Their occurrence is unpredictable and their return uncertain. Where do comets come from? Do they even belong to the Solar System? Researchers have attempted to answer these questions throughout history.
The comet's coma
The coma is the comet's atmosphere. Unlike Earth's atmosphere it is extremely thin, comparable to a high vacuum chamber in one of our laboratories on Earth. It is also unstable.
The comet nucleus releases a constant stream of molecules and dust particles with high velocities, developing a thin coma. The complex interaction with solar radiation and the solar wind forms the tails. Ground-based spectrometers have been used to measure the composition of the coma, but instruments on spacecraft deliver more precise results. Although chemical and physical processes in the coma alter the molecules, they provide important information about the composition of the nucleus. The comparison of data from interstellar molecular clouds, Earth's composition and the make-up of stars delivers clues to how our planetary system developed. The proportionate quantities of isotopes, such as deuterium and hydrogen, are particularly significant here.
Approximately 30 so-called parent molecules found in cometary comae were known before the Rosetta mission. Often, it is the decay products – the daughter molecules – that are measured, as they provide indications of the original compounds. Numerous organic substances were found in addition to the main constituents of water, carbon monoxide and carbon dioxide. It has long been speculated that comets introduced important building blocks of life to Earth when they collided with the planet.
How did the Solar System form?
Our planetary system formed 4.56 billion years ago from a rotating cloud of hydrogen, helium and dust. Gas and dust originated from the earlier explosion of one or several stars. Rock samples from meteorites – or more precisely the decay products of the radioactive elements they contain – from the Solar System provide an indication of its age.
Gravity subsequently caused the cloud to contract, while a centrifugal force compressed it into the shape of a disc. The Sun formed at the centre of this disc, capturing almost all (99.8 percent) of the mass contained in the cloud. The temperature of the disc decreased gradually from the interior to the exterior, causing solid bodies with high melting temperatures (rock, iron) to develop close to the early Sun, while other solid bodies with lower melting temperatures (frozen water, ammonia, methane) formed further away. Initially small, the solid bodies became ever-larger objects – so-called planetesimals – before finally forming the planets and their moons. At the outer boundaries of the disc, the rocky and icy protoplanets that went on to become giant planets initially captured most of the remaining gas, growing to their current dimensions. Mercury, Venus, Earth and Mars – the rocky inner planets formed in the interior of the disc, which contained very little gas.
Comets and asteroids in all shapes and sizes are considered almost untarnished remnants of former planetesimals. They are witnesses of the birth of the Solar System, which makes them particularly fascinating for researchers.
The early Earth: Oceans and burgeoning life
We know very little about what the early Earth 3.8 billion years ago was like. We do not have any rock samples from this period. But it is possible to get an approximate idea by analysing meteorites and rocks from the Moon and Mars. These samples indicate that an ocean of magma covered the early Earth. Back then, the scorching atmosphere contained rock vapour, hydrogen and helium. Earth eventually lost this original atmosphere. Violent solar eruptions during the socalled T Tauri phase, which occurred when the early Sun continued to contract and a thermonuclear fire began at its core, simply swept the original atmosphere away.
The origin of the carbon-dioxide-rich, secondary atmosphere and the oceans is a controversial topic. Some researchers believe they both emerged approximately four billion years ago during a bombardment of the Earth by planetesimals (asteroids and comet nuclei) containing water. Other scientists argue that Earth's original building blocks already contained enough water and carbon dioxide and that the oceans and secondary atmosphere are the product of volcanic activity.
Today, we know that the isotope chemistry of the water found on Earth does not match the frozen water on most comets. This would lean more towards the theory of outgassing, asteroid bombardment, or a mixture of both.
The composition of the atmosphere changed substantially 3 to 2.5 billion years ago after the emergence of life. The Earth's carbon dioxide is largely bound in limestone, and the atmosphere now consists mainly of nitrogen and biologically produced oxygen.
Where did comets form? How have their orbits developed over time? Astronomers have discovered three reservoirs in entirely different regions of the Solar System.
The hypothetical spherical Oort Cloud (named after Jan Hendrik Oort, 1900-1992) is located in the outermost regions of the Solar System, up to 100,000 astronomical units (1 AU = distance from Earth to the Sun, approximately 150 million kilometres) from the Sun. This cloud is predicted to have many thousands of billions of comets with a total mass several times greater than Earth. The ring-shaped Kuiper-Edgeworth Belt (predicted by Gerard Peter Kuiper, 1905-1973, and Kenneth Essex Edgeworth, 1880-1972) is situated in the outermost planetary region of the Solar System and the total mass of its comets is merely a fraction of Earth's.
The comets in these two reservoirs developed in the icy regions of the protoplanetary disc during the formation of the Solar System. Later on, changes in the orbits of the larger planets and the corresponding gravitational forces moved the comets to their current locations. The Main Asteroid Belt, located between Mars and Jupiter, is home to another recently discovered, smaller reservoir. These comets formed in the warmer regions close to the Sun and do not display much activity.
Gravitational forces exerted by large planets, especially Neptune, or even by objects drifting in the Milky Way sometimes knock comets out of their orbits, bringing them close to the Sun and Earth, where they become active and therefore visible. Depending on where and how they were formed, comets have different trajectories and orbital periods. Long-period comets with orbital periods of more than 200 years generally come from the Oort Cloud.
Asteroids and comets
Along with comet nuclei, asteroids are considered leftover planetesimals or minor planets and, as such, witnesses to the origin of the Solar System. Most asteroids orbit the Sun and are located in the Main Asteroid Belt between Mars and Jupiter. There are also near-Earth asteroids with orbits that can intersect the Earth's, as well as asteroids with orbits that draw even closer to the Sun. The orbits of asteroids and near-Jupiter comets are therefore closely adjacent to one another.
The chemical composition of asteroids differs from that of cometary nuclei in that the former contain fewer volatile elements, such as water, carbon oxides, methane and ammonia, but more metals and silicates. However, the transitions of the composition of comets and asteroids can be very gradual. Furthermore, among asteroids we find bodies that formed significantly more quickly – in several million years – than cometary nuclei and, in the process, have been heated to the melting temperature of iron by the decay of short-lived radioactive elements. Such asteroids are differentiated, similar to planets. Some of these have been subject to collisions, so that we see asteroids that consist almost entirely of iron. On the other hand, the cometary nuclei we have studied formed a lot more slowly, in a few tens of millions of years. As such, they have remained very cold and pristine and have been involved in very few collisions.
Asteroids and comets have been linked to the origins of life on Earth and are speculated to have seeded the Earth with organic compounds upon impact billions of years ago. They may have been the building blocks of the first simple organisms on Earth.
First comet exploration spacecraft
In 1957, the dawning age of space travel brought with it new opportunities to explore the Solar System. Robotic spacecraft were sent to the Moon, Venus and Mars to study our cosmic neighbourhood from orbit and directly on site.
Many scientists were also keen to visit a comet. But compared with planets, comets are extremely small, which makes approaching them a far more complicated affair. What is more, the scientists only knew with certainty of a few comets that would soon come close to Earth. Fortunately, Halley's Comet was scheduled to return in 1986.
In total, five spacecraft were sent on their way to rendezvous with Halley: the Soviet Vega 1 and Vega 2, the Japanese Suisei and Sakigake, and the European Giotto probe. On 14 March 1986, travelling at a speed of 247,000 kilometres per hour, Giotto flew past Halley at a distance of just 600 kilometres, where it was able to take measurements and produce images of the nucleus and the coma. But collisions with dust particles quickly destroyed most of the instruments. Giotto also flew past Comet Grigg-Skjellerup in 1992.
A few years later, significant results were produced by three NASA spacecraft: Deep Space 1 reached the Comet Borrelly in 2001. Just three years later, NASA's Stardust mission collected dust particles from the coma of Wild 2. The NASA Deep Impact mission was truly spectacular – the spacecraft fired a 372-kilogram projectile at Comet Tempel 1 on 4 July 2005, using cameras and measuring devices to record the impact from a distance of 8600 kilometres.
The open questions
Comets contain the primordial material of the Solar System. They are leftovers from the period – over 4.5 billion years ago – when the first planets began to form around the young Sun. Due to their small size and their location in cold regions far from the Sun, comets have changed very little. The planets, on the other hand, have changed greatly since their formation – in particular, there are hardly any traces of the earliest periods in Earth's history. Hence, we do not know exactly if the water in the oceans came from the interior of the Earth or was brought here by other bodies such as comets. We also still do not know how life arose on our planet.
Consequently, comets are important witnesses of the early development of our planetary system. The Rosetta mission was launched to answer the big questions regarding the origin of comets and the development of the Solar System.
- What is the nucleus of a comet made of, what shape does it have and how does it evolve?
- What chemical elements, molecules and isotopes make up the nucleus, and what minerals can be found there?
- What physical properties, such as density, solidity, structure and thermal behaviour does the cometary nucleus possess?
- How does the comet's activity that leads to the development of the coma and the tail in the vicinity of the Sun occur?
- How were comets formed – and where? Do they bear a resemblance to materials in interstellar space?
- Are comets really witnesses to the origin of our Solar System, and what can we learn from them?
To answer these questions, the Rosetta mission has been accompanying and investigating Comet 67P/Churyumov-Gerasimenko for two years – from a distance of several kilometres with the orbiter, and directly on the surface with the Philae lander.