What's in the heart of a comet?

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?

Deep Impact blasted lots of material from beneath the surface into the comet's coma. Remember, the comais 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 impact crater.

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.