The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 393 2015 February 15
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METEORITE MAY REPRESENT BULK OF MARS' CRUST
NWA 7034, a meteorite found a few years ago in the Moroccan desert, is like no other rock ever found on Earth. It has been shown to be a 4.4-billion-year-old piece of the crust of Mars and, according to a new analysis, rocks just like it may cover vast swaths of Mars. In a new paper, scientists report that spectroscopic measurements of the meteorite are a spot-on match with measurements from orbit of the Martian dark plains, areas where the planet's coating of red dust is thin and the rocks beneath are exposed. The findings suggest that the meteorite is representative of the 'bulk background' of rocks on the Martian surface. When scientists started analyzing the meteorite in 2011, they knew that they had something special. Its chemical make-up confirmed that it came from Mars, but it was unlike any other Martian meteorite. Previously, all the Martian rocks found on Earth were classified as SNC meteorites (shergottites, nakhlites, or chassignites). They are mainly igneous rocks made of cooled volcanic material, but the new object is a breccia, a mash-up of different rock types welded together in a basaltic matrix. It contains sedimentary components that match the chemical make-up of rocks analyzed by the Mars rovers. Scientists concluded that it is a piece of Martian crust -- the first such sample to be found on the Earth.
Scientists thought it might help to clear up a long-standing enigma: spectra obtained from SNC meteorites never quite match remotely-sensed spectra from the Martian surface. So after acquiring a chip of the meteorite, they used a variety of spectroscopic techniques to analyze it. The researchers say that the spectral match suggests that the 'dark plains' on Mars are dominated by brecciated rocks similar to the new meteorite. Because the dark plains are dust-poor regions, they are thought to be representative of what lies beneath the red dust on much of the rest of the planet. The researchers claim that, in the light of what is known about Mars, the idea that the surface would be rich in such breccias makes sense. Mars has more than 400,000 impact craters more than 1 km in diameter. Because brecciation is a natural consequence of impacts, it is to be expected that material similar to NWA 7034 has accumulated on Mars over time. In other words, many of the rocks on the surface of Mars are probably very similar to the meteorite.
MERGING STARS DESTINED TO BECOME SUPERNOVA
Astronomers observing the planetary nebula Henize 2-428 have identified a close pair of white-dwarf stars -- tiny, extremely dense stellar remnants -- that have a total mass of about 1.8 times that of the Sun. It is the most massive such pair yet found, and when the two stars merge, as they seem destined to do about 700 million years from now, they will create a runaway thermonuclear explosion leading to a Type Ia supernova. The team that found the massive pair actually set out with a different interest. They wanted to find out how some stars produce strangely shaped and asymmetric nebulae late in their careers. One of the objects they studied was the unusual planetary nebula known as Henize 2-428. The team looked at the object's central star with the Very Large Telescope, and found not just one but a pair of stars at the heart of that strangely lopsided glowing cloud. Further observations, made with telescopes in the Canary Islands, allowed them to determine the orbit of the two stars and to deduce their masses and their separation. They found that each of the stars has a mass slightly less than that of the Sun and that they orbit one another every four hours. They are so close together that, according to Einstein's theory of general relativity, they will get closer and closer, spiralling in owing to loss of energy through the emission of gravitational waves, before eventually merging into a single star that will promptly explode.
NEARBY SUPERNOVA REMNANT HAS FROTHY INTERIOR
Harvard-Smithsonian Center for Astrophysics
Cassiopeia A, or Cas A for short, is one of the best-studied supernova remnants in the Galaxy, but we do not know everything about it yet.Astronomers have generated a new 3-D map of its interior. They found that the remnant includes a collection of about half a dozen great cavities. About 340 years ago a massive star exploded in the constellation Cassiopeia. As it blew itself apart, extremely hot and radioactive matter rapidly streamed outward from the star's core, mixing and churning debris. The complex physics behind such explosions is difficult to model. However, by studying relatively young supernova remnants like Cas A, astronomers can try to investigate the processes that drive such titanic stellar explosions.
To make the 3-D map, astronomers examined Cas A in near-infrared wavelengths with the Mayall 4-m telescope at Kitt Peak. Spectroscopy allowed them to measure expansion velocities of extremely faint material in Cas A's interior, which provided the crucial third dimension. They found that the large interior cavities appear to be connected to -- and nicely explain -- the previously observed large rings of debris that make up the bright and easily seen outer shell of Cas A. The two best-defined cavities are 3 and 6 light-years in diameter, and the entire arrangement has a structure like a Swiss cheese. The bubble-like cavities were probably created by plumes of radioactive nickel generated during the stellar explosion. Since the nickel will decay to form iron, the astronomers think that Cas A's interior cavities should be enriched with as much as a tenth of a solar mass of iron. Such enriched interior debris has not been detected by previous observations, however, so next-generation telescopes may be used to look for the 'missing' iron and confirm the origin of the cavities.
NEW DETAILS IN WHIRLPOOL GALAXY
Case Western Reserve University
Astronomers have discovered new features of a galaxy that has been sketched and photographed for 170 years. The researchers were able to see faint plumes extending from the northeast and south of the nearby spiral galaxy M51a, also called the 'Whirlpool Galaxy', by taking a 20-hour-exposure photograph. The image also provides new details of the linear northwest plume, which is nearly 120,000 light-years long, and reveals a lack of stars in a portion of the southeast tail. M51a was the first galaxy whose spiral structure was recognized, when it was identified and sketched by William Parsons, the Earl of Rosse, in 1845. The whirlpool and its small companion, M51b, are about 31 million light years away, in the constellation of Canes Venatici. The images were taken from the Burrell Schmidt telescope at Kitt Peak in 2010 and 2012. The team aimed the telescope at M51 on moonless nights and exposed its digital camera to the light from the galaxy at 20-minute intervals, recalibrating in between. For a total of 10 hours, light was filtered to reveal younger stars; for another 10 hours it was filtered to reveal older stars. The two exposures were merged to create the final image.
The northwest plume was seen in the 1970s, but the technology provided limited detail. The astronomers found that it is dominated by older, redder stars and has little gas, found in small patches. Owing to the age of the stars and the extreme length of the plume, they suggest that the plume was created by the interaction of an outer disc of M51 with another galaxy 200 million years or more ago. The southern plume is an oddity. It has no morphological similarities to the surrounding parts of M51, and no gas. The plume has comparatively few stars and, therefore, mass, and little total light. One possibility, the researchers suggest, is that the plume could be the remnant of a third satellite or body in the M51 system. The northeast plume has about the same total light as the southern one and may be an extension of the north side of the galaxy, but that is impossible to tell. Other researchers discovered the southeastern gas tail in 1990 and assumed that it was pulled out during an interaction with another galaxy. The new, deeper view still found no stars. That is unusual for such a tail, but it provides a clear test for future interaction models. The astronomers are now devising other ways to look at M51, particularly to gather more detail about the faint plumes. The northwest plume is bright enough that it may be a good candidate for further study with the Hubble telescope.
STARS ARE YOUNGER THAN PREVIOUSLY THOUGHT
The latest data release from the Planck satellite consortium reveals a surprise: star formation in the Universe may be more recent than previously indicated by the analysis of Planck's predecessor, the WMAP satellite. Thanks to new maps of cosmic background radiation (in particular, those containing 'polarization anisotropies' of radiation) scientists have found that the 're-ionization' process could be more recent than was estimated until now. Re-ionization is one of the most important processes in cosmology, as it is associated with star formation, which cosmologists date back to after the 'dark ages' of the Universe, when there was still no star light. The WMAP satellite, launched in 2001, had given an initial estimate of the period when the process may have taken place. The discovery, which still requires validation by the measurements that Planck is still able to provide and that will be published in about a year's time, is associated with the publication of maps of polarized cosmic background radiation (the first light in the Universe produced by the Big Bang). WMAP was the first satellite to attempt to provide such a map, but today new Planck data suggest that re-ionization may have occurred approximately 550 million years after the Big Bang, i.e., 100 million years later than WMAP had estimated. According to Planck's observations, stars may be younger than believed, in keeping with other independent astrophysical indicators, and that finding may have major consequences for our attempts to understand the dark components of the Universe.
GRAVITATIONAL WAVES AND COSMIC INFLATION REMAIN ELUSIVE
NASA/Jet Propulsion Laboratory
A joint analysis of data from the Planck space mission and the ground-based experiment BICEP2 has found no conclusive evidence of gravitational waves from the birth of the Universe, despite earlier reports of a possible detection. The collaboration between the teams has resulted in the most precise knowledge yet of what signals from the ancient gravitational waves should look like, aiding future searches. Planck and BICEP/Keck [Keck, in this item, does not refer to the large telescopes in Hawaii, but to an array of small telescopes set up at the South Pole. - ED] were both designed to measure relic radiation emitted by the Universe shortly after its birth 13.8 billion years ago. An extraordinary source of information about the Universe's history lies in that 'fossil' radiation, called the cosmic microwave background (CMB). Planck mapped the CMB over the entire sky from space, while BICEP/Keck focused on one patch of sky over the South Pole. In 2014 March, astronomers presented intriguing data from the BICEP2/Keck experiments, finding what appeared to be a possible signal from the Universe when it was just born. If the signal were indeed from the early cosmos, then it would have confirmed the presence of ancient gravitational waves. It is hypothesized that those waves were generated by an explosive and very rapid period of growth in the Universe, called inflation, which took place when the Universe was only a tiny of a fraction of a second old. Specifically, the BICEP/Keck experiments found evidence for a 'curly' pattern of polarized light called B-modes. Those patterns would have been imprinted on the CMB light as the gravitational waves slightly squeezed and stretched the fabric of space. Polarization describes a particular property of light. Usually, the electric and magnetic fields carried by light vibrate in all orientations equally, but when they vibrate preferentially in a certain direction, the light is polarized.
The swirly polarization pattern, reported by BICEP2, was also clearly seen with new data from the Keck Array. Searching for that unique record of the very early Universe is as difficult as it is exciting, since its subtle signal is hidden in the polarization of the CMB, which itself represents only a feeble few per cent of the total light. One of the trickiest aspects of identifying the primordial B-modes is separating them from those that can be generated much closer to us by interstellar dust in our Milky Way galaxy. The Milky Way is pervaded by a mixture of gas and dust shining at frequencies similar to those of the CMB, and that closer, or foreground, emission affects the observation of the oldest cosmic light. Very careful data analysis is needed to separate the foreground emission from that of the CMB. The BICEP2/Keck experiments collected data at a single microwave frequency, making it difficult to separate the emissions coming from the dust in the Milky Way and the CMB. On the other hand, seven of the Planck channels were also equipped with polarization-sensitive detectors. Some of those frequencies were chosen to make measurements of dust in the Milky Way. By careful analysis, those multi-frequency data can be used to separate the various contributions of emissions. The Planck and BICEP2/Keck teams joined forces, combining the space satellite's ability to deal with foregrounds by means of observations at several frequencies, with the greater sensitivity of the ground-based experiments over limited areas of the sky.
The final results showed that most of the original BICEP2/Keck B-mode signal, but not necessarily all of it, could be explained by dust in the Milky Way. As for signs of the Universe's inflationary period, the question remains open. The joint Planck/BICEP/Keck study sets an upper limit on the amount of gravitational waves from inflation, which might have been generated at the time but at a level too low to be confirmed by the present analysis. The new upper limit on the signal due to gravitational waves agrees well with the upper limit obtained earlier with Planck from the temperature fluctuations of the CMB. But the gravitational-wave signal could still be there, and the search has definitely not been abandoned.
UK 'TWINKLE' SATELLITE TO UNVEIL EXOPLANET ATMOSPHERES
UK scientists have announced plans for a small satellite, named 'Twinkle', that should give radical new insights into the chemistry, formation and evolution of planets orbiting other stars. The mission should be launched within four years. Nearly two thousand exo-planets (orbiting stars other than the Sun) have been discovered to date, but we know very little about them. We can measure their mass, density and distance from their star. From that, we can deduce that that some are freezing cold, some are so hot that they have molten surfaces, some are balls of gas, like Jupiter, or small and rocky, like the Earth. When an exo-planet passes in front of the star that it orbits, a tiny amount of starlight is filtered through the molecules and clouds in the planet's atmosphere, if it has one. Twinkle will observe that light to see if it shows the characteristic spectral signatures of gases such as water vapour or methane that might be present on the planet. Knowledge of the chemical composition of exo-planet atmospheres is important for understanding whether a planet was born in the orbit in which it is currently observed or whether it has migrated from a different part of its planetary system. The make-up, evolution, chemistry and physical processes driving an exo-planet's atmosphere are strongly affected by its distance from its parent star. The atmospheres of small, terrestrial-type planets may have evolved quite dramatically from their initial composition. The loss of lighter molecules, impacts with other bodies such as comets or asteroids, volcanic activity, or even life can significantly alter the composition of primordial atmospheres. Atmospheric composition is therefore a tracer of an exo-planet's history as well as whether it might be 'habitable'.
Twinkle will analyze at least 100 exo-planets in the Milky Way. Its infrared spectrograph will enable observations to be made of a wide range of planet types including super-Earths (rocky planets 1-10 times the mass of the Earth) and 'hot Jupiters' (gas giants orbiting very close to their stars). Some of the target planets are orbiting stars similar to our Sun and some are orbiting cooler red dwarfs. For the largest planets orbiting bright stars, Twinkle is expected even to be able to produce maps of clouds and temperature. The spacecraft will be built to operate for a minimum of three years, with the possibility of an extension to five years or more. The mission will be funded through a mixture of private and public sources. With a total cost of around £50 million, including launch, Twinkle is reckoned to be 10 times less expensive to build and operate than other astrophysical spacecraft developed through international space-agency programmes. The relatively moderate development time-scale and budget are made possible through expertise already developed at UK institutions and the use of off-the-shelf components.
Bulletin compiled by Clive Down
(c) 2015 the Society for Popular Astronomy
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