(meteorobs) (Meteorobs) comment on being green

[MstrEman]

FWIW, Actually, we largely agree   Meteoroid composition only impacts
spectra measurements, not perceived color. There is also a distinction
between the meteor, the train, and the trail.  I believe most who do
incidental observing do not distinguish color regions.  I think color
comes from the bow-wave, when visible, and the zone twixt meteoroid
head itself and the shockwave where most compressive heating is
presumed to take place.  The trail(aka wake zone) is almost always
white/white yellow, full of "smoke" .  We infer it contains a molten
silicate and/ or molten iron so true incandescence is probably a major
component of color inside the wake zone--that and some chemical
reaction products if we correctly assume the persistent "smoke trail"
is truly "smoke". I'd like to hear more about color indicators going
UP in the atmosphere however.

As visual observation is a matter of perception, the overriding source
for color (vs spectra) is photon emissions from excited atmospheric
gases (normally). It has been suggested that some as of yet to be
determined density of fresh volcanic ash/dust in the atmosphere, might
skew normal spectra but could also-- in theory-- affect apparent
color. I don't know if this reference was to actual excitation of dust
particles or if it was the scattering/blocking by particles throughout
the atmosphere affecting apparent magnitudes as well. We assume we
know that it affects the intensity of sunsets after all.

There was a long- accepted belief in the meteorite community that the
green color was simply owing to "nickel content".  I guess you run to
what is most familiar when you really have no data.   My personal
opinion in having read extensively on the topic of meteor color and
elemental analysis via  "blowpipe and bead" mineralogical analysis is
that meteoroid composition indicators are imperceptible outside
spectrograms.

As to the color change at end of flight: an alternate explanation is
involves the change in spectra emission lines for nitrogen in the
lower atmosphere vs entry into dark flight.  According to some things
I read, both oxygen and nitrogen in single atom and normal molecule (
O vs O2 and N vs N2)--presumably owing to natural sorting in the
atmosphere-- have emission lines which show a change in color with
altitude.    Annotations do not always distinguish between spectral
lines for oxygen/nitrogen molecules and free O and N atoms.

 We've always accepted oxygen as the green producer in this arena but
now we know it is more complex than that and nitrogen is a big
component the deeper a meteor makes it into the atmosphere.  Even from
my readings in both aurora and meteor distribution of typical spectra
there are ample emissions of  blues and yellows--which are perceived
by the eye as "green".  Ergo, there are a lot of opportunities for
green to be the most often mentioned color of any fireball.  This
appears to be borne out by observation.

Also previously discussed is that at lower levels in the atmosphere,
"nitrogen's" spectral lines shift to the magenta end.  Hence the idea
that a "green fireball" which changes over to a
"reddish-orange-magenta color-cascade" has reached into the "lower"
atmosphere( below 100km???).  Meteoroids reaching into the lower
atmosphere by necessity had to have started  with a mass that was
larger and more characteristic of that from an asteroid, rather than
the dust/grain- sized typical cometary meteor.  It is not just green
fireballs-- as your survey of reports show they are common.  I think
the distinction are those that were green and show a color change at
the end of flight are those we can make course assumptions about--As I
understand what I believe I've picked up in digging into the subject.
Applying what I know about the extreme shallow depth of heating in on
a meteoroid and rapid chilling below solidus I think the output of
color is too weak and too short lived to be easily observed.  Occum'
Razor might suggest looking instead to this reported nitrogen spectra
swing as the source.

As to George's list of reference colors seen in elemental blow pipe
analysis--most of the elements you outline are only found in
meteorites in super trace levels. They can show up in x-ray
diffraction/ spectrography analysis, but many in your list are in too
small quantities to detect as in a "normative mineralogy"  Ignition
Test, for example.

Elton

On Mon, Oct 17, 2011 at 11:51 AM, Chris Peterson <clp at alumni.caltech.edu> wrote:
> I don't believe there is much evidence to support the idea that meteor
> color (as seen with the eye) has much relationship to the meteoroid
> composition- at least, when we are talking about fireballs. There is
> good evidence, however, that the color is mainly the from ionization of
> atmospheric gas- especially oxygen. I've personally collected images of
> several bright fireballs through a 501 nm narrow band (6 nm) filter,
> which argues for a very strong [OIII] component to the light.
>
> FWIW, a quick review of the meteor reports (nearly all fireballs) I've
> received in the last 11 years shows this:
>
> 9110 reports total
> 3735 (41%) report some sort of color
> 3069 (82% of those reporting color) report some shade of green
>
> I've long since concluded that bright fireballs are almost always green.
> The exceptional cases are those which are not (and these are almost
> always reported as white).
>
> The only other color that tends to show up in witness descriptions is
> red/orange, and a close look reveals that this is almost always at the
> end of the path, when it is easily explained as the output of a cooling
> blackbody radiator.
>
> Chris
>
> *******************************
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
>
> On 10/17/2011 9:33 AM, drobnock wrote:
>> It has been suggested that the green meteor trails are from the
>> ionization of oxygen or the excitation of the oxygen atoms. So is it
>> proper to state -- for those interested in green trails - that  green
>> meteor trails are green  at 100 km up to about 150 km. Red oxygen meteor
>> trails  are 150 km upwards to 250 km and more rarely to 600 km plus?
>>
>> Is the composition of the meteor and the colour of the trail
>> attribured to the mineral composition? Per flame test of minerals. For
>> example  a red meteor may be  -  Red because Both lithium and strontium
>> produce a red flame and the strontium flame is brighter red.  or
>> Orange/yellow Calcium burns with an orange flame. Sodium burns bright
>> yellow. or Green A number of elements generate flames in the green
>> spectrum. Barium and molybdenum produce yellowish-green flames. Boron
>> creates a bright green colour. Thallium burns pure green. Phosphorus and
>> zinc burn bluish green. or Blue Selenium, indium and arsenic burn blue.
>> http://www.ehow.com/info
>>
>> George John Drobnock