APOD: Saturn's Moon Helene in Color (2023 Apr 30)

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APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by APOD Robot » Sun Apr 30, 2023 4:06 am

Image Saturn's Moon Helene in Color

Explanation: Although its colors may be subtle, Saturn's moon Helene is an enigma in any light. The moon was imaged in unprecedented detail in 2012 as the robotic Cassini spacecraft orbiting Saturn swooped to within a single Earth diameter of the diminutive moon. Although conventional craters and hills appear, the above image also shows terrain that appears unusually smooth and streaked. Planetary astronomers are inspecting these detailed images of Helene to glean clues about the origin and evolution of the 30-km across floating iceberg. Helene is also unusual because it circles Saturn just ahead of the large moon Dione, making it one of only four known Saturnian moons to occupy a gravitational well known as a stable Lagrange point.

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by Ann » Sun Apr 30, 2023 5:34 am


It's a colorful world out there! :mrgreen: 🙃

Ann

Edit: Yes, I know that the Saturn picture has been color-enhanced to bring out details in Saturn's atmosphere. I'll never forget my shock at first seeing it, because the concept of false or mapped color was unknown to me. I disliked Saturn for a while after seeing that image. Oh well, that's me... :roll:

Edit 2: Can we learn anything in particular from the extremely subtle dilute tan hue of Saturn's moon Helene?
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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by gmPhil » Sun Apr 30, 2023 8:17 am

Ann wrote: Sun Apr 30, 2023 5:34 am Yes, I know that the Saturn picture has been color-enhanced to bring out details in Saturn's atmosphere. I'll never forget my shock at first seeing it, because the concept of false or mapped color was unknown to me. I disliked Saturn for a while after seeing that image. Oh well, that's me... :roll:
Thing is - there's no such thing as "true color". What we like to call that is somewhat anthropocentric; it is just how our human brains interpret light at different frequencies. Maybe an alien brain will look at Saturn and see it just as in that image you posted, and think that as being "true color". :ssmile:

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by gmPhil » Sun Apr 30, 2023 8:20 am

Is a stable Lagrange point really a "gravitational well"? It is only so because the centrifugal forces balance the gravitational ones, not because of the g-forces alone - no? Happy to be corrected by anyone better informed...

alex555

Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by alex555 » Sun Apr 30, 2023 9:40 am

The smooth parts and streaks are absolutely incredible.

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by Chris Peterson » Sun Apr 30, 2023 1:18 pm

gmPhil wrote: Sun Apr 30, 2023 8:17 am
Ann wrote: Sun Apr 30, 2023 5:34 am Yes, I know that the Saturn picture has been color-enhanced to bring out details in Saturn's atmosphere. I'll never forget my shock at first seeing it, because the concept of false or mapped color was unknown to me. I disliked Saturn for a while after seeing that image. Oh well, that's me... :roll:
Thing is - there's no such thing as "true color". What we like to call that is somewhat anthropocentric; it is just how our human brains interpret light at different frequencies. Maybe an alien brain will look at Saturn and see it just as in that image you posted, and think that as being "true color". :ssmile:
Especially that image, taken through green, violet, and ultraviolet filters!
Chris

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by gmPhil » Sun Apr 30, 2023 1:45 pm

Chris Peterson wrote: Sun Apr 30, 2023 1:18 pm
gmPhil wrote: Sun Apr 30, 2023 8:17 am
Ann wrote: Sun Apr 30, 2023 5:34 am Yes, I know that the Saturn picture has been color-enhanced to bring out details in Saturn's atmosphere. I'll never forget my shock at first seeing it, because the concept of false or mapped color was unknown to me. I disliked Saturn for a while after seeing that image. Oh well, that's me... :roll:
Thing is - there's no such thing as "true color". What we like to call that is somewhat anthropocentric; it is just how our human brains interpret light at different frequencies. Maybe an alien brain will look at Saturn and see it just as in that image you posted, and think that as being "true color". :ssmile:
Especially that image, taken through green, violet, and ultraviolet filters!
Yes, but that only makes it "false" color according to how nature has evolved our eyes and our brains to see and interpret light waves. A different being might well look at Saturn as see it much as we can only through such filters. (We already know for instance. that many animals see light differently from us. A cat's view of Saturn is different from ours.) My point is, there is no such thing as absolute "true color" - such "truth" is only relative to the being observing it.

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by Chris Peterson » Sun Apr 30, 2023 1:54 pm

gmPhil wrote: Sun Apr 30, 2023 1:45 pm
Chris Peterson wrote: Sun Apr 30, 2023 1:18 pm
gmPhil wrote: Sun Apr 30, 2023 8:17 am
Thing is - there's no such thing as "true color". What we like to call that is somewhat anthropocentric; it is just how our human brains interpret light at different frequencies. Maybe an alien brain will look at Saturn and see it just as in that image you posted, and think that as being "true color". :ssmile:
Especially that image, taken through green, violet, and ultraviolet filters!
Yes, but that only makes it "false" color according to how nature has evolved our eyes and our brains to see and interpret light waves. A different being might well look at Saturn as see it much as we can only through such filters. (We already know for instance. that many animals see light differently from us. A cat's view of Saturn is different from ours.) My point is, there is no such thing as absolute "true color" - such "truth" is only relative to the being observing it.
Well, in a sense there is "true" color, because "color" is a physiological phenomenon, not a physical one. So "true color" is reasonably defined as a human perception.
Chris

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by gmPhil » Sun Apr 30, 2023 2:13 pm

Chris Peterson wrote: Sun Apr 30, 2023 1:54 pm
gmPhil wrote: Sun Apr 30, 2023 1:45 pm
Chris Peterson wrote: Sun Apr 30, 2023 1:18 pm

Especially that image, taken through green, violet, and ultraviolet filters!
Yes, but that only makes it "false" color according to how nature has evolved our eyes and our brains to see and interpret light waves. A different being might well look at Saturn as see it much as we can only through such filters. (We already know for instance. that many animals see light differently from us. A cat's view of Saturn is different from ours.) My point is, there is no such thing as absolute "true color" - such "truth" is only relative to the being observing it.
Well, in a sense there is "true" color, because "color" is a physiological phenomenon, not a physical one. So "true color" is reasonably defined as a human perception.
Tell that to the LGM when they arrive :)

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by Chris Peterson » Sun Apr 30, 2023 2:24 pm

gmPhil wrote: Sun Apr 30, 2023 2:13 pm
Chris Peterson wrote: Sun Apr 30, 2023 1:54 pm
gmPhil wrote: Sun Apr 30, 2023 1:45 pm
Yes, but that only makes it "false" color according to how nature has evolved our eyes and our brains to see and interpret light waves. A different being might well look at Saturn as see it much as we can only through such filters. (We already know for instance. that many animals see light differently from us. A cat's view of Saturn is different from ours.) My point is, there is no such thing as absolute "true color" - such "truth" is only relative to the being observing it.
Well, in a sense there is "true" color, because "color" is a physiological phenomenon, not a physical one. So "true color" is reasonably defined as a human perception.
Tell that to the LGM when they arrive :)
You mean the L[510 nm]M?
Chris

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by Sa Ji Tario » Sun Apr 30, 2023 2:27 pm

gmPhil wrote: Sun Apr 30, 2023 8:20 am Is a stable Lagrange point really a "gravitational well"? It is only so because the centrifugal forces balance the gravitational ones, not because of the g-forces alone - no? Happy to be corrected by anyone better informed...
Lagrange points

Potential curves in a two-body system (here the Sun and the Earth), showing the five Lagrange points. The arrows point to the direction of potential increase around the L-points – moving towards or away from them. Counterintuitively, points L4 and L5 are minimal.
Lagrange points, also called L points or libration points, are the five positions in an orbital system where a small object, affected only by gravity, can theoretically be stationary relative to two larger objects, such as a Artificial satellite with respect to the Earth and the Moon. The Lagrange points mark the positions where the combined gravitational pull of the two large masses provides the necessary centripetal force to rotate synchronously with the smaller one. They are analogous to geosynchronous orbits that allow an object to be in a "fixed" position in space rather than in an orbit where its relative position is continually changing.

A more precise but technical definition is that the Lagrange points are the stationary solutions of the three-body problem constrained to circular orbits. If, for example, we have two large bodies in a circular orbit around their common center of mass, there are five positions in space where a third body, of negligible mass compared to the other two, can be located and maintain its position. relative to the two large bodies. Viewed from a rotating reference frame that rotates with the same period as the two coorbital bodies, the gravitational field of the two large bodies combined with the centrifugal force balances out at the Lagrange points, allowing the third body to stabilize stationary with respect to the two. first.


Index
1 History and concept
2 Complications to Kepler's laws
3 Lagrange points
3.1 The point L1
3.2 The point L2
3.3 The point L3
3.4 Points L4 and L5
4 Stability
5 values of the solar system
6 Space missions at libration points
7 Natural Examples
7.1 Other coorbital examples
8 See also
9 References
10 External links
History and concepts
In 1772, the Italian-French mathematician Joseph-Louis Lagrange was working on the famous three-body problem when he discovered an interesting peculiarity. Originally, a way was discovered to easily calculate the gravitational interaction of an arbitrary number of bodies in a system. Newtonian mechanics determines that such a system rotates chaotically until either a collision occurs, or one of the bodies is expelled from the system and mechanical equilibrium is achieved. It is very easy to solve the case of two bodies that orbit around the common center of gravity. However, if a third body, or more, is introduced, the mathematical calculations are very complicated, being a situation in which one has to calculate the sum of all the gravitational interactions on each object at each point along its trajectory. .

However, Lagrange wanted to make this simpler, and modified it by a simple assumption: The trajectory of an object is determined by finding a path that minimizes the action over time. This is calculated by subtracting the potential energy from the kinetic energy. Developing this hypothesis, Lagrange reformulated Newton's classical mechanics to give rise to Lagrangian mechanics. With his new way of calculating, Lagrange's work led him to hypothesize a third body of negligible mass orbiting two larger bodies that would already be spinning in a quasi-circular orbit. In a reference frame that rotates with the larger bodies, he found five specific fixed points at which the third body, following the orbit of the larger bodies, is subjected to zero force. These points were called Lagrange points in his honor.

In the more general case of elliptical orbits there are no stationary points but rather a Lagrange «area». The successive Lagrange points, considering circular orbits at each instant, form stationary elliptical orbits, geometrically similar to the orbit of larger bodies. This is due to Newton's second law (


/


=

{\displaystyle d\mathbf {p} /dt=\mathbf {F} }), where p = mv (p is momentum, m is mass, and v is velocity). p is an invariant if the force and position are multiplied by the same factor. A body at a Lagrange point orbits with the same period as the two large bodies in the circular case, implying, as it happens, that they have the same ratio between gravitational force and radial distance. This fact is independent of the circularity of the orbits and implies that the elliptical orbits described by the Lagrange points are solutions of the equation of motion of the third body.

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by VictorBorun » Sun Apr 30, 2023 3:01 pm

Ann wrote: Sun Apr 30, 2023 5:34 am

It's a colorful world out there! :mrgreen: 🙃

Ann
I disliked Saturn for a while after seeing that image. Oh well, that's me... :roll:
I feel like teasing
Click to view full size image 1 or image 2
Saturn's Moon Helene in Color..jpg
Saturn's Moon Helene in Color-.jpg
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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by VictorBorun » Sun Apr 30, 2023 7:46 pm

Ann wrote: Sun Apr 30, 2023 5:34 am Can we learn anything in particular from the extremely subtle dilute tan hue of Saturn's moon Helene?
My first thought is that
1) most substances are more white than coloured: H₂O, CO₂, SiO₂. Now H₂O gets aqua and dark-blue, but you need 1 meter or 300 meters with no bubbles or crystallines; you can't seriously expect a surface ocean here
2) if you have more carbon than oxygen, there can be black C—C sooth (and blue CH₄, but only if there is thick layer, like on Neptune)
3) if you have nitrogen, there can be some fox tail colours
4) if you have a little iron, there will be red Fe₂O₃

This APOD is gray and a little brown. Gray is for sooth dust.

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by orin stepanek » Sun Apr 30, 2023 7:47 pm

helene2_cassini_1024.jpg
Well shades of gray is a color! The photo was taken in color! :evil:
curious-cat-looking-in-a-box-960x540.jpg
Watch out little kitty! :mrgreen:
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Smile today; tomorrow's another day!

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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by VictorBorun » Sun Apr 30, 2023 8:54 pm

orin stepanek wrote: Sun Apr 30, 2023 7:47 pm Watch out little kitty! :mrgreen:
I feel like teasing
Click to view full size image 1 or image 2
glean clues.jpg
glean clues-.jpg
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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by Ann » Mon May 01, 2023 4:12 am

Chris Peterson wrote: Sun Apr 30, 2023 1:54 pm
gmPhil wrote: Sun Apr 30, 2023 1:45 pm
Chris Peterson wrote: Sun Apr 30, 2023 1:18 pm

Especially that image, taken through green, violet, and ultraviolet filters!
Yes, but that only makes it "false" color according to how nature has evolved our eyes and our brains to see and interpret light waves. A different being might well look at Saturn as see it much as we can only through such filters. (We already know for instance. that many animals see light differently from us. A cat's view of Saturn is different from ours.) My point is, there is no such thing as absolute "true color" - such "truth" is only relative to the being observing it.
Well, in a sense there is "true" color, because "color" is a physiological phenomenon, not a physical one. So "true color" is reasonably defined as a human perception.
Yes, Chris, that's exactly my point. "Color" is a human (or biological) concept.


I once saw a fantastic documentary that explained why tigers and many domestic cats are orange-colored. The reason for this color of large and small cats is that their prey, typically large or small mammals, don't have receptors for red light in their retinas. Almost all mammals except humans and some apes only have two types of color receptors, one for blue and one for yellow-green light.

According to the documentary, this means that, to its prey, a tiger hiding in green grass blends in perfectly with the vegetation! Because the orange tiger and the green grass is the same color to the antelopes and other mammals that the tiger will hunt! :shock:

But, gmPhil, what you said in your posts is perfectly correct, of course. Except I define "color" in a different way than you do.

(Or maybe we even define it the same way, but "interpret" it differently.) :)

Ann
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Re: APOD: Saturn's Moon Helene in Color (2023 Apr 30)

Post by VictorBorun » Mon May 01, 2023 9:27 am

Ann wrote: Mon May 01, 2023 4:12 am
Chris Peterson wrote: Sun Apr 30, 2023 1:54 pm
gmPhil wrote: Sun Apr 30, 2023 1:45 pm Yes, but that only makes it "false" color according to how nature has evolved our eyes and our brains to see and interpret light waves. A different being might well look at Saturn as see it much as we can only through such filters. (We already know for instance. that many animals see light differently from us. A cat's view of Saturn is different from ours.) My point is, there is no such thing as absolute "true color" - such "truth" is only relative to the being observing it.
Well, in a sense there is "true" color, because "color" is a physiological phenomenon, not a physical one. So "true color" is reasonably defined as a human perception.
Yes, Chris, that's exactly my point. "Color" is a human (or biological) concept.
Ann
1. Colour is in fact a culturally adopted system of identifying objects in a scene by their substance or material
2. Metaphorically colour is referred in discussing clouds and stars as if they were stage prop made of coloured substance or material

Some confusion is about if colour is in the eyes of a beholder or in the pigments of a thing. Well, colour must be in both places: a tiger must have pigment to not be white and a human or a big ape must have L, M and S cone cells in their retina to tell orange from lime and make tyger tyger burn bright in the forests of the night.

Some confusion is about if RGB system is correctly identifying objects in a scene by their substance or material. Well, RGB system is enough to replay the scene but it is not user-friendly to calculate the sort of a pigment. A graphical editor with a gamma slider lets you thicken or thin all the pigments in the scene without changing their colour: coffee, chocolate, auburn all stay recognizable as long as you neither thicken the scene to almost black nor thin it to almost white. This proves that what matters is not the reflection ratio in Long-, Media- and Short-waves but rather logarithm of such.
So a user-friendly system of colour must use logarithm or, better still, log ̣₅(log ̣₅(the reflection ratio in Long-, Media- or Short-waves)) to map a culturally adopted system. When using double logarithms instead of the reflection ratio in three wavelength bands you simply add 1 to all the three values when you dilute a pigment by half.