4.5
FIREBALLS AS AN ATMOSPHERIC SOURCE OF METEORITIC DUST
Zdenek Ceplecha
Astronomical Institute
Czechoslovak Academy of Sciences
251 65 Ondrejov Observatory
This note emphasizes the importance of very bright fireballs as a
powerful source of meteoritic dust in the upper atmosphere and on the
Earth's surface. Recently we obtained a direct observational evidence
when we photographed the Sumava Fireball, an object of -22 absolute
stellar magnitude, on Dec. 4, 1974 at 17 h 57.5 m UT. This is the
brightest object on the European Network photographic records during
almost 12 years of systematic fireball observations. We obtained 4 .
all-sky-camera records of the Sumava Fireball from two Czech and one
German station of the Network. The most important combination for the
trajectory computations proved to be station No. 5 at Ondrejov
(Astronomical Institute of the Czechoslovak Academy of Sciences) and
station No. 65 at Bernau (Max-Planck-Institut fiir Kernphysik,
Heidelberg). The results are given in Table 1. At Ondrejov we also
obtained 4 spectral records of the Sumava Fireball; the most detailed
record has a dispersion of 5 A/mm.
The fireball began the luminous trajectory at a height of 99 km
with a velocity of 27 km/s. A very steep increase of brightness took
place starting at 82 km height. The maximum brightness of -22 stellar
absolute magnitude was achieved at 73 km height. The maximum was a
flare of long duration: 14 additional shorter flares were observed:
four of them were extremely intense, occurring at heights of 73> 68,
65 and 61 km. The meteor ended its luminous trajectory at 55 km
height with a small flare and with almost unchanged initial velocity.
The trajectory was inclined by 27° to the horizon of the terminal
point. The 90 km long luminous trajectory was traversed in 3.4 seconds.
Using the luminous efficiency given by Ayers et al. (1970) and by
McCrosky (1973)> the initial mass was computed as 200 metric tons.
This might seem to be an excessively large value, but we note that
the measured energy radiated within the panchromatic bandpass (from
3600 to 6600 8) is lO 1 ^ erg (300 000 kWh). If the entire kinetic
energy of the meteoroid would be radiated within the panchromatic
bandpass, 3 metric tons are still needed to produce the Sumava Fireball. Taking the experimentally measured value of the luminous
efficiency (between 1 and 2 % for this velocity), we see that 200 tons
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386
were necessary to produce the fireball. Owing to an insignificant
change of velocity and a very high termination height the terminal
mass had to be virtually zero. Any significant mass at a height of
55 km cannot move with velocity higher than 20 km/s without ablation
being observed. The entire mass has ablated during the short interval
of 3.4 seconds. The structure of the body was very fragile and it
resembles the weakest known meteoroids of the Draconid shower. A
striking difference between two photographic fireballs with the same
velocity is demonstrated in Table 2, which compares the Sumava Fireball with a fireball photographed within the Prairie Network by
McCrosky (1973). PN 4o66o B. (This is the closest fireball in velocity
and in inclination to the surface, which we could find among all PN
and EN photographic records). The ratio of brightness is almost 5
orders of magnitude, still the terminal point of the fainter PN-fireball is 26 km lower!
Ceplecha and McCrosky (1976) have recently finished a thorough
study of PN-fireballs. Three groups according to the structure of
meteoroids were found: group I has the structural strength and density
of ordinary chondrites; group II belongs to the carbonaceous chondrites;
group III consists of weak material of cometary origin. Group III
contains meteoroids of type IIIA, the "stronger" cometary material,
and of type IIIB, the "Draconid type" cometary material. The number
of fireballs observed in each group is about the same, i.e. about 1/3
of all observed fireballs are of type I, 1/3 of type II and 1/3 of
type III. The Sumava Fireball is of type IIIB and the Prairie Network
fireball 4o66o B is of type I. The §umava Fireball probably belongs
to the meteor shower of Northern Chi Orionids. (Table 1 ) .
The spectral records of the Sumava Fireball do not differ from
spectra of fireballs of about -10 stellar magnitude. The emission line
spectrum contains Fel, Cal, Call, Mgl, Nal, CrI, Mnl, All (preliminary
indentification). There is some hope that the 5 8/mm spectrum might
permit a study of the line profiles, namely for the brightest feature,
the sodium D-doublet. Iron is the most important radiator: more than
2/3 of the observed lines belong to it; multiplet 15 of Fel is the
strongest one, not much fainter than the sodium D-lines.
Let us now study the contribution of such big bodies to the Earth's
environment. The total area-time coverage of the European Network is
about 4 x 10
cm s up to now; (this is approximately equivalent to a
1 day coverage of the entire Earth's surface). If N is the number of
2
bodies per cm per second having 200 metric tons and more, we have
log N « -23.6. If we extrapolate the data given by McCrosky (I968;
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REFERENCES
Ayers, W.G., McCrosky, R.E., Shao, C.-Y.: 1970, Smithson.Astrophys.
Spec.Rep. 317.
Ceplecha, Z., McCrosky, R.E.: 1975* Fireball End Heights: A Diagnostic
for the Structure of Meteoritic Material, in prep, for JGR
Lindblad, B.A.: 1971, Smlthson.Contr.Astrophys., No. 12.
McCrosky, R.E.: 1968, Smithson.Astrophys.Obs.Spec.Rep., No. 280.
McCrosky, R.E.: 1973:, private communication.
Table 1.: Sumava Fireball ( EN 041274 )
date
time UT
max.brightness
initial velocity
height
beginning\ A
height
end
9
inclination to
the surface
length
duration
fireball
initial velocity
COS Zo
beginning height
terminal height
max. brightness
initial mass
terminal velocity
terminal mass
probable structure
type
Dec 4, 1974
17h 57.5m
-22 abs.stel.mag.
27 km/s
99 km
14°38'24" E.Gr.
49°16'38"
55 km
13°3l'20" E.Gr.
49°08'l0"
27"
2 x 108 g
virtually zero
radiant * app. 75"
28c
shower(Lindblad) Northern ^Orionid
2.0 a.u.
orbit
a
0.76
e
0.47 a.u.
q
Q
3.5 a.u.
282°
Si11950.0 251.9°
initial mass
terminal mass
2°
90 km
3.4 s
174°
lfj
spectral records F e l , C a l , C a l l
Mgl, N a l , C r I ,
linl. All
Table 2 .
EN 041274 5umava
27 km/s
0.46
99 km
55 km
-22 abs.mag.
200 000 kg
» 2 6 km/s
SJO
very weak cometary
material
IIIB (Oraconid type)
PN 40660 B
26.5 km/s
0.60
93 km
29 km
-10 abs.mag.
8 kg
6.5 km/s
30 g
compact stone
I (ordinary-chondrite
type)
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