APOD: Phobos over Mars (2023 Jul 31)

[APOD Robot]

Phobos over Mars

Explanation: Why is Phobos so dark? Phobos, the largest and innermost of the two Martian moons, is the darkest moon in the entire Solar System. Its unusual orbit and color indicate that it may be a captured asteroid composed of a mixture of ice and dark rock. The featured assigned-color picture of Phobos near the edge of Mars was captured in late 2021 by ESA's robot spacecraft Mars Express, currently orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. From images like this, Phobos has been determined to be covered by perhaps a meter of loose dust. Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, while from other places it would not be visible at all. Phobos' orbit around Mars is continually decaying -- it will likely break up with pieces crashing to the Martian surface in about 50 million years.

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[Ann]

Phobos over Mars. Image Credit: ESA, DLR, FU Berlin, Mars Express; Processing & CC BY 2.0 License: Andrea Luck

---

That is an amazing image! It looks positively unreal!

Near bottom of the image, we can clearly see the typical orange-colored Martian land mass, with interspersed craters. But what is the undulating whitish-beige "tapestry" that covers much of the rest of the picture?

Is that the Martian cloud cover? Does Mars have so many clouds?

Near top of the image, the undulating structure remains, but the color turns orange again, then black. What is the orange thing here? And is the black stuff the blackness of space?

Than there is Phobos itself. I swear it looks like a giant asteroid just minutes away from crashing on Mars!

Asteroid on its way to crashing on the Earth. Image credit: I don't know.

---

I find it hard not to think of the asteroid that killed the dinosaurs on Earth when I look at today's APOD!

And of course, Phobos is going to crash on Mars. Assuming there are any people left on the Earth to watch the spectacle when it happens, what would it look like from the Earth?

Ann

[FLPhotoCatcher]

Phobos over Mars. Image Credit: ESA, DLR, FU Berlin, Mars Express; Processing & CC BY 2.0 License: Andrea Luck

---

I find it hard not to think of the asteroid that killed the dinosaurs on Earth when I look at today's APOD!

And of course, Phobos is going to crash on Mars. Assuming there are any people left on the Earth to watch the spectacle when it happens, what would it look like from the Earth?

Ann

Phobos probably will not crash into Mars, if we have any say in the matter. I imagine that in the future, it would not be that difficult to raise its orbit, over a century or two.

But if it did crash on the earth-facing side of Mars, I'm guessing that from Earth it would look like Mars gets about 10 times brighter.

[Rauf]

Phobos over Mars. Image Credit: ESA, DLR, FU Berlin, Mars Express; Processing & CC BY 2.0 License: Andrea Luck

---

I find it hard not to think of the asteroid that killed the dinosaurs on Earth when I look at today's APOD!

And of course, Phobos is going to crash on Mars. Assuming there are any people left on the Earth to watch the spectacle when it happens, what would it look like from the Earth?

Ann

Phobos probably will not crash into Mars, if we have any say in the matter. I imagine that in the future, it would not be that difficult to raise its orbit, over a century or two.

But if it did crash on the earth-facing side of Mars, I'm guessing that from Earth it would look like Mars gets about 10 times brighter.

Why (and how) Mars is gonna get 10 times brighter? An asteroid (or an small moon) crashing on a big planet is sufficient to make it 10 times brighter??

[Chris Peterson]

I find it hard not to think of the asteroid that killed the dinosaurs on Earth when I look at today's APOD!

And of course, Phobos is going to crash on Mars. Assuming there are any people left on the Earth to watch the spectacle when it happens, what would it look like from the Earth?

It will actually be a rather mild event. The impact speed will only be a couple of kilometers per second, and since this just a pile of rubble, not a solid body, it will likely break up into small pieces from tidal stresses before it can fall as a single object. It might not even be capable of producing a crater. From Earth what we're most likely to see is a lot of dust, and possibly water clouds for a few months or years afterwards.

[Chris Peterson]

Phobos probably will not crash into Mars, if we have any say in the matter. I imagine that in the future, it would not be that difficult to raise its orbit, over a century or two.

The more common scenario that people predict is that we will deliberately drop it onto Mars to get it out of the way. And that's a lot easier than moving it higher.

[Ann]

Phobos probably will not crash into Mars, if we have any say in the matter. I imagine that in the future, it would not be that difficult to raise its orbit, over a century or two.

The more common scenario that people predict is that we will deliberately drop it onto Mars to get it out of the way. And that's a lot easier than moving it higher.

I'm sure you're right, Chris. I said it because I was thinking of Comet Holmes.

Comet Holmes. Credit: Iván Éder

---

Wikipedia wrote: Comet Holmes /ˈhoʊmz/ (official designation: 17P/Holmes) is a periodic comet in the Solar System, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. Although normally a very faint object, Holmes became notable during its October 2007 return when it temporarily brightened by a factor of a million, in what was the largest known outburst by a comet, and became visible to the naked eye.[5] It also briefly became the largest object in the Solar System, as its coma (the thin dissipating dust ball around the comet) expanded to a diameter greater than that of the Sun (although its mass remained minuscule).

But I'm sure that neither Mars nor Phobos is anything like Comet Holmes.

Ann

[Bird_Man]

Ann asked:

"Near bottom of the image, we can clearly see the typical orange-colored Martian land mass, with interspersed craters. But what is the undulating whitish-beige "tapestry" that covers much of the rest of the picture?

Is that the Martian cloud cover? Does Mars have so many clouds?

Near top of the image, the undulating structure remains, but the color turns orange again, then black. What is the orange thing here? And is the black stuff the blackness of space?"

If Phobos was near the edge of Mars, then we are looking through the atmosphere on a line tangent to the surface. This would be the thickest way possible to look through the atmosphere and a few thin clouds might have the appearance we see in this image. The dark at the top of the image would be space above the atmosphere.

[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?

[johnnydeep]

Ann asked:

"Near bottom of the image, we can clearly see the typical orange-colored Martian land mass, with interspersed craters. But what is the undulating whitish-beige "tapestry" that covers much of the rest of the picture?

Is that the Martian cloud cover? Does Mars have so many clouds?

Near top of the image, the undulating structure remains, but the color turns orange again, then black. What is the orange thing here? And is the black stuff the blackness of space?"

If Phobos was near the edge of Mars, then we are looking through the atmosphere on a line tangent to the surface. This would be the thickest way possible to look through the atmosphere and a few thin clouds might have the appearance we see in this image. The dark at the top of the image would be space above the atmosphere.

That seems right. And I'm sure someone with more investigative skill than I could determine from the array of craters in the image just where this part of the surface is on Mars.

[Chris Peterson]

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?

An insulating material's surface will generally heat or cool more quickly because the thermal mass of the bulk material isn't easily transported to the surface.

In making comparisons, keep in mind that the temperature of materials on Earth is commonly determined primarily by convective energy transfer, while that of materials in space is determined by radiative energy transfer. So the dynamics can be quite different.

[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?

An insulating material's surface will generally heat or cool more quickly because the thermal mass of the bulk material isn't easily transported to the surface.

In making comparisons, keep in mind that the temperature of materials on Earth is commonly determined primarily by convective energy transfer, while that of materials in space is determined by radiative energy transfer. So the dynamics can be quite different.

Ah, interesting, so it's precisely BECAUSE an insulating material prevents easy transfer/loss of heat to/from the surface that it must itself heat up or cool down quickly! That is, the energy has nowhere else to go to.

[Lasse H]

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

[Chris Peterson]

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?

An insulating material's surface will generally heat or cool more quickly because the thermal mass of the bulk material isn't easily transported to the surface.

In making comparisons, keep in mind that the temperature of materials on Earth is commonly determined primarily by convective energy transfer, while that of materials in space is determined by radiative energy transfer. So the dynamics can be quite different.

Ah, interesting, so it's precisely BECAUSE an insulating material prevents easy transfer/loss of heat to/from the surface that it must itself heat up or cool down quickly! That is, the energy has nowhere else to go to.

You need to consider what "insulating" even means. It's a property of bulk. A 3D property. But radiative heat loss/gain from a surface is a 1D process. What matters is material properties like emissivity much more than thermal resistance (insulation).

[orin stepanek]

Phobos over Mars. Image Credit: ESA, DLR, FU Berlin, Mars Express; Processing & CC BY 2.0 License: Andrea Luck

---

That is an amazing image! It looks positively unreal!

Near bottom of the image, we can clearly see the typical orange-colored Martian land mass, with interspersed craters. But what is the undulating whitish-beige "tapestry" that covers much of the rest of the picture?

Is that the Martian cloud cover? Does Mars have so many clouds?

Near top of the image, the undulating structure remains, but the color turns orange again, then black. What is the orange thing here? And is the black stuff the blackness of space?

Than there is Phobos itself. I swear it looks like a giant asteroid just minutes away from crashing on Mars!

Asteroid on its way to crashing on the Earth. Image credit: I don't know.

---

I find it hard not to think of the asteroid that killed the dinosaurs on Earth when I look at today's APOD!

And of course, Phobos is going to crash on Mars. Assuming there are any people left on the Earth to watch the spectacle when it happens, what would it look like from the Earth?

Ann

With mars' thin atmosphere, I think it would hit the planet without much ado!
'

[johnnydeep]

An insulating material's surface will generally heat or cool more quickly because the thermal mass of the bulk material isn't easily transported to the surface.

In making comparisons, keep in mind that the temperature of materials on Earth is commonly determined primarily by convective energy transfer, while that of materials in space is determined by radiative energy transfer. So the dynamics can be quite different.

Ah, interesting, so it's precisely BECAUSE an insulating material prevents easy transfer/loss of heat to/from the surface that it must itself heat up or cool down quickly! That is, the energy has nowhere else to go to.

You need to consider what "insulating" even means. It's a property of bulk. A 3D property. But radiative heat loss/gain from a surface is a 1D process. What matters is material properties like emissivity much more than thermal resistance (insulation).

So is the "thermal bulk" of Styrofoam small or large? Less dense matter seems like it would be more insulating than higher density matter, or is that only true in some circumstances.

And a 1D process? Not 2D (as in a 2D surface)?

[Chris Peterson]

Ah, interesting, so it's precisely BECAUSE an insulating material prevents easy transfer/loss of heat to/from the surface that it must itself heat up or cool down quickly! That is, the energy has nowhere else to go to.

You need to consider what "insulating" even means. It's a property of bulk. A 3D property. But radiative heat loss/gain from a surface is a 1D process. What matters is material properties like emissivity much more than thermal resistance (insulation).

So is the "thermal bulk" of Styrofoam small or large? Less dense matter seems like it would be more insulating than higher density matter, or is that only true in some circumstances.

And a 1D process? Not 2D (as in a 2D surface)?

Well, Styrofoam is a good insulator. And it has low density, meaning it doesn't have much thermal mass. Those two often go together, but they don't have to.

Yes, I meant a 1D process, in the sense that radiative loss is in one direction, away from the surface. A point on the surface doesn't radiate into the surrounding material, either in three dimensions or in two.

[Chris Peterson]

With mars' thin atmosphere, I think it would hit the planet without much ado! :shock:

The behavior of meteors on Mars is surprisingly similar to that of meteors on Earth. Heating, ablation, burning up. The important points in the case of Phobos are that it has very low material strength, so it will likely fragment into much smaller pieces first, and it will fall very slowly compared with a meteor.

[johnnydeep]

You need to consider what "insulating" even means. It's a property of bulk. A 3D property. But radiative heat loss/gain from a surface is a 1D process. What matters is material properties like emissivity much more than thermal resistance (insulation).

So is the "thermal bulk" of Styrofoam small or large? Less dense matter seems like it would be more insulating than higher density matter, or is that only true in some circumstances.

And a 1D process? Not 2D (as in a 2D surface)?

Well, Styrofoam is a good insulator. And it has low density, meaning it doesn't have much thermal mass. Those two often go together, but they don't have to.

A low density would always mean poor (kinetic) heat transfer capability since there would be fewer atoms available to do the transferring.

Yes, I meant a 1D process, in the sense that radiative loss is in one direction, away from the surface. A point on the surface doesn't radiate into the surrounding material, either in three dimensions or in two.

Why wouldn't a point be capable of radiating a photon in any arbitrary 3D direction?

[Chris Peterson]

So is the "thermal bulk" of Styrofoam small or large? Less dense matter seems like it would be more insulating than higher density matter, or is that only true in some circumstances.

And a 1D process? Not 2D (as in a 2D surface)?

Well, Styrofoam is a good insulator. And it has low density, meaning it doesn't have much thermal mass. Those two often go together, but they don't have to.

A low density would always mean poor (kinetic) heat transfer capability since there would be fewer atoms available to do the transferring.

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

Yes, I meant a 1D process, in the sense that radiative loss is in one direction, away from the surface. A point on the surface doesn't radiate into the surrounding material, either in three dimensions or in two.

Why wouldn't a point be capable of radiating a photon in any arbitrary 3D direction?

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.)

[johnnydeep]

Well, Styrofoam is a good insulator. And it has low density, meaning it doesn't have much thermal mass. Those two often go together, but they don't have to.

A low density would always mean poor (kinetic) heat transfer capability since there would be fewer atoms available to do the transferring.

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

Yes, I meant a 1D process, in the sense that radiative loss is in one direction, away from the surface. A point on the surface doesn't radiate into the surrounding material, either in three dimensions or in two.

Why wouldn't a point be capable of radiating a photon in any arbitrary 3D direction?

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?)

[Chris Peterson]

A low density would always mean poor (kinetic) heat transfer capability since there would be fewer atoms available to do the transferring.

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

Why wouldn't a point be capable of radiating a photon in any arbitrary 3D direction?

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.

[VictorBorun]

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

[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.

[Chris Peterson]

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

Well, it's not like any images, and especially astronomical ones, are photometrically accurate. And in fact, if you create a photometrically accurate image it will actually look nothing like what we'd see with our eyes due to their highly non-linear response. Today's APOD probably is close.