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  • richardmitnick 8:29 am on August 14, 2019 Permalink | Reply
    Tags: "A brief astronomical history of Saturn’s amazing rings", , , , , In the four centuries since the invention of the telescope rings have also been discovered around Jupiter Uranus and Neptune., , Pioneer 11, Saturn's rings, , The magnificent ring system of Saturn is between 10 meters and one kilometer thick., The shepherd moons Pan; Daphnis; Atlas; Pandora and Prometheus measuring between eight and 130 kilometers across quite literally shepherd the ring particles keeping them in their present orbits.   

    From The Conversation: “A brief astronomical history of Saturn’s amazing rings” 

    Conversation
    From The Conversation

    August 14, 2019
    Vahe Peroomian, University of Southern California

    1
    With giant Saturn hanging in the blackness and sheltering Cassini from the Sun’s blinding glare, the spacecraft viewed the rings as never before.

    Many dream of what they would do had they a time machine. Some would travel 100 million years back in time, when dinosaurs roamed the Earth. Not many, though, would think of taking a telescope with them, and if, having done so, observe Saturn and its rings.

    Whether our time-traveling astronomer would be able to observe Saturn’s rings is debatable. Have the rings, in some shape or form, existed since the beginnings of the solar system, 4.6 billion years ago, or are they a more recent addition? Had the rings even formed when the Chicxulub asteroid wiped out the dinosaurs?

    I am a space scientist with a passion for teaching physics and astronomy, and Saturn’s rings have always fascinated me as they tell the story of how the eyes of humanity were opened to the wonders of our solar system and the cosmos.

    Our view of Saturn evolves

    When Galileo first observed Saturn through his telescope in 1610, he was still basking in the fame of discovering the four moons of Jupiter. But Saturn perplexed him. Peering at the planet through his telescope, it first looked to him as a planet with two very large moons, then as a lone planet, and then again through his newer telescope, in 1616, as a planet with arms or handles.

    Four decades later, Giovanni Cassini first suggested that Saturn was a ringed planet, and what Galileo had seen were different views of Saturn’s rings. Because of the 27 degrees in the tilt of Saturn’s rotation axis relative to the plane of its orbit, the rings appear to tilt toward and away from Earth with the 29-year cycle of Saturn’s revolution about the Sun, giving humanity an ever-changing view of the rings.

    But what were the rings made of? Were they solid disks as some suggested? Or were they made up of smaller particles? As more structure became apparent in the rings, as more gaps were found, and as the motion of the rings about Saturn was observed, astronomers realized that the rings were not solid, and were perhaps made up of a large number of moonlets, or small moons. At the same time, estimates for the thickness of the rings went from Sir William Herschel’s 300 miles in 1789, to Audouin Dollfus’ much more precise estimate of less than two miles in 1966.

    Astronomers understanding of the rings changed dramatically with the Pioneer 11 and twin Voyager missions to Saturn.

    NASA Pioneer 11

    NASA/Voyager 1

    NASA/Voyager 2

    Voyager’s now famous photograph of the rings, backlit by the Sun, showed for the first time that what appeared as the vast A, B and C rings in fact comprised millions of smaller ringlets.

    2
    Voyager 2 false color image of Saturn’s B and C rings showing many ringlets. NASA

    The Cassini mission to Saturn, having spent over a decade orbiting the ringed giant, gave planetary scientists even more spectacular and surprising views.

    NASA/ESA/ASI Cassini-Huygens Spacecraft

    The magnificent ring system of Saturn is between 10 meters and one kilometer thick. The combined mass of its particles, which are 99.8% ice and most of which are less than one meter in size, is about 16 quadrillion tons, less than 0.02% the mass of Earth’s Moon, and less than half the mass of Saturn’s moon Mimas. This has led some scientists to speculate whether the rings are a result of the breakup of one of Saturn’s moons or the capture and breakup of a stray comet.

    The dynamic rings

    In the four centuries since the invention of the telescope, rings have also been discovered around Jupiter, Uranus and Neptune, the giant planets of our solar system. The reason why the giant planets are adorned with rings and Earth and the other rocky planets are not was first proposed by Eduard Roche, a French astronomer in 1849.

    A moon and its planet are always in a gravitational dance. Earth’s moon, by pulling on opposite sides of the Earth, causes the ocean tides. Tidal forces also affect planetary moons. If a moon ventures too close to a planet, these forces can overcome the gravitational “glue” holding the moon together and tear it apart. This causes the moon to break up and spread along its original orbit, forming a ring.

    The Roche limit, the minimum safe distance for a moon’s orbit, is approximately 2.5 times the planet’s radius from the planet’s center. For enormous Saturn, this is a distance of 87,000 kilometers above its cloud tops and matches the location of Saturn’s outer F ring. For Earth, this distance is less than 10,000 kilometers above its surface. An asteroid or comet would have to venture very close to the Earth to be torn apart by tidal forces and form a ring around the Earth. Our own Moon is a very safe 380,000 kilometers away.

    3
    NASA’s Cassini spacecraft about to make one of its dives between Saturn and its innermost rings as part of the mission’s grand finale. NASA/JPL-Caltech

    The thinness of planetary rings is caused by their ever-changing nature. A ring particle whose orbit is tilted with respect to the rest of the ring will eventually collide with other ring particles. In doing so, it will lose energy and settle into the plane of the ring. Over millions of years, all such errant particles either fall away or get in line, leaving only the very thin ring system people observe today.

    During the last year of its mission, the Cassini spacecraft dived repeatedly through the 7,000 kilometer gap between the clouds of Saturn and its inner rings. These unprecedented observations made one fact very clear: The rings are constantly changing. Individual particles in the rings are continually jostled by each other. Ring particles are steadily raining down onto Saturn.

    The shepherd moons Pan, Daphnis, Atlas, Pandora and Prometheus, measuring between eight and 130 kilometers across, quite literally shepherd the ring particles, keeping them in their present orbits. Density waves, caused by the motion of shepherd moons within the rings, jostle and reshape the rings. Small moonlets are forming from ring particles that coalesce together. All this indicates that the rings are ephemeral. Every second up to 40 tons of ice from the rings rain down on Saturn’s atmosphere. That means the rings may last only several tens to hundreds of millions of years.

    Could a time-traveling astronomer have seen the rings 100 million years ago? One indicator for the age of the rings is their dustiness. Objects exposed to the dust permeating our solar system for long periods of time grow dustier and darker.

    Saturn’s rings are extremely bright and dust-free, seeming to indicate that they formed anywhere from 10 to 100 million years ago, if astronomers’ understanding of how icy particles gather dust is correct. One thing is for certain. The rings our time-traveling astronaut would have seen would have looked very different from the way they do today.

    See the full article here .

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    The Conversation launched as a pilot project in October 2014. It is an independent source of news and views from the academic and research community, delivered direct to the public.
    Our team of professional editors work with university and research institute experts to unlock their knowledge for use by the wider public.
    Access to independent, high quality, authenticated, explanatory journalism underpins a functioning democracy. Our aim is to promote better understanding of current affairs and complex issues. And hopefully allow for a better quality of public discourse and conversation.

     
  • richardmitnick 10:41 am on July 17, 2019 Permalink | Reply
    Tags: , , , , , , Saturn's rings   

    From Eos: “The Cassini Mission May Be Over, but New Discoveries Abound” 

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    Eos news bloc

    From Eos

    7.17.19
    Sarah Derouin
    sarah.derouin@gmail.com

    NASA/ESA/ASI Cassini-Huygens Spacecraft

    New analysis of high-resolution images shows ring textures and disruptions within Saturn’s rings in unprecedented detail.

    1
    The embedded moon Daphnis creates three waves in Saturn’s rings in this image taken by the spacecraft Cassini during its grand finale. Credit: NASA/JPL-Caltech /Space Science Institute
    After more than a decade observing Saturn, Cassini completed its mission in style—a grand finale sent the spacecraft on almost 2 dozen dives between the planet and the rings before it took its final descent into Saturn’s atmosphere.

    During these ring-grazing trips during summer 2017, Cassini collected high spatial resolution images and spectral and temperature scans of the rings. Years after its crash, researchers are still working with the piles of data Cassini collected, making new discoveries about the ringed planet.

    In a new paper in Science, researchers dove into these high-resolution data, and their synthesis revealed new features inside the rings that hadn’t been seen before. They found sculpted areas within the rings—including banded textures and disturbances from embedded bodies—that can be used to help theorists narrow in on how Saturn and its rings may have formed.

    Ring Disruptions

    During the grand finale, Cassini took high-resolution images of all the rings, from the ring closest to the planet (ring A) to the F ring, one of the most distant. The images are the highest-fidelity images ever to be taken of the rings, and they revealed some surprises to the research team.

    “We found a number of things that are new—a number of structures that we’d never been close enough to see before,” says Matthew Tiscareno, a senior research scientist at the SETI Institute in Mountain View, Calif., and lead author of the paper.

    The team explored disturbances within the rings related to moons or smaller moonlike debris embedded in the rings. The moon Daphnis, for example, leaves a wide trail of disruption in its wake, including a wide gap in the ring and trailing waves of debris.

    These waves, Tiscareno explains, are created by the rings moving at different speeds: The rings orbiting closer to Saturn move at a faster speed than those farther from the planet. This process creates a sheer flow, and “on the outward edge, the ring part, the ring material is falling behind Daphnis and its orbit,” he says.

    2
    This propeller, informally called Bleriot, formed within Saturn’s rings. Propellers are caused by a central moonlet that alters the ring as it orbits around the planet. Credit: NASA/JPL-Caltech/Space Science Institute

    But it’s not just big moons like Daphnis that disrupt the rings. Smaller objects are trying their best to create ring gaps as well, but with less success. “These are objects that are 10 times smaller, which means they’re a thousand times less massive,” says Tiscareno.

    Instead of forming a gap, Tiscareno says these objects form a propeller-shaped disturbance. The swirled structure forms briefly but doesn’t stick around long enough to create a true gap in the ring.

    The researchers knew the propellers existed, so they asked Cassini to perform some targeted flybys to get a closer look.

    “The details of that [propeller] structure [are] telling us exactly how big the moons are at the center of the propeller…about a kilometer across,” says Tiscareno, adding that at that size, it’s not possible to see the actual moon with these image resolutions.

    Ring Textures

    Cassini’s instruments also revealed new details on textures within the rings. “We knew that there were textures before, but we had not seen them as comprehensively,” says Tiscareno. The team noted that the ring textures ranged from strawlike clumps to feathery regions, with sharp edges on their borders.

    One idea for the different textures within the rings is a changing particle composition, says Douglas Hamilton, an astronomer at the University of Maryland who was not involved with the paper. For example, one part of the ring could be more silicate rich, whereas another area has more ice. But Hamilton says the researchers “make a good case” for these textures being caused by something other than composition.

    The team inferred that the sharp borders along the ring textures were not due to a composition change, says Tiscareno, but instead result from the physical properties of the ring particles.

    One physical property might be the roughness of ring particles. Tiscareno explains: Is a ring particle more like a billiard ball or more like a snowball? Roughness can affect not only the reflection of light but also how particles interact with each other. “Do they bounce off of each other, like billiard balls do?” he asks. “Or are they kind of sticky, like a snowball would be?”

    Forming a Ring

    Getting close-up data from Cassini gives researchers information that reaches beyond our nearby ringed planet.

    “Rings are our only natural laboratory to understand disk processes more generally,” says Tiscareno. “And that goes to understanding baby solar systems, which are disk systems where you have massive objects that are embedded in the disk.”

    “We’re seeing massive objects embedded in the rings, and we’re seeing the disk itself doing things that we didn’t expect,” he adds.

    Hamilton says that papers like this help uncover how features like propellers might form. “The theory is our imagination,” Hamilton says. Work like this paper, he adds, allows theoretical researchers to be able to test their models on Saturn’s rings against observed data.

    “[These data are going] to be the basis for 10 years of effort by the entire field in trying to figure out how to make all this [propellers, textures] happen,” says Hamilton.

    See the full article here .

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    Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.

     
  • richardmitnick 1:08 pm on January 23, 2019 Permalink | Reply
    Tags: , , , , , Saturn's rings, Saturn’s icy rings reveal another secret: they’re young   

    From Cornell Chronicle: “Saturn’s icy rings reveal another secret: they’re young” 

    Cornell Bloc

    From Cornell Chronicle

    January 23, 2019
    Blaine Friedlander

    1
    Cassini’s wide-angle camera captures the sunlit side of Saturn’s rings June 26, 2016, offering a good view of the B ring from about 940,000 miles away.
    NASA/JPL-Caltech/Space Science Institute

    Data from the last days of the NASA spacecraft Cassini show that Saturn’s beautiful, extensive rings are relatively young – perhaps created when dinosaurs roamed the Earth – because the ring’s mass is less than previously thought and its frozen components are surprisingly bright and free from dusty cosmic impurities, according to a study published Jan. 17 in Science.

    “Based on previous research, we suspected the rings were young, but not everyone was convinced,” said Phil Nicholson, Cornell professor of astronomy and a co-author of “Measurement and Implications of Saturn’s Gravity Field and Ring Mass”[Science above].

    Before Cassini’s demise when it crashed into Saturn in September 2017, the spacecraft passed repeatedly between the rings and the planet’s cloud tops to study Saturn’s gravity field and the rings’ mass.

    NASA/ESA/ASI Cassini-Huygens Spacecraft

    Cassini (and two Voyager spacecraft) had studied Saturn’s rings from afar, but no craft had yet ventured into the rings to obtain up-close data.

    NASA/Voyager 1

    NASA/Voyager 2

    Before its final planetary plunge, Cassini dove through the rings 22 times, using six passes to measure the gravity field by tracking the radio signal from the spacecraft. (The technique is similar to a police radar, but more precise; Cassini’s velocity was measured with an accuracy of better than 0.1 millimeter per second.)

    The scientists found that the rings – particularly the dense B-ring, one of the three main rings and the brightest visible in a telescope – had lower masses than many had expected, indicating a relatively young age. While Saturn is about 4.5 billion years old, the new Cassini data indicate that the rings probably formed between 10 million and 100 million years ago, according to the lead researchers from Sapienza University in Rome.

    Had the rings been contaminated and darkened by interplanetary debris over a longer period, they would appear much darker, according to NASA’s Jet Propulsion Laboratory.

    “The new mass measurement is firm, because Cassini was able to pass inside the rings. In our prior research, we used waves driven in the rings by Saturn’s moons to indirectly estimate their mass density at several locations, which we then extrapolated to estimate the total mass of the rings,” said Nicholson, who had conducted that earlier research at Cornell with Matt Hedman, now an assistant professor of astronomy at the University of Idaho. “Our final result was very close to the new measurement, but lower than most earlier estimates.”

    “From what we know based on Cassini’s spectral and radar measurements, the rings are also less contaminated than previously thought – probably less than 1 percent,” said Nicholson. “They are close to pure water ice.”

    In 2016, Zhimeng Zhang, Ph.D. ’16, led work examining the dust content of Saturn’s C ring. This research determined that the C ring, once thought to have formed in the primordial era, was less than 100 million years old. In 2017, she reported on similar measurements of the A and B rings, obtaining similarly young ages.

    “Think of an unused desk in an unused room. The more it sits there, the more it collects dust,” Zhang said when she published her work. “The C ring is the same way. While it is composed mostly of water ice, it collects silicate-containing dust from the far-off Kuiper Belt. … [I]n this case, the dust – in terms of the age of the solar system – has not been here a long time.”

    Among ring scientists, Nicholson and others had wagered what Cassini might find in terms of ring mass. The result was close to Nicholson’s prediction. He said: “This is quite gratifying from a scientific and personal point-of-view that we got close to the real number when Cassini finally measured it.”

    See the full article here .


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    Once called “the first American university” by educational historian Frederick Rudolph, Cornell University represents a distinctive mix of eminent scholarship and democratic ideals. Adding practical subjects to the classics and admitting qualified students regardless of nationality, race, social circumstance, gender, or religion was quite a departure when Cornell was founded in 1865.

    Today’s Cornell reflects this heritage of egalitarian excellence. It is home to the nation’s first colleges devoted to hotel administration, industrial and labor relations, and veterinary medicine. Both a private university and the land-grant institution of New York State, Cornell University is the most educationally diverse member of the Ivy League.

    On the Ithaca campus alone nearly 20,000 students representing every state and 120 countries choose from among 4,000 courses in 11 undergraduate, graduate, and professional schools. Many undergraduates participate in a wide range of interdisciplinary programs, play meaningful roles in original research, and study in Cornell programs in Washington, New York City, and the world over.

     
  • richardmitnick 1:55 pm on February 24, 2017 Permalink | Reply
    Tags: , Saturn's rings   

    From NAOJ: “Saturn’s Rings Viewed in the Mid-infrared Show Bright Cassini Division” 

    NAOJ

    NAOJ

    1
    Figure 1: A three-color composite of the mid-infrared images of Saturn on January 23, 2008 captured with COMICS on the Subaru Telescope. The Cassini Division and the C ring appear bright. Color differences reflect the temperatures; the warmer part is blue, the cooler part is red. (Credit: NAOJ)

    A team of researchers has succeeded in measuring the brightnesses and temperatures of Saturn’s rings using the mid-infrared images taken by the Subaru Telescope in 2008. The images are the highest resolution ground-based views ever made. They reveal that, at that time, the Cassini Division and the C ring were brighter than the other rings in the mid-infrared light and that the brightness contrast appeared to be the inverse of that seen in the visible light (Figure 1). The data give important insights into the nature of Saturn’s rings.

    The beautiful appearance of Saturn and its rings has always fascinated people. The rings consist of countless numbers of ice particles orbiting above Saturn’s equator. However, their detailed origin and nature remain unknown. Spacecraft- and ground-based telescopes have tackled that mystery with many observations at various wavelengths and methods. The international Cassini mission led by NASA has been observing Saturn and its rings for more than 10 years, and has released a huge number of beautiful images.

    Subaru Views Saturn

    The Subaru Telescope also has observed Saturn several times over the years. Dr. Hideaki Fujiwara, Subaru Public Information Officer/Scientist, analyzed data taken in January 2008 using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the telescope to produce a beautiful image of Saturn for public information purposes. During the analysis, he noticed that the appearance of Saturn’s rings in the mid-infrared part of the spectrum was totally different from what is seen in the visible light.

    Saturn’s main rings consist of the C, B, and A rings, each with different populations of particles. The Cassini Division separates the B and A rings. The 2008 image shows that the Cassini Division and the C ring are brighter in the mid-infrared wavelengths than the B and A rings appear to be (Figure 1). This brightness contrast is the inverse of how they appear in the visible light, where the B and A rings are always brighter than the Cassini Division and the C ring (Figure 2).

    2
    Figure 2: Comparison of the images of Saturn’s rings in the 2008 view in the mid-infrared (left) and the visible light (right). The visible light image was taken on March 16, 2008 with the 105-cm Murikabushi telescope at Ishigakijima Astronomical Observatory. The radial brightness contrast of Saturn’s rings is the inverse between the two wavelength ranges. (Credit: NAOJ)

    “Thermal emission” from ring particles is observed in the mid-infrared, where warmer particles are brighter. The team measured the temperatures of the rings from the images, which revealed that the Cassini Division and the C ring are warmer than the B and A rings. The team concluded that this was because the particles in the Cassini Division and C ring are more easily heated by solar light due to their sparser populations and darker surfaces.

    On the other hand, in the visible light, observers see sunlight being reflected by the ring particles. Therefore, the B and A rings, with their dense populations of particles, always seem bright in the visible wavelengths, while the Cassini Division and the C ring appear faint. The difference in the emission process explains the inverse brightnesses of Saturn’s rings between the mid-infrared and the visible-light views.

    Changing Angles Change the Brightnesses

    3
    Figure 3: Comparison of the mid-infrared images of Saturn’s rings on April 30, 2005 (top) and January 23, 2008 (bottom). Although both of the images were taken in the mid-infrared, the radial contrast of Saturn’s rings is the inverse of each other. (Credit: NAOJ)

    The team concluded that the “inversion” of the brightness of Saturn’s rings between 2005 and 2008 was caused by the seasonal change in the ring opening angle to the Sun and Earth. Since the rotation axis of Saturn inclines compared to its orbital plane around the Sun, the ring opening angle to the Sun changes over a 15-year cycle. This makes a seasonal variation in the solar heating of the ring particles. The change in the opening angle viewed from the Earth affects the apparent filling factor of the particles in the rings. These two variations – the temperature and the observed filling factor of the particles – led to the change in the mid-infrared appearance of Saturn’s rings.

    The data taken with the Subaru Telescope revealed that the Cassini Division and the C ring are sometimes bright in the mid-infrared though they are always faint in visible light. “I am so happy that the public information activities of the Subaru Telescope, of which I am in charge, led to this scientific finding,” said Dr. Fujiwara. “We are going to observe Saturn again in May 2017 and hope to investigate the nature of Saturn’s rings further by taking advantages of observations with space missions and ground-based telescopes.”

    This research is published in Astronomy & Astrophysics, Volume 599, A29 and posted on-line on February 23, 2017 (Fujiwara et al., 2017, Seasonal variation of the radial brightness contrast of Saturn’s rings viewed in mid-infrared by Subaru/COMICS). This work is supported JSPS KAKENHI Grant Numbers JP23103002 and JP26800110.

    The research team:

    Hideaki Fujiwara: Subaru Telescope, National Astronomical Observatory of Japan, USA
    Ryuji Morishima: University of California, Los Angeles/Jet Propulsion Laboratory, California Institute of Technology, USA
    Takuya Fujiyoshi: Subaru Telescope, National Astronomical Observatory of Japan, USA
    Takuya Yamashita: National Astronomical Observatory of Japan, Japan

    See the full article here .

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    The National Astronomical Observatory of Japan (NAOJ) is an astronomical research organisation comprising several facilities in Japan, as well as an observatory in Hawaii. It was established in 1988 as an amalgamation of three existing research organizations – the Tokyo Astronomical Observatory of the University of Tokyo, International Latitude Observatory of Mizusawa, and a part of Research Institute of Atmospherics of Nagoya University.

    In the 2004 reform of national research organizations, NAOJ became a division of the National Institutes of Natural Sciences.

    NAOJ Subaru Telescope

    NAOJ Subaru Telescope interior
    Subaru

    ALMA Array
    ALMA

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    Solar Flare Telescope

    Nobeyama Radio Telescope - Copy
    Nobeyama Radio Observatory

    Nobeyama Solar Radio Telescope Array
    Nobeyama Radio Observatory: Solar

    Misuzawa Station Japan
    Mizusawa VERA Observatory

    NAOJ Okayama Astrophysical Observatory Telescope
    Okayama Astrophysical Observatory

    The National Astronomical Observatory of Japan (NAOJ) is an astronomical research organisation comprising several facilities in Japan, as well as an observatory in Hawaii. It was established in 1988 as an amalgamation of three existing research organizations – the Tokyo Astronomical Observatory of the University of Tokyo, International Latitude Observatory of Mizusawa, and a part of Research Institute of Atmospherics of Nagoya University.

    In the 2004 reform of national research organizations, NAOJ became a division of the National Institutes of Natural Sciences.

     
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