Lesson 32: Comets
In this lesson, we will look at what comets are made of, why they have tails,
and where they come from.
As we learned previously, rocky planetesimals are called asteroids, and
ice-rich planetesimals are comets.
Just as Ceres is a large asteroid, Pluto and
other large objects of the Kuiper belt are essentially large comets.
The vast majority of comets are at the outer reaches of our solar system, far
from the Sun, and on the surface, they look similar to asteroids. Instead of
being made out of rock, however, they are basically chunks of ice mixed with
rocky dust and some other chemicals. That is why comets can be compared to
"dirty snowballs."
Far out in space, a comet is basically just an ice ball. It is only if a comet
comes in closer to the Sun that it develops the tail that we normally associate
with comets. When the comet starts accelerating towards the inner solar
system, closer to the Sun, then this ice ball becomes the 'nucleus' of the comet.
As its surface temperature increases, ices begin to vaporize into gas and
dust is also released,
forming the dusty atmosphere around the nucleus called a 'coma'.
There are
two tails on a comet. The plasma tail is made up of ionized gas that is pushed
outward by the solar wind. That is why the plasma tail extends directly away
from the Sun. The dust tail consists of dust-size particles that the comet
leaves behind and curve back in the direction the comet came from.
Larger particles, from the size of a grain of sand to
a pebble, may also be ejected from comets. When Earth crosses a comet's orbit,
these particles enter our atmosphere causing meteor showers.
There are two reservoirs of comets in
the distant outer solar system. They are the Kuiper belt and the Oort cloud.
We cannot see these reservoirs of comets since they are so far away,
but their existence has been determined by tracing the orbits of observed
comets that have reached our inner solar system and figuring out which
direction they came from.
The comets in
the Kuiper belt are the leftover planetesimals that
originated on the outer edges of where the planets formed, beyond the orbit of
Neptune. Since they formed so far away, they were most unaffected by the
gravity of the jovian planets and were able to maintain their original ecliptic
plane and orbital direction as the planets.
The comets of the Oort cloud, however, may have actually formed much closer --
in between the orbits of Jupiter, Saturn, Uranus, and Neptune. Due to
collisions or close gravitational encounters with the jovian planets, they were
flung off at high speed to end up at the outer limits of the solar system.