ARE M-TYPE ASTEROIDS METAL CORES? EYIDENCE FROM LIGHTCURVE DATA. A.W. Harris, Jet Propulsion Laboratory, Pasadena, CA 91103.
It has been suggested (1,2) that M-type asteroids are metal rich, to the exteniz that they may be of greater material strength and/or higher mean density than other taxonomic classes of asteroids. In particular, it has been suggested (3) that members of this class of asteroids may be the iron cores of asteroidal parent bodies. Harris (4) has refined the collisional theory of asteroid rotation to the point of offering specific predictions in regard to rotational properties of different classes of objects: the mean rotation rate should be proportional to the square root of density and the mean rotation rate should increase with decreasing size where material strength becomes important - at a size ofs50 km for pure iron. Earlier compilations of rotational data (5,6) suggested more rapid rotation among M objects, but contained too few M objects. During 1978, 5 additional M objects (87, 135, 224, 441, and 516) were observed at Table Mountain observa- tory. The rotation rates and amplitudes of variation are reported here (Table I) along with a compilation of earlier data. Taxonomic identification
TABLE 1
Asteroid Diameter, km Period, hrs. Amplitude, mag. 16 Psyche 252 4.303 0.34 21 Lutetia 112 6.133 0.15 22 Kalliope 178 4.147 0-14 87 Sylvia 130 5.186 0.42 97 Klotho 94 16 0.07 110 Lydia 90 10.927 0.18 129 Antigone 115 4.957 0.30 135 Hertha 79 16 0.1 224 Oceana 59 18.91 0.1 441 Bathilda 61 10.25 0.13 516 Amherstia 64 6.7 0.18
as M class and diameters are from the TRIAD file (7,8). The logarithmic mean rotation frequency,
From Harris' theory (4), the mean density is proportioned to < f >2. The values of < f >c and
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A. W. Harris
This value, while somewhat greater than the mean density of S asteroids, is only marginally so and in any event is much less than would be expected for pure or nearly pure metallic bodies.
A log-log plot of rotation frequency vs diameter is presented in Figure 1. The dashed line is the linear least squares fit of log(£) vs log@). One expects, theoretically, a slope of zero if gravitational binding is the dominant force or a slope of -1 if material strength domi- nates (1,4). The observed slope for all asteroids is near zero (5,6). The only explanation which I can offer for the apparent slope of very nearly +1 for M asteroids is random noise due to the small sample size. (The formal significance of the slope is 1.30.) There appears to be no theoretical justification for this result.
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A. W. Harris
The distribution of rotation frequencies about the mean is presented in figure 2. The distribution is strikingly not Maxwellian, as is expected and observed for other classes of asteroids (5). I suggest that this may be due to two distinct groups of M class asteroids which are not distinguishable from one-another by the present taxonomic classification system (7): one of high mean density - the true iron cores - and one of low mean density. A strong word of caution is in order, however: the available statistics are insufficient to regard this as a conclusion, but only as a hypothesis to be tested by further observations.
REFERENCES Chapman, C. R. (1976) Geochimica et Cosmochimica Acta 40, 709-719. Chapman, C. R., Williams, J. G., and Hartmann, W. K. (1978) Ann. Rev. Astron. Astrophys. 16, 33-75. Gradie, J., and Zellner, B. (1977) Science 197, 254-255. Harris, A. W. (1979) Icarus, in press. Harris, A. W., and Burns, J. A. (1979) Icarus, in press. Tedesco, E. F., and Zappalii, V. (1979) Subm. to Icarus. Bowell, E., Chapman, C. R., Gradie, J. C., Morrison, D., and Zellner, B. (1978) Icarus 35, 313-333. The classification of 87 Sylvia was changed t0.M after publication of the TRIAD file in ref. 7 (Bowell, private communication, 1978).
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