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September 1, 2019 at 12:42 pm #9906Anonymous
THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 497 2019 September 1
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 http://www.popastro.com/
STARDUST IN ANTARCTIC SNOW
Technical University of Munich (TUM)
The rare isotope iron-60 is created in massive stellar explosions. Only a very small amount of that isotope reaches Earth from distant stars. Now, a research team has discovered iron-60 in Antarctic snow for the first time. The scientists suggest that the iron isotope comes from the interstellar neighbourhood. The quantity of cosmic dust that trickles onto the Earth each year ranges between several thousand and ten thousand tons. Most of the tiny particles come from asteroids or comets within our solar system. However, a small percentage comes from distant stars. There are no natural terrestrial sources for the iron-60 isotope contained therein; it originates exclusively as a result of supernova explosions or through the reactions of cosmic radiation with cosmic dust. The first evidence of the occurrence of iron-60 on Earth was discovered in deep-sea deposits 20 years ago. Scientists hypothesized that traces of stellar explosions could also be found in the pure, untouched Antarctic snow. In order to verify that assumption, researchers collected 500 kg of snow at the Kohnen Station, a container settlement in the Antarctic, and had it transported to Munich for analysis. There, scientists melted the snow and separated the meltwater from the solid components, which were processed using various chemical methods. The team found five iron-60 atoms in the samples using the accelerator laboratory in Garching near Munich. Analyses allowed the team to rule out cosmic radiation, nuclear weapons tests or reactor accidents as sources of the iron-60. As there are no natural sources for this radioactive isotope on Earth, the iron-60 must have come from a supernova.
The research team was able to make a relatively precise determination as to when the iron-60 has been deposited on Earth: the snow layer that was analyzed was not older than 20 years. Moreover, the iron isotope that was discovered did not seem to come from particularly distant stellar explosions, as the iron-60 dust would have dissipated too much throughout the universe if that had been the case. On the basis of the half-life of iron-60, any atoms originating from the formation of the Earth would have completely decayed by now. The team therefore assumes that the iron-60 in the Antarctic snow originates from the interstellar neighbourhood, for example from an accumulation of gas clouds in which our solar system is currently located. Our solar system entered one of these clouds about 40,000 years ago and will leave it in a few thousand years. If the gas-cloud hypothesis is correct, then material from ice cores older than 40,000 years would not contain interstellar iron-60. That would enable scientists to verify the transition of the solar system into the gas cloud — that would be a groundbreaking discovery for researchers working on the environment of the solar system.
AMERICA'S LARGEST METEOR IMPACT
About 35 million years ago, an asteroid travelling at nearly 231,000 km/h smashed into the Atlantic Ocean near the modern-day town of Cape Charles, Virginia. The space rock vaporized instantly, but its impact triggered a gargantuan tsunami, cast up a monsoon of shattered rocks and molten glass that spanned hundreds of miles and carved out the single largest crater in the United States — the so-called Chesapeake Bay impact structure. Today,
that 40-kilometre crater is buried half a mile below the rocky basement of Chesapeake Bay — the 320-km-long estuary linking Virginia and Maryland on the East Coast. That hasn't stopped scientists from trying to piece together the site's mysterious history since it was first discovered during a drilling project in 1990. In a recent study of ocean sediment cores taken
almost 400 km northeast of the impact site, researchers found traces of radioactive debris dating from the time of the strike, providing fresh evidence of the impact's age and destructive power. When the Chesapeake Bay impactor smashed into the Atlantic, it showered the surrounding land and water with shards of molten glass (known as tektites) for hundreds of miles in every direction. That rain of meteoric debris formed what scientists call the North American tektite strewn field, which stretches from Texas to Massachusetts to Barbados, covering about 10 million square km of terrain. By studying shards of meteoric rock buried deep within that sweeping field of impact wreckage, scientists can gather clues about the asteroid's key characteristics, including its age.
In their recent study, researchers from Arizona State University dated 21 microscopic shards of zircon — a durable gemstone that can survive underground for billions of years. The zircons were lodged in a sediment core taken from roughly 655 metres below the surface of the Atlantic Ocean. Not only is zircon commonly found in tektites, but it is also a choice mineral for radiometric dating, thanks to some of its radioactive elemental components. In this case, the researchers used a dating technique called uranium–thorium–helium dating, which looks at how radioactive isotopes of uranium and thorium decay into helium. By comparing the ratios of specific helium, thorium and uranium isotopes in each mineral sample, the researchers
calculated approximately how long ago the zircon crystals solidified and started to decay. The team found that the 21 crystals ranged widely in age, running the gamut from about 33 million to 300 million years old. The two youngest samples, which had an average age of about 35 million years, fit in with previous studies' estimates for the time of the Chesapeake Bay impact.
A closer examination showed that the zircons also bore a cloudy appearance and deformed surface, two signs the minerals were kicked through the air and water by a great impact. The team concluded that those two young crystals were part of the Chesapeake impact's path of destruction, confirming that the impact occurred about 35 million years ago. Moreover, the researchers wrote, it showed that uranium–thorium–helium dating is a useable method for constraining the age of ancient impact events, giving scientists a tool to reveal our planet's long and violent past.
MISSION TO EUROPA CONFIRMED
An icy ocean world in our solar system that could tell us more about the potential for life on other worlds is coming into focus with confirmation of the Europa Clipper mission's next phase. The decision allows the mission to progress to completion of final design, followed by the construction and testing of the entire spacecraft and scientific payload. The mission will
conduct an in-depth exploration of Jupiter's moon Europa and investigate whether that icy moon could harbour conditions suitable for life, honing our insights into astrobiology. To develop the mission in the most cost-effective fashion, NASA is targeting to have the Europa Clipper spacecraft complete and ready for launch as early as 2023.
SECOND PLANET ORBITING BETA PICTORIS
A team of astronomers has discovered a second giant planet in orbit around beta Pictoris, a star that is relatively young (23 million years old) and close (63.4 light years), and surrounded by a disc of dust. The Beta Pictoris system has fascinated astronomers for the last thirty years, since it enables them to observe a planetary system in the process of forming around its star. Comets have been discovered in the system, as well as a gas giant, Beta Pictoris b, detected by direct imaging and described by the team in 2009. This time, the team had to analyse more than 10 years of high-resolution data, obtained with the HARPS instrument at ESO's La Silla Observatory in Chile, in order indirectly to detect the presence of Beta
Pictoris c2. That second giant planet, which has a mass nine times that of Jupiter, completes its orbit in roughly 1,200 days, and is relatively close to its star (approximately the distance between the Sun and the asteroid belt, whereas Beta Pictoris b is 3.3 times more distant). The researchers hope to find out more about the planet from data from the GAIA spacecraft and from the Extremely Large Telescope now under construction in Chile.
RARE LOOK AT ROCKY EXOPLANET'S SURFACE
A new study using data from the Spitzer Space Telescope provides a rare glimpse of conditions on the surface of a rocky planet orbiting a star other than the Sun. The study shows that the planet's surface may resemble those of Earth's Moon or Mercury: the planet probably has little to no atmosphere and could be covered in the same cooled volcanic material found in the dark areas of the Moon's surface, called maria. Discovered in 2018 by the Transiting Exoplanet Satellite Survey (TESS) mission, planet LHS 3844b is located 48.6 light-years from Earth and has a radius 1.3 times that of the Earth. It orbits a small, cool type of star called an M dwarf — especially noteworthy because, as the most common and long-lived type of star in the Milky Way galaxy, M dwarfs may host a high percentage of the total number of planets in the galaxy. TESS found the planet via the transit method, which involves detecting when the observed light of a parent star dims because of a planet orbiting between the star and Earth. Detecting light coming directly from a planet's surface — another method — is difficult because
the star is so much brighter and drowns out the planet's light. But during follow-up observations, Spitzer was able to detect light from the surface of LHS 3844b. The planet makes one full revolution around its parent star in just 11 hours. With such a tight orbit, LHS 3844b is most likely 'tidally locked', which means that one side of a planet permanently faces the star. The star-facing side, or dayside, is about 770 degrees Celsius. Being extremely hot, the planet radiates a lot of infrared light, and Spitzer is an infrared telescope. The planet's parent star is relatively cool (though still much hotter than the planet), making direct observation of LHS 3844b's dayside possible. This observation marks the first time Spitzer data have been able to provide information about the atmosphere of a terrestrial world
around an M dwarf.
By measuring the temperature difference between the planet's hot and cold sides, the team found that there is a negligible amount of heat being transferred between the two. If an atmosphere were present, hot air on the dayside would naturally expand, generating winds that would transfer heat around the planet. On a rocky world with little to no atmosphere, like the
Moon, there is no air present to transfer heat. Understanding the factors that could preserve or destroy planetary atmospheres is part of how scientists plan to search for habitable environments beyond our solar system. The Earth's atmosphere is the reason liquid water can exist on the surface, enabling life to thrive. On the other hand, the atmospheric pressure of Mars is now less than 1% of Earth's, and the oceans and rivers that once dotted the Red Planet's surface have disappeared. Compared to Sun-like stars, M dwarfs emit high levels of ultraviolet light (though less light overall), which is harmful to life and can erode a planet's
atmosphere. They are particularly violent in their youth, belching up a large number of flares, or bursts of radiation and particles that could strip away budding planetary atmospheres. The Spitzer observations rule out an atmosphere with more than 10 times the pressure of Earth's. (Measured in units called bars, Earth's atmospheric pressure at sea level is about 1 bar.) An atmosphere between 1 and 10 bars on LHS 3844b has been almost entirely ruled out as well, although the authors note that there is a slim chance it could exist if the stellar and planetary properties were to meet some very specific and unlikely criteria. They also argue that with the
planet so close to a star, a thin atmosphere would be stripped away by the star's intense radiation and outflow of material (often called stellar winds). Spitzer and the Hubble Space Telescope have previously gathered information about the atmospheres of multiple gas planets, but LHS 3844b appears to be the smallest planet for which scientists have used the light coming from its surface to learn about its atmosphere (or lack thereof). Spitzer previously used the transit method to study the seven rocky worlds around the TRAPPIST-1 star (also an M dwarf) and learn about their possible overall composition; for instance, some of them probably contain water ice. The authors of the new study went one step further, using LHS 3844b's surface albedo (its reflectiveness) to try to infer its composition. The study shows that LHS 3844b is 'quite dark', which suggests that the planet is covered with basalt, a kind of volcanic rock.
Bulletin compiled by Clive Down (c) 2019 The Society for Popular Astronomy
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