Although Mars has been the focus of planetary exploration over the past three decades, most effort has centered on evaluating the distribution and history of near-surface liquid water as a marker for potential habitability – for ancient and extant life and for future exploration – and so much of the data are more relevant to questions of surficial processes over geological time. These findings are important, but not central to our investigation. Accordingly, we have resisted the temptation of focusing too much attention on these results, as impressive as they are. Instead, we consider them where they address major questions of crustal evolution that are the subject of this enquiry. There are several up-to-date reviews of the recent findings from the Mars exploration programs for those so interested.

Martian crustal evolution represents a near-perfect intermediary between the simple and mostly ancient crustal histories of Mercury and the Moon, where primary crusts dominate, and the extended evolution of Earth. On Earth any primary crust that may have existed is long since lost from the geological record and both secondary and tertiary crusts formed, but at very different rates, over some four billion years.

Sampling martian crust

Mars presents unique challenges in obtaining representative sampling of the crust.

This work is not intended as a textbook, or as a review, but represents an enquiry into the problem of how and why solid planets produce crusts. As this seems to have happened at many different scales throughout the Solar System, we were curious to see whether some general principles might emerge from the detail. The formation of the planets themselves is the outcome of essentially random processes, constrained mainly by the history of the inner nebula and by the cosmochemical abundances of the chemical elements. But perhaps the production of crusts might be a simpler or more uniform process, a notion supported by the frequent appearance of basaltic lavas of assorted types on the surfaces of rocky bodies.

This book is also written from geochemical and geological perspectives, the areas with which the authors are most familiar. We were immediately faced with the problems of ordering the discussion in a logical sequence because “good reasons could be found for placing every chapter before every other chapter”. Although one might reasonably expect to begin such a book with a discussion of the continental crust on which we are standing, this useful feature, like the Earth itself in a wider planetary context, is one of the least enlightening places from which to discover how planets form crusts. For this reason, our familiar continental crust appears late in the discussion. We decided instead to begin with simpler examples.

Too many variables

On commencing this study, we were hopeful of reaching some general conclusions about the origin and evolution of crusts, at least on the terrestrial planets. However a survey of the previous chapters reveals little that might assist one in predicting any of the details of crustal development. Crusts of many types are present but they are characterized by differences rather than similarities; there are more variables than there are planets. As with most aspects of planetology, reaching general conclusions or deriving some widely applicable principles remains elusive. Rather than the terrestrial planetary crusts representing points on a continuum of evolutionary style, crustal evolution is governed largely by stochastic processes that also influenced the origin and evolution of the planets themselves. So there are too many variables and too few outcomes to allow for any kind of statistical treatment, just as the accretion of the terrestrial planets, as we have seen in Chapter 1, is essentially a stochastic process, with outcomes impossible to predict.

Although it is possible to classify crusts on the terrestrial planets into “primary”, “secondary” and “tertiary” (Section 1.5), this does not imply any logical or inevitable sequence of development. Thus both the primary anorthositic crust of the Moon and the tertiary continental crust of the Earth are unique (Fig. 14.1). So like many classifications, distinguishing the different types of crusts provides convenient pigeonholes but has little predictive power.

The planet

Mercury is a unique planet even by the standards of the Solar System. Like Mars, it is a survivor of many similar bodies, possibly a dozen or more, that formerly were present in the inner Solar System before the hierarchical assembly of the Earth and Venus. Thus the investigation of this planet may yield important insights into the early stages of the accretion of planets between the assembly of kilometer-size objects and of the Earth-sized bodies (Chapter 1).

Because so little is known about this smallest planet, one might question the wisdom of including here a separate chapter on Mercury. Although it was tempting to include this discussion in a section under minor bodies (Chapter 13), we decided on separate treatment. This conclusion was driven by the similarities between the mercurian crust and that of the lunar highlands, so that this chapter follows on naturally from those dealing with the Moon. It also provides some interesting problems about primary or secondary crusts. Further, the Messenger mission is already en route to this innermost planet, so that it is useful at this stage to summarize more thoroughly our current understanding.

The geology of Mercury, a one-plate planet like Mars and Venus, shows some similarities with that of the Moon.

The composition of the upper part of the continental crust is well established, but it is so enriched in incompatible elements and the heat-producing elements K, U and Th in particular, that it cannot be representative of the entire crust. Unfortunately the inaccessible and largely unknown nature of the lower continental crust makes it more difficult to determine the overall crustal composition so that elements of model-dependency enter the discussion. Because the crust is a significant reservoir for many elements, understanding its overall chemical composition is of fundamental importance to geochemistry as these data place constraints on the basic processes of crustal growth, differentiation and evolution of the mantle.

Because of these restrictions, indirect evidence from the geophysical disciplines (e.g. heat flow, seismology) has to be employed mostly to obtain the bulk composition of the continental crust. So in contrast to upper crustal abundances where there is a consensus, the chemical composition of the bulk crust is much more controversial, with recent models covering a broad range from basalt through to dacite (Fig. 12.1).

However, compositions at both extremes encounter a variety of problems that are difficult to reconcile with known crustal characteristics. In our opinion, the combination of constraints imposed by the upper crustal composition, heat flow and geochemistry yields reliable compositions for the bulk crust.

Mars is the only body in the Solar System, apart from the Earth and Moon, to which we devote more than one chapter in this enquiry of planetary crusts. Information now available for Mars, from telescopic observations, orbiters, landed missions and martian meteorites, is enormous and accordingly details now known about the martian crust are considerable. An important finding is that Mars has been geologically active throughout its history and yet still retains a rock record dating back to about 4.5 Gyr, the age of the oldest martian meteorite. Sedimentary deposits are recognized both in some of the oldest and youngest exposed terrains. Accordingly, Mars may well have the most completely preserved geological record of any terrestrial planet.

For both the Moon and Earth, chapters are broken out according to crustal types (primary, secondary, tertiary) and age (Hadean, Archean, Post-Archean). For Mars, we take a different approach. Mars differentiated into core, mantle and crust very early in its history, likely due to magma ocean processes. Unlike Earth, there is unambiguous evidence for this early differentiation. The composition and subsequent evolution of the crust in turn has been greatly influenced by this early history.

The maria

The dark lunar maria form a type example of a secondary crust, derived by partial melting from the mantle during ongoing planetary evolution. These enormous plains cover 17% (6.4 × 106 km2) of the surface of the Moon and constitute the familiar dark areas that form the features of the “Man in the Moon” and various other imaginary figures. But despite their prominent visual appearance, the maria form only a thin veneer on the highland crust (Fig. 3.1).

The lavas are mostly less than 500 meters thick, reaching thicknesses of up to 4 km only in the centers of the circular maria such as Imbrium. They cover an area that is only a little more extensive than that of the submerged Ontong Java basaltic plateau, which lies north east of the Solomon Islands in the Pacific Ocean. The total volume of the maria, about 1.8 × 107 km3, is trivial compared to the anorthositic crust or the whole Moon. This compares with the total volume of the current terrestrial oceanic crust of 1.7 × 109 km3, two orders of magnitude larger.

Planetary formation

Although this book is concerned with the crusts of the solid bodies in the Solar System, it is necessary to delve a little deeper into the interiors of the planets, to see how the planets themselves came to be formed and why they differ from one another. It is only possible to understand why and how crusts form on planets if we understand the reasons how these bodies came to be there in the first place and why they are all different from one another. Following 40 years of exploration of our own Solar System, the discovery of over 200 planets orbiting stars other than the Sun has brought the question of planetary origin and evolution into sharp focus. The detailed study of planets is in fact a very late event in science and has required the prior development of many other disciplines.

This highlights a basic problem in dealing with planets, at least in our Solar System, that are all quite different, so that it is difficult to extract some general principles that might be applicable to all of them.

Minor bodies in the Solar System

Apart from the major planets, a host of minor bodies are also in orbit around the Sun or around the planets. However, no commonly agreed definitions can be found for the bodies that formed by a variety of stochastic processes in the nebula. Furthermore the extra-solar planetary systems mostly do not resemble our own. This has led to sharp debates over what does or does not constitute a planet (Chapter 1, Ref. 2) and the matter has been resolved by adding qualifiers. So the Solar System consists of eight major planets, each one distinct and a huge variety of objects that include the rocky bodies that inhabit the asteroid belt, the satellites of the major planets and the multitude of small icy bodies in the Edgeworth–Kuiper Belt that are usually referred to as Trans-Neptunian Objects or TNOs that possibly represent material from the primordial solar nebula. Perhaps the most interesting observation about the small bodies is that there is little uniformity. Dave Stevenson has noted that “the four giant planets exhibit a startling diversity of satellite systems” while Brad Smith has remarked that “the sense of novelty would probably not have been greater if we had explored a different Solar System”.

Minor bodies are typically composed of various mixtures of the ice and rock components of the original nebula. Gases are strongly depleted.

The period of 1933–59 brought forth several improvements in the study of comets, which led to more discoveries and longer periods of visibility. The greatest advances came in the area of telescopes and photography.

Comet discoveries

The USA continued its dominance in discovering comets during this period, with amateur and professional astronomers being given official credit for 60 discoveries. Following the USA were South Africa (24 discoveries), Slovakia (19 discoveries), Japan (9 discoveries), Russia (8 discoveries), and Finland (7 discoveries).

The most prolific comet discoverer of this period was A. Mrkos (Slovakia), who found 11 new comets. Next in line were M. Honda (Japan) and L. C. Peltier (USA), who each found 7 new comets, M. J. Bester (South Africa), who found 6, and R. Burnham Jr. (USA) and D. du Toit (South Africa), who each found 5. Honda and Peltier were both amateur astronomers, while Burnham discovered comets as both an amateur and a professional astronomer.

Another important point concerning comets discovered during this period was that many were found during surveys. The most successful were the National Geographic–Palomar Observatory Sky Survey, which found 11 comets during the period of 1949–55, and the Skalnaté Pleso binocular comet search program, which found 19 comets during the period of 1948–59.

Comet observations

Several very active comet observers mentioned in Cometography volume 3 continued to observe during most, if not all, of the period covered by this volume. The most notable include G. van Biesbroeck, H. M. Jeffers, and M. Beyer.

1. C/1933 D1Discovered: 1933 February 16.1 (Δ = 0.60 AU, r = 1.01 AU, Elong. = 75°)

2. (Peltier)Last seen: 1933 April 14.21 (Δ = 1.52 AU, r = 1.49 AU, Elong. = 68°)

3. Closest to the Earth: 1933 February 23 (0.5575 AU)

4. 1933 I = 1933aCalculated path: CEP (Disc), CAS (Feb. 17), PER (Feb. 23), TAU (Mar. 8), ORI (Mar. 18)

L. C. Peltier (Delphos, Ohio, USA) was involved in a routine comet-sweeping session on 1933 February 16.1, when he came across an object of magnitude 8.6 at α = 22h 48m, δ = +62°. He immediately wired G. van Biesbroeck (Yerkes Observatory, Wisconsin, USA) for confirmation, but cloudy skies were prevalent. Peltier sent a telegram to Harvard College Observatory (Massachusetts, USA) the next morning announcing his discovery. Confirmation came on February 17.05, when van Biesbroeck detected the comet in hazy skies. He described it as 9th magnitude, with a round centrally condensed coma 5′ across. H. M. Jeffers (Lick Observatory, California, USA) independently confirmed the comet with the 30-cm refractor on February 17.23. He estimated the magnitude as 9, and said the centrally condensed coma was 2′ across, but contained no stellar nucleus. Additional confirmation came on February 17.81, when R. Carrasco (Madrid Observatory, Spain) estimated the photographic magnitude as 8. The comet attained its most northerly declination of +62° on February 17. The comet was discovered a few days after it had passed perihelion, but was approaching Earth.