A Two-Toned Wonder from the Saturnian Outskirts

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 Images credit: Saturn & the Phoebe Ring (middle) – NASA / JPL-Caltech / Keck; Iapetus (top left) – NASA / JPL / Space Science Institute / Cassini Imaging Team; Phoebe (bottom right) – NASA / ESA / JPL / Space Science Institute / Cassini Imaging Team.

By Dr. Ethan Siegel

Although Saturn has been known as long as humans have been watching the night sky, it’s only since the invention of the telescope that we’ve learned about the rings and moons of this giant, gaseous world. You might know that the largest of Saturn’s moons is Titan, the second largest moon in the entire Solar System, discovered by Christiaan Huygens in 1655. It was just 16 years later, in 1671, that Giovanni Cassini (for whom the famed division in Saturn’s rings—and the NASA mission now in orbit there—is named) discovered the second of Saturn’s moons: Iapetus. Unlike Titan, Iapetus could only be seen when it was on the west side of Saturn, leading Cassini to correctly conclude that not only was Iapetus tidally locked to Saturn, but that its trailing hemisphere was intrinsically brighter than its darker, leading hemisphere. This has very much been confirmed in modern times!

 

In fact, the darkness of the leading side is comparable to coal, while the rest of Iapetus is as white as thick sea ice. Iapetus is the most distant of all of Saturn’s large moons, with an average orbital distance of 3.5 million km, but the culprit of the mysterious dark side is four times as distant: Saturn’s remote, captured moon, the dark, heavily cratered Phoebe!

 

Orbiting Saturn in retrograde, or the opposite direction to Saturn’s rotation and most of its other Moons, Phoebe most probably originated in the Kuiper Belt, migrating inwards and eventually succumbing to gravitational capture. Due to its orbit, Phoebe is constantly bombarded by micrometeoroid-sized (and larger) objects, responsible for not only its dented and cavity-riddled surface, but also for a huge, diffuse ring of dust grains spanning quadrillions of cubic kilometers! The presence of the “Phoebe Ring” was only discovered in 2009, by NASA’s infrared-sensitive Spitzer Space Telescope. As the Phoebe Ring’s dust grains absorb and re-emit solar radiation, they spiral inwards towards Saturn, where they smash into Iapetus—orbiting in the opposite direction—like bugs on a highway windshield. Was the dark, leading edge of Iapetus due to it being plastered with material from Phoebe? Did those impacts erode the bright surface layer away, revealing a darker substrate?

 

In reality, the dark particles picked up by Iapetus aren’t enough to explain the incredible brightness differences alone, but they absorb and retain just enough extra heat from the Sun during Iapetus’ day to sublimate the ice around it, which resolidifies preferentially on the trailing side, lightening it even further. So it’s not just a thin, dark layer from an alien moon that turns Iapetus dark; it’s the fact that surface ice sublimates and can no longer reform atop the leading side that darkens it so severely over time. And that story—only confirmed by observations in the last few years—is the reason for the one-of-a-kind appearance of Saturn’s incredible two-toned moon, Iapetus!

 

Learn more about Iapetus here: http://saturn.jpl.nasa.gov/science/moons/iapetus.

 

Kids can learn more about Saturn’s rings at NASA’s Space Place: http://spaceplace.nasa.gov/saturn-rings.

 

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Rare Sight: See Venus During the Day | Space.com

Venus, February 2014

Venus reaches its point of greatest brilliancy on February 15 when its increasing distance and the broadening width of its crescent reach an optimum combination.
Credit: Starry Night Software

PHOENIX — One of the more amazing sites in our sky is the planet Venus. At its best, Venus is brighter than all other celestial objects except the sun and moon. Right now, the brilliant planet is so bright that you can actually see it in the daytime, if you know where to look.

Venus doesn’t make any visible light of its own. It shines by reflecting sunlight. Right now, Venus is well up in the morning sky before sunrise, and any time this week, if the sky is clear, you’ll have no trouble finding it. Just go out, look east. Venus will be the brightest thing you see. If you stretch your arms out, this “morning star” will be about two fists above the horizon as the sky begins to lighten.

By the time the sun rises, Venus will be reduced to a pale pinprick of light. The trick to seeing it during the day is to prepare. Here’s what you do:

via Rare Sight: See Venus During the Day (Photo) | Space.com.

Great Science YouTube Channel

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Here is one of the videos I liked.