Re: APOD: Moon Struck (2019 Jan 25)
Posted: Sat Jan 26, 2019 4:27 am
Very shallow indeed.
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We're not talking about warming my cold ~300K hands by Moonlight.
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.
The new crater (the Blood Red Wolf Moon's lair?) will no doubt be found ... just not in the IR.
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).
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?Chris Peterson wrote: ↑Sat Jan 26, 2019 1:23 amYes... 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.neufer wrote: ↑Sat Jan 26, 2019 1:18 amMarkBour wrote: ↑Sat Jan 26, 2019 12:02 am
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.
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.
- That cuts both ways.
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.
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.BDanielMayfield wrote: ↑Sun Jan 27, 2019 4:28 pm My thanks to JohnD, Mark, Art and Chris for the interesting responses to my crater detectablity in IR question.
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?Chris Peterson wrote: ↑Sat Jan 26, 2019 1:23 amYes... 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.neufer wrote: ↑Sat Jan 26, 2019 1:18 amRock 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.
- That cuts both ways.
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.
Bruce
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.