Home            Comets
Are Comets Made of Antimatter?
If you look on the internet you will find many websites saying that comets are made of antimatter.  Some of these sites have what sounds like good evidence to back up the basic idea. These sites have mathematical formulae, and the things you would expect from serious scientific sites. 
I would note that one of the things said in several of these sites is that the body of the comets appears to be black.  I cannot see the logic of this.  Antimatter should reflect light in the same way as normal matter.  Whether comets are made of matter or antimatter, if they appear black, it presumably simply means their surface in covered with black material, perhaps something like tar.  I do not consider the colour of comets to be evidence one way or the other.
Most sites about comets say they are made of ordinary matter.
There are a lot of comets each year, and if you consider the billions of years that our Solar System has existed, there have been many collisions with planets.
If a comet of even moderate size hit a planet, the explosion from the annihilation of the comet and an equal amount of matter would be enough to blow the planet apart. The mere fact that we have a reasonable number of planets is good evidence to me that comets are not made of antimatter.
Mathematical explanation of the last paragraph
(can be skipped if preferred):
The Explosive power of thermonuclear weapons is measured in Megatons.  One megaton is the equivalent of the annihilation of 46.5 grams of mass.  A moderate sized comet has a mass of about100,000,000,000,000 Kilograms.  If an antimatter comet this size hit a planet, this mass as well as the same mass of normal matter would be annihilated, becoming energy. 
So 200,000,000,000,000 Kilograms of mass would be changed to energy.  There are 1000 grams in a Kilogram, so 200,000,000,000,000,000 grams of mass would become a 4,297,323,135,755,260 megaton explosion. This number is a little too big to easily comprehend, but it would certainly destroy a planet.
Although I personally am convinced that the comets we can see are not made of Antimatter, we have no way of being sure that no antimatter comets exist in other star systems. However, I am inclined to doubt it.
What Are Comets Made of
Comets are made of normal matter, not antimatter. 
Dirty Snowballs
Carl Sagan once described comets as dirty snowballs. I do not know if this was original when he first said this, but it is a quite good description.  Comets have a lot of water ice, together with things like Methane and other hydrocarbons.  They also have varying amounts of rock.
As the comet gets closer to the Sun, the outer layers will warm up, and particles from the Sun also act on the Comet, causing some of the comet to be removed from the comet nucleus, becoming the tail of the comet. 
The matter that is left after each passage near the sun will contain, near the surface of the comet, more of the heavier components of the comet, and less of the lighter ones.  This means that the comet will have a skin of things like the heavier hydrocarbons. 
This is a bit like tar pits where oil has lost its more volatile components leaving the heavier ones which make up tar.
Like tar, the surface skin of comets tends to be black, reflecting very little light.
Comets contain all the things that other astronomical bodies do.  This will include things like Ammonia as well as varying amounts of all the elements. This means that all the elements necessary for life will be found in comets.
Nearly all scientists would say that it is too cold inside a comet to support active life, but the possibility of life in a dormant form is more difficult to dismiss.
Even if comets contain no actual life, they certainly supply to all the planets the chemical elements necessary to support life.

Halley's Comet in 1910
Picture taken by The Yerkes Observatory.
Comet Hale-bopp.
Photograph by NASA
Comet Wild 2
This image was taken during the close approach phase of Stardust's Jan 2, 2004 flyby of comet Wild 2. It is a distant side view of the roughly spherical comet nucleus. One hemisphere is in sunlight and the other is in shadow analogous to a view of the quarter moon. Several large depressed regions can be seen. Comet Wild 2 is about five kilometers (3.1 miles) in diameter.