EVIDENCE FOR THE TIMING OF THE EARLY BOMBARDMENT OF MARS.  R. D. Ash, S. F. Knott, and G. Turner, Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK.

Published in Meteoritics, 30, p. 483.

The analysis of the SNC meteorite, ALH 84001, which now appears to be a sample of the ancient crust of Mars [1], provides a new avenue to investigate the period of intense planetary bombardment of the early solar system. The sharp cut-off in lunar highland ages close to 3900 Ga has been widely used as a time marker for heavily cratered objects throughout the solar system. However, it is not yet established whether the heavy bombardment that affected the Moon was a solar-system-wide event or was local to the Earth-Moon system. There is some evidence, from the distribution of Ar-Ar ages of meteorites, of extensive resetting by impacts on their parent bodies, in the interval between 4500 and 3500 Ma, but the distribution is broader and less well established than the lunar case [2]. The full answer will only appear when the detailed chronology of the martian highlands is established. In the meantime we have carried out Ar-Ar analyses of ALH 84001 (Table 1). These show clearly that this meteorite was involved in outgassing events at a time close to the time of the so-called lunar cataclysm.

TABLE 1.  Results of analyses of three samples of ALH 84001.

ALH 840010.1270.1110.110
Weight (mg)
K (ppm)9164101
Ca (wt%)1.460.921.07
Cl (ppm)
Cl/36Ar9.49 × 1051.27 × 1061.28 × 106
40Ar*/K (×10-6)107.1 ± 5.798.4 ± 4.1100.7 ± 0.9
Age (Ga)4110 ± 903940 ± 704010 ± 14

The K contents of all three samples analyzed are extremely low, ~100 ppm, consistent with the origin of ALH 84001 as a mafic cumulate. The major carrier of K is maskelynite and it is the formation and outgassing of this that our analyses presumably date. The release patterns of 40Ar/39Ar as a function of temperature show structure that is interpreted as the result of 39Ar recoil. There is evidence of only minor recent loss of radiogenic 40Ar associated with the low-temperature fractions. The 40Ar/K ratio and inferred age are calculated by summing all the gas released above these low-temperature fractions. The exposure age of ALH 84001 inferred from the ratio of the high-temperature 38Ar and Ca-derived 37Ar is close to 15 Ma. Chlorine abundances are inferred from the low-temperature neutron-induced 38Ar and appear to correlate roughly with the trapped 36Ar, which mainly derives from the terrestrial atmosphere. This suggests that an appreciable fraction of the Cl is terrestrial, although some of indigenous origin is present in the carbonate. One feature that is difficult to understand is the apparent absence in our samples, based on a 40Ar/36Ar vs. 39Ar/36Ar, of any clear evidence of 40Ar from the martian atmosphere. The 40Ar-39Ar age inferred for ALH 84001 is clearly similar to those typical of the lunar highlands. This result provides the earliest evidence from Mars in support of the view that the late bombardment was a widespread event.

References:  [1] Mittlefehldt D. W. (1994) Meteoritics, 29, 214-221. [2] Turner G. (1988) in Meteorites and the Early Solar System, 276-288.