APOD: Mars Rises above the Lunar Limb (2022 Dec 09)

[APOD Robot]

Mars Rises above the Lunar Limb

Explanation: On the night of December 7 Mars wandered near the Full Moon. In fact the Red Planet was occulted, passing behind the Moon, when viewed from locations across Europe and North America. About an hour after disappearing behind the lunar disk Mars reappears in this stack of sharp video frames captured from San Diego, planet Earth. With the Moon in the foreground Mars was a mere 82 million kilometers distant, near its own opposition. Full Moon and full Mars were bright enough provide the spectacular image with no exposure adjustments necessary. In the image Mars appears to rise just over ancient, dark-floored, lunar crater Abel very close to the southeastern edge of the Moon's near side. Humboldt is the large impact crater to its north (left).

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[Ann]

That is such an attractive image, and it was posted just recently in the Recent Submissions thread of December here at Starship Asterisk*! I saw it there just yesterday, and I was about to write a comment about it, but then I had other things to do, and I forgot.

Mars Rises above the Lunar Limb Image Credit & Copyright: Tom Glenn

---

So, the picture! The color and apparent size contrast between the Moon and Mars is striking. Of course the size difference is a consequence of the Moon's great proximity to the Earth.

As a Color Commentator, I am of course more interested in the color difference between these two celestial bodies. Mars is famous for its reddish appearance, so the orange hue of Mars is not surprising. By contrast, the Moon looks all gray.

What I find mystifying is the fact that Mars looks darker than the Moon. It appears to reflect light less efficiently than the Earth's Moon.

That is surprising. I found a list of the reflectivity, the albedo, of the planets of the Solar system, plus the albedo of the Moon. Interestingly, Mars is quite dark, with an albedo of only 0.16. Only Mercury and, yes, the Moon, are darker, with albedos of 0.12.

But in this APOD, Mars looks darker than the Moon. Is that because the Moon is closer to the Sun than Mars, so that it receives more sunlight?

Ann

[daddyo]

I think that makes sense, plus the reflected light back from Mars spreads radially again. The visual comparison between red and gray brightness might make it more confusing too, maybe its worth converting the image to monochrome.

[Chris Peterson]

That is such an attractive image, and it was posted just recently in the Recent Submissions thread of December here at Starship Asterisk*! :D I saw it there just yesterday, and I was about to write a comment about it, but then I had other things to do, and I forgot.

Mars Rises above the Lunar Limb Image Credit & Copyright: Tom Glenn

---

So, the picture! The color and apparent size contrast between the Moon and Mars is striking. Of course the size difference is a consequence of the Moon's great proximity to the Earth.

As a Color Commentator, I am of course more interested in the color difference between these two celestial bodies. Mars is famous for its reddish appearance, so the orange hue of Mars is not surprising. By contrast, the Moon looks all gray.

What I find mystifying is the fact that Mars looks darker than the Moon. It appears to reflect light less efficiently than the Earth's Moon.

That is surprising. I found a list of the reflectivity, the albedo, of the planets of the Solar system, plus the albedo of the Moon. Interestingly, Mars is quite dark, with an albedo of only 0.16. Only Mercury and, yes, the Moon, are darker, with albedos of 0.12.

But in this APOD, Mars looks darker than the Moon. Is that because the Moon is closer to the Sun than Mars, so that it receives more sunlight?

Ann

It is complicated. There is the matter of distance. There is the matter of color. And it is important to keep in mind that albedo is a linear metric: a body with half the albedo of another reflects half the light (although even that is complicated by color). But we don't perceive a body that reflects half as much light as half the brightness. Our vision is highly nonlinear in its response to intensity. And then we add in the effects of image processing, where we almost always adjust the intensity curve nonlinearly to use most of the intensity space. And there is the gamma of our display devices. And let's not forget that the lunar albedo is an average across the entire surface, and varies considerably with location... and we're only seeing a section of it here.

Well, you get the idea! In most cases an astronomical image, processed for aesthetics, is not very useful for making photometric assessments.

[orin stepanek]

Wow; awesome! I find this image quite beautiful! Kudo's Tom Glenn!

[JohnD]

Quote, Ann: "Of course the size difference is a consequence of the Moon's great proximity to the Earth."

Gosh, Ann, you make it so much clearer than with Ted and Dougal!
https://www.youtube.com/watch?v=MMiKyfd6hA0

[Chris Peterson]

Quote, Ann: "Of course the size difference is a consequence of the Moon's great proximity to the Earth."

Gosh, Ann, you make it so much clearer than with Ted and Dougal!
https://www.youtube.com/watch?v=MMiKyfd6hA0

Mars is only about twice the size of the Moon. The apparent size is about 1% of the size of the Moon. Which makes sense because Mars is about 200 times farther away.

[Avalon]

Does molten lava/magma still exist under the moon's cold surface?

[johnnydeep]

Does molten lava/magma still exist under the moon's cold surface?

Apparently, yes, but it's deep down and too dense to rise. From https://www.universetoday.com/93696/if- ... -erupting/ (yes, it's from 2012, so it might be outdated):

Last year, scientists took another look at the seismic data collected by Apollo era experiments and discovered that the lower mantle of the Moon, the part near the core-mantle boundary, is partially molten (e.g., Apollo Data Retooled to Provide Precise Readings on Moon’s Core, Universe Today, Jan. 6, 2011). Their findings suggest that the lowest 150 km of the mantle contains anywhere from 5 to 30% liquid melt. On the Earth, this would be enough melt for it to separate from the solid, rise up, and erupt at the surface. We know that the Moon had volcanism in the past. So, why is this lunar melt not erupting at the surface today? New experimental studies on simulated lunar samples may provide the answers.

It is suspected that the current lunar magmas are too dense, in comparison to their surrounding rocks, to rise to the surface. Just like oil on water, less dense magmas are buoyant and will percolate up above the solid rock. But, if the magma is too dense, it will stay where it is, or even sink.

[Tom Glenn]

That is such an attractive image........

What I find mystifying is the fact that Mars looks darker than the Moon. It appears to reflect light less efficiently than the Earth's Moon.

Ann, thanks for your nice comment about the image. I share your fascination about the perceived difference in brightness between the two objects. As Chris points out above, there are many difficulties inherent to interpreting photometric information from images. However, I processed this image in such a way that some general interpretations can be made, including your observation of relative brightness. The image was processed using only "global" operations, which is to say that every operation was applied equally across the frame and there were no selective enhancements (eg brightening of certain features). This is important because many astro images we see published do contain such selective enhancements, and although the result is often aesthetically pleasing, it is then impossible to interpret relationships in the data.

Of note, geometric albedo compares the brightness of an object to an idealized Lambertian reflector, and both the Moon and Mars are non-Lambertian. Furthermore, the brightness of the Moon has a significant dependence on phase (due to an opposition surge), with the Full Moon at 100% illumination appearing much brighter than a gibbous Moon even just slightly below 100% illumination. Interestingly, the occultation of Mars occurred while the Moon was 99.9% illuminated and therefore experiencing a peak in brightness. Mars was also approaching opposition, and consequently exhibiting an opposition effect of its own, so the interplay here is difficult to predict. I have seen images from previous Mars lunar occultations in which Mars does not appear as dark, but this interpretation is complicated by the fact that we don't usually know many details about how the images were processed. However, it would certainly be possible for Mars to appear brighter than the lunar surface depending on the phase.

The visual comparison between red and gray brightness might make it more confusing too, maybe its worth converting the image to monochrome.

This is an excellent idea, although it is surprisingly more complicated than one may originally assume. The RGB values for each pixel must be converted to a grayscale value, and there are many ways to do this, each producing a different result. One of the common "default" settings is to use a weighting of R=21%, G=72%, and B=7%, to reflect the sensitivity of the human eye to perceived brightness that varies with wavelength. Shown below are three panels. The left is the color panel, using the raw data having only applied a color balance and gamma transformation (to make it look "normal" on most screens). The middle panel applies the "default" grayscale transformation described above, whereas the right panel applies a grayscale transformation in which only the red channel data is used to assign grayscale value. This approximates how the scene would look if using a red filter and a monochrome camera. In this case, Mars appears brighter than in the middle panel, because of the strong contribution from the red channel. In both cases Mars appears less bright than the average surface of the Full Moon, although the effect differs depending on the grayscale transformation used.

[Ann]

That is such an attractive image........

What I find mystifying is the fact that Mars looks darker than the Moon. It appears to reflect light less efficiently than the Earth's Moon.

Ann, thanks for your nice comment about the image. I share your fascination about the perceived difference in brightness between the two objects. As Chris points out above, there are many difficulties inherent to interpreting photometric information from images. However, I processed this image in such a way that some general interpretations can be made, including your observation of relative brightness. The image was processed using only "global" operations, which is to say that every operation was applied equally across the frame and there were no selective enhancements (eg brightening of certain features). This is important because many astro images we see published do contain such selective enhancements, and although the result is often aesthetically pleasing, it is then impossible to interpret relationships in the data.

Of note, geometric albedo compares the brightness of an object to an idealized Lambertian reflector, and both the Moon and Mars are non-Lambertian. Furthermore, the brightness of the Moon has a significant dependence on phase (due to an opposition surge), with the Full Moon at 100% illumination appearing much brighter than a gibbous Moon even just slightly below 100% illumination. Interestingly, the occultation of Mars occurred while the Moon was 99.9% illuminated and therefore experiencing a peak in brightness. Mars was also approaching opposition, and consequently exhibiting an opposition effect of its own, so the interplay here is difficult to predict. I have seen images from previous Mars lunar occultations in which Mars does not appear as dark, but this interpretation is complicated by the fact that we don't usually know many details about how the images were processed. However, it would certainly be possible for Mars to appear brighter than the lunar surface depending on the phase.

The visual comparison between red and gray brightness might make it more confusing too, maybe its worth converting the image to monochrome.

This is an excellent idea, although it is surprisingly more complicated than one may originally assume. The RGB values for each pixel must be converted to a grayscale value, and there are many ways to do this, each producing a different result. One of the common "default" settings is to use a weighting of R=21%, G=72%, and B=7%, to reflect the sensitivity of the human eye to perceived brightness that varies with wavelength. Shown below are three panels. The left is the color panel, using the raw data having only applied a color balance and gamma transformation (to make it look "normal" on most screens). The middle panel applies the "default" grayscale transformation described above, whereas the right panel applies a grayscale transformation in which only the red channel data is used to assign grayscale value. This approximates how the scene would look if using a red filter and a monochrome camera. In this case, Mars appears brighter than in the middle panel, because of the strong contribution from the red channel. In both cases Mars appears less bright than the average surface of the Full Moon, although the effect differs depending on the grayscale transformation used.

color_comparison.jpg

Thank you for your explanation! That's very interesting.

Ann

[Chris Peterson]

However, I processed this image in such a way that some general interpretations can be made, including your observation of relative brightness. The image was processed using only "global" operations, which is to say that every operation was applied equally across the frame and there were no selective enhancements (eg brightening of certain features).

That said, even global operations will change the ratio between regions of different brightness. I assume you applied a non-linear transfer function to maximize (or at least, optimize) the dynamic range. That, of course, changes the brightness differently for different elements of the image. Nothing wrong with that, of course... our eyes do the same thing!

[Tom Glenn]

However, I processed this image in such a way that some general interpretations can be made, including your observation of relative brightness. The image was processed using only "global" operations, which is to say that every operation was applied equally across the frame and there were no selective enhancements (eg brightening of certain features).

That said, even global operations will change the ratio between regions of different brightness. I assume you applied a non-linear transfer function to maximize (or at least, optimize) the dynamic range. That, of course, changes the brightness differently for different elements of the image. Nothing wrong with that, of course... our eyes do the same thing!

All correct statements. If you wanted to make any sort of measurements, you would need to use the linear data. Shown below is the image before any non-linear operations have been performed, and after converting to grayscale using default parameters. The mean pixel value (8 bit 0-255 scale) for the lunar surface is 84, and for Mars is 40.

[VictorBorun]

The mean pixel value (8 bit 0-255 scale) for the lunar surface is 84, and for Mars is 40.
Moon_Mars_linear_TGlenn.jpg

In Mars's sky the disk of Sun is smaller, so Mars is poorly illuminated compared to Earth or Moon.
You can think of Mars' orbit as a backstage half-shadowed from the lime light.

As to a relation of angular size to brightness, there is none as long as every object of interest occupies many pixels of the scene.
Only hair-width lines or star-like specks in a scene do mix with their background and do lose their contrast at greater distances

[VictorBorun]

It's not to say that plants from Earth would not thrive on Mars because of low light.
Plants on Earth have to deal with cloudy days which are often darker than Mars' days

[Chris Peterson]

It's not to say that plants from Earth would not thrive on Mars because of low light.
Plants on Earth have to deal with cloudy days which are often darker than Mars' days

The Sun is a bit under half as bright at Mars as it is on Earth. Our vision would not even notice a difference, nor would it have much impact on plants.