What is a comet?

Now and then, a bright comet appears in the sky. It develops a fuzzy head and a long tail while it slowly drifts through the constellations. Such a sight has always fascinated humanity and given rise to stories, myths and superstition. It has also inspired scientists to investigate the nature of comets, and it turns out that they are not only beautiful to watch, but they can also reveal important secrets about the early history of the Solar System.

Comets are among the oldest and least altered bodies that revolve around the Sun, and therefore they constitute a unique source of knowledge about the origin of the Solar System and its early evolution. They constitute left-overs of the material that built the giant planets and their satellites. If one seeks to understand the earliest phase of the planet formation process, and the chemical and physical properties of the environment in which the giant planets formed, one must study comets. We also know that comets bombarded the young Earth and that a substantial fraction of the water we drink every day once were part of comet nuclei that circled the Sun outside the orbit of Neptune. We know that comets are rich in organic species, and that these may have been necessary for the emergence of life on Earth. We also know that comets impact Earth on rare occasions, which has led to local or global changes to the environment during shorter or longer periods, which has forced the ecosystems to adapt. These impacts are therefore parts of our own evolutional-biological history.

Below I will describe the physical and chemical properties of the comet nucleus, the characteristics of the orbits of visible comets and how comets are transported from such relatively nearby orbits from more distant parts of the Solar System. Then I will describe the components of the active comet, i.e., the properties of the coma and the tail. Finally, I will describe the reasons why comets are important from a scientific point of view.

The comet nucleus

The impressive head and tail of a comet originates from a small solid body called the nucleus. A typical comet nucleus is less than 10 kilometers in diameter and is darker than charcoal, since the nucleus only reflects 2-4% of the incoming sunlight. The comet nucleus has a very irregular shape and displays a variety of detail on its surface – highly irregular terrain mixed with smooth surface, valleys, mountains, ridges, hills and craters. The comet nucleus is extremely porous and a large fraction of its volume (60% or more) is just empty space. This makes comets very fragile, and dozens of comets have been seen to split or pulverize. The high porosity and the low strength is due to the fact that the nucleus is made up of weakly bound grains, that typically are a micrometer across (i.e., one part of a thousand of a millimeter).

Halley

The nucleus of Comet 1P/Halley photographed by the European Space Agency (ESA) spacecraft Giotto in 1986.
Copyright: ESA/MPAE, 1986, 1996
Original image: http://www.esa.int/spaceinimages/Images/2002/01
/A_composite_image_of_the_nucleus_of_comet_P_Halley

The grains are primarily consisting of four different types of material. About a third of the mass is silicates and sulfides, another third is organic species, while the rest are volatile species. I will now describe these in some detail.


Silicates constitute a large family of minerals that are rich in silicon, oxygen and various metals, and is the stuff that rock primarily is made of. About half of the comet silicates is olivine, which consists of two metal atoms, one silicon atom and four oxygen atoms. If the two metal atoms are magnesium, we have an olivine called forsterite. If the two metal atoms instead are iron, we have another olivine called fayalite. Comets seem to be rich in forsterite and contain less fayalite. The Earth mantle is made of different types of rocks that are very rich in olivine.
The other half of the comet silicates is pyroxene, that consists of a metal atom, a silicon atom and three oxygen atoms. If the metal atom is magnesium, we have a pyroxene called enstatite, but if the metal is in the form of iron we have ferrosilite. Comets seem to be richer in enstatite than ferrosilite, so on average the comet silicates are magnesium-rich. Pyroxene is an important component in the basalt that makes up most of Earth’s ocean floors.

Sulfides are chemical species consisting of sulfur mixed with iron and nickel. Troilite is the most simple member among the sulfides. It consists of an iron atom and a sulfur atom and is very common in comets. The most complex sulfide found so far in comet material is pentlandite that contains eight sulfur atoms and a total of nine metal atoms that are a mixture of iron and nickel. On Earth, large amounts of sulfur and iron are found in molten form in the outer core.

We now consider the organic species, that all have one thing in common – they contain carbon. Carbon is the most important element in the periodic table since it readily binds to other atoms, which means that carbon can form an extreme variety of molecules. It is this diversity that makes organic molecules suitable as building-blocks of life. The living organism needs a large “tool box” of molecules to solve all sorts of tasks, and only the family of organic species is large enough to provide a sufficiently rich variety.

An example of organic species found in comets are polycyclic aromatic hydrocarbons or PAHs. The most simple PAH, benzene, has six carbon atoms that form a ring, to which six hydrogen atoms are connected. By joining such rings, other PAHs can be formed, such as naphthalene (two rings), phenanthrene (three rings) and pyrene (four rings). All these PAHs have been found in comet material. On Earth, PAHs form during incomplete combustion of carbon-rich material, e.g., when wood is burning. The fact is that naphthalene is extracted from charcoal (this molecule happens to be the active substance in mothballs). Other environments where PAHs are formed are burning cigarettes, car exhaust fumes, and in the frying pan. Comets also contain other forms of organic species, e.g. glycine, the most simple amino acid. Living organisms use amino acids to manufacture proteins, i.e., macro-molecules that perform all sorts of tasks in the cell. To find such pre-biotic molecules in interplanetary space is extremely fascinating.

However, it is the large amount of volatile species that make comets special. Volatile species are basically substances that are liquid or gaseous at room temperature, but that turn solid at the low temperatures found in interplanetary space, i.e., they have turned into ice. Water is the most common volatile in a comet, while carbon monoxide and carbon dioxide come in second and third. Methanol, hydrogen sulfide, formic acid, methane, ammonia and hydrogen cyanide have concentrations of about a percent each relative to water. Methanol is the most simple alcohol, while it is hydrogen sulfide that give rotten eggs their unpleasant smell. Formic acid is used as a disinfectant and during industrial production of plastic, while methane (on Earth) is formed during putrefaction, i.e., when bacteria decompose organic material. Ammonia gives window polish its strong and irritating smell, while hydrogen cyanide is a deadly poison. This rich chemistry has formed in the Solar Nebula, the cloud of gas and dust that surrounded the young Sun, from which the planets formed.

The comet orbits

Objects that are gravitationally bound to the Sun move along trajectories shaped as ellipses. The degree of flattening of the ellipse, or the eccentricity, is very small for the planets (their orbits are almost circular), but is generally large for the comets. The Sun is not located at the center of the ellipse, but in one of the two focal points of the ellipse. These are located on either side of the center, on the the largest of the diameters of the ellipse (the major axis), at distances from the center that are determined by the eccentricity. For this reason, the distance between the Sun and a comet may vary significantly during a revolution, which is not the case for the planets. The point in the orbit where the comet is closest to the Sun is called the perihelion, while the most distant point is called the aphelion. The orbital planes of the planets more or less coincide with a plane called the ecliptic. However, comet orbits can tilt significantly with respect to the ecliptic – they are said to have high inclination.

ellipse

Comets move along elliptical paths around the Sun. The ellipse has two focus points, f1 and f2. The shape of the ellipse is determined by the fact that the distance l1 from the focus f1 to the ellipse, plus the distance l2 from the focus f2 to the ellipse, remains constant for all points on the ellipse. The Sun is here located in f1, while the comet is located at the point P. The smallest distance between the comet and the Sun (q) is called the perihelion, while the longest distance (Q) is called the aphelion.

Visible comets are comets that have orbits that bring the sufficiently close to the Sun and the Earth for us to be able to see them. Visible comets are grouped into a number of families based on the properties of their orbits. Jupiter-family comets have orbital periods shorter than 20 years and move in orbits around the Sun that are near the ecliptic plane. They regularly pass in the vicinity of Jupiter’s orbit. When passing close to Jupiter, they may be subjected to strong gravitational perturbations that may change the perihelion distance, the eccentricity and the inclination. It is these perturbations from Jupiter that have given the group its name.


We also have Halley-type comets that differ from the Jupiter-family comets by having longer orbital periods (up to 200 years), and often significantly higher inclinations. Comet 1P/Halley itself has such a high inclination that the orbit has “flipped over” so that the comet moves clockwise around the Sun (as seen from a point high above Earth’s north pole), while all planets, asteroids and most comets move counter-clockwise.

New comets are barely gravitationally bound to the Sun, and approach the Sun from large distances along an orbit shaped as a parabola. If the comet manages to cross the inner Solar System without suffering a perturbation of a planet, it will continue to move along its parabolic orbit and leaves the Solar System for good, never to return. However, it is not unusual that smaller perturbations take place that transforms the orbit into an extremely eccentric ellipse. This is a long period comet, with an orbital period that could be thousands of years.

How come there are so many different types of comet orbits? Why do some comets belong to the Jupiter family while others are Halley-type comets or new comets? These populations arise for two reasons – there are different reservoirs of comets at very large distance from the Sun, and there are different mechanisms that transport comets from these distant reservoirs to orbits close enough to Earth so that we can see them. There are several large reservoirs of comets that constantly feed objects into the inner part of the Solar System – the Edgeworth-Kuiper belt, the scattered disk, and the Oort cloud.


Distant reservoirs

The Edgeworth-Kuiper belt is a population of icy bodies located beyond the orbit of Neptune. The largest known member is called Eris. The second largest, and the first to be discovered, is Pluto. Both Eris and Pluto are dwarf planets, a category that was introduced in 2006 to distinguish between the largest of the Solar System bodies (the planets), the smallest bodies (asteroids, comets and meteoroids), and intermediate-sized bodies (dwarf planets).

Currently, about 1,200 objects in the Edgeworth-Kuiper belt are known, that all have been discovered after 1992, except Pluto that was found already in 1930. The Edgeworth-Kuiper belt has an inner edge that coincides with the 3:2 mean motion resonance with Neptune, which means that these objects move twice around the Sun in the same time as Neptune completes three revolutions. This corresponds to a distance of 39 AU (one AU, or Astronomical Unit, is the average distance between Sun and Earth, or roughly 150 million kilometers). This can be compared to the outermost planet Neptune, located 30 AU from the Sun. The outer edge is located at the 2:1 mean motion resonance with Neptune, which means that objects near this edge orbit the Sun once during the time it takes Neptune to complete two revolutions. This corresponds to a distance of roughly 48 AU.

Beyond the Edgeworth-Kuiper belt we find the scattered disk. It consists of objects that have had their originally circular orbits strongly perturbed by Neptune. They are characterized by large eccentricities and often have substantial inclinations. The perihelion distances typically fall between 30-40 AU, i.e., between the orbit of Neptune and the inner edge of the Edgeworth-Kuiper belt. The aphelion distances can be larger than 80 AU from the Sun.

However, these distances are very modest compared to that of the largest reservoir of comets in the Solar System – the Oort cloud. The comets in the Oort cloud have more or less circular orbits located 10,000-50,000 AU from the Sun. At such distances the gravitational attraction of the Sun is rather weak, and galactic tides start to become comparable to the pull of the Sun. The galactic tide is essentially the combined gravitational force of the stars and molecular clouds scattered throughout the disk of our galaxy, the Milky Way. The gravitational attraction from individual stars that temporarily come close to the Sun may also become comparable to that of the Sun. Due to such perturbations, the orbital planes of the comets have obtained random tilts, so that a given comet may have any possible inclination. Therefore, the comets in the Oort cloud are spread within a more or less spherical envelope at very large distances from the Sun.

Transport routes

The comets in the Jupiter family, and most Halley-type comets, are believed to originate from the scattered disk or the outer parts of the Edgeworth-Kuiper belt, but they have taken very different routs to reach their current orbits.

Computer simulations of the dynamics of comets show that comets in the Jupiter family slowly are dragged into the inner parts of the Solar System from the scattered disk or the outer parts of the Edgeworth-Kuiper belt, due to the influence of the gas giants. This process normally starts when Neptune changes the orbit of a distant object in such a way that it starts to feel the gravitational force of Uranus at the inner parts of its orbit. Thereafter, Uranus is modifying the orbit further, and is passing on the object towards Saturn. Finally, Saturn directs the object towards Jupiter, which subsequently creates the typical orbit of a Jupiter family comet. This is a very slow process that may take hundreds of thousand or millions of years to complete. The fact is that we can observe objects that are in the midst of this transport route. They are called Centaurs and orbit the Sun at distances that typically fall between the orbits of Saturn and Neptune. Some Centaurs even display comet activity in spite of their large distance to the Sun, like the comets 95P/Chiron and 29P/Schwassmann-Wachmann 1. Both objects are unusually large for being comets (Chiron has a diameter of about 200 kilometers), which is why we can see them over such large distances. This means that the inner Solar System has been visited by extremely large and bright comets during its long history.

Halley-type comets follow a completely different orbital evolution. Typically, Neptune starts to change the orbit of an object in the scattered disk or the Edgeworth-Kuiper belt in such a way that the aphelion distance increases dramatically, while the perihelion distance remains at about 30-40 AU from the Sun. Eventually, such objects can be 10,000 AU from the Sun at aphelion, where they are exposed to the galactic tide and the gravitational force of nearby stars. These forces may change the inclination of the orbit, but may also decrease the perihelion distance. This means that the comet periodically approaches much closer to the Sun than previously, and it may cross the orbits of Jupiter or Saturn. If this happens, these gas giants may change the orbit further by bringing the aphelion point back to the planetary region, while the perihelion distance remains roughly the same. In such a way, another Halley-type comet has been created.

However, it is not only the scattered disk and the Edgeworth-Kuiper belt that provides comets to the inner Solar System. Galactic tides and the gravity of nearby stars can perturb the orbits of comets in the Oort cloud, so that they start to fall towards the inner parts of the Solar System on parabolic orbits. Eventually, when they reach our part of space, we see them as new comets. If a new comet is unaffected by the planets it will simply return to interstellar space, and it is very likely that it never will come back again. However, a small disturbance from Jupiter may slow the comet down a bit, which forces it to return repeatedly although one need to wait for hundreds or thousands of years between each return – the comet has become long-periodic. Some long period comets can be transformed to Halley-type comets over time, which means that some of these object originally may have come from the Oort cloud.

The active comet

When a comet nucleus is far from the Sun (about three times as distant from the Sun as Earth), the temperature is too low for the frozen volatiles to sublimate at a high rate. The comet nucleus is then said to be inactive, and it can only be seen with the largest telescope if visible at all. If the comet instead approaches sufficiently close to the Sun it starts to heat up and the frozen species are vaporized – we say that the comet has become active. Solid grains of silicates, sulfides and organic species are liberated from the surrounding ice and are dragged along with the outwelling gas that rushes out into space. A dusty gas cloud is formed around the comet nucleus which is called a coma. A coma can have a diameter of 100,000 kilometers, which is ten times larger than Earth. The coma contains large-scale structures like jets since the nucleus outgassing is not evenly distributed across the surface of the nucleus. The coma is sufficiently thick to hide the nucleus from view. If we also remember that the inactive nucleus is distant and faint, it means that comet nuclei rarely can be observed at all, except from flyby spacecraft.

The solid grains soon lose contact with the gas, and their future orbits in space are only determined by two factors – the solar gravity and the solar radiation pressure. If solar gravity alone would influence the grains, they would follow trajectories around the Sun that resemble the orbit of the nucleus itself. However, when the radiation pressure is added, it means that the grains are pushed further away from the Sun compared to the nucleus, which means that they are smeared into a curved structure called the dust tail. This tail can be seen from the Earth due to the sunlight reflected by the grains. Color photographs shows that the dust tail has a yellow or white color, i.e., the same color as the Sun.

Hale_Bopp

Comet C/1995 O1 (Hale-Bopp). The yellow-white comet dust tail consists of small dust grains that reflect the solar light. The blue plasma tail consists partially of ionized carbon monoxide that absorbs and re-emit the blue light of the Sun.
Copyright: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria
Original image: http://en.wikipedia.org/wiki/File:Comet_Hale-Bopp_1995O1.jpg


The gas molecules in the coma have arrived to a very hostile environment. No longer protected in the nucleus interior, they are exposed to hard ultraviolet radiation from the Sun that literally smash them to pieces. Molecules and their fragments (radicals and atoms) are also ionized by the solar light, which means that they loose one or several electrons. This process make them electrically charged, which means that they start to interact with the solar wind. The solar wind consists of fast electrically charged particles that emanates from the Sun and drags along the solar magnetic field. The ions from the comet are picked up by this outwelling magnetic field, are swept backwards and therefore forms a structure known as the plasma tail. The plasma tails of comets have a clearly blue color in photographs. The blue color originates from singly ionized carbon monoxide, that only absorbs and re-emits the blue light of the Sun. However, the most common gaseous species in the coma in terms of number is atomic hydrogen and hydroxyle (a radical consisting of a hydrogen atom and an oxygen atom). These are the photodissociation products of the water molecule, and form when water is smashed to pieces by the ultraviolet radiation. The solar radiation that is absorbed and re-emitted by these species cannot be seen by the human eye, but can be detected with ultraviolet detectors on spacecraft.

Comet tails can become huge. In some cases, they stretch over distances larger than that between the Sun and Earth, i.e., more than 150 million kilometers. When a bright comet with such tails passes Earth, it gives rise to a spectacular show. Historical sources speak of comets that were bright enough to be seen in full daylight, and comets have been seen with tails so long that they stretched across the sky, from one horizon to the opposite one.

Why are comets important from a scientific point of view?

One of the most fascinating problems of astrophysics is to understand our Solar System. When did it form, and how did it look like when it was very young? How did it evolve and why does it look the way it does today? What events led to the formation of an environment that was suitable for the emergence of life (i.e., our planet)? Will Earth remain to be an environment suitable for life, or are there processes in the Solar System that threaten our survival?

Some more specific questions we would like to answer are the following. What was the chemical composition of the Solar Nebula, i.e., the could of gas and dust from which the Solar System formed? How did dust grains form out of the cooling hot gas? How did the properties of the Solar Nebula change with distance from the Sun, and to what degree did material from different parts of the Solar Nebula mix? Why and when did planetesimals start to form, that later grew to embryos and eventually to planets? What was the internal structure and physical properties of these planetesimals?

To answer this type of questions today, 4.6 billion years after the Solar System formed, is not easy. The Solar System has changed beyond recognition during its lifetime and there are not many things from its earliest history left to study. Among all the bodies in the Solar System, comets appear to be the ones that have changed the least. Comets look more or less the same as when they formed 4.6 billion years ago, which makes them unique. If we want to learn about the earliest epochs of the Solar System, the study of comets is extremely important.

What makes us think that comets are primordial and rather unaltered bodies? Firstly, due to their sizes. Comets are too small to have experienced a high level of geologic activity. Basically, their content of radioactive material has been too small to generate the heat that is necessary to drive such activity. Secondly, due to their low heat conductivity. For active comets, the solar heat is strong enough to erode the very surface by sublimation. However, the high porosity of comets means that they conduct heat very poorly, which means that the solar heat does not penetrate very far. Ice at rather shallow depths are therefore probably completely pristine. Thirdly, it is not likely that comets have experienced substantial alterations due to collisions. The reason is that the number of objects in the scattered disk is rather low, which means that they collide rarely with each other.

The best evidence that comets have not experienced substantial heating or other forms of alteration, and therefore contain more or less unchanged Solar Nebula material, is that they still are so rich in highly volatile species like carbon monoxide. Therefore we can learn about the chemical and physical properties of the Solar Nebula by studying comets from ground or by the aid of spacecraft. By studying the cometary grains and the internal structure of nuclei, we learn about the earliest stages of planetary formation. By studying comets up close, we can therefore learn about the very earliest stages in the history of our own planet.

Another fascinating thing about comets is their high content of organic materials and water. Without organics and water on the young Earth, life would never have formed. The question is to what extent carbon and water in the biosphere was a natural component of the material from which the Earth formed, and how much that was brought here subsequently. For example, we know that Earth formed through the mergers of planetesimals and embryos during a period of time that lasted for 50-150 million years, starting 4.6 billion years ago. But we do not know how much water that could be found at the surface of Earth at that time, or what types of organic species that were available.

Subsequently, about 0.6 billion years after the formation of Earth, the number of large impacts increased dramatically during an epoch known as the Late Heavy Bombardment (LHB). The LHB was most likely caused by a fundamental change of the giant planet orbits, from an original rather compact configuration close to the Sun, to the distant and well-separated orbits we see today. During this process, thousands of asteroids and comets were sent towards the inner parts of the Solar System, where they collided with Earth and the other terrestrial planets. The large impact structures seen on the Moon today were formed during the LHB. It is reasonable that a large amount of organic substances and water was brought to Earth during this event. We also know that the first evidence of life on Earth comes from the time right after the LHB. The question is therefore – how important was the water and organic compounds that comets brought to Earth during the LHB for the emergence of life that followed? Would life have emerged also without these cometary impacts, or did they bring vital components not present previously?

The fact is that large objects still impact on Earth now and then (typically a few hundred of thousands years pass between large impacts). Along with super volcanos, such impacts are the most violent natural disasters we have on Earth. The effects on Earth’s climate will be global if the impacting object is about a kilometer in diameter or larger.

If the object impact takes place on land, enormous amounts of dust is launched into the atmosphere, furthermore, wildfires are ignited that may spread over entire continents. What follows is a so called atomic winter, when the dust and ash in the atmosphere prevents the sunlight from reaching the surface of Earth. Our planet then becomes very cold, plants can no longer survive, which also causes a massive extinction of animal life. If the impact occurs at sea, there is in addition a huge tsunami with waves reaching tens or hundreds of meters that can flood vast areas. It is believed that the cause of the extinction of the dinosaurs 65 million years ago was a large impact at the Yucatan peninsula in Mexico.

The risk of impacts is yet another reason for studying comets. How many comets are there, what orbits do they have, and do any of them threaten Earth? How large are the comets, what masses do they have, and what would happen if they entered Earths atmosphere? How does the effects of an impact depend on the physical properties of the nucleus?

Comet research is a rather young science. Normally we consider 1950 as the year when the modern comet astronomy was born. This is the year when Fred Whipple for the first time made an accurate description of the properties of comet nuclei, and when Jan Oort discovered the distant reservoir of comets that bears his name. Another important era began in 1986 with the first spacecraft to visit Comet 1P/Halley, e.g., Giotto. These were followed by other spacecraft in 2001 (Deep Space 1 to Comet 19P/Borrelly), in 2004 (Stardust to Comet 81P/Wild 2), in 2005 (Deep Impact to Comet 9P/Tempel 1), and in 2010 (EPOXI to Comet 103P/Hartley 2). Every new spacecraft to a comet has made fascinating discoveries. A new era in the exploration of comets starts in 2014 when the European Space Agency (ESA) spacecraft Rosetta arrives at Comet 67P/Churyumov-Gerasimenko. Rosetta will not only fly past the comet, as has been the case with previous spacecraft, it will go into orbit around the nucleus, and even send a lander to the nucleus surface. Rosetta will undoubtedly revolutionize our understanding of the oldest members of the Solar System – the mysterious comets.

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