Astronomy Ch.9

Meteorites from the Moon and Mars

- A few meteorites arrive on Earth from the Moon and Mars -Their composition differs from those from the asteroid fragments -The group comes from the Moon has rocks similar to those brought back by Apollo & Luna -The group comes from Mars has elemental and isotpic composition similar to rocks and atmosphere analyzed by space crafts

Meteorite

- A rock from space that falls through Earth's atmosphere

Asteroid Facts

- Asteroids are rocky leftovers of planet formation - The largest is Ceres, diameter ~1000 km -There are 150,000 listed in catalogs, and probably over a million with diameter >1 km - Small asteroids are more common than large asteroids

Orbital Resonance

- It occurs when two orbiting objects have their orbital periods in a ratio of integers (for example, 3:2) - they exert a regular, periodic gravitational perturbation to each other (think about pushing a child on a swing) -It could stabilize the orbit if the two objects never closely approach -It more often destabilizes the orbit, in which case the smaller objects are "ejected" away to a nearby orbit where the resonance does not occur -It happens everywhere in the Solar system (e.g., the wide gapes in Saturn's ring)

Asteroids with Moons

- Some large asteroids have their own moons

Meteor

- The bright trail left by a meteorite

QUESTION: Why do comets show two tails?

- The two tails are composed of different materials, one mostly plasma and one mostly dust, and they are separate because of different forces acting on them

QUESTION: Take the diameter of the Earth as 6400km, and there are 1 trillion (1012) objects in the Oort cloud, each with a diameter of 1km. How is the total mass of the Oort Cloud compared to that of the Earth, roughly?

-About a few Earth's mass

Naming of Asteroids

-An observation event is reported to the Minor Planet Center - tied to any possible events previously reported -If the orbit can be determined, it receives a catalog number - The first observer of this asteroid with a determined orbit is declared the discoverer, and gets the honor of naming this asteroid

Anatomy of a Comet

-Coma is atmosphere that comes from heated nucleus. -Plasma tail is gas escaping from coma, pushed by solar wind. -Dust tail is pushed by photons - lagging behind in its orbit as dust particles are heavier

Hale-Bopp, 1997

-Discovered 1995 -Passed perihelion April 1 1997 -Visible to naked eyes for 18 months (!)

Shoemaker-Levy

-Discovered in 1993 -The first comet discovered to orbit another planet (Jupiter) -It fragmented and hit Jupiter on March 17, 1994

Comet Facts

-Formed beyond the frost line, comets are icy counterparts to asteroids. -The nucleus of a comet is like a "dirty snowball." -Most comets do not have tails. -Most comets remain perpetually frozen in the outer solar system. -Only comets that enter the inner solar system "grow" tails.

Asteroid Orbits

-Most asteroids orbit in a belt between Mars and Jupiter -Trojan asteroids follow Jupiter's orbit -Orbits of near-Earth asteroids cross Earth's orbit

Where do comets come from?

-Only a tiny number of comets enter the inner solar system; most stay far from the Sun. -Oort Cloud: Comets on random orbits extending to about 50,000 AU -Kuiper Belt: Comets on orderly orbits at 30-100 AU in disk of solar system

Meteorite Types

-Primitive: Unchanged in composition since they first formed 4.6 billion years ago - Processed: Younger, have experienced processes such as volcanism or differentiation

QUESTION: Which explanation for the asteroid belt seems the most plausible? A. The belt is where all the asteroids happened to form. B. The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. C. The belt is where all the asteroids happened to survive

-The belt is where all the asteroids happened to survive

Origin of Asteroid Belt

-The planetesimals in the asteroid belt evolved in the normal way until Jupiter came close to its current mass -OR caused by Jupiter ejected > 99% of them -The survived are the asteroids we see today in this belt, whose orbits leave gaps in between -Rocky planetesimals between Mars and Jupiter did not accrete into a planet - Jupiter's gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.

Asteroids

-fast-moving objects against the "stationary" sky background - Very high-speed ones can leave trails on a single long exposure; normally their motions can be detected on exposures separated by a few days

How did comets get there?

• Kuiper Belt comets formed in the Kuiper Belt. - Flat plane aligned with the plane of planetary orbits - Orbiting in the same direction as the planets • Oort Cloud comets were once closer to the Sun, but they were kicked farther out by gravitational interactions with jovian planets. - Spherical distribution - Orbiting in any direction