by Chris Peterson » Mon Jul 20, 2015 3:16 pm
bkebackstrom wrote:Why does a comet leave a dust trail in the first place? Are not all the comet's content travelling with the same speed?
Well, yes. Mainly. But not exactly.
When a comet gets near the Sun, there's enough energy to volatilize its ices, which results in trapped dust and rock being ejected. This creates a cloud of debris around the nucleus. In the absence of other forces, this cloud would continue expanding outward, generally following the same orbit as the comet, but distorted by the fact that particles drifting outside the comet's orbit would lag, and particles drifting inside the comet's orbit would lead (Kepler's Second Law).
However, there
are other forces acting on the ejected material- a number of them. The simplest is radiation pressure. This affects very small particles- typically 1 µm or less- and pushes them outward. This is the dominant force creating a cometary dust trail (which always lags the comet, being in a larger orbit). These small particles are an important source of interplanetary dust. Larger particles, however, are not pushed outward, but rather experience a form of drag called the
Poynting-Robertson effect. This results in their moving inward, and eventually into the Sun. But since small particles are much more numerous than larger ones, we mainly see those, and therefore the typical tail.
Quite a lot of larger material stays in nearly the same orbit as the parent comet for many orbits, only slowly dissipating due to the above mentioned forces. This material produces meteor showers in the case of comets which have orbits intersecting Earth's. And as the cometary material dissipates, it regenerates the interplanetary dust cloud (which we observe directly in zodiacal light and gegenschein, in many meteors, and instrumentally in IR light), which is constantly being swept into the Sun or out of the Solar System.
[quote="bkebackstrom"]Why does a comet leave a dust trail in the first place? Are not all the comet's content travelling with the same speed?[/quote]
Well, yes. Mainly. But not exactly.
When a comet gets near the Sun, there's enough energy to volatilize its ices, which results in trapped dust and rock being ejected. This creates a cloud of debris around the nucleus. In the absence of other forces, this cloud would continue expanding outward, generally following the same orbit as the comet, but distorted by the fact that particles drifting outside the comet's orbit would lag, and particles drifting inside the comet's orbit would lead (Kepler's Second Law).
However, there [i]are [/i]other forces acting on the ejected material- a number of them. The simplest is radiation pressure. This affects very small particles- typically 1 µm or less- and pushes them outward. This is the dominant force creating a cometary dust trail (which always lags the comet, being in a larger orbit). These small particles are an important source of interplanetary dust. Larger particles, however, are not pushed outward, but rather experience a form of drag called the [url=https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect]Poynting-Robertson effect[/url]. This results in their moving inward, and eventually into the Sun. But since small particles are much more numerous than larger ones, we mainly see those, and therefore the typical tail.
Quite a lot of larger material stays in nearly the same orbit as the parent comet for many orbits, only slowly dissipating due to the above mentioned forces. This material produces meteor showers in the case of comets which have orbits intersecting Earth's. And as the cometary material dissipates, it regenerates the interplanetary dust cloud (which we observe directly in zodiacal light and gegenschein, in many meteors, and instrumentally in IR light), which is constantly being swept into the Sun or out of the Solar System.