Origin and Evolution of Comets

Ten Years after the Nice Model and One Year after Rosetta

Hans Rickman

• Publisher: WSPC
• Year: 2017
• ISBN:
  9789813222595
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

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Since several decades, comets have been considered as key witnesses of solar system formation. Their nature has been explored using the modern arsenal of Earth- and space-based observations, and they hold a central place as dynamical arbiters of the planetary system in the new paradigm of solar system evolution known as the Nice Model. Thus, they have the potential to test the various ideas, using the detailed data recently gathered by the ESA/Rosetta mission. This requires an understanding of their origin and evolution, which form the subject of the present book. All the relevant issues are covered, describing both the background and the current frontiers of research.

--> Contents:

• Introduction
• Physical and Chemical Properties
• Comet Dynamics
• Physical Evolution in Observable Comets
• Capture of Comets
• Formation of Comet Reservoirs
• Origin of Comet Nuclei
• Outlooks

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--> Readership: University students and researchers interested in astrophysics. -->
Keywords:Comets;Nice Model;Rosetta Mission;Comet DynamicsReview: Key Features:

• Currently this is the only book that discusses the Nice Model
• This book presents a comprehensive assessment of the recent progress in the whole, broad field of research
• It offers the first picture of comet origin and evolution that appears after the amazing results of Rosetta have been digested

Chapter 1

Introduction

In most parts of the world, it would be difficult to find people of age, who have no idea what a comet is. However, those ideas are generally different from the concepts that scientists have in mind when using the word comet. In fact, the scientific definition of a comet is a non-trivial issue, which we had better tackle before describing their physical properties and how they originated and evolved.

1.1.What is a Comet?

There are two interpretations of the word. Closest to the layman’s impression is the one about the phenomenon observed on the sky, and according to this, a comet is a diffuse object whose technical term is coma (usually, including a bright spot called the central condensation), from which a tail may extend. This object is in orbit around the Sun. Referring to the orbit is often necessary to distinguish the comets from Galactic nebulae and external galaxies. In 1771, the first catalogue of such diffuse nebulae (the famous Messier catalogue) was in fact produced by Charles Messier in order to avoid wasting time on these objects when hunting for comets, because one sort of diffuse object was often difficult to distinguish from the other.

The second interpretation refers to a physical object belonging to the solar system. In 2006, the International Astronomical Union (IAU) at its 26th General Assembly adopted a classification of solar system objects in terms of planets, dwarf planets and small bodies. In this definition, comets are counted with the small bodies together with, for instance, the asteroids. Here, the word comet means the solid object, orbiting around the Sun, which gives rise to the diffuse phenomena mentioned above. The mechanism whereby this occurs is another matter.

Generally, it is a question of ice sublimation due to heating by absorption of sunlight, which leads to an outflow of gas and dust into space. This outflowing material is seen as the coma with a possible, more or less anti-sunward extension, called the tail. The solid object, from which the coma and tail would emanate, is called the nucleus, and thus, the word comet is used as a synonym to the nucleus. It is fair to say that this usage dominates in recent scientific literature, and the same practice will be followed here. This is natural, because the concepts of origin and evolution always refer to the nucleus. The coma and tail evolve very rapidly and they typically come and go, as the comet moves around the Sun. But the nucleus persists and typically evolves on much more significant time scales encompassing many orbits.

The physical definition of a comet, as briefly sketched above, is in fact a bit ambiguous. If we consider ice sublimation as the cause of the outflow, it is obvious that this is strongly temperature dependent. Hence, it will depend on the distance between the object and the Sun. It is indeed a well-known fact that comets develop their comae and tails essentially in the innermost parts of their orbits. Thus, when comets are observed far from the Sun, they may appear starlike, and we may actually see the bare nucleus. Comets do not have to produce the diffuse cloud all the time, but reliable observations of such activity on at least one occasion are required. Thus, official status as a comet is recognized only after such observations, but it is not withdrawn if an established activity ceases.

Hence, the comet is in reality an object that has the potential to develop a coma and a tail under the right circumstances — essentially, when it comes close enough to the Sun. This means that the object has to contain ice but also that the ice must be found near the surface, so that sublimation may lead to an outflow of gas and dust. Moreover, the perihelion distance must be small enough for this to happen. One can easily see that this is problematic. If the orbit is perturbed because of a close approach to Jupiter, the perihelion distance can change appreciably. An object may thus be called a comet, if it is discovered before an increase of the perihelion distance but not if the discovery happens afterwards. In addition, even slight modifications of the surface layers without any orbital change may imply that the gas production subsides or resumes, and hence, what is essentially the same object may or may not be called a comet depending on when it is observed — if not, it would probably be called an asteroid.

The only reasonable solution to these problems would be to call an icy object a comet, even if its perihelion distance is too large, or the ice is too deeply buried beneath the surface. The essential property of a comet would hence be its ice content: comets are icy, while asteroids are rocky or metallic. This definition is attractive in theory, because the criterion used is intrinsic and more or less quantifiable. Moreover, it helps to convey an important message, namely, that the small bodies of the solar system belong together, even though there is a range of chemical compositions depending on their formation temperatures. Comets and asteroids are not fundamentally different — they are different incarnations of the small body population, representing objects that were formed at different distances from the Sun and thus have different ice content. However, the definition is not useful in practice as long as we cannot measure the ice content by probing the interior of the objects, and we therefore have to require observed cometary activity as an objective criterion when distinguishing comets from asteroids.

In any case, it is clear that we have to be open minded about the objects to discuss. The border between comets and asteroids is somewhat fluent, and there may be transitional objects that are difficult to classify. When limiting ourselves to “real” comets that have exhibited comae or tails, we must recognize that they have siblings that sometimes need to be discussed in the same context.

1.1.1.The comet nucleus

Let us now pay some more attention to what a comet nucleus is thought to be, as an introduction to all the recent findings to be described below. As mentioned, the starting point is the gas and dust forming the comae and tails in comets. In the early 20th century, the old concept of a solid object within this cloud was no longer a dominating idea. Comets had been seen to split and disappear, and prominent meteor streams had been shown to trace the orbits of well known comets. It thus seemed natural to imagine a comet as nothing but a concentration of grains moving together in space. There was also a theory that claimed to show, how such comets could be formed by interstellar material captured by the gravity of the Sun, as it travels through the denser regions of interstellar space.

However, computations had shown that the observed comets do not show a tendency to arrive along hyperbolic orbits. Moreover, comets had been found to approach Jupiter closely without being dispersed and losing their identity, as one would expect from large clouds without much internal gravity. It thus seems, in retrospect, that there was no physical basis for the picture of comets as loose clouds. However, this was clear to some but not to all.

One problem was how to explain the origin of the coma by ice evaporation. In the 1940s there was little information about the chemical composition of the coma, but some radicals had been identified in comet spectra and shown to provide much of the light that is observed. In 1948, the Belgian astronomer Pol Swings proposed that these radicals were produced by the release and dissociation of ices made of polyatomic molecules. One obvious way to release these molecules was proposed by Fred Whipple (1950) in the paper that introduced the modern concept of a comet nucleus, namely, a solid body consisting of an icy conglomerate composed of ices and refractories in an intimate mix. Sublimation of the H₂O-dominated ice in the solar heat would release the parent molecules, from which the radicals emanate.

Whipple’s paper dealt with one particular comet. This is Encke’s comet, which was known since the early 19th century. With an orbital period of only 3.3 years it had been observed on many returns, and scientists had noticed that each of these returns occurred a little too early, compared to the best predictions that could be made by integrating the orbit from the preceding apparitions. This nongravitational effect needed an explanation, and Whipple’s solid nucleus offered a good explanation using the same concept as Bessel (1836) had used. This was a jet force acting on the nucleus due to the asymmetric outflow of material feeding the coma. Since observations of Encke and other comets indicated the outflow to occur mainly in the solar direction, this would mainly accelerate the nucleus in the radial direction outward from the Sun, and Bessel focused on this aspect. In Whipple’s model, the asymmetry followed directly from the fact that the heating of the ice is strongest at the subsolar point, but a thermal lag due to the rotation of the nucleus could in principle add a transverse component to the radial acceleration. In principle, the latter could act persistently over time, if the rotation is markedly prograde or retrograde, and the effect would then be either too late or too early arrival at perihelion.

While Whipple’s theory appeared to offer a good foundation for understanding the behavior of comets and thus seemed clearly preferable compared to its competitors, one problem would remain for decades. To explain the observed amounts of material in cometary comae, a km-sized nucleus was generally enough. The problem was that such a small object is very difficult to detect at the typical distances of observed comets, and the long-lasting absence of any clear observational verification of Whipple’s nucleus caused some lingering skepticism by the proponents of alternative theories.

1.2.Comet Designations

In both media reports, popular descriptions and scientific literature, comets are referred to by names and designations. These are not always consistent and may appear confusing, so a brief guide may be helpful.

Comet orbits span an enormous range of revolution periods from just a few years to millions of years. Thus, comets can be subdivided into two categories: the single-apparition comets and the returning comets. The former often have so long orbital periods that, essentially, astronomers have only had a single occasion to observe them in connection with one perihelion passage. The latter, on the other hand, have periods short enough to present at least two such occasions. Those categories have traditionally been referred to as long-period versus short-period comets (see Sec. 1.4), and the limit has been placed at orbital period P = 200 years.

The present designation system dates back to a resolution passed by the IAU 22nd General Assembly in 1994.¹ Here, the returning category is referred to as periodic comets. These are defined to have revolution periods of less than 200 years or confirmed observations at more than one perihelion passage.² Upon discovery, all previously unknown comets get a designation, consisting of the year of discovery followed by an upper-case letter denoting the halfmonth in question and a numeral indicating the sequential order of this discovery announcement within the relevant halfmonth. Before this, a letter is applied, which indicates the category of the comet: “P/” denotes a periodic comet, and “C/” denotes a comet that is not periodic.

In 1999, the IAU Minor Planet Center decided to call single-apparition comets periodic only when their orbital periods are less than 30 years. Meanwhile, there is another way to designate returning comets, which is independent of the orbital period. This is a permanent, serial number followed by the letter “P”, and it is assigned to comets that have been observed to return or have had their periodicity established otherwise. The name of the comet may be added, separated by a slash. The list of such comets is basically chronologic, starting with 1P/Halley. As of 1 January 2016, there were 330 comets listed as periodic in this way, and on 1 January 2017 this number had grown to 347.

A category of special interest for the evolutionary aspect of comets is those that have been deemed not to exist any more. In most cases, the reason is that they have not been found in spite of deep exposures of the sky area where they would certainly have been according to the ephemerides. For these comets, the letter P is replaced by D in the designation. The list of comets with permanent numbers currently hosts eight such members, the most famous of which is 3D/Biela.

In addition, there are single-apparition comets with “D/” designations. Among these, the most notable is the famous, very special comet D/1993 F2 (Shoemaker-Levy 9), which was discovered in a jovicentric orbit and collided with Jupiter in 1994. This is the only comet that is definitely deceased, even though the letter D generally stands for ‘dead’ or ‘disappeared’. In other cases, one has to consider the possibility that the comet will reappear. A case in point is that the record of single-apparition comets used to contain two members with designations D/1783 W1 (Pigott) and D/1819 W1 (Blanpain), which were long lost comets with orbits known to be of short period and were thought to have disappeared. However, both were recently rediscovered and are now known as 226P/Pigott-LINEAR-Kowalski and 289P/Blanpain, respectively.

Comets are often referred to by names. For some of them, this is almost indispensible since the names are so deeply rooted in people’s minds. For instance, it would be strange to discuss comet 1P without adding the name Halley or D/1993 F2 without clarifying that this is comet Shoemaker-Levy 9. However, in c...

Table of contents

• Cover Page
• Advances in Planetary Science
• Title
• Copyright
• Preface
• Contents
• 1. Introduction
• 2. Physical and Chemical Properties
• 3. Comet Dynamics
• 4. Physical Evolution in Observable Comets
• 5. Capture of Comets
• 6. Formation of Comet Reservoirs
• 7. Origin of Comet Nuclei
• 8. Outlooks
• Bibliography