress, but it is weak enough for those stresses to cause it to fail. That implies that the tidal forces acting on Phobos can produce more than enough stress to fracture the surface. Stress fractures predicted by that model line up very well with the grooves seen in images of Phobos. The explanation also fits with the observation that some grooves appear younger than others, which would be the case if the process that creates them is ongoing. The same fate may await Neptune's moon Triton, which is also slowly falling inward and has a similarly fractured surface.
GIGANTIC ICE CLOUD IN TITAN'S SOUTH POLAR REGION
New observations made near the south pole of Titan by the Cassini spacecraft add to the evidence that winter comes in like a lion on that moon of Saturn. Scientists have detected an enormous new cloud of frozen compounds in the moon's low- to mid-stratosphere -- a stable atmospheric region above the troposphere, or active weather layer. Cassini's camera had already imaged an impressive cloud hovering over Titan's south pole at an altitude of about 300 km. However, that cloud, first seen in 2012, turned out to be just an initial instalment. A much more massive ice-cloud system has now been found lower in the stratosphere, peaking at an altitude of about 200 km. The new cloud was detected by Cassini's infrared instrument -- the Composite Infrared Spectrometer, or CIRS -- which obtains profiles of the atmosphere at invisible thermal wavelengths. The cloud has a low density, similar to terrestrial fog but probably flat on top. For the past few years, Cassini has been catching glimpses of the transition from autumn to winter at Titan's south pole -- the first time any spacecraft has seen the onset of a Titan winter. Because each Titan season lasts about 7.5 years on our calendar, it will still be winter at the south pole when the Cassini mission ends in 2017.
The ice clouds at Titan's pole do not form in the same way as the Earth's rain clouds. For rain clouds, water evaporates from the surface and encounters cooler temperatures as it rises through the troposphere. Clouds form when the water vapour reaches an altitude where the combination of temperature and air pressure is right for condensation. The methane clouds in Titan's troposphere form in a similar way. However, Titan's polar clouds form higher in the atmosphere by a different process. Circulation in the atmosphere transports gases from the warm hemisphere to the pole in the cold hemisphere. At the cold pole, the warm air sinks, almost like water draining out of a bathtub, in a process known as subsidence. The sinking gases -- a mixture of smog-like hydrocarbons and nitrogen-bearing chemicals called nitriles -- encounter colder and colder temperatures on the way down. Different gases will condense at different temperatures, resulting in a layering of clouds over a range of altitudes. Cassini arrived at Saturn in 2004 -- mid-winter at Titan's north pole. As the north pole has been coming into springtime, the ice clouds there have been disappearing. Meanwhile, new clouds have been forming at the south pole. The build-up of the southern clouds indicates that the direction of Titan's global circulation is changing.
PLUTO MAY HAVE ICE VOLCANOES
Two possible ice volcanoes have been identified on the surface of Pluto. They are seen in images returned from the New Horizons probe, which flew past Pluto in July. The mountains are several km high and tens of km across, and each has what appears to be a depression in the top. Unlike Earth volcanoes that spew molten rock, Pluto's volcanoes - if that is what they are - would be likely to erupt an icy slush of substances such as water, nitrogen, ammonia or methane. The scientists still need to do further work to confirm their volcano idea. What would help in particular is information on the composition of the materials making up the local surface. That information may come in due course, since the probe has not yet returned all of its data from the July encounter. Only about 20% of its observations have so far been downlinked to Earth. But if cryo-vulcanism can be established, it would be an exciting discovery. While the phenomenon has been postulated to occur on several outer-Solar-System bodies, nothing really convincing has been detected, certainly not in terms of mountain-building. The two candidates on Pluto are found just south of Sputnik Planum, the smooth plain on the planet's equator. They have been informally called Wright Mons and Piccard Mons. Their shape recalls that of shield volcanoes on the Earth and Mars -- broad, tall edifices that develop from repeated eruptions of low-viscosity fluids.
As well as their putative calderas, they display a hummocky texture on their flanks that may represent old 'lava' flows. How recently they might have been active, though, no-one can say. Pluto is a small body that should have lost most of its heat to space over the course of Solar-System history. That said, not a lot of energy would be required to melt and mobilise the cocktail of ices that coat the planet. It is now almost four months since New Horizons made its historic fly-by of Pluto and its moons. In that time, the probe has moved 140 million km further into the outer Solar System, some five billion km from the Earth. Mission managers last week oversaw the last of four engine burns that put the spacecraft on course to meet its next target -- a 45-km icy body known simply as 2014 MU69 -- in just over three years' time. However, the team does not yet formally have funding from the US space agency to operate the probe there; a proposal to win that support is likely to be submitted to NASA some time next year.
MOST DISTANT OBJECT IN SOLAR SYSTEM
Researchers have just discovered an icy object, designated V774104, a 'mini-Pluto' which is about 103 times farther away from the Sun than the Earth is (103 AU). V774104 is three times farther away than Pluto is, a new record for the most distant object ever seen in the Solar System. The record was previously held by the 'dwarf planet' Eris, which is 97 AU from the Sun. Eris was first observed in 2003, by a team of astronomers led by Michael Brown, and confirmed in 2005, and regarded then as the most distant known object of the Solar System. It is located well beyond the Kuiper Belt, the home to Pluto and thousands of other icy bodies, a region that stretches from the orbit of Neptune out to about 50 AU from the Sun. V774104 is believed to be between 500 and 1000 km across and is probably made of ice. It is currently 15400 million km from the Sun. That extreme remoteness suggests that its orbit may have remained undisturbed for thousands of millions of years. The object could be one of thousands of distant objects that are thought to form what is known as the Oort cloud (a sphere of icy objects surrounding the Solar System). To date, only two objects are known to lie in the inner Oort cloud, one called Sedna and another known as 2012 VP113, nicknamed Biden. Sedna never comes closer to the Sun than 76 AU; VP113's approach is to 80 AU. If the newly found object never comes closer to the Sun than 103 AU it will join the other two scientifically fascinating objects residing in the inner Oort cloud. The team of astronomers also discovered about a dozen other objects, with distances ranging from 80 to 90 AU. They travel slowly, so gathering orbital data about them is also slow.
NEW-FOUND VENUS-LIKE PLANET
Harvard-Smithsonian Center for Astrophysics
Our knowledge of rocky planets orbiting distant stars is increasing, and the latest discovery is the most intriguing one to date. The new-found object, although hot as an oven, is not too hot to hold an atmosphere. If it does hold one, it is close enough (only 39 light-years away) that we could study that atmosphere in detail with the Hubble telescope and future observatories such as the Giant Magellan Telescope. GJ 1132b, as the planet is known, orbits a red dwarf star only one-fifth the size of the Sun. The star is also cooler and much fainter than the Sun, emitting just 1/200th as much light. GJ 1132b circles its star every 1.6 days at a distance of 1.4 million miles (much closer than the 36-million-mile orbit of Mercury around the Sun). As a result, GJ 1132b is baked to a temperature of about 230 C. That would boil off any water the planet may once have held, but still allows for the presence of an atmosphere. It is also significantly less than that of any other exo-planet confirmed to be rocky. In comparison, well-known objects like CoRoT-7b and Kepler-10b possess scorching temperatures of 1000 C or more. GJ 1132b was discovered by the MEarth-South array, which is dedicated to the hunt for terrestrial planets orbiting red-dwarf stars. MEarth-South consists of eight 40-cm robotic telescopes at the Cerro-Tololo observatory in Chile. It monitors several thousand red-dwarf stars up to 100 light-years away. It looks for planets that transit in front of their host stars. When a planet transits its star, the star's light is dimmed by a small but possibly detectable amount. The amount of dimming gives an indication of the planet's physical size.
After MEarth-South detected a transit of the new object, additional observations were gathered by the array and the Magellan Clay telescope in Chile. The team also measured the host star's gravitational wobble with the HARPS spectrograph to determine the planet's mass. They found that GJ 1132b is 16% larger than the Earth, with a diameter of about 9,200 miles. It has a mass 60% greater than the Earth's. The inferred density indicates that the planet has a rocky composition similar to the Earth's, and also has an Earth-like force of gravity. Since the red-dwarf star is small, the relative size of the planet to the star is larger than it would be for a Sun-like star. That, combined with the star's moderate distance, makes it easier to detect and study any planetary atmosphere that may exist. The team has requested follow-up observations with the Hubble and Spitzer space telescopes. Future observatories like the James Webb space telescope also will undoubtedly take a look at GJ 1132b.
GLOWING HALO OF DEAD STAR
The remains of an interaction between a dead star and a planet associated with it have been studied by astronomers using the Very Large Telescope at the Paranal Observatory in Chile. They give a glimpse of the possible far-future fate of the Solar System. The team used data from the VLT and other observatories to study the shattered remains of an asteroid around a stellar remnant -- a white dwarf called SDSS J1228+1040. Using several instruments, including the Ultraviolet and Visual Echelle Spectrograph (UVES) and X-shooter, both attached to the VLT, the team obtained detailed observations of the light coming from the white dwarf and its surrounding material over a period of twelve years between 2003 and 2015. Observations over periods of years were needed to probe the system from multiple viewpoints. The team used a technique called Doppler tomography -- similar in principle to medical tomographic scans of the human body -- which allowed it to map in detail the structure of the glowing gaseous remains of the material orbiting J1228+1040. While large stars -- those more massive than around ten solar masses -- suffer a spectacularly violent climax as a supernova explosion at the ends of their lives, smaller stars are spared such dramatic fates. When stars like the Sun come to the ends of their lives they exhaust their fuel, expand as red giants and later expel their outer layers into space. The hot and very dense core of the former star, a white dwarf, is all that remains.
But would the planets, asteroids and other bodies in such a system survive the star's evolution? What would be left? The new observations help to answer those questions. It is rare for white dwarfs to be surrounded by orbiting discs of gaseous material -- only seven have ever been found. The team concluded that an asteroid had strayed dangerously close to the dead star and been ripped apart by the immense tidal forces it experienced, to form the disc of material that is now visible. The orbiting disc was formed in a similar way to the photogenic rings seen around planets closer to home, such as Saturn. However, while J1228+1040 is more than seven times smaller in diameter than Saturn, it has a mass over 2500 times greater. The team learned that the distance between the white dwarf and its disc is also quite different -- Saturn and its rings could comfortably sit in the gap between them. The new long-term study has now allowed the team to watch the disc precess under the influence of the very strong gravitational field of the white dwarf. It has also found that the disc is somewhat lopsided and has not yet become circular. Remnants such as J1228+1040 can provide clues to understanding the environments that exist as stars reach the ends of their lives. That can help astronomers to understand the processes that occur in exo-planetary systems and even forecast the fate of the Solar System when the Sun meets its demise in about 7 billion years.
ANCIENT WHITE DWARFS IN MILKY WAY
Space Telescope Science Institute (STScI)
Looking deep into the Milky Way's crowded central hub of stars, Hubble researchers have uncovered for the first time a population of ancient white dwarfs -- smouldering remnants of stars that inhabited the core. Finding such relics at last can yield clues to how our Galaxy was built, long before the Sun and the Earth formed. The observations are the deepest, most detailed study of the Galaxy's foundational structure -- its vast central bulge that lies in the middle of a pancake-shaped disc of stars that includes the Solar System. Like an archaeological relic, the white dwarfs contain the history of a bygone era. They contain information about the stars that existed about 12000 million years ago that burned out to form the white dwarfs.As those dying embers of former stars cool, they serve as time-pieces that tell astronomers something about the Milky Way's early years. An analysis of the Hubble data supports the idea that the Milky Way's bulge formed first and that its stars were born quite quickly -- in less than roughly 2 billion years. The rest of the Galaxy's sprawling disc of second- and third-generation stars grew more slowly further out, encircling the central bulge.
It is important to observe the Milky Way's bulge because it is the only bulge we can study in detail. We can see bulges in distant galaxies, but cannot resolve the very faint stars, such as the white dwarfs. The Milky Way's bulge includes almost a quarter of our Galaxy's stellar mass. Characterizing the properties of the bulge stars can then provide information important to understanding the formation of the entire Milky Way galaxy and that of similar, more distant galaxies. The Hubble survey also found slightly more low-mass stars in the bulge, compared to those in the Galaxy's disc population. That result suggests that the environment in the bulge may have been different from the one in the disc, resulting in a different star-formation mechanism. The observations were so sensitive that the astronomers also used the data to pick out the feeble glow of white dwarfs. The team based its results on an analysis of 70 of the hottest white dwarfs detectable by Hubble in a small region of the bulge among tens of thousands of stars. Those stellar relics are small and extremely dense. They are about the size of the Earth but 200,000 times denser. A teaspoonful of white-dwarf material would weigh about 15 tons. Their tiny size makes them so dim that to see them at the distance of the Galactic Centre is as challenging as looking for the glow of a pocket torch at the distance of the Moon. Astronomers used the Hubble images to separate the bulge stars from the myriad stars in the foreground of our Galaxy's disc by tracking their movements over time. The team did that by analysing Hubble images of the same field of 240,000 stars, taken 10 years apart.
The region surveyed is part of the 'Sagittarius Window Eclipsing Extra-solar Planet Search' ('SWEEPS') field and is located 26,000 light-years away. The unusually dust-free location on the sky offers a keyhole view into the Galaxy's bulge. Hubble's 'Advanced Camera for Surveys' made the observations in 2004 and 2011-2013. The astronomers identified the white dwarfs by analysing the colours of the bulge stars and comparing them with theoretical models. The extremely hot white dwarfs appear blue relative to Sun-like stars. As white dwarfs age, they become cooler and fainter, becoming difficult even for Hubble to detect. Astronomers estimate that the total number of white dwarfs in the tiny Hubble view of the bulge is about 100,000.
NEW INSTRUMENT FOR FINDING WATER IN THE UNIVERSE
A new instrument attached to the 12-m Atacama Pathfinder Experiment (APEX) telescope at 5100 metres elevation in the Chilean Andes, which observes at mm/sub-mm wavelengths, is opening up a previously unexplored window on the Universe. The Swedish-ESO PI receiver for APEX (SEPIA) will detect the faint signals from water and other molecules within the Milky Way, other nearby galaxies and the early Universe. Installed on APEX earlier this year, SEPIA is sensitive to radiation with wavelengths in the range 1.4-1.9 mm. The exceptional observing conditions on the extremely dry Chajnantor Plateau in northern Chile mean that, although such wavelengths are blocked by water vapour in the atmosphere at most places on the Earth, SEPIA is still able to detect the faint signals coming from space. That wavelength region is of great interest to astronomers, as signals from water in space are found there. Water is an important indicator of many astrophysical processes, including the formation of stars, and is believed to play an important role in the origin of life. Studying water in space - in molecular clouds, in star-forming regions and even in comets within the Solar System - is expected to provide critical clues to the role of water in the Milky Way and in the history of the Earth. In addition, SEPIA's sensitivity makes it also a powerful tool for detecting carbon monoxide and ionized carbon in galaxies in the early Universe.
The new SEPIA receiver has been used to make test astronomical observations on APEX during 2015. Identical receivers are being installed in the ALMA antennae. Results from the new detector on APEX have shown it to be working well. With that validation, SEPIA is being made available to the wider scientific community. Just as darkskies are essential to see faint objects in visible light, a very dry atmosphere is needed to pick up the signals from water in the cosmos at millimetre wavelengths. But dry conditions are not the onlyrequirement, the detectors need to be cooled to the very low temperature of -269 C, just 4 degrees above absolute zero, for themto work. Recent technological advances have made such detectors practical. APEX, which is a collaboration between the Max PlanckInstitute for Radio Astronomy (MPIfR), the Onsala Space Observatory (OSO) and ESO, is the largest single-dish sub-mm telescope operating in the southern hemisphere and is based on a prototype antenna constructed for the ALMA project.
Bulletin compiled by Clive Down
(c) 2015 the Society for Popular Astronomy
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