(meteorobs) Green Fireballs and NEOs Part 1

[MEM]

Part 1 The significance of Green Fireballs 


Long time list members on 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 mucg statistical evaluation og 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 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 
the 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 the 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.

Elton
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