Transcript The Structure of Comets - Oregon State University

This set of slides.
• This material finishes up our brief tour
of the solar system: trans-neptunian
objects, comets, meteors, meteorite
impacts, and asteroids.
• Units covered: 46, 47, 48, 41
Io
• Io is closest to Jupiter.
• Io has a very active interior.
– Experiences tidal forces from Jupiter and
gravitational tugs from Europa, squeezing it.
– Similar to bending a metal clothes hanger
back and forth until it heats up.
• Volcanoes erupt
constantly on Io’s
surface.
– Sulfur spews out
from the surface.
– Low escape
velocity means
that the sulfur
escapes.
Europa
• Europa is covered with
cracks in its surface.
– Red, mineral-rich water
seeps out to make the
crack visible.
• Very few craters.
• Metallic core,
surrounded by a thick
rocky layer.
• Weak magnetic field.
• Heat from the interior is
likely enough to keep a
liquid ocean of water
beneath its icy crust.
Saturn’s Moon Titan
• Titan is Saturn’s largest
moon
– Larger than Mercury.
– Thick atmosphere of nitrogen
(which gives it the orange color.)
– Temperature of 95K
– Atmospheric pressure similar to
Earth
– Huygens probe sent back pictures
of what looks like rivers and lakes
of methane.
Neptune’s Moon Triton
• Triton is larger than Pluto
and almost as big as
Europa.
• Orbits backwards relative
to Neptune’s rotation.
– Likely a captured icy
planetesimal.
• Has an atmosphere that
freezes out on the night
side of the planet.
• Odd surface features.
Pluto and its Moons
• Pluto is the only planet in the solar system that
we have not visited.
• Pluto and its largest moon Charon would fit
comfortably within the United States.
• Charon orbits Pluto at a steep angle to the
ecliptic, and Pluto’s axis is heavily tipped as well.
• Two new moons were discovered in 2005, and
were named Nix and Hydra in 2006.
• Pluto is a mix of water ice, rock, methane and
frozen nitrogen.
• When Pluto is within Neptune’s orbit, it has an
atmosphere.
– As the planet moves further out in to the solar
system, the atmosphere snows out onto the surface.
Trans-Neptunian Objects
• More than 130 have
been discovered, one of
them larger than Pluto.
• Plutinos are bodies that
orbit the Sun at more or
less the same distance
as Pluto.
• Small icy bodies
orbiting the Sun
beyond Neptune’s orbit
are called TransNeptunian Objects, or
TNOs.
The Structure of Comets
•
•
Comets have two primary
parts, the head and the tail.
The head consists of
– The nucleus, a lump
of frozen gas mixed
with loose rock and
dust.
• Only about 10 km
across.
• Dark in color,
probably from
dust and other
materials.
– The coma, the cloud
of evaporated ices and
gases streaming from
the surface of the
nucleus.
• May be 100,000
km wide.
• The tail can be hundreds of
millions of km long, and streams
directly away from the Sun.
Visiting Comets
Comet Tempel 1, visited by
Deep Impact
Comet Wild 2, visited by Stardust
Comet Halley, visited by Giotto
The Origin of Comets
• Comets may originate in
either the Oort Cloud or
the Kuiper Belt.
– Oort cloud is a cloud of
comet-like planetesimals
more than 100,000 AU
from the Sun.
– Oort cloud objects may
have formed near the giant
planets and then were
tossed outwards by
gravitational forces.
• Passing stars or other
gravitational influences
nudge the comets into the
inner Solar System.
How a comet becomes visible
• As a comet moves into the
inner solar system, it is
warmed by the sun.
– Ices on the surface
sublimate (go from solid
to gas) and stream away
from the comet nucleus.
– The sublimated gases
form the coma.
– Escaping gas carries
dust particles outward.
• Solar photons strike the dust
particles, pushing them
away
– Process is called
radiation pressure.
– This forms the dust tail.
• Gas and ions in the coma are pushed
away from the nucleus by the solar wind.
– This forms the ion tail, and usually points
directly away from the Sun.
• Gas in the coma and tail are lit up by the
Sun, making them visible (fluorescence).
Another View of the Process
Meteor Showers
• As a comet orbits the sun,
it leaves a trail of dust
behind it.
• Occasionally, the Earth
passes through one of
these dust trails
– Dust particles enter
Earth’s atmosphere
and burn up
– We see them as
meteors, in a meteor
shower
• The meteors all appear to
be coming from the same
point in the sky called the
radiant.
The Names of Meteor Showers
• We name meteor
showers after the
constellation the
radiant is located
in.
• Perseus –
Perseids
• Leo
Leonids
and so on
2010 Meteor Showers
Name
Date of Peak
Quadrantids January 3, 4
Lyrids
April 21/22
Aquarids May 5, 6
Perseids
August 12, 13
Orionids
October 21, 22
Leonids
November 17/18
Geminids
December 13/14
Moon Phase
First quarter
First quarter
Third quarter
New Moon
Near full
Near full
First quarter
The Heating of Meteors
• When a meteoroid (small
chunk of rock in space)
enters the Earth’s
atmosphere, it heats up and
begins to glow.
– It is now called a meteor.
• Meteors can
leave a trail of
glowing gas
from their
passage
• Large meteors
(> 4 cm or so)
are called
fireballs
Meteorites
• Most meteors burn up in
the atmosphere.
• Some of them survive all
the way to the ground,
hitting the surface.
– These are called meteorites.
• Three kinds of meteorites:
– Iron meteorites: made of
iron, probably came from
the cores of differentiated
asteroids
– Stony meteorites: Made of
silicates, probably from the
outer layers of asteroids
– Stony-iron meteorites: a
mixture of the previous two
• Some stony meteorites look like
loose material fused together
– These are called chondrites
– The clumps are called chondrules
– Sometimes the chondrules are
embedded in a sooty, carbon rich
material (carbonaceous chondrites)
The Energy of Impacts
• Every so often, a large meteoroid
(>10 meter) will strike a planet.
• Energy released by the impact is:
1
EK  m  V 2
2
• Here, m is the mass of the
meteoroid, and V is its velocity
on impact.
• For a 100 kg meteoroid traveling
at 30 km/s, the energy released is
equivalent to 10 megatons of
dynamite!
• And this was a small meteoroid.
Giant Meteor Craters
• Giant meteor
craters can be
found on Earth.
• Barringer Crater
(left) in Arizona.
– Meteor was 50
meters in
diameter
– Crater is a mile
across!
• Manicouagan
Crater (right)
– Meteor was 5
km in diameter
– Crater is 73 km
across!
Mass Extinction Events
• About 65 million
years ago, a 10 kmwide meteoroid
struck the Yucatan
Peninsula
• This impact caused
massive climate
changes, leading to
the extinction of
the dinosaurs and
other forms of life
• Iridium found in a
layer of soil all
over the world is
the “smoking gun”
The Chicxulub Crater
The Asteroid Belt
• Using Bode’s Rule (a simple
mathematical formula that
seemed to predict where
planets would be found), the
asteroid Ceres was discovered
between the orbits of Jupiter
and Mars.
• Soon many more small bodies
were discovered.
• Now we know of 21,785
asteroids, located in the
asteroid belt.
• The asteroids are not the
remains of a shattered planet
– All the asteroids mass
added together is ~1% of
the Earth’s mass.
The Shapes and Sizes of Asteroids
• Asteroids come in all
shapes and sizes
– Ceres is the largest, only
930 km across
– Ceres is massive enough to
pull itself into a sphere
– Most asteroids are not
massive enough
– Eros is potato shaped
• Spacecraft have only
recently visited asteroids
– NEAR landed on Eros
– Discovered craters and a
regolith-covered surface
• Most asteroids are very small 10’s of km across
• Would still cause tremendous
damage if one impacted Earth.
Origin of Asteroids
• A collision with another
• Asteroids are probably
asteroid could have
fragments of planetesimals.
shattered the planetesimal.
• The planetesimal had collected
– Fragments of the inner core
a mixture of rock and metals,
would form the iron-nickel
asteroids.
and then differentiated, creating
– Fragments of the outer shell
a dense metallic core and a
would form the silicate
lighter, silicate-rich outer shell.
asteroids