[meteorite-list] What makes a meteor glow?

From: Phil Whitmer <prairiecactus_at_meteoritecentral.com>
Date: Mon, 28 Jun 2010 21:32:38 -0400
Message-ID: <D21CA2935C8748BF9793245EBFD856BA_at_whitmerjbqtim1>

Sterling:

Thanks for the clear concise explanation. I've often wondered about the
exact process that makes meteors glow. I've read bits and pieces but
couldn't quite put it all together. I didn't realize that combustion was the
primary reason. (Duh!)

Phil Whitmer




--------------------------------------------------------------------------------
 Bob, Phil, List,

The entire process is degrading kinetic and potential energy
into --- at the end --- heat, through which all that energy is
dissipated. "Ablative loss" includes energy released as "light"
of various frequencies, but the principle reason for so much
mass to be lost (90%+) is good, old-fashioned combustion.

The atmosphere functions like a gigantic blowtorch being fed
with oxygen blowing at incredible velocity on a very hot object.
(There's no difference between the case where the object moves
and the air stands still and where the air moves and the object
stands still; it's all relative.)

If you could construct an oxygen-nitrogen jet with 30-40 Mach
speed and direct it at a stationary object, the target would heat
up and burn exactly like a meteroid/meteorite does. What's left
over? Well, the combustion products: tiny smoke-like particles
of oxides of all the elements heated hot enough to oxidize
(and tiny smoke-like particles of all the elements that won't
oxidize). Even some of the nitrogen in the air forms compounds
if the thing is hot enough.

And naturally all that energy release heats the air surrounding
the event but the principal source of light in the "streak" behind
a meteor is produced by "burning" or oxidizing the material. In
so energetic an event, every pathway for energy to degrade is
utilized: heating, oxidation, ionization, re-radiation, re-combination,
and so forth.

The event changes as it proceeds. In very thin air at very high
speeds (like an annual shower meteor), the quantity of oxygen
is limited and radiative effects predominate. It is harder to
dissipate energy those ways, so a tiny cometary particle produces
a streak that's brighter, thicker and more persistent than
its small combustion trail.


At lower altitudes and lesser speeds, combustion is the dominant
way of releasing and dissipating the energy. Meteoroids/meteorites
that leave a thick, pronounced "trail" (like Carancas did) are
combustion-dominated events. The deeper the meteoroid bores
into the atmosphere, the greater the supply of oxygen. (In the
case of Carancas, the thick opaque "smoke" was said to have
followed the object all the way to the ground!)

Is anything left? Well, when you cut the size of a particle down
to half, you lose 7/8ths of the mass, but only 3/4rs of the frontal
area, so the mass loading against the atmosphere is halved. There's
less friction and less energy released. Keep on halving the size,
and you end up with tiny micron-sized particles that are quickly
cooled and slowed to a standstill, as meteoritic dust particles --
extra-terrestrial "smoke."

Your question made me think: what if the Earth's atmosphere
had NO oxygen? I suppose if you had an atmosphere of say,
argon, you would still have a luminous streak produced entirely
by radiative processes. It would last longer, like a high altitude
cometary particle, all the way down and possibly more of the
meteoroid would survive. (So, if you're ever on a planet with an
argon atmosphere, keep an eye out.)

On the other hand, if a planet had an atmosphere with fluorine
in it (not likely, but possible), there'd just be a sudden brilliant
flash and -- Poof! No more meteor. Same goes to a lesser degree
for atmospheres with chlorine or even bromine (all unlikely).


Sterling K. Webb
Received on Mon 28 Jun 2010 09:32:38 PM PDT