Module 17: Ice Worlds Activity 2: Comets

In this Activity the main topics covered will be: Summary: (a) observing comets; (b) comets and the Dirty Snowball Model; (c) structure of comets; (d) comet orbits; (e) Shoemaker-Levy 9; (f) Stardust mission.

Comets are mysterious fuzzy objects, that first appear (completely unpredictably) from the beyond the very edges of the Solar System. The first you see is a chance discovery of a faint fuzzy smudge that moves: Here is one of the first pictures taken of Comet Hale Bopp. (a) Observing Comets Dr Paul Francis, ANU/MSSSO, took this picture on the Siding Spring 1m telescope a day after the comet was discovered.

A few comets never get any closer: their orbits turn around and they depart back into the darkness from whence they came. Next they develop a tail, which always points away from the Sun (it may not be behind them in their motion). Others, however, keep on approaching. They sweep past the orbit of Jupiter, past the asteroid belt, and advance into the inner Solar System. By this time, they are beginning to swell, becoming much larger and fuzzier.

As they approach the Sun, the tail grows and brightens. Soon, this comet may be visible to the naked eye.

What are these things? Observations show us that the tails are made of gas and water. They always point away from the Sun, so presumably they are being blown off the comet’s nucleus by solar radiation and the solar wind.

Far from the Sun, temperatures are low enough for water, ammonia, carbon dioxide and methane to exist as ices. Debris of this nature was created during the formation of the Solar System. (b) Comets and the Dirty Snowball Model So what, if anything, lurks in the middle of this cloud of gas? Until the mid 1980s, nobody really knew. Some sort of giant snowball was the theory: a frozen chunk of ice and frozen gases (water, methane, carbon dioxide etc). They had to contain some complex organic chemicals as well – we detected their presence in the tail.

Occasionally these ice-rock objects from the outer limits of the Solar System “swing in” past the Sun on highly elliptical orbits. Comet Halley Near the Sun, as sunlight heats the ices in these comets, jets of gas and dust particles are released. For a great majority of their lives these objects are dim lumps of frozen material, “dirty snowballs”, with diameters which are typically only a few kilometers across. This makes them very difficult to find with telescopes.

(c) Structure of comets The nucleus of the comet is modelled as being an irregular shaped lump of dirty ice, mainly frozen water but also carbon monoxide, carbon dioxide and formaldehyde. Microscopic dust particles are present in the loosely packed clump of ice. In the 1980s, the European Space Agencey’s Giotto space probe ventured into the central fuzz of Halley’s comet for a look. The camera was rapidly destroyed by impacts with dust grains, but it got one good picture of the nucleus first...

The nucleus appeared to be a ‘lump’ about 10 km long, which was very dark in colour (perhaps due to black organic chemicals coating the surface, like tar?). Jets of gas could be seen blowing off the sunny side in huge gusts. Presumably this side is hot, causing frozen gases to evaporate.

Within 2–3 AU of the Sun, water and other molecules evaporate and, along with dust particles, stream away from the nucleus to form the coma and tails of the comet.

A comet near the Sun : Head or coma – gas & dust up to 1,000,000 km in diameter Tail – typically 10 7 –10 8 km, (can be up to 1AU) – usually points away from the Sun Nucleus – central lump of dirty ice, perhaps 10s of km across – darker than coal A “halo” of hydrogen gas surrounds the entire comet when near the Sun Comet Halley, 1986 Over 6 degrees

The coma is deceptively sparse, with gas and dust molecules from a thin outer layer of the nucleus spread over the enormous volume of space covered by the coma. The coma is in fact so sparse that it would be considered a vacuum in a laboratory on Earth. The combination of sunlight reflected from the dust and emissions from the gases results in the spectacularly bright appearance of the coma. The ball of gas and dust which surrounds the nucleus is called the coma. The coma can have diameters up to a million kilometers; far larger than that of the nucleus.

The yellow or white dust tail contains dust particles expelled from the coma by radiation pressure due to sunlight. Comets have 2 types of tails : Plasma tail Dust tail The plasma (ion) tail contains ionised molecules dragged from the comet’s coma by magnetic fields associated with the solar wind. Comet Halley © David Malin, AAO

A misconception about comets is that their tails trail behind their direction of motion. In fact the dust tail points in the direction of the radiation which causes them, i.e. away from the Sun. The ionised methane (CH 4 ) in the plasma tail emits strongly in the blue part of the spectrum resulting in the plasma tail’s blue appearance. The solar wind can sweep the plasma tail to distances up to 1 AU, again pointing away from the Sun rather than trailing the direction of motion. Gravitational forces affecting the dust tail result in the two tails pointing in slightly different directions.

Surrounding the nucleus, not visible to the human eye, is a huge sphere of hydrogen referred to as the hydrogen envelope. The source of the hydrogen is water molecules which evaporate from the nucleus and are broken apart by ultraviolet radiation from the Sun. The hydrogen envelope, seen using ultraviolet photography, can be 10 million km or more across.

Some comets are observed periodically as they make repeated visits into the Solar System. (d) Comet orbits Comet Halley, Mt Wilson Observatory

Pluto’s orbit Neptune’s orbit Uranus’s orbit Saturn’s orbit Jupiter’s orbit Mars orbit Earth’s orbit Comet Halley’s orbit Position at given dates indicated. Note that Comet Halley is in a retrograde orbit, meaning its orbit is in the opposite sense to the orbits of the planets around the Sun.

Comet Halley is a short-period comet as it has a period of less than 200 years. It never travels further from the Sun than the average distance to Pluto. Most short period comets orbit the Sun in under 7 years. Comets cannot survive many close encounters with the Sun. A comet typically loses between 1/60 th and 1/100 th of its mass each perihelion (point of closest approach to the Sun), as its ices evaporate. Eventually, all that may be left will be a pile of rubble and organic gunk: this rubble may disintegrate and form a meteoroid swarm, e.g. 1st orbit2nd orbit3rd orbit4th orbit

Comet Hale-Bopp is a long-period comet as it has orbital period greater than 200 years. After passing through the inner Solar System in 1997, Hale-Bopp is not due to return for 2400 years. Image of Hale-Bopp ©Michael Brown

Some long period comets may take millions of years to return. Comet Hyakutake has an orbital period of 30,000 years and travels as far as 2000 AU from the Sun in its orbit. Image of Hyakutake ©Michael Brown

Both the short and long period comets orbit the Sun on elliptical orbits, even if the really long period comets only seem to ever visit in the inner Solar System once… We’ll discuss the long and short period comets in more detail in the next Activity.

(e) Shoemaker-Levy 9 The impact of the ninth short period comet found by Eugene and Carolyn Shoemaker and David Levy with Jupiter was explored in Activity Jupiter, the Dominant Gas Giant Planet. Shoemaker-Levy 9 was broken apart by tidal forces caused by Jupiter’s gravitation when the comet made a close approach (96,000 km) to Jupiter around 7 July HST image of fragments of Shoemaker-Levy 9

Even though Shoemaker-Levy 9 had been orbiting Jupiter, previous approaches were no closer than 9 million km where the tidal forces were much weaker. The breaking up of comet Shoemaker-Levy 9 led some astronomers to propose that the nucleus of comets is made up of several smaller ice balls held together by their mutual gravitational attraction.

As the comet gets close to a planet, the tidal forces separate the fragments, which subsequently form a crater chain when they collide with the larger body. Many crater chains have been found on the Moon, Ganymede and Callisto. Crater chain on Ganymede

Could a comet collide with Earth? Yes, but as with Shoemaker-Levy 9, Jupiter acts like a “giant vacuum cleaner”, protecting us from most comets which enter the inner solar system. This question will be further addressed in the Activity Solar System Debris and its effects on Earth.

(f) Stardust mission Launched in February 1999, Stardust collected interstellar grains in 2000 and In January 2004 it successfully rendezvoused with Comet Wild 2 and collected cometary material. It will return these samples to Earth in 2006, using a capsule that will parachute to the surface after re-entry. Artist’s rendering of the Stardust probe Stardust is a NASA spacecraft designed to collect samples of interplanetary material, including interstellar dust and cometary dust grains and gasses.

The nucleus of Comet Wild 2 was photographed by Stardust’s navigation camera, from a distance of only 236 km. Large impact craters and 100 m high cliffs can clearly be seen. Stay tuned for the data analysis in 2006 when the samples are returned to Earth! Comet Wild 2 was chosen for the Stardust mission because it is a relatively pristine comet, only having entered the inner Solar System recently. Unlike comet Halley, which has orbited the Sun about 100 times, comet Wild 2 has only experienced 5 orbits. It is hoped that the captured comet samples, barely altered since the birth of the Solar System, will reveal information about our origins.

Animation of comet Wild 2 as seen by Stardust

In the next Activity we’ll investigate theories of the origin of comets.

Image Credits Comet Halley - NASA Comet Hale Bopp © Paul Francis, ANU/MSSSO, used with kind permission Siding Springs 40 inch (1m) telescope - MSSSO Comet Hyakutake © Michael Brown, University of Melbourne, used with kind permission Comet Halley © David Malin, AAO, used with permission Nucleus of Comet Halley - Giotto Mission, NASA Comet Halley - courtesy Mt. Wilson Observatory Halley’s plasma and ion tails © David Malin, AAO, used with kind permission

Comet Halley - NASA Comet Hale Bopp © Michael Brown, University of Melbourne The fragmenting of Comet Shoemaker-Levy 9 - HST Crater Chain on Ganymede - NASA Artist’s rendering of the Stardust probe - NASA/JPL-Caltech The nucleus of Comet Wild 2 - NASA Image Credits

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