Something more than boulders: A geological comment on the nomenclature of megaclasts on extraterrestrial bodies

doi:10.1016/j.pss.2016.11.006

Planetary and Space Science

Volume 135, January 2017, Pages 37-42

https://doi.org/10.1016/j.pss.2016.11.006 Get rights and content

Highlights

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Nomenclature of clasts should be the same for extraterrestrial bodies and Earth.
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Boulders identified on extraterrestrial bodies do not correspond to boulders on Earth.
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Boulders and megaclasts should be distinguished on extraterrestrial bodies.
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Blocks, megablocks and superblocks have to be distinguished among megaclasts.

Abstract

Large clasts are common on extraterrestrial bodies, and these are traditionally termed “blocks” and “boulders”. These two terms can easily raise confusion, however, because they are used in a sense that differs from geological definitions. Several classifications of large clasts are currently in use in the Earth sciences, and they differ only in detail. They restrict the size of boulders to 1–4 m; larger particles are called "megaclasts". The analysis of the published information on large clasts on planet satellites, asteroids, and comets imply that the particles often described as "boulders" actually are megaclasts; boulders, as the term is used in the Earth sciences, are too small to be detected given the limited resolution of most images obtained. It were therefore scientifically preferable if the established geological literature were applied in the modern planetary and space research. It appears sensible to distinguish boulders from megaclasts; the latter comprise bodies that might be subdivided granulometrically into blocks, megablocks, and superblocks. It is also shown that the abundance of megaclasts on extraterrestrial bodies may itself be beneficial for our understanding of such particles, which are rare on Earth.

Introduction

Outstanding achievements in the exploration of the Solar System have been recorded since the Sumerian and Assiro-Babylonian cultures, about 2000 years ago, when the cosmic objects were studied for early civilizations needs. Talete of Mileto (624 a.C.), a Greek philosopher, was the first whose whose intuitions led to astronomical hypotheses. Further, observations and then investigations led to major discoveries in the Universe. However, modern planetary and space research employs a lot of knowledge obtained from the Earth.

During the past decades, the development of what can be termed provisionally "planetary geology" (Tanaka and Hartmann, 2012, de Pater and Lissauer, 2015) increased our insight into the age, morphology and structure of extraterrestrial bodies. Some analogies with the Earth have been noted, above all for the nearest planets, their satellites, and some asteroids. It is well demonstrated that the approaches used for geological investigations on Earth can be applied with equal success in studies of other cosmic bodies. A typical example can be found in the work of Pondrelli et al. (2008), who recognized deltaic facies in the Eberswalde crater on Mars. It is worth to add that morphological evidence of fluvial and fluvial-like landforms have been recorded on the surfaces of the inner planets and on some of their satellites (Baker et al., 2015) and also crater size, debris flows, and gullies have been studied in detail (Krishna and Kumar, 2016).

Considering that much of this research has dealt with morphological and structural features, the widespread distribution of large clasts (called "blocks" and "boulders") on extraterrestrial bodies has received close attention (e.g., Lee et al., 1986; Michikami et al., 2008). These large clasts have evident analogues on the Earth's surface (Fig. 1). Recently, the successful Rosetta mission to comet 67P/Churyumov-Gerasimenko and its flyby asteroid 21 Lutetia has obtained evidence that has increased the interest in both the morphology of these bodies and their "sedimentary cover" (Küppers et al., 2012, Pajola et al., 2015). Such research requires the use of proper terminology for description of the different clasts constituting this cover.

With regard to the success of extraterrestrial geology (exogeology), it is logical to consider the suitability of the Earth-based clastic rock nomenclature for extraterrestrial bodies. This option has been expressed already, particularly, by the developers of one classification (Blair and McPherson, 1999) and was practically employed by Miyamoto et al. (2007). However, it is evident that these and other classifications are not used widely in the modern planetary and space research. The main objective of the present paper is to discuss the terminology that should be applied to large clasts defined provisionally as block and boulders on extraterrestrial bodies in the light of the Earth-based nomenclature.

Section snippets

Theory: megaclast classifications

Clastic sedimentary rocks are widespread in the geological record of the Earth. The development of their classification was critical to the development of sedimentology as an individual discipline. The Udden-Wentworth grain-size classification, proposed more than a century ago (Udden, 1898; Wentworth, 1922), is still widely used and accepted (e.g.,Boggs, 2006; Nichols, 2009; Tucker, 2011). According to this scheme, all clasts (particles) larger than 256 mm (~0.25 m) in size are boulders (cf.Dutro

Results

Large clasts, termed "blocks" (Basilevsky et al., 2014, Lee et al., 1986, Thomas et al., 2000), "boulders" (Basilevsky et al., 2013, Kickapoo Lunar Research Team and Kramer, 2014, Krishna and Kumar, 2016), or "clasts" (Wilson and Head, 2015), are reported from the Moon, Mars, and the satellites of Mars. The resolution of the analyses is relatively high, and the smallest particles that can be distinguished as individual pieces are a few meters in size. The number of clasts is large. For

Discussion and conclusions

The reconsideration of large clasts from different cosmic bodies presented above permits several general conclusions. First, there is inconsistency in the application of terms such as boulders or blocks. Second, these large clasts are rarely true boulders and only sometimes blocks in the light of the modern sedimentological classifications. Third, the low resolution of the surface images of planets, planet satellites, asteroids, and comets often does not allow recognition of boulders as

Acknowledgements

The authors gratefully thank the journal editor, A.J. van Loon (Spain), and two anonymous reviewers for their valuable suggestions, R. Mauldin (USA) for his improvements, as well as A.-T. Auger (France), M. Küppers (Spain), T. Michikami (Japan), M. Pondrelli (Italy), W. Riegraf (Germany), and many other colleagues for help with literature.

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Something more than boulders: A geological comment on the nomenclature of megaclasts on extraterrestrial bodies

Highlights

Abstract

Introduction

Section snippets

Theory: megaclast classifications

Results

Discussion and conclusions

Acknowledgements

Planet. Space Sci.

Planet. Space Sci.

Geomorphology

Icarus

Sediment. Geol.

Icarus

Planet. Space Sci.

Icarus

Planet. Space Sci.

Icarus

Icarus

Icarus

Planet. Space Sci.

Geomorphology of the Imhotep region on comet 67P/Churyumov-Gerasimenko from OSIRIS observations

Astron. Astrophys.

Grain-size and textural classification of coarse sedimentary particles

J. Sediment. Res.

Processes and forms of alluvial fans

Particle size scales and classification of sediment types based on particle size distributions: review and recommended procedures

Sedimentology

Principles of Sedimentology and Stratigraphy

Planetary Sciences

HiRISE observations of new impact craters exposing Martian ground ice

J. Geophys. Res.: Planets