APOD: Moon Struck (2019 Jan 25)

[Nitpicker]

Very shallow indeed.

[JohnD]

neufer, I'm glad to accept that the albedo of the Earth from the Moon is 40 times the opposite, but still. 40 times a very small number is still a very small number. To repeat my analogy of before, ever tried warming your hands in moonlight?

Mark's point about the low heat conductivity of rock is valuable, but then we have Chris' that most of the heated regolith and rock will have been vaporised and ejected, leaving a crater base at near 'normal' temperature. So it seems that exciting as it is, a search for a glowing crater is a bit Hollywood.

But we may see it! The Bad Astronomer, Phil Plait, in the link posted by neufer, wrote that a colleague hopes to use the Lunar Reconnaissance Orbiter to image it, and is seeking other observations to narrow down the location. Watch this space?

And, isn't this an interesting conversation? Much more so, I dare to say, than emoting on the undoubted beauty of astronomical images.
John

[neufer]

An infrared camera image of the Moon taken with the Lunar Crater Observation and Sensing Satellite (LCROSS) mid-infrared camera. This gives some indication of how rapidly the far side of the Moon's surface radiatively cools down from ~390K as it slowly rotates into lunar night.

---

neufer, I'm glad to accept that the albedo of the Earth from the Moon is 40 times the opposite, but still. 40 times a very small number is still a very small number. To repeat my analogy of before, ever tried warming your hands in moonlight?

We're not talking about warming my cold ~300K hands by Moonlight.
We're talking about warming a ~26K starlit Moon by Earthlight.

Mark's point about the low heat conductivity of rock is valuable, but then we have Chris' that most of the heated regolith and rock will have been vaporised and ejected, leaving a crater base at near 'normal' temperature.

Chris's scenario is no doubt more realistic than Mark's. But even given Mark's scenario Mark's logic is flawed because his heated regolith and rock are quickly covered over with a cool sheath of low heat conductive rock.

But we may see it! Phil Plait, in the link posted by neufer, wrote that a colleague hopes to use the Lunar Reconnaissance Orbiter to image it, and is seeking other observations to narrow down the location.

The new crater (the Blood Red Wolf Moon's lair?) will no doubt be found ... just not in the IR.

[Chris Peterson]

neufer, I'm glad to accept that the albedo of the Earth from the Moon is 40 times the opposite, but still. 40 times a very small number is still a very small number. To repeat my analogy of before, ever tried warming your hands in moonlight?

Albedo is entirely the wrong way to be thinking about this. Albedo is a measure of reflectivity- the Earth reflects about 30% of the light that hits it, the Moon reflects between about 10% and 50%, depending upon phase. What's at issue here isn't reflectivity, but emissivity. Both bodies are warm- much warmer than the cosmos- so they are radiating lots of IR. The Moon radiates significant heat towards the Earth, but we don't feel it on the ground because we are blanketed by a much warmer atmosphere. But from above the atmosphere you could aim an IR thermometer at the Moon and note how warm it is compared to the rest of the sky. On the Moon, however, the radiant heat of the Earth makes it right to the surface. You could indeed warm your hands by earthlight (just as an astronaut in orbit above the Earth could warm hers by moonlight).

[JohnD]

There, I'm told, and will say no more.
I'm educated, me, and thank you for it!

Except, please, NOT "The Blood Red Moon Wolf's lair"? (See viewtopic.php?f=9&t=39101, post 2)
John

[BDanielMayfield]

My thanks to JohnD, Mark, Art and Chris for the interesting responses to my crater detectablity in IR question.

As deep as the rest of this discussion is getting, I'm guessing that an impact crater will start out quite hot and partially molten, though perhaps in some cases not all that much (if most of the material vaporizes). Then it would begin to equiliibrate with its surrounding lunar regolith while radiating some heat into space. I would think this would follow a Newtonian cooling curve. Of course the constant for the equation is the big question. I would have guessed that you could tell the crater was hot for days. It was a wild guess. I did find one rather different article that might help. It gives hope that one could detect heat in this brand-new crater for a lot longer than my guess.

• That cuts both ways.

Rock being a poor conductor also means that the hot (partially molten) rock will quickly radiate off from its top surface thereby covering these hot pieces with a thin poorly conducting cool layer that will rapidly make them very hard to see in the IR.

Yes... but this presumes there is any hot rock at the bottom of the hole in the first place. I think that the the molten material is immediately ejected, and there is a shockwave which propagates somewhat deeper at supersonic speed, breaking up and flinging out big chunks, and what's left after a second or two is a hole with the bottom about the same temperature as the rock at that depth was already at.

It's also worth noting that radiation is the least efficient way of cooling something off, so I don't know that most materials will cool all that quickly on the Moon, at least not compared with the Earth.

So then the main area of the newly formed crater won't be hot for long at all (except perhaps the central point of impact?) but what about the ejecta blanketing the area surrounding the new crater? Won't that glow for quite some time?

Bruce

[Chris Peterson]

My thanks to JohnD, Mark, Art and Chris for the interesting responses to my crater detectablity in IR question.

• That cuts both ways.

Rock being a poor conductor also means that the hot (partially molten) rock will quickly radiate off from its top surface thereby covering these hot pieces with a thin poorly conducting cool layer that will rapidly make them very hard to see in the IR.

Yes... but this presumes there is any hot rock at the bottom of the hole in the first place. I think that the the molten material is immediately ejected, and there is a shockwave which propagates somewhat deeper at supersonic speed, breaking up and flinging out big chunks, and what's left after a second or two is a hole with the bottom about the same temperature as the rock at that depth was already at.

It's also worth noting that radiation is the least efficient way of cooling something off, so I don't know that most materials will cool all that quickly on the Moon, at least not compared with the Earth.

So then the main area of the newly formed crater won't be hot for long at all (except perhaps the central point of impact?) but what about the ejecta blanketing the area surrounding the new crater? Won't that glow for quite some time?

Bruce

The first thing that gets ejected is just molten microscopic droplets, which cool before they even land. That's where most of the impact energy goes. That's followed by bigger pieces, but those are just being ejected by the shock wave, with nowhere near enough energy to heat up significantly. The macroscopic ejecta is cold even as it leaves the newly forming crater.

[bystander]

Image of the explosion on the Moon during the beginning of the totality of the eclipse
of January 21, 2019, captured by the camera of the LaLoma Observatory in San Vicente
Ferrer (Antioquia, Colombia). (Jonathan Ospina, Mauricio Gaviria, Sergio López)

Impact on the Moon!
Astronomia Universidad de Antioquia | 2019 Jan 31

A group of professional and amateur astronomers from the Dominican Republic and Colombia recorded an impact on the Moon during the last total eclipse and now submitted a scientific analysis of their images.

Location, Orbit and Energy of a Meteoroid Impacting the Moon
During the Lunar Eclipse of January 21, 2019 ~ Jorge I. Zuluaga et al

• arXiv.org > astro-ph > arXiv:1901.09573 > 28 Jan 2019

[Nitpicker]

Thanks bystander. Most satisfying.