Small Bodies of the Solar System

Pluto, Comets, Asteroids,

Meteors and Zodiacal Light

PLUTOGod of the Underworld

Pluto Physical Data

• Discovered by Clyde W. Tombaugh in 1930

• Diameter: 2294 km (0.18 Dearth)

• Mass: 1.2x1025 g (0.002 Mearth)

• Density: 2.03 g/cm3

• Rotation Period: 6.39 days

• Tilt of Axis: 96o (retrograde!)

• Surface Temperature: 43 K (-382o F)

Pluto Physical Data

• Orbital Semi-Major Axis: 39.44 AU

• Orbital Period: 247.7 years

• Orbital Inclination: 17.2o

• Orbital Eccentricity: 0.250

• Surface Gravity: 0.06 Earth gravity

• Satellites: 3 known

• Magnetic Field: unknown

The Discovery of Planet X?

• Discovered Feb 8, 1930 by then 24-year old Clyde Tombaugh by coincidence.

• The existence of a planet X was predicted by Percival Lowell using the same techniques that Leverrier used to find Neptune – but turned out to be based on measurement noise.

The Discovery of Pluto

The Plates on which Clyde Tombaugh found Pluto

January 23, 1930 January 29, 1930

Pluto’s Surface• This image was taken by

the Hubble Space Telescope and is the best surface map yet made

• Only contrasts are seen, no features

• Pluto has not yet been visited by spacecraft but “New Horizons” is on the way. Stay tuned for 2015.

• Pluto’s surface is probably a methane and water ice mixture (1) covering a water ice mantle (2) with an ice/rock core (3).

Surfaces of Charon, Nix and Hydra

• This image was taken by the Hubble Space Telescope and is the best surface map yet made

• Only contrasts are seen, no features

• Charon’s surface is mostly water ice due to lower surface gravity.

• Nix and Hydra probably also have water ice surfaces

Nix and Hydra discovered in 2005 by the Hubble Space Telescope's Pluto Companion Search Team

Pluto’s Atmosphere

• Very thin atmosphere (when closest to Sun)

• Envelopes its moon too

• Mostly made of Nitrogen and Methane

• Atmosphere condenses and snows to the surface of Pluto and its moon when they are farther from the Sun

Pluto’s MoonCHARON:• Discovered by James

Christy in 1978• Saw a bump move from

one side of Pluto to the other about every six days

• About half the size of Pluto, orbits retrograde

One of Christy’s images

Named after the ferryman who rowed souls across the River Styx to Pluto's realm in the underworld of Greek and Roman mythology.

Pluto’s Moon

• As seen by Hubble Space Telescope

19,700 km

Pluto

CharonDensity 2.03 g/cm3

70% rock and 30% water ice

much like Triton

Density 1.65 g/cm3

Pluto system to be visited by the New Horizons mission in 2015.

Nix and hydra discovered in 2005 by the Hubble Space Telescope's Pluto Companion Search Team

NOT picture. Best picture currently possible from the ground.

Possible formation of Charon

Charon’s orbit within 0.001 deg of Pluto’s equator supports this scenario.

Pluto’s Orbit not so Strange anymore?

• Pluto’s Orbit is highly eccentric (0.250), highly inclined to the plane of the solar system (17.2o), and its orbit crosses Neptune’s!

• From Jan 23, 1979 and until March 15,1999 Pluto was closer to the Sun than Neptune!

Pluto’s Orgin

• Since Pluto and Charon are thought to have the same composition as Neptune’s Triton, it is suggested that they might be escaped satellites of Neptune (dynamicists say no)

• Or Triton, Pluto and Charon all formed near Neptune as many other Kuiperbelt objects did and only Triton was captured

Many more Trans-Neptunian objects now discovered

Neptune’s satellite Triton

Methane surface sprinkled by impact craters and geysers

Asteroids

The Asteroid BeltTheory 1:• Material between Jupiter and Mars tried to form a

planet when the Solar System was forming, but Jupiter’s gravitational influence wouldn’t let it happen

Theory 2:• A planet did form between Jupiter and Mars

(Asteroidia), and some catastrophic event destroyed the planet, leaving the asteroids

The Discovery of Asteroids

• On the first day of the 19th C. (Jan 1,1801), Ceres (the largest known asteroid) was discovered by Giuseppe Piazzi

• By the end of the 19th C., several hundred were known

• We now know of more than 10,000

Particular AsteroidsCERES: • The largest asteroid by far• Diameter : 914 km• Contains 25% of the mass of all the asteroids combined.• The water asteroid – having more water than the Earth• Dawn to arrive at Ceres 2015, Vesta in 2011.

The next largest are Pallas, Vesta, and Hygiea which are between 400 and 525 km in diameter

All other known asteroids are less than 340 km across

Particular Asteroids

• Ida was encountered by the Galileo spacecraft in 1993

• Ida was found to have a small satellite, Dactyl

• Dactyl’s size is 1.6 x 1.2 km, and orbits 90 km above Ida

IIDA and DACTYL:

Dactyl was blasted of Ida and will eventually fall back. Many asteroids were smashed-up and reassembled.

Comets

Comet West

Comet Basics• Comets are mostly (~50%) water ice

• Comets are referred to as “dirty snowballs”

• They are a mixture of water ice, frozen gases, and silicate materials

• Comets have very eccentric orbits – because their orbits have been recently perturbed sending them much closer to the Sun.

• They only have tails when close to the Sun

• The tails are made of gas and dust released by the comet.

Anatomy of a Comet

Dust Tail

Ion TailComaDirection

of Comet’s Motion

To Sun

Nucleus

Hydrogen Cloud

Comet Halley

• Orbits Sun every 76 years

• Furthest point in its orbit is just beyond the orbit of Neptune

• Nucleus: 15 x 8 x 8 km• Rotates every 7.6 days

1986 Halley mission – first picture of a comet nucleus

Recent Comets

Comet Hyakutake in March of 1996

Image taken by J. De Buizer and J. Radomski of the University of Florida Department of Astronomy

Recent Comets

Comet Hale-Bopp in March 1997

Image taken by J. DeBuizer and J.Radomski of the University of Florida Department of Astronomy

Meteors, Meteorites, and Meteor Showers

Meteor NomenclatureMeteoroids - interplanetary debris

Meteor - Also called “shooting star”

When a meteorite has entered the

atmosphere creating a streak of light

Meteorite - Those few meteoroids that make

it to the Earth’s surface

Types of Meteorites

• Mostly made of iron with about 9% nickel

IRONS:

STONES:• Primarily silicates

similar to Earth rocks

STONY-IRONS:• Mixture of the above

two types

Iron Meteorite Lodged in Tree TrunkIron Meteorite Lodged in Tree Trunk

Meteorite interiors remain cold throughout atmospheric flight and can therefore preserve valuable information on how the solar system formed.

Some inclusions formed in an oxygen-rich environment and date to 4.567 billion years old, while other chondrules were formed in an oxygen setting much like that on Earth and date to 4.565 billion, or less, years old.

Example of what we LearnedHeat from

Shock waves in gas

… so gas was thereT

emp.

[K

]

Meteor Showers

• Associated with debris left behind by comets

• Typically very small meteoroids, so no meteorites are produced during a shower

• Can be as many as 100 meteors per hour in a good shower

Meteor Impacts• One catastrophic meteor impact every 26

million years on average• Might have been responsible for dinosaur

extinction• Earth population of 7.6 billion means fatalities

are <3000/yr>. Watch out for statistics …. (compare 2004 Asian tsunami above 226,000 fatalities)

Barringer crater in Arizona (1200 m in diameter, 200 m deep)

Fatality Probability

2004 Tsunami

Zodiacal Light and Gegenschein

• Zodiacal light is due to dust, concentrated in the plane of the solar system, that reflects the light of the Sun towards our eyes

• Reflection from the dust also causes a patch of light directly opposite the Sun, called the gegenschein

Ecliptic

Sun

Horizon

ZodiacalLight

Zodiacal Light and Gegenschein

• The zodiacal light in the eastern sky before the beginning of morning twilight.

• The planet Venus and the open cluster M44 are also visible in this photograph.

ZodiacalLight