[meteorite-list] Where does the Color in a Fireball come from?

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:29 2004
Message-ID: <20040227175354.30944.qmail_at_web60302.mail.yahoo.com>

All too often, when a fireball is being reported, the
Press feels compelled to print an "explanation" about
what produces the colors seen in a bright meteor.
Since it is hard to find a definitive explanation for
this phenomenon, the answer to the following "FAQ" is
attached below.

The general consensus is this:

A. About 95% of the light from a fireball comes from
the surrounding atmospheric gases (ionized), while the
remainder comes from the ablated material.

B. Most bright meteors produce a wide blend of
emissions, giving the meteor an overall white color.

Bob V.
References -
http://www.amsmeteors.org/fireball/faqf.html

------------- Attached Text -------------

Frequently Asked Questions (FAQ)
About Fireballs and Meteorite Dropping Fireballs -
Question List:
 
5. Can fireballs appear in different colors?

Vivid colors are more often reported by fireball
observers because the brightness is great enough to
fall well within the range of human color vision.
These must be treated with some caution, however,
because of well-known effects associated with the
persistence of vision. Reported colors range across
the spectrum, from red to bright blue, and (rarely)
violet. The dominant composition of a meteoroid can
play an important part in the observed colors of a
fireball, with certain elements displaying signature
colors when vaporized. For example, sodium produces a
bright yellow color, nickel shows as green, and
magnesium as blue-white. The velocity of the meteor
also plays an important role, since a higher level of
kinetic energy will intensify certain colors compared
to others. Among fainter objects, it seems to be
reported that slow meteors are red or orange, while
fast meteors frequently have a blue color, but for
fireballs the situation seems more complex than that,
but perhaps only because of the curiousities of color
vision as mentioned above.

The difficulties of specifying meteor color arise
because meteor light is dominated by an emission,
rather than a continuous, spectrum. The majority of
light from a fireball radiates from a compact cloud of
material immediately surrounding the meteoroid or
closely trailing it. 95% of this cloud consists of
atoms from the surrounding atmosphere; the balance
consists of atoms of vaporized elements from the
meteoroid itself. These excited particles will emit
light at wavelengths characteristic for each element.
The most common emission lines observed in the visual
portion of the spectrum from ablated material in the
fireball head originate from iron (Fe), magnesium
(Mg), and sodium (Na). Silicon (Si) may be
under-represented due to incomplete dissociation of
SiO2 molecules. Manganese (Mn), Chromium (Cr), Copper
(Cu) have been observed in fireball spectra, along
with rarer elements. The refractory elements Aluminum
(Al), Calcium (Ca), and Titanium (Ti) tend to be
incompletely vaporized and thus also under-represented
in fireball spectra.


------------- Attached Message -------------

From: richardson_at_digitalexp.com (Jim Richardson)
To: meteorobs_at_latrade.com
Subject: Re: (meteorobs) Fireball mags...

Date: Sat, 06 Sep 1997 17:57:40 -0500

-----------------------------------------------------

I realize that I have been eating up more bandwidth
than usual this week,
but I thought I might post an answer to the several
meteor
light/magnitude/train questions that have been
cropping up. Most of this
comes out of the FAQ's posted on the AMS web site,
which include reference
lists. Part of my motivation for answering is so that
if you have a better
answer (and a reference), please let me know.

>From the AMS Meteors FAQ:


4. Where does a meteor's light and color come from?
What is a meteor train?

The majority of light from a meteor radiates from a
compact cloud of
gaseous atoms and molecules immediately surrounding
the meteoroid or
closely trailing it. This cloud consists of a mixture
of atoms and
molecules ablated from the meteoroid itself as well as
from the surrounding
air. These excited particles will emit light at
wavelengths characteristic
for each element/compound. The most common emission
lines from meteors
originate from iron (Fe), oxygen (O), magnesium (Mg),
sodium (Na), nitrogen
(N), and calcium (Ca). Less frequently seen are the
emission lines of
hydrogen (H), Silicon (Si), Manganese (Mn), and
Chromium (Cr).

While most meteors produce a wide blend of these
emissions, giving the
meteor an overall white color, specifically colored
meteors are often
reported by meteor observers. Usually, such colors are
rather weak in
appearance; however, vivid colors are occasionally
reported, especially
with fireballs. Reported colors range across the
spectrum, from reds,
yellows, greens, and blues, to gold, orange, and
(infrequently violet. The
velocity of the meteor also plays an important role,
since a higher level
of kinetic energy will excite the atoms/molecules to a
higher degree. Slow
meteors are often reported as red or orange, while
fast meteors frequently
have a blue color. Due to the nearly identical
composition and velocity of
meteors belonging to a particular shower, several
showers are known for
their characteristically colored meteors.

Often, a brief glow will remain after the passage of
the meteor. If this
glow persists for less than 0.5 seconds, it is called
a wake. This residual
glow is caused by the same atoms which produced the
original light from the
meteor, only at lower excitation energies.

If the glow from the meteor trail persists for a
longer period, this is
called a meteor train. Trains are most often seen from
fast, bright
meteors, in the altitude band from about 100 to 120 km
(62 - 75 miles).
This type of train usually lasts about 1-2 seconds,
and is primarily
generated by the green emissions of the neutral
nitrogen atom. On very rare
occasions, a train may persist for several minutes,
and will be observed to
change shape as the trail is blown by upper atmosphere
winds. Such
persistent meteor trains provided scientists with
their first data on winds
in this region.


Personal notes:

Most of the light-emitting cloud is made up of
atmospheric gases (something
like 95 %), while the remainder is ablated material.

As the atoms are ionized and excited, they also
generate a cloud of free
electrons which remain in the wake of the meteor as a
long, thin,
cylindrical-paraboloid trail. It is this trail of
free electrons which is
capable of reflecting radio waves. Immediately upon
trail formation, the
negative electrons repel each other, causing the trail
radius to increase
over time. This diffusion process causes most trails
to rapidly dissipate,
keeping radio reflections relatively short in
duration.

Note also that the long duration trails (those lasting
several minutes) are
still not very well understood. Some aurora-like
mechanism is still needed
in order to maintain the excitation of the oxygen and
other atoms for this
extended period of time.

Steve's question also reminded me of smoke trails.

>From the AMS Fireball FAQ:


The second type of trail is called a smoke trail, and
is more often seen in
daylight fireballs than at night. Generally occurring
below 80 km of
altitude, smoke trails are a non-luminous trail of
particulate stripped
away during the ablation process. These appear similar
to contrails left
behind by aircraft, and can have either a light or
dark appearance.

Personal Notes:

The question of how to relate meteor magnitudes to
meteoroid size and mass
is a very sticky one, and it seems that every paper or
textbook has its own
approach to this fundamental problem. Estimates for
the 0 magnitude meteor
range from 0.1 grams to 10 grams depending upon your
source, with most
accepting 1 gram as about ball park.

Fireball experts use several measures to derive the
initial mass of the
meteoroid, such as trajectory, light curve, spectrum,
initial velocity, and
deceleration characteristics. Non-meteorite dropping
fireballs probably
range from about 100 g in mass up to a few kg, with
densities ranging from
0.3 g/ml for the lightest cometary meteoroids to 3.7
g/ml for the typical
stony asteroidal meteoroids. Thus, a 100 g meteoroid
could have a
spherical diameter ranging from 3.7 cm up to 8.6 cm,
encompasing a range of
ablation characteristics.

Take care, everyone,

     Jim


James Richardson
Graceville, Florida
richardson_at_digitalexp.com

Operations Manager / Radiometeor Project Coordinator
American Meteor Society (AMS)
http://www.serve.com/meteors/

--------- Original Message ---------

Re: [meteorite-list] Meteor Seen In Alaskan Sky
Ron Baalke baalke_at_zagami.jpl.nasa.gov
Fri, 27 Feb 2004 08:27:49 -0800 (PST)

---------------------------------------------------

http://www.adn.com/alaska/story/4788567p-4731415c.html

Meteor seen in Mat-Su sky

BLUISH FIREBALL: Witnesses say burn lasted six or
seven seconds.

By DOUG O'HARRA
Anchorage Daily News
February 27, 2004

++++++

Thousands of meteors burn up over Earth each day, most
above
oceans or uninhabited areas or during daylight when
they
can't be seen, according to a Web site maintained by
the
American Meteor Society.

The color comes from the meteor's composition and
velocity
as it heats up and disintegrates. Magnesium generates
blue-white
light, and so do meteors that are streaking especially
fast.
The superheating of atmospheric gases around the
object can
also influence the color.


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Received on Fri 27 Feb 2004 12:53:54 PM PST