Comets are packages of rock and ice, speeding around the Sun in numberless highly elliptical orbits, with the star at one of the foci. They reside in the furthest outer areas of the solar system, either in the Kuiper Belt or the Oort Cloud. Kuiper Belt comets have short and intermediate orbital periods, not less than 20 and not more than 200 years, therefore visiting Earth more often. Their paths are eccentric, direct orbits, on the ecliptic plane, and most of them have an enemy in the giant Jupiter, as its gravity can completely derail their well-established trajectories. [1] On another hand, Oort Cloud comets have very long orbital periods, taking thousands of years and even more to fly by the inner planets, that is only if some external force, such as the gravity of another star pushes them towards the inner solar system. They have orbits randomly inclined to the ecliptic plane, and therefore they arrive in Earth’s neighborhood from different directions. [2] Comets are all about their tails. In fact, David Levy, one of the discoverers of the Shoemaker-Levy Comet believes that “comets are like cats: they have tails, and they do precisely what they want”. [3] As a comet approaches the Sun, solids and gases are released, and it develops a glowing atmosphere known as a coma, blown outward away from the Sun, as well as an ion and a meteoric dust tail reflecting light. In the words of astronomer Fred L. Whipple, comets are “dirty snowballs” [4], and some of that dirt is made of organic compound. [5] The ion tail is developed when gases are ionized by solar radiation. Since the solar wind travels very fast by the comet, the orientation of the ion tail is always against the Sun. The dust tail has a slightly different orientation than the ion tail. This material stays in individual orbit around the Sun, and therefore the dust tail is curved. [6] In contact with Earth’s atmosphere, it is the source of one of the most spectacular cosmic shows: meteor showers, popularly known as falling stars. Most importantly, comets are leftovers of the solar system formation, and hold the secrets of its beginnings. They formed slowly as leftover debris clumped together. Comets contain the building blocks of the solar nebula, material that has been pushed from the inner to the outer solar system. Some of these materials were organic compounds: carbon, oxygen, nitrogen, and hydrogen, but also interstellar silicates and ices such as water, ammonia, methane, carbon monoxide, and recently discovered ethane. Comet dust is composed of silicates and some metals. [7]
What about life? Where did it come from? How did it come into existence? These are difficult, unresolved questions tormenting scientists for centuries, while the origin of life remains one of the most puzzling and controversial issues. When Earth was formed over 4.5 billions years ago, the planet was inhospitable and inhabitable. From that point on until a billion years later, some processes occurred that led to a planet teeming with life. [8] Life on Earth started 3.8 billion years ago, as soon as the environment and the conditions became slightly favorable to ignite the spark of life. These favorable conditions allowed life to become firmly rooted in less than half a billion years. [9] To figure out how this happened and how could non-living matter turn into living matter, biologists imagined the process of evolution in reverse. Reversing evolution concluded that life on Earth started in the oceans, and in simple forms, such as carbon-chained molecules capable of copying themselves. [10] These organisms did not have bones or shells that could have been fossilized and preserved, and therefore the evidence of their existence is scarce. However, ancient rocks dating over three billion years ago have revealed stromatolites, fossilized colonies of these single cell organisms. Life was thriving in the oceans just a billion years after its beginnings. [11] But where did it come from?
The Russian biochemist A.I. Oparin, and the English biologist J.B.S. Haldane introduced the so-called primordial soup theory of the origin of life. This theory states that a primordial soup of amino acids was formed on Earth a little under four billion years ago from molecules generated by chemical reactions between gases like nitrogen, ammonia, methane, and hydrogen. These amino acids would have later developed into organisms. This theory however was demolished when it was discovered that the early atmosphere on Earth had an oxidizing nature, meaning no life could have formed in such conditions. [12]
But what do comets have to do with the origins of life? It appears that the speculation about their connection started when Halley’s Comet last visited, in 1986. At that time the Vega and Giotto missions observed that the famous comet had in its composition carbon, hydrogen, oxygen and nitrogen in proportions just like on Earth. [13] Comets also have in their structure organic molecules. The Deep Impact mission has revealed evidence of organic and clay particles in the comet Tempel 1. Similar organic molecules were discovered by the Stardust mission in the comet Wild 2. [14] However, the samples returned by Stardust did not contain clay, and therefore the existence of clay in comets is still controversial. [15] The Herschel Infrared Space Observatory has recently discovered that the comet Hartley 2 contains the same kind of water as Earth’s oceans, allowing the scientist to assume that comets must have played a significant role in the origin of water on the Blue Planet. [16] The data brought by all these missions led to the conclusion that comets do contain all the prerequisites for life: liquid water, organic molecules, and clay surfaces that allow catalytic reactions. Life could have first developed on a comet, and travel later to Earth and other planets or moons. [17] It sounds like a process of contamination: Earth could have been literally contaminated with life four billion years ago. Since life on Earth started rapidly in geologic terms, it could have arrived on the surface of the planet ready-made. An extreme theory supporting this idea is called the Panspermia Theory, claiming that life originated elsewhere and spread by spores or bacteria to habitable planets all over the universe, developing wherever it found a suitable environment. [18]
But the connection between life and comets has been made centuries before in popular beliefs. It appears that Sir Isaac Newton has several times expressed his belief that the appearance of comets in the sky was related to the spontaneous generation of plants. [19] However his belief has been demolished by the theory of evolution. The publication of this theory by Darwin
The idea that comets had a role in the formation of planets was already proposed by both Newton Newton
In 1992 Carl Sagan expressed that “comets may be the bringers of life”. [23] It was already known at that time that life has developed a little under four billion years ago during the heavy bombardment phase, and that comets do carry organic molecules, which, if given energy, can break apart and re-combine in more complex forms, such as amino acids, proteins and potentially DNA. [24] To all this was added the knowledge that organic molecules on board a comet could have survived a violent collision with the planet, especially since 74 different amino acids unknown on Earth have been found in chondrites, and hence such amino acids could have easily survived a minor impact. [25] After all, thermofilic organisms are the common ancestors of the oldest forms of life found on Earth, which means that life has evolved in high temperatures. Prebiotic molecules could have also been sowed even if a comet exploded in the atmosphere and the debris would have fallen on the surface of the planet, or even from such interplanetary debris. [26]
The similarity in composition between humans and comets suggests at the very least that there must be a connection between life and comets. Humans contain 9.5 percent carbon, 63 percent hydrogen, 26 percent oxygen, and one percent nitrogen. Halley’s Comet contains 11 percent carbon, 55 percent hydrogen, 28 percent oxygen, and two percent nitrogen. [27] Recent studies have revealed organic compounds, clay and light water on board of comets. All of these seem to be enough to allow the spark of life to ignite. However, there is not yet enough evidence to support the theory that life has hitched a ride on a comet and arrived on Earth ready-made, and the fact that life is still yet to be discovered elsewhere is a drawback of such a hypothesis. If life arrived onboard of a comet, it must have contaminated other worlds too. But, if life turns out to be widely spread everywhere in the universe, the cometary contamination of all planets with life seems likely, and it is just a matter of time until such evidence will come up. Until then, panspermia through comets seems to be a fascinating theory on the origins of life.
