[meteorite-list] Green Fireballs and NEOs Part 1

[MEM]

Part 1 The significance of Green Fireballs (double posted but not cross posted)


Long time list members of the meteorobs list are aware of the concentration 
of reported fireballs this past spring and of “green” fireballs-- there has been much 
discussion.  This two part post is to address the possible significance and 
relationship of green fireballs and Near Earth Objects(NEOs) especially 
Earth-crossing bodies. Be it remembered that all meteoroids entering 

our atmosphere are/were in Earth-crossing orbits and represent the 

closest of the close of NEOs!  The meteorite chasers community may 

take interest as the mix of successive colors of a meteor my provide clues as to the 
meteorite dropping potential of a particular event.  There is a 
documented higher flux of both fireballs and meteorite falls in early 
spring( citation later)

The purpose of meteor observation traditionally has been to 

document meteor flux from cometary sources and identify 

new showers to identify the existence of long period comets.
 Thinking wholly within a particular box, the long standing 

focus has been to take snapshots of  meteor flux with not much
statistical evaluation of the bulk data.  I surmise someone, 

somewhere, someday, might eventually correlate this data to 

comets  past and present, to whatever ends that 
research might tell us of recent comet populations and their orbits in the 
inner solar system.

 Virtually no effort is going on in the meteor observer community 

to identify asteroid-ally derived debris streams as the low 

density/frequency of observations places any potential pattern
there may be into the category “sporadic”.  Couple this with 

lock box thinking that “such a thing could not exist” and 

no one may ever undertake such an attempt to make annual 

correlations. More on that in part two.

A minuscule portion of observational effort has been capturing meteor 
spectral emissions.  Without a spectrometer, fireball colors are not 

easy to reliably categorize beyond basic color sequences as only 

the spectrometer and camera can record them with specificity. 

We should all realize how difficult it is to even capture a fireball 

on camera and how even more rare it is to capture the spectral
emissions of a meteor through an even smaller aperture.  

In fact, most  “color discussions”, have favored the 

“subjective perception of the observer" paradigm and not the 

specific atomic sources of the light emissions.  When 
color is noted, it is largely taken as a “whole” rather than collection 
of specific emission lines of light viewed through a spectrometer and 
that is just the way the eye perceives color--by blending multiple 
wavelengths into a single hue.  When we mix enough wavelengths together 
we will always favor the "white fireball" when describing a meteor--unless
there is a specific overriding hue. Well frequently there is such a 
natural bias and frequently it is green.

What vignette examples  we do have of meteor spectra do not 

seem to have been methodically worked into a sound scientific 

theory regarding the significance of meteoritical light. 

Better late than never.

For clarity a quick review of the spectral sources might be in order:
I find at least three "sources" of light in a meteor’s flare and 

two of those overlap.

1)
The spectral emission lines of the constituent elements/molecules of 
the meteoroid proper when they change phase into gas/plasma.

2) 
The excited state of the atmospheric gases/dust which the meteoroid acts
upon-- mainly 5-6 species: N2, O2, O3,NO, N and O but at high velocity 
expands to at least 19 species of atoms/molecules including CO2, argon, 
H2O and so forth.( NOTE: this has implications for a different "typical"
color of a meteor when viewed from the surface of Mars)

3) The 
change in chemical composition as existing molecules are disassociated 

and  new molecules form in a flash( pun intended) owing to recombination 

which may form species such as (CN)2 ,CO, Fe2O, FeC, Mg2O etc. not 
normally seen in the auroral spectra.

The atoms/molecules from a meteoroid emit light because they are 

heated in an induced plasma stream when entering the atmosphere; they 

incandesce as well as chemically oxidize, emitting a more complicated 

assembly of spectral lines; loosely conceptualized such as the way that
different compounds in fireworks provides for different colors. 

The atoms/molecules of the atmosphere are ionized in the super-hot 

bow wave ahead of the meteoroid, causing them to emit photons of 

certain specific wavelengths, depending on what elements are present
--and what compounds reform in the furnace of entry as a mist of 

melted meteoroid enters the slip stream. 

Color saturation/intensity is also a function of density for the various 
atoms/molecules.  Nitrogen will tend to dominate over oxygen which will 
dominate over CO2 owing to bulk percentages in the makeup of the 

atmosphere. We will tend to see colors associated with Fe, Mg, N2 and O2 

with a generic meteor, counter-mixed with atmospheric plasma.  

To the eye, it will seem that there no more than two colors at 

once but usually a single overall hue within a bright overall flash. 

( NOTE: human eye physiology-- cone and rod density, night vision, 

dazzle--  all come to play in perceived color)

Moving on to specific spectral emissions, the common 
emissions for metallic atoms in meteors and for atmospheric atoms can be
seen at. 
<http://leonid.arc.nasa.gov/meteor.html>
Combinations of  two sources of emissions’ produce the colors one sees in the 
fireball. “Colors of meteors: The color of many meteors is caused 
(sic)by light emitted from metal atoms from the meteoroid (blue, green, 
and yellow) and light emitted by atoms and molecules of the air (red). 
The metal atoms emit light much like in our sodium discharge lamps: 
sodium (Na) atoms give an orange-yellow light, iron (Fe) atoms a yellow 
light, magnesium (Mg) a blue-green light, ionized calcium (Ca+) atoms 
may add a violet hue, while molecules of atmospheric nitrogen (N2) and 
oxygen atoms (O) give a red light.( Note: See discussion of 
spectral changes in  atomic vs molecular oxygen and nitrogen with 

decreasing altitude below)  The meteor color depends on whether 

the metal atom emissions or the air plasma emissions dominate“...NASA

This simplistic model so far  described is good for starters but the 

atmosphere is more dense with lower altitude and a “Real 

(non-equilibrium) gas model” is required to explain emission behavior.  

<http://en.wikipedia.org/wiki/Atmospheric_entry> 

We might be able to stop here were it not for the fact that some atoms 
actually “color shift” their emissions with altitude.  A curious paradox
exists for atmospheric oxygen and nitrogen emissions which vary with 
altitude owing perhaps to atomic and molecular densities and the effects
partial pressures might have on average atomic radii.( e.g. O2 vs O and
N2 vs N) Illustration at:
<http://www.flickr.com/photos/11304375@N07/2844511020/>. 

As I annotate: "Nitrogen Oxygen Emissions with changes in altitude: 
Density of nitrogen and oxygen varies by altitude. This affects the 
colors of an aurora" (which is a substitute for meteor spectral 
behavior--not because it is complete but because it is far more deeply 
studied).

"Oxygen atoms above 200 km produces a red hue, while 
below 200km a green hue is produced. Below 100 km not enough atomic oxygen 
exists to have an effect".( Curiously, O2  has about 19 spectral lines 
which are distributed seesaw fashion towards the ends of the visible 
spectrum with 3 green lines at the fulcrum.  Single atoms of oxygen 

have about three peaks under lab conditions).

"Nitrogen produces blue and violet when it decays( e.g. molecular 

bonding broken by going plasma) at the middle altitudes and 

magenta at the lowest altitudes."

The few meteor spectrographs captured so far suggest a 

combination of peaks.  An generic illustration can be seen at 

<http://leonid.arc.nasa.gov/meteor.html>.  

What is not differentiated in this illustration is the fact that the 

total spectral output of a given meteor ( meteoroid plus 

atmosphere) are going to change as velocity and altitude decreases.

When we combine what we understand from auroral and 

meteor spectra, we may infer that the deeper a meteoroid makes it 

into the atmosphere, the color will trend from reddish white, 

then longer at green perhaps capped by orange/red/magenta 

at the end of incandesence. Meteor spectra will 
differ from auroral spectra being doped by the composition of the 
meteoroid itself (i.e. Fe blue and Mg green, Na yellow). Be it also 
remembered that blue and yellow are seen as green.  In fact there are 
several factors which tend to make a fireball appear green much of the 
time below 200 km --more likely than not!

All that said, I believe we may be able to use "color" as a coarse 

indicator of the depth of penetration into the 
atmosphere. To survive a deeper plunge requires a larger mass.  Ergo a 
green/bluish green fireball  “tends to suggest” an asteroidal origin vs 
the sand grain-sized meteor of cometary origin-- all other things being 
equal.  Couple color with sporadics  and these are the fireballs which 
need to be evaluated against similar sightings around the same time
year after year to seek out orbits of asteroidal debris streams.  

As I’ll debate in part two, these presently “random” fireballs may 

ultimately be associated with heliocentric earth-crossing NEOs. 

It should not be a surprise that asteroid debris will be strung out 

along an orbit stream much as a cometary stream only less densely 

and  spread more widely as it is not refreshed by new material each orbit.

Elton