ICARUS 28, 537--542 (1976)

Comments on the Paper by C. R. Chapman: Chronology of Terrestrial Planet Evolution--The Evidence from Mercury

GEORGE W. WETHERILL

Department of Terrestrial Magnetism, Carnegie Institution of Washington,

5241 Broad Branch Road, N.W. Washington, D.C. 20015

I~e~eived September 24, 1975; revised December 8, 1975

Despite the negative tone of Chapman's article, it is pointed out that there is substantial agreement between his opinions and those given in my previous publications on this subject. These areas of agreement are identified and distin- guished from those which require further investigation or discussion. I t is also pointed out that the apparent discrepancy between the small crater population on the Mercury intercrater terrain and the lunar plains may argue against a simul- taneous late heavy bombardment. Suggested ways to resolve this discrepancy should be thoroughly evaluated. The primary need at this t ime is to identify further specific observational, experimental, or theoretical tests in order to advance beyond the stage of agreeing that the available evidence permits a simultaneous late heavy bombardment, to the more difficult task of determining whether or not it probably occurred.

Chapman (1976) has discussed the inter- pretation of recent photogeological and dynamical studies of Mercury cratering. He is particularly critical of interpretations presented by Murray et al. (1975) and Wetherill (1975ab). At least insofar as my own work is concerned, I feel that the negative tone of Chapman's discussion may obscure the large areas of agreement between us. I believe it may be useful to those attempting to follow this discussion to distinguish those points for which there appears to be substantial agreement from those which represent unsolved problems in order that attention may be focused on the latter. In addition, there are several matters which seem to be in dispute even though they should be, at least in large part, resolvable with currently available data.

I f I interpret Chapman's remarks cor- rectly we agree on the following points.

1. I t would be most unwise to seek to establish a "ruling hypothesis" for solar system history at this time. Rather, alter- Copyright © 1976 by Academic Press, Inc. All rights of reproduction in any form reserved. Pr/nted in Great Britain

native hypotheses should be clearly stated and both their obvious and more indirect consequences carefully examined. In parti- cular I don't think any of the alternatives discussed in my paper (~¥etherill, 1975b) should be considered as ruled out simply because I discuss difficulties associated with these alternatives.

2. A peak in lunar impact rate m a y have occurred about 3.9b.y. ago as a consequence of natural dynamic processes, rather than necessarily being regarded "as an unaccountable, unexpected or extra- ordinary event." For this reason, although I do not consider the matter very import- ant, I personally prefer to use the term "late heavy bombardment" to describe this episode rather than the more dramatic expression "cataclysm." Even though the impact rate on the earth and moon during this ~100m.y. period was probably ~104 times that at present, a hypothetical terrestrial resident at that time could have easily lived a normal lifetime without witnessing the fall of a single meteorite.

3. I f the impacting bodies of N4b.y. 537

5 3 8 GEORGE W. WETHERILL

ago were derived from distant locations in the solar system in highly eccentric orbits, the bombardment chronology on all the planets would have been similar (ending about 3.9b.y. ago).

4. The cratering history of Mars is much less certain than tha t of Mercury and the Moon.

5. I f an episode of late heavy bom- bardment by bodies in heliocentric orbit did occur on the Moon, it must have necessarily happened on Mars, Mercury, and the other terrestrial planets at the same time.

6. Considerably more photogeological s tudy of Mercury, including studies of crater morphologies, are needed in order to permit progress in understanding its early impact and volcanic history.

In my view, these are significant areas of agreement, and represent most of the conclusions I have wished to make. In addition, several proposals which I have emphasized still appear to me to be legitimately controversial, even though they are not specifically evaluated by Chapman :

1. Residual material from the outer- most solar system played an important role in the bombardment history of the inner solar system.

2. Tidal fragmentation of ~1023g planetesimals following dynamically prob- able close approaches to the terrestrial planets was a significant cause of frag- mentation of these bodies.

Further investigation and discussion of these problems are clearly needed. I hope, however, that in my papers I have made the point that items (1) and (2) above are not far-fetched or ad hoc ideas proposed simply to account for the cratering records. On the contrary, they are processes which should be evaluated in any serious at tempt to understand the early history of the solar system.

The principal thrust of Chapman's discussion appears to be that the dynamical history I have presented is not a unique one, and that alternatives exist which in some cases lead to significantly different impact histories on the various planets. I have explicitly drawn the same conclusion

myself (e.g. Wetherill, 1975c), and have presented alternative models (Wetherill, 1975b). Chapman prefers the alternative that Mars-crossing bodies were responsible for most of the early Martian and lunar cratering. I have pointed out attractive features of this possibility, as well as several difficulties which deter me from persuading others to prefer this alternative. I have also discussed areas of investigation which could possibly, but by no means neces- sarily, remove these difficulties. Although Chapman reiterates this possibility, it seems to me that what is needed to make these alternatives presentable is to inte- grate them into a reasonably consistent and fairly quantitative dynamical picture of the orbital evolution of the early solar system. This may prove possible, and such a contribution would be valuable.

The recent model proposed for the evolu- tion of the size distribution in the asteroid belt of Chapman and Davis (1975), when combined with the Chapman's present discussion, can be considered the beginning of such a contribution. I feel it still to be unsatisfactory in that I have been unable to find a way to obtain a sufficiently rapid decline in lunar flux at 3.9b.y. from a model of this kind. On more general grounds, I think the Chapman and Davis model falls far short of explaining other major features of the asteroids, particu- larly their remarkable velocity distribu- tion. Chapman and Davis propose that the present mass of the asteroid belt resulted from grinding down of a much larger mass of asteroids by mutual collisions. Unless a change in mean relative velocity accom- panied the beginning of this eollisional history, these same collisions would have precluded the accretion of the asteroids in the first place. Gurevich and Lebedinskii (1950) and Safranov (1969) have presented calculations showing that the mean rela- tive velocity of a planetesimal swarm resulting from these mutual perturbations is in the range 0.2 to 1.0 times the escape velocity of the largest body. For the asteroid belt this largest escape velocity will be less than 500m/see and much less than the observed mean relative velocity of ~5 km/see. In our a t tempt to understand

COMMENTS ON PAPER BY CHAPMAN 5 3 9

the evolution of the asteroid belt, an important "piece of the puzzle" seems to be missing. I t appears likely that inter- vention of some mechanism originating external to the asteroid belt is required to produce the velocity distribution. Until the nature of this mechanism is understood, it seems to me tha t the initial mass and mass distribution to be associated with the present velocity distribution is entirely an open question. For this reason, conclu- sions based on a model such as that of Chapman and Davis, which assumes these quantities to be known, are not very com- pelling. I have no particular quarrel with the idea that the original mass in the asteroid region may have been much greater than that at present; in fact, I have speculated along these lines myself (Wetherill, 1972). However, the way in which this initial large mass could have evolved into the present asteroid belt remains very obscure.

There are also several points which I think should be amenable to agreement with present data. Although I may be mistaken that all of these differences can be reconciled, the effort to do so would be worthwhile if it should prove possible to clear future literature of some superfluous discussion.

1. There seems to be some mis- understanding regarding my interpretation of the photogeological data. I agree that the crater densities do not in themselves yield impact rates. Perhaps, at least in a preliminary version of my manuscripts, I contributed to this misunderstanding by using the term "flux" imprecisely when I meant "integral flux" without stating this explicitly, and readers may have thought I meant by that term "impact rate." However, it is certainly true that the Mariner 10 data did suggest to many workers, including myself, tha t the possi- bility of similar impact rates on Mercury and the Moon was a topic worthy of inves- tigation and discussion, even though it was recognized that this conclusion is not speci- fically implied by the observations. I still believe this to be the case. In my earlier discussions I also emphasized tha t careful distinction should be made between the

question of whether or not simultaneous late heavy bombardment was consistent with photogeological and dynamical evi- dence, and the question of whether or not it actually occurred.

2. A 4b.y. late heavy bombardment has no necessary connection to the "onset of (second stage) volcanism". In fact, I believe many or even most of the breceiated highland rocks to be originally of volcanic, or at least magmatic origin, and that some of these had a similar origin to the mare basalts.

3. Acceptance of the rapid decline in lunar integral mass flux between 3.9 ~: 0.1 and 3.8 ± 0.1 b.y. does not depend upon controversial photogeological interpreta- tion. I t is simply inferred from the gross contrast in lithology and radiochrono- metric disequilibrium found between the 3.9 b.y. highland Apollo sites and the oldest (~3.Sb.y.) mare sites.

Almost all the rocks returned from the Apollo premare sites have been highly brecciated and cataclastic rocks including a large number of annealed breccias. Those few rocks which bear a claim to be genuine igneous rocks (e.g. 62295 and 14310) may well have crystallized from impact-related melts, as indicated by their trace-element concentrations (Ganapathy et al., 1975). Detailed isotopic studies of these breccias and cataclastic rocks show that the 3.9 m.y. "age" is frequently but a metamorphic overprint partially obscuring an earlier history. In contrast, impact melts, cata- clastic rocks, and annealed breceias are absent from the mare collections. The degree of Rb-Sr equilibration of the mare basalts is greatly superior to that usually found for terrestrial rocks. Evidence for resetting by impact metamorphism or any other cause is absent. The nearest mare analogue to the highland breccias are the "soil breccias." These are not annealed and represent unmetamorphosed aggrega- tions of mare regolith. Little chronological work has been done on this material, but the two age measurements available on mare basalt clasts from these breccias (Stettler et al., 1973; Mark et al., 1974) indicate no significant resetting of the mare basalt ages. The impact history of

5 4 0 GEORGE W. WETHERILL

these contrasting terrains is clearly differ- ent. How much different? I t is hard to be very quantitative about this. Fortunately this isn't necessary. I f the difference were say only 5-fold, there should be abundant mare basalt samples, i.e., ~40% of them, which resemble the highland impact- related rocks, assuming that 5 to 10% of the highland rocks escaped resetting at 3.9b.y. Perhaps 3 to 5% could resemble highland rocks in their impact history, and this smaller quanti ty has escaped our attention because of incomplete sampling. This would imply an integral flux at 3.9 b.y. 50 to 100 times that of the older maria surfaces.

The oldest post-Imbrium surfaces iden- tified by photogeological studies are plains units such as the Fra Mauro formation and the Cayley plains. There is some variation in the relative age and impact flux history of the regions given by various authors, but their primary characteristic is that they are only slightly more cratered and/or degraded (2 to 4 fold) than the older mare surfaces. This conclusion is reached by all methods of cratering chronology, and is found over a wide range of crater diameters, including the largest size for which adequate statistics are available (~Skm). I t is not likely that this first-order result should be attributed to difficulties (such as volcanic or secondary craters) associated with any particular method or size of crater. The data support- ing this has been reviewed by Neukum et al. (1975) and Chao et al. (1975). In view of the discussion of the previous paragraph, neither the majority of the brecciation nor the associated 3.9b.y. ages found for rocks from these plains can be reasonably attributed to this relatively mild post- Imbrium impact history. This conclusion is independent of the factor of about two difference found by different authors for the integral cratering rates of the plains units, and discussion of these differences is not relevant in this context. Therefore the ~3.9b.y. ages found for most highland rocks are either Imbrium or slightly pre-Imbrium ages, and the total integral mass flux (including Imbrium and Orien- tale) declined by a factor of ~50 or

more between about 3.9 and 3.Sb.y. ago.

As calculated elsewhere (Wetherill, 1975b) this is nearly the same ratio obtained by comparing the energy of the Imbrium and Orientale projectiles with the integral energy flux on the mare surfaces, as inferred from conventional expressions relating diameter to impact energy. The integral flux on the mare surfaces is not significantly affected by the suggested possibility (Chapman, 1970) that many of the smaller (~300m diameter) mare craters may be volcanic. Almost all of the impact energy is related to craters > 1 km in diameter, and elimination of even 100% of the smaller craters from the total would have a negligible effect on the result.

Therefore it can be maintained tha t the 3.9 b.y. ages are the result of Imbrium and Orientale alone. I primarily wish to emphasize that whether or not this is the case is a question of greater difficulty than the one discussed above of whether or not the 3.9 b.y. ages might primarily represent post-Imbrium impact events. Much pre- vious discussion has confused these two questions, and I think it desirable that this confusion be eliminated. Even if it is maintained, implausibly in my opinion, that the 3.9b.y. ages date post-Imbrium and Orientale events, it should be possible for authors to agree that this is a different problem from that of deciding whether these ages result from Imbrium and Orientale alone, or from Imbrium and Orientale together with other smaller, pre-Imbrium impacts. Although the latter distinction is also of considerable signifi- cance, the inferred ~ 50-fold decrease in integral flux between 4.0-3.9b.y. and 3.8-3.7b.y. does not depend upon resolu- tion of this distinction.

The question of whether or not the observed phenomena can be largely attri- buted to Imbrium and Orientale alone is definitely worthy of further discussion and I would not presume to be firm about this. My view is that it probably cannot be. Experience with terrestrial craters indi- cates that only a small part of the ejecta from any particular event records the metamorphic age of tha t event. The

COMMENTS ON P A P E R BY CHAPMAN 541

unmetamorphosed condition of the mare basalts, in spite of small-scale saturation cratering, supports this. Brett (1975) has reached similar conclusions on the basis of geochemical data.

The direct photogeological evidence for a small-body flux accompanying the Im- brium and Orientale projectiles would be difficult to identify, particularly if the Imbrium and Orientale projectiles were relatively late members of the population producing the late heavy bombardment and if the size-frequency distribution of this population were such that the mass was primarily concentrated in the largest bodies. I am uncertain about Chapman's position on the plausibility of an associated small body population. On the one hand he regards my suggestion that the late heavy bombardment involved a complete size- spectrum of bodies as "an even more encompassing hypothesis with wider im- plications" than the "cataclysm" hypo- thesis of Tera et al. (1974). On the other hand, he seems uncomfortable with my choice of a size distribution in which the mass is almost entirely concentrated in a few of the largest bodies because he feels tha t the crater frequency distributions on the oldest post-Imbrium (and Orientale) surfaces may not support this view. As stated previously, this is a difficult question and firm opinions would be improper. I t seems reasonable to me that since all observed planet-crossing populations con- tain a size spectrum, and since all of the proposed mechanisms for producing a late heavy bombardment involve popula- tions for which a size spectrum would be expected, that it would be rather ad hoc if I were to assume that the size distribu- tion of projectiles of the late heavy bom- bardment suddenly was cut off at the size of the Orientale projectile. On the other hand, the relative mass of the Imbrium and Orientale projectiles, and the low density of craters on the Fra Mauro and Cayley surfaces argue against a size distribution in which the small bodies represent a large part of the mass. I have simply tried to adopt an intermediate size distribution which was most consistent with all the available evidence.

As I mentioned in the panel discussion at the Mercury conference, the size distri- bution of craters on Mercury and the Moon might prove to contain evidence against a simultaneous late heavy bombardment. Murray £t al. 1975) have interpreted the intercrater terrain on Mercury as being a pre-Caloris surface with a deficiency of small craters < 30km in diameter, but argue against these small craters being removed by an erosional event. The alterna- tive is that the projectile population which produced these craters was deficient in small bodies. I f one accepts the conclusion of Neukum £t al. (1975) tha t the lunar formations which should be nearly equiva- lent in age according to the simultaneous late heavy bombardment hypothesis do not show this deficiency, then it appears tha t the lunar and Mercurian projectiles had a different size distribution. Trask (draft manuscript, 1975) and Oberbeck et al. (1975) have suggested that the lunar flux may also have been similarly deficient and that many of the < 3Okm craters on the lunar highland plains may be basin secondaries. The differences between the Moon and Mercury would then be attri- buted to the effect of the different gravita- tional acceleration on the range of secon- daries. This whole matter deserves careful attention as it could provide evidence for or against the hypothesis tha t all of the terrestrial planets were impacted by the same population of bodies.

4. The cumulative power law expo- nent of 0.5 which I have used for the mass spectrum of impacting bodies responsible for craters > 10km in diameter is the same as that obtained for lunar craters in an extensive compilation by Chapman and Haefner (1967). I t appears to me that in spite of the difficulties inherent to this area of investigation this value is sufficiently different from the steady-state value of 0.83 to represent a potential difficulty in models which imply steady-state cratering size distributions. To ignore this in dis- cussions of such models would be improper. In my papers I explicitly stated tha t the steady-state law may well not be applicable to the fragments produced in a single event and did not propose that this distinction

542 GEORGE W. WETHERILL

w a s m e a n i n g f u l for m o d e l s in w h i c h s ingle- e v e n t d i s r u p t i o n was d o m i n a n t .

T h e r e a r e a n u m b e r o f o t h e r p o i n t s r a i s e d b y C h a p m a n on w h i c h c o m m e n t s cou ld b e m a d e . T h e s e i n c l u d e such m a t t e r s as t h e t i m e scale for t h e f o r m a t i o n o f Mars , t h e ef fec t o f r e s o n a n c e s on p l a n e t a r y i m p a c t h i s to r i e s , a n d t h e p l a u s i b i l i t y o f t h e p r o j e c t i l e s o f t h e l a t e h e a v y b o m b a r d m e n t p r i m a r i l y cons i s t i ng o f b o d i e s in E n c k e - l i k e o r b i t s (pe r ihe l ion w i t h i n t h e o r b i t o f M e r c u r y ) . W h i l e t h e s e t o p i c s a r e r e l e v a n t t o t h i s d i scuss ion , I d o n ' t feel t h e y a re su f f i c i en t ly c e n t r a l to t h e m a i n p o i n t s a t i s sue t o j u s t i f y l e n g t h e n i n g t h e s e com- m e n t s .

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