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Preliminary text (published in Nature380, 23-24, 1996)
A most unusual rock (called ALH84001), picked up from the Antarctic ice over a decade ago, may tell us something profound about Mars...and about the first billion years of solar system history. At first, ALH84001 was thought to be a normal meteorite. But a few years ago, studies (1) revealed textural features and oxygen isotopes proving that it is one of a clan of a dozen meteorites (called SNC's) believed to be from Mars (2). Now, on page xxx of this issue, R. D. Ash, S. F. Knott, and G. Turner (University of Manchester) report that ALH84001 has an age (measured by the Ar-Ar technique) that is within 100 million years of 4.0 Ga, far older than the other Martian meteorites, all of which are younger than 1.3 Ga. It is only one rock, but ALH84001 may be the tip of an iceberg.
At issue is the nature of a profound epoch in planetary history termed the Late Heavy Bombardment (LHB), first identified by analyses of the ages of Moon rocks (3). Evidently, the Moon was intensely bombarded 3.8 - 4.0 Ga ago, long after the Moon formed, differentiated, and cooled (all before 4.45 Gy). The large multi-ringed impact basins on the Moon were formed during that interval. Subsequent volcanic flooding of those basins created the flat, dark ("man in the Moon") maria, whose very flatness testifies to a sharp drop in the cratering rate, ending the LHB.
Most lunar highland craters were also formed during the cataclysm. The same projectiles should also have struck the nearby Earth; indeed, gravitational scattering of these bodies would ensure that they struck Mercury, Venus, and Mars as well. Thus the heavily cratered terrains on Mercury and Mars have been presumed to date from the same LHB, a little less than 4 Ga ago. (A special Mercury-specific population of so-called vulcanoids has been hypothesized, though not yet discovered, which could have formed that planet's cratered terrains more recently.) The LHB must have had profound consequences for the Earth, especially for the ocean and atmosphere. Indeed, until the LHB ended, our planet would have been unsuitable for the origin and survival of life (4).
The nature of the LHB, and its pervasiveness throughout the solar system, have been disputed since it was proposed. One reason is due to the absence of enough relevant planetary samples to date by analysis of the decay of radioactive isotopes. We also have incomplete knowledge of the present population of comets and asteroids, and even less knowledge about what shorter-lived populations might have existed in the period following planetary accretion.
A chance break-up of a large comet or asteroid, perhaps by planetary tidal disruption during a close encounter (as happened to Comet Shoemaker-Levy 9 in 1992), could have caused the LHB. The size distribution of LHB craters (dominated by big ones), which differs from that produced by present-day asteroids and comets, indicates such an unusual origin for the projectiles. But several researchers disagree. One is W. K. Hartmann, who even regards the cataclysm itself as mostly an illusion (5). He believes there was a fairly gradual decline in the bombardment rate as the remnant planetesimals, which failed to accrete immediately into planets, were swept up. He believes that we can't "see" earlier rock ages or craters because the saturation bombardment masks earlier epochs.
The opposite extreme is represented by Ryder (6), who believes the LHB was so brief (less than 100 Ma) that it must have been caused by projectiles in geocentric orbit, perhaps created by the collisional break-up of additional moons. In that case the LHB was unique to the Earth-Moon system, and the supposition that Mercurian and Martian craters were formed 4 Gy ago (on which those planets' geological chronologies are based) is spurious.
Thus it is very tantalizing, indeed, that ALH84001 should be dated at the same epoch as the LHB. From a single age, we can hardly deduce a duration for the Martian LHB. Nor can Mars settle the dispute about whether rock age distributions are biased by saturation. However, a Martian LHB could hardly be caused by Ryder's hypothetical geocentric projectiles. Ar-Ar ages for meteorites (7) (especially for the basaltic meteorites that are probably derived from the asteroid Vesta, but also for ordinary chondrites) indicate an LHB also occurred in the asteroid belt during the period 3.5 - 4.1 Ga. The data argue against Ryder's short duration for the LHB and probably also against Hartmann's saturation masking. So an LHB cataclysm dominated at least the inner solar system out to the asteroid belt. Moreover the unique signature of the LHB in meteorites implies that it had a very different cause from the usual inter-asteroid collisional break-ups.
Resetting of the Ar-Ar radiometric clock is facilitated by large, high velocity impacts on bodies greater than several hundred km diameter (7). Evidently, there was a real, dramatic increase in the flux of such large projectiles long after the planets had formed. So the LHB was pervasive and it may be unrelated to processes of planetary accretion. If a giant comet broke up at 4 Ga, maybe another comet could create another LHB in the future.
However, ALH84001 has not yet told its full story. Recently, A.J.T. Jull, S. Cloudt, and C.J. Eastoe (Univ. of Arizona) described measurements of carbon isotopes in several SNC meteorites (8). One interpretation of their unexpected data is that ALH84001 may not be from the same parent body (i.e. not from Mars!) as other SNC's. Until samples are eventually returned by a mission to Mars, we will have to hope that the Antarctic meteorite expeditions will find more SNC meteorites to further elucidate the Red Planet's early history.
CLARK R. CHAPMAN has recently joined the Southwest Research Institute, 1050 Walnut Street, Boulder CO 80302 USA.
1. Mittlefehldt, D.A. Meteoritics 29, 214-217 (1994).
2. McSween, H.Y. Meteoritics 29, 757-779 (1994).
3. Tera, F., Papanastassiou, D. & Wasserburg, G.J. Earth Planet. Sci. Lett. 22, 1-21 (1974).
4. Maher, K.K. & Stevenson, D.J. Nature 331, 612-614 (1988).
5. Hartmann, W.K. Icarus 24, 181-187 (1975).
6. Ryder, G. Eos (March 6, 1990), 313 + 322-323 (1990).
7. Bogard, D.D. Meteoritics 30, 244-268 (1995).
8. Presented at the Workshop on Evolution of Martian Volatiles, Lunar & Planetary Institute, Houston, 12-14 Feb. 1996.
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