Nov. 4th - Comets & Outer Solar System

Question 1

Some comets return periodically, such as…

Answer 1

Halley’s comet
* Halley realized (in 1705) that a 1682 comet had returned.
* Became first predicted comet, in 1758

Question 2

Parts of a comet

Answer 2

• 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

Question 3

Parts of a comet

Tail - What direction does it point to?

Answer 3

Always point away from the Sun due to radiation pressure, rather than “behind” comet in its orbit

Question 4

Parts of a comet

Dust Tail

Answer 4

• white: neutral dust in orbit
• being pushed backwards

Question 5

Parts of a comet

Ion Tail

Answer 5

• Blue: ionized dust
• Made of smaller particles, pushed harder - making a straighter tail

Question 6

What are active comet orbits like?

Answer 6

Big elliptical orbits (unlike asteroids)

Question 7

Comet Composition

Answer 7

• 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

Question 8

Comets vs Asteroids

Answer 8

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

Question 9

There are the two cometary “reservoirs” of inactive comets

Answer 9

Comet nuclei:
* An inactive (above) and active (left) cometary nucleus
* It seems only a fraction of surface is active at any one time

Question 10

Kuiper Belt

Cometary origins:

Answer 10

• Originally formed in the outer solar system
• Kuiper Belt is similar to the asteroid belt: leftover planetesimals

Question 11

Oort Cloud

Cometary origins:

Answer 11

• Leftover planetesimals from the jovian planets
• Many of the comets flung out from between the giant planets ended up in the Oort cloud

Question 12

READINGS

Question 13

Comet Composition

Spectra confirm the distant origin of comets because…

Answer 13

they show the presence of compounds that could have condensed only in the cold outer regions of the solar nebula

Question 14

Nucleus

Answer 14

For a comet plunging inward, we call this frozen center (the “dirty snowball” in solid form) the nucleus of the comet.

Question 15

Coma

Answer 15

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.

Question 16

Tail

Answer 16

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

Question 17

2 distinct tails:

Plasma Tail

Answer 17

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

Question 18

2 distinct tails:

Dust Tail

Answer 18

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.

Question 19

What 2 outcomes occur to a comet after its ices can no longer vaporize into gas and escape

Answer 19

• 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.

Question 20

How did spacecraft inform us about comet nature/behaviour?

Answer 20

• 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 **

Question 21

What makes the “third, invisible tails” of comets?

Answer 21

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

Question 22

When can we see meteors?

Answer 22

On any clear night, but more visible on nights when our planet is crossing a comet’s orbit

Question 23

How/when do meteor showers occur?

Answer 23

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

Question 24

How can we figure out where comets come from?

Answer 24

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.

Question 25

2 Reservoirs of Comet Origin

Oort Cloud Origin

Answer 25

These comets must come plunging sunward from the vast spherical region of space that we call the Oort cloud.

The planetesimals that escaped being swallowed therefore tended to be flung off in all directions.
Some may have been cast away at such high speeds that they completely escaped the solar system. The rest ended up on orbits with very large average distances from the Sun, becoming the comets of the Oort cloud. The random directions in which these comets were flung explain why the Oort cloud is roughly spherical in shape.

Be sure to note that the Oort cloud is not a cloud of gas, but rather a collection of many individual comets

Question 26

2 Reservoirs of Comet Origin

Kuiper Belt Origin

Answer 26

A smaller number of the comets that visit the inner solar system have a pattern to their orbits. They travel around the Sun in the same direction and in nearly the same plane as the planets, and their elliptical orbits carry them no more than about twice as far from the Sun as Neptune.

These comets must come from the donut-shaped Kuiper belt that lies beyond the orbit of Neptune.

Question 27

Orbit IN RELATION TO Neptune

What’s Pluto like?

Answer 27

• Pluto orbits the Sun at an average distance of nearly 40 AU, and it takes about 248 years to complete a single orbit.
• Pluto sometimes comes closer than Neptune to the Sun, although there is no danger of collision: Neptune orbits the Sun precisely three times for every two Pluto orbits, and this stable orbital resonance means that Neptune is always a safe distance away whenever Pluto approaches its orbit.

Question 28

MOONS

What’s Pluto like?

Answer 28

• Hypothesized that Pluto’s moons were created by a giant impact similar to the one thought to have formed our Moon.
• A large comet crashing into Pluto may have blasted away its low-density outer layers, which then formed a ring around Pluto and eventually re-accreted to make Charon and the smaller moons. Such an impact may also explain why Pluto rotates almost on its side.

Question 29

1978 - How did observations of Charon’s orbit allow scientists to calculate Pluto’s precise mass?

Answer 29

• 1978 - Observations of Charon’s orbit allowed scientists to calculate Pluto’s precise mass by applying Newton’s version of Kepler’s third law
• From 1985 to 1990, Pluto and Charon happened to be aligned in a way that made them eclipse each other every few days as seen from Earth—something that happens only about every 120 years.

Question 30

Pluto’s Atmosphere

Answer 30

Pluto is very cold, with an average surface temperature of only 40 K, as we would expect at its great distance from the Sun.

Nevertheless, Earth-based observations showed that Pluto has a thin atmosphere of nitrogen, methane, and carbon monoxide, formed by vaporization of surface ices.

The amount and composition of atmospheric gas can change due to seasonal effects on condensation and vaporization that arise both from Pluto’s large axis tilt and its varying distance from the Sun along its elliptical orbit.

Question 31

Tidal Forces between Charon and Pluto

Answer 31

The mutual tidal forces acting between Pluto and Charon long ago made them rotate synchronously with each other, which means that Pluto’s “day” is the same length as Charon’s “month” (orbital period) of 6.4 Earth days