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Early February Astronomy Bulletin
« on: February 07, 2021, 11:34 »
PART OF EARTH’S NITROGEN WAS LOCALLY SOURCED
Rice University

The isotopic signatures of nitrogen in iron meteorites reveal that Earth likely gathered its nitrogen not   only from the region beyond Jupiter's orbit but also from the dust in the inner protoplanetary disk. Nitrogen is a volatile element that, like carbon, hydrogen and oxygen, makes life on Earth possible. Knowing its source offers clues to not only how rocky planets formed in the inner part of our solar system but also the dynamics of far-flung protoplanetary disks. Researchers have always thought that the inner part of the solar system, within Jupiter's orbit, was too hot for nitrogen and other volatile elements to condense as solids, meaning that volatile elements in the inner disk were in the gas phase. Because the seeds of present-day rocky planets, also known as protoplanets, grew in the inner disk by accreting locally sourced dust, it appeared they did not contain nitrogen or other volatiles, necessitating their delivery from the outer solar system. An earlier study by the team suggested much of this volatile-rich material came to Earth via the collision that formed the Moon. But new evidence clearly shows only some of the planet's nitrogen came from beyond Jupiter. In recent years, scientists have analyzed nonvolatile elements in meteorites, including iron meteorites that occasionally fall to Earth, to show dust in the inner and outer solar system had completely different isotopic compositions. Iron meteorites are remnants of the cores of protoplanets that formed at the same time as the seeds of present-day rocky planets, becoming the wild card the authors used to test their hypothesis.

The researchers found a distinct nitrogen isotopic signature in the dust that bathed the inner protoplanets within about 300,000 years of the formation of the solar system. All iron meteorites from the inner disk contained a lower concentration of the nitrogen-15 isotope, while those from the outer disk were rich in nitrogen-15. This suggests that within the first few million years, the protoplanetary disk divided into two reservoirs, the outer rich in the nitrogen-15 isotope and the inner rich in nitrogen-14. The volatile elements were present in the inner disk dust, probably in the form of refractory organics, from the very beginning. This means that contrary to current understanding, the seeds of the present-day rocky planets -- including Earth -- were not volatile-free. Even if other protoplanetary disks don't have the kind of giant planet migration resulting in the infiltration of volatile-rich materials from the outer zones, their inner rocky planets closer to the star could still acquire volatiles from their neighbouring zones.


PHOSPHINE ON VENUS WAS PROBABLY SULPHUR DIOXIDE
University of Washington

In September, British astronomers announced that they had detected the chemical phosphine in the thick clouds of Venus. The team's reported detection, based on observations by two Earth-based radio telescopes, surprised many Venus experts. Earth's atmosphere contains small amounts of phosphine, which may be produced by life. Phosphine on Venus generated buzz that the planet, often succinctly touted as a "hellscape," could somehow harbour life within its acidic clouds. Since that initial claim, other science teams have cast doubt on the reliability of the phosphine detection. Now, researchers have used a robust model of the conditions within the atmosphere of Venus to revisit and comprehensively reinterpret the radio telescope observations underlying the initial phosphine claim. Instead of phosphine in the clouds of Venus, the data are consistent with an alternative hypothesis: They were detecting sulphur dioxide which is the third-most-common chemical compound in Venus' atmosphere, and it is not considered a sign of life. The team shows that sulphur dioxide, at levels plausible for Venus, can not only explain the observations but is also more consistent with what astronomers know of the planet's atmosphere and its punishing chemical environment, which includes clouds of sulfuric acid. In addition, the researchers show that the initial signal originated not in the planet's cloud layer, but far above it, in an upper layer of Venus' atmosphere where phosphine molecules would be destroyed within seconds. This lends more support to the hypothesis that sulphur dioxide produced the signal. Both the purported phosphine signal and this new interpretation of the data centre on radio astronomy. Every chemical compound absorbs unique wavelengths of the electromagnetic spectrum, which includes radio waves, X-rays and visible light. Astronomers use radio waves, light and other emissions from planets to learn about their chemical composition, among other properties.

In 2017 using the James Clerk Maxwell Telescope, or JCMT, the U.K.-led team discovered a feature in the radio emissions from Venus at 266.94 gigahertz. Both phosphine and sulphur dioxide absorb radio waves near that frequency. To differentiate between the two, in 2019 the same team obtained follow-up observations of Venus using the Atacama Large Millimeter/submillimeter Array, or ALMA. Their analysis of ALMA observations at frequencies where only sulphur dioxide absorbs led the team to conclude that sulphur dioxide levels in Venus were too low to account for the signal at 266.94 gigahertz, and that it must instead be coming from phosphine. In this new study, the researchers started by modelling conditions within Venus' atmosphere, and using that as a basis to comprehensively interpret the features that were seen -- and not seen -- in the JCMT and ALMA datasets. This is what's known as a radiative transfer model, and it incorporates data from several decades' worth of observations of Venus from multiple sources, including observatories here on Earth and spacecraft missions like Venus Express. The team used that model to simulate signals from phosphine and sulphur dioxide for different levels of Venus' atmosphere, and how those signals would be picked up by the JCMT and ALMA in their 2017 and 2019 configurations. Based on the shape of the 266.94-gigahertz signal picked up by the JCMT, the absorption was not coming from Venus' cloud layer, the team reports. Instead, most of the observed signal originated some 50 or more miles above the surface, in Venus' mesosphere. At that altitude, harsh chemicals and ultraviolet radiation would shred phosphine molecules within seconds. Phosphine in the mesosphere is even more fragile than phosphine in Venus' clouds. If the JCMT signal were from phosphine in the mesosphere, then to account for the strength of the signal and the compound's sub-second lifetime at that altitude, phosphine would have to be delivered to the mesosphere at about 100 times the rate that oxygen is pumped into Earth's atmosphere by photosynthesis.


SATURN’S TILT CAUSED BY ITS MOONS
CNRS

Rather like David versus Goliath, it appears that Saturn's tilt may in fact be caused by its moons. This is the conclusion of recent work carried out by scientists from the CNRS, Sorbonne University and the University of Pisa, which shows that the current tilt of Saturn's rotation axis is caused by the migration of its satellites, and especially by that of its largest moon, Titan. Recent observations have shown that Titan and the other moons are gradually moving away from Saturn much faster than astronomers had previously estimated. By incorporating this increased migration rate into their calculations, the researchers concluded that this process affects the inclination of Saturn's rotation axis: as its satellites move further away, the planet tilts more and more. The decisive event that tilted Saturn is thought to have occurred relatively recently. For over three billion years after its formation, Saturn's rotation axis remained only slightly tilted. It was only roughly a billion years ago that the gradual motion of its satellites triggered a resonance phenomenon that continues today: Saturn's axis interacted with the path of the planet Neptune and gradually tilted until it reached the inclination of 27° observed today. These findings call into question previous scenarios. Astronomers were already in agreement about the existence of this resonance. However, they believed that it had occurred very early on, over four billion years ago, due to a change in Neptune's orbit. Since that time, Saturn's axis was thought to have been stable. In fact, Saturn's axis is still tilting, and what we see today is merely a transitional stage in this shift. Over the next few billion years, the inclination of Saturn's axis could more than double. The research team had already reached similar conclusions about the planet Jupiter, which is expected to undergo comparable tilting due to the migration of its four main moons and to resonance with the orbit of Uranus: over the next five billion years, the inclination of Jupiter's axis could increase from 3° to more than 30°.


TITAN’S LARGEST SEA IS 1,000-FEET DEEP
Cornell University

Far below the gaseous atmospheric shroud on Saturn's largest moon, Titan, lies Kraken Mare, a sea of liquid methane. Astronomers have estimated that sea to be at least 1,000-feet deep near its centre - enough room for a potential robotic submarine to explore. Kraken Mare contains about 80% of the moon's surface liquids. Titan is cloaked in a golden haze of gaseous nitrogen. But peeking through the clouds, the moonscape has an Earthlike appearance, with liquid methane rivers, lakes and seas, according to NASA. The data for this discovery was gathered on Cassini's T104 flyby of Titan on Aug. 21, 2014. The spacecraft's radar surveyed Ligeia Mare -- a smaller sea in the moon's northern polar region -- to look for the mysteriously disappearing and reappearing "Magic Island." While Cassini cruised at 13,000 mph nearly 600 miles above Titan's surface, the spacecraft used its radar altimeter to measure the liquid depth at Kraken Mare and Moray Sinus, an estuary located at the sea's northern end. The Cornell scientists, along with engineers from NASA's Jet Propulsion Laboratory, had figured out how to discern lake and sea bathymetry (depth) by noting the radar's return time differences on the liquid surface and sea bottom, as well as the sea's composition by acknowledging the amount of radar energy absorbed during transit through the liquid. It turns out that Moray Sinus is about 280 feet deep, shallower than the depths of central Kraken Mare, which was too deep for the radar to measure. Surprisingly the liquid's composition, primarily a mixture of ethane and methane, was methane-dominated and similar to the composition of nearby Ligeia Mare, Titan's second-largest sea.

Earlier scientists had speculated that Kraken may be more ethane rich, both because of its size and extension to the moon's lower latitudes. The observation that the liquid composition is not markedly different from the other northern seas is an important finding that will help in assessing models of Titan's Earth-like hydrologic system. Beyond deep, Kraken Mare also is immense -- nearly the size of all five Great Lakes combined. Titan represents a model environment of a possible atmosphere of early Earth. One puzzle is the origin of the liquid methane. Titan's solar light -- about 100 times less intense than on Earth -- constantly converts methane in the atmosphere into ethane; over roughly 10 million-year periods, this process would completely deplete Titan's surface stores. In the distant future, a submarine -- likely without a mechanical engine -- will visit and cruise Kraken Mare.


SOLAR SYSTEM FORMATION IN TWO STEPS
University of Oxford

Researchers have discovered that a two-step formation process of the early Solar System can explain the chronology and split in volatile and isotope content of the inner and outer Solar System. They present a new theoretical framework for the formation and structure of the Solar System that can explain several key features of the terrestrial planets (like Earth, Venus, and Mars), outer Solar System (like Jupiter), and composition of asteroids and meteorite families. The team's work draws on and connects recent advances in astronomy (namely observations of other solar systems during their formation) and meteoritics -- laboratory experiments and analyses on the isotope, iron, and water content in meteorites. The suggested combination of astrophysical and geophysical phenomena during the earliest formation phase of the Sun and the Solar System itself can explain why the inner Solar System planets are small and dry with little water by mass, while the outer Solar System planets are larger and wet with lots of water. It explains the meteorite record by forming planets in two distinct steps. The inner terrestrial protoplanets accreted early and were internally heated by strong radioactive decay; this dried them out and split the inner, dry from the outer, wet planetary population. This has several implications for the distribution and necessary formation conditions of planets like Earth in extrasolar planetary systems.

The numerical experiments performed by the interdisciplinary team showed that the relative chronologies of early onset and protracted finish of accretion in the inner Solar System, and a later onset and more rapid accretion of the outer Solar System planets can be explained by two distinct formation epochs of planetesimals, the building blocks of the planets. Recent observations of planet-forming disks showed that disk midplanes, where planets form, may have relatively low levels of turbulence. Under such conditions the interactions between the dust grains embedded in the disk gas and water around the orbital location where it transitions from gas to ice phase (the snow line) can trigger an early formation burst of planetesimals in the inner Solar System and another one later and further out. The two distinct formation episodes of the planetesimal populations, which further accrete material from the surrounding disk and via mutual collisions, result in different geophysical modes of internal evolution for the forming protoplanets. The different formation time intervals of these planetesimal populations mean that their internal heat engine from radioactive decay differed substantially. Inner Solar System planetesimals became very hot, developed internal magma oceans, quickly formed iron cores, and degassed their initial volatile content, which eventually resulted in dry planet compositions. In comparison, outer Solar System planetesimals formed later and therefore experienced substantially less internal heating and therefore limited iron core formation, and volatile release. The early-formed and dry inner Solar System and the later-formed and wet outer Solar System were therefore set on two different evolutionary paths very early on in their history. This opens new avenues to understand the origins of the earliest atmospheres of Earth-like planets and the place of the Solar System within the context of the exoplanetary census across the galaxy.


SIMILARITY OF THE 7 ROCKY TRAPPIST PLANETS
ESO
The red dwarf star TRAPPIST-1 is home to the largest group of roughly Earth-size planets ever found in a single stellar system. Located about 40 light-years away, these seven rocky siblings provide an example of the tremendous variety of planetary systems that likely fill the Universe. A new study shows that the TRAPPIST-1 planets have remarkably similar densities. That could mean they all contain about the same ratio of materials thought to compose most rocky planets, like iron, oxygen, magnesium, and silicon. But if this is the case, that ratio must be notably different than Earth’s: The TRAPPIST-1 planets are about 8% less dense than they would be if they had the same makeup as our home planet. Based on that conclusion, researchers hypothesized a few different mixtures of ingredients could give the TRAPPIST-1 planets the measured density. All seven TRAPPIST-1 planets, which are so close to their star that they would fit within the orbit of Mercury, were found via the transit method: Scientists can’t see the planets directly (they’re too small and faint relative to the star), so they look for dips in the star’s brightness created when the planets cross in front of it.

Repeated observations of the starlight dips combined with measurements of the timing of the planets’ orbits enabled astronomers to estimate the planets’ masses and diameters, which were in turn used to calculate their densities. Previous calculations determined that the planets are roughly the size and mass of Earth and thus must also be rocky, or terrestrial – as opposed to gas-dominated, like Jupiter and Saturn. The new research offers the most precise density measurements yet for any group of exoplanets – planets beyond our solar system. The more precisely scientists know a planet’s density, the more limits they can place on its composition. Consider that a paperweight might be about the same size as a baseball yet is usually much heavier. Together, width and weight reveal each object’s density, and from there it is possible to infer that the baseball is made of something lighter (string and leather) and the paperweight is made of something heavier (usually glass or metal). The densities of the eight planets in our own solar system vary widely. The puffy, gas-dominated giants – Jupiter, Saturn, Uranus, and Neptune – are larger but much less dense than the four terrestrial worlds because they’re composed mostly of lighter elements like hydrogen and helium. Even the four terrestrial worlds show some variety in their densities, which are determined by both a planet’s composition and compression due to the gravity of the planet itself. By subtracting the effect of gravity, scientists can calculate what’s known as a planet’s uncompressed density and potentially learn more about a planet’s composition. The seven TRAPPIST-1 planets possess similar densities – the values differ by no more than 3%. This makes the system quite different from our own. The difference in density between the TRAPPIST-1 planets and Earth and Venus may seem small – about 8% – but it is significant on a planetary scale. For example, one way to explain why the TRAPPIST-1 planets are less dense is that they have a similar composition to Earth, but with a lower percentage of iron – about 21% compared to Earth’s 32%, according to the study. Alternatively, the iron in the TRAPPIST-1 planets might be infused with high levels of oxygen, forming iron oxide, or rust. The additional oxygen would decrease the planets’ densities. The surface of Mars gets its red tint from iron oxide, but like its three terrestrial siblings, it has a core composed of non-oxidized iron. By contrast, if the lower density of the TRAPPIST-1 planets were caused entirely by oxidized iron, the planets would have to be rusty throughout and could not have solid iron cores.

The answer might be a combination of the two scenarios – less iron overall and some oxidized iron.
The team also looked into whether the surface of each planet could be covered with water, which is even lighter than rust and which would change the planet’s overall density. If that were the case, water would have to account for about 5% of the total mass of the outer four planets. By comparison, water makes up less than one-tenth of 1% of Earth’s total mass. Because they’re positioned too close to their star for water to remain a liquid under most circumstances, the three inner TRAPPIST-1 planets would require hot, dense atmospheres like Venus’, such that water could remain bound to the planet as steam. But this explanation seems less likely because it would be a coincidence for all seven planets to have just enough water present to have such similar densities.


PUZZLING SIX-EXOPLANET SYSTEM
ESO

Using a combination of telescopes, including the Very Large Telescope, astronomers have revealed a system consisting of six exoplanets, five of which are locked in a rare rhythm around their central star. The researchers believe the system could provide important clues about how planets, including those in the Solar System, form and evolve. The first time the team observed TOI-178, a star some 200 light-years away in the constellation of Sculptor, they thought they had spotted two planets going around it in the same orbit. However, a closer look revealed something entirely different. Through further observations astronomers realised that there were not two planets orbiting the star at roughly the same distance from it, but rather multiple planets in a very special configuration. The new research has revealed that the system boasts six exoplanets and that all but the one closest to the star are locked in a rhythmic dance as they move in their orbits. In other words, they are in resonance. This means that there are patterns that repeat themselves as the planets go around the star, with some planets aligning every few orbits. A similar resonance is observed in the orbits of three of Jupiter’s moons: Io, Europa and Ganymede. Io, the closest of the three to Jupiter, completes four full orbits around Jupiter for every orbit that Ganymede, the furthest away, makes, and two full orbits for every orbit Europa makes. The five outer exoplanets of the TOI-178 system follow a much more complex chain of resonance, one of the longest yet discovered in a system of planets. While the three Jupiter moons are in a 4:2:1 resonance, the five outer planets in the TOI-178 system follow a 18:9:6:4:3 chain: while the second planet from the star (the first in the resonance chain) completes 18 orbits, the third planet from the star (second in the chain) completes 9 orbits, and so on. In fact, the scientists initially only found five planets in the system, but by following this resonant rhythm they calculated where in its orbit an additional planet would be when they next had a window to observe the system. More than just an orbital curiosity, this dance of resonant planets provides clues about the system’s past. The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth. If the system had been significantly disturbed earlier in its life, for example by a giant impact, this fragile configuration of orbits would not have survived. But even if the arrangement of the orbits is neat and well-ordered, the densities of the planets are much more disorderly. It appears there is a planet as dense as the Earth right next to a very fluffy planet with half the density of Neptune, followed by a planet with the density of Neptune. It is not what we are used to. In our Solar System, for example, the planets are neatly arranged, with the rocky, denser planets closer to the central star and the fluffy, low-density gas planets farther out.

To investigate the system’s unusual architecture, the team used data from the European Space Agency’s CHEOPS satellite, alongside the ground-based ESPRESSO instrument on ESO’s VLT and the NGTS and SPECULOOS, both sited at ESO’s Paranal Observatory in Chile. Since exoplanets are extremely tricky to spot directly with telescopes, astronomers must instead rely on other techniques to detect them. The main methods used are imaging transits and radial velocities — observing the star’s light spectrum for small signs of wobbles which happen as the exoplanets move in their orbits. The team used both methods to observe the system: CHEOPS, NGTS and SPECULOOS for transits and ESPRESSO for radial velocities. By combining the two techniques, astronomers were able to gather key information about the system and its planets, which orbit their central star much closer and much faster than the Earth orbits the Sun. The fastest (the innermost planet) completes an orbit in just a couple of days, while the slowest takes about ten times longer. The six planets have sizes ranging from about one to about three times the size of Earth, while their masses are 1.5 to 30 times the mass of Earth. Some of the planets are rocky, but larger than Earth — these planets are known as Super-Earths. Others are gas planets, like the outer planets in our Solar System, but they are much smaller — these are nicknamed Mini-Neptunes. Although none of the six exoplanets found lies in the star's habitable zone, the researchers suggest that, by continuing the resonance chain, they might find additional planets that could exist in or very close to this zone. ESO’s Extremely Large Telescope (ELT), which is set to begin operating this decade, will be able to directly image rocky exoplanets in a star’s habitable zone and even characterise their atmospheres, presenting an opportunity to get to know systems like TOI-178 in even greater detail.


GALAXY SHEDS LIGHT ON HOW STARS FORM
University of Bath

A lot is known about galaxies. We know, for instance, that the stars within them are shaped from a blend of old star dust and molecules suspended in gas. What remains a mystery, however, is the process that leads to these simple elements being pulled together to form a new star. But now scientists have taken a significant step towards understanding how a galaxy's gaseous content becomes organised into a new generation of stars. Their findings have important implications for our understanding of how stars formed during the early days of the Universe, when galaxy collisions were frequent and dramatic, and star and galaxy formation occurred more actively than it does now. For this study, the researchers used the Chile-based Atacama Large Millimeter Array (ALMA) -- a network of radio telescopes combined to form one, mega telescope -- to observe a type of galaxy called a tidal dwarf galaxy (TDG). TDGs emerge from the debris of two older galaxies colliding with great force. They are actively star-forming systems and pristine environments for scientists trying to piece together the early days of other galaxies, including our own -- the Milky Way (thought to be 13.6-billion years old). From their observations, the researchers learnt that a TDG's molecular clouds are similar to those found in the Milky Way, both in terms of size and content. This suggests there is a universal star-formation process at play throughout the Universe. Unexpectedly, however, the TDG in the study (labelled TDG J1023+1952) also displayed a profusion of dispersed gas. In the Milky Way, clouds of gas are by far the most prominent star-forming factories. Because of the vast distance that separates Earth from TDG J1023+1952 (around 50 million light years), individual clouds of molecular gas appear as tiny regions in the sky when viewed through the naked eye. However, ALMA has the power to distinguish the smallest details.


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