[meteorite-list] Very Bright Fireball Over Europe on Halloween Night

From: Chris Peterson <clp_at_meteoritecentral.com>
Date: Wed, 04 Nov 2015 08:02:05 -0700
Message-ID: <563A1DED.3020203_at_alumni.caltech.edu>

The whole issue of meteor color is complex. We now have many examples of
high resolution meteor spectra... but "color" is a physiological
phenomenon that isn't always easy to relate to physical spectra.

The light of meteors consists mostly of thermally broadened atomic
emission lines- lots of them- from both the meteoritic material and the
atmosphere. As has been noted, the atmospheric contributions tend to
dominate. But there are often strong lines from meteoritic material, as
well. In the case of large fireballs (where we have much less
spectroscopic data) there may be a blackbody contribution as well,
either from the ablating surface or from a supercompressed plasma. And
since this is mostly driven by thermal effects, the speed of the body
makes a big difference in perceived color. Throw into all of this the
complexities of human vision- differences in retinal response,
persistence effects, psychological effects given typically short
observation times- and it's little wonder this entire area remains
poorly understood.

After large fireballs, when I get many witness reports submitted, I
review color. It's common for about half the witnesses who report color
to agree on one in particular (green is by far the most common), while
the other half see red, orange, yellow, or blue.

My takeaway is that we should generally assume that most color is coming
from atmospheric contributions, probably modified slightly by meteoritic
components (often too subtly for people to report accurately), and that
above all, it's almost impossible to make any assumptions about
meteoroid composition from color.

Chris

*******************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On 11/4/2015 5:21 AM, Beatty, Kelly via Meteorite-list wrote:
> Marco, Rob...
>
> this discussion is timely. what you've noted is exactly my understanding. just yesterday I came across a high-profile blog about these fireballs, and the writer stated that most of the light comes from the superheated vaporized particle as it ablates. suspecting this was wrong, I looked in several places for the correct information -- IMO, AMS, RASC Handbook, etc -- and yet I didn't really find the physics spelled out explicitly. (maybe I was looking in the wrong places?) the closest I came was this post by Peter Jenniskens (http://leonid.arc.nasa.gov/meteor.html), which was equivocal.
>
> clear skies,
> Kelly
>
> *******************************
> J. Kelly Beatty
> Senior Editor, Sky & Telescope
> SkyandTelescope.com
> (a division of F+W, a Content + eCommerce Company)
>
> 617-864-7360 x22168
> _at_NightSkyGuy
>
>
> -----Original Message-----
> From: Meteorite-list [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of Rob Matson via Meteorite-list
> Sent: Wednesday, November 04, 2015 3:54 AM
> To: 'meteorite-list'
> Subject: Re: [meteorite-list] Very Bright Fireball Over Europe on Halloween Night
>
> HI All,
>
> Marco took the words out of my mouth. Getting tired of hearing that a green meteor tells you anything about its composition. I know that it's natural for people to think the most important thing they can report about a meteor is its color, but I wish various broadcast media would do the public a service and disabuse them of this notion. It would be far better if witnesses could be trained to get in the habit of counting the duration accurately, and noting the exact time of the meteor to the nearest minute. Seeing as how almost everyone has a cell phone these days, and all cell phones have accurate clocks, there really is no excuse to get the time wrong. Yet even a casual browse of the AMS fireball site reveals that people clearly don't think getting the time right is important. And even more obvious is that most people have no business reporting anything about fireball starting and ending bearings and elevation angles. --Rob
>
> -----Original Message-----
> From: Meteorite-list [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of Marco Langbroek via Meteorite-list
> Sent: Wednesday, November 04, 2015 12:06 AM
> To: meteorite-list at meteoritecentral.com; baalke at zagami.jpl.nasa.gov
> Subject: Re: [meteorite-list] Very Bright Fireball Over Europe on Halloween Night
>
>> A lot of folks say it looked green to them, which means it may have
>> been metallic;
>
>
> It is a perpetuated misunderstanding that meteor colours are primarily due to their composition. It's a science myth inspired by High School Bunsen burner experiments that appears hard to kill.
>
> While composition in some cases does have some influence on the colour, it is actually the composition of the atmosphere that is usually dominant for our perception of meteor colours.
>
> That certainly is true for green colours. Meteor spectra show that meteors usually are very strong at the "forbidden" Oxygen line at 5577 Angstrom (557.7 nm). This line is due to atmospheric Oxygen, the same atmospheric Oxygen exitation line also responsible for the green colours of Aurora.
>
> So green meteor colours are likely atmospheric in origin and say little about the meteoroids' composition.
>
> - Marco
>
> -----
> Dr Marco (asteroid 183294) Langbroek
> Dutch Meteor Society (DMS)
>
> e-mail: dms at marcolangbroek.nl
> http://www.marcolangbroek.nl
>
> ______________________________________________
>
> Visit our Facebook page https://www.facebook.com/meteoritecentral and the Archives at http://www.meteorite-list-archives.com
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> https://pairlist3.pair.net/mailman/listinfo/meteorite-list
> ______________________________________________
>
> Visit our Facebook page https://www.facebook.com/meteoritecentral and the Archives at http://www.meteorite-list-archives.com
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> https://pairlist3.pair.net/mailman/listinfo/meteorite-list
>
Received on Wed 04 Nov 2015 10:02:05 AM PST