things that fall from the sky

Things That Fall From the Sky

Comets & Meteors

Questions from Before

• What is Jupiter made of?– Gas (hydrogen-rich gasses and helium), but in a

liquid form.

• Can rocks fall from the sky?– Yup.

Questions for Today

• What is a meteor?

• How old is the Earth?

• Will we all be killed by a giant asteroid?

Meteors: Things That Fall From the Sky

Meteors

• There are many small chunks of matter orbiting the Sun. A piece that is in space is a meteoroid. A piece that burns up in the Earth’s atmosphere

is a meteor (a bright streak of light). A piece that lands on Earth is a meteorite.

Meteors

• Many “meteor showers” are associated with comets.

Dust from Comets

• The dust tail contains small particles evaporated from the comet.

• These particles remain in orbit about the Sun.

• If the Earth passes through the “dust cloud”, then several meteors may be seen.

Meteor Showers

• During periods of high meteor activity, most of the events appear to come from one spot on the sky.

Meteor Showers


• This point is roughly where the comet’s tail was.

Meteor Showers


• This point is roughly where the comet’s tail was.

Dust particles enter the atmosphere and burn up, causing astreak of light.

•

Rocks from Space

• Some early cultures were aware that rocks sometimes fell from the sky. These items had great religious value, e.g. the Black Stone of Ka’aba.

Rocks from Space

• Some early cultures were aware that rocks sometimes fell from the sky. These items had great religious value, e.g. the Black Stone of Ka’aba.

• “Enlightened” scientists in the 18th and 19th centuries declared that stones cannot possibly fall from space. It was all primitive superstition.

Rocks from Space

• Thomas Jefferson said: “It is easier to believe that two Yankee professors [Profs. Silliman and Kingsley of Yale] would lie than that stones would fall from the sky.”

Rocks from Space

• Thomas Jefferson said: “It is easier to believe that two Yankee professors [Profs. Silliman and Kingsley of Yale] would lie than that stones would fall from the sky.”

• Jefferson was wrong: stones do fall from the sky.

Rocks from Space

• Evidence that rocks fall from space:

Rocks from Space

• Evidence that rocks fall from space: There have been eyewitness accounts of

impacts.

Rocks from Space


impacts. In many cases, the mineral composition of

samples indicates the material cannot be native to Earth.

Rocks from Space


impacts. In many cases, the mineral composition of

samples indicates the material cannot be native to Earth.

Most older samples are iron, most “fresh” samples are stony material.

Rocks from Space

Where to Find Meteorites

• Antarctica is one of the best places to find meteorites on Earth, owing to the high contrast (black rocks on white snow).

http://www-curator.jsc.nasa.gov/curator/antmet/program.htm











Where to Find Meteorites

• Over time, meteorites tend to get concentrated in certain areas because of large-scale ice flows.












Meteorites

• Most older samples are iron.

Meteorites

• Most older samples are iron. Iron is dense and not easily weathered.

Meteorites


• Most “fresh” samples are composed of stony materials.

Meteorites


• Most “fresh” samples are composed of stony materials. This material is easily weathered and does not

last long on the Earth’s surface.

Rocks from Space

• Why is are meteorites useful?

Rocks from Space


• They are material samples from outside the Earth that can be analyzed in the laboratory.

Rocks from Space



• We can measure the age of the solar system by studying meteorites.

Radioactive Decay

• A chemical element is uniquely determined by the number of protons its nucleus has. For example, hydrogen has 1 proton, carbon has 6 protons, etc.

Radioactive Decay

• A chemical element is uniquely determined by the number of protons its nucleus has. For example, hydrogen has 1 proton, carbon has 6 protons, etc.

• Different isotopes of the same element differ only in their number of neutrons. For example 12C has 6 protons and 6 neutrons and 14C has 8 neutrons and 6 protons.

Radioactive Decay


• A radioactive isotope is an isotope prone to spontaneous change.

Radioactive Decay


• A radioactive isotope is an isotope prone to spontaneous change.– 14C changes into 14N– 40K changes into 40Ar

Radioactive Decay

• A radioactive isotope is an isotope prone to spontaneous change.– 14C changes into 14N– 40K changes into 40Ar

• The decay rate for a given isotope is fixed and can be measured in the laboratory. The rate is usually given as a “half life”, which is the amount of time required for half of a given sample to decay.

Radioactive Decay

• The decay rate for a given isotope is fixed and can be measured in the laboratory. The rate is usually given as a “half life”, which is the amount of time required for half of a given sample to decay.

• The half life can be as short as a fraction of a second or as long as billions of years.

Radioactive Decay

• For a given atom, there is a certain probability that it will decay.

Radioactive Decay

• For a given atom, there is a certain probability that it will decay.

• For a large collection of atoms, a well-determined half life emerges from the statistics of a large number of events.

Image from Nick Strobel (http://www.astronomynotes.com)

http://www.astronomynotes.com/



Radioactive Decay

• Example: the half life of 40K is 1.25 billion years. Suppose we start with 1 kg. In 1.25 billion years, we have 1/2 kg of 40K and

1/2 kg of 40Ar.

Radioactive Decay


1/2 kg of 40Ar. In 2.50 billion years, we have 1/4 kg of 40K and

3/4 kg of 40Ar.

Radioactive Decay




7/8 kg of 40Ar.

Radioactive Decay

• My measuring the relative amounts of the radioactive “parent” isotope to the resulting “daughter” isotope in a rock, one can measure the amount of time since the rock sample solidified.

Radioactive Decay

• My measuring the relative amounts of the radioactive “parent” isotope to the resulting “daughter” isotope in a rock, one can measure the amount of time since the rock sample solidified.

• In practice one looks at many parent/daughter combinations, and also looks at stable isotopes of the parent and/or daughter.

Radioactive Decay

• The oldest rocks on the Earth were solidified about 4 billion years ago.

Radioactive Decay


• The oldest rocks from the Moon were solidified 4.4 billion years ago.

Radioactive Decay



• The oldest meteorites solidified 4.55 billion years ago.

Radioactive Decay



• The oldest meteorites solidified 4.55 billion years ago. The Sun and the solar system are about 4.6 billion years old.

Rocks from Space



• We can measure the age of the solar system by studying meteorites.

Next:

Minor Planets or Asteroids


• The region between between Mars and Jupiter is populated by thousands of small rocky bodies called minor planets or asteroids.



• Ceres, the largest one with a diameter of 1000 km, was discovered in 1801.



• Ceres, the largest one with a diameter of 1000 km, was discovered in 1801.

• Only 6 are known with a diameter larger than 300 km.

Where Asteroids Are

• Most asteroids are confined to orbits between Mars and Jupiter.

• Some have orbits in Jupiter’s orbit.

• Some have orbits that cross the Earth’s orbit.





Where Asteroids Are

• There are about 150,000 asteroids cataloged. The total population is perhaps 1 million with a diameter of more than 1 km.





Where Asteroids Are

• There are about 150,000 asteroids cataloged. The total population is perhaps 1 million with a diameter of more than 1 km.

• The total mass, however, is small: much less than the mass of the Earth.





Where Asteroids Are

• In spite of what this diagram might imply, the asteroid belt is relatively empty.





Where Asteroids Are

• In spite of what this diagram might imply, the asteroid belt is relatively empty.

• The average distance between any 2 is more than 1 million km.





What Asteroids Are

• Asteroids are basically chunks of rock left over from the formation of the solar system.

What Asteroids Are

• Asteroids are basically chunks of rock left over from the formation of the solar system.

• There are three basic groups: stony, carbon rich, and iron rich.

What Asteroids Look Like

• Asteroids have irregular shapes, and typically have craters and other features.


• A probe crashed into Eros on February 12, 2001.


• A probe crashed into Eros on February 12, 2001.• The chemical composition of Eros is similar to that of

old meteorites, indicating Eros contains “primitive” material.

Next:

The Big One

The Big One

• We know that rocks can fall from the sky. One can ask at least three questions:

The Big One


1) How big can they get?

The Big One



2) How often does it happen?

The Big One



2) How often does it happen?

3) Does it matter?

Evidence from the Past

• The Moon has suffered collisions with large bodies in its history.

• The largest craters are a few hundred km across.


• The Moon has suffered collisions with large bodies in its history.

• The largest craters are a few hundred km across. These require impacting bodies a few dozen km across.


• Mercury has also suffered from bombardment by large bodies in its history.


• The Moon and Mercury are covered with impact craters, which is evidence of a large number of collisions in the past.



• There is no reason to think that the Earth was not also bombarded.



• There is no reason to think that the Earth was not also bombarded.– However, surface features on the Earth are

subject to weathering, so older features are sometimes hard to find.

Craters on Earth

• It is possible to find impact craters on Earth.

• Some are obvious, such as this one in Arizona.

Craters on Earth


• Some are obvious, such as this one in Arizona.

• The impacting body was about 50 meters across, and it fell about 50,000 years ago.

Craters on Earth


• Some are not so obvious, like this one in Quebec.

• It is 100 km across, and about 250 million years old.

http://www.unb.ca/passc/ImpactDatabase/








Craters on Earth

• Other craters are not at all obvious.• This one is near Decaturville, Missouri. It is about 6

km across and about 300 million years old.http://www.unb.ca/passc/ImpactDatabase/








Craters on Earth

• There are more than a hundred documented impact sites on Earth.









What Happens When One Hits?

• The falling body has energy of motion, where E = 0.5 x (mass) x (velocity)2. This energy of motion is converted (rapidly) into other forms of energy upon impact.



• For small objects, most of this energy can be dissipated in the upper atmosphere.



• For small objects, most of this energy can be dissipated in the upper atmosphere.

• For larger objects, some of this energy will be released at the ground level.

Does it hurt?

• The amount of damage depends on the mass of the impacting body and on its speed.

• Bodies with diameters less than a few meters burn up in the atmosphere.

• Bodies larger than a few dozen meters across usually hit the ground, leaving a crater roughly 10 times larger.

Does it hurt?

• Bodies with diameters less than a few meters burn up in the atmosphere.


• Bodies around 50 to 100 meters cause significant local damage (similar to a H bomb).

Does it hurt?


• Bodies around 50 to 100 meters cause significant local damage (similar to a H bomb).

• Bodies larger than 1km cause damage on a global scale.

Does it hurt?

• It is believed that the impact of an asteroid 12 to 15 km in diameter caused the extinction of the dinosaurs.


• Check out the Solar Systems Collisions Page:

http://janus.astro.umd.edu/astro/impact/




How Often?

• The rate of impacts was higher in the early history of the solar system (e.g. about 4 billion years ago).

How Often?


• Eventually, most of the small bodies were used up, so the impact rate dropped.

How Often?


• Eventually, most of the small bodies were used up, so the impact rate dropped.

• However, the present-day impact rate is NOT zero.

How Often?


• The impact rate depends on how big the impacting body is. Smaller bodies are much more common than larger bodies.

How Often?


• The impact rate depends on how big the impacting body is. Smaller bodies are much more common than larger bodies.

• The impact rates are uncertain and are based on a small number of events “observed” in the past.

How Often

• Every day: bodies a few meters across and smaller. These explode in the atmosphere.

How Often


• Once a year: bodies around 10 meters across. Most explode in the atmosphere, but a few make small craters.

How Often



• Once a century: bodies a few 10s of meters across. Atomic bomb-like energies.

How Often?

• A body with a diameter of about 40m hit Tunguska, Siberia in 1908. Trees were knocked down over an area 200 km across.









How Often



• Every million years: bodies around 1 km across. Widespread damage.

How Often


• Every million years: bodies around 1 km across. Widespread damage.

• Every 100 million years: bodies around 10 km across. Mass extinctions.

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