SP-467 Planetary Geology in the 1980s
 

7

The Geology of Small Bodies

 

[133] Given that there are thousands of asteroids and probably a hundred thousand million comets, these small bodies must be considered essential components of the solar system. Certainly objects closely similar to the small bodies that remain today were involved in the agglomeration of the larger planets and satellites some 4.5 billion years ago, and much of the importance of the small bodies today derives from the clues that they may contain about the processes that took place in the early solar system. This importance is magnified when we realize that asteroid-like parent bodies are the only solar system objects (other than Earth and the Moon) of which we have samples for detailed laboratory studies.

Although our understanding of small bodies is relatively limited, we know enough to realize that geologically these objects are best studied separately from the larger bodies, such as Earth and the Moon. For one thing, gravity is so much smaller on these bodies that it is difficult to extrapolate our experiences with surface processes on larger objects with any great confidence. For another, many of the small objects are irregular and call for mapping and geodetic techniques quite distinct from those commonly used for the larger (usually almost spherical) planets and satellites.

 

7.1. What is a Small Body?

It is not easy (nor is it necessary) to give a rigorous definition of a small body. Certainly implicit in the term is that the object has a low surface gravity and small escape velocity. Rather arbitrarily, we can take the largest small body to be the size of the biggest asteroid, Ceres, which has a diameter of some 1000 km. Most small bodies are considerably smaller; the two satellites of Mars, Phobos (21 km) and Deimos (12 km), are more representative.

[134] For an object the size of Phobos, surface gravity is only about 1 cm sec-2, and the escape velocity is some 10 m sec-1. Weak gravity has several important implications. Since such bodies cannot have atmospheres, their regoliths are immune to weathering processes involving the presence of an atmosphere. On the other hand, they are directly exposed to the whole spectrum of meteoroidal impacts, cosmic rays, solar radiation, and the solar wind. Low gravity also makes it impossible for the body to achieve or retain a spherical shape during its history, and many small bodies tend to be irregular in shape. Additionally, low gravity affects the development of the surface under meteoroidal bombardment. Craters probably tend to remain deeper, ejecta become more dispersed, and the proportion of strongly shocked material retained is smaller than on larger bodies. Furthermore, the chances that an asteroid-like small body will suffer a catastrophic, or nearly catastrophic, impact during its history are non-negligible.

The study of meteorites has provided incontrovertible proof that some small parent bodies underwent differentiation (Dodd, 1981). In addition, there is strong evidence of subsurface aqueous processes in some parent bodies (Kerridge and Bunch, 1979) and of surface eruptions of lavas on others (Drake, 1979). The realization of the importance of short-lived nuclides such as 26AI as possible heat sources early in the solar system's history has made it quite plausible that some small bodies should have had early histories of melting and other internal activity (Sonett and Reynolds, 1979). Thus, whereas some small bodies (comet nuclei?) may have had dull evolutionary histories and may rightly be regarded as primitive, others have probably experienced histories almost as complex and certainly as interesting as some larger objects.

 

7.2. Inventory

The solar system's small bodies can be divided conveniently into three broad categories: (1) rocky objects (asteroids and some small satellites), (2) icy objects (mostly small satellites, but perhaps including such objects as Chiron), and (3) comet nuclei.

The inventory of known small bodies includes thousands of asteroids in the main belt, as well as about 60 Amor, Apollo, and Aten objects. Only about 35 asteroids are larger than 200 km across, although physical measurements have been made of objects as small as 200 meters (Gehrels, 1979). None has yet been studied by spacecraft.

 


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Mars' outer satellite Deimos, a body only 12 km across.

Mars' outer satellite Deimos, a body only 12 km across.

 

The inventory also includes the small satellites of Mars and of the outer planets. Phobos and Deimos, the two tiny satellites of Mars, are the only very small bodies that have been investigated sufficiently by spacecraft (Mariner 9 and Viking) to permit meaningful discussions of surface geologic processes (Veverka and Thomas, 1979).

Jupiter has at least a dozen small satellites. Except for a few low-resolution images of Amalthea obtained by Voyager, we know [136] almost nothing about the geology of these bodies. There are also at least 70 known Trojan asteroids near the libration points of Jupiter's orbit, and speculations exist that some of Jupiter's outer satellites may be related to them (Degewij and van Houten, 1979).

Recent Earth-based and Voyager observations have greatly expanded our list of Saturn's small satellites, and at least in the case of Mimas and Enceladus, the Voyager data are adequate to support geologic investigation. Beyond Saturn, most of the satellites of Uranus, Neptune's Nereid, and Pluto's Charon probably fall within our definition of small bodies. However, it will be at least 1986 before any spacecraft data on any of these objects are available.

It is worthwhile to stress that the above list is almost certainly incomplete and that new small bodies will continue to be discovered. In addition, there are indications that small, so far undetected satellites are associated with the rings of Uranus and perhaps those of Saturn and Jupiter as well.

Comets are the most abundant small bodies in the solar system: one estimate is that some 1011 exist in the Oort cloud at the fringes of the solar system (Wilkening, 1982). From the geologic point of view, it is only the nuclei of comets that are of interest and not the comas and tails that develop when the nucleus approaches close enough to the Sun for its surface ices to vaporize. Most comet nuclei are believed to be bodies of rock and ice less than 10 km across, but very little direct information about them exists. None has been studied by spacecraft yet. They could be the parent bodies of some volatile-rich meteorites, and there may be an evolutionary connection between them and some asteroids. For example, it has been suggested that some Apollo asteroids are the remnants of extinct short-period comets (Shoemaker and Helin 1977 Kresak 1979).

In summary, three facts about small bodies must be kept in mind: (1) their vast number, (2) their great diversity, and (3) our lack of knowledge concerning them.

The next two decades of solar system exploration should remedy our current lack of information about small bodies. We cannot gain a true understanding of the solar system's evolution by ignoring them. They are of interest not only in their own right, but as the solar system's most abundant projectiles, they have influenced, m some cases probably dramatically, the evolution of the surfaces of the larger planets and satellites.

 

[137] 7.3. Why Study Small Bodies?

At least four major reasons for studying small bodies in the geologic context can be given:

 
1. Small-body impacts have significantly modified most planetary and satellite surfaces. Data on present and past fluxes and size-frequency distribution are essential to a thorough understanding of the chronology and evolution of such surfaces.
 
2. It is likely that there are geologic processes that are effective on small bodies, but that cannot be studied adequately on larger objects.
 
3. Owing to the wide range of surface characteristics (g varying from about 100 cm sec-2 to less than 0.1 cm sec-2, and composition varying from metallic to icy), small bodies provide a unique laboratory for calibrating the dependence of basic planetary processes, such as cratering, on different variables.
 
4. A better understanding of the geologic evolution of small bodies is needed to better define their possible interrelationships and to take full advantage of the information provided by the meteorite record.

 

It could also be argued that another important reason for studying small bodies is that their geologic record may extend further back in time than that preserved on the surfaces of the larger bodies. Also, many small bodies (including satellites) probably are collisional fragments of large bodies and in some instances could provide accessible information on the differentiation of large parent bodies.

 

7.3.1. Effects of Small Bodies on Larger Objects

Surfaces in the solar system continue to be modified by impacts, and there is abundant evidence that during the first half billion years of the solar system's existence, the surfaces of planets and satellites were influenced dramatically by collisions with small bodies. From the geologic point of view, we are interested in the time history of the flux and population (size and composition) of the impacting objects at different distances from the Sun. The early fluxes appear to have had a profound influence on the evolution of the crusts of larger bodies, and subsequent fluxes are important in [168] determining relative chronologies of different surface units (chapter 3). The actual nature of the impacting bodies (whether volatile-rich or volatile-poor) may have played a role in determining the evolution of some atmospheres and perhaps even of subsequent weathering processes. For instance, it has been proposed that a significant fraction of some gases in the atmospheres of the terrestrial planets were brought in by comets.

Some of the important questions to be addressed are:

 

1. What is the present flux of small bodies in Earth's neighborhood? What is the current proportion of volatile-poor and volatile-rich objects?
 
2. How have this flux and the proportion of volatile-poor to volatile-rich objects changed with time?
 
3. How do these quantities presently vary throughout the solar system? How did they vary in the past?
 
4. What have been the mechanisms responsible for the changes in the impact flux, if such changes have occurred?

 

In the above, the term "flux" should be understood to mean not only total flux of bodies of all sizes (or masses) but also information about the relative fluxes of bodies of various sizes (or masses).

A vigorous program of searching for Apollo, Aten, and Amor asteroids, as well as for comets, can answer the first of these questions. The second and third questions are more difficult, but considerable progress is being made in addressing some aspects of them by theoretical calculations.

A closely related issue involves the orbital evolution of the various classes of impacting objects (origin, lifetime, and eventual fate). For example, how do objects end up in Apollo orbits? How long do they stay? What happens to them?

 

7.3.2. Unique Surface Features and Processes

Not surprisingly, there are processes that are important on small bodies but impossible to predict from an extrapolation of our terrestrial or lunar experience. In fact, it is sometimes even difficult to predict a priori what form a well-known process will take in the small-body environment. For example, a decade ago, there was a legitimate discussion about whether or not there would be recognizable craters on bodies as small as the satellites of Mars. A more serious debate developed about whether appreciable regoliths would form on such small objects. Although we have now learned....

 


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The unexpected grooves of Phobos.

The unexpected grooves of Phobos.

 

....the answers to such rudimentary questions, we cannot pretend to fully understand the process of cratering and regolith formation on small bodies (Cintala et al., 1978; Housen and Wilkening, 1982). For example, we have no convincing explanation for the gross y different appearance of the surfaces of Phobos and Deimos. Why is it that the surface of the smaller Deimos appears to have retained considerably more regolith than that of the larger Phobos?

[140] Our very limited experience in exploring small bodies has already confirmed that unique and unexpected surface features and processes come into play. No one anticipated the existence of grooves on Phobos, yet this type of feature may well be a common one on many small bodies (Thomas and Veverka, 1979). There is every reason to expect that additional, important surface features and processes will be discovered as our exploration of small bodies proceeds, especially in the cases of small icy satellites and the nuclei of comets.

 

7.3.3. Small Bodies as Natural Laboratories

Due to their great diversity in size and composition, small bodies provide ideal testing grounds for studying various processes especially those involving cratering. In principle, one can find small....

 


Mimas, a small icy satellite of Saturn.

Mimas, a small icy satellite of Saturn.

 

[141] ....bodies of similar surface gravity but drastically different surface composition (rock versus ice), or bodies of similar composition but very different surface gravity, to test the importance of such variables on crater morphology, ejecta patterns, etc. Much could be learned by comparing surface features and regolith characteristics on three small asteroids of similar surface gravity but of different composition (carbonaceous, stony, or metallic). As a next step, one could investigate the effects of rotation rate on regolith characteristics by comparing two asteroids that are identical in all bulk characteristics except their spin rates. Full exploitation of such possibilities would require an aggressive program of future solar system exploration.

 

7.3.4. Evolution and Interrelationship

There is ample evidence that some small bodies have had complicated evolutionary histories that involved processes of high interest to planetary geologists. The meteorite record proves that some parent bodies experienced internal differentiation, aqueous metamorphism, and even the eruption of lava onto their surfaces (Dodd, 1981). In many cases, very mature and very complex regoliths were developed (Housen and Wilkening, 1982). Understanding the geologic evolution of such interesting bodies is not only worthy in its own right, but would improve our understanding of the possible interrelationships among small bodies and between the small bodies and larger planets. First, there are questions of the following type to be considered: what styles of eruption and what types of volcanic constructs would one expect on a body as small as Vesta? Or, what kinds of structure control the local emission of gases from a comet nucleus? Second, there are the interrelationship questions; for example, is it geologically reasonable that a comet nucleus can evolve into something like an Apollo asteroid or that some volatile-rich carbonaceous chondrites could come from comets? Unfortunately, in many cases we still lack key observational data to address such important questions meaningfully.

 

7.4. Summary

The small bodies of the solar system are of great intrinsic geologic interest that goes beyond their original role as building blocks of planets and their subsequent role as projectiles. They are characterized by vast numbers and by their diversity.

[142] So far, their geologic study has been hampered by a lack of first-hand information of the sort that can be obtained only by direct spacecraft exploration. Even after Viking and Voyager, our inventory of small objects about which enough is known to carry out detailed geological investigations is very meager. It is restricted to a few icy satellites of Saturn and to the two rocky moons of Mars. We have yet to carry out a geologic reconnaissance of an asteroid or a comet nucleus. Although our accumulated knowledge may be adequate to guess what asteroid surfaces may be like in a general way, we really know next to nothing about comet nuclei. Thus, a first-order requirement for progress in our understanding of small bodies is the exploration of at least one asteroid and one comet nucleus during the coming decade. Some important questions, however, can be addressed only by studying a variety of objects.

In the meantime, it is important to continue the ongoing active programs of Earth-based observations of small bodies as well as related laboratory and theoretical investigations. It is especially crucial to continue monitoring the neighborhood of Earth's orbit for small comets and asteroids, since there is no other way of obtaining adequate statistics on the population of such objects.

In terms of data analysis and interpretation, there are enough unresolved questions concerning the small satellites of Mars and of the outer planets to justify a healthy program of analysis of Viking and Voyager data in these areas. For example, the Viking IRTM * measurements of Phobos and Deimos must be fully correlated with imaging data to gain information on regolith characteristics. We must also develop techniques for mapping irregular satellites and making accurate measurements of their topography and volume. We should make a special effort to apply the many lessons we have learned from comparative planetology during the past two decades to considerations of surface and near-surface processes on small bodies. Such extrapolations from our experience with larger bodies will have to be done judiciously, but the effort should prove beneficial to our general understanding of the solar system.

 


*Infrared Thermal Mapper.


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