APOD: The Map of Dione (2014 Nov 07)

APOD Robot · Post by **APOD Robot** » Fri Nov 07, 2014 5:10 am

The Map of Dione

Explanation: This cylindrical projection global map is one of six new color maps of Saturn's midsized icy moons, constructed using 10 years of image data from the Cassini spacecraft. Discovered by Cassini (the astronomer) in 1684, Dione is about 1,120 kilometers across. Based on data extending from infrared to ultraviolet, the full resolution of this latest space-age map is 250 meters per pixel. The remarkable brightness difference between the tidally locked moon's lighter leading hemisphere (right) and darker trailing hemisphere clearly stands out. Like other Saturn moons orbiting within the broad E-ring, Dione's leading hemisphere is kept shiny as it picks up a coating of the faint ring's icy material. The E-ring material is constantly replenished by geysers on moon Enceladus' south pole. Lighter, younger surface fractures also appear to cross the dark, cratered trailing hemisphere.

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Nitpicker · Post by **Nitpicker** » Fri Nov 07, 2014 6:16 am

Presented in this projection, I might have said that Dione is about 3520 km across (equatorial circumference) with a diameter of about 1120 km. (Although it did make me look it up, which isn't a bad thing.)

Interesting to note that five of the six moons newly mapped, are in the vicinity of the E ring and have dark trailing hemispheres (especially Tethys, Dione and Rhea, where it almost looks like mildew). Of the six, only Iapetus has a dark leading hemisphere.

storyteller · Post by **storyteller** » Fri Nov 07, 2014 7:29 am

tell me about earth's second moon

the earth has two moons - and, a third moon may eventually evolve as a result of asteroid capture

eros, the bean shaped asteroid, is expected to enter earth orbit sometime in the 22nd century

Nitpicker · Post by **Nitpicker** » Fri Nov 07, 2014 7:39 am

storyteller wrote:tell me about earth's second moon

the earth has two moons - and, a third moon may eventually evolve as a result of asteroid capture

eros, the bean shaped asteroid, is expected to enter earth orbit sometime in the 22nd century

http://en.wikipedia.org/wiki/Other_moons_of_Earth

Although the Moon remains the Earth's only known natural satellite, there are a number of near-Earth objects (NEOs) with orbits that are in resonance with the Earth. These have been called, inaccurately but provocatively, "second", "third" or "other" moons of Earth.

Boomer12k · Post by **Boomer12k** » Fri Nov 07, 2014 8:50 am

The bottom left area of the image looks like wave ripples....

:---[===] *

ChrisKotsiopoulos · Post by **ChrisKotsiopoulos** » Fri Nov 07, 2014 9:11 am

This is a real 'little planet'!

Markus Schwarz · Post by **Markus Schwarz** » Fri Nov 07, 2014 10:51 am

Boomer12k wrote:The bottom left area of the image looks like wave ripples....

:---[===] *

I noticed the ripples in the pole regions, too. They are an artifact of the cylindrical projection. It is a mathematical fact that you can't map the surface of a sphere onto a plane without distortions. Usually, these distortions are more severe near the poles and negligible near the equator. This occurs on Earth maps as well. If you look at google maps it appears that Great Britain is about as large as Saudi Arabia. In truth, Britain's area is only about a tenth of Saudi Arabia's.

Coil_Smoke · Post by **Coil_Smoke** » Fri Nov 07, 2014 1:33 pm

An obvious question...Why is it green? I think this image attempts rendering natural colors. What chemical activity is responsible for this unusual coloring?

bystander · Post by **bystander** » Fri Nov 07, 2014 3:02 pm

Orthographic projections of the leading and trailing hemispheres as well as the Northern and Southern
hemispheres can be found at the JPL Photojournal catalog page PIA18434: Color Maps of Dione - 2014.

Guest · Post by **Guest** » Fri Nov 07, 2014 6:12 pm

Discounting the areas at the top and bottom of the image where the graphic distortion makes observation a little useless. I did note that the central portion of the image (centered vertically on the X axis), the surface generally appears fairly rough. However, in the 'dark area', just to the upper left of its center (of the dark area) there appears to be a couple of craters that have a much (apparently) smoother surface, and what appears to be a similar surface that radiates to the upper portion of the image. That is, until the distortion of the image margins prevents any real appreciation of the surface characteristics. I wonder if this body took a couple of hits big enough to allow some sub-surface liquid to flow out and flood these regions, or low lying features, before 'freezing' up?

Ann · Post by **Ann** » Fri Nov 07, 2014 9:13 pm

Coil_Smoke wrote:An obvious question...Why is it green? I think this image attempts rendering natural colors. What chemical activity is responsible for this unusual coloring?

It isn't green, of course. The link that bystander gave us a link to shows that Dione was photographed through three filters, an infrared one, a green one and an ultraviolet one. Almost certainly, the infrared filter was mapped as red, and the ultraviolet filter was mapped as blue. The blue-gray parts of Dione reflect ultraviolet light very efficiently, and they are probably covered in fresh ice from the E ring ( and ultimately from Enceladus). The yellowish patch to the right is fairly bright in infrared light, most likely because it is centered on a (fresh) meteorite impact which led to a local injection of heat.

The green patch is not very infrared at all. It is cold, and quite possibly colder than the rest of Dione. It does a lousy job of reflecting ultraviolet light, too. This part of Dione is really only detected strongly by the green filter, which is why this part of Dione looks green here. In my opinion, we have no reason to suspect that Dione is green. The dark green color probably just means that this part of Dione reflects a bit of visible light (such as, for example, green) but not much else.

Ann

chuckster · Post by **chuckster** » Fri Nov 07, 2014 9:27 pm

Sometimes, rather than just go ahead and dive into details and discussions, I am moved to step back and just marvel at the age of wonders we live in. No, I'm not wrapped in some naive space picture bubble of rapture, but despite everything wrong with early 21st century Earth, there are some things that are just plain good-old-fashioned incredible going on in planetary science.
And it's good to see what humans can do when they answer the higher call.

Chris Peterson · Post by **Chris Peterson** » Fri Nov 07, 2014 11:24 pm

Ann wrote:The green patch is not very infrared at all. It is cold, and quite possibly colder than the rest of Dione... The dark green color probably just means that this part of Dione reflects a bit of visible light (such as, for example, green) but not much else.

I agree with your conclusion, but your initial comment could be misunderstood. The moon was imaged in near-IR, so we can infer nothing about the temperature from the IR signal. That is, we are only seeing reflected IR, not emitted IR.

Nitpicker · Post by **Nitpicker** » Fri Nov 07, 2014 11:52 pm

Chris Peterson wrote:
Ann wrote:The green patch is not very infrared at all. It is cold, and quite possibly colder than the rest of Dione... The dark green color probably just means that this part of Dione reflects a bit of visible light (such as, for example, green) but not much else.
I agree with your conclusion, but your initial comment could be misunderstood. The moon was imaged in near-IR, so we can infer nothing about the temperature from the IR signal. That is, we are only seeing reflected IR, not emitted IR.

With the filters described as "IR-Green-UV" (I can't see any more details than that), does that typically mean that the IR filter also allows some of the visible red to pass, and the UV filter also allows some of the visible blue to pass? As such, is it still fair to call the darker regions green, but not as intensely green as shown in the enhanced colour APOD?

geckzilla · Post by **geckzilla** » Sat Nov 08, 2014 12:06 am

It's weird but what the human mind interprets as green is actually a dark, saturated yellow in this case.

Chris Peterson · Post by **Chris Peterson** » Sat Nov 08, 2014 12:33 am

Nitpicker wrote:With the filters described as "IR-Green-UV" (I can't see any more details than that), does that typically mean that the IR filter also allows some of the visible red to pass, and the UV filter also allows some of the visible blue to pass? As such, is it still fair to call the darker regions green, but not as intensely green as shown in the enhanced colour APOD?

No way to know without more info. The filters could be broad or narrow, with crossovers or non-overlapping.

geckzilla · Post by **geckzilla** » Sat Nov 08, 2014 12:53 am

Looks like most of Cassini's ISS imagery for Dione was done with its IR1, GRN, and UV3 filters. A convenient chart listing the transmission of these filters is available in one of Emily's posts here: http://www.planetary.org/explore/space- ... ssing.html (second to the last chart)

Nitpicker · Post by **Nitpicker** » Sat Nov 08, 2014 1:35 am

geckzilla wrote:Looks like most of Cassini's ISS imagery for Dione was done with its IR1, GRN, and UV3 filters. A convenient chart listing the transmission of these filters is available in one of Emily's posts here: http://www.planetary.org/explore/space- ... ssing.html (second to the last chart)

With those filters (mapped to RGB), there would be no overlaps and virtually no visible light from IR1 and UV3 filters. The GRN filter seems to let through greens, yellows, oranges and some reds, which seems fairly broad. The only thing missing in ROYGBIV is BIV.

The fact that the "green" in the image is really dark yellow (or dark red-green) suggests there is a comparable signal from the IR1 and GRN filters in the dark regions of Dione, but virtually nothing from the UV3.

ellesmith · Post by **ellesmith** » Sat Nov 08, 2014 1:43 pm

The "cylindrical projection global map" - can we have a link to its definition? I was on a work break, and did not "get it" till this morning: " oh, yes of course.....a "cylindrical projection global map" ! Eureka ! "

Starship Asterisk*

APOD: The Map of Dione (2014 Nov 07)

APOD: The Map of Dione (2014 Nov 07)

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