April 10 lecture

Atmospheres in orbit

Before we talk about comets, let's talk about atmospheres some more.

Recall that we noted that all atmospheres are escaping into space, albeit slowly for planets as big or bigger than the Earth.

If the "planet" is really small, any secondary atmosphere that is formed can escape very quickly into space. For example, the escape velocity from the Earth's surface is 11 km/sec, but the escape velocity from the Moon's surface is about 2 km/sec. The escape velocity from Io's surface is about the same, about 2 km/sec. So if the escaping gas is warm enough (has a large enough scale height), it will go right into space in a hyperbolic orbit with respect to Io or the Moon.

Now the Moon has long ago lost any volatiles from its surface, but Io is being cooked to a high temperature in its outer layers, high enough to melt rock! So maybe stuff is being cooked out of Io. What happens to it? Some small fraction will be moving at speeds in excess of 2 km/sec, so it sails off into space from Io. However, it doesn't escape Jupiter!

The escape velocity from Jupiter's surface is about 60 km/sec. The escape velocity from a planet goes as 1/r^1/2 , where r is the distance from the center of a planet. So at Io's distance from Jupiter, about 6 Jupiter radii, the escape velocity from Jupiter would be 1/6^1/2 less than at Jupiter's surface, or about 24 km/sec. Therefore, almost all the material that is cooked out of Io does not escape from Jupiter. It stays in orbit around Jupiter, tending to follow Io's general orbit path. In fact, this cooked-off atmosphere forms a giant donut (torus) around Jupiter.

We have already looked at a movie of this "orbiting atmosphere" (the yellow donut in the simulation).

Some questions:
Do other solar system bodies have "orbiting atmospheres"?
Can Jupiter's ring system be thought of as an "orbiting atmosphere"?
Under what circumstances will an "orbiting atmosphere" extend only partially along the orbit?

Now let's look at possibilities for cooking "orbiting atmospheres" out of very small bodies in the outer solar system. To understand how this works, we need to look at other frost lines besides that of water:

water phase
diagram with frostline

Phase diagrams of water, methane, and nitrogen superimposed:

combo phase
diagram

So there will be a frost line for each of these substances. Schematically:

If there were methane in the outer layers of a small body, the methane would burst out of the surface somewhere between the water frost line and the nitrogen frost line.

In summary, if the body is big enough (say as big as Triton or Pluto), it may be able to hold much of the cooked-off volatiles as a secondary atmosphere. But if it is very small, say only a few tens of kilometers in radius, the volatiles are likely to form an "orbiting atmosphere".

COMETS

A comet can be thought of as a small body, perhaps a few tens of kilometers in size, with boiled-off volatiles as a secondary atmosphere. Because the body cannot hold these gases, the volatiles form an "orbiting atmosphere". Where the atmosphere forms and what it is made of, is determined by where the comet is with respect to a frost line.

comet West of 1975

Wilson-Hubbard 1961

comet Wilson-Hubbard of 1961

Comets, the Kuiper Belt, and the Oort Cloud

What Is a Comet?

The Solar System is made up of much more than just the 8 planets, dwarf planets, and their satellites. Tens of thousands of minor planets also share the same space. A minor planet is simply any body (other than the 8 major planets) that directly orbits the Sun. The two main classes of minor planets are asteroids and comets. Comets are characterized by icy surfaces (in contrast to asteroids, which are generally rocky). Many different species of ice can be found on the surfaces of these bodies: not just H₂O (water), but also CO₂ (carbon dioxide), CH₄ (methane), N₂ (nitrogen), CO (carbon monoxide), and many others. Some comets have orbits that occasionally take them within a few AU of the Sun, causing their water ice to rapidly vaporize. In the most spectacular cases, this creates some of the brightest objects in the sky.

Humans have known about comets since ancient times. Ancient astronomers in China, Babylon, Egypt, and Greece wrote down their observations of them.

Because comets were unpredictable, spectacular, and relatively brief celestial events, they were often feared as evil omens. Some people thought that comets foreshadowed great events. The appearance of Halley's Comet in the year 1066 is commemorated in the ancient Bayeaux Tapestry. Later that same year, William the Conqueror invaded England and established the British monarchy that still exists today. The appearance of Halley's Comet in late 1835 was followed a few months later by the successful rebellion of the Mexican state of Texas.

Halley's Comet

One of the first men to think about comets scientifically was an Englishman named Edmund Halley. By the 17th century, the advances of Kepler, Galileo, and Newton had shown the great orderliness of the heavens, and Halley thought that comets should be no different. Halley observed the "great comet of 1682" and noticed that it had similarities to comets that had appeared in 1531 and 1607. He predicted that the same comet would return again in 1758. When his prediction came true, it was the first proof that comets were regular and predictable. The comet was dubbed "Halley's Comet." It returns to the inner Solar System approximately every 76 years, most recently in 1986 (one of the least spectacular apparitions of this comet).

The Parts of a Comet

When passing close to the Sun, a comet develops 4 major parts: the nucleus, the coma, the dust tail, and the ion tail. The nucleus is the comet itself, an icy minor planet. During a close approach to the Sun, the rapid vaporization of ices from the nucleus creates the coma, which is basically a kind of atmosphere. However, because the nucleus is so small, it does not have the gravity to hang onto an atmosphere. Thus, the coma streams out behind the comet, forming the two tails. The ion tail is made up of gas molecules that become ionized by the Solar Wind and "blown" behind the comet. Thus, the ion tail always points directly away from the Sun. The dust tail, on the other hand, is made up of heavier particles that tend to stream away from the comet in roughly the direction that the comet is moving.

On some comets, the ion tail and the dust tail are not easily distinguishable. On the other hand, Comet Hale-Bopp, which appeared in 1997, showed off its two tails beautifully. The ion tail is the blue one, the color coming from fluorescence of the ionized gases.

comet diagram comet Hale-Bopp

In the past two decades, several spacecraft have had rendezvous with the nucleus of an active comet. This has provided the first pictures of coma formation. It turns out that the vaporization of gases is not spread evenly over the surface. Rather, it is concentrated in a few "jets." Most of the surface is extremely dark, reflecting only 3% of the light that shines on it. The dark surface is made up of rocky dust that is left behind when the ices vaporize, while the jets are areas where vaporizing gases have broken through this dark crust.

This picture is of the nucleus of Halley's Comet, The nucleus is only 16 km across, and the few bright jets are prominently visible. The image was taken by Giotto, a European spacecraft that encountered the comet in 1986.

Halley nucleus

and here is a picture of the nucleus of Comet Tempel 1, taken by the NASA Deep Impact spacecraft just before it launched a projectile at the nucleus. Icy patches are color-enhanced to blue. The Deep Impact projectile impacted the comet on July 4, 2005:
Comet Tempel 1

And finally, here is a movie (Quicktime format) of the November 4 encounter of the Deep Impact spacecraft with Comet Hartley 2.

Two Classes of Comets

Comets can be divided into short-period and long-period varieties. Short-period comets stay closer to the Sun, their orbits tend to have lower inclinations to the plane of the ecliptic, and their orbits are usually prograde. Long-period comets, on the other hand, can stray tens of thousands of AU away from the Sun. Long-period comets often have very high inclinations, and are just as likely to have retrograde orbits as prograde.

Incidentally, some short-period comets are on orbits that cross the orbit of Earth. Debris from these comets periodically enters the Earth's atmosphere, causing the phenomenon of meteors. The Leonid meteor shower, which occurs in November, is caused by debris from Comet Tempel-Tuttle. Most meteors are no bigger than a grain of sand, but their high velocity causes them to flash brightly as they burn up in the atmosphere.

Where Do Comets Come From?

Comets don't last long, once they begin to come close enough to the Sun to develop a coma and a tail. A comet like Halley can only survive through several dozen encounters or so. Thus, it has long been hypothesized that "reservoirs" of comets exist further out from the Sun. These theories have been dramatically confirmed in the last few decades, with the discovery of Centaurs and Kuiper Belt Objects (KBOs). In general, short-period comets have their origins in the Kuiper Belt, while long-period comets originate in the Oort Cloud. Click here for a movie showing an artist's conception of the formation of a comet nucleus from ice-rich particles in the outer solar system.