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COMETS - RELICS FROM THE BIRTH OF THE SOLAR SYSTEM

Now and then a bright comet appears in the night sky. It develops a fuzzy head and a long tail while it slowly drifts across the constellations. Such a sight has always fascinated mankind and given rise to stories, myths, and superstition. It has also inspired scientist to explore the nature of comets and it turns out that they are not only beautiful to watch but also are extremely interesting objects!

For example, comets are the oldest and least processed bodies orbiting the Sun and therefore constitute a unique source of knowledge about the birth and early evolution of our Solar System. We know that comets bombarded the young Earth and that much of the water we drink daily once was part of cometary nuclei orbiting the Sun outside the orbit of Neptune. We know that comets are rich in organic molecules of which some could have been involved in the formation of life on Earth. We also know that comets impact Earth and in principle could end human civilization in a gigantic blast.

Below you will find some interesting information about comets, starting with the physical and chemical properties of the comet nucleus. Then the orbits of comets are described and how the comets are transported to those orbits from their ancient birth places. Next, the characteristics of an active comet are described, i.e. the properties of the coma and tails. Finally, it is described why comets are scientifically interesting.

The nucleus of Comet 9P/Tempel 1 imaged by Deep Impact. Credit: Univ. of Maryland, JPL-Caltech, NASA.

The Comet Nucleus

The impressive head and tails of a comet originates from a small solid body called the "nucleus". A typical comet nucleus is less than 10 kilometers across and is darker than charcoal, only reflecting 2-4% of the incoming solar light. The nucleus has a highly irregular shape and displays a variety of surface features, such as mixtures of either very rough or very smooth areas, depressions, ridges, hills, and craters. The nucleus is very porous, which means that a large fraction of its volume (60% or more) is just empty space. This makes comets very fragile, and indeed dozens of comet nuclei have been observed to break up and sometimes disintegrate completely. The high porosity and low strength is a consequence of the fact that comets are made up of weakly bounded grains, typically being around a micrometer in size (i.e., one part in 1000 of a millimeter).

The grains themselves basically consist of four different kinds of material. Roughly one third of the mass is in the form of silicates and sulfides, another third consist of organic species, while the rest is volatiles. The rest of this section is devoted to a more detailed description of these substances.

Silicates is a large family of substances rich in silicon, oxygen, and various metals, and is the stuff that rocks and stones are made of. About half of the cometary silicate is in the form of 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. Instead, if the two metal atoms are iron we have another olivine called fayalite. Comets seem to be very rich in forsterite but contain less fayalite. Another half of the cometary silicate is in the form of pyroxene, which consists of one metal atom, one 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 appear to be richer in enstatite than ferrosilite, hence comet silicates are generally magnesium-rich. Olivine and pyroxene can also be found on Earth, and in fact, they dominate the material found in Earth's upper mantle.

Sulfides are chemical compounds consisting of sulfur mixed with iron and nickel. Troilite is the simplest member of the sulfides. It consists of one iron atom and a sulfur atom, and is common in comets. The most complex sulfide identified in a comet is pentlandite, with eight sulfur atoms, and nine atoms of iron and nickel in total.

Moving on to the organic species, they all have one thing in common - they are all based on the chemistry of the carbon atom. Actually, carbon is the single most important atom in the periodic table since it is so efficient in binding itself to other atoms, which means that it can form endless variants of molecules. It is this diversity which makes organic molecules the most suitable building blocks of life. The living organism needs a large "tool box" of molecules in order to solve all sorts of tasks, and only the family of organic species is large enough to provide what is needed.

A particular group of organic molecules is common in comets, namely the polycyclic aromatic hydrocarbons, or PAHs. The most simple PAH, benzene, consists of six carbon atoms forming a ring, with six hydrogen atoms attached to the carbon. By combining such rings other PAHs are formed, such as naphthalene (two rings), penanthrene (three rings), and pyrene (four rings). All those PAHs have been found in comet material. On Earth, PAHs are formed during incomplete combustion of carbon-rich material, e.g. when burning wood. In fact, naphthalene is extracted from charcoal (and happen to be the active substance in moth balls). Other environments where PAHs are formed are in burning cigarettes, in car exhaust fumes, and in the frying pan! Comets also contain other organic substances, of which the most interesting discovery so far is glycine - the simplest amino acid. Living organisms use amino acids to build proteins, which are macromolecules that perform all sorts of tasks in the cell. To find such pre-biotic molecules in interplanetary space is extremely interesting!

However, the thing that makes comets very special is that they are rich in volatiles. Volatiles are basically substances that are liquid or gaseous at room temperature, but have solidified at the low temperatures of interplanetary space and become ice. Ordinary water ice is the most common volatile in comets, while carbon monoxide and carbon dioxide come in second and third. Methanol, hydrogen sulphide, formaldehyde, methane, ammonia, and hydrogen cyanide are present on a percent level. Methanol is the simplest alcohol, while hydrogen sulphide gives rotten eggs their unpleasant smell. Formaldehyde is a disinfectant which also is used in industry to produce plastic, while methane (on Earth) is formed by bacterial putrefaction of organic substances. Ammonia gives window polish its strong and irritating smell, while hydrogen cyanide is a highly toxic substance. If we would bring back a sample of comet material to Earth it would certainly have a repulsive smell!

Jupiter, here imaged by the Hubble Space Telescope, has a major influence on the orbits of many comets. Credit: NASA, STScI.

The Cometary Orbits

Objects that are gravitationally bound to the Sun move on orbits with the same shape as the mathematical figure called the ellipse. The degree of elongation, or the eccentricity, is very small for the planets (their orbits are almost circular) but is generally large for the comets. It means that the distance between the comet and the Sun can change dramatically during an orbital period, 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 farther point is the aphelion. The orbits of the planets are more or less confined to the same plane, called the ecliptic. However, the orbits of comets can be substantially inclined with respect to the ecliptic - they are said to have a high inclination.

Comets can be grouped into different families based on the properties of their orbits. The Jupiter Family Comets typically have orbital periods shorter than 20 years, and move on orbits rather close to the ecliptic. They have got their name from the fact that their aphelia are located at similar distances from the Sun as Jupiter, and this massive planet occasionally modifies the orbits of these comets, which are completely under its gravitational control.

We also have the Halley Type Comets which differ from the Jupiter Family Comets by having longer orbital periods (up to 200 years), and in general having substantially larger inclinations. Comet Halley itself has such a large inclination that the orbit has "flipped over" and the comet moves clockwise around the Sun (as seen from a point high above Earth's north pole), while all planets, asteroids, and most other comets move counter-clockwise.

Finally, there are long-period comets with orbital periods that can be measured in thousands of years. There are even non-periodic comets that no longer are gravitationally bound to the Sun, and move on orbits with the same shape as the mathematical figure called the parabola. Such objects generally only visit the Sun once, never to return again.

How come that we have this rich variety of cometary orbits? Why are some comets in the Jupiter Family, while others are Halley type or non-periodic comets? To understand that we need to study the birth place of most comets, the Edgeworth-Kuiper belt, and how the comets got from that belt to their present orbits.

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

Presently more than 1000 Edgeworth-Kuiper belt objects are known, all discovered after 1992 except Pluto which was discovered in 1930. The Edgeworth-Kuiper belt consists of two different parts. The "cold disk" contains objects that never had their orbits strongly affected by the gravity of Neptune. They typically move on circular orbits near the ecliptic, at the places where they were born as they condensed from the Solar Nebula. The inner edge of the cold disk is located at the 3:2 resonance with Neptune, meaning that these objects move twice around the Sun in the same time as Neptune makes three revolutions. This is equivalent to 39 AU from the Sun (one AU, or Astronomical Unit, is the distance between Sun and Earth, 150 million kilometers). The outer edge of the cold disk is located at the 2:1 resonance with Neptune, meaning that these objects move once around the Sun in the same time as Neptune makes two revolutions, which corresponds to about 48 AU.

The other part of the Edgeworth-Kuiper belt is called the "scattered disk" and consist of objects that have had their original orbits changed by Neptune. They are characterized by high eccentricities and higher inclinations than cold disk objects. The perihelion distances generally fall between 30-39 AU, i.e. between the orbit of Neptune and the inner edge of the cold disk. It is believed that both Jupiter Family Comets and Halley Type Comets originate from the scattered disk, although they use different routes to reach their present orbits.

Jupiter Family Comets are believed to be slowly dragged in from the Edgeworth-Kuiper belt by the gas giants. Typically, Neptune starts changing the orbit of a scattered disk object in such a way that the object start to feel the gravitational influence of Uranus in parts of its orbit. Next, Uranus is modifying the orbit further, passing the object on to Saturn. Finally, Saturn passes the object on to Jupiter, which then creates the typical Jupiter Family Comet orbit. This is a slow process, needing millions of years for completion. The fact is that we actually can observe the objects that are in transition from the scattered disk to the Jupiter Family Comet population. They are called Centaurs and orbit the Sun at distances stretching from that of Saturn to that of Neptune. Some of the Centaurs even display cometary activity although being far from the Sun, such as 95P/Chiron and 29P/Schwassmann-Wachmann 1. Both these objects are unusually large to be comets (Chiron has a diameter of 200 kilometers), which is why we see them at all over these great distances. With time, as they come closer to Sun and activity increases, such objects will evolve into monstrously large comets that will put on a very impressive show!

The Halley Type Comets follow a different path. Typically, Neptune will start to change the orbit of a scattered disk object, so that the perihelion distance remains in the 30-39 AU range, but increasing the aphelion distance dramatically by giving the orbit a very high eccentricity. Eventually, such objects can be 10000 AU from the Sun when at aphelion! At such large distances the gravitational pull from the Sun is very weak, and so-called galactic tides start to be comparable in strength. The galactic tides are basically the combined gravitational pull from the stars and molecular clouds located throughout the disk of our galaxy, the Milky Way. The galactic tides may change both the inclination of the orbit, as well as decreasing the perihelion distance. This means, that the next time the object returns to the planetary region, it may not stop just outside Neptune but perhaps cross the orbits of Saturn and Jupiter. If so, those giant planets can modify the orbit further, by bringing the aphelion point back to the planetary region while not changing the perihelion distance substantially. Thereby, yet another Halley Type Comet has been formed.

What about the long- and non-periodic comets? They come from yet another reservoir of icy objects - the Oort cloud. The Oort cloud itself has been formed by processes similar to those forming Halley Type Comets, i.e. that the aphelion distance of objects originally orbiting in the gas giant region of the Solar System are increased dramatically, to 50000 AU or more. In this case, however, the galactic tides also increase the perihelion distance, forming objects that permanently resides very far from the Sun. The Oort cloud comets are on the verge of leaving the Solar System completely (in physical language their kinetic energy is almost equally large as their potential energy). They are sensitive to disturbances, such as the gravity of passing stars, which can cause them to fall back towards the inner Solar System on parabolic orbits. When they finally reach our region of space, we see them as non-period comets. If a non-periodic comet is not affected by the planets, it will simply return to interstellar space, most likely never to return again. However, a slight perturbation from Jupiter may slow down the object and force it to return repeatedly, although one might have to wait several hundred our thousand years for the return - the comet is a long-periodic one.

Comet C/1995 O1 (Hale-Bopp) with its yellow dust tail and blue plasma tail. Credit: Robert Allevo.

The Active Comet

When a comet nucleus is far from the Sun (roughly more than three times farther from the Sun than is 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 can only be seen through the largest telescopes, if visible at all. However, if the comet nucleus gets close enough to the Sun it starts to heat up and the volatiles rapidly turn to vapor - the comet is then said to be active. Solid grains of silicates, sulphides, and organic material become liberated from the surrounding ice and is dragged along with the gas which rush into space. As a result, a dusty gas cloud is forming around the comet nucleus, that is called the "coma". A typical full-grown coma is on the order of 100000 kilometers in diameter, which is ten times larger than Earth. It often contains large-scale structures since outgassing on the nucleus is not evenly distributed on the surface. The coma is thick enough to hide the nucleus from view. Remembering that inactive nuclei are far away and faint, it means that comet nuclei are rarely observed directly at all, except during spaceprobe flybys.

The solid dust particles soon lose contact with the gas and their future trajectories through space are only affected by two things - solar gravity and solar radiation pressure. If only solar gravity was active the dust grains would have started to orbit the Sun on trajectories similar to that of the comet nucleus itself. However, adding the solar radiation pressure means that the grains are pushed farther away from the Sun than is the nucleus, thereby being dragged out in a large curved structure called the "dust tail". This tail can be seen from Earth due to the sunlight reflected by the grains. On color photographs the dust tail looks yellow or white, i.e. it has more or less the same color as the Sun.

The gas molecules in the coma find themselves in a rough environment. No longer protected by the safe haven of nucleus interior, they are exposed to hard ultraviolet radiation from the Sun that literally tear them apart. Molecules becomes ionized by the solar radiation, which means that they lose one or several electrons. That process makes them electrically charged which in turn means that they start to interact with the solar wind. The solar wind consist of fast electrically charged particles that emanates from the Sun, dragging the solar magnetic field with them. The ions from the comet get picked up by this outwelling magnetic field, therefore they are swept outward and fall into a structure called the "plasma tail". Cometary plasma tails have a distinct blue color on photographs. The blue color comes from singly ionized carbon monoxide, which absorb and re-emit only the blue light from the Sun. However, the most common gas species in the coma by number is hydrogen, atomic oxygen, and hydroxyl (the latter consisting of one oxygen atom and a hydrogen atom). These are the photodissociation products of water molecules, which are smashed to pieces by ultraviolet solar radiation. The solar radiation absorbed and re-emitted by these species cannot be seen by the human eye, but can be observed with ultraviolet detectors on spacecraft.

The tails of a comet can become enormous. In some cases they can stretch longer than the distance between the Sun and Earth, i.e. more than 150 million kilometers. When a bright comet with such tails passes close to Earth it can be a spectacular show. The historical record tells about comets bright enough to be seen in broad day light, or comets having tails that stretch from one part of the horizon to the opposite side!

Artist's concept of a large asteroid impacting Earth. Also comets impact Earth and pose a threat to human civilization. Credit: Don Davis.

Why Comets are Scientifically Important

One of the most fascinating problems in 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 did it end up looking the way it does today? Which were the events that led to the creation of an environment that was suitable for the formation of life (i.e. our planet)? Will Earth continue being a suitable place for life, or are there processes active in the Solar System that threatens our survival?

Some more specific questions that we would like to answer are the following. What was the chemical composition of the Solar Nebula, i.e. the cloud of gas and dust particles that collapsed to form the Solar System? Were the dust grains in the Solar Nebula predominantly formed here or did they come from somewhere else in the Galaxy? How did the properties of the Solar Nebula change with distance to the Sun, and did different parts of the Solar Nebula exchange material? Why and how did protoplanetesimals form, that later grew to planetesimals and later planets? What was the internal structure and properties of those protoplanetesimals?

To answer those questions today, 4.6 billion years after the formation of the Solar System, is not easy. The Solar System has changed beyond recognition since those days, and there are not many evidence left behind for us to study. However, of all the bodies in the Solar System, comets appear to be the least evolved ones. In fact, we believe they look pretty much the same as when they formed 4.6 billion years ago, which makes them truly unique. Therefore, if we want to understand the earliest time of our Solar System, then we have to study comets! The comet astronomer is a sort of astro-archaeologist which study comets in order to learn about our earliest history.

What makes us think that comets are pristine unaltered bodies? Firstly, because of their size. Comets are too small to have experienced any major geological activity. They simply do not contain enough radioactive material to generate the heat necessary for such processes. Secondly, because of their low heat conductivity. For active comets, solar heating is strong enough to erode the very surface by sublimation. However, since comets are very porous it means that their heat conductivity is very low, and the solar energy does not penetrate very far. The interior of comets should therefore be rather unaffected by solar radiation. Thirdly, because comets are not likely to have experienced a lot of collisional processing. The reason for that is that objects in the scattered disk are few and rarely collide with each other.

The best evidence that comets never have experienced strong internal or external heating or other forms of processing, and therefore still contain virtually unaltered Solar Nebular material, is that they still contain a lot of very volatile species such as carbon monoxide. Therefore, by studying comets from Earth and with the aid of spacecraft, we can learn about the chemical and physical properties of the Solar Nebula. By studying cometary dust grains and the internal structure of comets, we learn about the very first steps of planetary growth. In fact, it would have been impossible for us to describe the earliest history of our home planet, had it not been for the comets!

Another fascinating thing about comets is their high content of carbon-rich material and water. Without the presence of organic substances and water on the young Earth, life would never have formed. The question is then to what extent the carbon and water in the biosphere was part of the material from which Earth itself was formed, and how much was provided later on. For example, we know that Earth formed by agglomeration of planetesimals about 4.6 billion years ago, and that the infall of material then decreased rapidly. However, we do not know how much water that could be found on the surface of Earth at that time, and what kind of chemical compounds that were available.

Then, about 0.6 billion years into the history of the Solar System, the number of large impacts suddenly increases explosively, in an event called the Late Heavy Bombardment (LHB). The large impact structures we see on the Moon were formed during the LHB. Recently, scientist have realized that the reason for the LHB probably was due to Jupiter and Saturn getting locked in a 1:2 resonance with each other, so that Jupiter orbited the Sun exactly twice as fast as Saturn. That created a lot of gravitational disturbances in the Solar System, sending thousands of minor bodies towards the terrestrial planets. First, a big shower of comets bombarded Earth, later came a wave with asteroids (in fact, it is believed that 99% of the original main asteroid belt population disappeared in the LHB, by collisions with the Sun and planets or being ejected out from the Solar System altogether).

We also know that the first signs of life on Earth appear soon after the end of the LHB. The question is then - how important was the water and organic substances delivered to Earth by comets during the LHB for the subsequent formation of life? Would life have formed anyway, without cometary impacts, or did the comets bring some important component not present earlier?

The truth is that the impact events that were so common during the formation of Earth and the LHB never stopped completely, just became more and more uncommon. In fact, large objects still impact Earth now and then (a few hundred thousands years generally passes between larger impacts). Such impacts are the most violent natural disasters occurring on Earth. For example, the impact of a kilometer-sized object with a velocity of 10 kilometers per second, is equivalent of detonating on the order of 100 million Hiroshima atomic bombs simultaneously! If impacting on land, such large explosions ignite wild fires that can spread over entire continents, and sends enormous amounts of dust and ashes into the atmosphere. What follows then is an "atomic winter", where dust and ashes in the atmosphere prevents solar light to reach the ground. Earth then becomes very cold, plants can no longer survive, which also causes animal mass extinction. If impacting at sea, an enormous tsunami will send floods reaching high up on land, killing everything in its way. It is believed that the reason for the mass extinction of dinosaurs 65 million years ago, was caused by a massive impact on the Yucatan Peninsula in Mexico.

The potential danger of impact is yet another reason for studying comets. How many comets are there, what are their orbits, and does anyone threaten Earth? What are the sizes and masses of comets, what happen when they enter Earth's atmosphere and how does the outcome of an impact depend on the physical properties of the nucleus?

Comet science is rather young. Normally, 1950 is considered to be the year of birth of modern comet astronomy, with the first accurate description of comet nuclei by Fred Whipple, and the discovery of the Oort cloud by Jan Oort. Another important era started in 1986 with the first spacecraft missions to Comet 1P/Halley. It has been followed by other spacecraft missions in 2001 (Deep Space 1 to Comet 19P/Borrelly), 2004 (Stardust to Comet 81P/Wild 2), and 2005 (Deep Impact to Comet 9P/Tempel 1). With each spacecraft mission, new fascinating discoveries are made. A new era in comet exploration will start in 2014 when the European Space Agency spacecraft Rosetta reaches Comet 67P/Churyumov-Gerasimenko. Rosetta will not only fly past the comet, as has been the case with previous spacecraft, but go into orbit around the nucleus and deliver a lander probe to the comet surface! So stay tuned, the adventure has just begun...

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