Chapter 12: Dwarf Planets and Small Solar System Bodies

Kirkwood gaps

Gaps in the main asteroid belt. + All of the Kirkwood gaps in the asteroid belt correspond to resonances. + The boundaries of the asteroid belt are set by some of these resonances. For example, the inner boundary of the asteroid belt, at 1.8 AU, corresponds to the 5:1 orbital resonance of Jupiter; the outer boundary, at 3.3 AU, corresponds to the 2:1 orbital resonance.

What made astronomer redefine Pluto's classification?

In 2005, astronomers identified an object more distant than Pluto, which is Eris, and then Eris's moon Dysnomia. They discovered Eris's mass is 28% greater than that of Pluto. Pluto and Eris have similar nitrogen and methane abundances and a relatively large moon. => Should Eris be a 10th planet? They decided to categorized Pluto and Eris as dwarf planets.

Where are the dwarf planets in the Solar System?

In the asteroid belt and the Kuiper Belt.

Fun fact about Pluto discovery

It is a coincidence. Astronomers, through a discrepancies of Neptune orbit, hypothesized the existence of the ninth planet, called Planet X, which has 6 times the mass of Earth. When they discovered Pluto (pointing to the region they thought it is planet X), they realized the mass of Pluto is too small. They also found that the orbital discrepancies of Neptune in 19th century were a mistake. => Pluto was discovered coincidently.

ALH84001

Meteorite found in Antarctica in 1984, thought to be from Mars (strong similarities with rocks on Mars). Surface contained suspected nanobacteria (still in debate). => Meteorites can be pieces of Mars (or even Moons) that were knocked into space by large asteroid impacts - so that researchers can study pieces of Mars in laboratories on Earth.

What is the NASA's mission to track NEOs?

NASA's Wide-field Infrared Survey Explorer (WISE), an infrared telescope in space, surveyed the entire sky during 2010.

Evidence from comets showing that water on Earth comes from Kuiper Belt

Observation by the Herschel Space Observatory shows that the water on the Comet Hartley 2 has the same ratio of hydrogen isotopes as that of the water in Earth's oceans. This suggests that some of Earth's water could have originated in the Kuiper Belt.

How do astronomers measure the rotation periods of asteroids?

Rotation periods for asteroids are measured by watching changes in their brightness as they alternately present their broad and narrow faces to Earth.

What does the remnants of volcanic activity on the asteroid Vesta indicate?

The asteroid is large enough to become differentiated and geologically active.

Hayabusa Spacecraft (Japan)

The first sample - return mission from an asteroid. (asteroid Itokawa/ S-type - 2005)

NEAR Shoemaker Spacecraft (NASA)

The first spacecraft to land on an asteroid (asteroid Eros - 2002)

Sedna

The unofficial name for object 2003 VB12, a distant planetoid with a very elliptical orbit, which takes it from 76 AU out to 937 AU. With such an extended orbit, Sedna requires more than 11,000 years to make a single trip around the Sun.

How many officially recognized dwarf planets are there?

There are 5 as of this writing in the Solar System: + Ceres (found in the asteroid belt) + Pluto + Eris + Makemake + Haumea (Except Ceres, all are found in the Kuiper Belt)

Why are asteroids pieces of the past?

They are primitive planetesimals that did not become part of the accretion process that formed planets. -> They are records of what the early Solar System was like. + Asteroids are composed of the same type of rocky material that became inner planets. + Comets are composed of the same type of icy material that became the outer planets.

Planetesimal

Tiny grains of primitive material stuck together to produce swarms of small bodies.

Why does Antarctica offer the best meteorite hunting in the world?

+ Because the only stones to be found on the ice are meteorites. + Because Antarctica is very dry, Antarctic meteorites also tend to show little weathering or contamination from terrestrial dust or organic compounds.

Pluto's surface

+ Contains an icy mixture of frozen water, CO2, methane, and CO, with flowing nitrogen ice + Few craters -> young surface.

Some characteristics of Pluto

+ Elliptical orbit + Its orbit periodically crosses inside of that of Neptune. + 2/3 the Moon size + Has 5 moons: Pluto and Charon are a tidally locked pair.

Types of comet tails

+ Ion tail: many of the atoms and molecules that make up the coma are ions. The solar wind pushes on these ions, rapidly accelerating them to speeds of more than 100 km/s - far greater than the orbital velocity of the comet itself. Because the particles that make up the ion tail are so quickly picked up by the solar wind, an ion tail is usually very straight. (point directly away from the Sun) + Dust tail: dust particles are much more massive than ions, so they are accelerated more gently and do not reach such high relative speeds as those of the ions. The dust tail often curves away from the head of the comet as the dust particles are gradually pushes from the comet's orbit in the direction away from the Sun.

Ceres

+ Largest body in the main asteroid belt. + Larger than most moons but smaller than any planets. + It contains 1/3 of the total mass in the asteroid belt, but only 1.3% of the Moon's mass. + Water ice mantle surrounds a rocker inner core (water ice take 1/4 of its mass) + Water vapor coming from 2 locations on Ceres indicates that there is water in specific locations on the surface.

Differences between meteoroids/ meteor/ meteorites

+ Meteoroids: cometary and asteroidal debris entering the planet's atmosphere. + Meteorites: meteoroids that survive to reach the planet's surface. + Meteor: the flash of light that we see in the night sky when a small chunk of interplanetary debris burns up as it passes through our atmosphere. "Meteor" refers to the flash of light caused by the debris, not the debris itself.

Category of Meteorites

+ Stony meteorites (90%): similar to terrestrial silicate rocks. It is characterized by the thin coating of melted rock that forms as it passes through the atmosphere. + Iron meteorites: comes from M-type asteroids. They can be recognized by their melted and pitted appearance generated by frictional heating as it streaked through the atmosphere. Heavier than stony meteorites. + Stony-iron meteorites: consist of a mixture of rocky material and iron-nickle alloys. They are relatively rare.

Comet Shoemaker - Levy 9 (from Kuiper Belt) collided with Jupiter

1. The comet orbit was perturbed by Jupiter, making it orbit closer to Jupiter. 2. Tidal stresses broke it into two dozen major fragments, which subsequently spread out along its orbit. (1992) 3. Finally, the entire string of fragments crashed into Jupiter (1994) 4. The impact left scars on Jupiter's surface that persisted for months.

Density of asteroids

1.3 and 3.5 times the density of water. The lower-density asteroids are shattered heaps of rubble, with large voids between the fragments.

Comet

A complex object consisting of a small, solid, icy nucleus, an atmospheric halo, a tail of dust and gas. A comet nucleus is the heart of the comet and contains most of the comet's mass.

What is the Kuiper belt?

A disk-shaped population of comet nuceli extending from Neptune's orbit to perhaps thousands of AU from the Sun.

Kuiper Belt

A disk-shaped population of comet nuclei that begins at about 30 AU from the Sun, near the orbit of Neptune, and extends outward to about 50 AU.

Oort Cloud

A spherical distribution of distant comets around the Sun, extending nearly halfway to the nearest star (~ 100,000 AU away)

When is the tail often formed?

A tail often forms as a comet crosses the orbit of Mars, where the increase in solar heating drives gas and dust away from the nucleus.

Near-Earth Objects (NEOs)

Asteroids and comet nuclei within 1.3 AU of the Sun. Near-Earth asteroids: + Apollo (cross the orbit of Earth and Mars) + Aten (cross Earth's orbit but not Mars) + Amor (cross the orbit of Mars but not Earth's)

Why couldn't early collisions between planetesimals create bodies large enough to differentiate?

Because Jupiter's tidal disruption and possible orbital migration prevented them from forming a larger objects.

Why do meteors in a shower appear to move away from a point?

Because the meteor paths are parallel. They appears to emerge from a vanishing point (called radiant) as in our view of the railroad tracks.

Where is the main asteroid belt?

Between the orbits of Mars and Jupiters

Sub-category of stony meteorites/ Chondrules

Chondrules: small round sphere on stony meteorites. They are once-molten droplets that rapidly cooled to form crystallized spheres ranging in size from that of sand grains to that of marbles. Stony meteorites containing chondrules are known as chondrites. Those without chondrules are known as achondrites. Carbonaceous chondrites are chondrites that are rich in carbon: these are the most primitive of the meteorites.

Names of the categorization of comets

Comets fall into two distinct groups named for scientist Gerard Kuiper and Jan Oort

Classification of Asteroids based on composition

+ C-type (carbon) asteroids: primitive, undifferentiated planetesimals that has largely been unmodified since the origin of the Solar System almost 4.6 billion years ago. + S-type (stony) asteroids: differentiated asteroids. Denser matter such as iron sinking to their centers. Lower-density material - such as compounds of calcium, silicon, and oxygen - floated toward the surfaces of these planetesimals and combined to form mantles and crusts of silicate rocks. They are chemically similar to volcanic rock on Earth. + M-type (metal) asteroids: fragments of the iron- and nickel-rich cores of one or more differentiated planetesimals that shattered into small pieces during collisions with other planetesimals.

Haumea

+ Has slightly larger orbit than Pluto's + Has 2 moons - Hi'iaka and Nakama, enabling astronomers to calculate the system's mass. + Its shape is flattened, since it spins so rapidly, with an equatorial radius is approximately twice its polar radius. + Oblateness of 0.5, the most distorted shape of any of the planets or dwarf planets. + Are covered with water ice. + Maybe the left over after a larger body broke up following a collision.

Eris characteristics

+ Highly eccentric orbit. + The most remote known object in the Solar System when it is furthest from the Sun. + High albedo: 0.96, showing a highly reflecting surface -> must have a coating of pristine ice. Eris is covered with methane ice.

Kuiper Belt Objects (KBOs)/ Trans-Neptunian Objects (TNOs)

+ Located in the innermost part of the Kuiper Belt. + The largest KBOs are similar in size to Pluto and Eris. + The size of KBOs are difficult to determine because although brightness and approximate distance are known, their albedos are uncertain.

Similarity/ Difference of Short-period and Long-period comets

+ Nearly all highly elliptical orbit. Differences: + Most short-period comets in the inner Solar System have prograde orbits. + Long-period comets have prograde and retrograde orbits.

Composition of an Active Comet

+ Nucleus: at the center, smallest component. It is the source of all the material that we see stretched across the sky as the comet nears the Sun. + Coma: The gases and dust driven from the nucleus of an active comet form a nearly spherical atmospheric cloud around the nucleus. + Head: the nucleus and the inner part of the coma. + Tails: long streamers of dust, gas, and ions.

Quaoar

+ One of the largest known KBOs. + One of the few whole-size KBOs that astronomers have independently measured. + More reflective than the nuclei of comets but far less reflective than Pluto. + Has volatile ices on its surface, including crystalline water ice, methane, and ethane.

Birghtness of short-period and long-period comets

+ Short-period comet: as the volatile ices are driven from a nucleus, some of the dust and organics are left behind on the surface. The buildup of this covering slows down cometary activity. + Long-period comet: relatively pristine. More of there supply of volatile ices still remains close to the surface of the nucleus, and they can produce a truly bright show. Example: McNaught (2007), Hale-Bopp (1997)

Type of comets based on the orbital period

+ Short-period comets: have periods less than 200 years. (400 known) + Long-period comets: have periods longer than 200 years. (3000 known)

Makemake

+ Slightly larger orbit than Pluto's + Does not have moons, so less is known about this dwarf planet than about Pluto, Haumea, or Eris.

Charon's surface and atmosphere

+ Surface: - It has deep canyons, which might have formed as an ancient ocean froze and pushed surface outward. - Few craters, young surface. + Atmosphere: no atmosphere

Comet Halley

+ The dust from Comet Halley was a mixture of light organic substances and heavier rocky material, and the gas was about 80 percent water and 10 percent carbon monoxide with smaller amounts of other organic molecules. + It nucleus is one the darkest objects in the Solar System, which means that it is rich in complex organic matter that must have been present as dust in the disk around the young Sun - perhaps even in the interstellar cloud from which the Solar System formed. + Lost 0.1% of its mass as it went around the Sun.

Motion of comets in the Oort Cloud

+ Weak gravitation attraction from the Sun. + The tug of a slowly passing star or interstellar cloud can compete with the Sun's gravity. => If the interaction adds to the orbital energy of a comet nucleus: the comet may move outward to an even more distant orbit or escape from the Sun completely. => A comet nucleus that loses orbital energy will fall inward.

Sungrazers

A member of a family comets, whose perihelia (closest point to the Sun) are located very close to the surface of the Sun. Many sungrazers fail to survive even a single orbit of the Sun. Example: Ikeya-Seki (1965)

Zodiacal dust

A mixture of cometary debris and ground-up asteroidal material.

24 Themis

An asteroid which has water ice covering its entire surface. Organic molecules were also found here. => This discovery may indicate that there is a continuum rather than a strict boundary between icy comets and rocky asteroids. The observations support the idea that both asteroids and comets bring water and organic material to the early Earth.

How does orbital resonance create Kirkwood gaps?

For an asteroid that has a orbital resonance with Jupiter, the tug from Jupiter comes at the same place in its orbit every time. The repeated tugs from Jupiter at the same place add up, changing the asteroid's orbit. Thus, an asteroid in such an orbit would not stay there long. The reason asteroids are not found in the Kirkwood gaps is that their gravitational interaction with Jupiter prevents them from staying there.

Dawn Spacecraft (NASA)

In orbit around Vesta, the second most massive body in the asteroid belt (after Ceres) The data indicate that Vesta is a leftover intact protoplanet that formed within the first 2 million years of the condensation of the first solid bodies in the Solar System. It has an iron core and is differentiated.

Zodiacal light

Just as you can "see" sunlight streaming through an open window by observing its reflection from dust drifting in the air, you can see the sunlight reflected off tiny zodiacal dust particles that fill the inner parts of the Solar System close to the plane of the ecliptic. This dust is visible as a faint column of light slanting upward from the western horizon along the path of the ecliptic, when it is short after dusk. This band is called the zodiacal light. In its brightness parts, the zodiacal light can be several times brighter than the Milky Way.

How many comet nuclei are there?

No one really knows that exact number, since most of these icy bodies are much too small and far away to be seen.

Density of the comet's gas

The gas in a comet's tail is even more tenuous than the gas in its coma, with densities of no more than a few hundred particles per cubic centimeter (compared to 10E19 of the Earth atmosphere). Dust particles in the tail are roughly the size of smoke particles.

Perihelion and Aphelion

The closest/farthest point from the Sun on a planet's elliptical orbit. Perihelion = A(1 - e) Aphelion = A(1 + e)

What is the asteroid belt?

The region between the orbits of Mars and Jupiter, contains most of the asteroids in the Solar System.

How does a comet change when it comes near the Sun?

When very distant from the Sun, the comet is entirely nucleus - frozen throughout. As it approaches the Sun, the coma forms first, and then the tail forms. When the are near enough to the Sun to show the effects of solar heating, they are called active comets.