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ASTA01 @ UTSC – Lecture 14
Chapter 12
The Origin of the Solar System
- Bolides
- Meteorites
- Age determination of solar system bodies
-The story of planet formation:
- From the beginning to planetesimal formation
- Accumulation into protoplanets
- Further collisional evolution
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Bolides = biggest meteors
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22 April 2012, over central California and Nevada
Minivan-sized
~70 ton mass, 5 kton TNT explosion
~1/yr
http://www.youtube.com/watch?v=RrL-cWaYdno
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Meteoroids, Meteors, and Meteorites
• Most meteoroids are specks of dust, grains of sand, or
tiny pebbles.
• Almost all the meteors you see in the sky are produced
by meteoroids that weigh less than 1 g.
• Only rarely is one massive enough and strong enough to
survive its plunge, and reach Earth’s surface.
• Such a rock is called a meteorite (this one was found in
Canada)
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Meteoroids, Meteors, and Meteorites
• Meteorites can be divided into three broad
categories.
• Iron meteorites are solid chunks of iron and nickel.
• Stony meteorites are silicate masses that resemble
Earth rocks.
• Stony-iron meteorites are iron-stone mixtures.
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Meteoroids, Meteors, and Meteorites
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• One type of stony meteorite called
carbonaceous chondrites has a chemical
composition that resembles a cooled lump
of the Sun gas with the hydrogen and
helium removed.
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Meteoroids, Meteors, and Meteorites
• These meteorites generally contain
abundant volatile compounds including
significant amounts of carbon and water.
• They may have similar composition to comet
nuclei.
• Allende meteorite
found in Mexico
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Meteoroids, Meteors, and Meteorites
• Heating would have modified and driven
off these fragile compounds.
• So, carbonaceous chondrites must not have
been heated since they formed.
• Astronomers conclude that carbonaceous
chondrites, unlike the planets, have not evolved
and thus give direct information about the early
solar system.
• Ca+Al inclusion in a chondrite
• Sign of original heating:
• Lightnings in the protoplanetary
disk?
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Meteoroids, Meteors, and Meteorites
• You can find evidence of the origin of meteors
through one of the most pleasant observations in
astronomy: a meteor shower, a display of meteors
that are clearly related
by a common origin.
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Meteoroids, Meteors, and Meteorites
• For example, the Perseid meteor shower
occurs each year in August.
• Orionids have a peak on 21 Oct (2012)
• During the height of the shower, you might
see 25-40 meteors per hour.
• The showers are so named because all their meteors
appear to come from a point in the constellation
Perseus and Orion, respectively
• The fall rate is ONLY <1 /min, so don’t expect to see
many meteors at the same time
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Meteoroids, Meteors, and Meteorites
• Meteor showers are seen when Earth passes
near the orbit of an active or a ‘dead’ comet.
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Meteoroids, Meteors, and Meteorites
• The meteors in meteor showers are produced by
dust and debris released from the icy head of
the comet.
• The Orionids, as well as Eta Aquarids meteor shower (best
viewed from the southern hemisphere) is caused by the Earth
passing through dust released by Halley’s Comet.
• Orbits of many meteorites have been calculated to
lead back into the asteroid belt.
• Asteroid collisions are the 2nd major source of
meteoroids and account for the background of
meteors: in a shower the number of meteors is
typically only 4 times higher than during other times.
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Empirical determination of ages
• Meteorites provide a specific clue
concerning the solar nebula: meteorites can
reveal the age of the solar system and the
relative ages of its components.
• The challenge for modern planetary
astronomers is to compare the characteristics of
the solar system with the solar nebula theory
and tell the story of how the planets formed.
They also need to explain the diverse features of
exoplanetary systems (extrasolar systems), that
is solar systems other than our own.
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The Age of the Solar System components
• The most accurate way to find the age of a
rocky body is to bring a sample into the
laboratory and determine its age by
analyzing the radioactive elements it
contains.
• When a rock solidifies, the process of cooling
causes it to incorporate known proportions of
the chemical elements.
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The Age of the Solar System
• A few of those elements are radioactive
and can decay into other elements, called
daughter elements or isotopes.
• The half-life of a radioactive element is the
time it takes for half of the radioactive atoms
to decay into the daughter elements.
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The Age of the Solar System
• For example, potassium-40 decays into
daughter isotopes calcium-40 and
argon-40 with a half-life of 1.3 billion
years.
• Also, uranium-238
decays with a half-life
of 4.5 billion years to
lead-206 and other
isotopes.
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The Age of the Solar System
• As time passes, the abundance of a
radioactive element in a rock gradually
decreases, and the
abundances of the
daughter elements
gradually increase.
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The Age of the Solar System
• Measuring the present abundances of the
parent and daughter elements allows you
to find the age of the rock.
• This works best if you have several
radioactive element “clocks” that can be used
as independent checks on each other.
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The Age of the Solar System
• To find a radioactive age, you need a sample
in the laboratory.
• The only celestial bodies from which scientists
have samples are Earth, the Moon, Mars, and
meteorites.
• This is no longer true!
• Hayabusa mission during 2003-2010 to asteroid
Itokawa brought back dust samples.
• Stardust mission in 2009 brought back pieces of
comet Wild 2.
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The Age of the Solar System
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The Age of the Solar System
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The Age of the Solar System
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Itokawa
• Asteroid Itokawa dust
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Haybusa returns from asteroid Itokawa, June 2010
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The Age of the Solar System
• Hayabusa mission during 2003-2010 to asteroid
Itokawa brought back dust samples.
• Reentry videos resemble a bolide
• http://www.youtube.com/watch?v=ZCdGQhYLo30
• http://www.youtube.com/watch?v=NoSpRoGNr5M
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Itokawa asteroid dust: battered by nono-impacts
• Asteroid Itokawa dust
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Stardust mission (1999-2006) to comet Wild 2
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Comet Wild 2 surprises
• The material of the comet formed in the
coldest parts of the solar system
• …from particles which were predominantly
not pristine stardust (extrasolar particles)
but from highly heated particles of solar
materials (olivines, pyroxenes etc.)
• How did the hot stuff end up in the outer
solar system?
• Perhaps the solar nebula (protoplanetary
disk) was initially very hot everywhere
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The Age of the Solar System
• The oldest Earth rocks so far discovered
and dated are tiny zircon crystals from
Australia, 4.4 billion years old.
• The surface of Earth is active, and the
crust is continually destroyed and
reformed from material welling up from
beneath the crust.
• The age of these oldest rocks informs you
only that Earth is at least 4.4 billion years old.
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The Age of the Solar System
• Unlike Earth’s surface, the Moon’s surface
is not being recycled by constant geologic
activity.
• So, you can guess that more of it might have
survived unaltered since early in the history of
the solar system.
• The oldest rocks brought back by the Apollo
astronauts are 4.48 billion years old.
The Moon must be at least that old.
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The Age of the Solar System
• Although no one has yet been to Mars,
over a dozen meteorites found on Earth
have been identified by their chemical
composition as having come from Mars.
• The oldest has an age of approximately 4.5
billion years.
• Mars must be at least that old.
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The Age of the Solar System
• The most important source for determining
the age of the solar system is meteorites.
• Carbonaceous chondrite meteorites have
compositions indicating that they have not
been heated much or otherwise altered since
they formed.
• They have a range of ages with a consistent
and precise upper limit of 4.56 billion years.
• The spread of ages between different
meteorite groups is a few million years, less
than +-0.01 billion years
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The Age of the Solar System
• That is in agreement with the age of the Sun,
which is estimated to be (5 +-1.5) Gyr
• This has been calculated using mathematical
models of the sun’s interior that are completely
independent of meteorite radioactive ages.
• Apparently, all the bodies of the solar system formed at
about the same time, some 4.56 billion years ago.
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The Origins of the Solar System
• According to the solar nebula theory, the planets
should be about the same age as the Sun. Here
is the brief timeline of events:
• The primordial gas cloud collapses in about 105 yrs,
forming a rotating disk/nebula
• The nebula cools down sufficiently for silicate rocks to
condense in the form of dust in ~105 yrs
• Turbulence in the disk dies down sufficiently to allow
settling of dust into a thin layer in the midplane of the disk
in several times 105 yrs (Remember Leukippos and
Democritos? They’ve predicted both the rotating nebula
and that process! Of course not the time scale.)
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The Origins of the Solar System
• During the settling, the dust agglomerates into sand and
pebbles by collisions and electrostatic forces.
• Next, the sub-layer undergoes instability: gravity of the
thin layer of dust and stones fragments it into dense
chunks which soon shrink to become km-size
planetesimals. Less than 1 Myr (1 million years passed at
this time from the beginning of the stellar formation
process).
• The nebula enters a period of slow evolution lasting 1-3
Myr, in most cases (although we have observations of
disks which still have a substantial amount of primordial
hydrogen+helium gas while 10 Myr old; one of them is
called TW Hydrae).
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The Origins of the Solar System
• During these several millions of years, terrestrial planets
and solid cores of giant planets assemble in mutual
collisions of smaller solid bodies, planetesimals. Since a
large portion of the nebula is at low temperatures, ices as
well as silicates dominate the chemical composition of
planetesimals.
• After the largest bodies reach the size > 10 km, their
gravity is substantial enough to speed up their buildup in
a “runaway” fashion. Isolated protoplanets grow in such a
way, out of reach of each other’s perturbing gravity force.
• The growth of cores is curbed by the lack of material
located on close enough orbits, which can be destabilized
and accreted (meaning: absorbed) by a protoplanet.
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The Age of the Solar System
• After several Myr, planets in the inner solar system
exhaust the supply of material and stop growing.
• At the same time, giant planet cores grow to the mass
~10 Earth masses. At that point, their massive hydrogen
& helium atmospheres become unstable and in ~0.1 Myr
they acquire a very massive gaseous envelope.
• After the lifetime of the disks expires (3-10 Myr), they are
dispersed, but giant planets are already gas-rich.
• Active planet formation is over after 10-30 Myr
• The next long stage is the removal of Kuiper belt bodies,
mainly comets in the region beyond Jupiter; it can last up
to 500 Myr.
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Chemical Composition of the Solar Nebula
• Everything astronomers know about the
solar system and star formation suggests
that the solar nebula was a fragment of an
interstellar gas cloud.
• Such a cloud would have been mostly
hydrogen (75% mass) with some helium
(23%) and minor traces of the heavier
elements (1.22-1.94)%.
• The lower value is the recently revised
average solar composition.
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Chemical Composition of the Solar Nebula
• Of course, in the sun nuclear reactions
have fused some hydrogen into helium.
• This, however, happens in the core and has
not affected its surface composition.
• Thus, the composition revealed in its spectrum is
essentially the same composition of the solar
nebula gases from which it formed.
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Chemical Composition of the Solar Nebula
• You can see that same solar nebula
composition is reflected in the chemical
compositions of the planets.
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Chemical Composition of the Solar Nebula
• The composition of the Jovian planets
resembles the composition of the Sun.
• Furthermore, if you allowed low-density gases
to escape from a blob of sun-stuff, the
remaining heavier elements would resemble
the composition of the other terrestrial planets
– as well as meteorites.
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Chemical Composition of the Solar Nebula
• The key to understanding the process that
converted the nebular gas into solid matter
is the observed variation in density among
solar system objects.
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Condensation of Solids
• The four inner planets are high-density,
terrestrial bodies.
• The outer, Jupiter-like planets are low-density,
giant planets.
• This division is due to the different ways gases are
condensed into solids in the inner and outer
regions of the solar nebula.
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Condensation of Solids
• Even among the terrestrial planets, you
find a pattern of slight differences in
density.
• The uncompressed densities – the densities
the planets would have if their gravity did not
compress them – can be calculated from the
actual densities and masses of each planet.
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Condensation of Solids
• In general, the closer a planet is to the
Sun, the higher is its uncompressed
density.
• This density variation is understood to have
originated when the solar system first formed
solid grains.
• The kind of matter that is condensed in a particular
region would depend on the temperature of the
gas there.
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Condensation of Solids
• In the inner regions, the temperature
seems to have been 1500 K or so.
• The only materials that can form grains at this
temperature are compounds with high melting
points, such as metal oxides and pure metals.
• These are very dense, corresponding to the
composition of Mercury.
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Condensation of Solids
• Farther out in the nebula, it was cooler.
• Silicates (rocky material) could condense.
• These are less dense than metal oxides and
metals, corresponding more to the compositions of
Venus, Earth, and Mars.
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Condensation of Solids
• Somewhere further from the Sun, there
was a boundary called the ice line –
beyond which the water vapour could
freeze to form ice.
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Condensation of Solids
• Not much farther out, compounds such as
methane and ammonia could condense to
form other ices.
• Water vapour, methane, and ammonia were
abundant in the solar nebula.
• So, beyond the ice line, the nebula was filled
with a blizzard of ice particles.
• Those ices have low densities like the Jovian
planets. satellites
• [The planets are low density due gases they
contain.]
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Condensation of Solids
• The sequence in which the different materials
condense from the gas as you move away from the
Sun is called the condensation sequence.
• It suggests that the planets,
forming at different
distances from the Sun,
accumulated from different
kinds of materials.
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Condensation of Solids
• The original chemical composition of the
solar nebula should have been roughly the
same throughout the nebula.
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Condensation of Solids
• The important factor was temperature.
• The inner nebula was hot, and only metals and
rock could condense there.
• The cold outer nebula could form lots of ices in
addition to metals and rocks.
• The ice line seems to have been between Mars and
Jupiter – it separates the formation of the dense
terrestrial planets from that of the low-density Jovian
planets.
• Astronomers have recently found that Jupiter is rather
poor in Oxygen but overabundant in Carbon. Isotopic
and elemental abundances (mass ratios of elements
and different sub-species of elements) are like those in
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asteroids not comets, which contradicts the importance
Condensation of Solids
• Astronomers have recently found that Jupiter is rather poor
in Oxygen but overabundant in Carbon. Isotopic and
elemental abundances (mass ratios of elements and
different sub-species of elements) in Jupitr are like those in
asteroids, not in comets, which contradicts the importance of
the ice line to inner-vs-outer planet division
• Other reasons for relative unimportance of ice line are:
• Migration and thus mixing of solids in the solar nebula
• Roughly 1:1 mass ratio of ice and rock in comets – not
important enough for locating Jupiter at 5.2 AU from sun.
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