The Short Answer:
A comet's nucleus is in the center of its coma. A comet's nucleus is like a snowball made of ice. As the comet nears the Sun, the ice starts to melt off, along with particles of dust. These particles and gases make a cloud around the nucleus, called a coma.
A comet's nucleus, or heart, is the solid chunk of something in the center of its fuzzy coma. As it approaches the Sun, some of its surface boils off and creates a long tail.
But what IS inside a comet's nucleus?
Marshmallows? Chewy caramel? Nuts?
Here is what scientists have found out.
Comets are part of the solar system. They orbit the Sun, just as planets do, except a comet usually has a very elongated orbit. Part of its orbit is very, very far from the Sun and part is quite close to the Sun.
A comet's nucleus is like a dirty snowball made of ice. As the comet gets closer to the Sun, some of the ice starts to melt and boil off, along with particles of dust. These particles and gases make a cloud around the nucleus, called a coma. The coma is lit by the Sun. The sunlight also pushes this material into the beautiful brightly lit tail of the comet.
Scientists have now had a look inside a comet's nucleus.
On July 4, 2005, NASA's Deep Impact spacecraft's "smart impactor" scooped out . . . well, more like blasted out a crater in the nucleus of Comet Tempel 1. What did they find? Was it dark and crusty like the surface, or soft and squishy like a marshmallow, or full of holes like Swiss cheese, or full of big rocks like nutty nougat?
Here is an animation of Deep Impact releasing the impactor, which then crashes into the nucleus of Comet Tempel 1.
Here's some of what they have found out so far from looking at Deep Impact's encounter with Tempel 1:
What we now know about Comet Tempel 1
The comet's nucleus is spongy, with lots of holes inside. No one knows yet whether there are a few large holes or many smaller ones.
If there are a few large holes, it might mean that the comet was formed from large, dirty ice boulders. If there are many smaller holes, it might mean the comet was formed from many more dirty ice snowballs.
Parts of the surface are very fragile and weak.
It may be that the comet's ice was the "glue" that held the comet dust and rocks together. Then, as the comet came closer to the Sun, the surface ice evaporated, leaving little or no "glue." The rocky and dusty structures would then be fragile and crumbly.
The surface of the nucleus is covered with fine dust, like baby powder.
What is this dust and where did it come from?
Originally, the comet's surface ice probably contained a lot of fine dust. When the orbit of the comet brings it close to the Sun, the ice evaporates into space, leaving some of the fine dust sitting on the surface. The dust is fine like talcum powder because comets are too small to have enough gravity to squeeze the dust together into larger particles.
The surface is very black.
What is this black stuff?
The very black material on the surface is carbon-based material similar to the greasy black goo that burns onto your barbecue grill. The comet originally formed from ices (mostly water ice), silicate dust (like powdered beach sand), and this type of black space gunk.
Some parts of the nucleus are smooth and young, while other areas are cratered and old.
The old-looking part of the surface has been battered for thousands of years by small, rocky asteroids or other comets. So why are some areas smooth? It is possible that as the comet has approached the Sun over the years, the ices on the surface have vaporized, and taken some of the embedded dust particles with it. Then, some of the dust particles could have settled back down on the surface, filling in some of the craters. Or, maybe the smooth surface areas that are covered with dust and dirty ice are disappearing as the comet repeadedly gets close to the Sun. After a long time, the smooth icy regions may have retreated, revealing the older cratered surface below.
The nucleus seems to have formed from overlapping layers of different materials.
The layers must have formed as the comet grew. As it got bigger, gravitational forces drew in ices, dust, and the black "space gunk" we talked about earlier from the comet's neighborhood.
There is ice beneath the surface, both water ice just below the surface and carbon dioxide ice (also known as "dry ice") farther down.
Why different kinds of ice at different depths?
Most of the ice in our solar system, including the ice in comets, is water ice. In Comet Tempel 1, almost all the ice is water ice, but some is carbon dioxide ice—or "dry ice." Carbon dioxide ice vaporizes faster than water ice. (That is why you might use "dry ice" to make "smoke" for a model volcano or "fog" for a stage play.) As the comet gets close to the Sun, the carbon dioxide ice will vaporize before the water ice. So, after thousands of years, even though the two kinds of ice were initially mixed together near the surface, only the water ice remains. The carbon dioxide ice a meter or so beneath the surface is more protected from the Sun's heat, so may survive, with water ice above it.
Tempel 1 contains materials from the outer, middle, and inner parts of the solar system.
We are not sure. Comets probably formed in the outer solar system. The inner solar system type of dust particles found in them could have traveled to the outer solar system where the comets formed. Or, not as likely, these dust particles could have arrived from other solar systems. Water and carbon dioxide ices are both found in the outer solar system, so comets could pick up both ices while forming.
This movie shows Comet Tempel 1's coma getting brighter after the Deep Impact collision. These pictures were taken by Caltech and JPL astronomers using the 200-inch Hale Telescope near San Diego, california.
Of course, not every comet may be just like Tempel 1.
Deep Impact blasted lots of material from beneath the surface into the comet's coma. Remember, the coma is the cloud of dust and gas that boils off the nucleus as the comet's orbit takes it closer and closer to the Sun.
The coma contains material from near the surface of the nucleus. This material is what the Sun heats up most and what boils off first. Scientists saw what was in the coma right after the impact, and compared that with what was there before the impact. This way, they could get an idea what was added from the material blasted out of the hole in the nucleus.
But, whether before or after the blast, how do the scientists know what the coma is made of? After all, the comet and its coma are millions of miles away!
Here's how: They observe the coma through a telescope equipped with a spectrometer.
Comet Tempel 1's coma before impact. Colored dots stand for
different materials that have boiled off the surface of comet's nucleus.
Comet Tempel 1's coma after impact. Added dots, some of
different colors, stand for materials from below the surface
of the nucleus that were "splashed" into the coma from the
A spectrometer creates something like a rainbow. Like droplets of water may do after a rain, a spectrometer breaks light apart into its different wavelengths, or "colors." Depending on what gases (such as those in air) the light has passed through, the "rainbow" will look different. That is because each gas absorbs one or more particular colors of the light that passes through it.