mixed in with the ice, snowballs.
Because yes they're made of H2O, although he was very clear that
these other components are an important part of what's going on in the comet too.
These days we now know a lot more about what comets are really made out of.
And we can make detailed tables that look like this.
These are the abundance of materials in comets relative to water percent,
relative to water, why?
Because water ice still is the dominant component in the comets and so
this is plotted at 100% H2O.
What's next?
Well, this shows you the range of different observations in comets
in the green and the amount that's here.
Over here is the number of comets in which it's been observed.
Some things have only been observed once and usually when it's once,
it's because it was Hale-Bopp.
Hale-Bopp was so bright that it was easy to observe.
Some things are so
bright that we see them all the time, greater than ten, greater than ten.
And what is the most abundant thing, after the H2O?
Well, CO and CO2.
Remember, Whipple said CO or CO2.
He was right on both counts.
There's the methane that Whipple proposed, and then some acetylene and
some ethane, and then some pretty complex hydrocarbons in through here.
CH3OH is methanol, but these other ones are ones that you might not recognize.
There's ammonia, these are things that have nitrogen in them.
Ammonia is the main nitrogen bearing species.
There's sulfur, H2S.
There are all of the materials that we find on the Earth, sulphur,
nitrogen, carbon.
But now rather than being locked up in rock form or being a gas,
these things are found as ices.
And I mean we know about them because we take these ices that would be hard to
detect otherwise and we bring these ices in close to the sun and Evaporate them and
make them very easy to see.
The existence of all these gasses is a big clue to what might be going on with
these comets.
First off, they can't have formed very close to the sun,
because as soon as they get close to the sun they start to evaporate like crazy.
We actually see comets eventually peter out and
die when they get too close to the sun.
The recent one, Comet ISON, got so
close to the sun, it was never seen again after we had a close passage.
So we know that they must have formed further out.
Not only that, we know the existence of something like the water line
we've talked about before, where frozen water becomes important.
Well, this thing had a lot of water in it.
So these things probably formed beyond the water line.
But they also probably form beyond most of these other important frozen ices
there own lines.
Everybody has their own equivalent to the water line.
So we have clues that these things formed in the outer part of the solar system and
are emissaries from this outer part of the solar system that we get to study up close
when we bring them in.
Like I said at the beginning comets are fascinating thing, there's so
much more we could talk about, about the compositions of comets.
About the fact that it looks like some of them have different compositions from
others, which is telling you maybe about where the different types form.
We haven't even talked at all about the dust in the comets,
that thing that's making them so bright for much of time.
They're fascinating stories, indeed they are.
Go out and see a comet if you can.
There's not a particularly bright one in the sky right now, but
pay attention, there will be.
Go see one, they're something that's worth seeing.
Everybody should see at least one real comet in their life,
well, because they're cool.
