THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 488 2019 April 28

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    THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 488 2019 April 28
    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

    Arizona State University

    A tiny piece of the building blocks from which comets formed has been discovered inside a primitive meteorite. The finding could offer clues to the formation, structure, and evolution of the Solar System. The meteorite was found in Antarctica's LaPaz Icefield and belongs to a class of primitive carbonaceous chondrites that have undergone minimal changes since they formed more than 4.5 billion years ago, probably beyond the orbit of Jupiter. Meteorites were once parts of larger bodies, asteroids, which broke up owing to collisions in space, and survived the trip through the Earth's atmosphere. Their makeup can vary substantially from one meteorite to another, reflecting their origins in diverse parent bodies that formed in different parts of the Solar System. Asteroids and comets both formed from the disc of gas and dust that once surrounded the young Sun, but they aggregated at different distances from it, which affected their chemical makeup. Compared to asteroids, comets contain larger fractions of water ice and far more carbon, and typically formed farther from the Sun where the environment was colder. By studying a meteorite's chemistry and mineralogy, researchers can unlock details about its formation and how much heating and other chemical processing it experienced during the Solar System's formative years.
    Inside the LaPaz meteorite, scientists found a very carbon-rich slice of primitive material. It bears some striking similarities to extraterrestrial dust particles that are thought to have originated in comets that formed near the Solar System's outer edges. Approximately 3 to 3.5 million years after the Solar System formed, but while the Earth was still growing, this tiny object — about a tenth of a millimetre across — was captured by the growing asteroid from which the meteorite originated. Meteorites like LaPaz are great places to hunt for pre-solar grains, microscopic pieces of stardust formed by stars that predate the Solar System. But none of the team expected also to find evidence for a surviving cometary building block inside a meteorite. By undertaking chemical and isotopic analysis of the material, the team was able to show that the encased material probably originated in the icy outer Solar System along with objects from the Kuiper Belt, where many comets originate. The existence of that primitive material captured inside the meteorite suggests that, owing to the drag caused by the surrounding gas, particles like it migrated from the outer edges of the Solar System, where comets and Kuiper Belt objects formed, to the closer-in area beyond Jupiter, where the carbonaceous chondrites formed. That reveals details about how our Solar System's architecture took shape during the early stages of planet formation.

    American Geophysical Union

    Scientists have long known that the Earth and Mercury have metallic cores. Like the Earth's, Mercury's outer core is composed of liquid metal, but there have been only hints that Mercury's innermost core is solid. Now, in a new study, scientists report evidence that Mercury's inner core is indeed solid and that it is very nearly the same size as the Earth's solid inner core. Some scientists compare Mercury to a cannonball because its metal core fills nearly 85 percent of the volume of the planet. That large core — huge compared to those of the other rocky planets in the Solar System — has long been one of the most intriguing mysteries about Mercury. Scientists had also wondered whether Mercury might have a solid inner core. The findings of Mercury's solid inner core help scientists to understand Mercury better, but also offer clues about how the Solar System formed and how rocky planets change over time. The team used several observations from NASA's MESSENGER mission to probe Mercury's interior, and to look, most importantly, at the planet's spin and gravity. The MESSENGER spacecraft entered orbit around Mercury in 2011 March and spent four years observing the planet until it was deliberately brought down to the surface in 2015 April. Scientists used radio observations from MESSENGER to determine Mercury's gravitational anomalies (areas of local increases or decreases in density) and the location of its rotational pole, which allowed them to understand the orientation of the planet.
    Each planet spins on an axis, between its two poles. Mercury spins much more slowly than the Earth, with its day lasting about 58 Earth days.  Scientists often use tiny variations in the way an object spins as clues to its internal structure. In 2007, radar observations made from the Earth revealed small shifts in Mercury's spin, called librations, that proved that some of the planet's core must be liquid — molten metal. But observations of the spin rate alone were not sufficient to give a clear measurement of what the inner core was like. Could there be a solid core lurking underneath, scientists wondered? As MESSENGER orbited Mercury over the course of its mission and got closer and closer to the surface, scientists recorded how the spacecraft accelerated under the influence of the planet's gravity. The density structure of a planet can create subtle changes in a spacecraft's orbit. In the later parts of the mission, MESSENGER flew about 120 miles above the surface, and less than 65 miles during its last year. The final low-altitude orbits provided the best data, and allowed the team to make the most accurate measurements about the internal structure of Mercury yet taken. The team put data from MESSENGER into a computer programme that allowed them to adjust parameters and calculate what the interior composition of Mercury must be like to match the way it spins and the way the spacecraft accelerated around it. The results showed that for the best match, Mercury must have a large, solid inner core. They estimated that the solid iron core is about 2,000 kilometres in diameter and makes up about half of Mercury's entire core, nearly 4,000 kilometres across. In contrast, the Earth's solid core is about 2,400 kilometres across, taking up a little more than a third of our planet's entire core.

    NASA/Jet Propulsion Laboratory

    On its final flyby of Saturn's largest moon in 2017, the Cassini spacecraft gathered radar data revealing that the small liquid lakes in Titan's northern hemisphere are surprisingly deep, perched on hills and filled with methane. The new findings are the first confirmation of just
    how deep some of Titan's lakes are (more than 100 metres) and of their composition. They provide new information about the way liquid methane rains on, evaporates from and seeps into Titan — the only planetary body in the Solar System other than the Earth known to have stable liquid on its surface. Scientists have known that Titan's hydrological cycle works similarly to the Earth's — with one major difference. Instead of water evaporating from seas, forming clouds and rain, Titan does it all with methane and ethane. We tend to think of those hydrocarbons as gases on Earth, unless they're pressurized in a tank. But Titan is so cold that they are liquids there, like petrol at room temperature on our planet. Scientists have known that the much larger northern seas are filled with methane, but finding the smaller northern lakes filled mostly with methane was a surprise. Previously, Cassini data measured Ontario Lacus, the only major lake in Titan's southern hemisphere. There they found a roughly equal mix of methane and ethane. Ethane is slightly denser than methane, with more carbon and hydrogen atoms in its makeup. Adding to the oddities of Titan, with its Earth-like features carved by exotic materials, is the fact that the hydrology on one side of the northern hemisphere is completely different than the that of other side.
    On the eastern side of Titan, there are big seas with low elevation, canyons and islands. On the western side there are small lakes, and the new measurements show that the lakes are perched on big hills and plateaus. The new radar measurements confirm earlier findings that the lakes are far above sea level, but they conjure a new image of landforms — like mesas or buttes — sticking hundreds of feet above the surrounding landscape, with deep liquid lakes on the top. The fact that these western lakes are relatively small — just tens of miles across — but very deep, also tells scientists something new about their 'geology'. It's the best evidence yet that they probably formed when the surrounding bedrock of ice and solid organics chemically dissolved and collapsed. On Earth, similar water lakes are known as karstic lakes. Occurring in areas that include Germany, Croatia and the United States, they form when water dissolves limestone bedrock.

    Centauri, a red dwarf, was first observed by Robert Innes in 1915. It is located approximately 4.2 light-years away, making it the closest star to our Solar System. Three years ago, a team at the European Southern Observatory discovered a planet orbiting the star; it was promptly named Proxima Centauri b. In a new effort, the researchers reported that they had found evidence suggesting that there might be another planet orbiting Proxima Centauri. Proxima Centauri b was identified by the slight wobbling of its host star. The team has been studying data received by HARPS, an instrument at the European Southern Observatory in Chile. The data covering the past 17 years revealed similar signs of the star wobbling, suggesting another planet. The planet would have a mass approximately six times that of the Earth, putting it in the category of a super-Earth planet, and would orbit approximately 1.5 AU from its star. It would also take the planet approximately five Earth years to make one orbit around its star. The team notes that such a long distance from a cooling star would probably mean very cold temperatures on the exoplanet — perhaps as cold as -234 degrees C.

    San Diego State University

    Astronomers have discovered a third planet in the Kepler-47 system, securing the system's title as the most interesting of the binary-star worlds. Using data from NASA's Kepler space telescope, a team of researchers detected the new Neptune-to-Saturn-size planet orbiting between two previously known planets. With its three planets orbiting two suns, Kepler-47 is the only known multi-planet circumbinary system. Circumbinary planets are those that orbit two stars. The planets in the Kepler-47 system were detected by the 'transit' method. If the orbital plane of the planet is aligned edge-on as seen from the Earth, the planet can pass in front of the host stars, leading to a measurable decrease in the observed brightness. The new planet, dubbed Kepler-47d, was not detected earlier because of the weak transit signals. As is common with circumbinary planets, the alignment of the orbital planes of the planets change with time. In this case, the middle planet's orbit has become more aligned, leading to a stronger transit signal. The transit depth went from undetectable at the beginning of the Kepler mission to the deepest of the three planets over the span of just four years.
    The San Diego researchers were surprised by both the size and the location of the new planet. Kepler-47d is the largest of the three planets in the Kepler-47 system. With the discovery of the new planet, a much better understanding of the system is possible. For example, researchers now know the planets in that circumbinary system have very low densities — less than that of Saturn, the Solar-System planet with the lowest density. While a low density is not unusual for the sizzling-hot Jupiter-type exoplanets, it is rare for mild-temperature planets. Kepler-47d's equilibrium temperature is roughly 10 C, while Kepler-47c is 32 C. The innermost planet, which is the smallest circumbinary planet known, is a much hotter 169 C. The inner, middle, and outer planets are 3.1, 7.0, and 4.7 times the size of the Earth, and take 49, 87, and 303 days, respectively, to orbit around their suns. The stars themselves orbit one another in only 7.45 days; one star is similar to the Sun, while the other has a third of the mass of the Sun. The entire system is compact and would fit inside the orbit of the Earth. It is approximately 3300 light-years away in the direction of the constellation Cygnus.

    Bulletin compiled by Clive Down
    (c) 2019 The Society for Popular Astronomy


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