Nov. 4th - Comets & Outer Solar System Flashcards
Some comets return periodically, such as…
Halley’s comet
* Halley realized (in 1705) that a 1682 comet had returned.
* Became first predicted comet, in 1758
Parts of a comet
- Gas and dust evaporated from comet nucleus - very diffused
- Most well-known feature: the tail
- Dust tail (white) - neutral dust in orbit
- Ion tail (blue) - ionized dust
Parts of a comet
Tail - What direction does it point to?
Always point away from the Sun due to radiation pressure, rather than “behind” comet in its orbit
Parts of a comet
Dust Tail
- white: neutral dust in orbit
- being pushed backwards
Parts of a comet
Ion Tail
- Blue: ionized dust
- Made of smaller particles, pushed harder - making a straighter tail
What are active comet orbits like?
Big elliptical orbits (unlike asteroids)
Comet Composition
- Mostly ice (water ice, carbon dioxide ice, methane ice, ammonia ice)
- Some rocky materials mixed in, and a few metals
- Ices imply: condensed in low temperature regions
- Comets are thus leftover planetesimals which formed outside Jupiter’s orbit
- Therefore, comets originally ‘froze out’ in the outer regions, as a rock/ice mix
Comets vs Asteroids
Asteroids: moderate eccentricity orbits, most between 2-4 au
* Rocky planetesimals, with varying degrees of metal
* Many escaped ones become Near Earth Asteroids (NEA)
Comets: active are big, eccentric orbits - not close enough to be affected by planet gravity?
* Some get inside earth’s orbit (with aphelion at or outside Jupiter) and get bright due to activity
* Some go out as far as 50,000 AU
There are the two cometary “reservoirs” of inactive comets
Comet nuclei:
* An inactive (above) and active (left) cometary nucleus
* It seems only a fraction of surface is active at any one time
Kuiper Belt
Cometary origins:
- Originally formed in the outer solar system
- Kuiper Belt is similar to the asteroid belt: leftover planetesimals
Oort Cloud
Cometary origins:
- Leftover planetesimals from the jovian planets
- Many of the comets flung out from between the giant planets ended up in the Oort cloud
READINGS
Comet Composition
Spectra confirm the distant origin of comets because…
they show the presence of compounds that could have condensed only in the cold outer regions of the solar nebula
Nucleus
For a comet plunging inward, we call this frozen center (the “dirty snowball” in solid form) the nucleus of the comet.
Coma
As the comet accelerates toward the Sun, its surface temperature increases, and ices begin to vaporize into gas that easily escapes the comet’s weak gravity.
Some of the escaping gas drags dust particles away from the nucleus, and the gas and dust create a huge, dusty atmosphere called a coma.
Tail
The coma grows as comets soar into inner solar system, and some of the gas and dust is pushed away from the sun - forming comet’s tails
2 distinct tails:
Plasma Tail
consists of gas that is ionized by ultraviolet light from the Sun and pushed outward by the solar wind; the plasma tail therefore extends almost directly away from the Sun at all times
2 distinct tails:
Dust Tail
Consists of dust-size particles that are unaffected by the solar wind and instead are pushed outward by the much weaker pressure of sunlight itself (radiation pressure)
The dust tail therefore also points generally away from the Sun, but has a slight curve back in the direction the comet came from.
What 2 outcomes occur to a comet after its ices can no longer vaporize into gas and escape
- In some cases, the dust layer may disguise the dead comet as an asteroid.
- In other cases, the comet may come “unglued” and break apart, or even disintegrate along its orbit.
How did spacecraft inform us about comet nature/behaviour?
- The Rosetta mission results show that the water ice that drives cometary activity near the Sun is hidden under a substantial crust of dusty material that is composed of rock and carbon-bearing molecules.
- Rosetta also measured the ratio of deuterium to ordinary hydrogen in water vapor streaming from Comet C-G, finding that the ratio is substantially higher than that for water on Earth. Several other comets also have high proportions of deuterium, suggesting that comets were not the primary source of the water delivered to the early Earth through impacts.
- What, then, was the source of Earth’s water? Primitive meteorites that contain water have a deuterium-to-hydrogen ratio more similar to Earth’s, **suggesting that Earth’s water came from impacts of asteroids that formed in the outer regions of the asteroid belt **
What makes the “third, invisible tails” of comets?
Comets also eject sand- to pebble-size particles that are too big to be affected by either the solar wind or sunlight.
These particles essentially form a third, invisible tail that follows the comet around its orbit. They are also the particles responsible for most meteors and meteor showers.
They enter the atmosphere at such high speeds that they make the surrounding air glow with heat. It is this glow that we see as the brief but brilliant flash of a meteor
When can we see meteors?
On any clear night, but more visible on nights when our planet is crossing a comet’s orbit
How/when do meteor showers occur?
You may see dozens of meteors per hour during a meteor showers, which recur at about the same time each year because: the orbiting Earth passes through a particular comet’s orbit at the same time each year.
The meteors of a meteor shower generally appear to radiate from a particular direction in the sky, for essentially the same reason that snow or heavy rain seems to come from a particular direction in front of a moving car
Thus, meteor showers are best observed in the predawn sky, when part of the sky faces in the direction of Earth’s motion
How can we figure out where comets come from?
The comets that we see in the inner solar system must come from somewhere, and we can figure out where that somewhere must be by tracing their orbits back.
We can then estimate numbers by figuring out how many must reside at great distances to explain the average number that enter the inner solar system each year.
