Download Comets and Asteroids

The Planets and the Solar System
VOCABULARY
comet
asteroid
Solar-System Debris
The solar system also includes debris such as
comets, asteroids, and meteoroids.
meteor
meteorite
meteor shower
Haley’s Comet
Asteroid 243 Ida
Comets
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
Appearance of Comets
• Observed since antiquity
• Typical comets appear as rather faint, diffuse spot
of light – smaller than the Moon, and many times
less brilliant.
• Small chunk of icy material that develop an
atmosphere as they get closer to the Sun.
• As they get “very close” they may develop a faint,
nebulous tail extending far from the main body of
the comet.
• Appearance seemingly unpredictable
• Typically remain visible for periods from a few days
to a few months.
Comet
structure
• Theory of comet structure first proposed by Fred
Whipple, Harvard, 1950.
• Nucleus is
– Solid object a few kilometers across
– Composed of mainly water ice, with traces of other ices,
mixed with silicate grains and dust.
– Model known as the “dirty snowball” model.
• Water vapor + other volatiles escape from the
nucleus when heated by sunlight.
• No large fragments of solid matter from a comet
ever survived passage through Earth’s atmosphere
– full composition of the nuclei - not known.
Comet Orbits
•Scientific study of
comets dates back
to Newton who
first recognized
their orbit as
elongated ellipse.
• Edmund Halley (a contemporary
of Newton) calculated/published
24 cometary orbits (1705).
• Noted that the orbits of bright
comets seen in 1531, 1607, 1682
were quite similar – and could be
the same comet – returning to the
perihelion every 76 years. He
predicted a return of the comet in
1758.
• When the comet did appear in
1758, it was given the name
Comet Halley.
Comet Halley
• Observed/Recorded on every passage at
intervals from 74 to 79 years since 239 B.C.
• Period variations caused by Jovian planets
• 1910, Earth was brushed by the comet tail. –
causing much public concern…
• Last appearance in our skies – 1986.
– Met by several spacecrafts
• Return in 2061.
• Nucleus approximately 16x8x8 kilometers.
Comet Components
• nucleus:
relatively solid and stable, mostly
ice and gas with a small amount of
dust and other solids;
• coma:
dense cloud of water, carbon
dioxide and other neutral gases
sublimed off of the nucleus;
• hydrogen cloud:
huge (millions of km in diameter)
but very sparse envelope of neutral
hydrogen;
•dust tail:
up to 10 million km long
composed of smoke-sized
dust particles driven off the
nucleus by escaping gases;
this is the most prominent part
of a comet to the unaided eye;
•ion tail:
as much as several hundred
million km long composed of
plasma and laced with rays
and streamers caused by
interactions with the solar
wind.
Two Types of Tails
Ion 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
Gas and Dust Tails of
Comet Mrkos in 1957
Comet HaleBopp in 1997
Discovery of C/Hale-Bopp
Alan Hale
• Discovered in 1995 by Alan
Hale (professional
astronomer in New Mexico,
and Thomas Bopp (amateur
astronomer in Arizona)
• Both were observing at
their home locations on the
evening of July 22nd-23rd,
1995 with their amateur
telescopes
• Hale-Bopp located at 7.15
AU, just outside the Jupiter
orbit!
“C” for Long Period
• ~ 4200 yrs ago since last
appearance…~2380 yrs
for next appearance
• Closest approach:
– Earth: March 27, 1997 @
1.315 AU
– Sun: April 1, 1997 @
0.914 AU
Some Properties of the Nucleus
of Hale-Bopp
• Known from measurements:
– R  30 km (2nd largest comet!)
– Spin Period  11.5
– Obliquity  86 degrees
• Unknown, but guess:
–
–
–
–
Top: Blue filter image. Bottom: false color version.
Image credit: http://www2.jpl.nasa.gov/comet/ampo145.html
Density  700 kg m-3
Specific heat  1400 J kg-1 K-1
Bond Albedo  0.04
Emissivity  0.9
The Geology of Comet Nuclei
Comet nuclei contain ices of water, carbon dioxide, methane, ammonia,
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.
Fragmentation of Comet Nuclei
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.
Fragmenting Comets
Comet Linear
apparently vaporized
during its sun passage
in 2000.
Only small rocky
fragments remained.
The Origin of Comets
Comets are believed to originate in the Oort cloud:
Spherical cloud of several trillion icy bodies, ~ 10,000
– 100,000 AU from the sun.
Oort Cloud
Gravitational influence
of occasional passing
stars may perturb some
orbits and draw them
towards the inner solar
system. Interactions
with planets may
perturb orbits further,
capturing comets in
short-period orbits.
Oort Cloud
• Estimated 1012 comets in the Oort cloud.
• 10 times this number of comets could be orbiting
the Sun between the planets and the Oort cloud.
• Such objects undiscovered because to small, to
reflect sufficient light to be detectable at large
distances, and because their stable orbit do not
bring them closer to the Sun.
• Total number of comets in the sphere of influence
of our Sun could be of the order of 1013!
• Represents a mass the order of 1000 Earths.
The Kuiper Belt
Second source of small, icy bodies in the outer solar
system:
Kuiper Belt, at ~
30 – 100 AU
from the sun.
Pluto and
Charon may be
captured
Kuiper-Belt
objects.
Kuiper Belt
• First object discovered in 1992.
– Diameter ~ 200 km.
– Period ~ 300 years.
• 60 objects found since then.
• Share orbital resonance with Neptune – two
orbits completed for three by Neptune.
• Nicknamed Plutinos for this reason.
• Speculated that Pluto is the largest example
of this group.
Fate of Comets
•
•
•
•
Comets spent nearly all their existence in the Oort cloud or Kuiper belt
– At a temperature near absolute zero.
As comet enter the Solar System, their “life” changes altogether!
– If they survive the initial passage near the Sun, they return towards
the cold aphelia – and may follow a quasi-stable orbit for a “while”.
– May impact the Sun
– May be completely vaporized as they fly by the Sun
– May interact with a planet
• Final impact
• Speed up and ejection
• Perturbed into an orbit of shorter period.
Each flyby the Sun reduces the size and mass of the nucleus of the
comets.
Few comets end their life catastrophically by breaking apart.
– Shoemaker-Levy 9 broke into ~20 pieces when it passed close to
Jupiter in July 1992.
– Fragments of Shoemaker-Levy captured into a very elongated 2 year
around Jupiter – In 1994 the comet fragments crashed into Jupiter.
Conclusion
• New examples are detected using spectra
analysis at an average rate of 5- 10 per year.
• Comparison of abundances to interstellar
sources showed similarities between comets
• There is a strong link between comets and
interstellar ices.
• Comets give clues about the origins of life,
despite their historical role as omens of death
and destruction.
Meteorites
• Meteoroid = small body in space
Distinguish
between:
• Meteor = meteoroid colliding with Earth and
producing a visible light trace in the sky
(shooting star)
• Meteorite = meteor that survives the
plunge through the atmosphere to strike the
ground
Meteorites
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).
The Origins of
Meteorites
Planetesimals cool and differentiate;
Collisions eject material from
different depths with different
compositions and temperatures.
Meteorites can not have been
broken up from planetesimals very
long ago
→ Remains of planetesimals
should still exist.
→ Asteroids
Meteorite Types
• Iron: primarily iron and nickel;
similar to type M asteroids
• Stony Iron: mixtures of iron and
stony material like type S asteroids
• Chondrite: by far the largest
number of meteorites fall into this
class; similar in composition to the
mantles and crusts of the terrestrial
planets
Meteorite Types
• Carbonaceous Chondritevery:
similar in composition to the Sun
less volatiles; similar to type C
asteroids
•Achondrite: similar to
terrestrial basalts; the
meteorites believed to
have originated on the
Moon and Mars are
achondrites
Meteorite Impacts on Earth
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
Meteor Showers
Most meteors appear in showers, peaking
periodically at specific dates of the year.
The Leonid Meteor
Shower in 2002
Meteoroid Orbits
Meteoroids contributing
to a meteor shower are
debris particles, orbiting
in the path of a comet.
Spread out all along the
orbit of the comet.
Comet may still exist or
have been destroyed.
Only few sporadic meteors are not associated with comet orbits.
Radiants of Meteor Showers
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.
The Perseid Meteor Shower
Asteroids
Asteroids
Last remains of
planetesimals
that built the
planets 4.6
billion years
ago!
Asteroids
• Asteroids are also categorized by their
position in the solar system:
• Main Belt: located between Mars and
Jupiter roughly 2 - 4 AU from the Sun;
further divided into subgroups:
Hungarias, Floras, Phocaea, Koronis, Eos,
Themis, Cybeles and Hildas (which are
named after the main asteroid in the
group).
• Near-Earth Asteroids (NEAs): ones that
closely approach the Earth
Asteroids
• Atens: semimajor axes less than 1.0
AU and aphelion distances greater
than 0.983 AU;
• Apollos: semimajor axes greater than
1.0 AU and perihelion distances less
than 1.017 AU
• Amors: perihelion distances between
1.017 and 1.3 AU;
Where do we find most asteroids
in the solar system?
1.
2.
3.
4.
5.
In a belt between the
Earth and Mars.
In a belt between Mars
and Jupiter.
In a belt far outside the
orbits of the planets.
On highly elliptical orbits,
coming as close to the
sun as Mercury’s orbit,
and reaching as far out as
Pluto’s orbit or beyond.
In elliptical orbits around
Jupiter.
The Asteroid Belt
Most asteroids
orbit the sun in a
wide zone
between the
orbits of Mars
and Jupiter.
(Distances and times reproduced to scale)
The Asteroid Belt
Small, irregular
objects, mostly in
the apparent gap
between the orbits
of Mars and
Jupiter.
Thousands of
asteroids with
accurateely
determined orbits
known today.
Sizes and shapes of the largest
asteroids, compared to the moon
Non-Belt Asteroids
Not all asteroids orbit within the asteroid belt.
Apollo-Amor
Objects:
Asteroids with
elliptical orbits,
reaching into
the inner solar
system.
Some
potentially
colliding with
Mars or Earth.
Trojans:
Sharing
stable orbits
along the
orbit of
Jupiter.
Non-Belt Asteroids
• Trojans: located near Jupiter's Lagrange points
(60 degrees ahead and behind Jupiter in its
orbit). Several hundred such asteroids are now
known; it is estimated that there may be a
thousand or more altogether. Curiously, there
are many more in the leading Lagrange point
(L4) than in the trailing one (L5). (There may
also be a few small asteroids in the Lagrange
points of Venus and Earth (see Earth's Second
Moon) that are also sometimes known as
Trojans; 5261 Eureka is a "Mars Trojan".)
Asteroid Types
Asteroids are classified into a
number of types according to
their spectra (and hence their
chemical composition and
albedo:
• C-type, includes more
than 75% of known
asteroids: extremely dark
(albedo 0.03); similar to
carbonaceous chondrite
meteorites; approximately
the same chemical
composition as the Sun
minus hydrogen, helium
Image of 253 Mathilde, a 66 by
and other volatiles;
48 by 46 km C-type asteroid.
Asteroid Types
On October 29, 1991, the Galileo spacecraft flew past
951 Gaspra, an S-type asteroid situated in the inner
asteroid belt. Gaspra measures 19 by 11 kilometers,
and its faceted shape suggests that it is a fragment
from a larger object that was shattered by collision
roughly 500 million years ago.
• S-type, 17%:
relatively bright
(albedo .10-.22);
metallic nickel-iron
mixed with iron- and
magnesium-silicates;
Asteroid Types
Shape model rendering from radar
data of the M-type asteroid 216
Kleopatra (NASA/JPL).
The refelectance characteristics of
M-type asteroids like Kleopatra
suggest that they may be
composed of iron-nickel which hints
at a possible source for iron
meteorites.
• M-type, most of
the rest: bright
(albedo .10-.18);
pure nickel-iron.
• There are also a
dozen or so other
rare types.
Beyond the Solar System