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April 5, 2020 at 1:24 pm #10092Anonymous
THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 512 2020 April 5
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
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METEORITES REVEAL MARS’ WATER HISTORY
University of Arizona
New research shows that Mars likely received water from at least two vastly different sources early in its history. The variability the researchers found implies that Mars, unlike Earth and the Moon, never had an ocean of magma completely encompassing the planet. A team of astronomers was able to piece together Mars' water history by looking for clues in two types, or isotopes, of hydrogen. One hydrogen isotope contains one proton in its nucleus; this is sometimes called “light hydrogen.” The other isotope is called deuterium, which contains a proton and a neutron in the nucleus; this is sometimes referred to as “heavy hydrogen.” The ratio of these two hydrogen isotopes signals to a planetary scientist the processes and possible origins of water in the rocks, minerals and glasses in which they're found.
For about 20 years, researchers have been recording the isotopic ratios from Martian meteorites, and their data were all over the place. There seemed to be little trend. Water locked in Earth rocks is what's called unfractionated, meaning it doesn't deviate much from the standard reference value of ocean water — a 1:6,420 ratio of heavy to light hydrogen. Mars' atmosphere, on the other hand, is heavily
fractionated — it is mostly populated by deuterium, or heavy hydrogen, likely because the solar wind stripped away the light hydrogen. Measurements from Martian meteorites — many of which were excavated from deep within Mars by impact events — ran the gamut between Earth and Mars' atmosphere measurements.
The team set out to investigate the hydrogen isotope composition of the Martian crust specifically by studying samples they knew were originated from the crust: the Black Beauty and Allan Hills meteorites. Black Beauty was especially helpful because it's a mashup of surface material from many different points in Mars' history. The isotopic ratios of the meteorite samples fell about midway between the value for Earth rocks and Mars' atmosphere. When the researchers' findings were compared with previous studies, including results from the Curiosity Rover, it seems that this was the case for most of Mars' 4 billion-plus-year history. The idea that Mars' interior was Earth-like in composition came from one study of a Martian meteorite thought to have originated from the mantle — the interior between the
planet's core and its surface crust. However, Martian meteorites basically plot all over the place, and so trying to figure out what these samples are actually telling us about water in the mantle of Mars has historically been a challenge. The fact that data for the crust was so different prompted the team to go back through the scientific literature and scrutinize the data. The researchers found that two
geochemically different types of Martian volcanic rocks -enriched shergottites and depleted shergottites — contain water with different hydrogen isotope ratios. Enriched shergottites contain more deuterium than the depleted shergottites, which are more Earth-like, they found. It turns out that if you mix different proportions of hydrogen from these two kinds of shergottites, you can get the crustal value. The team think that the shergottites are recording the signatures of two different hydrogen — and by extension, water — reservoirs within Mars. The stark difference hints to them that more than one source might have contributed water to Mars and that Mars did not have a global magma ocean.
STRANGE ORBITS OF ‘TATOOINE’ PLANETARY DISKS
National Radio Astronomy Observatory
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found striking orbital geometries in protoplanetary disks around binary stars. While disks orbiting the most compact binary star systems share very nearly the same plane, disks encircling wide binaries have orbital planes that are severely tilted. These systems can teach us about planet formation in complex environments. In
the last two decades, thousands of planets have been found orbiting stars other than our Sun. Some of these planets orbit two stars just like Luke Skywalker's home Tatooine. Planets are born in protoplanetary disks — we now have wonderful observations of these thanks to ALMA — but most of the disks studied so far orbit single stars. 'Tatooine' exoplanets form in disks around binary stars, so-called
circumbinary disks. Studying the birthplaces of 'Tatooine' planets provides a unique opportunity to learn about how planets form in different environments. Astronomers already know that the orbits of binary stars can warp and tilt the disk around them, resulting in a circumbinary disk misaligned relative to the orbital plane of its host stars. For example, in a 2019 study ALMA found a striking circumbinary disk in a polar configuration. The astronomers compared the ALMA data of the circumbinary disks with the dozen 'Tatooine' planets that have been found with the Kepler space telescope. To their surprise, the team found that the degree to which binary stars and their circumbinary disks are misaligned is strongly dependent on the orbital period of the host stars. The shorter the orbital period of the binary star, the more likely it is to host a disk in line with its orbit. However, binaries with periods longer than a month typically host misaligned disks.
There is a clear overlap between the small disks, orbiting compact binaries, and the circumbinary planets found with the Kepler mission. Because the primary Kepler mission lasted 4 years, astronomers were only able to discover planets around binary stars that orbit each other in fewer than 40 days. And all of these planets were aligned with their host star orbits. A lingering mystery was whether there might be many misaligned planets that Kepler would have a hard time finding. We now
know that there likely isn't a large population of misaligned planets that Kepler missed, since circumbinary disks around tight binary stars are also typically aligned with their stellar hosts. Still, based on this finding, the astronomers conclude that misaligned planets around wide binary stars should be out there and that it would be an exciting population to search for with other exoplanet-finding methods like direct imaging and microlensing. (NASA's Kepler mission used the transit method, which
is one of the ways to find a planet.) Astronomers now want to find out why there is such a strong correlation between disk (mis)alignment and the binary star orbital period.
WE MAY BE IN A VAST BUBBLE
Université de Genève
The Earth, solar system, entire Milky Way and the few thousand galaxies closest to us move in a vast “bubble” that is 250 million light years in diameter, where the average density of matter is half as large as for the rest of the Universe. This is the hypothesis put forward by a theoretical physicist from the University of Geneva (UNIGE) to solve a conundrum that has been splitting the scientific community for a decade: at what speed is the Universe expanding? Until now, at least two independent calculation methods have arrived at two values that are different by about 10% with a deviation that is statistically irreconcilable. This new approach erases this divergence without making use of any “new physics.” The Universe has been expanding since the Big Bang occurred 13.8 billion years ago — a proposition first made by the Belgian canon and physicist Georges Lemaître (1894-1966), and first demonstrated by Edwin Hubble (1889-1953). The American astronomer discovered in 1929 that every galaxy is pulling away from us, and that the most distant galaxies are moving the most quickly. This suggests that there was a time in the past when all the galaxies were located at the same spot, a time that can only
correspond to the Big Bang. This research gave rise to the Hubble-Lemaître law, including the Hubble constant (H0), which denotes the Universe's rate of expansion. The best H0 estimates currently lie around 70 (km/s)/Mpc (meaning that the Universe is expanding 70 kilometres a second more quickly every 3.26 million light years). The problem is that there are two conflicting methods of calculation.
The first is based on the cosmic microwave background: this is the microwave radiation that comes at us from everywhere, emitted at the time the Universe became cold enough for light finally to be able to circulate freely (about 370,000 years after the Big Bang). Using the precise data supplied by the Planck space mission, and given the fact that the Universe is homogeneous and isotropic, a value of 67.4 is obtained for H0 using Einstein's theory of general relativity to run through the scenario. The second calculation method is based on the supernovae which appear sporadically in distant galaxies. These very bright events provide the observer with highly precise distances, an approach that has made it possible to determine a value for H0 of 74. These two values carried on becoming more precise for many years while remaining different from each other. It didn't take much to spark a scientific controversy and even to arouse the exciting hope that we were perhaps dealing with a 'new physics'. To narrow the gap, it has been suggested that the Universe is not as homogeneous as claimed, a hypothesis that may seem obvious on relatively modest scales. There is no doubt that matter is distributed differently inside a galaxy than outside one. It is more difficult, however, to imagine
fluctuations in the average density of matter calculated on volumes thousands of times larger than a galaxy. If we were in a kind of gigantic 'bubble' where the density of matter was significantly lower than the known density for the entire Universe, it would have consequences on the distances of supernovae and, ultimately, on determining H0. All that would be needed would be for this “Hubble bubble” to be
large enough to include the galaxy that serves as a reference for measuring distances. By establishing a diameter of 250 million light years for this bubble, the physicist calculated that if the density of matter inside was 50% lower than for the rest of the Universe, a new value would be obtained for the Hubble constant, which would then agree with the one obtained using the cosmic microwave background. The probability that there is such a fluctuation on this scale is 1 in 20 to 1 in 5 which means that it is not a theoretician's fantasy. There are a lot of regions like ours in the vast Universe.
EVIDENCE FOR ELUSIVE MID-SIZED BLACK HOLE
NASA/Goddard Space Flight Center
Astronomers have found the best evidence for the perpetrator of a cosmic homicide: a black hole of an elusive class known as “intermediate-mass,” which betrayed its existence by tearing apart a wayward star that passed too close. Weighing in at about 50,000 times the mass of our Sun, the black hole is smaller than the supermassive black holes (at millions or billions of solar masses) that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of a massive star. These so-called intermediate-mass black holes (IMBHs) are a long-sought “missing link” in black hole evolution. Though there have been a few other IMBH candidates, researchers consider these new observations the strongest evidence yet for mid-sized black holes in the Universe. The team used Hubble to follow up on leads from NASA's Chandra X-ray Observatory and ESA's (the European Space Agency) X-ray Multi-Mirror Mission (XMM-Newton). In 2006 these satellites detected a powerful flare of X-rays, but they could not determine whether it originated from inside or outside of our galaxy. Researchers attributed it to a star being torn apart after coming too close to a gravitationally
powerful compact object, like a black hole. Surprisingly, the X-ray source, named 3XMM J215022.4?055108, was not located in a galaxy's centre, where massive black holes normally would reside. This raised hopes that an IMBH was the culprit, but first another possible source of the X-ray flare had to be ruled out: a neutron star in our own Milky Way galaxy, cooling off after being heated to a very high
temperature. Neutron stars are the crushed remnants of an exploded star. Hubble was pointed at the X-ray source to resolve its precise location. Deep, high-resolution imaging provides strong evidence that the X-rays emanated not from an isolated source in our galaxy, but instead in a distant, dense star cluster on the outskirts of another galaxy — just the type of place astronomers expected to find an
IMBH. Previous Hubble research has shown that the mass of a black hole in the centre of a galaxy is proportional to that host galaxy's central bulge. In other words, the more massive the galaxy, the more massive its black hole. Therefore, the star cluster that is home to 3XMM J215022.4?055108 may be the stripped-down core of a lower-mass dwarf galaxy that has been gravitationally and tidally disrupted by its close interactions with its current larger galaxy host.
IMBHs have been particularly difficult to find because they are smaller and less active than supermassive black holes; they do not have readily available sources of fuel, nor as strong a gravitational pull to draw stars and other cosmic material which would produce telltale X-ray glows. Astronomers essentially have to catch an IMBH red-handed in the act of gobbling up a star. The team combed through the XMM-Newton data archive, searching hundreds of thousands of observations to
find one IMBH candidate. The X-ray glow from the shredded star allowed astronomers to estimate the black hole's mass of 50,000 solar masses. The mass of the IMBH was estimated based on both X-ray luminosity and the spectral shape. This is much more reliable than using X-ray luminosity alone as typically done before for previous IMBH candidates. The reason why we can use the spectral fits to estimate the IMBH mass for our object is that its spectral evolution showed that it has been in the thermal spectral state, a state commonly seen and well understood in accreting stellar-mass black holes. This object isn't the first to be considered a likely candidate for an intermediate-mass black hole. In 2009 Hubble teamed up with NASA's Swift observatory and ESA's XMM-Newton to identify what is interpreted as an IMBH, called HLX-1, located towards the edge of the galaxy ESO 243-49. It too is in the centre of a young, massive cluster of blue stars that may be a stripped-down dwarf galaxy core. The X-rays come from a hot accretion disk around the black hole. The main difference is that this object is tearing a star apart, providing strong evidence that it is a massive black hole, instead of a stellar-mass black hole as people often worry about for previous candidates including HLX-1. Finding this IMBH opens the door to the possibility of many more lurking undetected in the dark, waiting to be given away by a star passing too close. The team plans to continue his meticulous detective work, using the methods his team has proved successful. Many questions remain to be answered. Does a supermassive black hole grow from an IMBH? How do IMBHs themselves form? Are dense star clusters their favoured home?
MISSION TO STUDY GIANT SOLAR PARTICLE STORMS
ASA has selected a new mission to study how the Sun generates and releases giant space weather storms – known as solar particle storms – into planetary space. Not only will such information improve understanding of how our solar system works, but it ultimately can help protect astronauts traveling to the Moon and Mars by providing better information on how the Sun's radiation affects the space
environment they must travel through. The new mission, called the Sun Radio Interferometer Space Experiment (SunRISE), is an array of six CubeSats operating as one very large radio telescope. NASA has awarded $62.6 million to design, build and launch SunRISE by no earlier than July 1, 2023. NASA chose SunRISE in August 2017 as one of two Mission of Opportunity proposals to conduct an 11-
month mission concept study. In February 2019, the agency approved a continued formulation study of the mission for an additional year. The mission design relies on six solar-powered CubeSats to simultaneously observe radio images of low-frequency emission from solar activity and share them via NASA's Deep Space Network. The constellation of CubeSats will fly within 10 kilometres of each other,
above Earth's atmosphere, which otherwise blocks the radio signals SunRISE will observe. Together, the six CubeSats will create 3D maps to pinpoint where giant particle bursts originate on the Sun and how they evolve as they expand outward into space. This, in turn, will help determine what initiates and accelerates these giant jets of radiation. The six individual spacecraft will also work together to map,
for the first time, the pattern of magnetic field lines reaching from the Sun out into interplanetary space.
Bulletin compiled by Clive Down
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