APOD: Enceladus Looms (2011 May 12)

[APOD Robot]

Enceladus Looms

Explanation: A sunlit crescent of Saturn's moon Enceladus looms above the night side of Saturn in this dramatic image from the Cassini spacecraft. Captured on August 13, 2010 looking in a sunward direction during a flyby of the icy moon, the view also traces layers in the upper atmosphere of Saturn scattering sunlight along the planet's bright limb. Closer to the spacecraft than Saturn, Enceladus is a mere 60,000 kilometers from Cassini's camera. The south polar region of the 500 kilometer-diameter moon is illuminated, including plumes of water vapor and icy particles spraying above the long fissures in the moon's surface. The fissures have been dubbed tiger stripes. First discovered in Cassini images from 2005, the plumes are strong evidence that liquid water exists near the surface of surprisingly active Enceladus.

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[nstahl]

Nice shot: dramatic, makes us think a little.

[Thinus vd Merwe]

What a stunnig image! Can you see in the upper right hand corner of the picture, in the light and shadow lines on Enceladus illuminated limb, the outlinse of a human face?!
Thank you

[Indigo_Sunrise]

I am a huge fan of the Cassini mission and visit their website almost everyday, so I especially enjoy APOD's that feature Saturn, it's moons, or any combinations thereof! Today's APOD would be no exception - incredible image! Enceladus is pretty awesome! It's interesting how the upper atmosphere of Saturn looks like it's divided into almost equal layers. Does this have something to do with the fact that Saturn is further away from Cassini (as written in the description), and the distance creates the 'illusion of equality'?


*I especially enjoy wasting time, er, playing the surprisingly fun game, even though here on Earth, the only golf I'm barely passable at is the 'miniature' kind.....

[biddie67]

The last link in today's APOD ("active Enceladus") described an electro-magnetic link between Saturn and Enceladus - does our Moon and Earth have something like this also?

Could there be both gravitational and electro-magnetic components in the dynamics of a planet and its moon(s) or even the Sun and the planets?

[neufer]

It's interesting how the upper atmosphere of Saturn looks like it's divided into almost equal layers. Does this have something to do with the fact that Saturn is further away from Cassini (as written in the description), and the distance creates the 'illusion of equality'?

While Saturn does have haze cloud layers (as well as a tropopause of sorts), this picture of Enceladus is probably an overlay of a number of consecutive shots from a moving Cassini all of which are then combined to put Enceladus in focus but not necessarily the more distant background of Saturn.

Saturn does have haze cloud layers.

---

[neufer]

The last link in today's APOD ("active Enceladus") described an electro-magnetic link between Saturn and Enceladus - does our Moon and Earth have something like this also?

The Full Moon's Passage Through Earth's Magnetotail
The Energetics of a Full Moon
by Nick Anthony Fiorenza

Once every lunar month, the Moon passes through Earth's magnetotail. The magnetotail extends well beyond the orbit of the Moon, thus the Moon enters the magnetotail during its orbit around the Earth. The Moon enters the magnetotail about three days before the Full Moon and exits about three days after. Excerpts below are from "The Moon and the Magnetotail" by Dr. Tony Phillips; NASA.:

"During the crossing, the Moon passes through a gigantic plasma sheet of hot charged particles trapped in the tail. The lightest and most mobile of these particles, electrons, pepper the Moon's surface and give the Moon a negative charge.

On the Moon's dayside this effect is counteracted to a degree by sunlight: UV photons knock electrons back off the surface, keeping the build-up of charge at relatively low levels. But on the nightside, in the cold lunar dark, electrons accumulate and voltages can climb to hundreds or thousands of volts.

The best direct evidence comes from NASA's Lunar Prospector spacecraft, which orbited the Moon in 1998-99 and monitored many magnetotail crossings. During some crossings, the spacecraft sensed big changes in the lunar nightside voltage, jumping "typically from -200 V to -1000 V," says Jasper Halekas of UC Berkeley who has been studying the decade-old data.

"It is important to note," says Halekas, "that the plasma sheet (where all the electrons come from) is a very dynamic structure. The plasma sheet is in a constant state of motion, flapping up and down all the time. So as the Moon orbits through the magnetotail, the plasma sheet can sweep across it over and over again. Depending on how dynamic things are, we can encounter the plasma sheet many times during a single pass through the magnetotail with encounters lasting anywhere from minutes to hours or even days."

"As a result, you can imagine how dynamic the charging environment on the Moon is. The Moon can be just sitting there in a quiet region of the magnetotail and then suddenly all this hot plasma goes sweeping by causing the nightside potential to spike to a kilovolt. Then it drops back again just as quickly."

The roller coaster of charge would be at its most dizzying during solar and geomagnetic storms....

Earth's magnetotail isn't the only source of plasma to charge the Moon. Solar wind can provide charged particles, too; indeed, most of the time, the solar wind is the primary source. But when the Moon enters the magnetotail, the solar wind is pushed back and the plasma sheet takes over. The plasma sheet is about 10 times hotter than the solar wind and that gives it more "punch" when it comes to altering the charge balance of the Moon's surface. Two million degree electrons in the plasma sheet race around like crazy and many of them hit the Moon's surface. Solar wind electrons are relatively cool at only 140 thousand degrees, and fewer of them zip all the way down to the shadowed surface of the Moon's nightside."

[Ann]

Enceladus is my favorite moon, although the Moon (Luna) comes in a close second. On second thought, I guess our own dear Moon is my favorite moon after all - I mean, I would certainly miss it if it was gone! And I can't help thinking that the Moon, or the object that smashed into the primordial Earth and later became the Moon - played a crucial part in making the Earth so wonderfully habitable. So thank you, Moon! But apart from you, Enceladus is my favorite moon. What an amazing little fountain (of youth?) it is! And today's APOD is a very good one.

Ann

[Ailean4321]

Holy icy moon, this picture is amazing!!!!
Love<3

[moonstruck]

Amazing photos.

[Chris Peterson]

Could there be both gravitational and electro-magnetic components in the dynamics of a planet and its moon(s) or even the Sun and the planets?

Depends what you mean by "dynamics". The forces involved are too small to significantly affect orbital dynamics, but they can certainly affect the transfer of material between the bodies, the behavior of material in the space around them, and in rare cases even produce significant energy dissipation inside one or both bodies.

[biddie67]

Chris - sorry that I tend to use words that can be so vague as I try to understand the ideas presented here ...

The diferences in magnitude of masses, and forces like gravity, magnetism, solar winds and plasma are hard for me to relate to - especially since the relatively simple and somewhat moderated forces that we deal with here in daily life are so different in scale from what the larger solar system and universe experience.

[biddie67]

..... During some crossings, the spacecraft sensed big changes in the lunar nightside voltage, jumping "typically from -200 V to -1000 V ......

Assuming the character in front of the voltage range is a negative sign, is that voltage range enough to cause problems to any electronic equipment that might be positioned on the moon's far side?

[Chris Peterson]

The diferences in magnitude of masses, and forces like gravity, magnetism, solar winds and plasma are hard for me to relate to - especially since the relatively simple and somewhat moderated forces that we deal with here in daily life are so different in scale from what the larger solar system and universe experience.

So true, and this is one of the reasons that I commented elsewhere on the danger of using "common sense" as a tool in understanding nature. The Universe simply doesn't behave in a way that is naturally intuitive very often.

[Chris Peterson]

Assuming the character in front of the voltage range is a negative sign, is that voltage range enough to cause problems to any electronic equipment that might be positioned on the moon's far side?

Probably not. Surfaces on the Earth, and even in your house pick up larger charges than that. Two things can damage electronics: strong electric fields (hundreds or thousands of volts per meter, which will not be present in this lunar case), or current discharges. The latter are possible when you have electronics sitting on a charged surface, but are generally easy to avoid using appropriate shielding. The real risk to electronics in space or on the Moon comes from highly energetic charged particles.

(BTW, this is a dayside/nightside phenomenon, not a nearside/farside one. So any issues affecting electronic equipment would be the same regardless of its location on the Moon.)

[neufer]

Assuming the character in front of the voltage range is a negative sign, is that voltage range enough to cause problems to any electronic equipment that might be positioned on the moon's far side?

Probably not...The real risk to electronics in space or on the Moon comes from highly energetic charged particles.

(BTW, this is a dayside/nightside phenomenon, not a nearside/farside one. So any issues affecting electronic equipment would be the same regardless of its location on the Moon.)

Since the magneto-tail voltage/plasma sheet Lunar problem happens ONLY within 3 days of a full moon it is BOTH a dayside/nightside and a nearside/farside phenomenon.

[billinger]

Best take of last week