Meteorites, Asteroids, and Comets

Chapter 19:

• Meteoroid = fragment of a comet or asteroid in space

• Meteor = meteoroid colliding with Earth and producing a visible light trace in the sky

• Meteorite = meteor that survives the plunge through the atmosphere to strike the ground

Comets leave a trail of debris behind them as

they orbit the sun.

Meteoroids contributing to a meteor shower are debris particles, orbiting in the path of a comet.

A meteor shower occurs when Earth passes through the orbital path of a comet. The comet may still exist or have been destroyed.

Meteor ShowersMost meteors appear in showers, peaking periodically at

specific dates of the year.

All of the meteors in a given shower have the same origin.

Shower Date R.A. Dec. Associated Comet

Perseids Aug. 10-14 3h4m 58o 1982 III

Leonids Nov. 14-19 10h12m 22o 1866 I Temp

Geminids Dec. 10-13 7h28m 32o

Most meteors we see, whether or not there is a shower, come from comets. Therefore, they are small specks of matter that burn up in the atmosphere.

Meteorites

About 2 meteorites large enough to produce visible impacts strike the Earth every day.

Statistically, one meteorite is expected to strike a building somewhere on Earth every 16 months.

Typically impact onto the atmosphere with 10 – 30 km/s (≈ 30 times faster than a rifle bullet).

Sizes from microscopic dust to a few centimeters.

Analysis of Meteorites3 broad categories:

• Iron meteorites• Stony meteorites• Stony-iron meteorites

• Iron Meteorites – Dense and heavy

– Dark rusted surfaces

– When sliced, polished, and etched with nitric acid, they reveal Widmanstatten patterns caused by crystals of nickel-iron alloys that have grown large. This indicates that the meteorite cooled slowly.

• Stony-iron meteorites are a mixture of iron and stone. They appear to have formed when a mixture of molten iron and rock cooled and solidified.

• Stony Meteorites – Chondrites

• Contain chondrules (rounded bits of glassy rock ranging from microscopic to pea size.)

– They formed from droplets of molten rock that cooled and hardened rapidly when the solar system was young.

– Their presence indicates that the meteorites have not melted since they formed.

• Some chondrites only have a few volatiles indicating they were heated slightly, which caused them to lose their volatiles, but not heated enough to destroy the chondrules.

• Carbonaceous chondrites contain both chondrules and volatile compounds including carbon. They have not been heated since the formation of the solar system.

– Achondrites contain no chondrules and lack volatiles. They appear to have been heated. They are similar to Earth’s lavas.

The Origins of Meteorites• Probably formed in the solar nebula, ~ 4.6 billion years ago.

• Almost certainly not from comets (in contrast to meteors in meteor showers!).

• Probably fragments of stony-iron planetesimals

Asteroids

Last remains of planetesimals that built the planets 4.6 billion years

ago!

Small, irregular objects,

mostly in the apparent gap between the

orbits of Mars and Jupiter.

Evidence for Collisions

Hirayama families: Groups of asteroids sharing the same orbits and spectroscopic characteristics

– apparently result of common origin through collisions.

Radar images of asteroids reveal irregular shapes, sometimes

peanut-like shapes:

Evidence for low-velocity collisions between asteroids

on very similar orbits.

• Not all asteroids are in the asteroid belt.

• A few thousand asteroids larger than 1 km cross Earth’s orbit. – Near Earth Objects (NEOs)– Searches are underway to find these NEOs.

The Origin of the Asteroids

• Ray blasts from Death Stars are unlikely to cause planets to explode as in Star Wars.

• Besides, the total mass of all the asteroids is only ~ 1/20 that of the moon.

• The asteroids probably are not the result of a planet exploding.• Asteroids are probably the remains of a planet that did not

form at 2.8 Au from the sun due to Jupiter’s gravity.• Therefore, asteroids are probably fragments of left over

planetesimals.– The ones in the outer belt formed where the solar nebula was cooler so

carbon could condense. That’s why type C asteroids are in the outer belt and type S are in the inner belt.

Comets

Comet C/2001 Q4

Throughout history, comets have been considered as portents of doom, even until very recently:

Appearances of comet Kohoutek (1973), Halley (1986), and Hale-Bopp (1997) caused great concern

among superstitious.

Comet Hyakutake in 1996

Comet Hale-Bopp in 1997

Comet NcNaught (2007) was visible in the southern sky. It will never return.

When a comet is far from the sun, it’s just the nucleus. When it gets close enough to the sun, it begins to sublime and a coma and tail form.

The coma of a comet is the cloud of gas and dust that surrounds the nucleus. It can be over a million km in diameter, which is bigger than the sun.

Two Types of Tails

gas tail: Ionized gas pushed away from the

comet by the solar wind. Pointing straight away

from the sun.

Dust tail: Dust set free from vaporizing ice in

the comet; carried away from the comet by the

sun’s radiation pressure. Lagging behind the

comet along its trajectory

Comet tails point generally away from the sun, but their precise direction depends on the flow of the solar wind and the orbital motion of the nucleus.

Comet Mrkos in 1957 shows howThe gas tail canchange from night to nightdue to changesin the magneticfield in thesolar wind.

• Comets cannot last more than 100 to 1000 orbits around the sun before all their ice is gone and there is nothing left but dust and rock.

• The comets we see today cannot have been orbiting close to the sun for 4.6 billion years.

• Where do new comets come from?

• Small meteorite impacts occur quite often.

• Every few years a building is damaged by a meteorite.

• A few years ago, a car was hit by a meteorite and then auctioned off for $10,000,000.

• Really large impacts are rare.

Impacts on Earth

In 1954 Mrs. E. Hulitt Hodges of Sylacauga, Alabama was hit by a meteorite while napping in her living room. This is the only known person to have been injured by a meteorite.

Over 150 impact craters found on Earth.

Famous example: Barringer

Crater near Flagstaff, AZ:

Formed ~ 50,000 years ago by a meteorite of ~ 80 – 100 m diameter

Barringer Crater: ~ 1.2 km diameter; 200 m deep

• Sediments from all over the Earth from 65 million years ago have an overabundance of iridium, an element common in meteorites but rare in the Earth’s crust.

• The impact of a large meteorite at that time may have altered the atmosphere and climate on Earth, which caused the extinction of the dinosaurs and 75% of the other species on the planet.

• The biggest extinction we know of occurred 250 million years ago – The Great Dying. – 95% of life in the oceans died out.– 80% of life on land died out.

• Data indicates that a large impact occurred off the shore of Australia 250 million years ago.

The 1908 Tunguska event in Siberia destroyed an area the size of a large city. Here the area of destruction is superimposed on a map of Washington, D.C., and its surrounding beltway. In the central area, trees were burned; in the outer area, trees were blown down pointing away from the center of the blast for as far as 30 km.

The Effects of a Large Impact on Earth

• If on land, the initial shock would be deadly.• If on sea, there would be tidal waves hundreds of

meters high that would devastate coastal regions.• Lots of dust would be thrown into the atmosphere.

– The hot dust falling back to Earth could start fires.

– The dust left in the atmosphere would block sunlight, making temperatures cooler for a time.

• In 1998, newspaper headlines read “Mile Wide Asteroid to Hit Earth in October 2028.”

• Rumors of Earth’s demise were greatly exaggerated. The asteroid will miss Earth by 600,000 miles.

• Now rumor is a 430 mile wide asteroid named Apophis will hit in 2029 or 2036. – Actually Apophis is not 430 miles in diameter but more like 250

METERS. – The future for Apophis on Friday, April 13 of 2029 includes an

approach to Earth no closer than 29,470 km (18,300 miles, or 5.6 Earth radii from the center, or 4.6 Earth-radii from the surface) over the mid-Atlantic, appearing to the naked eye as a moderately bright point of light moving rapidly across the sky.

– Updated computational techniques and newly available data indicate the probability of an Earth encounter on April 13, 2036, for Apophis has dropped from one-in-45,000 to about four-in-a million.

http://www.nasa.gov/home/hqnews/2009/oct/HQ_09-232_Apophis_Update.html