APOD: Saturns Moon Dione in Slight Color (2012 Nov 05)

[APOD Robot]

Saturns Moon Dione in Slight Color

Explanation: Why does one half of Dione have more craters than the other? Start with the fact that Saturn's moon Dione has one side that always faces Saturn, and one side that always faces away. This is similar to Earth's Moon. This tidal locking means that one side of Dione always leads as the moon progresses in its orbit, while the other side always trails. Dione should therefore have undergone a significant amount of impacts on its leading half. Strangely, the current leading half of Dione is less cratered than the trailing half. A leading explanation is that some crater-forming impacts were so large they spun Dione, changing the part that suffered the highest impact rate before the moon's spin again became locked. The above detailed image of Dione highlighting the moon's subtle hues is a meticulously-constructed mosaic -- by an dedicated amateur -- of pictures taken during the April 2010 flyby of Dione by NASA's robotic Cassini spacecraft.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Guest]

My knowledge in astronomy is almost nonexistent, maybe more than half of what I know I learned from here, APOD.

That is why for sure my supposition is wrong. Who am I to contradict an astronomer? BUT, what I point here is more about common physics.

The explanation about leading face would be true is meteorites will be floating around and then the Dione will hit them with the leading face. But since the meteorites came with high velocity this explanation is not sustainable unless the speed of the entire system, Saturn/Dione is or the solar system the one responsible for collecting the meteorites . I do not know the directions of streams of meteorites the plane of Dione around Saturn, and so on, so I am on a wild guess. But since Dion is rotating around Saturn, the leading face will collect from only one plane, and if is the same as of the Saturn's vector, then the leading face will be leading only half of time, from the point of view of Saturn. If the stream of meteorites comes perpendicular on the plane on which Dione spin then the leading face does not matters. If the streams of meteorites comes from many directions or from all around, than I suppose that not the leading face matters but the face hidden behind Saturn. The side facing Saturn will be protected.

Please explain me where I am wrong.

[FLPhotoCatcher]

I have no idea why the trailing side has been hit more often, nor do I know why there is an equal sign on Dione at about the 10 o'clock position.

[gorade]

To me it looks like the leading face is scarred by craters too, but it has been powdered over by dust. Dione belongs to Saturn and there may have been other rings before our time. I think Dione's leading face has met with relatively fine dust that has done cosmetic to it hiding it's embarrassing pockmarks. Beneath that cover the surface might be quite like the other side.
What do you think?

[FLPhotoCatcher]

I don't think that dust falling onto Dione could hide any of the craters except the smallest. And if dust had fallen heavily enough to hide them on one side, an area that got less dust would show evidence of hidden craters underneath, like a chair shows up as a smooth bump after a very heavy snowfall.

[FLPhotoCatcher]

Also, around the equal sign on Dione at about the 10 o'clock position is what looks like a few faint lines of text, like a watermark on a photo.

But there is something that is actually on Dione that could get the conspiracy theorists excited. I don't know if I should mention it... Well here goes! To the upper-right of the "equal sign" is what looks a lot like a face or a mask. But, of course, it's not.

[Ann]

One hemisphere - the leading one? - is yellower than the other. This might make sense if Dione is collecting a thin coating of reddish dust particles in its path. On the other hand, Dione is highly reflective, with an albedo of 0.998 ± 0.004, meaning that the moon must be very white. The likeliest reason for the white color of Dione might be that it has picked up a coating of snow-white ice particles emanating from Enceladus.

According to a Cassini enhanced-color composite of Dione, it is in fact the trailing hemisphere that is yellower than the other. Perhaps color on Dione is such a very subtle thing that it mostly has to do with changes in illumination.

Ann

[gorade]

I don't think that dust falling onto Dione could hide any of the craters except the smallest. And if dust had fallen heavily enough to hide them on one side, an area that got less dust would show evidence of hidden craters underneath, like a chair shows up as a smooth bump after a very heavy snowfall.

To me it looks just like there were hidden craters below snow or something..

[RedFishBlueFish]

...crater-forming impacts were so large they spun Dione, changing the part that suffered the highest impact rate before the moon's spin again became locked...

This would nicely explain the differential cratering at the leading and trailing surfaces.

But, to my mind, this leaves unexplained the large craters on the sides of the moon perpendicular to the path of the orbit - halfway between the leading/trailing parts (on the plane perpendicular to the path) - looking the same as those on the leading portion.

To me it seems that if there are all of these impactors coming from one direction hitting the moon dead-on and leaving the round craters we see, then there should be furrowed craters from glancing strikes along the sides - which is not what is seen on the side shown in the image, as the craters on the side look the same as those on the leading edge.

Dust on the now-leading edge seems attractive, if so, then one should see a smooth shading of the craters on side of the moon between leading/trailing edges. Whilst the most superior of those craters in this image suggests such blurring none of the others do - nor does inspection in this image of the now-leading yellow edge even hint at obscured craters lying below.

The reason that I always start my day with APOD is that the image is often beautiful and the subject always makes one think.

How did the 'dedicated amateur' gain access to NASA's images?

[BDanielMayfield]

To the question raised by Guest and others wondering about why the leading face of a tidally locked moon will have more craters than the trailing face an analogy might be of help. Think of a vehicle driving through rain. Some raindrops will hit the rear window, but many more will hit the front windshield, and these will hit with greater velocity.

Dione is orbiting Saturn with an average velocity of 10.005787 km/s, and that must be added to the average velocity of Saturn’s orbit around the Sun which is 10.04991 km/s. Yes, impactors can fall in from any direction, but the ones that make the biggest craters per impactor size will be the ones that hit the leading side.

This principle is also the same reason why observing meteors here on earth is more productive before dawn than it is after dusk.

The fact that in Dione this crater count is reversed means that a large impact must have rotated the moon, like a car that’s been spun 180 degrees by a collision. Hope this helped.

Bruce

[biddie67]

Good Morning. Many of the craters seem to have a little "hill" in the middle of them. Is this a usual effect when some debris hits the surface?

[Boomer12k]

http://system.solaire.free.fr/saturnesat2.htm

Did an image search of Dione. Above is an interesting page...video of Dione rotating, and two face view of Dione front and back, I guess...seems to be in French.

I observe that not only are there less craters on one side, but there appears to be less in the bottom part of the pictures. I am not sure if UP IS NORTH in these photos.

Go down the page a little and watch the video of RHEA...it shows a very similar feature....one side pockmarked, and a smoother side...

:---[===] *

[BDanielMayfield]

Good Morning. Many of the craters seem to have a little "hill" in the middle of them. Is this a usual effect when some debris hits the surface?

Yes it is, if the impact is forceful enough. If you can, try looking at the (our) moon through a small telescope or even binoculars and you will see many lunar craters with central peaks.

On Mars, the rover Curiosity's mission is to include a climb up a massive central peak insde Gale Crater.

Bruce

[orin stepanek]

Here's to Cassini! This little space probe has given us so much! http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygenshttp://www.nasa.gov/mission_pages/cassi ... index.html

[Tszabeau]

I've always assumed that photos of other planets and moons are oriented, like we orient maps and depictions of the Earth, with the north pole at the top and the south pole at the bottom. Is that true or false? I know that some planets and moons have been knocked on their sides and thus spin perpendicular relative to most planets and think it might be appropriate to depict them with poles on the right and left of the image. Is there a convention in this regard in astro photography or is it an aesthetic consideration, only?

[bystander]

How did the 'dedicated amateur' gain access to NASA's images?

http://saturn.jpl.nasa.gov/photos/raw/

[kyokugaisha]

Hi guys,

I used images downloaded from NASA's Planetary Data System http://pds.nasa.gov/ where you will find full, calibrated img files with complete metadata from pretty much all the planetary missions since voyager.

You can use their search tool here - http://pds-rings.seti.org/search/ and there's a great starter guide if you fancied doing some yourself here - http://www.planetary.org/explore/space- ... rials.html.

The raw image site is great and more up to date but they are not the highest quality and require more processing to get a decent final result.

- Marc Canale

http://www.flickr.com/photos/76658917@N04

How did the 'dedicated amateur' gain access to NASA's images?

http://saturn.jpl.nasa.gov/photos/raw/

[rudminjd@jmu.edu]

It makes perfect sense to me that Dione should be most heavily cratered on its trailing side, because that is the direction from which the vast majority of material comes, including the vast majority of fast impacts. Smaller orbits have a faster angular velocity than larger orbits. They approach the trailing side of an object in a higher orbit. As one can see from the diagram at the link below, Dione resides very high in Saturn's system of moons and rings. So, the vast majority of material hitting Dione comes from smaller orbits. The faster impacts probably come from objects ejected by lower residing moons, or by errant comets.

http://media.web.britannica.com/eb-medi ... 7682B4.jpg

Joe Rudmin

[neufer]

It makes perfect sense to me that Dione should be most heavily cratered on its trailing side, because that is the direction from which the vast majority of material comes, including the vast majority of fast impacts. Smaller orbits have a faster angular velocity than larger orbits. They approach the trailing side of an object in a higher orbit.

Smaller orbits have less angular momentum than larger orbits.

Were the inner moons somehow pushed out to the orbit of Dione
they would be expected to have the less angular velocity than Dione.

Since Dione's escape velocity (0.5 km/s) is negligible as compared to its orbital velocity (10 km/s)
Dione would be expected to be most heavily cratered on its leading side on purely kinetic arguments.

[earendil]

While I don't have a problem with a large impactor, could it be possible that the smoothness of the leading side is due to ablation from more smaller impactors? It seems to me that with the ring formations around Saturn, there should be a higher amount of small debris and enough small impacts should smooth the features and add dust layers that further disguise the surface. I don't know much about the atmospheric makeup of Dione, is there enough there to stop small debris?

[neufer]

While I don't have a problem with a large impactor, could it be possible that the smoothness of the leading side is due to ablation from more smaller impactors? It seems to me that with the ring formations around Saturn, there should be a higher amount of small debris and enough small impacts should smooth the features and add dust layers that further disguise the surface.

Excellent point

Dione would constantly receive rain from the E ring: http://media.web.britannica.com/eb-medi ... 7682B4.jpg

[... possibly focused by the moderately sized leading (L₄ trojan) moon Helene.]

[gorade]

While I don't have a problem with a large impactor, could it be possible that the smoothness of the leading side is due to ablation from more smaller impactors? It seems to me that with the ring formations around Saturn, there should be a higher amount of small debris and enough small impacts should smooth the features and add dust layers that further disguise the surface. I don't know much about the atmospheric makeup of Dione, is there enough there to stop small debris?

Yes, I agree. That seems intuitively plausible, doesn't it?

[FloridaMike]

...Dione should be most heavily cratered on its trailing side, ...

...Dione would be expected to be most heavily cratered on its leading side ...

Oh, Snap. Only an Epic Rap Battle of Astonomy can resolve this dispute.....

[Guest]

Dione is orbiting Saturn with an average velocity of 10.005787 km/s, and that must be added to the average velocity of Saturn’s orbit around the Sun which is 10.04991 km/s. Yes, impactors can fall in from any direction, but the ones that make the biggest craters per impactor size will be the ones that hit the leading side.


Bruce

Thanks for the answer. It is obvious now why the leading face matters. In my ignorance I supposed that the speed of asteroids is hundreds or thousands times bigger than the speed of Dione. So actually a rock traveling with 20 km / sec will hit the leading face with 40 km/sec, while coming from behind speeds are almost the same so the will be barely kind of a free fall.

Now illuminated as I am, I will have a better sleep.

[rudminjd@jmu.edu]

Art Neuendorffer said,
"Smaller orbits have less angular momentum than larger orbits. Were the inner moons somehow pushed out to the orbit of Dione they would be expected to have the less angular velocity than Dione."

True. My reasoning was unclear. I agree that Dione will overtake some particles scattered upward, which have just barely enough energy to reach Dione's orbit. But, when two particles scatter from lower in the disk, those that reach Dione's orbit are very likely to have a higher angular momentum than Dione's. Especially the high impact particles will have higher angular momentum.

Joe