Planetology and classification of the solar system bodies
Introduction
Present terminology and classification of the Solar System (SS) bodies was shaped by many factors: scientific investigation, philosophy, religion and even astrology. Up to the times of Galileo, most SS bodies were just wandering stars. For the next few centuries, they were merely faint disks seen through telescopes. The best known quantities concerning celestial bodies were parameters of their orbits, consequently the orbits plays a major role in classifications. Note that differences between planets and satellites, inner planets and outer planets, regular satellites and irregular satellites describe properties of orbits, not properties of bodies. Space missions and modern methods of observation gave data that lead to the formation of planetology, which includes geophysics, geology, meteorology etc. of extraterrestrial bodies. Unfortunately, the present day planetary sciences still use terminology that does not correspond to our knowledge about SS bodies. Moreover, the terminology is not coherent and there are many inconsistencies. Let us consider two examples.
Example 1. Is Mimas a medium or a small satellite?
Answer1: According to Schubert et al. (1986): “Accretional heating is probably insufficient to have differentiated the interiors of the small icy Saturnian satellites Mimas, Tethys, Dione, Rhea and Iapetus.”
Answer2: According to (de Pater and Lissauer (2001), p. 203): “5.56.2 Medium-sized Saturnian Satellites […] These moons range in radius from a little over 100 km (Phoebe, Hyperion) up to 750 km (Rhea).”
Conclusions: Mimas is a small or a medium-sized satellite. Note also that Phoebe could be a very small satellite according to A1 or a medium satellite according to A2.
Example. 2. How many terrestrial planets (terrestrial bodies) are there in the SS?
In addition to Earth, Venus, Mars, and Mercury, we have the following candidates: the Moon, Vesta, Io, and Europa. Different approaches and different answers to this question could be found in (Wood, 1979, Glass, 1982, de Pater and Lissauer, 2001, p. 203), Keil (2002).
The main purpose of the paper is to indicate inconsistencies of the present day terminology and to suggest some way of solving them. Usually, these suggestions are not original; they are based on the common-sense and practical approach found in papers of many scientists. Suggestions are intended mainly for unification of terminology in planetary science. We think that it is a proper time to discuss that subject.
A good classification should have the following properties. (i) It should be comprehensive (i.e., all celestial bodies should be grouped using criteria of the classification). (ii) It should have few dubious cases; i.e., few cases for which we are not sure to which class a given body belongs. (iii) It should use scientific criteria (i.e., physical, geological, geochemical etc.) instead of arbitrary criteria like the 100 km limit for radius. (iv) It should guarantee similarity of bodies belonging to the same group. (v) It should be possible to use for bodies that will be discovered in the future (e.g., for Kuiper belt bodies, bodies in other star systems etc.). (vi) It should be easy to use. (vii) Compatibility with traditional classification would be an advantage.
Section snippets
Stars, planets, satellites, and planetary bodies
The International Astronomical Union has never had a definition of planets. Encyclopedia Wikipedia (http://en.wikipedia.org/wiki/Planet) gives the following definition: “A planet (from the Greek πλανη´της, planetes or “wanderers”) is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion.” Additional information states that there are nine planets in the SS. The main drawback of this definition is that it does not specify the lower limit
Small bodies versus large and medium bodies
Accreted bodies are usually classified according to their sizes as: large, medium and small bodies. The criteria are rather arbitrary (see Example 1 in Section 1). We suggest to use criterion of shape. Words ‘large’ and ‘medium’ will be reserved for regular, spherical (in fact: spheroidal) bodies while ‘small bodies’ could be of irregular shape. By spherical shape we mean the shape corresponding to the shape of an isosurface of potential gravity and non-inertial forces. Usually, it is a shape
Large bodies versus medium bodies
There are considerable size gaps between spherical bodies. For spherical icy bodies one can find several gaps: 250–530 km (Enceladus–Tethys), 593–718 km (Charon–Iapetus), 789–1151 km (Titania–Pluto). However, there is little doubt that Kuiper Belt bodies will fill all these gaps. For rocky bodies, we have two gaps: 457–1561 km (Ceres–Europa) and 1821–2440 km (Io–Mercury). In the present paper, the gaps: 789–1151 km (Titania–Pluto) and 457–1561 km (Ceres–Europa) are used as a temporary criterion for
Basic groups
Any body could be disrupted by tidal forces or be broken by a catastrophic impact. A number of fragments could be produced by such an event. The fragments could be bodies of similar size like a parent body (bodies) or could be significantly smaller. These significantly smaller fragments cannot be classified according to their size and densities because they have some signatures (e.g., mineralogical composition) of the parent bodies. Therefore, a class (or classes) ‘fragments’ is introduced here.
Interplanetary interactions: additional dimensions of the classification
Many processes on celestial bodies depend on interactions with other celestial bodies. There are several physical processes through which celestial bodies can act on each other. (i) Gravity (leads to perturbations of orbits and to tidal effects including disruption). (ii) Impact (leads to surface cratering or breaking of celestial bodies). (iii) Electromagnetic radiation (leads to surface heating). (iv) Solar wind (only during T Tauri phase).
Other ways of interaction (e.g., by electric field,
Some examples
Basic properties of bodies belonging to each group listed in Section 5 are described (possibly under other names) in monographs of planetary sciences (e.g., Wood, 1979, Glass, 1982, de Pater and Lissauer, 2001). Most of the major planets and satellites directly correspond to their descriptions. Let us consider some cases where additional dimensions of the classification are useful for better description.
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Mercury is a large rocky body (LR) modified by impact (responsible for inhomogeneity of mass
Tidal interaction and tidal parameter
The tidal interactions are important for many bodies. Most satellites rotate synchronously as a result of tidal forces. These forces could result in disruption of a body (e.g., comet Schumaker-Levy 9) and tidal heating. The tidal heating is especially intensive and long lasting if an orbital resonance keeps significant eccentricity of the orbit (e.g., resonances of Io, Europa, and Ganymede). Tidal heating is also important for medium size icy satellites of Saturn and Uranus. Six of them
Conclusions
Classification of celestial bodies that disregard their orbital motion is simple and comprehensive. Most problems arise from our limited knowledge about asteroids. Physical criteria for discrimination between medium and large rocky bodies are also necessary.
Acknowledgements
This paper was supported by Grant 2 P03D 010 25 received from Polish Ministry of Scientific Research and Information Technology.
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