Aerodynamic heating of the Grant meteorite

Qeochimica et Cosmochimica Acta, 1960, Vol.18, pp. 157 to 161. PerpmonPress

Ltd.

Printedin NorthernIreland

Aerodyrmmict heating of the Chant meteorite* R. E. MARINGER and G. K. MANNING Battelle Memorial Institute, Columbus, Ohio

Ab&&--The

“heat-affected zone” near the surface of the Grant meteorite hss been examined by

metallographio means. This zone, produced by mrodynamic heating during the fall of the meteorite, serves az a record of the thermal gradient which produced it. By comparison of structure and hardness within this zone with arti&ally heat-treated meteorite specimens, this thermal gradient was reconstructed. dictations baaed on the shape of the thermal gradient have permitted estimates to be made of the rate of heat transfer into the meteorite and of the associated rates of ablation.

METEORITES enter the atmosphere of the earth at very high velocities, probably of the order of 11 kmfseo or more. At such velocities, meteorites are subject to severe aerodynamic heating. The surface is rapidly heated to the melting point, and the molten metal is swept away by the air flow around the meteorite. Between the molten surface and the cold core of the meteorite, however, there is a thermal gradient. This thermal gradient causes certain structural changes in the metal near the surface of an iron meteorite, producing what is called a “zone of alteration” or a “heat-affected zone”. Such a zone is, in effect, a record of the thermal environment through which the meteorite has passed. Through the use of appropriate metallographic techniques, it is possible to reconstruct the thermal gradient as it existed in the final stages of flight, and to estimate from the gradient the rates of heat flow and ablation. EXCREMENTAL

WORK

The meteorite used in this study is known as the Grant. It was supplied to this laboratory through the courtesy of Mr. E. P. HENDERSON of the U.S. National museum in Washington B.C. The Grant is an iron containing 9.35 per cent Ni (HENDERSON, 1958), and is classed as a fine octahedrite. It was roughly cone shaped, measuring about 22 in. across the base, and about 20 in. from base to apex. Its original weight was about 480 kg. The slice studied was cut through the approximate axis of the cone. On polishing and etching, the slice revealed a visible heat-affected zone (but only in the kamacite) over most of its perimeter. An example of this zone is shown in Fig. 1. The width of the zone varied irregularly from zero to 0.38 cm, averaging about O-18 cm. The lack of a regular variation in zone thickness suggests that the Grant may have been tumbling as it fell through the atmosphere. Knoop hardness measurements were made through the kamaoite of the visible heat-affected zone and in toward the core of the meteorite. In general, the hardness was observed to decrease with distance in from the surface, reaching a minimum near the edge of the visible heat-affected zone, and then to increase * This research was supported in whole or in part by the United States Air Force under Contract No. AF 33(6~6)-6080, monitored by the Aeronautical Research Laboratory, Wright Air Development Center.

1

157

again to a maximum. Examples of such hardness gradients are shown in Fig. 2. On t,he basis of these two observations (structural changes ;~l hardru~ss gradients) it is possible to reconsixuct the tZhermal gradient which xvas reqtlired to produce these changes. This was done by the col~l~arisol~ of the structure and hardness of artificially heat-treat,ed specimens with the stSructuro and hardness encountered near the surface of the meteorite. Artificial heat treatment was carried out on small (0.3 x (1.5 .,.; 1 cm) specimens cut from positions in the Grant slice well below the meteorite surface where aerodynamic heat had not affected t!he structure or hardness. ~~~ecimens were immersed in molten salt, at controlled ten~peratures, agitated vigorously, then removed from the salt and air cooled. Total time in the salt was held constant at 20 sec. The time required for the specimen t,o reach the temperature of the salt was from 5 to 10 sec. Within the precision of the results, no effects of small time variations couId be distinguished. After cooling, the specimens were mounted, ground, polished and etched to show their metallographic structure. Visual estimates were made of the degree of From these it was found that the temperature visible change of structure. associated with the edge of the visible heat-affected zone was about 750 & 2O”O. Following this, hardness measurements were made. Because of the scatter in hardness data, as is evident from Fig. 2, no less than 20 Knoop impressions, from at least four separate kamaoite bands, were averaged for hardness comparison. The effetits of artificial heat treatment on hardness are shown in Fig. 3. By direct comparison of the data in Fig. 3 with hardness gradients such as shown in Fig. 2, the thermal gradients can be reconstructed. ~sTI~I~~T~~~ OF HEAT

FLOW AND ABLATIW

It is postulated that during passage through the earth’s atmosphere the front surface of the meteorite is quickly raised to its melting point and that the molten material is immediately swept away. If this is the case, then it is possible to estimate the rate of heat transfer from the thermal gradient that is developed just below the surface. Such a relationship has been developed by II. G. LANDAU (19~0-19~1). He showed that, when a. steady state of melting had been estabiished at the surface, the temperature at some distance beneath the surface as measured from the interface between the molten and solid metal could be given by the equation :

where T is the temperature (“K) ; T, is the temperature of meteorite before it entered the atmosphere; T, is the melting point of the metal g1800”K; Q is the rate of heat transfer across the interface (cal cm-2 see-l); e is the specific heat (cal g-l “C-l); g is the thermal conductivity (eal see-l cm-2 cm “C-l); x is the distance from the molten-solid interface (cm); L is the latent heat of melting (cal g-r),

Fig. 1. Visible heat-affected

zone of Grant meteorite.

158

Aerodynamic heating of the Grant meteorite 300 360 340 320 300 200 260 240 220 200 180 Depth below surface,

cm

Fig. Z(S). Hardnese gradient on Grant meteorite, specimen B-I. 380 360 340

/ 320 300 280 260 @ 240 0” L 220

180 0

0.2

04

06

0.8

I.0

Depth below surface,

1.2

16

cm

Fig.Z(b). Hardness gradient on Grant meteorite, specimen A-64.

3

3

0

0

0

0

0

h 0

e

I

500

600

Temperature.

700

I

3

900

1000

ilO

“C

Fig. 3. Effect of arti6cial heating on hardness of Grant meteorite.

159

It can be seen that the term [L $. c(T,, -~- 1’,)] in equation (1) is the hcl:tt required to melt a gramme of met,al whell heating front some startirlg temperature T,. If T, is ta,ken as O”K, this term will be (for cc-iron) about, 3%) Cal/g. E’or T, = 3OO”K, this value decreases to 330 cal/g. Unfortunately, t,he t,rue value of T, is unknown. However. since the calculation is not’ very sensit*ive 2-

30 8 -e-

4-

. 2

i

30-

,

0

Fig. 4. Reconstructed

r--h

3.L

Distance

from surface,

I.0

I.2

cm

thermal gradient for Grant meteorite specimen B-l.

of T,, it will be assumed for the present discussion that T, = 300°K. The thermal conductivity of a specimen of the Grant meteorite was measured and found to be roughly independent of the temperature between 90 and 835”C, averaging about 0.08 cal see-l cm-2 cm0 C-l. Using these data, equation (1) reduces to: In (T -

300) = 7.3 -

0.038 Qcx

(2)

Where the specific heat c is a constant, a plot of In (T - 300) vs. x should be a straight line whose slope is determined by the rate of heat flow Q. Such a plot has been made (Fig. 4) by comparison of hardness gradients with the hardness of the artificially heat treated specimens as shown in Fig. 3. (It should be noted that the abscissa values in Fig. 3 are not generally equal to x, since the present surface of the meteorite does not necessarily coincide with 2 = 0.) It will be noted that, rather than being straight, the gradient is slightly concave upwards. This is what would be expected if the specific heat increases as the temperature increases. Experimentally, such an increase is observed in almost all iron-base alloys,but data which refer to a specific alloy with the composition and structure of the 160

Aerodynamic heating of the Grant meteorite

kamacite in the Grant meteorite are not available. However, moat iron-base alloys have heat capacities which vary from O-11 to O-14 at 4OO’C to anywhere from 0.12 to 0.35 at 750°C. Assuming c to vary from 0.11 cal g-i ‘K-1 at 400°C to O-35 calg-l “K-l at 750°C will account for the curvature, and leads to a value of 220 cal om-2 se@ for the rate of heat flow. It requires 330 cal to melt a gramme of iron when starting at T, = 300”K, or about 2000 cal to melt 1 om3. For a steady state of heat flow, then, the surface would be melting away at a rate of 22012600 cm/see or about 0*85 mm/see. Thermal gradients from other surface areas on the meteorite vary, suggesting melting rates greater or less than this value, It is estimated that the average rate of ablation is of the order of l-2 mmjsec. This is in all probability a minimum estimate, since the metallographic record of higher rates of heat flow would be destroyed by subsequent lower rates, If the Grant spent 20 or 30 see in passing through the atmosphere, the estimated total amount of ablation would be of the order of from 2 to 6 cm. HOFF~A~~ and NIER (1958) have tentatively estimated that the Grant lost about 15 cm on the basis of some preliminary analyses of He3 and He4 contents. HENDERSON and PERRY (1958) have estimated, from the shape and dimensions of holes on a specimen of the Cation Diablo fall, a loss of less than 10 cm. While each of these estimates is very tentative, and based on entirely different arguments, the &greement is most interesting. These estimates suggest losses due to ablation may be considerably less than has been theoretically estimated. CONCLUSIONS (I) The metallurgical structure near the surface of B meteorite has been altered by the effects of heat generation and flow during its passage through the atmosphere. This altered structure provides a permanent record of the thermal environment through which the meteorite has passed. (2) Comparison of hardness and visible structural changes in meteoritic iron after heat treatment to those in the aerodynamically heated zone near the original surface permits the reconstruction of the thermal gradient which at one time existed near the surface. (3) Born the experimentally established thermal gradients, certain physical properties and certain mathematical assumptions, it is possible to estimate rates of heat flow into the meteorite, and the associated rates of ablation. Ac~w~dg~~~~~-~t ia a pleasure for the writers to ack.nowIedgethe continued help end interest of Mr. E. P. HENDERSON of the U.S. N&ional Museum in Wonton. Thanka me also due Mr. I;. R. JACKSON ctndMr. C. R. SIMCOE for many interestingdiscussions,Mr. 31. DEEM for the determirmtionof thermal conductivity, and Mr. A. D. FRIDAY and Mr. P, R. HELD for their careful performance of much of the experimental work.

REFERENCES HENDEMON E. P. (1958) Private co~~c&tion. HENDERSON E. P. and PERRYS. H. (1958) .Proc. U.S. Nut. Mua. 362. HOFFMANN J. H. and Nnm A. 0. C. (1958) Private communication. LANDAU EC.G. (1950-1951)Quart. Appl. Math. 8, 81. 161