Comets as Arks

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Donati's comet, as imaged in 1858 (Image © Wikimedia Commons)

Mysterious travellers of the solar system

By YD-Bar-Ness

In every society there are those who do not play by the rules, and who dream of taking long journeys of pilgrimage. In our Solar System, these maverick renegades are also the most mysterious; they are the comets that bring magic and wonder to our night skies. Unexpectedly, and beyond our wildest imaginings, they are the source of all true magic – they hold the key to the origins of life on Earth.

All objects in our Solar System planets travel on predictable, elliptical paths around the Sun. Newton’s physical mathematics have enabled us to chart out the movements of the planets, moons and asteroids, but the comets surprise us from afar. Swooping in to visible range from the outer limits of observational distance, they travel along the most extreme of orbital paths.

Zooming past the Sun and then back out into the edge of the Solar System, their “year”, or orbital period, can last for millions of years. With high-precision observations, comets are indeed mathematically predictable. However, sometimes comets break up, collide with other objects or are gravitationally deflected out into the cold distances between the stars. Humans have so far observed and catalogued more than five thousand comets – a small number when compared to the trillions of them theorised to be orbiting the Sun.

Omens of disaster

Comets have been recorded by humans for thousands of years, and consistently recognised as omens of great significance. Written observations appear globally and are often linked to subsequent events: in the 4,000-year-old Mesopotamian epic Gilgamesh as the bringer of flood, as Chinese illustrations of “vile stars” in 240 BC, in connection with the assassination of Julius Caesar in 44 BC, in advance of the birth of Christ, in the European Bayeux Tapestry depicting battles of the year 1066, and in the Incan Empire in advance of Pizarro’s 1533 conquest. Deriving their name from the Greek word for “long-haired”, they were unexplainable until much more recent times.

In 1705, the British mathematician Edmund Halley detected a pattern in the historical records of comets. There had been sightings in 1531, 1607 and 1682. Observing the 76-year interval, he correctly predicted a comet would arrive in the year 1758. Unfortunately, Halley did not live long enough to see it, but he established that many of the world’s historical comet sightings were in fact, one singular object. Curiously, the American author Mark Twain was born on the exact day of its arrival in 1835, and died on the exact day of its arrival in 1910. You may be old enough to remember its last visit, or young enough to look forward to its next arrival in 2061.

Various views of Donati’s comet in the night sky (Image © Wellcome Library, London)

What is a comet?

Comets, like the planets and moons, were born from the cloud of dust and gas before the Sun flared into existence. Often described as “dirty snowballs”, they consist of a solid nucleus up to ten kilometres across, with a vaporous tail that can be millions of kilometres in length. The dramatic tail is actually composed of two, often overlapping parts: a curved dust tail trailing behind, and an ion gas tail pushed by the solar wind pointing directly away from the Sun.

The observed comets are classified as either short or long period. The short period comets orbit in regions of space between 35 and 100 astronomical units (AU) from the Sun; each AU is the distance from Earth to the Sun, or about 150 million kilometres. Just beyond the orbit of Neptune, these regions known as the Kuiper Belt and the Scattered Disk are filled with objects similar to the asteroids that exist between Mars and Jupiter. These comets are flung onto their dramatic orbits by the gravity of Neptune, and every so often one is deflected towards the Earth and the Sun.

From the Oort Cloud

Even more exotic are the long period comets from the Oort Cloud. This distant region of space between 3,000 and 50,000 AU – almost a quarter of the way to the closest other star – is hypothesised from the orbits of these far-travelling comets with orbital periods of millions of years. In it, trillion of comets are travelling on their distant ellipses, still gravitationally tied to the Sun. In some instances, such as Halley’s Comet, they are captured by the gravity of the outer planets and bound to much shorter orbits.

When a comet comes within the orbit of Mars, the energy of the Sun begins to heat up its atmosphere to form the dust and gas tails. When a comet comes close enough to Earth, we can train our telescopes on them. If we plan and cooperate, we can send robots after them to take a more careful look.

Research so far

Until very recently, research on comets has been done with the aid of telescopes, but starting in 1986, robotic spacecraft have been able to make observations at much closer ranges. In that year, co-operation between European, Japanese and Soviet missions resulted in imagery from within 600 kilometres of Halley’s Comet. In 1994, the spectacular collision of the comet Shoemaker-Levy 9 with Jupiter was observed with the satellite Galileo.

In 2004, the NASA Stardust mission sampled and returned to Earth material from the tail of a comet. In 2005, the NASA Deep Impact spacecraft blasted an impact sensor into a cometary nucleus to learn about the surface and internal characteristics. Most recently, the 2014 European Rosetta mission landed a robot on the surface of a comet, providing a wealth of chemical and structural data about these mysterious objects.

Life in Comets

The core of the nucleus, like the asteroids between Mars and Jupiter, is formed of ices and rocky metallic minerals. The surface, atmosphere and tail contains chemicals familiar from our own biochemistry; water ice, carbon dioxide methane, ammonia, oxygen.

The abundance of water in comets may even be the original source of the water on our own planet; in a much earlier stage of Earth’s existence a bombardment by comets or asteroids could have supplied Earth with the water that is so critical to organic life.

The composition of the nucleus itself is a source of great scientific interest; in the core is potentially immaculate material from the formation of the Solar System to be found, and in the dust of the surface, the biochemical precursors to life.

There is growing evidence that we owe our existence to these comets, and that our own origin story includes the distant reaches of the Solar System. The Rosetta mission has detected organic molecules that are also the building blocks of our own ecology; alcohols, ethanes, acetone, formaldehyde and even the amino acid glycine.

Comets have been detected in ten other star systems, and they may have a connection to any interstellar biology. As strange and unfamiliar as they are, it seems that they are intimately linked with our own story. Our most distant ancestors may have arrived on Earth travelling vast distances on these mysterious travellers.

For more stories and photos, check out Asian Geographic Issue 119.

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