Tales of Comet Tails

(This entry caps off my Facebook “fun facts” series from my studying for a brutal comprehensive exam for graduate school, and is inspired by an old comps problem on Comet Hyakutake. And it’s an excuse to look at so many pretty pictures of comets. Enjoy!)

Going back to the early days of recorded history, mankind has always been fascinated by comets. Despite their incredible beauty, they were almost always seen as bad omens. Big events can be traced back to events of comet sightings…and then there are comets in popular culture. Overall, comets are a huge source of wonder to people of all ages, but have you ever wondered why they look so beautiful? What causes those long, majestic tails? And have you ever noticed that comets actually have two tails? That’s right—comets have two tails! Each tail is caused by a different process…one is caused by the light from the Sun, while the other is caused by the solar wind.

Let’s first look at a comet’s dust tail. Comets are essentially large balls of ice with long, elliptical orbits—as they sweep inwards towards the inner solar system, the icy material on their surface heats up and begins to sublimate (go from solid to gas) off the surface. This forms a gas cloud of gaseous water, carbon dioxide, and other icy molecules around the comet nucleus. This cloud is called the comet’s coma. If you remember from my entry on the Crookes Radiometer, light exerts a pressure, and can push on things. Similar to how it pushed on the radiometer panes, radiation pressure pushes out the gasses of the coma as well as the dust on the comet’s surface, forming the dust tail! The dust tail is typically redder in color, because it preferentially reflects red sunlight. This radiation pressure not only creates the dust tail, but also pushes it into a curved shape!

Light, however, is not the only thing comets have to deal with as they hurl into the inner solar system. In addition to sunlight, our beloved star is constantly ejecting energetic charged particles outward, at speeds of  approximately 250 miles/second! Comets are therefore getting rammed by this energetic solar wind, and it doesn’t go unnoticed. As solar wind particles slam into the comet’s nucleus and coma, they undergo a process with the gas called charge exchange, in which the charged wind particle steals an electron from a neutral gas particle. The now neutral wind particle flies off as an energetic neutral particle no longer affected by electromagnetic forces, but the now-charged gas particle gets swept up into the solar wind!

The solar wind is a real ion player, picking up all those ions.

Because these charged particles are swept up into the solar wind, and are now going to spiral around the interplanetary magnetic field lines (yes, there’s a magnetic field that permeates the solar system!), the new pick-up ions from the comet form a distinct, sculpted tail called the ion tail!

Unlike the dust tail, a comet’s ion tail preferentially reflects blue light, so it will be bluish in color. And since its formation and movement depend on totally different processes than the dust tail (light pressure versus particle ram/magnetic interactions), it is often a bit separated from the dust tail. As a result, most comets will have two very distinct tails that you can tell apart. These tails are huge, and can be up to an entire astronomical unit in size (1 astronomical unit is roughly 93 million miles long — it’s the distance between the Sun and Earth).

But there’s a final piece to our dazzling story of comets. Comets are often confused with meteors, and for good reason—both are objects in the sky that are relatively small and have long streaked tails. The only difference (visually) is that meteors disappear really fast, since they are actually chunks of dust/rock/ice that enter Earth’s atmosphere and burn up, whereas comets are not actually in Earth’s atmosphere but are just physically large and bright. But these two seemingly unrelated objects share a very intimate connection…meteor showers are actually the debris of comets!

How is this possible? Well, we already discussed the comet tails and how they form, but what actually happens to the material in the tail after it gets blown off the comet? The ion tail is the easiest—since it’s swept into the solar wind, it does what all solar wind particles do—follow along magnetic field lines and sweep out to the far reaches of the solar system. But what about the dust tail? The dust tail is neutral, and so all the dust, rock, and ice ejected from the comet just goes into orbit around the Sun, trailing behind the comet. Since the tail is so long, this dust reaches all the way out into Earth’s orbit and beyond.

You can think of comets as being like dirty children coming into the house after rolling around in dirt. As they run in and begin wreaking havoc, they start spraying dirt all over the floor. You can track where they’ve been by the trail of dirt that they leave behind. So what do you do? You clean it up! In some sense, the Earth is like a giant vacuum cleaner. As it follows its orbit around the Sun, all the left over comet debris will fall into the atmosphere and burn up—causing a meteor shower! So when you see these meteor showers, you are actually just seeing the burn up of cometary debris. So even though meteors are not comets, they do originate from comets!

Let’s face it…comets are awesome. And though they may be a big threat to our existence (they may have killed the dinosaurs), we love them anyway. Next time a comet passes by the Earth’s neighborhood (and they do often), get your butt to a telescope!