1. The world is not coming to an end in 2012.
• The story started with claims that a planet called Nibiru is headed
toward Earth. This catastrophe was initially predicted for May 2003,
but when nothing happened the doomsday date was moved forward to
December 2012. Then this was linked to the end of one of the cycles
in the ancient Mayan calendar at the winter solstice in 2012 -- hence
the predicted doomsday date of December 21, 2012.
• Nibiru and other stories about wayward planets are an Internet hoax.
There is no factual basis for these claims. If Nibiru or Planet X were
real and headed for an encounter with the Earth in 2012, astronomers
would have been tracking it for at least the past decade, and it would
be visible by now to the naked eye.
• The Mayan calendar isn’t ending on December 21, 2012 any more
than our calendar ends on December 31. The first day of a new cycle
will begin on December 22, 2012.
http://www.nasa.gov/topics/earth/features/2012.html
2. 0
Chapter 19:
Meteorites, Asteroids,
and Comets
3. • Compared with planets, the comets and
asteroids are unevolved objects. They are
much as they were when they formed 4.6
billion year ago.
• By studying comets and asteroids we can
learn about the conditions in the solar
nebula from which the planets formed.
– Comets are the icy remains of the solar nebula.
– Asteroids are the rocky remains of the solar
nebula.
– Meteors are the fragments of comets and
asteroids that fall into Earth’s atmosphere.
4. • 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
5. 0
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.
6. Meteor Showers 0
Most 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
7. Radiants of Meteor Showers
0
Tracing the tracks of meteors in a shower backwards,
they appear to come from a common origin, the radiant.
↔ Common direction of
motion through space.
8. 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.
9. Meteorites 0
Sizes from microscopic dust to a few centimeters.
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).
10. Analysis of Meteorites 0
3 broad categories:
• Iron meteorites
• Stony meteorites
• Stony-iron meteorites
11. • 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.
12. • 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.
13. (Volatiles, are elements and compounds with low
boiling points. Examples include nitrogen, water,
carbon dioxide, ammonia, hydrogen and methane.)
While meteorites can tell us about the origin of the
solar system, they also contain grains of
interstellar matter that predate our solar system.
14. 0
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
15. 0
Planetesimals cool and differentiate;
Forming iron-nickel cores and rocky
mantles.
Collisions broke up the bodies to
form different kinds of meteorites:
Iron meteorites – iron cores
Stony Meteorites – stony mantel.
Meteorites can not have been broken
up from planetesimals very long ago
→ Remains of planetesimals
should still exist.
→ Asteroids
16. Asteroids 0
Small,
irregular
objects,
mostly in the
apparent gap
between the
orbits of Mars
and Jupiter.
Last remains of
planetesimals
that built the
planets 4.6
billion years
ago!
17. Evidence for Collisions 0
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.
18. Colors of Asteroids 0
“Colors” to be interpreted as albedo (reflectivity) at different wavelengths.
M-type: Brighter,
less reddish
asteroids, probably
made out of metal-
rich materials;
S-type: Brighter,
probably iron cores
of fragmented redder asteroids,
asteroids probably made
out of rocky
materials; very
C-type: Dark common in the
asteroids, probably inner asteroid belt
made out of carbon-
rich materials
(carbonaceous
chondrites);
common in the outer
asteroid belt
19. 0
Distribution: C-type asteroids in the outer asteroid belt; S-type asteroids in
inner asteroid belt → may reflect temperatures during the formation process.
However, more
complex
features found:
Vesta shows
evidence for
impact crater
Images of the and lava flows.
Asteroid Vesta
show a complex Heat for
surface, existence of
including a large lava flows
impact crater. probably from
radioactive
decay of 26Al.
Meteorite probably fragmented from Vesta
21. • 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.
22. 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.
24. Throughout history, comets have been considered 0
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
27. • Five spacecraft flew past the nucleus of
Comet Halley when it visited the inner
solar system in 1985 and 1986.
• Since then, spacecraft have visited the
nuclei of Comet Borrelly, Comet Wild 2,
and Comet Temple 1.
28. The Geology of Comet Nuclei 0
Comet nuclei contain ices of water, carbon dioxide, methane, ammonia, etc.:
Materials that should have condensed from the outer solar nebula.
Those
compounds
sublime
(transition from
solid directly to
gas phase) as
comets
approach the
sun.
Densities of comet
nuclei: ~ 0.1 – 0.25 g/cm3
Not solid ice balls, but
fluffy material with
significant amounts of
empty space.
29. Deep Impact Spacecraft 2005
• Released a probe that
Comet Temple 1 slammed
into.
• The nucleus is rich in dust
finer than talcum powder.
It is not solid rock, but has
the density of fresh fallen
snow.
30. Stardust spacecraft
Collected particles
from Comet Wild 2
that were parachuted
back to Earth.
31. 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.
32. Two Types of 0
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
33. 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.
34. Comet Mrkos in
1957 shows how
The gas tail can
change from
night to night
due to changes
in the magnetic
field in the
solar wind.
35. Fragmenting Comets 0
Comet Linear
apparently completely
vaporized during its
sun passage in 2000.
Only small rocky
fragments remained.
36. Fragmentation of Comet Nuclei
0
Comet nuclei are very fragile and are easily fragmented.
Comet Shoemaker-Levy was disrupted by tidal forces of Jupiter
Two chains of impact
craters on Earth’s
moon and on Jupiter’s
moon Callisto may
have been caused by
fragments of a comet.
37. Comets Holmes Eruption
Spectacular outbursting Comet Holmes exploded in size and brightness on
October 24, 2007. It expanded in size until it was bigger than the Sun. This
amazing eruption of the comet is produced by dust ejected from a tiny solid
nucleus. http://www.ifa.hawaii.edu/faculty/jewitt/holmes.html
38. Composite of 19 snapshots of Comet Holmes, showing its changing brightness and
position spanning the period from October 2007 until March 2008. During its
outburst in late October 2007, the comet brightened by a factor of about 500,000 as
a large pocket of volatile material exploded through its crust and spread into
space.
39. The Origin of Comets
• Long period comets
– Most comets are long period comets
– Have long elliptical orbits with periods greater than 200
years
– Orbits are randomly inclined
– Some orbit the sun clockwise and some orbit the sun
counter clockwise.
• Short period comets
– About 100 known
– Periods less than 200 years
– Inclinations within 30 degrees of the plane of the solar
system.
– Most revolve counterclockwise.
40. • 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?
41. The Origin of Comets 0
Many comets are believed to originate in the Oort cloud:
Spherical cloud of several trillion icy bodies,
~ 10,000 – 100,000 AU from the sun.
Gravitational influence
of occasional passing
stars may perturb some
orbits and draw them
10,000 –
100,000
AU towards the inner solar
system.
Interactions with
planets may perturb
Oort Cloud orbits further,
capturing comets in
short-period orbits.
42. Is there a large planet lurking in the Oort Cloud?
Can Wise Find the Hypothetical Tyche?
43. • While some short period comets, such as Comet
Halley, may have been comets from the Oort
cloud that were captured by Jupiter, this cannot be
true of all the short term comets.
• It isn’t possible for some of the short period
comets to obtain the orbits they have if they were
Oort cloud comets captured by a planet.
• There must be another source of comets in our
solar system then the Oort cloud.
44. • In 1951 astronomer Gerard Kuiper proposed that
the formation of the solar system should have left
behind a disk shaped belt of small, icy
planetesimals beyond the Jovian planets and in the
plane of the solar system.
• This is what we now call the Kuiper belt, which is
at ~ 30 – 100 AU from the sun.
• Hundreds of Kuiper belt objects have been found
in orbits extending from Neptune (30 AU) out to
about 50 AU.
• Pluto and Charon are Kuiper-belt objects.
• This is the second source of small, icy bodies in
the outer solar system.
45. Similar belts have been detected around other stars
e.g. Beta Pictoris.
Objects in the Kuiper belt can be perturbed into the
inner solar system and be captured into smaller
orbits becoming short term comets.
The two places where comets originate are the
Oort cloud and the Kuiper belt.
46. How did the Oort Cloud and
Kuiper Belt Form?
• The Kuiper belt probably formed when the solar
system did.
• The objects in the Oort cloud could not have
formed where they are now from the solar nebula.
– They are too far away.
– They aren’t in the plane of the solar system.
– These objects may have formed in the solar system
amongst the orbits of the Jovian planets and then were
ejected from the solar system by the gravity of the
Jovian planets.
47. Impacts on Earth
• 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.
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.
48. 0
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
50. • 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.
51. • 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.
52. 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.
53. 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.
54. • 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
