Lecture
Comets

Learning Objectives

1. Describe the major characteristics of a comet.

2. State the difference between the nucleus, coma, ion tail, and dust tail.

3. Explain the "dirty snowball" model of the nucleus of a comet.

4. List the major "ingredients" of a comet.

5. Define what the Oort Cloud and Kuiper Belt are: Where they are located and the kind(s) of comets each provides.

6. Contrast what a comet looks like when in the outer part of the solar system (well beyond 5 AUs) versus the inner part of the solar system (inside 5 AUs and the reason for these differences in appearance.

7. Explain why comets are of such high interest when considering the source of Earth's water and organic molecules.

8. Explain why comets are so important to our understanding of solar system history.

9. Outline the Stardust Mission to comet Wild 2 (pronounced "vilt" two).

Concepts Covered

What is a comet? (Could you eat one?)	Make a comet
Take a look at the image of a comet to the right (view the full image if possible). What do you observe? What questions are brought to mind?	Full image (1.4 megabytes)
Ancients (hundreds or thousands of years ago) thought of comets as portents of catastrophies, death, and black magic. Fortunately, we are much more knowledgeable today. Who would ever consider that aliens were riding in on one, decide to dress in purple, and do away with oneself? Yes, we've come a long way in 1000 years......	Harbingers of Doom
Modern-day comets
Halley	Halley's Comet in false color to emphasize varying brightness; from Views of the Solar System An excellent site for information, including an animation of the Giotto satellite fly-by of Halley.
Hyakutake Hale-Bopp Ikeya-Seki and others.	This image of comet Hale-Bopp was taken by John Laborde with his home designed and built, 8.8" f/3.7 Wright Schmidt Camera. The picture was taken on March 15, 1977 at the Tierra Del Sol Observatory site in San Diego County with a 25 minute exposure on Kodak PPF400 film.
Origins of comets	Oort Cloud: lies well beyond Pluto's orbit, out to maybe 50,000 AU. No Oort Cloud object has ever been detected at that distance; however, it is thought that there is a reservoir of millions of comets there. Probably the source of the long-period (orbits of thousands of years) comets. Kuiper Belt: located roughly at Neptune's and Pluto's distance from the Sun. Thought to be the source of the short-period (orbits of around 100 years) comets. Kuiper Belt objects are slowly being discovered; Pluto is rumored to be the "King of the Kuiper Belt" although it retains its planetary status.

Comet Observation Home Page

Structure of Comets	Nucleus	This picture is from the Giotto spacecraft. In this picture, the Sun is on the left. Three jets can be seen blowing molecules toward the Sun. A crater can also be seen in the middle right. This image shows that evaporation occurs along specific portions of the comet. Data taken by a suite of spacecraft suggests that the comet is mostly made of ice.
	Ion Tail and Dust Tail	This image of comet West was taken by John Laborde at the Tierra Del Sol Observatory site in San Diego County. It was taken on morning of March 9, 1975 looking almost due east. The exposure was 30 minutes with a 135 mm Nikon lens.
	Coma and Hydrogen Envelope	The parts of a comet from Views of the Solar System
Physical Properties of Comets	Mass	1012 - 1016 kg
	Size	1500 x 1000 x 1000 meters
	Density	0.1 g/cm3
	Composition	water ice, CO2 ice, methane, ammonia, dust

What are comets? Comets are not harbingers of ill fortune and plagues. They are not atmospheric phenomenon. They are small, primitive objects, thought to most closely resemble a frozen "stockpile" of ices: H₂O, CO₂, CO, and dirt (carbon soot). They also contain traces of alcohol, methanol, ammonia, silicates, carbonaceous particles, and other cosmic seasonings. The comets are frozen fossils of the formation of our solar system. Sooty snowballs or fluffy, dirty snowdrifts are phrases used to describe the nucleus. If and when a comet approaches the inner solar system, it develops a coma and two tails: an ion or plasma tail and a dust tail.

Comets have a nucleus. When comets are far from the Sun, all they have is a nucleus. The albedo (reflectivity) is lower than one might expect from ices: 20% due to the presence of carbon. The density is roughly that of water. They are irregularly shaped and have very low mechanical strength. It would not take much to pull one apart. Relatively speaking they are very small, maybe 10 - 20 km or so.

As a comet approaches a distance from the Sun where temperatures rise above 200 Kelvin, it starts to sublimate (goes directly from a solid to a gas). The comet develops a coma, which consists of gas and dust from the nucleus. The inner coma is perhaps 100 - 1000 km in diameter, while the outer coma may extend millions of kilometers. An ordinary comet has enough volatile material for maybe 1000 solar passes. AFter it runs out of ices, what remains will closely resemble an asteroid. Chiron is an object that may have once been a glorious comet.

As the comet gets closer to the Sun, it develops a tail--sometimes a spectacular tail. The tail is actually two: a plasma (ion) tail and a dust tail. The plasma tail consists of ionized gases that "glow" with molecular and atomic emissions. It may extend 100 million kilometers or more, and flows straight out away from the Sun. The plasma tail is the "celestial wind sock" for the solar wind, and is affected by the Sun's magnetosphere. The dust tail is perhaps 10 times shorter than the ion tail and is made up of cake-flour like particles that reflect the Sun's light. These dust grains follow their own orbits, and fight against the pull of the Sun's gravity and the push of the Sun's radiative pressure. Comet dust is responsible for meteor showers when the Earth crosses a comet's path.

The Life of a Comet in a still frame and animated (speed reading required). Image from A Cosmic Perspective, Bennett et al.

Comets have the most eccentric orbits of any solar system object. Their orbits may take thousands of years or less than 100. The orbits may have high inclination (orbit at an angle far from the ecliptic plane), and they may orbit either in the same direction as the planets (prograde) or in the opposite direction (retrograde). Comets follow Kepler's Laws.

Comets must come from somewhere. Theory predicts that long-period comets come from the Oort Cloud--a reservoir of maybe a trillion ice balls. The Oort Cloud resembles a spherical shell around our solar system, with a concentration close to the ecliptic plane. The long-period comets can come from any direction or orientation, indicative of a spherical distribution. Oort Cloud comets account for a number of new comets each year. All have highly eccentric orbits. The source for short-period comets is theory-confirmed-by-observations. These objects come from what is known as the Kuiper belt at the Neptune/Pluto distance from the Sun. These comets are more likely to have prograde, lower inclined orbits indicative of a flattened source region. Long-period comets may become short-period comets if they have a gravitational interaction with Jupiter (a process thought to populate the comet reservoirs originally).

Comets may die a sensational death. Comet Shoemaker-Levy crashed into Jupiter with a force that would have spelled instant doom for life on Earth. Other comets come too close to the Sun and are "swallowed whole." These comets are given the appropriate designation of "Kamikaze Comets." Take a look at two heading in to the Sun in these SOHO satellite images from June 1, 1998.

Comets: bringers of life? takers of life?

Life's Far-Flung Raw Materials, Scientific American, July 1999, pp. 26-33

"Since the early 1960's, space scientists have speculated that comets and other remnants of solar system formation hauled in gas and water molecules and that these components provided the atmosphere and oceans that made the planet habitable."

"...only 100 million years or so after the earliest possible point when Earth could have safely supported life, organisms were already well enough established that evidence of them remains today."

"When Deamer (a scientist) mixed organic compounds from the (Murchison) meteorite with water, they spontaneously assembled into spherical structures similar to cell membranes. Our colleague Jason Dworkin has shown that these capsules are made up of a host of complex organic molecules."

Earth would have been too hot and inhospitable for delicate organic molecules to survive.

Comets are full of organic compounds: kerogen (polycyclic aromatic hydrocarbons), methane, ethane

Meteorites have about 70 varieties of amino acids, of which 8 are part of the 20 used by life. Examination of carbonaceous chondrite meteorites have shown a slight surplus of left-handed amino acids.

NASA Science News for April 5, 2001
A new experiment suggests that comet impacts could have sowed the seeds of life on Earth billions of years ago. FULL STORY at NASA Headlines for 5 April 2001

Comet Missions in the Works
Stardust: First comet sample from deep space. A probe will fly through thegaseous coma of comet Wild 2 in 2004 and will use a silicon-based substance called aerogel to collect dust samples that it will return to Earth in 2006. Launched 7 February 1999	Space Technology 4 / Champollion: First comet landing A satellite orbiting comet Tempel 1 will send a small vehicle to land on the comet's rocky nucleus in 2005. The lander will take photographs and analyze subsurface samples. Launch: 2003 (NASA)	Rosetta: Most thorough comet study ever A satellite will rendezvous with comet Wirtanen in 2013 and will spend 11 months making measurements from orbit while a lander probes the comet's surface. Launch: 2003 (European Space Agency)

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