by Ann » Sat Jul 11, 2020 6:34 pm
johnnydeep wrote: ↑Sat Jul 11, 2020 5:21 pm
Ann wrote: ↑Sat Jul 11, 2020 5:07 pm
johnnydeep wrote: ↑Sat Jul 11, 2020 4:56 pm
So, I have to ask: what element(s) are causing the blue ion tail?
Cosmos - The SAO Encyclopedia of Astronomy wrote:
While the dust tails of comets shine by reflected sunlight and are therefore yellow in colour, the gas tails shine through fluorescence. In particular, the dominant CO+ molecule absorbs sunlight which it then re-emits at a wavelength of 4,200 angstroms as it de-excites. For this reason, ion tails tend to be blue in color.
Ann
Thanks - cool site! (and
very appropriate emojis) And now I have to ask the obvious follow-up: is it
always primarily carbon monoxide ions (and why would that be, unless all comets are made up of the same stuff, which I find hard to believe), or have there been other comets with different colored ion tails caused by different molecules?
Ethan Siegel of Forbes wrote:
Comets are made out of a mix of rocky components, similar to what makes up the Earth's mantle, dust, and ices. Ice doesn't just mean water-ice (H2O), but also volatile components like dry ice (solid CO2), methane (CH4), ammonia (NH3), and carbon monoxide (CO). The full suite of cometary ices was investigated by the Rosetta mission, but these are the big five. Under typical, cold conditions, the ices remain frozen, but as the comet nears the Sun, they start to heat up.
The first thing that happens to a comet, as it approaches the Sun, is that the amount of ultraviolet light striking it becomes great enough that it can start ionizing the weakest molecule there: carbon monoxide. This creates an abundance of the CO+ ion, which streams directly away from the Sun. This turns into a blue ion tail, and is the first comet-like feature to appear as a comet begins to heat up.
...
As the nucleus of the comet gets hot, more of the ices melt and diffuse away from the surface, creating a large, diffuse set of particles around the nucleus. This diffuse region is known as the coma of a comet, and is made of a mix of gas and dust.
...
But the coma is more than dust. There is also gas, created from the sublimated compounds that were part of the comet. There aren't merely simple ices and rocks on this body, but more complex molecules made out of these fundamental building blocks: mostly hydrogen, oxygen, carbon, and nitrogen. Two molecules that are of particular interest are cyanide/cyanogen (CN: a carbon-nitrogen bond) and diatomic carbon (C2: a carbon-carbon bond).
...
When you see that green color, it's an indicator of a combination of things:
that the coma contains large amounts of CN and C2 molecules,
that the comet is active (outgassing) and warm (close to the Sun), and
that the potential for a schism or eruption is at its highest.
So, to summarize: The ion tail is
blue because ultraviolet photons from the Sun excite
CO molecules and make them emit blue light of 420 nm. The comet coma is
bluish green because ultraviolet photons from the Sun excite
CN and
C2 molecules.
Note that Comet NEOWISE doesn't seem to have a green coma. This is unusual. I suspect that it does have a green coma after all, because pretty much all comets do (I believe). Maybe Comet NEOWISE is illuminated in such a way that the sunlight that strikes it has passed through the Earth's atmosphere first? So that, from our point of view, the sunlight lighting up Comet NEOWISE has been reddened?
Or something?
Ann
[quote=johnnydeep post_id=304029 time=1594488082 user_id=132061]
[quote=Ann post_id=304028 time=1594487276 user_id=129702]
[quote=johnnydeep post_id=304027 time=1594486581 user_id=132061]
So, I have to ask: what element(s) are causing the blue ion tail?
[/quote]
[quote][url=https://astronomy.swin.edu.au/cosmos/C/Cometary+Gas+Tail]Cosmos - The SAO Encyclopedia of Astronomy[/url] wrote:
While the dust tails of comets shine by reflected sunlight and are therefore yellow in colour, the gas tails shine through fluorescence. In particular, the dominant CO+ molecule absorbs sunlight which it then re-emits at a wavelength of 4,200 angstroms as it de-excites. For this reason, ion tails tend to be blue in color.[/quote]
🌞☄️
Ann
[/quote]
Thanks - cool site! (and [i]very [/i]appropriate emojis) And now I have to ask the obvious follow-up: is it [i]always [/i]primarily carbon monoxide ions (and why would that be, unless all comets are made up of the same stuff, which I find hard to believe), or have there been other comets with different colored ion tails caused by different molecules?
[/quote]
[quote][url=https://www.forbes.com/sites/startswithabang/2018/08/09/this-is-why-comets-glow-an-eerie-green-color/#5b4f5c2c1d24]Ethan Siegel of Forbes[/url] wrote:
Comets are made out of a mix of rocky components, similar to what makes up the Earth's mantle, dust, and ices. Ice doesn't just mean water-ice (H2O), but also volatile components like dry ice (solid CO2), methane (CH4), ammonia (NH3), and carbon monoxide (CO). The full suite of cometary ices was investigated by the Rosetta mission, but these are the big five. Under typical, cold conditions, the ices remain frozen, but as the comet nears the Sun, they start to heat up.
The first thing that happens to a comet, as it approaches the Sun, is that the amount of ultraviolet light striking it becomes great enough that it can start ionizing the weakest molecule there: carbon monoxide. This creates an abundance of the CO+ ion, which streams directly away from the Sun. This turns into a blue ion tail, and is the first comet-like feature to appear as a comet begins to heat up.
...
As the nucleus of the comet gets hot, more of the ices melt and diffuse away from the surface, creating a large, diffuse set of particles around the nucleus. This diffuse region is known as the coma of a comet, and is made of a mix of gas and dust.
...
But the coma is more than dust. There is also gas, created from the sublimated compounds that were part of the comet. There aren't merely simple ices and rocks on this body, but more complex molecules made out of these fundamental building blocks: mostly hydrogen, oxygen, carbon, and nitrogen. Two molecules that are of particular interest are cyanide/cyanogen (CN: a carbon-nitrogen bond) and diatomic carbon (C2: a carbon-carbon bond).
...
When you see that green color, it's an indicator of a combination of things:
that the coma contains large amounts of CN and C2 molecules,
that the comet is active (outgassing) and warm (close to the Sun), and
that the potential for a schism or eruption is at its highest.[/quote]
So, to summarize: The ion tail is [b][size=120][color=#0035ff]blue[/color][/size][/b] because ultraviolet photons from the Sun excite [b][size=120]CO[/size][/b] molecules and make them emit blue light of 420 nm. The comet coma is [b][size=120][color=#00ff92]bluish green[/color][/size][/b] because ultraviolet photons from the Sun excite [b][size=120]CN[/b] and [b][size=120]C2[/size][/b] molecules.
Note that Comet NEOWISE doesn't seem to have a green coma. This is unusual. I suspect that it does have a green coma after all, because pretty much all comets do (I believe). Maybe Comet NEOWISE is illuminated in such a way that the sunlight that strikes it has passed through the Earth's atmosphere first? So that, from our point of view, the sunlight lighting up Comet NEOWISE has been reddened?
Or something?
Ann