APOD: Phobos over Mars (2023 Jul 31)

[Ann]

Phobos doesn't look so dark to me. Maybe a darker shade of gray should have been used to color the moon.

pixels in this APOD's Phobos are like RGB = 115 125 137 of 255, far too bright for albedo 0.071, or 18 of 255

In the same time Mars's clouds at grazing angles should be as white as they are presented, so you can not just darken the whole picture to get an accurate velvet-black Phobos
PhobosMars_MarsExpress_1500 Phobos-.jpg

Fascinating, Victor! I especially like that you have figured out the RGB values of Phobos in the APOD:

Click to view full size image

So the ARG values of Phobos in the APD is R = 115, G = 125, and B = 137.

Such an RGB value would actually make Phobos bluish (since the B value is higher than either R or G). And it does look bluish in the APOD! We can be sure that this is not correct. Almost all asteroids and asteroid-like bodies in the solar system are either dull gray or reddish:

John Hopkins Applied Physics Laboratory wrote:

Many airless rocky bodies, including Arrokoth, appear dull gray to rusty red thanks to cosmic and ultraviolet rays that hit the surface and “toast” any organic, carbon-based materials, “much like the stuff you burn in your kitchen,” he explained. The rays also smelt the rock, forcing up tiny dark-red iron crystals just a few billionths of a meter long.

This gray-and-red dichotomy is particularly noticeable in Pluto's giant moon Charon:

Charon in reasonably true color. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

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Charon in enhanced color. Credit: NASA/JHUAPL/SwRI

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We have very good reasons to believe that Phobos is reddish, and it is, too.

ENHANCED COLOR VIEW OF PHOBOS FROM MARS RECONNAISSANCE ORBITER. Credit: NASA / JPL / U. Arizona

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The Planetary Society wrote about the color of Phobos in the Mars Reconnaissance Orbiter image:

The color is from infrared, red, and blue-green channels on the camera, so it represents light shifted slightly longer in wavelength than human eyes can see, which emphasizes subtle colorations on the moon. The color view shows that the material surrounding the giant crater Stickney (on the left side of the moon) appears gray while the rest of the moon appears reddish. The grayer material is likely fresher material. From this vantage point Phobos appears about 21 kilometers (13 miles) in diameter. (The irregularly-shaped moon has a minimum diameter of about 19 kilometers and a maxiumum diameter of about 27 kilometers.)

So the true color of Phobos would be reddish. Or rather, it would be more like pitch black with the barest hint of red! Not like this color,███, but much darker.

Ann

[johnnydeep]

The link about there likely being a meter thick layer of loose dust on the surface of Phobos says "Evidence comes from infrared pictures that indicate the rapid speed that Phobos' surface cools after sunset."

I suppose my intuition about a loose layer of dust providing insulation and so not warming or cooling quickly is incorrect? Does a Styrofoam cup cool and warm quickly here on Earth despite preventing whatever is in it from doing the same?

what infrared pics tells you of a porous material in vacuum is the temperature of a thin layer which is thermally isolated from the bulk.
Earth's Sahara sand cools down by 42°C during a clear sky night, while Phobos goes farther, -4°C to -112°C.

"Thermal isolation" - a concept that almost makes sense to me. But why would a less dense surface layer not be thermally isolated both from the body it surrounds as well as from the vacuum of space? Is it purely due to the ability of such a thin layer to quickly radiate away to space any heat it has?

[johnnydeep]

But there is more to thermal transfer than the number of atoms.


Only away from the material, not into it. And all external directions are nominally equivalent in the simple case, so there's really only a single metric that defines the radiative gain/loss. So it's best understood as a one-dimensional process. (And yes, we could make a complex analysis where photons are emitted into the material, resulting in some messy combination of effects. But I think we're just considering the basic, first-order process here. Heat transfer inside the material is primarily conductive, and is weak in this insulating material. Heat transfer at the surface is primarily radiative, and is what is most important in considering the rate that the surface temperature changes with radiative environment.)

Ok. I'll have to add thermal properties of materials to optics as things I don't understand well. Which is a list that already had electromagnetism and relativity on it for quite a long time! (My latest confusion is how come a molten iron core generates a magnetic field due to rotation (like in the core of the Earth), but a rotating solid iron ball or ring does not? Electrons are "flowing" in both cases are they not?)

Well, the full answer requires a gruesome dive into magnetohydrodynamics, but the key difference is that in order to generate a magnetic field in a planet or star you need convection, and that obviously requires a fluid conductor, not a solid one. Rotation is also required, but is not enough by itself.

Ok. And of course Wikipedia has more about this process happening in the Earth. Pretty complex stuff!

Earth's core and the geodynamo

The Earth's magnetic field is believed to be generated by electric currents in the conductive iron alloys of its core, created by convection currents due to heat escaping from the core.

The Earth and most of the planets in the Solar System, as well as the Sun and other stars, all generate magnetic fields through the motion of electrically conducting fluids.[51] The Earth's field originates in its core. This is a region of iron alloys extending to about 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core, with a radius of 1220 km, and a liquid outer core.[52] The motion of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary, which is about 3,800 K (3,530 °C; 6,380 °F).[53] The heat is generated by potential energy released by heavier materials sinking toward the core (planetary differentiation, the iron catastrophe) as well as decay of radioactive elements in the interior. The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core.[54]

The mechanism by which the Earth generates a magnetic field is known as a dynamo.[51] The magnetic field is generated by a feedback loop: current loops generate magnetic fields (Ampère's circuital law); a changing magnetic field generates an electric field (Faraday's law); and the electric and magnetic fields exert a force on the charges that are flowing in currents (the Lorentz force).[55]

...

The motion of the fluid is sustained by convection, motion driven by buoyancy. The temperature increases towards the center of the Earth, and the higher temperature of the fluid lower down makes it buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is plated to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This is called compositional convection. A Coriolis effect, caused by the overall planetary rotation, tends to organize the flow into rolls aligned along the north–south polar axis.[54][56]

A dynamo can amplify a magnetic field, but it needs a "seed" field to get it started.[56] For the Earth, this could have been an external magnetic field. Early in its history the Sun went through a T-Tauri phase in which the solar wind would have had a magnetic field orders of magnitude larger than the present solar wind.[57] However, much of the field may have been screened out by the Earth's mantle. An alternative source is currents in the core-mantle boundary driven by chemical reactions or variations in thermal or electric conductivity. Such effects may still provide a small bias that are part of the boundary conditions for the geodynamo.[58]

The average magnetic field in the Earth's outer core was calculated to be 25 gauss, 50 times stronger than the field at the surface.

Click to view full size image

[VictorBorun]

The link about there likely being a meter thick layer of loose dust on the surface of Phobos says "Evidence comes from infrared pictures that indicate the rapid speed that Phobos' surface cools after sunset."

I suppose my intuition about a loose layer of dust providing insulation and so not warming or cooling quickly is incorrect? Does a Styrofoam cup cool and warm quickly here on Earth despite preventing whatever is in it from doing the same?

what infrared pics tells you of a porous material in vacuum is the temperature of a thin layer which is thermally isolated from the bulk.
Earth's Sahara sand cools down by 42°C during a clear sky night, while Phobos goes farther, -4°C to -112°C.

"Thermal isolation" - a concept that almost makes sense to me. But why would a less dense surface layer not be thermally isolated both from the body it surrounds as well as from the vacuum of space? Is it purely due to the ability of such a thin layer to quickly radiate away to space any heat it has?

I meant thermally isolated just when you compare Sahara's sand or Phobos's material to centimetres or even a meter of nightly/daily affected layer of high thermal conductivity inside monoliths or, say, water in a lake on Earth.
Yes, there is a radiation thermal equilibrium between the sky and the surface of Sahara's sand or the surface of Phobos, lagged back by the heat storing capacity of a few grain thick surface layer: for a minute I think by the upper grain, for a night or for a day by a few lower grains

[johnnydeep]

what infrared pics tells you of a porous material in vacuum is the temperature of a thin layer which is thermally isolated from the bulk.
Earth's Sahara sand cools down by 42°C during a clear sky night, while Phobos goes farther, -4°C to -112°C.

"Thermal isolation" - a concept that almost makes sense to me. But why would a less dense surface layer not be thermally isolated both from the body it surrounds as well as from the vacuum of space? Is it purely due to the ability of such a thin layer to quickly radiate away to space any heat it has?

I meant thermally isolated just when you compare Sahara's sand or Phobos's material to centimetres or even a meter of nightly/daily affected layer of high thermal conductivity inside monoliths or, say, water in a lake on Earth.
Yes, there is a radiation thermal equilibrium between the sky and the surface of Sahara's sand or the surface of Phobos, lagged back by the heat storing capacity of a few grain thick surface layer: for a minute I think by the upper grain, for a night or for a day by a few lower grains

Ok.