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Saturn’s largest moon has electrified sand, castles could last for weeks
by Ron Duwell
April 18, 2017
Saturn has been stealing the imaginations of scientists and laymen alike in recent weeks with a host of new discoveries in the planet’s vast satellite system. NASA’s Cassini spacecraft is working overtime as of late, capturing images of potentially life-bearing chemicals found in Enceladus’ enormous water plumes and a recent discovery made on the planet’s largest moon, Titan.
Apparently, the planet’s sand would be amazing for sand castles if enthusiasts were ever to arrive there.
A recent study on the planet’s surface concludes that the sand on Titan is electrified with such strong static that sand particles form an extremely tight bond with one another. Joshua Méndez, a granular dynamicist at the Georgia Institute of Technology in Atlanta, says that the planet’s geology is like Earth only at surface value.
At first glance, if you look at images from NASA’s Cassini spacecraft, Titan looks very Earth-like, with dunes, lakes, oceans, mountains and potentially volcanoes, and it has a dense, nitrogen-rich atmosphere like Earth’. But once you start looking at the details, you realize that it is an alien and exciting world.
These differences come from the sand’s ability to pick up an electric charge through the “triboelectric effect,” the same process in which we pick up static and shock one another. Earth’s sand is able to undergo the same effect, but it does not stick as Titan’s sand does thanks to its strong gravitational pull, which overpowers any cling generated by the static.
On Titan, where the gravity is seven times weaker than Earth’s, the cling is able to hold thanks to it not being so easily overpowered. Méndez also attributes this shaping ability to the sand’s lighter composition, which is fluffier than the silicate minerals that make up the sand on Earth.
Need to rethink how we perceive the surface of other planets
In fact, the cling is so strong that not even the winds can knock it down. Méndez’s study might finally provide an explanation as to why the moon’s prevailing winds don’t flow with the direction of its giant dunes.
Our findings highlight that caution is needed when applying models from Earth to other environments. We have to rethink our assumptions with a world as different as Titan.
hanks to its Nitrogen-rich atmosphere, Titan is often seen as one of the stopping points for human colonization as space exploration expands to the outer reach of our solar system.
Devourer of planets? Researchers dub star 'Kronos'
October 12, 2017
https://phys.org/news/2017-10-devourer- ... ronos.html
In mythology, the Titan Kronos devoured his children, including Poseidon (better known as the planet Neptune), Hades (Pluto) and three daughters.
So when a group of Princeton astronomers discovered twin stars, one of which showed signs of having ingested a dozen or more rocky planets, they named them after Kronos and his lesser-known brother Krios. Their official designations are HD 240430 and HD 240429, and they are both about 350 light years from Earth.
Read more at: https://phys.org/news/2017-10-devourer- ... s.html#jCp
The keys to the discovery were first confirming that the widely separated pair are in fact a binary pair, and secondly observing Kronos' strikingly unusual chemical abundance pattern, explained Semyeong Oh, a graduate student in astrophysical sciences who is lead author on a new paper describing Kronos and Krios. Oh works with David Spergel, the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation and director of the Flatiron Institute's Center for Computational Astrophysics.
Other co-moving star pairs have had different chemistries, Oh explained, but none as dramatic as Kronos and Krios.
Most stars that are as metal-rich as Kronos "have all the other elements enhanced at a similar level," she said, "whereas Kronos has volatile elements suppressed, which makes it really weird in the general context of stellar abundance patterns."
In other words, Kronos had an unusually high level of rock-forming minerals, including magnesium, aluminum, silicon, iron, chromium and yttrium, without an equally high level of volatile compounds—those that are most often found in gas form, like oxygen, carbon, nitrogen and potassium.
Kronos is already outside the galactic norm, said Oh, and in addition, "because it has a stellar companion to compare it to, it makes the case a little stronger."
Kronos and Krios are far enough apart that some astronomers have questioned whether the two were in fact a binary pair. Both are about 4 billion years old, and like our own, slightly older sun, both are yellow G-type stars. They orbit each other infrequently, on the order of every 10,000 years or so. An earlier researcher, Jean-Louis Halbwachs of the Observatoire Astronomique of Strasbourg, had identified them as co-moving—moving together—in his 1986 survey, but Oh independently identified them as co-moving based on two-dimensional astrometric information from the European Space Agency's Gaia mission.
During a group research discussion at the Flatiron Institute, a colleague suggested pooling their data sets. John Brewer, a postdoctoral researcher from Yale University visiting at Columbia University, had been using data from the Keck Observatory on Mauna Kea, Hawaii, to calculate the spectrographic chemistries and radial velocities of stars.
"John suggested that maybe we should cross-match my co-moving catalogue with his chemical-abundance catalogue, because it's interesting to ask whether they have the same compositions," Oh said.
Binary stars should have matching radial velocities, but that information hadn't been available in the Gaia dataset, so seeing their matching velocities in Brewer's data supported the theory that Kronos and Krios, though two light years apart, were a binary set.
Then the researchers noticed the extreme chemical differences between them.
"I'm very easily excitable, so as soon as they had the same radial velocities and different chemistry, my mind already started racing," said Adrian Price-Whelan, a Lyman Spitzer, Jr. Postdoctoral Fellow in Astrophysical Sciences and a co-author on the paper.
Oh took more convincing, both scientists recalled. "Semyeong is careful and was skeptical," said Price-Whelan, so her first step was to double-check all the data. Once simple error had been ruled out, they began entertaining various theories. Maybe Kronos and Krios had accreted their planetary disks at different times during stellar formation. That one can't be tested, said Price-Whelan, but it seems unlikely.
Maybe they only started moving together more recently, after trading partners with another pair of binary stars, a process known as binary exchange. Oh ruled that out with "a simple calculation," she said. "She's very modest," Price-Whelan noted.
Oh's skepticism was finally overcome when she plotted the chemical abundance pattern as a function of condensation temperature—the temperatures at which volatiles condense into solids. Condensation temperatures play a key role in planetary formation because rocky planets tend to form where it's warm—closer to a star—while gas giants form more easily in the colder regions far from their star.
She immediately observed that all of the minerals that solidify below 1200 Kelvin were the ones Kronos was low in, while all the minerals that solidify at warmer temperatures were abundant.
"Other processes that change the abundance of elements generically throughout the galaxy don't give you a trend like that," said Price-Whelan. "They would selectively enhance certain elements, and it would appear random if you plotted it versus condensation temperatures. The fact that there's a trend there hinted towards something related to planet formation rather than galactic chemical evolution."
That was her "Aha!" moment, Oh said. "All of the elements that would make up a rocky planet are exactly the elements that are enhanced on Kronos, and the volatile elements are not enhanced, so that provides a strong argument for a planet engulfment scenario, instead of something else."
Oh and her colleagues calculated that gaining this many rock-forming minerals without many volatiles would require engulfing roughly 15 Earth-mass planets.
Eating a gas giant wouldn't give the same result, Price-Whelan explained. Jupiter, for example, has an inner rocky core that could easily have 15 Earth masses of rocky material, but "if you were to take Jupiter and throw it into a star, Jupiter also has this huge gaseous envelope, so you'd also enhance carbon, nitrogen—the volatiles that Semyeong mentioned," he said. "To flip it around, you have to throw in a bunch of smaller planets."
While no known star has 15 Earth-sized planets in orbit around it, the Kepler space telescope has detected many multi-planet systems, said Jessie Christiansen, an astronomer at the NASA Exoplanet Science Institute at the California Institute of Technology, who was not involved in the research. "I see no problem with there being more than 15 Earth masses of accretable material around a solar-type star." She pointed to Kepler-11, which has more than 22 Earth masses of material in six planets with close orbits, or HD 219134, which has at least 15 Earth masses of material in its inner four planets.
"At the moment, we are still at the stage of piecing together different observations to determine how and when exoplanets form," said Christiansen. "It's difficult to directly observe planet formation around young stars—they are typically shrouded in dust, and the stars themselves are very active, which makes it hard to disentangle any signals from the planets. So we have to infer what we can from the limited information we have. If borne out, this new window onto the masses and compositions of the material in the early stages of planetary systems may provide crucial constraints for planet formation theories."
The research also has implication for stellar formation models, noted Price-Whelan.
"One of the common assumptions—well-motivated, but it is an assumption—that's pervasive through galactic astronomy right now is that stars are born with [chemical] abundances, and they then keep those abundances," he said. "This is an indication that, at least in some cases, that is catastrophically false."
Explore further: What kinds of stars form rocky planets?
Provided by: Princeton University
Grote hoeveelheid hagel valt uit ringen rond Saturnus
04 oktober 2018 20:15
https://www.nu.nl/wetenschap/5496815/gr ... urnus.html
Hagel valt met grote hoeveelheden uit de ringen rond de planeet Saturnus. Soms valt ongeveer 10.000 kilo hagel per seconde, heeft ruimtesonde Cassini ontdekt.
Cassini verging volgens planning vorig jaar op Saturnus, aan het einde van een jarenlange missie.
Aan het einde van de ruimtereis scheerde de ruimtesonde meer dan twintig keer dicht langs de planeet en onder de ringen door. Met die duikvluchten wilden wetenschappers ontdekken waar de bovenkant van de atmosfeer van Saturnus uit bestaat en hoe die reageert op materiaal dat afkomstig is uit de ringen.
Wetenschappers hadden al een vermoeden van de opvallende regenbuien, maar die blijken groter te zijn dan ze dachten. "De regen is eerder een wolkbreuk'', aldus een van de onderzoekers. Het gaat om een mengsel van waterijs en organisch materiaal.
Ruimtesonde is 21 jaar geleden gelanceerd
De Cassini was in 1997 gelanceerd. Na zeven jaar kwam hij aan bij Saturnus en zijn minstens 62 manen. De sonde ontdekte daar onder meer oceanen op de Saturnusmaan Enceladus en zeeën van vloeibaar methaan op de maan Titan. Bij zijn ondergang vloog de sonde de dampkring van Saturnus binnen, waar hij uit elkaar viel.
In de laatste momenten verrichtte de sonde nog allerlei metingen, op een plek waar de mensheid nog nooit was geweest.
Vluchtleiding wilde crash voorkomen
De ondergang was nauwkeurig voorbereid. De vluchtleiding besloot de sonde te laten vergaan, omdat de brandstof aan boord (plutonium) langzaam opraakte.
De crash voorkwam dat de sonde ooit stuurloos zou inslaan op een van de manen van Saturnus. Daar zijn misschien sporen van leven te vinden. De vluchtleiding wil die niet radioactief vervuilen.